Theme 5: The deeper earth

5.3 Subduction processes and the subcontinental lithosphere

A597

5.3.11

THEME 5: THE DEEPER EARTH

Session 5.3: Subduction processes and the subcontinental lithosphere CONVENED BY: JAMES CONNOLLY STEFANO POLI

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At subduction zones, rocks get pushed, or maybe pulled, down into the Earth. Regardless of whether they are pushed or pulled, things happen to the rocks as they get deeper. This session invites papers that examine those things as well as what might happen in the surrounding rocks as a consequence. We look forward to lively discussions that shed more light on some of the most controversial issues in the field.

Trace element distribution between antigorite and fluid near the dehydration conditions K.T. KOGA1, B. REYNARD,1 AND I. DANIEL2 1

Laboratoire de Sciences de la Terre, ENS Lyon, 69364 Lyon Cedex 7 ([email protected]) 2 Laboratoire de Sciences de la Terre, Univ. C. Bernard Lyon 1, Bat Geode, F-69622 Villeurbanne cedex We are currently conducting a set of experiments to characterize trace element distribution between antigorite and fluid at conditions relevant to subduction zone tectonic setting (3 to 5 GPa and 500 to 700 ˚C), because antigorite is one of key minerals contributing the fluid release from subducting slab to mantle wedge below the arc. The partitioning data help construct the quantitative geochemical model accounted for fluid release and element segregation, which are ultimately constrained by observations of arc lavas and subducting slab compositions. The synthesis experiments were conducted with anhydrous mineral-oxide mixture. The fluid form of water was introduced into the capsule with quantity ranging from 10 to 60 weight percent of samples, which are up to 6 mg in total. The synthesis conditions were maintained by a belt-type apparatus at Univ. C. B. Lyon 1. The pressure was calibrated against room temperature metal transition (Ce, Hg, Bi, and Tl) and high temperature coesite synthesis. Temperature is monitored by chromel-alumel thermocouples. When thermocouples failed, a power-temperature calibration curve is used to determine the run temperature. The press has shown excellent stability up to 100 hours at the conditions, 3 to 4 GPa at 550˚C. The stability over long duration enabled us to synthesize relatively large crystals of antigorite (10 to 50 microns). Presence of free fluid during experiments is confirmed by direct observation of fluid release from the sealed capsule upon puncturing. Formation of antigorite is confirmed by x-ray diffraction and Raman spectroscopy. We have tested “bulk” analysis method to reconstruct the fluid composition at high-pressure temperature conditions. Elements dissolved in fluid during experiments were recovered by leaching pre-dried samples using diluted acid and base. The remaining leached samples were then dissolved in acid for analysis. Concentrations of elements in these solutions were measured by ICPMS. Our preliminary results show a systematic trace element distribution at conditions 3.3 and 3.9 GPa at 550˚C. The partition coefficients between antigorite and fluid are: (Tl)~0.005 > (Li, Pb, Sr)~0.05 > (Ba, Zn)~0.5 and Ni, Cu, Al, Co, Fe, Cr, Mn, Ga, and In are compatible to antigorite.

A598

Goldschmidt 2004, Copenhagen

5.3.12

The subduction blender – the role of slab-derived hybrid rock-types for volatile and trace element recycling in subduction zones C.J. SPANDLER1, J. HERMANN2, R.J. ARCULUS1 AND J.A. MAVROGENES1,2 1

Department of Earth and Marine Sciences, Australian National University, Australia ([email protected]) 2 Research School of Earth Sciences, Australian National University, Australia The fluxing of H2O, CO2 and trace elements through subduction zones is one of the most important geochemical cycles on Earth. Volatile components released during progressive metamorphism of subducting oceanic crust are thought to cause melting of the overlying mantle wedge, producing arc magmas. The petrological evolution and devolatisation of the principle components of subducting slabs (MORB, sediments, serpentinite) are now well constrained. However, volatile and trace element flux through subduction zones remains highly contentious. The blueschist to eclogitefacies Pouebo Terrane of northern New Caledonia represents oceanic crust that was previously subducted to depths of ~60 km. Within the terrane are high-pressure melange zones consisting of a range of mafic, metasedimentary and ultramafic lithologies that have been strongly sheared and intermixed. These melange zones have been observed on submetre to kilometre scales and comprise a significant proportion of the terrane. The melanges contain hybrid rocktypes, such as carbonate-rich talc and chlorite schists, that are not equivalent to any protolith at the Earth’s surface. Using field relations, bulk-rock geochemistry and oxygen isotopes, we suggest that these hybrid rocks formed during subduction by metasomatism and mixing of serpentinites, mafic rocks and metasediments. Subducted mafic rocks and serpentinites may deliver H2O to sub-arc depths, but fluid release occurs at temperatures too low (>600 ˚C) to dissolve significant amounts of the slabsourced elements that are found in arc magmas. Hybrid rocktypes may comprise a significant proportion of subducting slabs and have the potential to transport large amounts of volatiles to sub-arc depths. Talc and chlorite-rich hybrid rocks will undergo dehydration at relatively high temperatures (~ 800 ˚C) allowing for elevated element solubility in fluids or partial melting of adjacent pelitic rocks. By contrast, carbonates and magnetite in these rocks are stable to very great depths and may be important for recycling some trace elements (REE, HFSE) into the deep mantle. Therefore, hybrid rock-types in subducting slabs may be critical for element and fluid recycling through subduction zones and the evolution of arc magmas.

5.3.13

Source compositional variability beneath the Bicol arc, the Philippines F. MCDERMOTT1, F.G. DELFIN JR2., M.J. DEFANT3, S. TURNER4 AND R. MAURY5 1

Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland ([email protected]) 2 School of Policy, Planning and Development, University of Southern California, Los Angeles, CA 90089, USA 3 Department of Geology, University of South Florida, Tampa, FL 33620, USA 4 Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia 5 Laboratoire de Petrologie et UA 1278, Universite Bretagne Occidentale, 29287 Brest, France Pliocene to Recent volcanic rocks from the Bulusan Volcanic Complex (BVC) in the southern part of the Bicol arc (Philippines) exhibit a wide compositional range (medium- to high-K basaltic-andesites, andesites and a dacite/rhyolite suite), but are characterised by LIL element enrichments and HFS element depletions typical of subduction-related rocks. They exhibit relatively low 87Sr/86Sr ratios (0.7036-0.7039) compared with most volcanics from the Philippine archipelago. Their Pb isotope ratios are also unlike those for other Philippine arc segments and typically plot within and below the data field for the Philippine Sea Basin, implying a pre-subduction mantle wedge similar to that sampled by the Palau Kyushu Ridge (PKR), east of the Philippine Trench. 143 Nd/144Nd ratios are moderately variable (0.51285-0.51300), and samples with lower 143Nd/144Nd tend to have high Th/Nd, high Th/Nb and low Ce/Ce* ratios. Unlike some other arc segments in the Philippines (e.g. the Babuyan-Taiwan segment of the Luzon arc) there is little evidence for the involvement of subducted terrigenous sediment. Instead, the moderately low 143Nd/144Nd ratios in some of the Bicol volcanics appears to result from subduction of pelagic sediment (low Ce/Ce*, high Th/Nd and high Th/Nb) and its incorporation into the mantle wedge as a partial melt. Apart from one sample that exhibits recent Th enrichment, the Useries results indicate minimal recent subduction-related U enrichment (230Th/238U) = 0.96-0.99). Two samples of historic lavas (1968 and 1984) from the Mayon volcano exhibit moderately high (226Ra/230Th) ratios indicating recent Ra enrichment and rapid transit of the magmas to the surface. .

5.3 Subduction processes and the subcontinental lithosphere

5.3.14

5.3.15

Boron isotope systematics in South Sandwich island arc

Heavy boron isotope compositions of back-arc lavas from the southern Lau-basin (Valu Fa Ridge)

S. TONARINI1, W.P.LEEMAN2 AND P.T. LEAT

M. ROSNER 1,2, D. RHEDE 1 AND J. ERZINGER1

1

Ist. Geoscienze e Georisorse. CNR Pisa, Italy Dept. Earth Sci., Rice University, Houston TX, USA 3 British Antarctic Survey, Cambridge CB3 0ET, UK ([email protected]; [email protected]; [email protected]) 2

The South Sandwich volcanic arc is characterized by a relatively simple tectonic context (primitive nature of the underlying crust, presence of low-K tholeiitic rocks, no complex mixture of pelagic and volcanogenic sediments on the down going slab) making it a natural laboratory to study the geochemical processes during transfer of subductionrelated fluids from the slab to the mantle wedge. Boron concentrations and isotope compositions are presented for previously well-characterized low-K tholeiite and tholeiite rocks. The samples show variation in fluid-mobile/fluidimmobile element ratios with high enrichments of B/Nb (2.7 to 55) and B/La (0.7 to 13), similar to that observed in Pacific arcs. δ11B is elevated (15-18‰) in the central part of the arc and decrease at the southern and northern ends of the arc (1214‰). δ11B is roughly positively correlated with B concentrations and with 87Sr/86Sr ratios, whereas no systematic variations are observed between δ11B and fluid mobile elements like Rb, Ba, Sr and Th. The δ11B measured in two forearc peridotites [1] is 9.5 and 10.2‰; these two samples show Rb, Ba and Th enrichment factors very close to those observed in arc-lavas. South Sandwich trench sediments (ODP 701) display a wide range of δ11B between +5 to –13‰, with negative values most common. Both, forearc peridotites and trench sediments, suggest that high δ11B of South Sandwich arc-magmas is not simply inherent to the slab material as measured prior to subduction. Studies on boron isotopes indicate substantial loss of boron (particularly 11B) from slab in the early stages of subduction by dehydration. Survival of the high unusually δ11B signature in SSI lavas implies the presence of 11Benriched fluids (probably formed in the forearc region) in forearc materials down dragged with the subducting slab beneath this arc.

Reference [1] Pearce J.A., Baker P.F., Edwards S.J., Parkinson I.J. and Leat P.T. (2000) Contrib. Mineral Petrol. 139, 36-53.

A599

1

GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany ([email protected]) 2 Present address: Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA ([email protected]) The chemical and isotopic compositions of back-arc volcanics is a window into the mantle behind volcanic arcs. We present new boron isotope and lithium, beryllium and boron concentration data of basaltic to dacitic fresh submarine glasses from the Valu Fa Ridge and the Central Lau basin. The analysis were performed on a Cameca 6f ion probe at the GeoForschungZentrum Potsdam using NIST 610 and 612 standard reference glasses for calibration. Overall δ11B values vary between –9.8 and +23.6 ‰. Replicate analysis on single glass shards indicate significant heterogenetity in boron isotope composition of typically a few permill on the µm scale, whereby the element concentrations are relatively homogeneous. Averaged δ11B values range only between –6.9 and +17.6 ‰ and Li, Be and B concentrations vary from 3.0 to 11.0 ppm, 0.13 to 0.78 ppm and from 0.8 to 13.0 ppm, respectively. In contrast to the MORB like boron characteristics of primitive basalts from the Central Lau basin, the Valu Fa samples range from basaltic to dacitic in composition and have dominantly heavy δ11B values ranging from +4.9 to +17.6 ‰ and elevated boron concentrations. Positive δ11B values and elevated B concentrations in fresh unaltered glassy samples relative to MORB require the involvement of a boron-rich, high δ11B component during magmagenesis at the Valu Fa rigde. Slab derived fluids from the downgoing Pacific plate and/or seawater or altered oceanic crust are possible candidates for the generation of the observed boron compositions. On the basis of the observed positive correlation of δ11B with B/Be it is very likely that positive δ11B values were generated by slab derived fluids. We speculate that serpentinized peridotite within or above the slab is a possible source, which undergoes phase transition of antigorite to prograde olivine at high pressures and temperatures, possibly releasing significant quantities of fluids with heavy boron isotope composition. In summary, δ11B values and B/Be ratios are very high for a back-arc position, indicative of proximity of the Valu Fa Ridge to the Tonga arc.

A600

Goldschmidt 2004, Copenhagen

5.3.16

5.3.17

Quantitative models for the composition of slab-derived fluids

Lawsonite eclogite from the Dominican Republic: Implications for cold subduction

M.D. FEINEMAN1,2, F.J. RYERSON2 AND D.J. DEPAOLO1,3

T. ZACK1, T. RIVERS2, R. BRUMM3 AND A. KRONZ4

1

University of California, Dept. of Earth and Planetary Science, Berkeley, CA 94720-4767 2 Lawrence Livermore National Laboratory, Dept. of Earth Sciences, L-206, Livermore, CA 94551 3 Lawrence Berkeley National Laboratory, Earth Sciences Division, MS 90R-1116, Berkeley, CA 94720 To predict the composition of slab-derived hydrous fluids, and how they change with depth in a subducting slab, requires knowledge of both slab mineralogy and fluid-mineral partition coefficients. New data on trace element partitioning for hydrous minerals, combined with similar data on other minerals from previous studies and phase equilibria experiments, now allow us to estimate bulk fluid-rock partition coefficients for the evolving mineralogy of a subducting slab. The bulk partition coefficients, in conjunction with measured trace element concentrations in altered oceanic crust, enable us to predict quantitatively the trace element compositions of subduction zone fluids and thus better assess the implications of trace element patterns in island arc lavas. Our models predict across-arc geochemical trends that have been found in some arcs (e.g. the Kuriles and Kamchatka). By comparing the across-arc geochemical trends with the fluid composition models, we can constrain the amounts of residual accessory phases permissible in the slab. For instance, residual mica must be less than 1% beyond 100 km slab depth in order to explain the Ba and K enrichment often observed in the back-arc. The thermal structure of the subduction zone plays a very important role in determining the amount and composition of fluids released as subduction proceeds. The Sr/Y ratio, for example, is strongly dependent on the relative proportions of lawsonite, zoisite, and garnet, the reactions governing which are strongly temperature dependent. A cold slab may never leave the lawsonite stability zone, in which case the Sr composition (and Sr/Y) of the fluid is relatively low. A warmer slab, in contrast, may pass out of the lawsonite stability field, and ultimately out of the zoisite stability field, releasing much larger amounts of Sr and H2O and generating fluids with very high Sr/Y. Quantitative predictions of slab-derived fluid compositions attained using our model can ultimately be incorporated into models for arc magma generation, improving our ability to model geochemical trends measured in subduction zone lavas.

1

Mineralogisches Institut, Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany ([email protected]) 2 Department of Earth Sciences, Memorial University of Newfoundland, St. John' s, Newfoundland, Canada 3 IRC, Moritz-von-Rohr-Strasse 1a, 07745 Jena, Germany 4 Geowissenschaftliches Zentrum, Universität Göttingen, Goldschmidtstr.1, 37077 Göttingen, Germany Lawsonite eclogite is a rare rock type that has been described from only five natural occurrences. In contrast, laboratory experiments and thermal models predict that lawsonite eclogite should be widespread in subducted oceanic crust deeper than 1.5 GPa. Here we report a new lawsonite eclogite find from the Dominican Republic [1] that provides further constraints on the conditions of subducted crust. In this sample, lawsonite coexisting with omphacite occurs as both inclusions in garnet and as large porphyroblasts, the latter being partly replaced at their margins by epidote and zoisite. Peak pressure conditions estimated from lawsonite-phengiteomphacite-garnet assemblages were ca 1.6 GPa at a temperature of 360 °C, implying formation under a geotherm of ca. 8 °C/km. Peak temperature conditions of 410-450 °C were in the zoisite eclogite field, suggesting that the sample crossed from the stability field of lawsonite eclogite into that of zoisite eclogite as a result of increasing temperature. A comparison with other reported occurrences indicates that most lawsonite eclogite exhumed at the Earth’s surface formed in accretionary wedges. The rarity of lawsonite eclogite at the Earth' s surface may be principally due to two factors: (i) that in ‘normal’ subduction settings lawsonite eclogite enters the subduction factory and hence is usually not exhumed; and (ii) that in accretionary wedge settings, where the PT path leaves the stability field of lawsonite eclogite due to heating, lawsonite eclogite is only preserved if the exhumation path is constrained to a narrow window where the terminal stability of lawsonite is not crossed. The few existing natural lawsonite eclogite samples therefore provide unique material where several geochemical key parameters can be extracted such as trace-element partition coefficients and stable-isotope compositions in rocks approaching the zone of arc-related fluid release.

References [1] Zack T., Rivers, T., Brumm, R., and Kronz, A. (2004) EJM, submitted.

5.3 Subduction processes and the subcontinental lithosphere

A601

5.3.18

5.3.21

Is the slab component in arc magmas produced by fluid-induced eclogitisation of subducted lower oceanic crust?

The role of fluids in an adakitic volcano: Constraints from U-series in lavas of Guagua Pichincha (Ecuador)

1,2

2

T. JOHN , E.E. SCHERER , K. HAASE

1

AND

1

V. SCHENK

1

Institut für Geowissenschaften and SFB 574, Universität Kiel, 24098 Kiel, Germany ([email protected]) 2 Institut für Mineralogie, Universität Münster, 48149 Münster, Germany Closely associated gabbros and eclogites represent relics of subducted lower oceanic crust within a ca. 200 km long Proterozoic suture zone in central Zambia. Gradual stages of the prograde gabbro-to-eclogite transformation are preserved by disequilibrium textures of incomplete reactions. No evidence for prograde blueschist- or amphibolite-facies mineral assemblages was found in the eclogites. Instead, finegrained intergrowths of eclogite-facies minerals replacing plagioclase indicate the direct eclogitisation of gabbroic precursors. Eclogitisation occurred at 630-690°C and 2.6-2.8 GPa and was accompanied by a channelised fluid flow that produced veins of the peak metamorphic assemblage. Although all of the mafic rocks were subducted, only the gabbros infiltrated by fluid were eclogitised. The eclogites and their veins thus represent relict fluid pathways through subducted oceanic crust, providing direct evidence of channelised fluid flow within a slab. The mafic rocks have MORB-like trace element and initial Nd and Hf isotope compositions. In some eclogites however, the LREE have been strongly fractionated from the HFSE and HREE, an effect that cannot be of magmatic origin but must have occurred during metamorphism. Garnet-whole rock ages based on the Sm-Nd (relatively mobile) and Lu-Hf (relatively immobile) isotope systems are identical within error, suggesting, in conjunction with the petrological evidence described above, that the LREE were fractionated during eclogitisation. Eclogitisation was limited by fluid availability, and the flow of fluids through the rock is the most likely mechanism of LREE fractionation. Fluid-rock ratios reveal that the fractionated rocks reacted with an amount of fluid up to 80% of their mass to create the most depleted REE patterns. The lower gabbroic part of the oceanic crust is an unlikely source for such a large volume of fluid and thus we hypothesise that the fluid originated in the underlying serpentinised lithospheric mantle. If this LREE-rich, HFSE+HREE-poor slab fluid reaches the zone of partial melting in the mantle wedge, it may contribute significantly to the arc signature. We speculate whether this process (i.e., trace element mobilisation during fluid-induced eclogitisation) could be generally responsible for producing the slab component in arc magmas.

J. CHMELEFF AND O. SIGMARSSON Laboratoire Magmas et Volcans, OPGC-Université Blaise Pascal–CNRS, 5 rue Kessler, 63038 Clermont-Ferrand, France ([email protected]; [email protected]) Adakites are geochemically well defined and thought to represent magmas resulting from melting of young (<25Ma) subducted oceanic crust. They often have (238U/230Th) < 1 due to the fact that eclogitic residue will retain uranium more than thorium during partial melting of a metabasaltic crust. However, it is not clear if all adakites have 230Th excess over 238 U. We studied radioactive disequilibria of historical lavas (between 550 and 2000 AD) from the Guagua Pichincha, an Ecuadorian stratovolcano located near the capital city Quito in the NVZ (Northern Volcanic Zone) related to the subduction of the Nazca plate, under the influence of the Carnegie ridge. Major and trace elements indicate that this volcano erupts adakitic lavas (high MgO dacites showing, for example, elevated La/Yb ratios (up to 18)). The historical lavas show 9% variations in (230Th/232Th) and 11% in (238U/230Th) and 226Ra-230Th disequilibria (all mesured by TIMS). All lavas have excess of 238U that increases toward present. Moreover a strong linear correlation exists between (238U/232Th) and (230Th/232Th) in the lavas similar to what is observed in normal volcanic arcs lavas. These observations indicate that the 238U-excesses most likely reflect a stronger role played by fluids than by melts of subducted crust during magma genesis. The fluid contribution vary with time and is the most important in products of the 2000 AD eruption. Furthermore these results suggest that adakites are not only formed by melting of oceanic basaltic crust but could result from a combination of processes. In the case of Guagua Pichincha, the lavas appear to be generated from fluid-induced flux melting of metasomatised mantle wedge (by adakitic melt and fluids from the slab, rich in mobile elements) and that the magma so formed quickly reach the surface (<8000 y) as indicated by Ra-Th disequilibria.

A602

Goldschmidt 2004, Copenhagen

5.3.22

5.3.23

Fluoride melts in subduction zones: Occurrence, origin and implications for mantle metasomatism

Immiscible liquids during high pressure melting of carbonaceous pelites and marls

S. KLEMME

T.B. THOMSEN AND M.W. SCHMIDT

Dept of Mineralogy, University of Heidelberg ([email protected])

Dept. of Earth Sciences, ETH, 8092 Zürich, Switzerland ([email protected]; [email protected] )

Hitherto unknown fluoride melts were found in metasomatized mantle xenoliths from New Zealand [1]. These fluoride melts were only preserved in rock fragments that were carefully polished using non-hydrous polishing liquids. The protogranular spinel wehrlites consist of mm-sized olivine, clinopyroxene, amphibole, accessory minerals as apatite and spinel and, on grain boundaries and in melt pockets on triple junctions, silicate and fluoride glasses. Fluoride glasses occur as veinlets and as thin films on grain boundaries, as well as in melt pockets on triple junctions. The fluoride glass is transparent, slightly yellowish and sometimes contains small secondary clinopyroxenes, and only rarely sulfide blebs or fluid inclusions. The fluoride melts are interpreted to be derived immiscibly from a precursor silicate melt and the most spectacular textural evidence for liquid immiscibility is found in one of the xenoliths. Minerals and melts in the xenoliths were analysed for major and trace elements using electron microprobe and Laser Ablation ICPMS. Trace elements are effectively partitioned between immiscible fluoride and silicate melts. For example, separation of immiscible silicate and fluoride melts fractionates light REE from heavier REE or HFSE from REE. [1]. In many cases, silicate glasses found within mantle xenoliths are products of infiltration of the host magma into the mantle xenoliths during ascend. This is, however, not the case here as comparison of the major and trace element composition of the host lava with the silicate glass indicates. The major element composition of the immiscible silicate glasses is characterised by extreme enrichment in Mg and Al, which places them close to high-Mg magmas that are commonly found in subduction zones. The genetic link to subduction zones is further substantiated by extremely low high-field strength element concentrations (e.g., Ti, Zr, Hf) that are characteristic for magmas observed in subduction zone settings.

Phase relations were studied in simplified model systems (KNCaFMAS-HC and KCaMAS-HC) at 2-5 GPa, 750-1300 °C, targeting the composition of melts, the conditions of melting, and just-below-the-solidus reactions in carbonaceous pelites and marls. Bulk compositions had saturation hierarchies quartz>kyanite>mica>carbonate, comparable to natural marls consisting of siliceous, pelitic and carbonaceous components. Experiments were carried out in end-loaded piston cylinders and Walker-type multi anvil apparati. Starting materials were mixtures of glasses or gels with mixed fluid (H2O-CO2) added as Al(OH)3 and CaCO3. For the KNCaFMAS-HC system a Fe-rich marl from the Antilles (Plank&Langmuir 1998, Chem.Geol.145,325f) was used. Melting relations were constructed from the experimental results. At fluid-absent conditions, 3.7 and 5 GPa, KNCaFMAS-HC system, phengitic micas disappear near 1000 °C forming a potassic granite melt coexisting with calcite, cpx, garnet and kyanite. At higher temperatures (~1100 °C) a Ca-carbonatite and a silicate melt coexist with garnet, kyanite and cpx defining a distinct two-melt field. Reaction textures and mass balance calculations imply achievement of equilibrium in the experiments. At 3.7 GPa and 1050 °C the calculated silicate melt fraction in the system of 0.32 is in agreement with modal estimates and results from incongruent melting out of phengite in a narrow temperature interval at the solidus. Very surprising, the silicate melts are poor in Na and coexist with a cpx that has up to 80% jadeite component (cpx/meltDNa up to 6.5). At 1100 °C the calculated silicate melt fraction is 0.43 and that of the carbonatite liquid 0.03, however, in the absence of segregation, analyses of the droplets of carbonatite liquid are difficult. At 2.0-3.0 GPa (in the simplified KCaMAS-HC), zoisite+ cpx are present in addition to the saturation phases at subsolidus conditions. Melting at 850-900 °C is still dominated by the micas, and results in potassic silicate melts. At higher temperatures, carbonatites are expected to appear at above 2-2.5 GPa. Depending on pressure, carbonates disappear during melting near 850 °C and kyanite saturated melts form garnet only at 150 oC higher temperatures. The observed melting of the carbonates provides a mean of transferring CO2 into the mantle wedge (a process which is completely inefficient with fluids), where the carbonatites and potassic granites would react and probably get stuck, thus metasomatizing the mantle.

References [1] Klemme S. (2004) Geology, in press.

5.3 Subduction processes and the subcontinental lithosphere

5.3.24

Non-slab melt origin for adakites from Mindanao, Philippines C.G. MACPHERSON1,2 , B.L.A. CHARLIER1, D.J. MORGAN1, S. DREHER1 D.G CHERTKOFF1 AND D.A. JERRAM1 1

Department of Earth Sciences, University of Durham, UK ([email protected]) 2 SE Asia Research Group, Department of Geology, Royal Holloway University of London Several suites of Plio-Pleistocene andesitic and dacitic stocks occur in the Surigao peninsula in north-eastern Mindanao. These are the first phase of magmatism resulting from initiation of subduction of the Philippine Sea Plate at the Philippine Trench. Whole rock geochemistry suggests that the majority can be classed as adakites (e.g. elevated Sr/Y and low Y and Yb). In some suites there is a relationship between Sr/Y and the modal abundance of plagioclase and crystal size distribution of plagioclase provides evidence for complex, multiple populations of phenocrysts. Furthermore, there is inter- and intra-crystal variation in 87Sr/86Sr; the cores of plagioclase crystals in one rock displaying a similar range of 87Sr/86Sr to whole rocks from the entire peninsula. This provides strong evidence for open system behaviour of Sr. Within individual suites trace element ratios correlate with SiO2 suggesting mixing of basic and evolved melts and the adakitic signal is associated with the latter. The geochemistry of the stocks varies systematically across the peninsula, with enrichment of more incompatible elements at greater distance from the trench. These variations mirror across-arc changes in the chemistry of Neogene basement volcanics providing a possible link between the two. Furthermore, the 187Os/188Os of rocks from throughout the peninsula are low, suggesting a relatively juvenile source for the magmas which is inconsistent with melting of 50Ma Philippine Sea Plate. The geochemical and textural evidence indicates that most Plio-Pleistocene magmas behaved as open systems in the arc lithosphere. Whole rock and crystal geochemistry is not consistent with melting of the subducted Philippine Sea Plate. The data suggest that the adakitic signature has a source in the arc lithosphere that has been incorporated into more basic magma generated by fluid fluxing of the mantle wedge above the current subduction zone. This adakitic source is probably remelting of basaltic material from a previous arc or the present day subduction zone.

A603

5.3.25

Experimental constraints on pelite melting in subduction zones: A new approach using HP metapelites B.J. LEVAY

AND D.M. KERRICK

Department of Geosciences, The Pennsylvania State University, University Park, PA 16802 ([email protected]; [email protected]) Pelitic sediments transport nearly all of the large-ion lithophile elements and continental isotope signatures entering a subduction zone, and these elements are released into the overlying mantle wedge creating the characteristic signatures of subduction magma. Whether the transfer occurs through aqueous fluids, silicate melt, or a supercritical silica-rich phase is still debated, but the discrepancies in previous sediment melting conditions have left doubts about whether all forms of element transfer are even possible in a subduction zone. To simulate a more realistic metamorphic environment, we have located the solidus and characterized the melt compositions for a high-pressure metapelite that has undergone subduction. Other sediment melting experiments have used highly metastable starting materials and have necessarily assumed closed system behavior. However, a growing body of evidence from UHP and HP terrains suggests that prograde metamorphism during subduction involves localized and regionalized metasomatism. The most appropriate starting material for subduction zone melting, therefore, would be a pre-anatectic metapelite that has undergone the chemical and mineral changes associated with prograde metamorphism. Our starting material is a minimally retrogressed sample of the Gåsetjørn gt-st-ky pelite from the ophiolite sequence in the Solund Basin, Western Gneiss Region of Norway. The metasediments were subducted to 50km and locked in peak metamorphic conditions of 1.5 GPa and 600 °C [1]. As observed in other HP and UHP ophiolites, our pelites were depleted in alkalis and calcium by metasomatism. Our results show that the solidus is located between 625 and 650 °C at 2 GPa, which is lower than any previously reported solidus for pelitic compositions. Thus, our results support sediment melting as a possible mechanism for arc magma enrichment.

References [1] Hacker B.R., Anderson T.B., Root D.B., Mehl L., Mattinson J.M., and Wooden J.L. (2003) J. Metamorphic Geol. 21, 613-629.

A604

Goldschmidt 2004, Copenhagen

5.3.31

5.3.32

Noble gases (He, Ne and Ar) in seafloor and high pressure metamorphic deep-sea sediments

Re-Os systematics of the UHP DabieSulu terrain, China: Results from Qinglongshan

W.H SCHWARZ, M. TRIELOFF AND R. ALTHERR

L.C. REISBERG1 , X. ZHI2 AND L. ZHENG2

Mineralogisches Institut, Universität Heidelberg, Im Neuenheimer Feld 236, D-69120 Heidelberg ([email protected]) It was suggested that solar type He and Ne in Earth´s mantle are due to subduction of extraterrestrial dust [1]. We measured He, Ne and Ar extracted by stepwise heating from magnetic separates of two pelagic seafloor sediments of a drillcore (LR44-GPC-3) in the pacific ocean and three metasediments (different layers) from Andros (Greece) and Laytonville (California/USA) both formed at c. 8kbar and 400°C suggesting a maximum subduction depth of c. 20km. The two seafloor sediment samples contain solar wind implanted He and Ne. The 3He/4He ratios vary between 1.71 ± 0.05 (1 ) and 2.47 ± 0.04 ·10-4 indicating SEP He. 20Ne/22Ne (between 10.75 ± 0.31 to 11.21 ± 0.35) and 21Ne/22Ne ratios also agree with SEP composition. The 40Ar/36Ar ratios range from 297 ± 7 to 578 ± 16 (due to 40Ar from 40K-decay) with 38 Ar/36Ar ratios from 0.184 ± 0.002 to 0.194 ± 0.008, indistinguishable from air composition. No 3He was detected in the five metasediment samples, while the 4He amount was very high suggesting radiogenic He (e.g. from alpha decay). The Ne isotope ratios were affected by nucleogenic production of 21Ne (from 18O) and 22Ne (from 19F). The 20 Ne/22Ne ratios are c. 9.80 (air ratio), 21Ne/22Ne ratios are between atmospheric composition and 0.1003 ± 0.0013. 40 Ar/36Ar ratios are always above atmospheric composition with a maximum of 19755 ± 51, with 38Ar/36Ar ratios within the range of air. While solar He and Ne are present in the seafloor sediments, these components could not be detected in the metasediments. Their He, Ne and Ar compositions were overprinted by radioactive decay (He from U or Th; 40Ar from 40 K) and nucleogenic reactions. These results do not support significant subduction of IDP-derived solar type noble gases into the Earth’s mantle.

Reference [1] Anderson D.L. (1993) Science 261, 170-176.

1

Centre de Recherches Pétrographiques et Géochimiques (CRPG/CNRS), BP 20, 54501 Vandoeuvre-les-Nancy, France ([email protected]) 2 School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China ([email protected]; [email protected]) The Dabie-Sulu terrain, located in eastern central China, is the largest ultrahigh pressure orogenic belt in the world. This UHP belt was formed by crustal subduction and subsequent exhumation during the collision of the Yangtze and SinoKorean plates at about 220 to 240 Ma. One particularly interesting feature of the Dabie-Sulu orogen is the remarkably 18 O-poor signature of many of the subducted rocks, which has been interpreted to result from the circulation of meteoric water prior to the time of subduction [1, 2]. This low δ18O signature is particularly well-developed in the Qinglongshan locality of the Sulu region. We report here preliminary results from a Re-Os isotopic study of the Qinglongshan eclogites. Whole rock samples, separated by several meters in outcrop, yield a Re-Os isochron with an age of about 228 Ma, consistent with the accepted age of subduction. This implies that Os isotopic equilibration occurred over distances of several meters during subduction. Interestingly, the initial 187Os/188Os ratio of the isochron (~ 0.3) is relatively unradiogenic. This is surprising given that the basaltic protoliths of these rocks are at least 700 Ma old. Basalts with typical Re/Os ratios would develop much higher 187 Os/188Os ratios in 700 Ma. However, the Re concentrations of these rocks (5 to 56 ppt) are much lower than those of most basalts. Thus the simplest explanation of the low initial Os ratio is that Re loss occurred soon after formation of the basaltic protolith, limiting the extent of radiogenic ingrowth of 187 Os. Assuming that this Re loss was provoked by the episode of pervasive, cold meteoric water circulation suggested by the oxygen data, the Re-Os results indicate that this event predated subduction by several hundred Ma. Thus the Re-Os data are consistent with the suggestion (e.g. [2]) that meteoric water circulation at Qinglongshan occurred during a Neoproterozoic "snowball Earth" glaciation.

References [1] Zheng Y.-F., Fu B., Li Y., Xiao, Y. and Li S. (1998) EPSL 155, 113-129. [2] Rumble D., Giorgis D., Ireland T. et al. (2002) GCA 66, 2299-2306.

5.3 Subduction processes and the subcontinental lithosphere

5.3.33

5.3.41

Re-Os behaviour during subduction of oceanic crust

Origin of the ultrapotassic volcanics within the Late Cretaceous to Early Tertiary Ulukı la Basin, Central Anatolia, Turkey

C.W. DALE1, A. GANNOUN1, T.W. ARGLES1, K.W. BURTON1, I.J. PARKINSON1 AND A.P. BOYLE2 1

Dept of Earth Sciences, Open University, Milton Keynes, MK7 6AA, United Kingdom. ([email protected])

2

Dept of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, U.K.

Fax: 0044-1908-655151

Ocean island basalts (OIB) often possess high 187Os/188Os isotopic ratios that are thought to result from a component of recycled oceanic crust in the mantle source (e.g. HIMU). Oceanic crust has a high Re/Os ratio and, over time, develops a high 187Os/188Os ratio; the system is therefore an excellent tracer for subducted crustal material in the mantle. However, the available data from basalts suggest that up to 60% of Re may be lost during high pressure metamorphism, resulting in lower Re/Os ratios in the subducted slab [1]. In this case, large quantities of oceanic crust or very long residence times in the mantle source would be required to generate the observed OIB Os compositions. This study offers a comprehensive account of the Re-Os budget of a portion of subducted oceanic crust. Data from metastable gabbros and corresponding eclogites from the Zermatt-Saas ophiolite, which have undergone very highpressure (HP) metamorphism (2.0-2.2 GPa), yield a best-fit regression consistent with a U-Pb zircon age of 163.5 Ma [2]. This suggests that gabbroic bodies remain closed, with respect to the Re-Os system, during subduction. However, data presented for metabasalts from the same ophiolite, lie to the upper left of this best-fit line, and have a median Re value of 254 ppt, which is substantially lower than published data for MORB glasses (870 ppt) [3]. This evidence indicates that these HP eclogite facies metabasalts have experienced Re loss, in agreement with a previous study [1]. A suite of pillow basalts of progressive metamorphic grade (chlorite, biotite, garnet and kyanite), from an ophiolite in northern Norway, has been analysed for Re and Os, in order to ascertain the metamorphic grade at which Re loss may occur. An isotope evolution diagram indicates that at all grades the 187Re/188Os ratio is insufficient to explain the measured 187Os/188Os. Therefore, the Re-Os system has been disturbed, and, as in the case of the eclogite facies basalts, these rocks have undergone substantial Re loss. The Re loss must first occur at chlorite or a lower grade, or possibly even as a result of seafloor hydrothermal alteration. References [1] Becker H. (2000) EPSL 177, 287-300. [2] Rubatto, D., Gebauer, D. and Fanning, M. (1998), Contrib. Mineral Petrol. 132, 269-287. [3] Schiano P., Birck J-L., Allegre C.J. (1997) EPSL 150, 363-379.

A605

M. ALPASLAN1, R. FREI2, M.A. KURT1, A. TEMEL3 AND D. BOZTUG4 1

Mersin University, Department of Geology, 33343, MersinTurkey 2 Geological Institute University of Copenhagen, Øster Voldgade 10 DK-1350 Copenhagen, Denmark 3 Hacettepe University, Department of Geology, Ankara, Turkey 4 Cumhuriyet University, Department of Geology, 58140, Sivas, Turkey Ulukı la basin is one of the post-collisional basins in the Central Anatolia that developed after collision between Tauride and Anatolide platelets. It includes the sedimentary units which reaches up to 5000 m. thickness, intercalating volcanics and mafic dykes. During the beginning of the basin development, alkaline volcanics occurs as lava flows which intercalate with sedimentary units. Ultrapotassic volcanics postdates the alkaline volcanics which occur as lava flows and dykes. Ultrapotassic volcanic rocks in the Ulukı la basin have geochemical characteristics belonging to Group III ultrapotassic rocks of the Foley et al. (1987) classification. These rocks have unusually high contents of large-ionlithophile elements (LILE) (e.g. Ba up to 5900 ppm, K2O up to 8 wt% in massive lava and 10 wt% in dykes). Incompatible trace element patterns exhibit a large Nb-Ta trough and large enrichments on LILE such as Ba, Th and U, and LREE, which indicate a subduction zone signature. Negative Nb and Ti anomalies and LREE enrichments relative to HREE on chondrite normalized trace and rare earth element patterns indicate that subduction related material is present in the mantle source region. Their high initial 87Sr/86Sr (0.70798-0.70917) and low εNd values suggest that they originated from enriched subcontinental lithospheric mantle sources with low Sm/Nd ratios. The elevated 207Pb/206Pb and low 143Nd/144Nd ratios and geochemical features such as low Nb/La and elevated Ce/Sr reflect a sedimentary signature. The relatively elevated 207 Pb/206Pb ratios (15.743-15.797) and steep trend on the 207 Pb/204Pb vs 206Pb/204Pb diagram imply involvement of an old radiogenic component in the source region.

A606

Goldschmidt 2004, Copenhagen

5.3.42

5.3.43

Source of radiogenic He in the mantle wedge: Constraints from Italian PlioQuaternary volcanism

HIMU-OIB magmatism in subduction zones: An example from the Italian south-eastern Alps

M. MARTELLI1, V. DI LIBERTO2, R.M. ELLAM3, P.M. NUCCIO1,2 AND F.M. STUART3

P. MACERA1, D. GASPERINI1, D. BOSCH2, C. FACCENNA3, C. PIROMALLO4, G. RANALLI5 AND R. MAHATSENTE5

1

INGV-Sez di Palermo, Italy ([email protected]; [email protected]) 2 Università di Palermo, Dip. CFTA, Italy ([email protected]) 3 SUERC, East Kilbride ([email protected]) In order to quantify the delivery of radiogenic He to the mantle by subduction we have undertaken a systematic study of the He and Sr isotopic composition of basaltic rocks from the Plio-Quaternary subduction-related volcanism of Italy (Roman Magmatic Province, Aeolian islands, Ustica, Etna). The 3He/4He of the olivine phenocrysts ranges to the lowest values recorded for mantle-derived rocks (6.8 – 0.44 Ra), although in many cases co-genetic pyroxene is slightly lower. The absence of any relation between 3He/4He and Mg content, and a strongly linear co-variation with 87Sr/86Sr (R2 > 0.9) rules out significant magma-crust interaction. The strong covariation between He and Sr isotopes displays a clear geographic trend as the radiogenic He and Sr generally increase northward from Eolian islands to Latium. The startlingly coherent He-Sr isotope relationship can be most simply explained as a mix between asthenosphere mantle with young HIMU affinities (as previously identified by Sr-Pb isotopes), and a radiogenic end-member, which is best explained as resulting from mantle enriched by subduction of the Ionian/Adriatic plate. The most primitive signatures in He and Sr isotopes are found at Alicudi (Eolian Islands), Ustica and Etna. Radiogenic He in the mantle can be generated by direct addition of crustal He by fluids dehydrated from the subducting crust or by post-metasomatic radiogenic He ingrowth from U and Th decay in the mantle wedge. Only if the He concentration of the asthenospheric mantle is two orders of magnitude lower than MORB-source mantle is slabderived radiogenic He unnecessary to explain the observed He-Sr isotope correlation. If the Sr isotopic composition of the enriched mantle is 0.715-0.72, as suggested by the geochemistry of Tuscan magmatism, the linear He-Sr isotope correlation cannot be a simple mixing relationship. The increase in the (He/Sr)EM/(He/Sr)yHIMU with increasing 87 Sr/86Sr required to explain the data can be accommodated, or volatile depletion prior to enrichment.

1

University of Pisa, Italy University of Montpellier 2, France 3 University of Roma Tre, Italy 4 INGV, Roma, Italy 5 Carleton University, Canada ([email protected]; [email protected]) 2

The geochemical features of the SE Alps volcanics (SEAV, Tertiary age) are comparable to the numerous volcanic eruptions of Tertiary-Quaternary age from the western Mediterranean area for which a plume-related origin has been assessed, and contrast to the widespread calcalkaline magmatism which developed northwestwards along the Periadriatic Lineament. The occurrence of a HIMU component, which is the hallmark of hotspot basalts, in a collision environment (the Tertiary convergence of Europe and Africa plates) is here explained in terms of slab breakoff. Evidence for the European slab breakoff comes from seismic tomography which shows that the present-day fast velocity material, interpreted as the European slab subducted below the Alpine chain, is shorter by about 300 km than the total length of the subducted slab estimated by paleotectonic reconstructions. Other piece of evidence comes from a kinematical model consisting in evaluating the time evolution of buoyancy of oceanic and continental lithosphere during subduction with both constant and time-varying convergence rates. If the subducted slab intercepts a rising plume from below the corresponding part of the slab is heated and therefore softened. The softening effect is enhanced if the slab includes continental material. The combination of changes in negative buoyancy caused by continental subduction, and softening of a part of the slab caused by slab-plume interaction, may act as a regulator for the time of slab breakoff and consequently for the time and type variations of magmatism in the overriding lithosphere above a subduction zone. In the Alpine region, we assume that the plume material interacted with the subducting slab causing its heating, softening, and finally its detachment. Ensuing upwelling of plume material through the resulting plate window is supposed to be the responsible for partial melting in the lithospheric mantle wedge and/or decompression melting of the ascending plume material. On the basis of geological, geophysical and geochemical data we conclude that both magmatic suites originated from a common and primary deep mantle plume the root of which was located beneath the Cape Verde-Madeira-Canary Islands region, while the head was dragged and frayed by the northeastward motion of the Eurasian plate.

5.3 Subduction processes and the subcontinental lithosphere

5.3.44

A607

5.3.51

Petrogenesis of late-Paleozoic highMg norites from North Xinjiang, China: Evidence for lower crust recycling in an intracontinental setting L.H. CHEN1,2 AND B.F. HAN2 1

Department of Earth Sciences, Nanjing University, Nanjing 210093, China ([email protected]) 2 School of Earth and Space Sciences, Peking University, Beijing 100871, China ([email protected]) Here we report geochemical and Sr-Nd-Pb isotopic composition of 280 Ma noritic plutons in Eastern Tianshan, North Xinjiang, China. Since ubiquitous 300 Ma A-type highepsilon Nd granites of North Xinjiang indicate an obviously crustal growth, the latish norite plutons along deep faults can provide compositional implications of the lithospheric mantle after the crustal growth. Eastern Tianshan norites show broad range in chemical compositions, with SiO2 (48.7-54.5%), MgO (4.1-13.3%), and MgO/(MgO+Fe2O3) ratios (0.340.61). In primitive mantle normalized spidergrams, Eastern Tianshan norites show a strong crustal signature, i.e. enrichment of LILE, depletion of Nb and Ta, strongly positive Pb anomaly. Good correlations between Sr, Nd, and Pb isotopes suggest mixing genesis for Eastern Tianshan norites. High-SiO2 norites have more radiogenic Sr, Pb isotopic composition, and lower radiogenic Nd isotopic composition than that of low-SiO2 norites. High-SiO2 norites show high Eu/Eu* ratios (>1.2) and obviously positive Sr and Ti anomaly in spidergrams, so we prefer source mixing to crust assimilation mode to interpret the mixing relationship. On the diagram of epsilon Sr vs. epsilon Nd, Eastern Tianshan norites deviate from normal mantle array and plot in the mixing line between depleted mantle component and 300 Ma A-type highepsilon Nd granites of North Xinjiang. Meanwhile, the complementary feature in some trace elements, i.e. Sr, Eu, and Ti, suggests a genetic relationship between them. Therefore, we propose that juvenile lower crust, with complementary trace element feature and similar Sr-Nd-Pb isotopic composition with A-type granites, had sunk into mantle and contributed to Eastern Tianshan high-Mg norites.

Mantle melting and metasomatism in European Lithosphere N. WITTIG1, J. BAKER1,2 AND HILARY DOWNES3 1

Danish Lithosphere Centre, 1350 Copenhagen K, DK ([email protected], [email protected]) 2 School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, NZ 3 School of Earth Science, Birkbeck College, University of London, UK ([email protected]) We have carried out a high-precision Pb (double-spike) and Lu-Hf isotope study of clinopyroxene from spinel-facies mantle xenoliths from the French Massif Central (FMC) and Pyrenean lherzolite massifs (PM). Pb isotope analyses (n = 35) define a heterogeneous sub-continental lithospheric mantle (SCLM) with 206Pb/204Pb = 16.9-20.3. Most replicate digestions (n = 26) fail to reproduce within double-spike errors (±100 ppm) due to difficulties in reproducing leaching procedures prior to digestion, inclusions of accessory minerals, or heterogeneity within the clinopyroxene. Clinopyroxene from the PM have heterogeneous Pb isotopes (206Pb/204Pb = 16.9-19.5) and are generally LREE depleted ([La/Nd]N < 0.55). In general, clinopyroxenes from the FMC with unradiogenic Pb (206Pb/204Pb < 18.6) are also LREE depleted, whereas radiogenic Pb samples (206Pb/204Pb > 18.6) are LREE enriched. LREE enrichment and radiogenic Pb in the FMC xenoliths reflects recent fluid-dominated metasomatism with radiogenic Pb and/or older increases in Th/Pb and U/Pb leading to development of radiogenic Pb. This metasomatism either occurred during Variscan subduction (400 Ma) or, more recently, related to FMC volcanism itself. Hf isotope ratios of clinopyroxene (n = 37) typically range between εHf = –0.4 to +40.9. However, in the northern FMC, clinopyroxenes from Montboissier xenoliths have very high εHf (+133 to +249) and 176Lu/177Hf (0.6190.929; Hf < 0.036 ppm). Clinopyroxene from Puy de Halle, also in the northern FMC, has even more extreme εHf (+468 to +1396). This Hf isotopic record of ancient melt depletion is accompanied by U-Th-Pb-LREE enrichment, unremarkable 143 Nd/144Nd (εNd = +2.1 to +18.1) and radiogenic Pb. However, radiogenic Nd (εNd = +91.2) is preserved in one sample with εHf = +220. The northern FMC clinopyroxenes have consistent Lu-Hf mantle depletion ages of 310-488 Ma (± 6 to 63 Ma; 2sd) recording melting during Variscan subduction beneath the FMC and subsequent incorporation of this material into European SCLM. Preservation of extreme εHf highlights the robustness of Hf to fluid-dominated mantle metasomatism that overprinted Sr-Pb-(Nd) isotopes and incompatible trace element (U-Th-Pb-LREE) abundances, and also the high Tc of Hf in clinopyroxene. The Lu-Hf isotope system has great potential for directly dating and constraining the formation of depleted mantle as recorded in SCLM lithologies from different lithospheric terranes.

A608

Goldschmidt 2004, Copenhagen

5.3.52

5.3.53

Dunite-pyroxenite xenoliths from South African kimberlites: Former cumulates of Archean oceanic crust?

Heterogeneous and metasomatized mantle recorded by mineral trace elements in Donghai garnet peridotites of the Sulu UHP terrane, eastern China

T. REHFELDT , S.F. FOLEY AND D.E. JACOB Universität Mainz, Becherweg 21, 55099 Mainz, Germany ([email protected]) Dunite, wehrlite and pyroxenite xenoliths are widespread in South African kimberlites, but are less well studied than lherzolites and eclogites. During the early Archean the Earth was probably hotter and more melt was generated at mid-ocean ridges, leading to a thicker crust with a more magnesian bulk composition [1]. Eclogites are widely accepted to represent basaltic and gabbroic parts of Archean oceanic crust, whereas rocks representing former ultramafic cumulates within the oceanic crust have not yet been identified. Following high pressure metamorphism, these ultramafic cumulates and picritic rocks would yield pyroxenites [2]. Four xenolith types in South African kimberlites are potential candidates for cumulates, namely pyroxenites, spinel-bearing wehrlites, spinel-free wehrlites and low-Mg dunites. These types differ in modal composition and structure, as well as major and trace element composition. Pyroxenites have equigranular structures with coarse grained garnet (Grt) in some samples and fine grained Grt in others, the latter crystallising along grain boundaries. Spinel-free wehrlites and dunites have equigranular or porphyroclastic structures, whereas spinel-bearing wehrlites have equigranular, porphyroclastic or foliated porphyroclastic structures. Silicate minerals in pyroxenites and spinel-free wehrlites have the lowest Mg# (100*Mg/(Mg+ΣFe)) of the four xenolith types; Ol has Fo89 and cpx Mg# 87 to 92. Pyroxenites also contain Opx with Mg# 88 to 91 and Grt with Mg# 72 to 77. Oxide minerals in pyroxenites and spinel-free wehrlites are magnetite, whereas dunites have Ol with Fo8993 and contain ilmenite and magnetite. Spinel-bearing wehrlites have silicate minerals with the highest Mg# of the four xenolith types: Ol = Fo90-93, Cpx = Mg# 90 to 93, Opx = Mg# 92 to 93 and Grt = Mg# 82 to 85. Cr-spinel is always present, and magnetite, ilmenite and/or rutile may occur in different abundances. Pyroxenites and wehrlites have Cpx, Grt and Opx with consistently different REE patterns. In comparison to wehrlitic Cpx, pyroxenitic Cpx is more enriched in LREE and has a steeper REE slope. Spinel-free and spinel-bearing wehrlites have similar REE patterns.

References [1] Zegers T.E. and van Keken P.E. (2001) Geology 29, 10831086. [2] Foley S.F., Buhre S., and Jacob D.E. (2003) Nature 421, 249-252.

J. ZHENG1,2,3, R.Y. ZHANG2, J.G. LIOU2, W.L. GRIFFIN3,4 3 AND S.Y. O’REILLY 1

Earth Sciences, Univ. of Geosci., Wuhan 430074, China Geol. and Env. Sci., Stanford University, CA 94305, USA 3 GEMOC ARC National Key Centre, Macquarie University, NSW 2109, Sydney, Australia 4 CSIRO Explor. and Mining, NSW 1067, Sydney, Australia 2

Garnet peridotites from the Sulu terrane of eastern China were derived from the mantle wedge above a subduction zone of the Yangtze craton beneath the Sino-Korean craton. They underwent Triassic ultrahigh-pressure (UHP) metamorphism with their country rock gneisses. Mineral trace elements of Donghai garnet peridotites were analyzed by laser ablation ICPMS. One Zhimafang peridotite (porphyroblastic texture) consists of olivine (Fo91.2-91.8), enstatite (En92.0), garnet (Prp66.5-70.7) and diopside (Di90.8). Four Xugou samples contain nearly equigranular olivine (Fo90.6-92.2), enstatite (En92.0-92.7), garnet (Prp63.2-87.1) and diopside (Di93.795.7) with additional minor phlogopite. Garnets from both localities have low LREE and show negative Ce anomalies, but the Zhimafang garnet has higher HREE (8-15) than those from Xugou (2-6). Zhimafang diopside shows sinusoidal REE pattern with flat HREE. Xugou diopsides, in contrast, have LREE-enriched patterns with large variation in HREE. Diopsides from both peridotites have low abundance of Yb, Y and Ti, and low Ti/Eu (517-1158), but high and variable Sr/Nd (4800-22000) and (La/Yb)n (40-100). Geochemical modeling indicates that the concentrations of moderately incompatible trace elements such as Y, Ti, Yb and Dy in diopside fit a fractional melting trend with about 30% partial melting of a primitive mantle source. However, Zr and Gd in the Zhimafang diopside, and the highly incompatible trace elements (such as Nb and especially LREE, Sr, Th and U) of diopside from both bodies, cannot be described by this model, and are attributed to carbonatitic metasomatism. The Sr isotopic composition of diopside and garnet, and marked negative Ce anomalies of garnets, suggest a sedimentary fluid source. Hydrous silicates (eg. Phlogopite) and low whole rock CaO/Al2O3 suggest a later silicate metasomatism related to subducted crustal materials. Systemic differences in the Ni content and Mg of olivine, and the Y, Al, HREE, Nd/Y and Sc/Y of garnet, show that the mantle volume represented by the Zhimafang peridotite is less depleted than the Xugou mantle. Such differences indicate that the garnet peridotites derived from a heterogeneous mantle in which different volumes had different evolution histories prior to their involvement in Triassic continental subduction.

5.3 Subduction processes and the subcontinental lithosphere

5.3.54

A609

5.3.55

Source area effects and crustal contamination in the Hekpoort Formation of South Africa

Metasomatic overprint of traceelement signatures of eclogites and peridotites in cratonic roots

P.C. BUCHANAN1, C. KOEBERL2 AND W.U. REIMOLD3

J.C.M. DE HOOG1 , D.J. SCHULZE2 AND K.S. VILJOEN3

1

National Institute of Polar Research, 1-9-10 Kaga, Itabashiku, Tokyo 173-8515 JAPAN ([email protected]) 2 Dept. of Geological Sciences (Geochemistry), University of Vienna, Althanstrasse 14 A-1090, Vienna, Austria 3 School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050 Johannesburg, South Africa The Hekpoort Formation represents a period of widespread volcanism prior to the Bushveld Magmatic Province (BMP). Crow and Condie [1] suggested that Hekpoort magmas were derived from a mantle source with a subduction-related component. However, subduction-related features may be difficult to distinguish from crustal contamination [1]. In an effort to discriminate between these two types of effects, we compare Hekpoort compositions with those of nearby Mesozoic Karoo continental flood basalts (Fig. 1), which experienced minimal contamination [e.g., 2]. Both units have chondrite-normalized Nb, Sr, P, and Ti depletions, which may be source-related. Abundances of Ba, Rb, and K are comparatively enriched for Hekpoort units and probably result from crustal contamination. A phlogopite-rich source and AFC processes have been suggested for the Montefiascone Volcanic Complex (Italy), which displays some similar compositional features [e.g., 3].

1

Geology, Göteborg U., Sweden ([email protected]) Earth Sci., U. Toronto, Canada ([email protected]) 3 GeoScience Centre, De Beers Consol. Mines, South-Africa ([email protected]) 2

The Kaalvallei kimberlite contains abundant eclogitic and lherzolitic mantle xenoliths. Garnet-lherzolites equilibrated at 2 pressures of 30-75 kbar and follow a ca. 40mW/m geotherm. They can be divided into a high-T, Ti-rich and a low-T, Tipoor group. A similar division, corresponding to Group I and Group II eclogites, respectively, can be made for the eclogites. Trace-element patterns of clinopyroxenes (LA-ICP-MS) show remarkable similarities between Ti-poor grt-lherzolites and Group II eclogites, and between Ti-rich grt-lherzolites and Group I eclogites (see Figure).

abundance/chondrite (except K, Rb, P)

1000 Hekpoort Formation

100 Karoo

Y

Ti

Tb

Hf

Zr

P

Sm

Sr

Nd

Ce

La

K

Nb

Ba

1

Rb

10

Fig. 1. Compositions of Hekpoort samples and of central Karoo basalts [from 2, 4], normalized according to Thompson et al. [5].

References [1] Crow C. and Condie K.C. (1990) Prec. Res. 47, 17-26. [2] Marsh J.S. (1987) J. Volcanol. Geotherm. Res. 32, 35-49. [3] di Battistini G., Montanini A., Vernia L. Bargossi G.M. and Castorina F. (1998) Lithos 43, 169-195. [4] Duncan A.R., Erlank A.J. and Marsh J.S. (1984) Geol. Soc. S. Afr. Spec. Pub. 13, 355-388. [5] Thompson R.N., Morrison M.A., Hendry G.L. and Parry S.J. (1984) Phil. Trans. R. Soc. Lond. A310, 549-590.

Ti-poor xenoliths are strongly depleted in more compatible elements, HFSE and Pb, but enriched in highly incompatible elements. Ti-rich xenoliths show less depletion in more compatible elements and HFSE, whereas incompatible elements are depleted. As eclogites and grtlherzolites must have very different origins, the similarity in their trace-element patterns suggests a strong metasomatic imprint. Lack of trace-element zoning suggests that metasomatism is ancient. We conclude that any inference of protoliths (e.g., eclogites from subduction origin or mantle cumulates) based on trace-element signatures alone is problematic. Detailed study of metasomatic processes in both xenolith suites may be the key to deciphering their initial compositions.

A610

Goldschmidt 2004, Copenhagen

5.3.56

5.3.57

The southern African Kaapvaal craton: Formation and modification of continental lithospheric mantle in Archaean subduction zones? 1

1

N.S.C. SIMON , G.R. DAVIES , D.G. PEARSON R.W. CARLSON3

2

AND

1

FALW, VU Amsterdam, De Boelelaan 1085, 1081HV Amsterdam, The Netherlands ([email protected]) 2 Dep. of Geological Sciences, Durham University, South Rd, Durham DH1 3LE, UK ([email protected]) 3 DTM, Carnegie Inst. Washington, 5241 Broad Branch Road N.W., Washington, D.C. 20015, USA ([email protected]) The Kaapvaal craton is underlain by an at least 200 km thick lithospheric keel. The sub-cratonic lithospheric mantle (SCLM) is distinct from oceanic mantle in that it is more depleted in magmaphile elements (Fe, Al, Ca, HREE), but at the same time strongly enriched in incompatible trace elements like LILE and LREE. The Kaapvaal SCLM also has a distinctly higher Si/Mg ratio and therefore higher modal opx content (at a given Mg/Fe) than oceanic and most continental upper mantle. We carried out a detailed petrological and geochemical study on low-temperature peridotite xenoliths for Kimberley and northern Lesotho in order to obtain information about the processes that led to the depletion and re-enrichment of the Kaapvaal SCLM. Samples have been characterized for Re-Os isotope systematics, major and trace element concentrations in whole rocks and minerals, and garnet and cpx Lu-Hf, Sm-Nd and Rb-Sr isotopes. The combined results require a multistage history of the Kaapvaal mantle. They are most consistent with a model that involves ancient trace element enrichment of the garnets and opx by aqueous fluids, possibly derived from a subducting oceanic slab. This is consistent with the observed enrichment in Si (opx) and the Re-Os systematics. Infiltration of hydrous fluids would also increase the degree of mantle melting and could therefore explain the strong major and HREE depletion of the SCLM. The Nd-Hf isotope characteristics of the garnets require the trace element enrichment to be ancient. We therefore suggest that melting and metasomatism of the Kaapvaal SCLM took place in subduction zone settings, probably during amalgamation of smaller pre-existing terranes in the Late Archaean. Trace element and Nd and Hf isotope disequilibrium between garnet and cpx is preserved in many samples and indicates that garnet and cpx are not co-genetic. Calculated equilibrium liquids and Hf-Nd isotopic compositions for cpx suggest that most diopside in the xenoliths studied crystallized from an infiltrating kimberlite-like melt, shortly prior to eruption.

Lu-Hf systematics of kimberlitehosted eclogite xenoliths from South Africa and Canada S.S. SCHMIDBERGER, L.M. HEAMAN, A. SIMONETTI AND R.A. CREASER Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3, Canada ([email protected]) Eclogite xenoliths hosted by kimberlites provide invaluable information on the chemical composition of the lithosphere beneath Archean cratons. The timing of eclogite formation, whether during stabilization or post-Archean modification of the subcratonic mantle, is currently poorly constrained. This ongoing study presents Lu-Hf, and in-situ Pb and Sr isotopic ratios of clinopyroxene obtained by laser ablation MC-ICP-MS for eclogite xenoliths from the Kaapvaal craton, South Africa (Roberts Victor, Kimberley Pool, Jagersfontein) and a suite of rare zircon-bearing eclogites [1] from the Slave craton, Canada (Jericho kimberlite). Epsilon Hf values for the South African eclogites show a large variation from -16 to +25 at the time of kimberlite emplacement (at ~90-120 Ma). The Roberts Victor kimberlite is characterized by an initial epsilon Hf value of -8.1. The Roberts Victor eclogites (n=6) define a Lu-Hf regression line corresponding to an errorchron age of 1.6±0.4 Ga. The initial Hf isotopic ratio of 0.28192 obtained from the regression is consistent with this age, since it intersects the temporal evolution line for a depleted mantle reservoir at 1.7 Ga. In-situ 87Sr/86Sr ratios obtained on single clinopyroxene grains selected from four Roberts Victor eclogite samples vary between 0.7059 and 0.7068, and are interpreted to reflect source characteristics. In-situ 206Pb/204Pb (15.9-17.2) and 207 Pb/204Pb (14.6-15.6) ratios for the clinopyroxenes are extremely variable and yield younger Pb-Pb model ages, possibly the result of metasomatic interaction with the host kimberlite. As noted in a previous study of a similar nature [2], the data obtained here support the more robust nature of the Lu-Hf chronometer in resisting chemical changes associated with metasomatic activity. Epsilon Hf values for the Jericho eclogites are much lower ranging from -28 to -33 (at ~170Ma). They are characterized by very low 176Lu/177Hf ratios (0.001 to 0.006) and fairly consistent 176Hf/177Hf values, and yield Paleoproterozoic Hf(depleted mantle) model ages (2.0 to 1.7 Ga). This is consistent with their origin involving Paleoproterozoic subduction as suggested by U-Pb zircon dating [1].

References [1] Heaman L.M., Creaser R.A., and Cookenboo H.O. (2002) Geology 30, 507-510. [2] Schmidberger S.S., Simonetti A., Francis D., and Gariépy C. (2002) EPSL 197, 245-259.

5.3 Subduction processes and the subcontinental lithosphere

5.3.58

A611

5.3.61

U-Pb age constraints on the subduction–extension interval in the Averoya–Nordoyane area, Western Gneiss Region, Norway 1

1

T. KROGH , Y. KWOK , P. ROBINSON

2

AND M. TERRY

3

1

Department of Geology, University of Toronto, 22 Russell St., Toronto, ON., Canada, M5S 3B1 ([email protected]) 2 Geological Survey of Norway, Trondheim, Norway 3 Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, Germany The interval between subduction and syn-uplift ductile extension has been determined using eclogite facies transformation zircons and those formed in amphibolite-facies extentional boudin-neck pegmatites. Eclogite zircons are typically small, rounded, low U with concordant U-Pb systems whereas those from infill pegmatites are commonly near concordant with abundant cores, hence multiple single tips are analyzed. Samples from 3 sites on or near Averoya and 9 sites in the Nordoyane area 70 km to the south-west were analyzed. Zircons from the Averoya eclogite display 2 stages of metamorphic zircon growth. Three fractions of small rounded typical eclogite grains have identical concordant 206/238 ages of 415+/-1 Ma whereas 2 tips from large euhedral grains have concordant ages of 411+/-1.5 and 410+/-1.5 Ma (all errors 2 ). The latter grain type probably formed during decompression that produced local cm-scale melt pods. Zircons from a post-uplift pegmatite that cuts the eclogite are sharp faceted, low U without visible cores but our initial results showed a ca. 1500 Ma inheritance. Grains selected after removal of tiny cores, revealed by HF-etching, gave 5 concordant analyses with a mean 206/238 age of 395.3+/-1 Ma. A second boudin-neck pegmatite 26 km to the east yielded 5 near concordant data with a mean 207/206 age of 395.5+/-2 Ma for the final stage of ductile flow in the region. Eclogite from the margin of the 1255+/-2 Ma Flem Gabbro gave 2 concordant 206/238 zircon ages of 408+/-1 Ma and 410+/-1 Ma whereas those from nearby Lepsoya gave 2 concordant fractions both with 206/238 ages of 411.5+/-1.2 Ma. Boudin-neck pegmatite from between 2 house-size eclogite rafts beside the Flem Gabbro gave a mean 207/206 age of 397+/-3 Ma for the late stage of ductile flow. In this case the 2 most concordant fractions have ages of 394 and 395 Ma. A second boudin-neck pegmatite adjacent to the UHP eclogite on Fjortoft gave an age of 394.5+/-2 Ma. Subduction-extension intervals of 20 m.y. (415 - 395 Ma) and 14 m.y. (410 - 396 Ma) are defined for the Averoya and Nordoyane regions with a remarkably uniform age for the cessation of ductile flow. Baddeleyite in the Flem and Haram Gabbros (1466+/-2 Ma) did not react with Si02 to make zircon until this 395 Ma event and rutile records ca. 400ºC cooling ages of 376+/-2 Ma. Region-wide unroofing at 395 Ma could explain coeval ductile flow and rapid cooling.

Hf-Nd-Sr-Pb isotope evidence for relict Indian mantle beneath Pacific crust at the Solomon island arc S. SCHUTH, A. ROHRBACH, C. MÜNKER AND C. BALLHAUS Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, D-48149 Münster, Germany At the Solomon island arc, the Australian Plate is presently subducted beneath the Pacific Plate. The collision of the Ontong Java Plateau with the old northeastern island arc caused a reversal in subduction polarity (ca. 12 Ma ago [1]) leading to the present day setting. A particular feature of the Solomon arc is the subduction of a spreading center (Woodlark Rise). Picritic rocks from the New Georgia group, Solomon Islands, occur only above the subducting Woodlark Rise and contain up to 29.7 wt.% MgO, whereas the most primitive basalts contain around 12 wt.% MgO. Linear trends defined by trace elements vs. MgO indicate that the picritic suite resulted from a mixing between a basaltic-picritic melt and a Mg- and Cr-rich source, probably mantle peridotites. LILE abundances in the New Georgia group magmas indicate a strong source enrichment by subduction components. 87Sr/86Sr and $Nd values range from 0.7033 to 0.7043 and +6.3 to +8.0, respectively. These values partially overlap with compositions of the Indian MORB field and lie between those of the neighbouring New Britain and Vanuatu arcs. $Hf values ranging from +12.2 to +14.6 can show in combination with the $Nd values that the examined magmas were most likely generated within the Indian mantle domain. The presence of relict Indian material in the mantle wedge that originates from N-S directed subduction prior to the subduction polarity reversal is confirmed also by $Nd vs. Nd systematics [2]. Pb isotope data (206Pb/204Pb ca. 18.45 to 18.62, 207Pb/204Pb ca. 15.51 to 15.56 and 208Pb/204Pb ca. 38.3 to 38.4) indicate a bimodal distribution of Pb isotope compositions. They can identify the enrichment of the mantle wedge by two types of subduction fluids that originate (1) from the old subduction components from the Pacific plate (> 12 Ma old) and (2) from more recent subduction components derived from the Australian plate.

References [1] Petterson, M. G. et al. (1999) Tectonophysics 301, 35 – 60. [2] Pearce, J. A. et al. (1999) J Petrol 40, 1579 – 1611.

A612

Goldschmidt 2004, Copenhagen

5.3.62

5.3.63

Mixing of high-Ca arc-related melts in Lombok (Indonesia)

Tracing crust-mantle interaction during Karakoram-Kohistan accretion (NW Pakistan)

M.A. ELBURG1, V.S. KAMENETSKY1,2, A.V. SOBOLEV1 3 AND J.D. FODEN

U. SCHALTEGGER1,2, S. HEUBERGER2, M. FRANK2, D. FONTIGNIE1, S. SERGEEV3 AND J.-P. BURG2

1

Max Planck Institute for Chemistry, Mainz, Germany ([email protected]) 2 School of Earth Sciences and CODES SRC, University of Tasmania, Australia ([email protected]) 3 Department of Geology and Geophysics, University of Adelaide, Australia Controversy exists whether ankaramitic magmas, with CaO/Al2O3>1, represent liquid compositions or reflect the presence of accumulated clinopyroxene in excess of its abundance on the liquidus. New data on mineral phases and melt inclusions in a volcanic sample from Lombok (Indonesia) confirms the existence of ankaramitic melts within an island arc setting, and shows evidence for mixing of different high-CaO melts with distinct CaO/Al2O3 ratios. The sample studied has 11.3% MgO, 0.9% K2O, CaO/Al2O3=1.03 and 2 wt% normative nepheline. Phenocrysts are olivine (~65 vol.%; with Cr-spinel inclusions), clinopyroxene (~35 vol.%) and rare plagioclase. Olivine Focontents vary from 67 to 91; at Fo=90, CaO ranges between 0.22 and 0.37 wt% and NiO 0.1-0.22 %. These variations at a given Fo-content suggest the presence of compositionally diverse melt fractions, although with similar Mg#. Clinopyroxene phenocrysts, with 100*Mg/(Mg+Fetot) 72-92, appear to belong to a single population. Melt inclusions in high-Fo olivine with CaO>0.3 have CaO/Al2O3 1.07-1.46; those in olivine with CaO<0.27 have CaO/Al2O3 0.85-1.02. Olivines with intermediate CaO contents have intermediate CaO/Al2O3 ratios, and there is a good correlation between CaO content of the olivine and CaO/Al2O3 of the melt. High CaO/Al2O3 inclusions have lower Na2O contents (1.9-2.7%) than low CaO/Al2O3 inclusions (2.4-3.4%). Major elements other than calcium, sodium, and alumina do not show distinctions between the two groups of melt inclusions; both types are silica undersaturated and relatively dry. Distinctions in trace element patterns do not bear a clear relationship to the CaO/Al2O3 ratio of the inclusion. Our results contrast with data from the Philippines [1], where melt inclusions show mixing between ne-normative high-Ca melts with ' normal'island arc magmas, whereas we find evidence for mixing between two types of ne-normative melts, which both classify as "high-Ca" melts (CaO>13%). The correlations between Ca, Al and Na suggest that modal abundances of clinopyroxene entering the melt control the behaviour of these elements.

References [1] Schiano, P. et al. (2000) G3 1999GC000032.

1

Dept of Mineralogy, Univ. of Geneva, 1205 Geneva, Switzerland ([email protected]; [email protected]) 2 Dept of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland ([email protected]; [email protected]; [email protected]) 3 All Russian Geological Research Institute, 199106 St.Petersburg, Russia ([email protected]) Plutonic rocks in the Karakoram (KK) and Kohistan (KO) terranes in NW Pakistan serve as probes for the evolution of mantle and crustal sources involved during convergencerelated magmatism, using U-Pb dating and the Hf isotope composition of zircon. Andean-type magmatism in the Karakoram active margin took place in a narrow age range, between 107 and 100 Ma, tapping sources yielding εHf values of -3 to +11. Analysed samples span 300 km along the KKKO (North-Kohistan) suture from the Drosh to the Hunza valleys. Gabbroic and dioritic intrusions entrained in the suture clearly are of KK affinity. KO terrane magmatism lasted for 120 m.y. (oceanic granitoids at 150 Ma, subductionrelated gabbros to granites at 110-90 Ma, intra-arc extensional norites at 85-70 Ma, and gabbro to granite batholith emplacement lasting till 30 Ma). The melts were tapping an increasingly enriched (metasomatized) mantle source evolving from εHf of +24 at 150 Ma to +10 at 30 Ma. Leucogranitic dykes, dated at 47, 39 and 30 Ma, represent either recycled arc basement (with εHf of +10 to +12), or partial melts of Indian crust with Paleozoic and Proterozoic inheritance. The 150 Ma old Matum Das tonalite represents extremely depleted oceanic magmatic components in the Tethys crust. The geochemical evolutionary trend of Hf isotopic compositions is interpreted as continuous additions of metasomatized mantle of Paleozoic age (KK continental lithosphere or metasomatized continental mantle predating the Tethys breakup?) to initially depleted (ultra-depleted?) oceanic mantle. Slab components cannot be used to explain the Hf isotope evolution since the geodynamic situation is not favourable for slab melting. Preliminary data for Pb isotopic composition are, however, characteristic of a DMII component in both KO and KK rocks. Leucogranites at 47 Ma (Beorai Gol, Drosh area) and 30 Ma (Indus confluence) likely mark the presence of Indian crust beneath the KK-KO suture and ascent of its partial melts into the overriding plate.

5.3 Subduction processes and the subcontinental lithosphere

A613

5.3.64

5.3.65

U-Th-Pa disequilibria constraints on the subduction processes beneath the South Sandwich island arc system

Subduction processes associated with back-arc opening; Hf isotope study of Tertiary NE Japan arc

A. HEUMANN1, T. ELLIOTT2, M. REGELOUS2, L. KIRSTEIN3 AND P. LEAT4

T. HANYU1, Y. TATSUMI1, S. NAKAI2, T. SHIBATA3, M. YOSHIKAWA3, Q. CHANG1, K. SATO1 AND T. YOSHIDA4

1

GZG-Geochemie, Goldschmidtstr. 1, 37077 Göttingen, Germany ([email protected]) 2 Earth Sciences, Queens Road, Bristol BS8 1RJ, UK ([email protected]; [email protected]) 3 Geosciences, West Mains Road, Edinburgh EH9 3JW, UK ([email protected] ) 4 British Antarctic Survey, Madingley Road, Cambridge CB3 0ET , UK ([email protected]) The presence of U-series disequilibria in magmas at island arcs provides first-order insights into the dynamics of material fluxes in subduction zones. In particular, the use of coupled U-series nuclide pairs offers the opportunity to apply simultaneous constraints to chemical fractionation processes of elements during fluid mobilization and melting in the mantle wedge beneath island arcs. The South Sandwich island arc system has developed in the southernmost Atlantic region during the past 3 Ma with westward subduction of the South American plate beneath the Sandwich micro-plate. Amongst the erupted tholeiitic and calc-alkaline magma series with typical chemical arc-signatures, the wide range in incompatible element contents at similar 143Nd/144Nd (e.g. REE) have been argued to require large source or dynamic melting variations [1]. We use U-series analyses to more fully assess their petrogenesis. Here we present the first U-Th-Pa dataset for the South Sandwich Islands, representative of the intra-island compositional variations from low-K tholeiitic basalts (Mg# = 70) to andesites (Mg# = 28) for which also new high-precision trace-element and Sr-Nd-Pb isotope data have been acquired. Large 238U excesses relative to 230Th are observed (>50% in some islands) as have been commonly observed in other depleted island arcs. This characteristic signature has been attributed to the addition of fluids from the down-going altered oceanic crust. The near constant (230Th/232Th) of the lavas despite their range in Th contents is difficult to reconcile with a dynamic melting model. Although distinguished by large 238U excesses, the samples all show 231Pa-235U excesses (up to 50%) as has also been observed in other island arcs. We argue that the production of 231Pa excesses reflect the result of melting and melt transport in the mantle wedge which overwhelme initial 235 U excesses.

References [1] Pearce J.A., Baker P.E., Harvey P.K. and Luff I.W. (1995) Journal of Petrology 36, 1073-1109.

1

IFREE, Japan Marine Science and Technology Center, Yokosuka, Japan ([email protected]) 2 Earthquake Research Institute, University of Tokyo 3 Beppu Geothermal Research Laboratory, Kyoto University 4 Graduate School of Science, Tohoku University Back-arc opening is one of the major tectonic features in island arc formation. In order to document the geochemical evolution of a subduction zone during back-arc opening, Hf isotope compositions coupled with other radiogenic isotopes and trace element compositions were determined for Tertiary and Quaternary volcanic rocks from NE Japan. The samples selected are from volcanoes along the volcanic front. The ages of the Tertiary samples range from 22 to 8 Ma, which overlaps with the period of opening of the Japan Sea back-arc basin (22-15 Ma). Hf isotope data demonstrate a secular variation. Among the samples, the 22 Ma high magnesian andesites (HMA) have the lowest εHf (+9). The 8 Ma tholeiitic basalts and andesites show the highest εHf (+14). Calc-alkaline basalts and andesites between 22 and 16 Ma have intermediate εHf from +10 to +13. Consequently, εHf was low (enriched) in the early stages of back-arc opening but became high (depleted) after back-arc opening ceased. The observed secular variation in Hf isotope ratios may be attributed to a change in magma source composition. (1) Injection of depleted asthenospheric mantle into previously enriched mantle wedge during back-arc opening is one possible process for such a secular variation. This occurred on the back-arc side of NE Japan, as indicated by a coupled decrease and increase of Sr and Nd isotope ratios, respectively, during and after back-arc opening. On the trench side, however, Sr and Nd isotope ratios were almost constant, suggesting that material flux was minimal on the trench side of the NE Japan arc[1]. (2) A hot asthenospheric injection into the mantle wedge, even if restricted to the back-arc side, should induce a high temperature gradient in the whole mantle wedge, which could affect the slab surface temperature, resulting in changing dehydration/melting conditions of the subducted slab. Melt from the subducted slab transports Hf more effectively into the mantle wedge than dehydrated fluid. The secular change in εHf is consistent with a model whereby slab melt was added to the mantle wedge during the early stage of back-arc opening, and slab dehydration was dominant after cooling of the mantle wedge.

References [1] Tatsumi Y., Nohda S., and Ishizaka K. (1988) Chem. Geol. 68, 309-316.

A614

Goldschmidt 2004, Copenhagen

5.3.71

5.3.72

Mantle source vs. contrasted continental crust contamination signatures in the NVZ revealed from an across-arc Sr-Nd isotopic study 1

1

2

E. BOURDON , M. F. THIRLWALL , M. MONZIER , P. SAMANIEGO3, C. ROBIN2 AND J.-P. EISSEN2. 1

Geology Dept., Royal Holloway Univ. of London, Egham Surrey, TW20 0EX, UK ([email protected]) 2 IRD UR 031 Lab. Magmas et Volcans, 5 Rue Kessler, 63038 Clermont-Ferrand, France 3 Depto. de Geofisica, EPN, AP 17-01-2759, Quito, Ecuador The Quaternary volcanism in the NVZ in Ecuador is characterized by a limited Sr-Nd isotope variation atypical of most continental arcs and despite a thick and contrasted continental crust. Highly magnesian lavas (up to 11% MgO) have been erupted throughout the arc, providing a direct window on the sub-continental isotopic signature. An across-arc Sr-Nd isotope study shows that the source of Ecuadorian arc lavas has a moderate 87Sr/86Sr-143/144Nd signature (0.704060.512910). Some of the lavas in the arc front are shown to have undergone some continental crust contamination by the unradiogenic material of oceanic origin forming the bulk of the crust beneath it. Most of the lavas though display the original isotopic signature of their source. In the Main Arc, higher 87Sr/86Sr and lower 143Nd/144Nd indicate significant but limited interaction (<15%) with both the upper and lower more mature continental crust. Lavas from the back-arc, despite being emplaced over the Guyana craton show no sign of significant contamination and share the exact same isotopic signature with those from the arc front lavas. Despite discrete variations, the homogeneous isotopic signature of Ecuadorian NVZ lavas confirms that the continental crust plays a minor role in the genesis of the magmas and that most of the unusual geochemical characteristics of the latter can be traced down to the deep source in both the subducted oceanic crust and/or sub-arc mantle. [1]. Due to the limited involvement of the continental crust in the evolution of the magmas and the possibility to pinpoint exactly the isotopic signature of the sub-arc mantle, the ecuadorian margin proves to be the best natural arc laboratory to study the effect of crustal contamination on a range of isotopic systems.

Reference [1] Bourdon E. et al. (2003) EPSL 205, 123-138.

Effects of subducting the Louisville Ridge and Osbourn Trough beneath the South Tonga arc T. WORTHINGTON, P. STOFFERS, C. TIMM, M. ZIMMERER AND D. GARBE-SCHÖNBERG Institut für Geowissenschaften, Universität Kiel, Germany ([email protected]; [email protected]; [email protected]; [email protected]; [email protected]) Subduction of both the plume-generated Louisville Ridge and the late Cretaceous Osbourn Trough paleo-spreading centre takes place beneath the 450 km-long South Tonga segment of the Tonga–Kermadec subduction system. The predominantly submarine South Tonga arc was previously unsurveyed and features only 1 volcanic island (Ata). Surveying and sampling during cruise SO 167 of the FS SONNE revealed a continuous volcanic front, 10–15 km wide, comprising 27 semi-regularly spaced active stratovolcanoes typically with basal diameters of 10–25 km and heights of 1–2 km. The volcanic front is disturbed only in the vicinity of Ata, where 10 major edifices and numerous smaller constructs extend the width of the front to 50 km. This greater width reflects a transient 40 km westward migration of the front and enhanced constructional volcanism as the Louisville Ridge dehydrates and passes under the arc. The South Tonga arc volcanoes show considerably more geochemical and isotopic variation than the better-known subaerial Central Tonga volcanoes. High-Mg basalt, andesite, and rhyodacite associated with calderas (often > 6 km in diameter) are common. Overall, these lavas define a low-K suite with strong enrichment in the fluid-mobile elements and extreme depletion in the fluid-immobile elements relative to MORB [e.g., (La/Yb)N < 1.1; Nb/Yb 0.09–0.27). Specific incompatible element ratios distinguish some volcanoes and are randomly distributed along the arc segment. These characteristics are long-lived, and best explained by interaction of ascending magma with sub-arc lithosphere in a volcano-specific manner. They provide support to models invoking significant lithospheric processing of magma in oceanic settings. Passage of the Louisville Ridge beneath the arc is associated with the eruption of lavas with (La/Yb)N > 1.1, higher Th/U, and other anomalies prior to and during the westward migration of the volcanic front. In contrast, lavas erupted during the eastward return of the front have relatively normal South Tonga compositions, suggesting rapid flushing of ridge-derived fluids from the sub-arc mantle. Volcanoes that overlie the subducting Osbourn Trough at 23.5–24.7°S exhibit a marked change in caldera-style to diatreme-like craters up to 1.1 km-deep. Associated changes in magmatic volatile content driving these eruptions probably reflect dehydration of mantle extensively serpentinised when spreading stalled at the Osbourn Trough.

5.3 Subduction processes and the subcontinental lithosphere

5.3.73

A615

5.3.P01

Variation of the slab input and its effects on partial melting along the Kermadec arc

Surface appearance of slab-derived helium in non-volcanic area in Kii Peninsula

K.M. HAASE , T.J. WORTHINGTON AND P. STOFFERS

TAKUYA MATSUMOTO1, TETSURO KAWABATA1, JUN-ICHI MATSUDA1, KOSHI YAMAMOTO2 AND KOICHI MIMURA2

Institut fuer Geowissenschaften, Universitaet Kiel ([email protected]; [email protected]; [email protected]) Along-arc variations in the chemical and isotopic composition of Kermadec-New Zealand arc lavas imply significant changes of the composition of the slab-derived component and the mantle wedge. Volcanic rocks north of about 30°S show the influence of a fluid from altered basaltic oceanic crust, whereas the southern lavas have chemical signatures implying the subduction of sedimentary material. Here we present new geochemical and isotopic data for lavas from the Kermadec-New Zealand arc. Co-variations among fluid-mobile element ratios indicate that the composition of the fluid derived from the subducting oceanic crust is not homogeneous but varies considerably. Because the element ratios do not correlate with radiogenic isotope ratios, we suggest that the fluid compositions are principally controlled by fractionation processes in the slab or mantle wedge rather than by variations in the composition of the subducting crust. The along-arc variation of the fluid sources is highlighted by the Cl/K ratios which decrease from the northern Kermadec arc to New Zealand, mainly due to an increasing input of K from sedimentary sources. These sediments are derived from the high-grade metamorphic rocks in the Southern Alps of New Zealand, and swept northwards to the Kermadec Trench by strong ocean currents. Element covariations are most consistent with the sediment-derived component being added to the sub-arc mantle as a fluid rather than a melt. The variable input of slab fluids also affects the partial melting behaviour of the sub-arc mantle, which is more depleted in the north than in the south. However, the southern Kermadec magmas apparently form by lower degrees of partial melting than the northern Kermadec magmas, probably reflecting less water input from the slab.

1

Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka 560-0043, Osaka, Japan ([email protected]) 2 Division of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan A total of 27 well gas samples collected from southwest Japan was subjected to precise analysis of He, Ne and Ar isotopic compositions [1]. The sampling area covers localities where mantle-derived helium emanates in the fore-arc of the Kii Peninsula, and is known as the “Kinki-spot” [e.g., 2]. The Kinki spot apparently is located within the fore-arc with no obvious magmatic activity, which contrasts to the occurrence of similarly high 3He/4He ratios along the volcanic front of NE Japan. Our high density sampling revealed more clearly that 3 4 the high He/ He ratio wells are distributed in the Kii Peninsula above the area where the young and hot Philippine Sea Plate is subducting with a relatively steeper dip than in the adjacent area; this situation favors the near-trench side of the subducting slab entering eclogite facies conditions involving slab dehydration, without inducing melting of the mantle wedge. The aqueous fluids derived from the slab acquire mantle-He during their passage through the mantle wedge. Thus, the observed association of mantle-He and the fore-arc dehydration of the Philippine Sea Plate indicates that fluid derived from the subducting slab is contributing to the occurrence of mantle-He in well gases of the Kii Peninsula. The high 3He/4He region of Kii Peninsula also coincides with the occurrence of long-period tremors. These are caused by the movement of fluids derived from the subducting slab. Localized stresses cause significant uplift in the area, interpreted as indicating that intra-crustal fractures provide pathways through the crust for the fluids derived from depth. Thus, magma is not necessarily required as the carrier of mantle-derived helium to the surface of the fore-arc region of Kii Peninsula.

References [1] Matsumoto et al., (2003) EPSL 216, 221-230. [2] Sano and Wakita (1985) JGR 90, 8728-8741.

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Goldschmidt 2004, Copenhagen

5.3.P02

5.3.P03

He-Sr isotope constraints on the mantle beneath the Monte Vulture volcano, southern Italy

Subduction-related metasomatic signature in noble gases in mantle derived xenoliths from Cheju Island, Korea

M. PATERNOSTER1, M. MARTELLI2, F.M. STUART3 AND P.M. NUCCIO1,2

H. SUMINO1, K.H. KIM2, H. KAGI1 K. NAGAO1 AND K. NOTSU1

1

Universita di Palermo, Dipartimento CFTA, Via Archirafi 36, 90123 Palermo, Italy ([email protected]) 2 INGV- Palermo, Via U. La Malfa 153, 90146 Palermo, Italy ([email protected]; [email protected]) 3 Isotope Geosciences Unit, SUERC, East Kilbride G75 0QF, UK ([email protected]) The Monte Vulture volcanic complex, southern Italy (<1Ma) is composed of dominantly silica undersaturated ultrabasic to intermediate lavas and pyroclastic flows. It is located east of the main axis of the Appennine chain, on the Adria plate margin. We have undertaken a He isotope study of olivine and pyroxene in lavas and pyroclastic flows from the major episodes of volcanism at Monte Vulture. Together with whole-rock and pyroxene 87Sr/86Sr and major and trace element chemistry we define the geochemical characteristics of the region, clarify the magmatic evolution and shed light on the relation between Vulture volcanism and the closely related Roman Magmatic Province (RMP). In vacuo crushing of olivine phenocrysts yields 3He/4He ranging from 6.0 Ra in the youngest deposits (c. 130 ka) to 4.4 Ra in the older samples (440-500 ka). This range is between the RMP (5.2 - 0.4 Ra: Martelli et al., submitted) and Etna (6.7 ± 0.4 Ra: Marty et al., 1994), suggesting a mantle source with HIMU affinities. Pyroxenes yield consistently lower 3He/4He than co-existing olivines. This is either due to crustal contamination or radiogenic He ingrowth in the magma. Strontium isotopes ratios (87Sr/86Sr =0. 7056) are more radiogenic than Etna (87Sr/86Sr= 0.7035). These results agree well with the enrichment of incompatible elements, displaying LILE/HFSE ratios lower than the RMP and intermediate between intraplate and subduction-related compositions (Peccerillo & Panza, 1999). Furthermore, in 143Nd/144Nd-87Sr/86Sr space, Monte Vulture volcanics fall between Vesuvius and Etna. Together with the other Italian volcanics the geodynamical meaning of Monte Vulture are discussed. In comparison with the RMP and Etna, the He and Sr isotopes suggest that the Monte Vulture volcanism was dominantly sourced in a asthenosphere mantle with HIMU affinities. However, the low 3He/4He and high 87Sr/86Sr relative to Etna imply that the parental magma has a small contribution of mantle that has been metasomatically-enriched during subduction of the Adriatic plate.

1

Laboratory for Earthquake Chemistry, University of Tokyo, Japan ([email protected]) 2 Dept. of Science Education, Ewha Womans University, Korea Cheju Island, a shield volcano composed of Quaternary alkaline basalt to trachyte, is located 90 km south of the Korean Peninsula. Ultramafic xenoliths are observed in lava flows of the alkaline basalts. Spinel lherzolites are the major constituent of the ultramafic xenoliths, and are derived from the subcontinental lithospheric mantle (SCLM) [1]. The maximum CO2 density of fluid inclusions in the xenoliths determined by micro-Raman densimeter is 1.10 g/cm3 for clinopyroxene, which corresponds to the depth of 31-35 km assuming temperature of 880-1040°C. Although the depth is shallower than that determined by mineralogical geobarometer (45-90 km) [1], probably resulting from plastic deformation during ascent of xenoliths with surrounding hot magma, it is deeper than the thickness of continental crust at this region (30 km) assuring mantle origin of the volatiles in the fluid inclusions. Noble gas isotopic compositions of minerals (olivine, orthopyroxene, and clinopyroxene) separated from the xenoliths were determined using both crushing and stepwise heating extraction methods. Except for high 3He/4He ratios observed at low temperature of the stepwise heating due to post-eruptive addition of cosmogenic 3He, all the samples showed uniform 3He/4He ratio of 6.51 ± 0.05 RA independently with the extraction method. The 3He/4He ratio lower than the MORB value (8 ± 1 RA) indicates higher (U+Th)/3He ratio of the SCLM than that of the MORB source mantle. The highest 40Ar/36Ar ratio was 5500, which is significantly lower than that of the MORB source mantle (>28000). Noble gas concentration of inclusion-rich olivines was one order of magnitude higher than the other samples, indicating that the fluid inclusions are trapping site of noble gases. Considering a rough correlation between mantlederived 3He and atmospheric 36Ar, the low 3He/4He and 40 Ar/36Ar ratios of the SCLM beneath Cheju Island are explained in terms of metasomatism by a slab-derived component, which subducted at the southeastern end of the continental margin of the Eurasian plate from Late Jurassic to Paleocene.

Reference [1] Choi S.H., Jwa Y.-J., and Lee H.Y. (2001) The Island Arc 10, 175-193.

A617

5.3 Subduction processes and the subcontinental lithosphere

Osmium in the Tonga subduction zone: Slab versus lithosphere W. BACH1, B. PEUCKER-EHRENBRINK1, T. ABBRUZZESE1 2 AND T. WORTHINGTON 1 2

WHOI, Woods Hole, MA 02543, USA ([email protected]) GPI, Univ. Kiel, Olshausenstr. 40, D-24118 Kiel, Germany

Recent studies of Os in island arc volcanics revealed striking correlations between 187Os/188Os and Os (Ni, Mg) concentrations primarily reflecting assimilation and fractional crystallization (AFC). The Tonga arc is relatively young (<30 Ma), has a thin crust (12 km), and U-series disequilibria studies suggest aqueous solutions are the dominant agents for mass transport and minimal interaction with old lithosphere.It may be less affected by AFC processes than many other arcs. We examined Os concentrations and 187Os/188Os ratios for a transect across the Tonga subduction zone to characterize sources and determine fluxes of Os (and PGE). The 70 m thick pile of pelagic and metalliferous sediments from DSDP Site 596 (1000 km E of the Tonga trench at 24º S) have a weighted Os concentration of 80±45 pg/g and 187 Os/188Os= 0.42±0.15. Pyroclastic sediments from Site 204 (100 km East of the Tonga trench at 25˚ S) have 110±20 pg/g Os and 187Os/188Os=0.15±0.01. Cretaceous upper altered ocean crust from Site 595 (adjacent to Site 596) reveal 4±2 pg/g Os and 187Os/188Os=1.6±0.8. Samples from the Tonga forearc are peridotites (~ 4 ng/g Os and 187Os/188Os=0.128) and boninites (24 pg/g Os, 187Os/188Os=0.146). Volcanics from the Tonga arc between Tafahi and Tofua range from 0.9 to 14.3 pg/g Os and 187Os/188Os=0.178-0.506. Lau Basin basalts and andesites vary from 1 to 225 pg/g Os and 187 Os/188Os=0.136-0.699. Both arc and back-arc samples form AFC-like trends in 187 Os/188Os vs. 1/Os diagrams, very similar to relationships found in arc volcanics from Mexico, Java, and the Andes. However, Os is not correlated with Ni and Mg. The main arc (Late to Tofua) displays a positive correlation between 187 Os/188Os and Ba/Nb (also U/Zr and Th/Nb). In contrast, rocks from Tafahi and Niuatoputapu as well as from the Valu Fa Ridge have constant Ba/Nb at variable 187Os/188Os. The horizontal trend is best explained by crustal contamination, although the Os isotope variability (0.136-0.699) is surprising, given that the lithosphere is young and thin. The co-variation between Ba/Nb and 187Os/188Os may indicate that slab-derived Os flux is significant beneath the main arc, although such a model does not account for the systematic 187Os/188Os vs. 1/Os relationship without invoking additional fractionation processes. The forearc peridotites are likely affected by slabderived fluids. If the initial 187Os/188Os of the mantle is between 0.120 and 0.125 and the slab-derived fluids have between 10 and 100 ppt Os of the composition of composite 595/596 crust (187Os/188Os= 0.76), small fluid fluxes (integrated water/rock ratios of 0.2 to 5) are sufficient to raise the 187Os/188Os to 0.128.

5.3.P05

Re-Os isotopes in mantle-derived pyroxenite xenoliths from Hannuoba North China X.C. ZHI1, Q. MENG1, Q.X. XIA1, L. ZHENG1 AND Y.G. XU2 1

School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China ([email protected]) 2 Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China ([email protected]) A set of very fresh pyroxenite xenoliths, composed of Alaugite spinel pyroxenite, Cr-diopside spinel pyroxenite and garnet pyroxenite sampled from Cenozoic alkaline basalts in Hannuoba, North China displays fairly heterogeneous Re-Os isotopic compositions. The Re and Os contents of the samples range from 0.031 to 1.551ppb and 0.126 to 4.163ppb respectively. The 187Os/188Os and 187Re/188Os ratios vary from 0.1270 to 2.352 and 0.114 to 177.1 respectively. When compared with peridotite xenoliths from Hannuoba and orogenic pyroxenites from Beni Bousera, Ronda, Lanzo, and Lherz etc., the Cr-pyroxenites and three of four garnet pyroxenites have Re and Os contents and isotopic compositions clustering around those of the Hannuoba xenoliths. Almost half of Al-pyroxenites have Re and Os contents similar to those of orogenic pyroxenites, but much lower 187Os/188Os and 187Re/188Os ratios. However, the 187 Os/188Os and 187Re/188Os ratios of these Al-pyroxenites are larger than those of peridotite xenoliths from the same location. The rest of the Al-pyroxenites have the same range of Re-Os contents as that of Cr-pyroxenites and mantle peridotite xenoliths. The Cr-pyroxenites and three garnet pyroxenites were formed from asthenospheric melt in multiple stages. The Alpyroxenites were derived from a mixture of asthenospheric melt and lower crust in various proportions during mantle melt underplating at the Moho boundary. 1.E+01

1.E+00

MORB OIB

Re (ppb)

5.3.P04

1.E-01

Continental mantle crust 1.E-02 1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

Os (ppb) Al-Py

Cr-Py

Gt-Py

Orogenic-Py

HNB-Peridotite

This work is supported by the National Nature Science Foundation of China (40173009, 49873005)

A618

Goldschmidt 2004, Copenhagen

5.3.P06

5.3.P07

Mantle modification and diamond genesis during continental accretion

Os isotopic systematics in peridotite xenoliths from the SW Japan arc

B.P. GARDEN1, S.B. SHIREY2, R.W. CARLSON2, M.D. SCHMITZ2, S.H. RICHARDSON1 AND J.J. GURNEY1

M.R. HANDLER1 S. ARAI2 AND K. SUZUKI1

1

Geological Sciences, University of Cape Town, Cape Town, South Africa 2 Dept of Terrestrial Magnetism, Carnegie Institution of Washington, Washington D.C., USA The Bellsbank Group kimberlite (120 Ma, group II kimberlite), Kaapvaal craton, lies ca. 200km west of the Colesberg Lineament, the proposed suture between the eastern and western Kaapvaal, where the eastern Kaapvaal was subducted against the western Kaapvaal during a period of craton accretion between 2.94 and 2.88 Ga [1]. The proximity of Bellsbank to this suture makes it an ideal locality in which to evaluate modification of the subcontinental lithospheric mantle (SCLM) during accretion and diamond genesis. Bellsbank peridotites are split into three groups based on petrography/mineral chemistry and Re-Os systematics 1) spinel peridotites (n=19) 2) garnet peridotites (n=6) 3) necklace textured garnet peridotites (n=3). Garnet bearing peridotites show more scatter in ages caused by post formation metasomatic events, in direct contrast to the shallower spinel peridotites that show consistent age systematics (TRD = 2.85 Ga ± 0.06, TMA = 3.01 Ga ± 0.09) and overlap with the age of accretion. Three peridotites give TRD ages >3.2Ga. As TRD ages are typically lowered by multiple depletion or metasomatic events, this supports the presence and subsequent modification of older SCLM in this region. Bellsbank eclogite (Group A, Group B & Group C) Re-Os concentrations suggest that Group B & C represent parts (residues?) of a subducted slab which has undergone partial melting. Their Re-Os ages cluster around a 2.9Ga isochron suggesting that they formed just prior to or during craton accretion. Group A eclogites are websteritic and may have formed by slab derived melt interaction within the overlying mantle wedge. Re-Os systematics for syngenetic eclogitic sulfide diamond inclusions give a ca. 2.9 Ga age. Their age and basaltic affinity support the subduction accretion model. Bellsbank is significant as the only locality so far where three lithospheric components (peridotites, eclogites, and diamonds) have been studied.

Reference [1] Schmitz et al, ESPL (in press)

1

Institute for Frontier Research in Earth Evolution, Japan Marine Science and Technology Center, Yokosuka, Japan 2 Department of Earth Sciences, Kanazawa University, Kakuma-machi, Kanazawa, Japan

Noyamadake volcano in the southwest Japan arc, has transported a varied suite of mantle-derived xenoliths to the surface, including residual peridotite and a range of cummulate rocks, providing direct samples of the mantle wedge. Mantle wedge-derived xenoliths can provide valuable insights into Os and other chalcophile element recycling through subduction zones, as well as constraining the composition and evolution of the mantle wedge. In particular, they have the advantage that they bypass potentially complicating crustal processes that are likely to effect arc lavas, such as crustal contamination and fractionation. Noyamadake is a 6-7.3 Ma [1] basanitic volcano that has sampled the backarc-side of the southwest Japan volcanic arc. The Noyamadake peridotite suite represents a hotter and less metasomatised mantle wedge [2] than previously reported arc xenoliths, including those from Ichinomegata in the northeast Japan arc [e.g. 3]. The peridotite xenolith suite comprises spinel bearing peridotites ranging from fertile lherzolite (23 vol% clinopyroxene) to highly depleted harzburgite (0.1 vol% clinopyroxene)[2]. These protogranular to porphyroclastic xenoliths record very high equilibration temperatures (11001250ºC)[2]. Secondary volatile-bearing phases, such as pargasite, are absent. The peridotites range from LREEdepleted to LREE-enriched, and trace element systematics suggest that the samples have interacted with two types of metasomatic agents: a hydrated silicate melt or hydrous fluid, and a more rare carbonate-rich melt or CO2-rich fluid [2]. We will present Re-Os isotopic data for fertile to depleted peridotites from Noyamadake to constrain the Re-Os isotopic composition of the sampled sub-arc mantle, and to investigate the behaviour of Re and Os in a high temperature, relatively dry region of the mantle wedge.

References [1] Uto, K. (1995) Bull. Volcanol. Soc. Japan - 40th Anniv. Iss. S27-46. [2] Abe, N., Hirai, H., Arai, S. and O`Reilly, S.Y. (2002) Abst. 4th Int. Workshop Orogenic Lherzolite & Mantle Proc., Hokkaido. [3] Brandon, A.D, Creaser, R.A. Shirey, S.B. and Carlson, R.W. (1996) Science 272, 861-864.

5.3 Subduction processes and the subcontinental lithosphere

5.3.P08

5.3.P09

Geochemical heterogeneity of south Alpine subcontinental lithosphere: Mantle xenoliths evidence

Petrology of the peridotite xenoliths from East Asia region: Implications for the evolution of the subcontinental lithosphere

D. GASPERINI1, D. BOSCH2, R. BRAGA3, M. BONDI3, P. MACERA1, AND L. MORTEN3 1

Dipartimento Scienze della Terra, Università di Pisa, Italy ([email protected]) 2 Laboratoire de Tectonophysique, Université de Montpellier 2, France 3 Dipartimento Scienze della Terra e Geologico Ambientali, Università di Bologna, Italy ([email protected]) The Tertiary Veneto Volcanic Province (VVP; SE Alps, northern Italy) includes Na-rich alkali basalts and basanites hosting many spl-peridotite xenoliths (mg#>88) among the ultramafic nodules which reflect a variably depleted mantle source [e.g. strong Cr/(Cr+Al) ratio increase at a slight Mg/(Mg+Fe2+) ratio decrease in spinels] subsequently enriched by different metasomatic processes (e.g. presence of spongy clinopyroxenes; large range of enrichment in LREE, K, Rb, Sr, and P over HREE and HFSE). New geochemical and isotopic (Sr, Nd, Pb, O) data on VVP whole rock xenoliths help evaluating the nature of the geochemical enrichment recorded in the south Alpine subcontinental lithosphere. Compared to various representative world-wide ultramafic xenoliths, the VVP xenoliths show geochemical similarities to those carried by alkaline basalts in tectonic settings dominated by the presence of hot spot magmatism (e.g. Canary and Kerguelen Islands, French Massif Central, Rhon, Eifel). Moreover, most of the enriched xenoliths display significant positive Nb and Ta anomalies, incompatible element ratios, and Sr-Nd-Pb and -O isotopic compositions similar to the host VVP alkaline basalts, thus with a marked OIB-HIMU signature. By contrast, among different agents proposed so-far to explain trace element enrichment in the VVP xenoliths, carbonatite-induced metasomatism does not appear as significantly responsible. Most of the depleted xenoliths are overall characterized by incompatible element patterns (e.g. troughs at Ba, Nb, and Ta) and ratios (e.g. high Rb/Nb, La/Nb, Th/Yb) likely related to a sedimentary component in their source, thus showing Sr-NdPb-O isotopic compositions falling between the field of MORB and altered oceanic crust ± pelagic sediments. Similar features have been observed for ultramafic xenoliths collected in basalts from subduction-related environments (e.g. Marianas, Philippines, and Papua Nuova Guinea). Lack of significant presence of hydrous phases (phlogopite, amphibole) in the spl-peridotite xenoliths (mg#>88), but persisting geochemical and isotopic heterogeneities, suggests that the effects of any eventual metasomatism inferred by fluids released from a supposed subducted plate beneath the Veneto region, have been subsequently veiled and overlapped by modification induced by mantle diapirism related to alkaline volcanism in the southern Alps.

A619

N. ABE1, S. ARAI2, M. NEDUKA2 AND M. KIDA2 1

Deep Sea Research Department, Japan Marine Science and Technology Center (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan ([email protected]) 2 Department of Earth Sciences, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan ([email protected]) There are a lot of mantle xenoliths localities in the East Asian continental margin [1]. This region had been the site of active the margin of the Asian continent in Mesozoic to Paleogene time. Therefore, the mantle xenoliths from this reagion give us useful information about the coplicated history of the Asian continent evolution. This paper discusses detailed petrologiccal characteristics of the spinel peridotite xenoliths from Cenozoic alkali basalts in Sikhote-Aline reagion, Far East Russia [2] and from Boun in South Korea [3] and combined with preveous studies, we summarize petrological characteristics of the upper mantle beneath the East Asian continental margin. The peridotite xenoliths from these two regions are mostly composed of clinopyroxene-rich peridotite, rarely of harzburgite. In the most samples, the Cr# of spinel are lower than 0.2 and the Na2O content in clinopyroxene are higher than 1.0 wt%. The chondrite normalized rare earth element patterns of clinopyroxene are slightly LREE (light rare earth element)-depleted to enriched patterns. These mineral chemistries in the samples suggest that they are metasomatized by some agent correlated with some continental type magmatism rather than arc magmatsm. It is noteworthy, however, that the peridotite xenoliths with arc or abyssal mantle signatures apparently have not been found from the eastern margin of the Asian continent, despite possible arc settings experienced through geologic time. It is highly speculative but is possible that the sub-arc type mantle material had not been accreted upon arc crust accretion. Alternatively the arc type mantle that had once been as suprasudction zone was replaced later by the continental rift-zone type mantle in the East Asian continental margin.

References [1] Nixon, P.N. (1987) Mantle Xenoliths. Pp. 844, John Wiley & Sons, NY. [2] Ionov, D.A., Prikhod’ko, V.S. and O’Reilly, S.Y. (1995) Chem. Geol. 120, 275-294. [3] Arai, S. Kida, M., Abe, N. and Yurimoto, H. (2001) J. Mineral. Petrol. Sci. 96, 89-99.

A620

Goldschmidt 2004, Copenhagen

5.3.P10

5.3.P11

Crystallization conditions of magmatic garnets from gabbroid xenolths (Siberian Region, Russia)

The petrogenesis and thermal history of lower crustal xenoliths from the Karnei-Hitin volcano, northern Israel

V. EGOROVA1, R. SHELEPAEV1 AND L. BUZLUKOVA2

O. BEN DOV1, O. NAVON1, L. HALICZ2 AND M. STEIN2

1

First Institute of Geology of SB RAS, Russia ([email protected]) 2 United Institute of Geology, Geophysics and Mineralogy of SB RAS, Russia (lucya@ uiggm.nsc.ru) Experiments on garnets show that they are generated under high pressure conditions in calc–alkaline basaltic magmas. We have studied garnet-bearing mafic xenoliths from the Ordovician lamprophyre of Western Sangilen (Siberian Region, Russia). Texture features of gabbroid xenoliths reflect their magmatic origin and belonging to abyssal layered intrusion. They have gabbroic and gabbroophitic texture and show layering of alternating plagioclase-rich and pyroxene-rich layers. The xenoliths consist of clinopyroxene, orthopyroxene, plagioclase and garnet (not exceed 25 vol.%). Garnet appears in the xenoliths as idiomorphic grains with kelyphite rims and sometimes contains inclusions of plagioclase and clinopyroxene. The kelyphite has largerly preserved the major element composition of the original garnet. Kelyphitisation is commonly attributed to heating of the affected rock, either during transport in the host magma [1] or decompression [2]. Garnet is pyrop-almandine (Prp22-52, Alm35-60, Grs15-20) with no zonation. The ratio Mg/(Mg+Fe) of garnets increase with increasing the ratio Mg/(Mg+Fe) of coexisting pyroxenes. For determination equilibration temperatures were used clinopyroxene-garnet [3], orthopyroxene-garnet pairs [4]. Equilibration pressures were estimated using orthopyroxenegarnet equilibria [4]. The temperature–pressure estimations indicate that equilibrium between garnet and pyroxenes is occurred near 990-1080°C and 1.15-1.3 GPa. Investigations of major and trace elements compositions and P,T-estimates of garnet-bearing xenoliths show their origin from calc–alkaline basaltic melt under high pressure and temperature near 1000°C. The work is supported by RFBR 04-05-64439, SS1573.2003.05, VMTK UIGGM SB RAS 1771, 1772.

References [1] Stosch H.G. et. al. (1995) Lithos, 36, 227-242. [2] Rudnick R.L., Jackson I. J. (1995) Geophys. Res. 100, 10211-10218. [3] Powell R. J. (1985) Metamorphic Geol. 3, 231-243. [4] Harley S.L. (1984) J. Petrology 5, 665-694.

1

The Hebrew University, Jerusalem, Israel ([email protected]) 2 Geological Survey of Israel, Israel ([email protected]) The Pliocene Karnei-Hitim volcano in northern Israel transported numerous and large size (10-30 cm) xenoliths of mafic granulite derived from the lower continental crust. The xenoliths are composed of mainly plagioclase (PLAG), clinopyroxene (CPX) and garnet (GT), and minor orthopyroxene (OPX), amphibole (AMP) and spinel. The rocks have granular or gneissic textures (showing CPX boudines within PLAG groundmass). Mineral thermobarometry yielded ~ 850 C and 6-8 kb, respectively, suggesting heating of the xenoliths source above the conductive steady state geotherm of the northern Arabian plate. SmNd isotope analyses yielded a late Proterozoic bulk rock isochron of ~700 Ma, which can be related to the production of the lower crust during the Pan African magmatism. However, 87Sr/86Sr isotope ratios of Plag-AMP pairs were reset and Rb was lost from the minerals probably due to the Cenozoic heating of the Arabian lithosphere that is also reflected by the elevated CPX-OPX temperatures. Initial εNd and 87Sr/86Sr ratios (+6 and 0.70312, respectively) and the patterns of major and trace element in the xenoliths (e.g. positive Eu anomalies) are consistent with their formation as cumulates from rising basaltic magmas.

5.3 Subduction processes and the subcontinental lithosphere

5.3.P12

A621

5.3.P13

Geochemistry of Neoproterozoic mafic-ultramafic rocks from northern Guiangxi, south China: Implications for arc magmatism along continental margin

Tectonic evolution of the central Higher Himalayan Crystallines in the Kharta area, southern Tibet: New constraints from geochronological data

JINCHENG ZHOU, XIAOLEI WANG AND JIANSHENG QIU

YAN LIU1, W. SIEBEL2, H.-J. MASSONNE3, XUCHANG XIAO1

Dept. of Earth Science, Nanjing University, Nanjing 210093, China ([email protected]; [email protected]; [email protected]) The western end of Proterozoic Jiangnan orogenic belt distributed along the southern margin of the Yangtze block is located at northern Guangxi, China, where the Mesoproterozoic Sibao group and the Neoproterozoic Danzhou group occur. A lot of mafic-ultramafic rocks are hosted in Danzhou group. The 828 7 Ma four mafic-ultramafic dykes intruded into Sibao group and the mafic rocks in Danzhou group have been considered to be correlative with 827 Ma Gairdner Dykes Swarm (GDS) and 824 Ma Amata suite in Australia and were thought to be an indicator for arrival of plume and breakup of the Rodinia supercontinent by some researchers [1]. The GDS and Amata suite in Australia show obvious continental food basalt and ocean island basalt, i.e. “within plate basalt” signatures, whereas the mafic-ultramafic rocks from Danzhou group are geochemically quite distinct.The most mafic-ultramafic rocks in Danzhou group have been plotted in the field of volcanic arc basalts in various tectonic discri-minative diagrams. Their primitive mantle-normalized patterns of selected HFSE and REE display negative Nb, Ta, Zr, Hf and Ti anomalies. They have higher Th/Lapm, lower Nb/La pm and Ti/Ti* documenting the characteristics of arc volcanic rocks. It is evident that they are products of subduction of the oceanic crust along the southern continental margin of the Yangtze block. A few samples fall in back-arc area in some discriminative diagrams but showing arcaffinities. Therefore, they should be generally considered as the products of magma activities in convergent plate margins, and could not be regarded as a sign of the breakup of Rodinia.

References [1] Li ,Z.X.,Li ,X.H.,Kinnh ,P.D.,Wang ,J. (1999) EPSL 173, 171-181.

1

Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 2 Institute of Geosciences, 72074 Tuebingen, Germany 3 Institute of Mineralogy & Crystallochemistry, 70174 Stuttgart, Germany Within the Kharta area, east of Mt. Qomolangma (Everest), garnet sillimanite gneisses and granites of the Higher Himalayan Crystallines were displaced beneath the North Col formation by a ductile normal fault (STD1) and above the upper Lesser Himalayan Crystallines by a ductile thrust (MCT1), respectively. Both the garnet sillimanite gneisses and granites contain overprinted eclogite lenses. Zircons from several samples of the lenses were dated by the U-Pb TIMS method. All samples plot close to a discordia line with Neoproterozoic upper intercept ages, suggesting a late Proterozoic age for the basic dike emplacement. Metamorphic monazite from the garnet sillimanite gneiss yields U-Th-Pb ages between 32 and 24 Ma, which are interpreted to indicate Barrovian-type metamorphism taking place during the Oligocene to early Miocene. Monazites from two granite bodies beneath the STD1 give very similar crystallization ages between 12-13 Ma, indicating the active time of the STD1. Monazites from a sample of highly sheared sillimanite gneisses beneath the MCT1 give a lower-intercept age of 13.4 1.3 Ma, which is interpreted as an active age of the MCT1. Our data show that the central Higher Himalayas comprises Proterozoic to early Paleozoic sediments and basic dikes. These rocks experienced HP-UHP metamorphism resulting in the formation of eclogite and HP metapelitic rocks during an early stage of India-Asia collision. Subsequently, these rocks underwent regional high-temperature and intermediate-pressure metamorphism and were converted into garnet sillimanite gneisses and garnet-bearing mafic lenses, respectively, at about 32 Ma. At around 13 Ma, these rocks were exhumed southwards to shallow depth via channel flow and to overlie the Lesser Himalayan Crystallines. Finally, both the Higher and Lesser Himalayan Crystallines underwent NNE-trending folding.

A622

Goldschmidt 2004, Copenhagen

5.3.P14

Geochemical characteristics and genesis of the Late Paleozoic metamorphic rocks in eastern Tianshan, Northwest China W. ZHU, L. SHU, M. ZHAO AND Y. SUN Department of Earth Sciences, State Key Laboratory of Mineral Deposit Research, Nanjing University, Nanjing 210093, P.R.CHINA ([email protected]) Two Late Paleozoic metamorphic belts have been identified in eastern Tianshan, northwest China . One is the Kushui metsmorphic belt to the south, and the other is the Harlik metamorphic belt to the north. The Kushui metamorphic belt consists of schistose epimetamorphic rocks, phyllite, schist, granulite and amphibolite. Metamorphism is largely of a middletemperature (465~640 C ) and middle-pressure (4Kb) facies with assemblages that include kyanite, staurolite and sillimanite. The study of chemical characteristics of metamorphic rocks displays that most metamorphic rocks are derived from volcanic protoliths, and others are from sedimentary protoliths. The compositions of the main rock forming minerals such as muscovite, hornblende, garnet and staurolite indicate that the Kushui metamorphic belt underwent hormblend facies metamoephism. A whole rock Rb-Sr ischron age of 291.8 28.5Ma implies that metamorphism occurred at the end of the Carboniferous. Although the strata of the Kushui metamorphic belt were deposited in an extensional setting, the middle-pressure type regional metamorphism is related to post-extension compressional tectonism. The Harlik metamoephic belt passed two metamorphoc stages, that is, thermal metamorphism and regional dislocation metamorphism. The thermal metamorphism, which formed metamorphic hornstones indicated by high-temperature (T=922~780 C and low-pressure mineral compositions, such as cordierite, andalusite etc, was closely associated with intra-island-arc tension caused by B-type subduction of the ancient oceanic plate. A zircon U-Pb age of 305.9 2.9Ma represents the age of the thermal metamorphism. The regional dislocation metamorphism, formed crystalline schists indicated by middle-temperature (T=450~670 C) and middlepressure (P=2.3~4.1Kb) mineral composition of almandine, staurolite and sillimanite, was closely related to regional faulting caused by A-type subduction in the process of collision orogeny. The zircon U-Pb dating ranges from 293.8 3.3Ma to 296.3 3.5Ma, which represents the age of the regional dislocation metamorphism. This research was supported by China Natural Science Foundation (Grant Nos: 49372102 and 40173001).

5.3.P15

PGE geochemistry of chromitites and dunites of the Nidar Ophiolite, Himalaya N.SIVA SIDDAIAH1

2

AND A.MASUDA

1

Wadia Institute of Himalayan Geology, Dehra Dun, India ([email protected]) 2 University of Electro-Communications, Tokyo, Japan ([email protected]) Numerous ophiolites occur in the Indus Suture Zone, and the Nidar ophiolite is the largest on the eastern Ladakh, Himalaya. It consists of radiolarian cherts, pillow lavas, dykes, cumulates, rodingites, peridotites and chromitites. Spinel bearing harzburgites and dunites are the dominant ultramafic rock types. Chromite, chalcopyrite, pyrrhotite, pyrite and arsenopyrite occur as disseminations in ultramafic rocks. Harzburgites and dunites are serpentinized to varying degrees and the sulfide assemblage suggests a reducing environment during serpentinization. Nidar peridotites have low TiO2 (0.02-0.03 wt. %), Al2O3 (2-3 wt. %), CaO (0.58 wt. %), Zr (40 ppm) and Y (30 ppm), and enriched in Cr (3000-5000 ppm) and Ni (3000 ppm). Ni positively correlates with MgO content. TiO2 and CaO concentrations decrease with increasing MgO content. They host podiform chromitites (Cr # = 0.82) of metallurgical grade. The geochemistry of peridotites and Cr # of the chromitites suggest its formation by high degrees of partial melting in a supra-subduction zone environment involving fluids/volatiles. PGE were determined by laser ablation ICP-MS after preconcentration into a NiS mini-bead. The chromitites have relatively higher concentrations of PGE (Pt = 0.275-0.716 ppm; Pd = 0.223-0.918 ppm; Ru = 0.186-0.595 ppm; Rh = 0.084-0.352 ppm; Os = 0.077-0.142 ppm; Ir = 0.053-0.106 ppm and Au = 0.057-0.270 ppm) with the total PGE (excluding Au) ranging from 0.898 to 2.75 ppm. The CI chondrite-normalized abundances are variable, fractionated and show a negative Pt anomaly. The host serpentinized dunites have very low concentrations of noble metals (total PGE = 0.5 ppm) and their chondrite-normalized PGE patterns are similar to those of the chromitites, except their low abundances which suggests that serpentinization has not affected the whole-rock PGE chemistry. The relative abundances of PGE are probably dependent on the number of PGE bearing phases present within the chromites. Such PGE patterns are common to the magmatic sulfide deposits suggesting a magmatic source for the PGE. The geochemistry of ultramafic rocks and the mineral chemistry of podiform chromite, indicate a fluid-rich suprasubduction zone environment for the formation of Nidar ophiolite, and a magmatic source for the PGE. Ophiolites formed in a supra-subduction zone environment constitute a potential host for mineral deposits.

A623

5.3 Subduction processes and the subcontinental lithosphere

5.3.P16

5.3.P17

Late Yanshanian mafic-ultramafic rocks in the coastal area of Fujian, SE China: A Sr-Nd-Os isotopic approach 1

CHING-YING LAN , JASON JIUN-SAN SHEN HUI-JEN YANG2

1

AND

Correlation of the Paleozoic porphyry magmatism of Cu-Mo deposits and global events on Earth V.A. PONOMARCHUK AND V.I. SOTNIKOV United Institute of Geology, Geophysics and Mineralogy, 630090, Novosibirsk, Russia. ([email protected])

1

Institute of Earth Sciences, Academia Sinica, Nankang P.O. Box 1-55, Taipei, Taiwan 11529, ROC ([email protected]; [email protected]) 2 Department of Earth Sciences, National Cheng Kung University, 1 University Road, Tainan, Taiwan 70101, ROC ([email protected]) Late Mesozoic igneous rocks occur over a vast area of SE China on the western Pacific margin. Among these rocks, granitoids are predominant, mafic-ultramafic rocks are rare and intermediate rocks are even less common. Along the coastal area of Fujian, late Yanshanian mafic rocks distribute sporadically along the east side of the Changle-Nanao deep fault. They consist of plagioclase, amphibole, pyroxene and opaques. Country rocks of these mafic intrusives are mostly granitic gneiss and migmatites. Mafic rocks in Daiqianshan show obvious cumulate layering structure, characterized by a chilled gabbros base that grades upward to medium grained gabbro or diorite. Those in Pingtan occur mainly as stocks and dykes or as enclaves within the granodiorite. Ultramafic rocks occur only in Changji which is situated to the west of the Changle-Nanao deep fault. Ultramafic rocks are strongly serpentinized and locally talc-magnesianized. The wall rocks of the ultramafic rocks are early Yanshanian volcanic rocks. All the mafic-ultramic rocks show slightly enriched Nd and Sr isotopic compositions ($Nd = -1.8 to -2.7; 87Sr/86Sr " 0.706 except ultramafic rocks of 0.710) which also overlap with those of the surrounding granitoids. Thus, Nd-Sr isotopes are not likely to be effective discriminants for maficultramafic rocks. However, Os isotopic compositions can be divided into three groups. Hence, the distinctions become clearer and enable us to differentiate ultramafic rocks (depleted) from cumulative (slightly enriched) and noncumulative (enriched) mafic rocks. This study further demonstrates the general belief that Re-Os isotope system is generally less disturbed than lithophile element-based (Nd and Sr) isotope systems.

Major mass extinctions, continental flood basalts, tectonic plate motion and variation in 87Sr/86Sr, 12C, 18O and 34S in Phanerozoic seawater are synchronous with global events. They mark stages in Earth’s evolution and are interpreted in terms of mantle processes. The investigations of the Paleozoic porphyry Cu-Mo deposits of the Siberia and Mongolia showed the necessity of including porphyry magmatism of the Cu-Mo deposits in that list of global events. The following sequence of age data was obtained for the Sora (South Siberia) ore-bearing center [1]: host rocks – gabbro 478-480 Ma; granosyenites 466 Ma; potash feldspar rocks 462 Ma; quartz monzonites 452 Ma; leucocratic granites 418-422 Ma; preore dikes 402-404 Ma; ore-bearing porphyries of the I rhythm 386-388 Ma; porphyries of the II rhythm -356 Ma. For the Aksug ore-bearing center (South Siberia ), the following endogenic events were established: host rocks - gabbro 532-522; 497; 490-488 Ma; quartz diorites 462 Ma; porphyry-like tonalities 404+/-7 Ma. Porphyries of the I rhythm (major ore formation) are dated as 364-356 Ma, while dikes of granites and aplites are 331-324 Ma. The hoste rocks of the large Cu-Mo-deposit Erdenetuin-Obo (North Mongolia) formed near 270 Ma, porphyries of I rythm (main mineralization) – 240 Ma ago, porphyries of II rythm -220 Ma. Porphyry magmatism of the Cu-Mo deposit TsaganSuvarga (South Mongolia) is dating 365 Ma. All above mentioned ages marked by bold italics are concurring with the time of development local minima on the global [2] the Phaleosoic seawater 87Sr/86Sr-evolution trend (Fig.1). 0.7100

6 8 7 8

rS0.7080 /r S 0.7060

500

380 360

240 220

0

M

Fig.1 87Sr/86Sr evolution of seawater. Modified from [2]. 87Sr/86Sr-minima of the seawater trend mark the time of ocean opening and transport of mantle materials to the surface of the Earth and also the time of manifestation of porphyry rocks on the Cu-Mo deposits in the Paleosoic.

References [1] Sotnikov V.I., et al. (2002) Geology & Geophysics, 43, 211-239. [2] Burke W.H., et al. (1982), Geology, 10, 516-519.

A624

Goldschmidt 2004, Copenhagen

5.3.P18

The subcontinental lithosphere beneath Central Srednogorie (Bulgaria): U-Pb and Hf-zircon, Nd and Sr whole rock constraints I. PEYTCHEVA 1,2 , A.V. QUADT2, M. FRANK2, B. KAMENOV3 AND C.A. HEINRICH2

1

Central Lab. of Mineral. & Crystall., BAS, Sofia, Bulgaria([email protected] ) 2 IGMR, ETH-Zurich, Switzerland ([email protected]) 3 Sofia University “St. Kliment Ohridski”, Sofia, Bulgaria ([email protected]) The Central part of the Srednogorie zone (Bulgaria) is characterized by high-metamorphic continental (mainly) and low-metamorphic island-arc basement rocks, overlain by Carboniferous to Cretaceous cover sequences. This succession is intruded by Late Cretaceous subduction related calcalkaline basic to acid (predominantly intermediate) igneous rocks, which host world-class Cu-Au-deposits. The goal of this study is to constrain the age and the nature of these basement rocks and to evaluate the role of both, the crustal protoliths and the subcontinental mantle lithosphere as source for some elements of the deposits. The following rock types were investigated using IDTIMS for the precise U-Pb single zircon (zr) and the Rb-Sr and Sm-Nd whole rock samples and MC-ICPMS for the analyses of the Hf isotopes: (1) Zircons from Amf-Bi Gneiss point to Lower Paleozoic age of the protoliths (460-500 Ma) and crustal dominated source (εNd (500) of -9.4 and εHf (500) of the concordant zr in the range of –1.14 to –2.55); (2) Concordant zircons of the green-schist facies overprinted diabase define an age of 443.0±1.5 Ma. The εNd (440) values of +2.5 to +2.8 are in agreement with the idea for island-arc geodynamic environment of formation. (3) Early Paleozoic events are recorded also in inherited zr of the Upper Cretaceous Velichkovo gabbro and the Medet gabbro-diorite. In the first case the age is 442.7±7.3, the εHf (440) ranging between –0.85 and +1.20 and marking a mixed mantle and crustal source for the protolithic magma. Old inherited zr grains and cores of the Medet gabbro-diorite define a discordia line with an upper intercept age of 456.5±5.5 Ma. The εHf (460) of the same zircons of +8.04 to +9.88 suggest mantle source of the old protoliths; (4) Upper Cretaceous basic rocks (most primitive magmatic compositions were chosen) reveal slightly positive εNd characteristics (< +1.55), initial Sr ratios of 0.7042 to 0.7049 and εHf of the concordant zircons in the range of +0.78 to +9.94, but mainly >+8.5. They argue for an enriched and probably not homogenous mantle source. Geochemical features give evidence for contribution of slab-derived components (Rb, Th, K, Na, P). The formation of economic deposits was a combination of factors, which lead to the concentration of the ore metals; the world-class Cu-Au-(PGE) deposits in Central Srednogorie are not connected with the most primitive Cretaceous magmas, but are related to processes of mantle-crustal interaction.

5.3.P19

Crustal processing: The Finnmarkian orogen reassessed F. CORFU1, R.J. ROBERTS2, T.H. TORSVIK3, T.B. ANDERSEN1, D. M. RAMSAY4 AND L.D. ASHWAL2 1

University of Oslo, Oslo ([email protected]) Univ. of the Witwatersrand, Johannesburg, South Africa 3 Geological Survey of Norway, Trondheim 4 27 Marlee Rd., Dundee, Scotland, UK 2

The geological history of the Finnmarkian orogen in the northern Scandinavian Caledonides has traditionally been the subject of many contrasting interpretations. Early geochronological work had provided evidence for a Cambrian orogenic event that predated the Siluro-Devonian nappe emplacement [1]. Subsequent studies [2] showed that there was also a record of Neoproterozoic deformation and plutonism in what had originally been taken to be the Late Neoproterozoic-Cambrian cover of the Baltic margin. Our new work confirms the evidence for a prolonged evolution but also leads to a more differentiated picture. It is now apparent that the metasedimentary succession is a composite of two distinct assemblages. The lower nappe complex had a multistage Neoproterozoic history and little Scandian overprint. The upper assemblage probably represents a sequence of latest Neoproterozoic or Palaeozoic deposits, which underwent deformation, metamorphism reaching anatexis and granitic magmatism in the Silurian as indicated by preliminary ages between 440 and 420 Ma for zircon, monazite and titanite in various rocks. It appears to correlate with the fossiliferous Palaeozoic units present on the island of Magerøy, that were intruded by gabbro and granite at 440-434 Ma. The lower nappe complex contains the syntectonic 850 Ma Litlefjord granite [2], the syn-tectonic (?) 706 Ma Sandøra granite on Skjervoy, the 602 Ma Gildetun granite, and the 580-560 (+530) Ma Seiland alkaline igneous complex. Titanite and monazite in various samples show evidence for metamorphic reworking during episodes that correlate with the granitic events, but there is only a very weak signature for the Siluro-Devonian nappe emplacement. This lower nappe complex, originally interpreted to represent the rifted Baltic margin, matches age relationships observed in the Moine succession of northern Scotland, that is considered to have been derived from Laurentia. Their distinct Neoproterozoic history suggests, however, that both represent terranes swept up from the Peri-Gondwanan realm, as Baltica was in the process of approaching Laurentia.

References [1] Sturt B.A., Pringle I.R. and Ramsay D.M. (1978) J. Geol Soc. 135, 547- 610. [2] Daly J.S., Aitcheson S.J., Cliff R.A., Gayer R.A. and Rice A.H.N. (1991) J. Geol. Soc. 148, 29- 40.

5.3 Subduction processes and the subcontinental lithosphere

5.3.P20

5.3.P21

Copper-Gold systematics and arc magmatism in the eastern Manus Basin W. SUN1, R. J. ARCULUS2 AND V.S. KAMENETSKY3 1

Max-Planck Institut f. Chemie, Postfach 3060, Mainz, 55020, Germany 2 Department of Earth and Marine Sciences, The Australian National University, Canberra, ACT 0200, Australia 3 Center for Ore Deposit Research and School of Earth Sciences, University of Tasmania, Hobart, Australia Most of the world’s large Cu-Au porphyry/epithermal ore deposits are closely associated with convergent margin magmatism; models to explain this association range from oxidation of sulfide and redistribution of Au and Cu in the mantle source by high fO2 melts or fluids released from subducted slabs to brine exsolution formed during magmatic evolution (1). Based on results for submarine volcanic glasses and phenocryst-hosted glass (formerly melt) inclusions from the eastern Manus Basin, Papua New Guinea, we propose that degassing of andesitic magma (at SiO2 content of ~58 wt%) may have played a major role. The samples studied here range in composition from basalt to rhyolite, with SiO2 from 50.6 to 73.7 wt%, and MgO contents from 10.6 to 0.3 wt%. This sample set was selected from a larger group recovered from the Eastern Rifts in the Basin, with coherent major and trace element fractionation patterns consistent with a common genesis. All samples have strong arc characteristics, with marked relative Nb, Ta depletion and Cs, Rb, Ba, U, Pb enrichment (2, 3). The results show that Au is moderately incompatible during the early stage of arc magmatism, similar to Cu. Remarkably, Au and Cu concentrations are dramatically elevated as SiO2 content increases, and then drop sharply at about 58 wt% SiO2, accompanied by other significant changes, e.g., Ti and Fe switch from relatively incompatible to highly compatible. This indicates that the abrupt changes of Au and Cu abundances are associated with titanomagnetite saturation: reduction in magmatic Fe content, led to a decrease in S solubility, saturation with sulfide, and enhanced partitioning of sulfide and chloride into the accompanying supercritical vapor. The released volatile-rich components extract considerable amount of Au and Cu from the magma, and may become Cu-Au oreforming fluids. The subsequent migration of these fluids is an important process that could form the crucial link between Cu-Au porphyry/epithermal ore deposits and convergent margin magmatism.

References [1] Mungall J. E. (2002) Geology 30 (10), 915-918. [2] Kamenetsky V. S., Binns R. A., Gemmell J. B., Crawford A. J., Mernagh T. P., Maas R. and Steele D. (2001) Earth and Planetary Science Letters 184 (3-4), 685-702. [3] Sun W. D., Arculus R. J., Bennett V. C., Eggins S. M. and Binns R. A. (2003) Geology 31 (10), 845-848.

A625

U-Th isotopes of explosive eruptions of the Campanian volcanic field B. SCHEIBNER1, A. HEUMANN1, I. ARIENZO2, L. CIVETTA2 AND G. WOERNER1 1

GZG - Universitaet Goettingen, Germany 2 Osservatorio Vesuviano - Naples, Italy ([email protected])

Introduction Campanian volcanoes (Vesuvius and Campi Flegrei, South Italy) are characterized by several large explosive eruptions during at least the past 40 ka. The largest known protohistoric eruptions in this area occured within the Campi Flegrei (Neapolitan Yellow Tuff, Campanian Ignimbrite) and the nearby Somma - Vesuvius volcanic complex. It is still a matter of debate, (1.) if the magma of both volcanoes derived from the same magma source and which modal composition the magma source(s) have; (2.) if they have a unique deep (2010 km) magma chamber and (3.) if the magma source of the young Campi Flegrei magmas differs from that of the other volcanic complexes and provinces of South Italy. Magma sources for Campanian Plain volcanoes For resolving the temporal evolution of the magma sources beneath the Campanian Plain, we present U-series analyses of whole-rock powder samples from the young deposits (<14 ka) of Campi Flegrei (Neapolitan Yellow Tuff, Averno, Mt. Procida, Nisida) and the Mt. Somma - Vesuvius volcanic complex (Mercato, Avellino). U-Th-isotopes compositions of the young eruptions of the Campi Flegrei (Neapolitan Yellow Tuff - 14 ka, Mt. Procida 10.8 ka, Nisida 10.8 ka, Averno 3.8 ka), measured by TIMS, show a range of (238U/232Th)-ratios from 0.8784 to 0.9895 and age-corrected (230Th/232Th)-ratios between 0.7929 and 0.8730. These results indicate for the young explosive Campi Flegrei eruptions a (230Th/232Th)i-ratio ("melt source composition") below the typical (230Th/232Th)i-ratio of the other Italian volcanic provinces and the Campanian Ignimbrite. Furthermore, a recently published study [1] of Useries from whole-rocks of the Avellino eruption (3.7 ka) indicates that protohistoric magmatism of the Somma Vesuvius volcanic complex (8000-2700 ka) shows higher (230Th/232Th)-ratios than the magma of Campi Flegrei, which was erupted during a comparable time period of the Avellinoeruption.

References [1] Voltaggio M., Branca M., Tedesco T., Tuccimei P., Di Pietro L. D. (2004). - GCA 68, 167-181

A626

Goldschmidt 2004, Copenhagen

5.3.P22

Subduction related rocks in Medet Cu-porphyry deposit: Sources and magma evolution

A.V. QUADT1, I. PEYTCHEVA 1,2 , M. FRANK1, R. NEDYALKOV3, B. KAMENOV3 AND C.A. HEINRICH1 1

IGMR, ETH-Zurich, Switzerland; [email protected] Central Lab. of Mineral. & Crystall., BAS, Sofia, Bulgaria ([email protected]) 3 Sofia University “St. Kliment Ohridski”, Sofia, Bulgaria ([email protected]) 2

The Europe’s world-class porphyry-copper deposit Medet is situated in the Central part of the Srednogorie zone (Bulgaria). The region of the deposit is characterized by highmetamorphic continental basement rocks, crosscut by Variscan granitoids. This succession is intruded by Late Cretaceous subduction related calc-alkaline basic to intermediate igneous rocks, bearing the deposit. The aim of this study is to constrain the age and the isotope-geochemical characteristics of the basement rocks and the Cretaceous magmatites and to evaluate the role of both, the crustal protoliths and the subcontinental mantle lithosphere as magma sources. ID-TIMS techniques are used for the precise U-Pb single zircon (zr) and Rb/Sr-Sm/Nd for whole rock samples, MCICPMS - for the Hf isotopes. Zircons of the Medet gabbro reveal an age of 305.6 ± 0.50 Ma, which is very close to the Smilovene graniodiorite (305.3 ± 1.3 Ma). U-Pb monazite age of the adjacent Koprivshtitsa granite gives an age of 304.8 ± 0.8 Ma. Furthermore the gabbro shows mixed crustal-mantle characteristics, according to the εHf (300) zircon value of +0.14 and whole-rock initial strontium ratio of 0.7043. These data fit very well together with the I-type affinity of the Smilovene and Koprivhtitsa Variscan plutons. The gabbro-diorite from the deeper parts of the deposit represents the oldest Upper Cretaceous rock variety: concordant zircons reveal a mean 206Pb/238U age of 90.36 ± 0.48 Ma (pre-ore formation). Old inherited zr grains and cores define a discordia line with an upper intercept age of 456.5 ± 5.5 Ma. The εHf (460) of the same zircons of +8.04 to +9.88 suggest mantle source of the old protoliths. It is noteworthy that the εHf (90) values for the old and young zircons are scattering in a narrow range between -0.81 and +2.12, whereas no correlation between inheritance and the negative values was found. The ore-related Q-monzodiorite of the Medet deposit shows an intrusion 206Pb/238U age of 89.61 ± 0.26 Ma. εHf (90) values of the concordant zircons change between +1.24 and +4.42 indicating additional input of mantle magma. One grain with an inherited core marks the assimilation of Lower Palaeozoic basement rocks with crustal characteristics (εHf90 of –3.78). Based on Sr-, Nd- and Hf-zircon isotope data at least two magma sources for the Cretaceous rocks in the Medet deposit could be constrained: the subcontinental enriched mantle and the pre-Variscan basement, which reveals mantle as well as crustal characteristics.

5.3.P23

Timing of Mesozoic magmatism in Khingan-Okhotsk volcano-plutonic belt (Russian Far East) I.M. DERBEKO1, A.A. SOROKIN1, V.A. PONOMARCHUK2 1 AND A.P. SOROKIN 1 2

Division of Regional Geology Blagoveshchensk, Russia Institute of Geology, Novosibirsk, Russia ([email protected]; [email protected])

The Khingan-Okhotsk volcano-plutonic belt is one of the most significant structures of the eastern margin of Asia but doubt remains about the succession and geodynamic setting of the magmatism. Recent 40Ar/39Ar data show two stages of magmatic activity ( 111-105 Ma and 101-99 Ma) within the Khingan-Okhotsk belt. The first is responsible for eruption of a basalt-andesite-dacite association, while the second is responsible for an acid-subalkaline and alkaline magmatism. Andesite, basaltic andesite and first stage dacite correspond to calc-alkali high-K series characterized by moderate Rb (50-150 ppm) and Ba (430-700 ppm). These increase in the basaltic andesite-dacite series, while Sr (170640 ppm) decreases. Zr (130-400 ppm), Hf (3-7 ppm), Nb (730 ppm) increase with increase of SiO2. REE pattern is characterized by (La/Yb)n=8.2-14.8. Andesite and basaltic andesite have (Eu/Eu*)n = 0.98-0.95, but this ratio in dacites is 0.6-0.4. The age datings range from 111 to 105 Ma. The overlying volcanic series are primarily composed of acid rocks with formation age of 101 to 99 Ma. According to chemical composition they are divided into three groups: I. Subalkaline rhyolites, SiO2=72-75%, Na2O/K2O=0.4– 0.7 %, ASI=1-1.25. Geochemical peculiarities lie in moderate contents of Rb (120-170 ppm), Ba (410-630 ppm), Th (11-13 ppm), Nb (28-38 ppm,) Hf (7-8.3 ppm), Zr (230-270 ppm), and in low Sr content (56-90 ppm). REE distribution is characterized by (La/Yb)n = 5.1-7.8, and (Eu/Eu*)n = 0.3-0.6. II. Subalkaline rhyolites, SiO2=75-78%, Na2O/K2O=0.5– 0.7 %, ASI=1-1.8. They are characterized by high content of Rb (120-320 ppm), Th (12-25 ppm), Nb (37-72 ppm,) Hf (814 ppm), and by low Sr contents (< 30 ppm), Ba (12-60 ppm). REE distribution is characterized by (La/Yb)n = 5.8-11.5 and (Eu/Eu*)n <0.1. III. Alkaline trachydacite with K2O+Na2O>10%. These rocks are characteristic of high content of Ba (1400-1700 ppm), Zr (610-660 ppm), Nb (70-80 ppm), Hf (18-19 ppm), and moderate Rb (134-139 ppm), Sr (120-160 ppm). REE distribution is characterized by (La/Yb)n=4.5–5 and (Eu/Eu*)n = 0.7-0.8. Subalkaline and alkaline granite-porphyry and syeniteporphyry are very important in the structure of the belt. Compositionally, they are identical to alkaline trachydacites. The age of one of the samples is 100.3+0.2 Ma. The peculiarities of these rocks indicate the possible participation of a source similar to enriched magma in the primary magmatic melt.

5.3 Subduction processes and the subcontinental lithosphere

A627

5.3.P24

5.3.P25

Study on the genesis of Kanggur gold deposit, NW China

Sorting out the variable roles of subduction and contamination in the generation of Late Archean alkaline intrusions, western Superior Province

W. CHEN1,2, Y.T. WANG3, Y. ZHANG2 AND C.M. HAN4 1

Institute of Geology and Geophysics, CAS, Beijing 100029, China ([email protected]) 2 Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, China ([email protected]) 3 Institute of Mineral Resources, Chinese Academy of Geological Science, Beijing 100029, China ([email protected]) 4 Institute of High-Energy Physics, CAS, Beijing 100039, China ([email protected])

Introduction The Kanggur gold deposit, located in the northern margin of Tarim plate, NW China, outcrops in the middle part of Quigemingtashi-Huangshan ductile shear zone, so it is, in the long time, considered to be the gold deposit of shear zone type. To inspect whether the above opinion is correct, we dated the relative rocks in Qiugemingtashi –Huangshan ductile shear zone and Kanggur gold deposit. RESULTS AND DISCUSSION Five ore samples are collected from the different parts of the well of No. Kanggur gold deposit vein. The K-Ar ages of the whole-rocks are from 290.1Ma to 286.4Ma. These ages are in accordance with the whole-rock Rb-Sr isochron ages of 290±5Ma of Aqishan Formation altered andesite, the ore mineral Sm-Nd isochron ages of 290.4±7.2Ma and Rb-Sr isochron ages of 282.3±5Ma of the fluid fragment contained in quartz. Therefore, we tend to interpret the ages of 290.1Ma282.3Ma as the ages of the gold metallization. Five mylonites are collected from the surrounding rock of Kanggur gold deposit. Two a little deformed samples of them show the ages of 236.6±3.8Ma and 231.5±3.6Ma respectively and the other three strongly deformed samples show the accordance ages of 224.8±3.5Ma, 225.7±6.7Ma and 224.8±3.7Ma. the K-Ar ages of ~225Ma is the youngest estimated value of cooling to 300 from 500 for the shear deformation. The above dating results show that the time of the shear deformation in Qiugemingtashi –Huangshan ductile shear zone is later than the time of the gold metallization. Conclusions The preliminary geochronology study indicates that the metallogenic epoch is discordant with the deformation age. If the metallization is considered to occur in Heicynian period, ductile shear deformation occurred in the later period is not the key factor of Kanggur gold deposit metallization and it is also limited on the alteration of the gold deposit. Therefore, Kanggur gold deposit is not that of shear zone type. Acknowledgements The work was supported by the National Key Basic Research Project of China (No 2001CB409807).

B. LASSEN Department of Earth Sciences, University of Ottawa, Ottawa, Canada ([email protected]) Late Archean igneous activity within the Quetico metasedimentary belt in the western Superior Province produced a chemically diverse alkaline rock suite. U-Pb ages range between 2683 and 2678 Ma. The intrusions include a wide range of rock types, which display arc-like trace element patterns with strong large ion lithophile element (LILE) and light rare earth element (LREE) enrichment ([La/Yb]N = 5 – 90) and variable high field strength element (HFSE) and heavy rare earth element (HREE) depletions ([Dy/Yb]N = 1.2 – 3.3). Nd and Hf isotope ratios suggest derivation from a depleted mantle source, but Pb-Pb isotope ratios are largely crustal in composition and reflect either input from a subduction-modified mantle source formed by fluid transport of sediment Pb into the mantle wedge or contribution from an older crustal source. The occurrence of near primary melt compositions provides a window on source compositions and conditions of melting and allows resolution of the effects of crustal contamination and subduction processes. The most primitive samples can be modeled by partial melting of a primitive mantle source mixed with 0.5% subduction component in the garnet-spinel transition zone. Evolved samples have undergone contamination with the surrounding country rocks, but simple bulk mixing models cannot generate the observed trace element variations and mixing models involving variable degrees of melting of the country rocks are incorporated to explain the range in data. The source region is evaluated with respect to mineralogy and types of metasomatizing agents. Time-integrated depleted Nd isotope signatures suggest that the metasomatic event occurred not long before melting. Superchondritic Nb/Ta is attributed to metasomatism by silicic melts that have left rutile in the residue and the metasomatic agent appears to be a mixture of slab-derived fluids and melts.

A628

Goldschmidt 2004, Copenhagen

5.3.P26

5.3.P27

Geochemistry, radiogenic and Useries isotopes in Andean magmas: Crustal assimilation and adakite-like magmatism in the Central and Southern Andes 1

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G. WÖRNER , M. MAMANI , R. MERCIER R. KILIAN2

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AND

1

Abt. Geochemie, GZG, Goldschmidtstr. 1, 37077 Göttingen, Germany ([email protected]) 2 Fachbereich VI Universität Trier, 54286 Trier Holocene to Tertiary Central Andean volcanism (CVZ: 12°40' S to 27°S) formed on continental crust that changed thickness from less than 40 to > 70 km. We analyzed close to 1000 major and trace elements and a large number of samples for Sr-Nd-, and Pb isotopes in CVZ samples over more than 1600 km along the arc. Holocene lavas from the CVZ as well as the Southern Volcanic and Austral Volcanic Zones in Southern Chile (SVZ, AVZ) were analyzed for U-Th- and Raisotopes. At any given location Sr-Nd-Pb isotopic compositions are similar and independent of age and delineate distinct crustal domains. Boundaries between regions are abrupt and correlate with changing isotopic composition of the assimilated basement. Geochemical signatures suggesting garnet in the residue during magma genesis (either deep crustal assimilation or slab melting) are expressed as Sm/Yb ratios. In the CVZ, this signature is generally restricted to young (< 5 Ma) and intermediate rocks whereas more mafic or more evolved (and older) rocks do not show this signature. CVZ magmas are thus strongly crustally contaminated where recent Th/Uenrichment is expressed as (238U/230Th) < 1. SVZ magmas are contaminated by crust of variable thickness but generally show a slab fluid signature expressed as (238U/230Th) > 1. In the AVZ, the garnet signatures increases from N to S where the subducted oceanic plate becomes progressively younger and a slab melt component may be expected. Here, the garnet signature is derived from partial melting of the down-going oceanic slab and it is related to (238U/230Th) < 1 and MORB-like Sr- and Nd- isotopes. U-series data combined with geochemical and isotopic systematics can thus distinguish between slab fluids and garnet signatures either caused by slab melting or deep crustal assimilation.

Petrogenesis of post-collision related Yengi-Spiran intrusives in the Nw of Iran (NW of Tabriz) A. JAHANGIRI AND Z. KHODAYARZADEH Dept of Geology, University of Tabriz, Tabriz 51665, Iran ([email protected]) Nw of Iran is an important component of AlpineHimalayan system that comprises voluminous volcanic and plutonic rocks from Eocene to pliocene age related to subduction and closure of Neo-Tethyan ocean and its branches. In the NW of Tabriz late-Miocene Yengi-Spiran plutonic rocks intrude flysch-type cretaceous rocks and metamorphosed these rocks in the epidote-hornfels facies. The intrusive roks ranging from gabbro-diorite to diorite and quartz-diorite, which exhibit meta-aluminous and calcalkaline characters and plots in the Sio2-K2O diagram in the medium potassium field. All intrusive rocks display enrichment in LILE (Ba, Th, U)and LREE relative to HFSE. Incompatible elements such as U, Ba, Th indicate high enrichment compared to K and Rb. Meanwhile high Mg numbers (51-79) of studied samples with relatively high content of Cr (53-499 ppm), Ni (47-287ppm), Co can be attributed to significant input of mantle derived magma in genesis of these plutons. Enrichment of U, Ba, Th, and LREE in compared with Rb and K could be explained by enrichment of mantle source due to fluids released from subducted slab with pelagic sediments. Geochemical data indicate minor contribution of crustal material in the genesis of these rocks either recycled with subducted slab or through assimilation process.