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The Monts Dore volcanics (MCF): Geochemical evolution in a crustal magma chamber
VARIATIONS OF THE INITIAL Nd-,Er-RATIOE IN THE BERGELL INTRUSIVE SEQUENCE (E-CENTRAL ALPE) F.v. Blanckenburq, P.Stille, K. Diethelm, E. Reusser Isotope Geochemistry, ETH, CH-8092 Z~rich The Oligocene calc-alkaline Bergell intrusive suite displays a wide range of mantle- to crustal initial ~Nd/cSr values. The inversely correlated Nd-Sr trend shows values within the mantle field in cumulatic hornblendites (~Nd/~Sr=+l.8/+17) and changes roughly with acidity of rocks to highly crustal values in the granodiorite (-6.6/+84). The strongest crustal values, however, are displayed by the western Bergell Melirolo-Augengneiss (-6/+145) and the Novate leukogranite (-8.5/+75), for both of which the genetic relation to the intrusive suite is not well known. Several varieties of gabbros show variable values ranging from pyroxene-rich samples with low Ndand Sr-concentrations and mantle- dominated (- values (+1.0/+37) to plagioclase-rich gabbros with higher Nd and Sr and crustal ~ (-5.7/+72). This variability indicates that complex intracrustal processes affected the mantle intrusions. In general, the E-values indicate stronger crustal participation in the Bergell than in the neighbouring South Adamello complex. In contrast to the Adamello magmas, which intruded cold Southern Alpine crust, the Bergell magmas intruded warm crust undergoing active Tertiary central Alpine metamorphism. Thus, additionally supplied metamorphic heat may account for the high contamination rates. Processes leading to magma contamination may involve i) anatexis and assimilation of wall rock, ii) metasomatism induced by the regional metamorphism, iii) entrainment of restitic minerals from the country rocks (plagioclase -> Sr, accessory minerals -> REE), all combined with fractional crystallization. Interestingly, crustal involvement in the chemically very homogeneous tonalite is low in the west (Iorio: -2.3/+39) and high 40km further east (Malenco: -4.4/+80). Tectonic models and igneous hornblende barometry suggest that the western Bergell tonalite intruded at a burial depth exceeding that of the eastern tonalite by some 10km (W: 7.5kbars, E: 5kbars). This may imply that the mantle magmas intruded at deep burial depths close to the Insubric lineament, while major contamination occurred in the upper parts of the magma chamber.
THE MONTS DORE VOLCANICS (MCF) : GEOCHEMICAL EVOLUTION IN A CRUSTAL MAGMA CHAMBER. D. BRIOT. J.M. CANTAGREL. (Laboratoire de gdologie, UA 10, C.N.R.S., ClermontFerrand). The Mont-Dore-Sancy volcano was built up in two main periods between 3 and 1.5 - 0.9 and 0.25 My, from an upper crustal magma chamber. The basic magmas are basanites, the more evolved lavas are rhyolites and phonolites but a great proportion of lavas is represented by i n t e r m e d i a t e heterogeneous truchyandesites. (1) 87Sr/86Sr and 143Nd/144Nd on basanites and hawaiites range from 0.70350 to 0.70420 and 0.512873 to 0.512798 and are positively and negatively correlated with SiO2 and K. ~ 180 values range from 5.6 to 6.1% and are positively correlated with 87Sr/O6Sr. (2) Sr and Nd isotopic ratios for trachyandesites, trachytes and rhyolites increase and decrease with SiO2 c o n t e n t s (0.70369-0.70537 and 0.512810-0.512596 respectively). (3) Tephrites and phonolites have isotopic ratios in the range of basanites (0.70380-0.512860). Olivine and clinopyroxene fractionation coupled with 10% assimilation of felsic granulites at the lower crust level occur from basanites to hawaiites. Pure fractional crystallization (amphibole - sphene - alkali feldspar) in a late stage ~ives phonolites. Large volume (3km j) of rhyolitic magma (pumice flows) are generated by AFC with 15-20% of granitic or gneissic component in relation with the setting up of an upper crustal magma reservoir. Repetitive refilling of the reservoir and complexe mingling, fractionation and varied assimilation processes generate the heterogeneous txachyandesites.
TSANSPORT AND COMPOSITIO§ OF MAITLE FLUID INT0 THE LOWER CRUST
K.
BUCHER-NURMINEN
(Deptarteent Of Geology, University of Oslo} Ultramaflc rocks occurlng In continental associations in the Scandinavian Caledonldes (SC) say represent fragments derlved from upper mantle beneath the continental crust. Typical continental geotheras result In HOBO temperatures vhich are veX1 vlthin the stah111ty field for hydrated and carbonated versions of perldotlte. Calculated phaserelatlonshlps in the MSEC system suggest that any mantle fluid (H20-C02) derlved from the lover parts of the mantle viii he consumed by chemical reactlons In the upper part of the mantle if the MOHO t e m p e r a t u r e l s below about 600 C. Under such c o n d i t i o n s no mantle f l u l d is a b l e to p e n e t r a t e
the HOHO. The c a l c u l a t i o n s ~ESPONSE OF THE OCEAN TO ANTHROPOCENIC LEAD, 1980-1986. E.A. BOYLE Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge MA 02139 USA Oceanic surface lead (Pb) concentrations have been determined in the region between New England and Bermuda in the period 1980-1986. Detailed surface water transects were undertaken in 1980 and 1984, and regular time-serles sampling has been undertaken near Bermuda since 1983. Both studies show significant decreases in oceanic lead over this time interval; between 1980-1984, Sargasso Sea lead decreased by 30 ± 13% (from 156 to 109 pmol/kg) while from 1983-1986, Pb near Bermuda decreased from 130 to 80 pmol/kg.
also
show t h a t o n l y
H20-rlch f l u i d s from the mantle can I n f i l t r a t e the l o v e r c r u s t a t high t e m p e r a t u r e s . E a r l y m a g n e s l t e Is very abundant In o l t r a m a f l c a from the SC. Many of t h e high grade sub-contlnetal type ultramaflcs from the SC bear vltness Of an e a r l y serpentlnlte stage. It Is therefore proposed that the hydrated/carbonated versions of perldotlte found in the SC (sub-contlnental type) represent mantle rocks (mantle serpentlnltes, soapstones, sagvandltes). The possible presence of partially hydrated/carbonated ultramafIcs In the upper mantle offers a geological explanation for observed seismic reflectors In the upper mantle and for the elevated topography of old continental shield areas. It questlons proprosed mantle C02 infiltration as an explanation for the formation of granullte facies terrains.
THE MONTS DORE VOLCANICS (MCF) : GEOCHEMICAL EVOLUTION IN A CRUSTAL MAGMA CHAMBER. D. BRIOT. J.M. CANTAGREL. (Laboratoire de gdologie, UA 10, C.N.R.S., ClermontFerrand). The Mont-Dore-Sancy volcano was built up in two main periods between 3 and 1.5 - 0.9 and 0.25 My, from an upper crustal magma chamber. The basic magmas are basanites, the more evolved lavas are rhyolites and phonolites but a great proportion of lavas is represented by i n t e r m e d i a t e heterogeneous truchyandesites. (1) 87Sr/86Sr and 143Nd/144Nd on basanites and hawaiites range from 0.70350 to 0.70420 and 0.512873 to 0.512798 and are positively and negatively correlated with SiO2 and K. ~ 180 values range from 5.6 to 6.1% and are positively correlated with 87Sr/O6Sr. (2) Sr and Nd isotopic ratios for trachyandesites, trachytes and rhyolites increase and decrease with SiO2 c o n t e n t s (0.70369-0.70537 and 0.512810-0.512596 respectively). (3) Tephrites and phonolites have isotopic ratios in the range of basanites (0.70380-0.512860). Olivine and clinopyroxene fractionation coupled with 10% assimilation of felsic granulites at the lower crust level occur from basanites to hawaiites. Pure fractional crystallization (amphibole - sphene - alkali feldspar) in a late stage ~ives phonolites. Large volume (3km j) of rhyolitic magma (pumice flows) are generated by AFC with 15-20% of granitic or gneissic component in relation with the setting up of an upper crustal magma reservoir. Repetitive refilling of the reservoir and complexe mingling, fractionation and varied assimilation processes generate the heterogeneous txachyandesites.
TSANSPORT AND COMPOSITIO§ OF MAITLE FLUID INT0 THE LOWER CRUST
K.
BUCHER-NURMINEN
(Deptarteent Of Geology, University of Oslo} Ultramaflc rocks occurlng In continental associations in the Scandinavian Caledonldes (SC) say represent fragments derlved from upper mantle beneath the continental crust. Typical continental geotheras result In HOBO temperatures vhich are veX1 vlthin the stah111ty field for hydrated and carbonated versions of perldotlte. Calculated phaserelatlonshlps in the MSEC system suggest that any mantle fluid (H20-C02) derlved from the lover parts of the mantle viii he consumed by chemical reactlons In the upper part of the mantle if the MOHO t e m p e r a t u r e l s below about 600 C. Under such c o n d i t i o n s no mantle f l u l d is a b l e to p e n e t r a t e
the HOHO. The c a l c u l a t i o n s ~ESPONSE OF THE OCEAN TO ANTHROPOCENIC LEAD, 1980-1986. E.A. BOYLE Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge MA 02139 USA Oceanic surface lead (Pb) concentrations have been determined in the region between New England and Bermuda in the period 1980-1986. Detailed surface water transects were undertaken in 1980 and 1984, and regular time-serles sampling has been undertaken near Bermuda since 1983. Both studies show significant decreases in oceanic lead over this time interval; between 1980-1984, Sargasso Sea lead decreased by 30 ± 13% (from 156 to 109 pmol/kg) while from 1983-1986, Pb near Bermuda decreased from 130 to 80 pmol/kg.
also
show t h a t o n l y
H20-rlch f l u i d s from the mantle can I n f i l t r a t e the l o v e r c r u s t a t high t e m p e r a t u r e s . E a r l y m a g n e s l t e Is very abundant In o l t r a m a f l c a from the SC. Many of t h e high grade sub-contlnetal type ultramaflcs from the SC bear vltness Of an e a r l y serpentlnlte stage. It Is therefore proposed that the hydrated/carbonated versions of perldotlte found in the SC (sub-contlnental type) represent mantle rocks (mantle serpentlnltes, soapstones, sagvandltes). The possible presence of partially hydrated/carbonated ultramafIcs In the upper mantle offers a geological explanation for observed seismic reflectors In the upper mantle and for the elevated topography of old continental shield areas. It questlons proprosed mantle C02 infiltration as an explanation for the formation of granullte facies terrains.