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The geometry of archean & proterozoic lithospheric mantle domains beneath the western U.S.A.
54 OSMIUM ISOTOPES IN OROGENIC LHERZOLITES AND ULTRAMAFIC SAMPLES J,M.LUCK. W.J. PEGRAM AND C.J. ALLEGRE (Lab. G~ochimie & Cosmochimie IPGP - 4 place Jussieu 75252 Paris Cedex 05 FRANCE) Improvements in the extraction (fusion of sample with ultra clean borate/carbonate flux in the presence of NiS) and purification technique for Re and Os from silicate samples allow us to determine both concentration and isotopic composition at the ppb to sub-ppb level . The Os isotopic composition and the concentrations (isotope dilution) are determined using a CAMECA ion-microprobe as an ion-sputtering mass spectrometer . The mantle evolution c u r v e for Os isotopes as derived from osmiridium samples extends 1o taday's value at 1,055. Dunites from "normal" ophiolites (Toba, Oman) plot very close to this value whereas samples from back-arc basin ophiolites show higher values possibly showing the involvement of highly radiogenic sediments . Peridotitic samples from orogenic Iherzoliles show a range spreading from radiogenic (187/186 Os=1,10) to low values (one sample from Beni Bousera : 1,00) through "chondritic" ones (1,05) . There seems to be a rough negative correlation between Nd and Os isotopes . The low values indicate that melting events have affected some parts of the mantle which have nol been rehomogeneized afterwards regarding Os isotopes : minimum model ages (based on theoretical Re/Os=0) are around 1-1,2 Ga , however the actual Re/Os ratios are higher than chondrilic . The pyroxenitic samples from those orogenic Iherzelites yield very high 187/186 Os between 2 and 9 that nre £1flaDorfpcl bv v~r v high Rn/Q£ r#fio£ . ,c:;Hch hinh Re/Os ratios have been reported on lholeites : these results therefore support lhe marble cake model lhal ultimalely derives the pyroxenile layers from stored subducted oceanic crust. Here too model ages are around 1,2 G a . .'.~,~ GEOMETRY OF ARC:~EAN & PROTEROZO[C L I T H O S P H E R I C MANTLE ~OMAffqS ~IEN.EATH ~ WESTERN U.S.A. ~,IARTIN ADRIAN MENZIES. ~e-'~" " ~ Geology, University Surrey TW2@ OEX, England.)
of
London
(RHBNC),
Egham,
The regthnal dismbutthfi of cratonic, eircum-cratonic & oceanic d o m a i n s , identified beneath the western U,S.A. on the basis of basaU-xenoJith geochemical-data correJates to some degree with heat flow data. c~stal age provinces & e~stat thickness data. The overall geometry has parallels in the Archean- Proterozoie of Scotland & South Africa. (~ratonie aOmaln~ underlie areas of thick (50 kin.). Arcbe~ (>2.7 b.y.) crust, where heat flow is <21"{FU. Sub-cratonth domains ~e charactensed by low Eb/Sr. high LIL/HFSE ratios, a m~ked earthhmem in LR~E/HREE and the following isotopes: Sr (.704 .706), Nd (< 05124). Pb206/204 (16.0-17.3) and Pb207/204 ( ] 5 3 15.6).Thermal disturbance of these rose.dips in post-Arehean umes may explain (a) the Archean-Prolerozoic ages (3.0-1,2 b.y.) rez~ed by xenoliths, and (b) the predominance, in Wyoming ~ d Montana, of Proterozoic ages (2.5-1.2 b.y.) [n volcanic rocks extracted from these sub-cratonie reservoirs. Within the craton of North America and South Africa sub-cratonic lithospheric sources dominate as the main reservoir for volcanic rocks. The participation of cratonic domains in volcanism outwith the Archean crust is limited possibly because ihey never formed under Proterozoic-Phanerozoic ~erraths. The chemical uniqueness and overall stability of the sub-Arcbean keel is perhaps a key (a) to the preservation cd the sub-cratomc domain (viz. EMIL ~ d (b) to its paucity within xeno0th suites and volcanic rocks erupted through post-Arcbean crust. Detachment of s~b-cratonic m~Ue, however, may toad to it acting as a contributory rose.dip to volcanism outwith Archean terrains (',ireum-cratp.ie domaDl~ underlie thinner (< 50 kin.), Proterozoic (2.7 - 0.6 by.) crust, where heat now is > 2HFU, Circ~-cratorlie domains (aeereted) ~e cbaracterised by high Rb/Sr, low LIL/HFSE ratios ~ d the following isotopes : Sr (>0.710), Nd (05120-05123), Pb206/204 (18.5-19.5) and Pb207/204 (> 15.8). Valcanic rocks that have tapped this rese~oir retain Proterozoic age information (2.5 1.0 b y ) ~ d xenolithic fragments of these domains also return ages of 1.8 /.O by. The similarity between island arc basafls and continental volcanic rocks that lap clreum-cratonic domains (viz. EM2) indicates a possible contribution frnm recycled material. Moreover these domains exist throughout the c~rcum cratomc toobile belts of North America ~ d Scotland where there is evidence of terrain accretion or subductthn processes. Cratome and circum-cratonth domains foxed at a time of major crustal production (40-50% of the crust in the Arcbean and 90% by the midPro~crozoic). E M I domains are believed lo have f o r m e d in the Archean during episodes of core.manlle-crusl differentiatioa when little or no Rb was being recycled into the mantle. EM2 domains may represent subductio,I modified lithosphere. Oceanic - OIB domains are tapped within Proterozoic-Phanerozoie
eominental te~ains particularly in areas of high heat flow (>>2HFU) and thin c~st t<30 km) Within the clrcum-cratomc regions of the western US.A. the regional distribution of OIB domains as a primary source for volcanic rocks cothcides exactly with the ~ea of high heat now. This contrasts with their paucity in areas of low" heat flow The Duped (3.0b.y.) ~ d M a n g a i a (1,7 b.y.) oceanic domains have counterp~ts within the contPlental lithosphere. DMM domains are common beneath Proterozoth crust and are frequently entrained as ~enoliths perhaps indicating that DMM constitutes the bulk of the upper asthenosphere. The similarity between circum-cratomc lithospheric mamle and sub-oceanic mantle may support the contention that the transition from the thick Archean keel to the thin lithosphere under the oce~s uppers to take place in the mobile belts.
STRONTIUM ISOTOPEAND TRACE ELEMENTEVIDENCEFOR INVOLVEMENTOF 'SLAB GENERATEDFLUIDS' IN QUATERNARY BASALTS FROM CENTRAL JAVA. D. VUKADINOVIC, R. SUKHYAR AND I.A. NICHOLLS. (Department of Earth Sciences, Monash University, Victoria, Australia).
Clayton,
Trace element and isotopic data for basalts from single large island-arc volcanoes (Gunung Slamet, Dieng, Sundoro and Sumbing, Central Java) indicate that multiple components are involved in magma genesis. SIamet basafts can be grouped into high and low abundance magmas (HAM and LAM respectively) or1 the basis of REE abundances, Zr/Nb, and 87Sr/86Sr. Stratigraph[cally, RAM are younger than LAM. Plots of immobile/_.mobileincompatible .[race element .[atios (IMITER; e.g. Zr/Rb or Nb/U) and 87Sr/86Sr show negative correlations for all four volcanic centres. At Slamet, Nb/U and 87Sr/86Sr systematics demonstrate that incorporation of continental material (as subducted sediment or via crustal assimilation) is not capable of lowering Nb/U mantle values of -47 to observed basalt values without overly raising 87Sr/86Sr. Ametasomatizing fluid (generated by the dehydration of subducted oceanic crust) whose IMITER -0 is ideally suited for lowering NbAJ of the mantle wedge magma source, whim not causing a major increase in 87Sr/86Sr. Similar patterns emerge at the other three volcanic complexes. The highest 87Sr/86Sr ratios are exhibited by basalts from Sumbing. These also possess high K20, St, Sa, Rb and slightly higher LREE concentrations compared to the olhef basaIts, causing them to classify with the high-K calcalkaline suite according to K2O vs SiC2 systematics. However, Sumbing REE abundances are low compared to the 'type' high-K ealcalkaline rocks from Ungaran VoIcano. This is probably due to different degrees of partial meJting of the manlle source, with magmas from Ungaran generated by a smaller degree of melting. The above data suggest that subduuted sediments of continental origin (1-2% or less) may have contributed to the magma source. Addition of subducted sediments cannot solely account for the high observed abundances of alkalies and alkaline earths. Fluid phases enriched in these elements from the subducIed slab may halve metasomatised the overlying mantle wedge contemporaneously with sediment addition prior to melting. Further contamination of magmas by the arc crust has probably occurred at shallow depths
C H E M I C A L C H A R A C T E R I S T I C S OF C O N T I N E N T A L L I T H O S P H E R I C M A N T L E AS R E F L E C T E D BY C O N T I N E N T A L THOLEIITES W I L L I A M J. P E G R A M (Laboratoire de G&~chimie et Cosmochimie, Insitut de Physique du Globe d e Paris, 4, Place Jussieu, 75252 Paris, France) G e o c h e m i c a l a n a l y s e s of d i k e s , sills, a n d flows of the Mesozoic Appalachian Tholeiite ( M A T ) Province of the easternmost U.S. support the i d e a that c o n t i n e n t a l tholeiites a r e d e r i v e d f r o m c o n t i n e n t a l lithospheric mantle sources that are geochronologically, geochemically, a n d g e n e t i c a l l y r e l a t e d to the o v e r l y i n g crust. O v e r a l l , 19 olivine tholeiites, d o m i n a n t l y from the southern half of the province, and 15 quartz tholeiites, m a i n l y from the north, yield an initial E TM range of +3.8 to -5.7. Initial 8 7 S r / 8 6 S r ratios r a n g e from 0.7044 to 0,7072. P b l C ' s v a r y as follows: 206/204 = 17.49-19.14; 207/204 = 15.5515.69; 208/204 = 37.24-39.11. In P b - P b s p a c e , the M A T define a linear a r r a y , d i s p l a c e d a b o v e the field for M O R B , w h i c h yields a s e c o n d a r y isochron age of = 1000 m.y. (~1=8.3), similar to S m / N d isochrons from the southern h a l f of the p r o v i n c e a n d similar to the radiometric a g e of the Grenville crest underlying eastern North America. T h e M A T d i s p l a y p r o n o u n c e d trace e l e m e n t ratio heterogeneity (e.g., S m / N d = 0 , 2 2 6 - 0 . 3 2 7 ) a n d h a v e t r a c e e l e m e n t traits r e m i n i s c e n t of c o n v e r g e n t m a r g i n m a g m a s (e.g., depletions of N b a n d Ti w.r.t. REE on normalized trace e l e m e n t d i a g r a m s , e l e v a t e d B a / N b (19-75), low %TIO2 (0.39-0.69)). Geochemical a n d geological considerations v e r y slzongly suggest that the M A T m a g m a s w e r e not c o n t a m i n a t e d b y the continental c r u s t during ascent. Further, isotopic a n d trace e l e m e n t variations are not consistent with the i n v o l v e m e n t of either c o n t e m p o r a n e o u s M O R B or O I B components; rather, the materials that control the M A T chemistry w e r e derived from the subcontinental lithospheric mantle. Thus, 1) the M A T / a r c m a g m a trace e l e m e n t similarities, 2) the Pb and N d isochron ages, and 3) the n e e d for a m e t h o d of introducing an a n c i e n t (>2-3 b.y.) P b c o m p o n e n t into subcontinental m a n t l e that c a n n o t b e much older than 1 b.y. leads to a m o d e l w h e r e b y the M A T were derived tom a s e d i m e n t - c o n t a m i n a t e d arc m a n t l e that w a s incorporated into the continental l i t h o s p h e r e d u r i n g arc a c t i v i t y p r e c e d i n g the continentcontinent collision o f the Grenville O r o g e n y (=1000 m.y.). T h e 1ow206/204 nature of s o m e M A T could reflect the dominance of their U-Pb systematics b y typically low-It marine sedimentary material.
of
London
(RHBNC),
Egham,
The regthnal dismbutthfi of cratonic, eircum-cratonic & oceanic d o m a i n s , identified beneath the western U,S.A. on the basis of basaU-xenoJith geochemical-data correJates to some degree with heat flow data. c~stal age provinces & e~stat thickness data. The overall geometry has parallels in the Archean- Proterozoie of Scotland & South Africa. (~ratonie aOmaln~ underlie areas of thick (50 kin.). Arcbe~ (>2.7 b.y.) crust, where heat flow is <21"{FU. Sub-cratonth domains ~e charactensed by low Eb/Sr. high LIL/HFSE ratios, a m~ked earthhmem in LR~E/HREE and the following isotopes: Sr (.704 .706), Nd (< 05124). Pb206/204 (16.0-17.3) and Pb207/204 ( ] 5 3 15.6).Thermal disturbance of these rose.dips in post-Arehean umes may explain (a) the Archean-Prolerozoic ages (3.0-1,2 b.y.) rez~ed by xenoliths, and (b) the predominance, in Wyoming ~ d Montana, of Proterozoic ages (2.5-1.2 b.y.) [n volcanic rocks extracted from these sub-cratonie reservoirs. Within the craton of North America and South Africa sub-cratonic lithospheric sources dominate as the main reservoir for volcanic rocks. The participation of cratonic domains in volcanism outwith the Archean crust is limited possibly because ihey never formed under Proterozoic-Phanerozoic ~erraths. The chemical uniqueness and overall stability of the sub-Arcbean keel is perhaps a key (a) to the preservation cd the sub-cratomc domain (viz. EMIL ~ d (b) to its paucity within xeno0th suites and volcanic rocks erupted through post-Arcbean crust. Detachment of s~b-cratonic m~Ue, however, may toad to it acting as a contributory rose.dip to volcanism outwith Archean terrains (',ireum-cratp.ie domaDl~ underlie thinner (< 50 kin.), Proterozoic (2.7 - 0.6 by.) crust, where heat now is > 2HFU, Circ~-cratorlie domains (aeereted) ~e cbaracterised by high Rb/Sr, low LIL/HFSE ratios ~ d the following isotopes : Sr (>0.710), Nd (05120-05123), Pb206/204 (18.5-19.5) and Pb207/204 (> 15.8). Valcanic rocks that have tapped this rese~oir retain Proterozoic age information (2.5 1.0 b y ) ~ d xenolithic fragments of these domains also return ages of 1.8 /.O by. The similarity between island arc basafls and continental volcanic rocks that lap clreum-cratonic domains (viz. EM2) indicates a possible contribution frnm recycled material. Moreover these domains exist throughout the c~rcum cratomc toobile belts of North America ~ d Scotland where there is evidence of terrain accretion or subductthn processes. Cratome and circum-cratonth domains foxed at a time of major crustal production (40-50% of the crust in the Arcbean and 90% by the midPro~crozoic). E M I domains are believed lo have f o r m e d in the Archean during episodes of core.manlle-crusl differentiatioa when little or no Rb was being recycled into the mantle. EM2 domains may represent subductio,I modified lithosphere. Oceanic - OIB domains are tapped within Proterozoic-Phanerozoie
eominental te~ains particularly in areas of high heat flow (>>2HFU) and thin c~st t<30 km) Within the clrcum-cratomc regions of the western US.A. the regional distribution of OIB domains as a primary source for volcanic rocks cothcides exactly with the ~ea of high heat now. This contrasts with their paucity in areas of low" heat flow The Duped (3.0b.y.) ~ d M a n g a i a (1,7 b.y.) oceanic domains have counterp~ts within the contPlental lithosphere. DMM domains are common beneath Proterozoth crust and are frequently entrained as ~enoliths perhaps indicating that DMM constitutes the bulk of the upper asthenosphere. The similarity between circum-cratomc lithospheric mamle and sub-oceanic mantle may support the contention that the transition from the thick Archean keel to the thin lithosphere under the oce~s uppers to take place in the mobile belts.
STRONTIUM ISOTOPEAND TRACE ELEMENTEVIDENCEFOR INVOLVEMENTOF 'SLAB GENERATEDFLUIDS' IN QUATERNARY BASALTS FROM CENTRAL JAVA. D. VUKADINOVIC, R. SUKHYAR AND I.A. NICHOLLS. (Department of Earth Sciences, Monash University, Victoria, Australia).
Clayton,
Trace element and isotopic data for basalts from single large island-arc volcanoes (Gunung Slamet, Dieng, Sundoro and Sumbing, Central Java) indicate that multiple components are involved in magma genesis. SIamet basafts can be grouped into high and low abundance magmas (HAM and LAM respectively) or1 the basis of REE abundances, Zr/Nb, and 87Sr/86Sr. Stratigraph[cally, RAM are younger than LAM. Plots of immobile/_.mobileincompatible .[race element .[atios (IMITER; e.g. Zr/Rb or Nb/U) and 87Sr/86Sr show negative correlations for all four volcanic centres. At Slamet, Nb/U and 87Sr/86Sr systematics demonstrate that incorporation of continental material (as subducted sediment or via crustal assimilation) is not capable of lowering Nb/U mantle values of -47 to observed basalt values without overly raising 87Sr/86Sr. Ametasomatizing fluid (generated by the dehydration of subducted oceanic crust) whose IMITER -0 is ideally suited for lowering NbAJ of the mantle wedge magma source, whim not causing a major increase in 87Sr/86Sr. Similar patterns emerge at the other three volcanic complexes. The highest 87Sr/86Sr ratios are exhibited by basalts from Sumbing. These also possess high K20, St, Sa, Rb and slightly higher LREE concentrations compared to the olhef basaIts, causing them to classify with the high-K calcalkaline suite according to K2O vs SiC2 systematics. However, Sumbing REE abundances are low compared to the 'type' high-K ealcalkaline rocks from Ungaran VoIcano. This is probably due to different degrees of partial meJting of the manlle source, with magmas from Ungaran generated by a smaller degree of melting. The above data suggest that subduuted sediments of continental origin (1-2% or less) may have contributed to the magma source. Addition of subducted sediments cannot solely account for the high observed abundances of alkalies and alkaline earths. Fluid phases enriched in these elements from the subducIed slab may halve metasomatised the overlying mantle wedge contemporaneously with sediment addition prior to melting. Further contamination of magmas by the arc crust has probably occurred at shallow depths
C H E M I C A L C H A R A C T E R I S T I C S OF C O N T I N E N T A L L I T H O S P H E R I C M A N T L E AS R E F L E C T E D BY C O N T I N E N T A L THOLEIITES W I L L I A M J. P E G R A M (Laboratoire de G&~chimie et Cosmochimie, Insitut de Physique du Globe d e Paris, 4, Place Jussieu, 75252 Paris, France) G e o c h e m i c a l a n a l y s e s of d i k e s , sills, a n d flows of the Mesozoic Appalachian Tholeiite ( M A T ) Province of the easternmost U.S. support the i d e a that c o n t i n e n t a l tholeiites a r e d e r i v e d f r o m c o n t i n e n t a l lithospheric mantle sources that are geochronologically, geochemically, a n d g e n e t i c a l l y r e l a t e d to the o v e r l y i n g crust. O v e r a l l , 19 olivine tholeiites, d o m i n a n t l y from the southern half of the province, and 15 quartz tholeiites, m a i n l y from the north, yield an initial E TM range of +3.8 to -5.7. Initial 8 7 S r / 8 6 S r ratios r a n g e from 0.7044 to 0,7072. P b l C ' s v a r y as follows: 206/204 = 17.49-19.14; 207/204 = 15.5515.69; 208/204 = 37.24-39.11. In P b - P b s p a c e , the M A T define a linear a r r a y , d i s p l a c e d a b o v e the field for M O R B , w h i c h yields a s e c o n d a r y isochron age of = 1000 m.y. (~1=8.3), similar to S m / N d isochrons from the southern h a l f of the p r o v i n c e a n d similar to the radiometric a g e of the Grenville crest underlying eastern North America. T h e M A T d i s p l a y p r o n o u n c e d trace e l e m e n t ratio heterogeneity (e.g., S m / N d = 0 , 2 2 6 - 0 . 3 2 7 ) a n d h a v e t r a c e e l e m e n t traits r e m i n i s c e n t of c o n v e r g e n t m a r g i n m a g m a s (e.g., depletions of N b a n d Ti w.r.t. REE on normalized trace e l e m e n t d i a g r a m s , e l e v a t e d B a / N b (19-75), low %TIO2 (0.39-0.69)). Geochemical a n d geological considerations v e r y slzongly suggest that the M A T m a g m a s w e r e not c o n t a m i n a t e d b y the continental c r u s t during ascent. Further, isotopic a n d trace e l e m e n t variations are not consistent with the i n v o l v e m e n t of either c o n t e m p o r a n e o u s M O R B or O I B components; rather, the materials that control the M A T chemistry w e r e derived from the subcontinental lithospheric mantle. Thus, 1) the M A T / a r c m a g m a trace e l e m e n t similarities, 2) the Pb and N d isochron ages, and 3) the n e e d for a m e t h o d of introducing an a n c i e n t (>2-3 b.y.) P b c o m p o n e n t into subcontinental m a n t l e that c a n n o t b e much older than 1 b.y. leads to a m o d e l w h e r e b y the M A T were derived tom a s e d i m e n t - c o n t a m i n a t e d arc m a n t l e that w a s incorporated into the continental l i t h o s p h e r e d u r i n g arc a c t i v i t y p r e c e d i n g the continentcontinent collision o f the Grenville O r o g e n y (=1000 m.y.). T h e 1ow206/204 nature of s o m e M A T could reflect the dominance of their U-Pb systematics b y typically low-It marine sedimentary material.