- Email: [email protected]
Crustal differentiation, granitoid segregation and migmatite genesis: An experimental approach
73 ORIGIN OF SILICIC VOLCANICS FROM THE SIERRA MADRE OCCIDENTAL, MEXICO, AND ITS BEARING ON CONTINENTAL CRUSTAL GROWTH JOAQUIN RUIZ, P. JONATHAN PATCHETT. (Department of Geosciences, University of Arizona, Tucson, A r i z o n a 85721, U.S.A.) The sourc~ of m e l t s in c o n t i n e n t a l m a r g i n magmatism is central to s t u d i e s of c r u s t a l genesis. If the p r e d o m i n a n t l y silicic and i n t e r m e d i a t e igneous rocks found in c o n t i n e n t a l margins are products of fractional crystallization of m a n t l e m a t e r i a l , then c o n s i d e r a b l e amounts of new crust are p r o d u c e d d u r i n g these m a g m a t l c periods. However, if the source of the melts is in the crust, then this type of m a g m a t i s e m o s t l y r e w o r k s o l d e r crust and c o n s t i t u t e s m i n o r new crustal growth. One of the largest accumulations of continental margin felsic volcanic rocks known is the Sierra Madre Occidental, Mexico, where up to 250,000 ke3 of silicic lavas and ignimbrites were erupted between 34 and 27 Ma. ago. The bulk of the v o l c a n i c s are c a l c - a l k a l i c d a c i t e s and b a s a l t i c andesites. H i g h s i l i c a r h y o l i t e s are c o m m o n l y found in the e a s t e r n edge of the S l e r r a M a d r e O c c i d e n t a l a s s o c i a t e d w i t h high K c a l c - a l k a l i c rocks. Generally there seems to be an increase in the alkalic nature of the rocks with distance from the west coast of N o r t h A m e r i c a and the t i m i n g of volcanism is coincident with the subduction of the Farallon Plate. The sillcic ignimhrites and lavas of the S i e r r a M a d r e O c c i d e n t a l h a v e i n i t i a l Sr ratios between 0.7044 and 0.7070 and c Nd values b e t w e e n -2 and + i. T h e s e v a l u e s are s i m i l a r to those of the Mexican lower crust in those areas, as indicated by data on xenoliths. The xenoliths seem to be Paleozoic or older and thus cannot represent cumulate material produced during the mid-Tertiary volcanic event. Based on these data, the silicic ignimbrites could comprise up to 100% lower crustal m a t e r i a l . Thus we b e l i e v e that the Sierra M a d r e O c c i d e n t a l igneous e v e n t was not a time of major continental crust formation.
CRUSTAL DIFFERENTIATION, GRANITOID SEGREGATION AND MIGMATITE GENESIS: AN EXPERIMENTAL APPROACH MICHAEL J. RUTTEB and PETER J. WYLLIE (University COllege, Belfie]d, Dublin 4, Ireland) Crustal anatexis and segregation of granitoid liquids is a fundamental mechanism of crustal differentiation. Vapour-absent melting experiments on a natural tonalite provide constraints on high grade metamorphic processes and melting reactions in the lower continental crust. Dehydrationmelting of biotite-hornblende-tona]ite containing 0.8 wt.% water at lO kbar generates iO vo].% peraluminous granite liquid at 875°C. Water released during biotite melting dissolves in waterundersaturated granite melt. Under granulite facies metamorphic conditions such liquids may be segregated on a metre scale to form migmatites, leaving an orthopyroxene-bearing residue. Nearsolidus granite liquids are also saturated with futile. Garnet crystallines subsoIidus and is present through the melting interval up to IO00°C. Amphibole persists to much higher temperatures than biotite, and more extensive melting involving amphibole is required for mobi]isation and ascent of granitoid magmas. In the probable absence of large volume aqueous fluid infiltration into low porosity deep crustal terranes temperatures greater than 1050°C (yielding a melt fraction > 0.4) are required for ascent of pluton-sized granodiorite liquids, which co-exist with plagioc]ase and clinopyroxene at a depth of 30 km. More evolved granites can form by crystal fractionation from granodioritic precursors. A substantial subcrustal heat supply is required to reach temperatures which are considerably in excess of those normally attained during high grade regional metamorphism.
THE KLYOUCHEVSKOY GROUP OF ACTIVE VOLCANOES IN C E N T R A L K A M T C H A T K A : A UNIQUE GEODYNAMICAL SETTING ? S.A. Fedotov, M.P. Semet, Bogoyavlenskaya, V.M. Okrougin, Khrenov, and J.L.Joron (Inst. Volcanol., Petropavlovsk, L.G.C.S., I.P.G.P., U.P.M.C., Paris; Pierre Sue, CNRS/CEA, Saclay)
G.E. A.P. USSR; Labo.
One o f the world's most active, the Klyouchevskoy group of volcanoes in eastcentral Kamchatka comprises several volcanic centers s p r e a d o v e r a b o u t 4 , 5 0 0 km2 and w h i c h have evolved in late Pleistocene to Recent t i m e s . Magma p r o d u c t i o n rate for t h e g r o u p h a s p r o b a b l y been w e l l i n e x c e s s o f 300 km~ o v e r t h e l a s t 1 0 4 a . A s in other parts of t h e e a s t e r n K a m chatka belt (Semet et al., this volume), three main t y p e s of v o l c a n i c e d i f i c e s a r e present. Recently active examples are: (1) K l y o u c h e v s k o y s.s., a regular cone, 4750 m a.s.l., with an a v e r a g e of a b o u t one eruption per y e a r ; (2) B e z y m i a n n i , a more subdued cone t r u n c a t e d in t h e c a t a clysmic 1956 eruption and permanently active since; (3) theTolbachik a r r a y of cones formed in the i0 km l o n g f i s s u r e e r u p t i o n of 1 9 7 5 - 7 6 . T y p e s (l) and (3) have high instantaneous and/or integrated lava production rates and erupt Mgand Al-rich basalts with The0.5, Ni>lO0 and The0.8, Ni=30 ppm respectively during the same event. Historical eruptions of B e z y m i a n n i , however, gave rise to a n d e s i t e s ( l o w Ni, T h a i ppm) becoming more basic with time. Contrasted magma compositions and p r o d uction rates at individual centers seems to r e s u l t from crustal plumbing systems. High growth rate of t h e g r o u p as a w h o l e m a y n o t be d u e to s a m p l e s u b d u E t l o n .
BORONq TOURMALINE CRATONS D. M. S H ~ W , University,
AND
WATER
D e p a r t m e n t of H a m i l t o n , ON.
IN C O N l I N E N T A L
Geology, McMaster Canada LSS4MI.
Continental cratons contain very little B: t h e s u r f a c e r o c k s of t h e C a n a o l a n P r e CamDrlan SnlelO average 9.2 ppm, lower crustal rocks (Kapuskaslng Structural Zone, O n t a r i o ) a v e r a g e 2.3 ppm. L o c a l l z e o enrichment in A u - D e a r l n g quartz veins (upto 5 9 0 0 ppm: R o P e r t & B r o w n , 1 9 8 0 ) , in b a s e metal Oeposlts, in t o u r m a l l n l t e seoiments (up to 1.9 w t . % : S l a c x , 1 9 8 4 ) a n o in pegmatlte, is q u a n t i t a t i v e l y trivial. The c o n t r o l s on B O e n a v i o u r a r e t o u r m a l i n e staDllltv and borate complex soluPility. Formation waters of all klnPS (continental, geothermal, ocean-crust) commonly c o n t a i n B ( u p t o l O O o p p m in t h e r m a l water;Henley, 1984). Diagenesls and lowgrade metamorpnlsm reaOlly form tourmallnIte a n q q u a r t z - t o u r m a l l n e segregations, u t i l i z i n g B in w a t e r , c l a y s a n d e v a p o r i t e s . Tourmaline synthesis at T K 350°C has not O e e n r e p o r t e o , b u t in d l a g e n e s l s it m u s t f o r m at l o w e r t e m p e r a t u r e s . Once formed, tourmaline is v e r y s t a p l e a n d a p p e a r s to r e s l s t b r e a k d o w n . It is c o n s e q u e n t l y a common r e s l s t a t e heavy m i n e r a l i n s e c o n d a r y s e d i m e n t s : i n d e e d one w o n d e r s w h y all t e r r e s t r i a l B is n o t l o c k e d up in t o u r m a l i n e . However the general a b s e n c e o f B m i n e r a l s in g r a n u l i t e r e g i o n s , a n d t h e r e s u l t s of l a P o r a t o r y s t a P i l l t y stuOies indicate that tourmaline must break d o w n u n d e r h i g h P a n o at n i g h T (> 7 5 0 ° C ; B e n a r o et a i . , 1 9 8 5 ) , the 8 returning once more t o metamorphic fluids. Such f l u i d s are generally aPsent from stabilized cratons and t h e r o l e o f P r e c a m P r i a n - n o s t e o w a t e r s ~ u p t o I 0 ppm; F r a p e & F r i t z , 1987~ i s t o leach what B Is left in the rocks.
CRUSTAL DIFFERENTIATION, GRANITOID SEGREGATION AND MIGMATITE GENESIS: AN EXPERIMENTAL APPROACH MICHAEL J. RUTTEB and PETER J. WYLLIE (University COllege, Belfie]d, Dublin 4, Ireland) Crustal anatexis and segregation of granitoid liquids is a fundamental mechanism of crustal differentiation. Vapour-absent melting experiments on a natural tonalite provide constraints on high grade metamorphic processes and melting reactions in the lower continental crust. Dehydrationmelting of biotite-hornblende-tona]ite containing 0.8 wt.% water at lO kbar generates iO vo].% peraluminous granite liquid at 875°C. Water released during biotite melting dissolves in waterundersaturated granite melt. Under granulite facies metamorphic conditions such liquids may be segregated on a metre scale to form migmatites, leaving an orthopyroxene-bearing residue. Nearsolidus granite liquids are also saturated with futile. Garnet crystallines subsoIidus and is present through the melting interval up to IO00°C. Amphibole persists to much higher temperatures than biotite, and more extensive melting involving amphibole is required for mobi]isation and ascent of granitoid magmas. In the probable absence of large volume aqueous fluid infiltration into low porosity deep crustal terranes temperatures greater than 1050°C (yielding a melt fraction > 0.4) are required for ascent of pluton-sized granodiorite liquids, which co-exist with plagioc]ase and clinopyroxene at a depth of 30 km. More evolved granites can form by crystal fractionation from granodioritic precursors. A substantial subcrustal heat supply is required to reach temperatures which are considerably in excess of those normally attained during high grade regional metamorphism.
THE KLYOUCHEVSKOY GROUP OF ACTIVE VOLCANOES IN C E N T R A L K A M T C H A T K A : A UNIQUE GEODYNAMICAL SETTING ? S.A. Fedotov, M.P. Semet, Bogoyavlenskaya, V.M. Okrougin, Khrenov, and J.L.Joron (Inst. Volcanol., Petropavlovsk, L.G.C.S., I.P.G.P., U.P.M.C., Paris; Pierre Sue, CNRS/CEA, Saclay)
G.E. A.P. USSR; Labo.
One o f the world's most active, the Klyouchevskoy group of volcanoes in eastcentral Kamchatka comprises several volcanic centers s p r e a d o v e r a b o u t 4 , 5 0 0 km2 and w h i c h have evolved in late Pleistocene to Recent t i m e s . Magma p r o d u c t i o n rate for t h e g r o u p h a s p r o b a b l y been w e l l i n e x c e s s o f 300 km~ o v e r t h e l a s t 1 0 4 a . A s in other parts of t h e e a s t e r n K a m chatka belt (Semet et al., this volume), three main t y p e s of v o l c a n i c e d i f i c e s a r e present. Recently active examples are: (1) K l y o u c h e v s k o y s.s., a regular cone, 4750 m a.s.l., with an a v e r a g e of a b o u t one eruption per y e a r ; (2) B e z y m i a n n i , a more subdued cone t r u n c a t e d in t h e c a t a clysmic 1956 eruption and permanently active since; (3) theTolbachik a r r a y of cones formed in the i0 km l o n g f i s s u r e e r u p t i o n of 1 9 7 5 - 7 6 . T y p e s (l) and (3) have high instantaneous and/or integrated lava production rates and erupt Mgand Al-rich basalts with The0.5, Ni>lO0 and The0.8, Ni=30 ppm respectively during the same event. Historical eruptions of B e z y m i a n n i , however, gave rise to a n d e s i t e s ( l o w Ni, T h a i ppm) becoming more basic with time. Contrasted magma compositions and p r o d uction rates at individual centers seems to r e s u l t from crustal plumbing systems. High growth rate of t h e g r o u p as a w h o l e m a y n o t be d u e to s a m p l e s u b d u E t l o n .
BORONq TOURMALINE CRATONS D. M. S H ~ W , University,
AND
WATER
D e p a r t m e n t of H a m i l t o n , ON.
IN C O N l I N E N T A L
Geology, McMaster Canada LSS4MI.
Continental cratons contain very little B: t h e s u r f a c e r o c k s of t h e C a n a o l a n P r e CamDrlan SnlelO average 9.2 ppm, lower crustal rocks (Kapuskaslng Structural Zone, O n t a r i o ) a v e r a g e 2.3 ppm. L o c a l l z e o enrichment in A u - D e a r l n g quartz veins (upto 5 9 0 0 ppm: R o P e r t & B r o w n , 1 9 8 0 ) , in b a s e metal Oeposlts, in t o u r m a l l n l t e seoiments (up to 1.9 w t . % : S l a c x , 1 9 8 4 ) a n o in pegmatlte, is q u a n t i t a t i v e l y trivial. The c o n t r o l s on B O e n a v i o u r a r e t o u r m a l i n e staDllltv and borate complex soluPility. Formation waters of all klnPS (continental, geothermal, ocean-crust) commonly c o n t a i n B ( u p t o l O O o p p m in t h e r m a l water;Henley, 1984). Diagenesls and lowgrade metamorpnlsm reaOlly form tourmallnIte a n q q u a r t z - t o u r m a l l n e segregations, u t i l i z i n g B in w a t e r , c l a y s a n d e v a p o r i t e s . Tourmaline synthesis at T K 350°C has not O e e n r e p o r t e o , b u t in d l a g e n e s l s it m u s t f o r m at l o w e r t e m p e r a t u r e s . Once formed, tourmaline is v e r y s t a p l e a n d a p p e a r s to r e s l s t b r e a k d o w n . It is c o n s e q u e n t l y a common r e s l s t a t e heavy m i n e r a l i n s e c o n d a r y s e d i m e n t s : i n d e e d one w o n d e r s w h y all t e r r e s t r i a l B is n o t l o c k e d up in t o u r m a l i n e . However the general a b s e n c e o f B m i n e r a l s in g r a n u l i t e r e g i o n s , a n d t h e r e s u l t s of l a P o r a t o r y s t a P i l l t y stuOies indicate that tourmaline must break d o w n u n d e r h i g h P a n o at n i g h T (> 7 5 0 ° C ; B e n a r o et a i . , 1 9 8 5 ) , the 8 returning once more t o metamorphic fluids. Such f l u i d s are generally aPsent from stabilized cratons and t h e r o l e o f P r e c a m P r i a n - n o s t e o w a t e r s ~ u p t o I 0 ppm; F r a p e & F r i t z , 1987~ i s t o leach what B Is left in the rocks.