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Petrology, geochemistry and geochronology of highly foliated amphibolites from the ophiolitic mélange beneath the Yarlung Zangbo ophiolites, Xigaze area, Tibet : geodynamical implications

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2005
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On retrouve localement des amphibolites fortement foliées dans le mélange ophiolitique sous les massifs ophiolitiques de la Zone de Suture du Yarlung Zangbo (ZSYZ). Ces blocs représentent la partie supérieure d’une semelle métamorphique démembrée. La géochimie des amphibolites (La/Yb = 0.65-0.97, Ta/Th = 0.33-0.65) est similaire à celle des roches mafiques provenant de l’ophiolite, suggérant une origine dans le même bassin d’arrière-arc. Le métamorphisme de haut grade (P=14 kbars, T= 800°C) subit par les amphibolites suggère un enfouissement pendant la naissance d’une subduction. Les âges voisins des amphibolites et de la croûte ophiolitique (121-130 vs 120±10 et 126 Ma, respectivement) suggèrent que la naissance de la subduction s’est déroulée dans le bassin arrière-arc Néo-Téthysien. Un tel événement n’avait pas encore été rapporté. La présence de dikes et le métasomatisme tardif responsable de la cristallisation de préhnite pourraient indiquer la subduction d’un centre magmatique. La composition en isotopes stables du fluide responsable confirmerait une telle hypothèse.
Blocks of highly foliated amphibolites are locally found within the serpentinite matrix mélange underlying the Yarlung Zangbo ophiolites near Bainang and Buma, Xigaze area, Yarlung Zangbo Suture Zone (YZSZ), Tibet. The mélange is thought to be the result of the tectonic dismemberment of the base of the ophiolitic napes during its obduction over the Indian passive margin, circa 50 Ma. Prior to dismemberment, amphibolites were probably parts of a coherent dynamothermal sole, as observed at the base of many ophiolites. Sampled amphibolites can be subdivided in three groups: garnet, banded and common amphibolites. Medium-grained garnet amphibolites contain the assemblage A) Hb+CPX+Gt+Pl±Rt and B) Gt+Hb+Pl (corona assemblage). Fine to medium-grained banded amphibolites contain the assemblage C) Hb+CPX+Pl+Ep±Sp+Qtz+Ap. Fine-grained common amphibolites contain facies D) Hb+Pl±Ep+Ap+Sp. In all assemblages, plagioclase is pseudomorphosed by an albite-prehnite simplectite. Retrograde cataclastic veins contain the assemblage E) Ab+Pr±Ch+Ep. The geochemistry of the garnet, banded and common amphibolites is very similar to the geochemistry of other mafic blocks in the mélange and of mafic igneous rocks within the ophiolitic massifs. When compared to MORBs, light depletion of LREE (La/Yb = 0.65-0.97) and mild HFSE depletion (Ta/Th = 0.33-0.65) would suggest a mixing between the IAT and MORB sources, as seen in back-arc basins and nascent intra-oceanic arcs. The amphibolites were buried at the inception of a subduction within the back-arc to peak metamorphism conditions of 11-14 kbars and ~800 °C. Ar/Ar analysis of amphiboles revealed a metamorphic age of 121-130 Ma, which is synchronous with ages obtained from the overlying ophiolites. Overlapping in ophiolite-sole age relationship reveals inception of the subduction near or at the spreading center from which originated the ophiolite. Subduction of a buoyant body could explain heterogeneous coronitization of pyrope-rich (up to 35 %) garnet by Al-Tschermakites (Al2O3 up to 21 wt %) at high-pressures. After exhumation, amphibolites were injected by very fine-grained diabasic dykes and were subject to percolation of a prehnite-precipitating fluid. Oxygen stable isotopes suggest that a magmatic fluid is responsible for prehnite precipitation. The magmatic and metamorphic history of the dynamothermal sole and field relationships with adjacent units seem to indicate that most of Neo-Tethys oceanic domain was subducted along this new Late Cretaceous subduction zone.
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