Abstract In southern New Caledonia, Late Oligocene granodiorite and adamellite are intruded into an ultramafic allochthon emplaced in the Late Eocene period. Previous studies of these granitoids proposed an origin associated with the melting of the underlying continental crust, but our new data show that these high‐K to medium‐K calc‐alkaline granitoids display the geochemical and isotopic features of volcanic arc magmas uncontaminated by crust‐derived melts. These magmas were probably generated in a post‐Eocene and pre‐Miocene subduction, the geophysical traces of which have been detected along the western coast of New Caledonia. Sr, Nd and Pb isotopic ratios indicate derivation from an almost isotopically homogeneous mantle wedge, but in contrast, some variation in trace element ratios uncorrelated to differentiation is indicative of source heterogeneity. Prominent heavy rare earth element (HREE) depletion of some of the younger granitoids may be the result of an equilibrium achieved with garnet‐bearing subcrustal material (granulite) found as xenoliths, while a relative Nb, Ta and Hf enrichment, irrespective of crystal fractionation, may be related to either a modest contamination by previously underplated mafic material, heterogeneous hydration of the mantle wedge, or mixing with uplifted Nb‐rich mantle. Post‐obduction slab break‐off can be proposed to have played a role in sublithospheric mantle mixing and the subsequent heterogeneity. The Late Oligocene subduction described here may be tentatively extended southward into northern New Zealand allochthons.
Some 800 km2 of the area lying between the Hararnosh Spur of the Himalaya and the Southern Karakorum Complex, centered on the Chogo Lungma glacier system, is presented on a preliminary geological map, after three expeditions. It includes three major units from SW to NE: the High Himalayan Nanga Parbat - Haramosh Crystallines (HHC), the Ladakh Paleo Island Arc, and the Karakorum Metamorphic Complex intruded by granitoids. The three units are thrust to the south. The High-Himalayan ortho- and para-gneisses are metamorphosed to granulite facies and migmatised. The Ladakh Arc is made up of two groups of formation, the Askore Amphibolite to the SW and the Greenstone Complex to the NE, separated by a screen of serpentinized ultramafics. It is intruded by numerous granitoids including a leuco-trondhjemite body cutting the Himalaya-Ladakh contact. The rnetasedimentary and metavolcanic sequences of the Greenstone Complex include limestone horizons that have yielded post-Valanginian fossils. The Karakorum Metamorphic Complex includes orthogneiss domes of granitoids intruded into the surrounding metasedimentary formations. A syenitic dome of granitoid at Hemasil is apparently syntectonic. Two phases of deformation are observed in Karakorum and Ladakh and seem to correspond, but with different grades of metamorphism. The late doming structures are characteristic of the Karakorum. The Himalaya-Karakorum contact varies from south to north, from a normal fault to a strike-slip shear zone, and finally a thrust fault. The original Ladakh Karakorum contact (Shyok Suture) is folded and reactivated by late brittle thrusting, possibly the latest large-scale deformation of the region. The closure of the Ladakh-Kohistan back-arc basin followed by the collision of India is mostly overprinted by the recent structures.
Metamorphism of the Askore Amphibolite, metabasaltic and metasedimentary medium-grade hornblendebearing schists at the northernmost portion of the Ladakh Terrane and of the Shyok Suture Zone, mainly a low-grade volcano-sedimentary series, has been studied in the area between the Chogo Lungma glacier and the Indus river halfway between Skardu and Rondu.In the Askore Amphibolite the peak assemblage in the amphibolite facies defines the regional metamorphic foliation, and is overprinted by a later static recrystallization at comparable P-T conditions. In spite of similar peak temperatures (630–650°C), geobarometry based on amphibole composition reveals a marked difference between garnet – epidote – andesine amphibolites exposed just above the Main Mantle Thrust at the head of Turmik valley, which equilibrated at high pressures (about 10 kbar) in late Miocene (Tortonian), and biotite – epidote – oligoclase amphibolites outcropping at the mouth of Turmik valley, which equilibrated at pressures of c. 6 kbar before late Eocene (Priabonian).The Dasu Ultramafite and other smaller lens-shaped bodies of low- to medium-grade metaperidotite separate the Ladakh Terrane from the Shyok Suture Zone. They are antigorite serpentinites, often with talc and magnesite, in which relict cumulitic structures are locally recognisable. The ultramafites may represent remnants of oceanic lithosphere separating the Ladakh-Kohistan island arc from the Asian plate, or they may be deep crustal rocks stripped from the basement of the arc.The mostly greenschist-facies Shyok Suture Zone shows the lithology of a calc-alkaline volcano-sedimentary series. It is supposed to be a remnant of a back arc basin of early Cretaceous age, separating the arc from the southern margin of Asia. Chloritoid, kyanite and biotite have been found in individual thrust sheets occurring at different structural levels and totally subordinate in volume to very low- and low-grade rocks. Such sharp differences in mineral paragenesis, together with field evidence of local shear, suggest a complex internal structure for the Shyok Suture Zone. From the head of Chogo Lungma glacier to the Basha valley, close to the contact with the Karakorum Metamorphic Complex, the rocks of the Shyok Suture Zone record a late Miocene metamorphic event at medium pressures and temperatures. Thermobarometric and geochronological evidence suggests that this event can be related to the exhumation and thrusting of the Karakorum metamorphic core over the Shyok Suture Zone.
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