GEOLOGY AND METAMORPHISM OF THE LADAKH TERRANE AND SHYOK SUTURE ZONE IN THE CHOGO LUNGMA-TURMIK AREA (NORTHERN PAKISTAN)

Metamorphism of the Askore Amphibolite, metabasaltic and metasedimenta ry medium-grade hornblende-bearing schists at the northernmost portion of the Ladakh Terrane and of the Shyok Suture Zone, mainly a low-grad e volcano-sedimentary series, has been studied in the area between the Chogo Lungma glacier and the Indus river halfway between Skardu and R ondu. 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 degrees C), geobarometry based on amphibole composition reveals a marked difference between ga rnet - epidote - andesine amphibolites exposed just above the Main Man tle Thrust at the head of Turmik valley, which equilibrated at high pr essures (about 10 kbar) in late Miocene (Tortonian), and biotite epido te - oligoclase amphibolites outcropping at the mouth of Turmik valley , which equilibrated at pressures of c. 6 kbar before late Eocene (Pri abonian). The Dasu Ultramafite and other smaller lens-shaped bodies of low-to medium-grade metaperidotite separate the Ladakh Terrane from t he Shyok Suture Zone. They are antigorite serpentinites, often with ta lc and magnesite, in which relict cumulitic structures are locally rec ognisable. The ultramafites may represent remnants of oceanic lithosph ere separating the Ladakh-Kohistan island are from the Asian plate, or they may be deep crustal rocks stripped from the basement of the are. The mostly greenschist-facies Shyok Suture Zone shows the lithology o f a calc-alkaline volcano-sedimentary series. It is supposed to be a r emnant of a back are basin of early Cretaceous age, separating the are from the southern margin of Asia. Chloritoid, kyanite and biotite hav e been found in individual thrust sheets occurring al different struct ural levels and totally subordinate in volume to very low-and low-grad e rocks. Such sharp differences in mineral paragenesis, together with field evidence of local shear, suggest a complex internal structure fo r the Shyok Suture Zone. From the head of Chogo Lungma glacier to the Basha valley, close to the contact with the Karakorum Metamorphic Comp lex, the rocks of the Shyok Suture Zone record a late Miocene metamorp hic event al medium pressures and temperatures. Thermobarometric and g eochronological evidence suggests that this event can be related to th e exhumation and thrusting of the Karakorum metamorphic core over the Shyok Suture Zone.