Tectonic and magmatic evolution of the eastern Karakoram, India

The Shyok suture zone separates the Ladakh terrane to the SW from the Karak oram terrane to the NE. Six tectonic units have been distinguished. From so uth to north these are: 1. Saltoro formation; 2. Shyok volcanites; 3. Salto ro molasse; 4. Ophiolitic melange; 5; Tirit granitoids; 6. Karakoram terran e including the Karakoram batholith. Albian-Aptian Orbitolina-bearing limes tones and turbidites of the Saltoro formation tectonically overlie high-Mg- tholeiites similar to the tectonically overlying Shyok vol canites. The hig h-Mg tholeiitic basalts and calcalkaline andesites of the Shyok volcanites show an active margin signature. The Saltoro molasse is an apron-like, mode rately folded association of red-green shales and sandstones that are inter bedded with similar to 50 m porphyritic andesite. Desiccation cracks and ra in-drop imprints indicate deposition in a subaerial fluvial environment. Ru dist fragments from a polygenic conglomerate of the Saltoro molasse documen t a post-Middle Cretaceous age. The calcalkaline andesites of the Shyok vol canites are intruded by the Tirit granitoids, which are located immediately south of the Ophiolitic melange and belong to a weakly deformed trondhjemi te-tonalite-granodiorite-granite suite. These granitoids are subalkaline, I -type and were emplaced in a volcanic are setting. The subalkaline to calca lkaline granitoids of the Karakoram batholith are I-and S-type granitoid. T he I-type granitoids represent a typical calcalkaline magmatism of a subduc tion zone environment whereas the S-type granitoids are crust-derived, anat ectic peraluminous granites. New data suggest that the volcano-plutonic and sedimentary successions of the Shyok suture zone exposed in northern Ladak h are equivalent to the successions exposed along the Northern suture in Ko histan. It is likely that the Kohistan and Ladakh blocks evolved as one sin gle tectonic domain during the Cretaceous-Palaeogene. Subsequently, collisi on, suturing and accretion of the Indian plate along the Indus suture (50-6 0 Ma) together with tectonic activity along the Nanga Parbat-Haramosh divid ed Kohistan and Ladakh into two arealy distinct magmatic are terranes. The activity and a dextral offset along the Karakoram fault (Holocene-Recent) d isrupted the original tectonic relationships. (C) 1999 Editions scientifiqu es et medicales Elsevier SAS.