THE CHANGE OF SOURCE COMPOSITION IN PLUTONIC ROCKS FROM THE KOHISTAN-LADAKH ARC CONSTRAIN THE ONSET OF COLLISION ALONG THE INDUS SUTURE IN THE WESTERN HIMALAYA

The Kohistan-Ladakh arc (KLA) exposed in NW India and NE Pakistan is considered an juvenile oceanic arc that was incorporated into the Himalayan collision system. It is separated in the south from the Indian plate by the Indus suture zone and in the North from the Karakoram by the Shyok suture zone. The Kohistan-Ladakh Batholith records ≥70 Ma of magmatic activity within the arc from ≥100Ma to ≤30 Ma, covering the large inferred time interval (~70-34 Ma) for which continental collision along the Indus Suture zone occurred. As no old continental basement was involved in the formation of the KLA the arrival of the leading edge of the old Indian continent should have a pronounced effect on the isotopic source composition of the magmatic rocks in the arc. Accordingly, we conducted an extensive in situ Hf and U-Pb isotopic analysis of zircons, coupled with bulk rock Nd isotopic composition of major granitoids plutons forming the Kohistan-Ladakh Batholith. Our results indicate a pronounced and abrupt shift in isotopic composition that, based on inherited zircons, can be related to the arrival of the Indian plate.