The early Palaeozoic magmatic event in the Northwest Himalaya, India: source, tectonic setting and age of emplacement

In the High Himalayan Crystalline Series of Northwest India, numerous peral uminous granites intruded the metasediments of the late Proterozoic to earl y late Cambrian Haimanta Group. Nd and Sr isotope systematics confirm that they were derived from heterogeneous crustal sources. New geochronological data from two plutons range in age from late Precambrian to early Ordovicia n: single zircon U-Pb dating yielded an age of 553 +/- 2 (2 sigma) Ma for t he Kaplas granite, whereas mineral Sm-Nd isotope systematics define a cryst allization age of 496 +/- 14 (2 sigma) Ma for the tholeiitic mafic rocks in the Mandi pluton, where evidence of magma mingling documents a close assoc iation between mafic and granitic melts. The end of this period of magmatic activity coincides with the depositional gap below the Ordovician transgre ssion, caused by surface uplift and erosion, that is an important feature i n the stratigraphy of the Northwest Himalaya. In Spiti, the transgression o f the Ordovician basal conglomerates on a normal fault indicates pre-Ordovi cian extensional faulting. Therefore, the early Palaeozoic magmatic activit ies in the Northwest Himalaya could be correlated with a late extensional s tage of the long-lasting Pan-African orogenic cycle which ended with the fo rmation of the Gondwana supercontinent.