Geochemistry of Precambrian mafic magmatic rocks of the Western Himalaya, India: petrogenetic and tectonic implications

Precambrian sequences of the Higher Himalayan Crystallines (Vaikrita Group) and the Lesser Himalaya (Chail, Jutogh and Jaunsar Groups), in the Garhwal and Himachal regions of the Western Himalaya, include abundant metamorphos ed mafic lavas and dykes (amphibolites). A gabbroic body within the Chail G roup has been dated at 1907 +/- 91 Ma (initial Sr-87/Sr-86 ratio of 0.7022 +/- 0.0008) by the whole rock Rb-Sr method. This age is consistent with sev eral age data (1800-2000 Ma: whole rock Rb-Sr method), assigned to the gran itoids and gneisses of this region. These amphibolites and gabbros exhibit low-Ti tholeiite characteristics. The Lesser Himalayan samples are enriched in light rare earth elements (LREE) and large ion lithophile element (LILE ), with distinct negative Nb, Sr, P and Ti anomalies. Conversely, the Vaikr ita Group samples are characterized by less enriched LREE-LILE and absence of the above anomalies but have distinct positive Sr anomalies. The chemica l characteristics of the Lesser Himalayan samples are remarkably similar to the basal Aravalli volcanics of the NW Indian shield, inferred to reflect significant components from enriched mantle sources and crustal contaminati on. Samples of the Vaikrita Group appear to have been influenced by an asth enospheric mantle and are less contaminated as indicated by the absence of negative Nb, P and Ti anomalies. These distinct geochemical characteristics are used to demarcate the Main Central Thrust along the Vaikrita Thrust. T he Chail and Jutogh Groups are affiliated with the Lesser Himalaya and the Chail Group gabbro body may represent portion of one among the numerous mag ma chambers that fed a large Precambrian magmatic province in the south of the Main Central Thrust. The similar emplacement ages (1800-2000 Ma) for th e mafic and felsic magmatic rocks, suggest that this region experienced a m ajor episode of crustal generation and evolution in a rift environment. (C) 1999 Elsevier Science B.V. All rights reserved.