GRANITOID ROCKS OF WANGTU GNEISSIC COMPLEX, HIMACHAL-PRADESH - AN EXAMPLE OF IN-SITU FRACTIONAL CRYSTALLIZATION AND VOLATILE ACTION

The Wangtu Gneissic Complex (WGC) of Himachal Himalaya represents the basement rocks which remained more or less in its original realm. The unique feature of this complex is that the various phases of deformed and undeformed granitoids are exposed in the area. The complex has a p eripheral zone of augen gneisses and core of non-foliated granitoids c omprising of Coarse Porphyritic Granites (CPG) and Fine Grained Granit es (FGG). In the field the FGG rocks have sharp intrusive contacts wit h CPG rocks. At places they are entrapped or have protrusions into CPG rocks. Compositionally they represent syenogranites. They are peralum inous to meta-aluminous with Sr initial ratio of 0.705. The major and trace elements of these rocks show unimodal distribution pattern on di fferent variation diagrams. They along with rare earth elements show m arked fractionation trends, with a significant negative Eu anomaly. Th e rocks more or less define an isochron, representing Rb-Sr age of 189 5+/-64 Ma. The petrochemical studies favour a fractionated rather than multiple intrusions of magma. The studies suggest that fractional cry stallisation of parental magma gave rise to early formed CPG rocks and the residual melts resulted in the crystallisation of FGG rocks. The last formed granitoids, the FGG rocks are interpreted as the final res idue which crystallised rapidly after build-up and loss of fluids and volatiles which migrated to higher levels to form pegmatite phase in t he aureol zone. The Rb-Sr systematics in the present case indicate tha t the granite have primary crystallisation melts derived from the deep crustal levels around 1895+/-64 Ma.