Executive Summary : | The Meghalaya plateau is considered as the north-eastern extension of the Precambrian shield of the Indian peninsula separated by the Garo-Rajmahal depression (Desikashar, 1974; Crawford, 1974). The Plateau is mainly composed of Proterozoic basement granite gneisses and migmatites known as the Assam Meghalaya Gneissic Complex (AMGC) (GSI, 2009). The AMGC is overlain by the Shillong Group (SG) of meta-sedimentary supra crustals (Mazumder, 1976; Nandy, 2001; Naik et al, 2019). The ensemble is intruded by the porphyritic to non -porphyritic granites and granodiorites more extensively in the eastern part of the AMGC. At few places SG includes metamorphosed mafic intrusives which are known as Khasi Greenstones. The Meghalaya plateau is bounded by the east-west trending Brahmaputra and Dauki fault systems in the north and south respectively (Nandy, 2001). The NW-SE trending Kopili lineament separates the Meghalaya plateau from the Mikir Hills (Acharyya et al, 1986; Evans, 1964). The southern part of the plateau is covered by Cretaceous Sylhet basalts and Tertiary shelf sediments (Fig.1). The granite gneisses and granites of Meghalaya plateau are dated by number of researchers by different methods of geochronology. Based on the whole rock Rb-Sr isotopic method, basement granite gneisses of the Meghalaya Plateau were dated ~1714–1150 Ma whereas the intrusive younger granite plutons range from 881 to 479 Ma (Chimote et al., 1988; Crawford, 1969; Ghosh et al., 1991, 1994, 2005; Selvam et al., 1995; Van Breemen et al., 1989). A new U-Pb zircon geochronology database suggested widespread granite magmatism from Archean to Cambrian (2650Ma to 500Ma) (Kumar et al, 2017). In the Meghalaya plateau, the thermal effect in the AMGC during the emplacement of the Meso to Neoproterozoic granites can be further studied as these granites are overwhelming in the eastern part of AMGC (Chatterjee, 2007). The AMGC is flanked both in the western and eastern sides by the Shillong Group of rocks. However, the gneissic rocks occurring in the east are sheared showing intrusive relationship with SG (Khonglah et al., 2010; Nath and Gogoi, 2020). A study of the lineaments indicate that NE-SW is the dominant set in the AMGC in the east and west of the Shillong Group with two other minor trends of N-S and E-W (Halder et al, 2022). However, a prominent trend of NW-SE lineaments in the AMGC in the west of SG is found to be absent in the east of SG (Fig 2). This is only possible if chronologically the gneissic rocks present on both the flanks of SG are not one and the same. Hence, the gneissic rocks occurring to the east of the SG lack a deformational event which is present in the gneissic rocks in the west of SG. Therefore, it is to be ascertained whether the gneissic rocks in the east are syn-to post tectonic intrusive granite pluton within SG, which may be later deformed /sheared bearing a gneissic character in the process. |