Episodic Proterozoic magmatism in Northwest Bangladesh: Implications for Columbia/Nuna and Rodinia reconstructions

Abstract

A diorite from drillhole GDH 53 at Voktipur in the Dinajpur Block in NW Bangladesh records an age of 1686 ± 4 Ma, whereas a tonalite from drillhole GDH 54 at Barapaharpur has an age of 1641 ± 7 Ma. Conversely, charnockite and monzogranite gneiss from drillhole GDH 31 at Gaibandha record considerably younger ages of 1144 ± 10 and 1133 ± 16 Ma, respectively. The diorite-tonalite-granodiorite suite is metaluminous, calc-alkaline, magnesian, with weak to moderate negative Eu anomalies and I-type characteristics. This suite is interpreted to have been derived from the melting of heterogeneous mafic crust, followed by a moderate degree of fractionation in an arc setting, identical to the source for similar rocks reported elsewhere in the basement of Bangladesh. The charnockite-monzogranite gneissic rocks from Gaibandha are ferroan, display weak negative Eu anomalies, belong to the high K calc-alkaline series, and plot as A-type granitoids (A2 subtype). It is considered that asthenospheric upwelling in an extensional/post-collisional setting provided the heat required to generate these rocks at the base of thinned crust. The geochemical data, lithostratigraphy and age of 1.73–1.64 Ga for the diorite-tonalite-granodiorite suite in NW Bangladesh do not match rocks of the Chotanagpur Granite Gneiss Complex and Central Indian Tectonic Zone to the west in India, negating their association with the collision that united the North and South Indian blocks. However, the Shillong-Meghalaya Gneissic Complex in NE India, with ~1.78–1.62 Ga granite gneiss and 1.28–1.08 Ga charnockite, provides a closer analogue. Events at these two times have also been recorded within the east Gondwana fragments in the Bunger and Obruchev Hills of East Antarctica. The similarities with both the Shillong-Meghalaya Gneissic Complex, lying to the east in India, and with East Antarctica, suggest that Proterozoic magmatic events in the Dinajpur Block of NW Bangladesh relate to events in both the Columbia/Nuna and Rodinia supercontinents and support a connection with these supercontinents throughout the Proterozoic.