Petrological and Geochemical Characteristics of the Charnockitic and Associated A-Type Granitoid Rocks from the Basement Complex of Bangladesh

Abstract:

We present the first discovery of charnockite and A-type Granitoid rocks from the basement rocks in Bangladesh. These lie within the Dinajpur block, located between the Indian Craton to the southwest and the Shillong Massif to the east. There are no surface outcrops and the rocks were intersected by drill holes. They consist of a suite of monzogranite and granitic rocks. The core samples encountered between the depths of 2897 ft to 3510 ft of drillhole GDH-31, Dariapur, Gaibandha district. They consist of a suite of charnockite, monzogranite and granitic rocks. Megascopic study of structure and fabric of rocks suggest that the basement rocks are largely felsic massive igneous with a minor gneiss. The microscopic study reveals that the monzogranite and granite contain plagioclase feldspar, quartz and K-feldspar as predominant mineral constituents and zircon, garnet, rutile, apatite, and opaque minerals as common accessories. In most of the felsic rocks, antiperthite, perthite, myrmekite, graphic, granophyric and micrographic intergrowth textures are seen. The basement rocks in Gaibandha therefore, are considered to have formed as a part of a granitoid, that belongs to a pluton overall displays metaluminous to peraluminous, high K calc-alkaline, ferroan, A2 subtype for the rocks of A-type suite. The rocks are enriched in large Ion Lithophile elements, such as Ba, Sr and moderate to low averaging Ni, Cr, Co, V contents as well as the Zr, Y, Ce, Ga/Al, SiO₂, Na₂O+K₂O and REE content is relatively high. Primitive Mantle–normalized incompatible trace element patterns are also broadly similar for all the rocks with moderate negative Th, Nb and Ti anomalies. The major and trace elements illustrate a post/late-collisional tectonic setting for the granitoid samples. In the first stage, partial melting in a collisional setting with mantle wedge influence and by assimilation of country rocks, the calc-alkaline felsic rocks were formed. Crustal thickening and delamination occurred, as the collision progressed resulting in invading of high-temperature mantle upwelling on the bottom of the thickened felsic crust. The high heat flux caused granulite facies metamorphism on the lower part of the A-type granitoid and eventually generated charnockite.