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The basic dykes in Mikir Hills have gabbroic composition. The prominent trends of these dykes are E-W in Southern part of Mikir Hills and NE-SW trend in the Northern Part of Mikir Hills. The geochemical characterization of dyke rock shows that they are sub-alkaline to alkaline basalt in nature, medium to high potassic, high titanium, Fe enriched tholeiites. The evolved nature of magma is represented by low Mg# 39.34 to 49.76 and are characterized by relatively low abundances of MgO, Cr, Ni, Zr, Y and high Rb, Ba, Th, Nb, indicating the within plate basalt (WPB) and continental rift basalt (CRB) chemical affinity. This dyke activity may be related to the extensional tectonics related to Proterozoic rifting hence, showing characteristics of continental rift basalt (CRB) type. The nine samples of gabbro define a five-point Rb-Sr isochron age of 1200±67 Ma with 87Sr/86Sr ratio of 0.70477±0.00042 with MSWD of 0.85. The Sm-Nd model ages (TDM) varies from 1600 to 2018Ma (εNd +0.11 to -2.04) indicates that the protolith from which these gabbroid rocks are derived is mostly Palaeo- to Meso-Proterozoic in age. The isotopic evolution of initial Sr, Nd and Pb reveals crustal contamination.

The Rb-Sr whole-rock isochron, age 1636 ± 66 Ma of Mirgarani granite, is the one of the oldest granite dated in the northwestern part of the Chhotanagpur Granite Gneiss Complex (CGGC). The initial Sr ratio is 0.715 ± 0.012 (MSWD = 0.11), showing an S-type affinity. The Mirgarani granite has intruded the migmatite complex of the Dudhi Group and forms the Mirgarani formation comparable to the granites of the Bihar Mica Belt around Hazaribagh (1590 ± 30 Ma). The present studies have established the chronostratigraphy of the Dudhi Group and adjoining areas in CGGC. Petro graphic and geochemical studies revealed that the granite is enriched in Rb (271 ppm), Pb (77 ppm), Th (25 ppm), and U (33 ppm) and depleted in Sr (95 ppm), Nb (16 ppm), Ba (399 ppm) and Zr (143 ppm) contents as compared to the normal granite. The Mirgarani granite is a peraluminous (A/CNK = 1.23), high potassic (K2O 6.42%), Calc-Alkalic to Alkali-Calcic {(Na2O+K2O)-CaO = 6.29} S-Type granite, a feature supported by the presence of modal garnet and normative corundum (2.68%). The Mirgarani granite is considered to have been formed by the anatexis of a crustal sedimentary protolith at a depth of approximately 30 km with temperatures ranging from 685-700 °C during the Co lumbian - Nuna Supercontinent.

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