Palaeostress and magma pressure measurement of granite veins in the Neoproterozoic Ambaji granulite, South Delhi terrane, Aravalli–Delhi mobile belt, NW India: Implication towards the extension-driven exhumation of the middle–lower crustal rocks

Neoproterozoic Ambaji granulite, in the South Delhi terrane (SDT) of the Aravalli–Delhi mobile belt (ADMB), is multiply deformed at different stages of its exhumation from the middle–lower crust. It was intruded by four phases of granites, \(\hbox {G}_{0-3}\), of which the \(\hbox {G}_{{3}}\) was syntectonic with brittle shearing. The orientation of the \(\hbox {G}_{{3}}\) granite veins was used, in this paper, to measure the palaeostress and magma pressure during its intrusion. The \(\hbox {G}_{{3}}\) granite has a porphyritic texture with quartz, microcline and biotite minerals and contains magmatic foliation; biotites are aligned oblique to such foliation, suggesting the syntectonic intrusion of the granite with normal faulting. Stereoplot of the poles of the granite veins shows concentration in the NW and SW quadrants and lack data over an elliptical area at the centre, indicating a girdle pattern. This implies that the magma pressure \(P_{\mathrm{m}}>\) \(\sigma _{2}\), compression \(\sigma _{1}\) was vertical and extension \(\sigma _{3 }\) was NW–SE horizontal. Values of \(\theta _{2} = 16{^{\circ }}\) and \(\theta _{3} = 40{^{\circ }}\) were obtained from the stereoplot, used to calculate stress ratio \(\Phi = 0.81\), driving pressure \(R^\prime = 0.92\) and the construction of the 3D Mohr plot for \(P_{\mathrm{m}}\) and principal stresses. Result suggests that the \(\hbox {G}_{{3}}\) granite intruded under extensional tectonics that probably contributed to the exhumation of the Ambaji granulite.