P.V. Nageswara Rao

Department of Geology, Acharya Nagarjuna University, Nagarjuna Nagar-522510, Guntur, Andhra Pradesh, India

(E-mail: drpvn.rao@gmail.com)

Abstract

The Chimakurthi Pluton is located on the western margin of the Eastern Ghats Mobile Belt which falls under the Ongole domain of Krishna province from southeastern India. It is an elliptically shaped and concentrically zoned pluton consisting of an olivine clinopyroxenite unit in the centre and olivine gabbronorite unit on the outer margin, with an arcuate shaped anorthosite body towards the northwestern part of the pluton. The anorthosite and olivine gabbronorite consists of variable amounts of cumulus and inter-cumulus phases of plagioclase, olivine, clinopyroxene, orthopyroxene, ilmenite, and magnetite assemblages, respectively. The rocks units registered several igneous and some deformational textures. Electron Probe Micro Analysis was performed to assess the chemical composition of clinopyroxene host (En44.2-36.1 Fs17.0-12.3 Wo46.5-43.3), orthopyroxene lamellae (En75.9-66.0 Fs30.7-23.0 Wo3.1-0.9), olivine (Fo81.0-72.0) and plagioclase (An74.0-56.0) from anorthosite, and clinopyroxene host (En42.2-38.8 Fs18.9-14.2 Wo46.9-40.2), orthopyroxene lamellae (En71.5-59.0 Fs40.0-25.0 Wo3.3-0.5), olivine (Fo73.0-60.0) and plagioclase (An81.0-72.0) from olivine gabbronorite, respectively. The KD values of the exsolved pyroxenes of all samples range from 0.53 to 1.85, consistent with igneous equilibrium conditions in the pluton. The KP values range from 0.68 to 1.97, suggesting the influence of post-cumulus solid-state deformation forces following crystallization of the high-temperature primary magma of the pluton. The conversion of pigeonite to orthopyroxene results in considerable amounts of orthopyroxene lamellae growing within the clinopyroxene host and undergoing exsolution due to the sub-solidus re-equilibration during a significant period of slow cooling. The temperature – pressure estimate of exsolved pyroxenes from the present rock units revealed very broad range from 874 to 12260C and 5 to 14 kbars, respectively. However, the overall pressure increase and temperature decrease from the middle zone anorthosite unit (11010C, 9.2 kbars) to the outer zone olivine gabbronorite unit (10790C, 11.2 kbars) suggested the syn-tectonic emplacement of the Chimakurthi pluton into metapelitic country rocks through the Terrane Boundary Shear Zone, associated with the impact of a regional shear-related ductile deformation event (D2) caused by the collisionbetweenthe juxtaposed Western Dharwar Craton and Eastern Ghats Mobile Beltof India during 1450-800 Ma.

Keywords: Anorthosite, Olivine Gabbronorite, Chimakurthi Pluton, Exsolution Textures, P-T Conditions, EGMB, South India.