Thermal and Chemical Evolution of an Archean Collision Zone: Insights from P–T–t History of Mafic Granulites from the Coorg Block, S. India

Abstract

Knowledge of pressure-temperature-time (P-T-t) evolution of Archean high-grade (deep crustal) metamorphic rocks is important for deciphering the nature of Archean tectonic processes. However, exposures of such rocks are limited in the present rock record. Here, we study a suite of high-grade, mafic rocks that are present along a crustal-scale shear zone (called the Mercara Shear Zone) between two Archean terrains of India, the Coorg block and the Dharwar Craton. Given that the Mercara Shear Zone is dated to be Mesoarchean, these shear zone rocks are well suited to elucidate Archean orogenic processes. Petrological investigation shows that these mafic rocks are characterized by a granulitic assemblage of orthopyroxene, clinopyroxene, plagioclase, quartz, amphibole ± garnet, and with accessory phases such as apatite, ilmenite, magnetite and rutile in some cases. We distinguish the investigated rocks into low-Mg and high-Mg varieties based on their whole rock composition as well as their mode of occurrence in the field and mineral chemistry. This difference in the bulk composition led to different reaction histories – for example, the low-Mg mafic granulites underwent partial melting while high-Mg granulites were less fertile. Combining these observations with the results of geothermobarometry, phase equilibria modelling, geochronology (U-Pb in zircon and Lu-Hf garnet geochronology) and diffusion modeling, we have reconstructed a multi-stage P-T-t history for these rocks. The first phase (Stage 1) is represented by granulite-grade metamorphism at ~750 – 900 °C and 8 – 13 kbar during ~3100 Ma (with uncertainties permitting a timing as recent as ~2700 Ma), after which they resided at T < 500 °C, likely at lower crustal levels (Stage 2). Subsequently, these rocks were reheated to a T of 700 – 750 °C at 7 – 10 kbar at ~2400 Ma (Stage 3) and then again cooled down to ~500 – 600 °C at 6 – 8 kbar (Stage 4). Application of diffusion chronometry shows that: (1) the cooling rates of these granulites at high temperatures (> 600 °C) varied in the range of 25 – 50 °C/Ma, and (2) the rocks resided for a long duration (~500 million years) at the Stage 2 metamorphic conditions i.e., at T < 500 °C. We infer that such a protracted, high-T metamorphic history involving at least two heating pulses, and the relatively slow cooling rates on the order of 10’s °C/Ma are consistent with the operation of peel-back styled orogenesis (an embryonic form of plate tectonics) on an early hotter Earth (Mesoarchean to Palaeoproterozoic). Moreover, the controls of bulk rock compositions on reaction histories provide a mechanism for intracrustal differentiation and generating Mg-rich, refractory material that may have eventually formed the lower continental crust over a protracted and pulsed thermal evolution spanning several hundred million years.