Petrology and geochemistry of charnockites (felsic ortho-granulites) from the Kerala Khondalite Belt, Southern India: Evidence for intra-crustal melting, magmatic differentiation and episodic crustal growth

Abstract

The Kerala Khondalite Belt (KKB) of the southern India encompasses volumetrically significant magmatic components. Among these, orthopyroxene-bearing, felsic ortho-granulites, popularly known as charnockites in Indian context, constitute an important lithology. In contrast to the well-known phenomena of arrested charnockitization, the geochemical characteristics and petrogenesis of these ortho-granulite suites remain poorly studied, leaving geodynamic models envisaged for the KKB highly conjectural. In this paper, we try to bridge this gap with detailed results on orthopyroxene-bearing, felsic ortho-granulites spread over the entire KKB and propose a new petrogenetic and crustal evolution model.Based on geochemical characteristics, the orthopyroxene-bearing, felsic ortho-granulites (charnockites sensu lato) of KKB are classified into (1) tonalitic (TC), (2) granitic (GC), and (3) augen (AC) suites. Members of the TC follow sodic (characterized by decreasing CaO/Na2O), whereas those of the GC and AC follow calc-alkaline trends of differentiation. Geochemical patterns of the TC resemble those of the Archaean tonalite–trondhjemite–granodiorite (TTG) suites, with slightly magnesian character (average Mg#=33), moderate LREE (average LaN=154), low HREE (average YbN=6) and Y (1–53ppm; average 11ppm). The TC is also characterized by positive to slightly negative europium anomalies (Eu/Eu*=0.7 to 1.67). The GC and AC suites, on the other hand, resemble post-Archaean arc-related granites. The GC displays ferroan nature (average Mg#=22), low to moderate degrees of REE fractionation (average [La/Yb]N=34.84), high contents of Y (5–128ppm; average 68), and low Sr/Y (1–98) ratios. Significant negative Eu anomalies (Eu/Eu*=0.18–0.91; average 0.50) and low Sr (65–690ppm) are also noted in the GC. Similar chemical characteristics are shown by the AC, with ferroan nature (average Mg#=21), low to moderate degrees of REE fractionation (average [La/Yb]N=26), high contents of Y (71–99ppm; average 87), and low Sr/Y (average 2) ratios with significant negative Eu anomalies (Eu/Eu*=0.03–0.31; average 0.23) and low Sr (average 160ppm) contents. The protoliths of the TC are interpreted as being derived from partial melting of thickened oceanic-arc crust composed of Archaean mafic composite source rocks (i.e., eclogite and/or garnet amphibolite) with a garnet amphibolite residue. Geochemical features of the GC, such as high Rb/Sr (average 1.80) and Ba/Sr ratios (average values >6), are considered as evidence for crustal reworking in their genesis, suggesting remelting of a quartzo-feldspathic (TTG) source, within the plagioclase stability fields.The geochemical features of the felsic ortho-granulite suite, substantiated with published geochronological data on members of the TC, GC, and AC suites, suggest a four-stage crustal evolution of the KKB. The first stage is marked by the formation of an over-thickened oceanic-arc. Zircon Hfc model ages of the TC and GC suites constrain the time of this juvenile magmatic crust-forming event as Meso- to Neoarchaean (2.8 to 2.6Ga). The second stage corresponds to the production of TTG magmas by melting of the over-thickened oceanic-arc crust, subsequent to basaltic underplating during Palaeoproterozoic (ca. 2.1Ga). The third stage was initiated by a transition in subduction style from shallow to steep due to continent-arc accretion. This stage is marked by the formation of granitic magmas through partial melting of the TTG crust and their differentiation into GC and TC. The zircon crystallization ages (1.89 and 1.85Ga) of the GC indicate arc accretion occurred during the Palaeoproterozoic. The fourth stage of crustal evolution is correlated with the Mesoproterozoic (~1.5Ga) emplacement of megacrystic K-feldspar granites (protoliths to the AC and augen gneisses). The distinct petrography, geochemistry and crystallization ages of the AC suggests recurrence of megacrystic, high-K calc-alkaline granitoids as the product of final phases of crustal-remelting marking subduction cessation. All these magmatic events are fairly well correlated with the major episodes of crustal growth observed in the once contiguous continental fragments (Sri Lanka and Madagascar) and worldwide events (2.7, 1.9, and 1.2Ga) implying similar episodic nature in the lower crustal evolution of the KKB.