Variations of geological processes and its products at the end of the Archaean are most important in Earth history, since it marks fundamental changes in planetary dynamics. Therefore, recognizing the records of variations in magmatism at Archaean–Proterozoic boundary will be much useful to understand the transition from Archaean to Proterozoic. This contribution envisages secular variations in geochemistry of granitoids, evolved from a common source with progressive changes in tectonic scenarios as a proxy to establish the time-space relation between Archaean and Proterozoic transition. The lower-crustal section exposed in southernmost part of the southern Indian granulite terrain (SGT) contains granitoid complex of contrasting geochemistry (Sreejith and Ravindra Kumar, 2013). The distinct granitoid groups comprise (i) metatonalites with characteristic Archaean tonalite–trondhjemite–granodiorite (TTG) affinity and (ii) metagranites showing geochemical patterns archetypal of the post-Archaean granites. The metatonalite groups show low-K, magnesian character with moderate to high SiO2 (57–76 wt.%) contents, moderate LREE (average LaN = 154), low HREE (average YbN = 6), characteristic high Sr/Y (27–934; average 331) and high [La/Yb]N (average 39) ratios, and strongly positive and/or no europium (Eu/Eu* = 0.7 to 1.67) anomalies. The geochemical features suggests that protolith for the metatonalites are derived from partial melting of a subducting slab/thickened lower continental crust composed of mafic composite source rocks (eclogitic or basaltic crust) in the Neoarchaean (ca. 2.73–2.89 Ga). The absence of volcanism associated with the tonalite genesis possibly indicates that the tectonic scenario was flat subduction of an old and cold lithospheric plate. On contrary, the metagranite group show ferroan nature, low to moderate degrees of REE fractionation (average [La/Yb]N = 20), high contents of Y, low Sr/Y ratios with significant negative Eu anomalies (average Eu/Eu* = 0.50). These geochemical features along with high ratios of Rb/Sr, Ba/Sr, and negative anomalies for HFS elements (e.g., Nb and Ti) characterises the metagranite rocks as crustal derivatives formed by remelting and differentiation of arc-accretionary complex crust during the Palaeoproterozoic (ca. 1.8 Ga). Thus, the metagranites show orogenic signatures compatible with subduction (continental-arc rocks). This stage indicates a change of style of the down going slab from a flat to subsequently steeper subduction (arc-accretion) during the Palaeoproterozoic producing calc-alkaline high-K granites. The contrasting magmatic episodes of the SGT, therefore, provide evidence for changeover in subduction style from flat to steep during the Archaean–Proterozoic transition. This assumption is favoured by the geological records all over the globe with evidence of a plagioclase-rich TTG dominant crust in the Archaean to more or less K-feldspar enriched granite magmatism in the Proterozoic. Thus, these kinds of significant geochemical variations in magmatic records provide opportunities to explore the timing of Archaean-Proterozoic transition.

Reference(s)

Sreejith C and Ravindra Kumar GR 2013, Petrogenesis of high-K metagranites in the Kerala Khondalite Belt, southern India: a possible magmatic-arc link between India, Sri Lanka, and Madagascar. Journal of Geodynamics, v.63, pp. 69–82.