Geochemical signatures of mid-crustal melting processes and heat production in a hot orogen: The Kerala Khondalite Belt, Southern India

Abstract

Garnet-bearing migmatites and associated leucogranites and leucosomes of the Trivandrum Block in the Kerala Khondalite Belt were formed through granulite facies dehydration melting of metasedimentary protoliths. Significant trace element depletions in Cs, Zr, Nb and the compatible transition metal elements V, Cr, Ni, Cu and Zn relative to average post-Archaean shales and model middle and upper crust, recorded in all samples, require their sedimentary protoliths to have been impure sandstones and greywackes, rather than shales. Leucogranites (70-75 wt%) and leucosomes (SiO2: 68-70 wt%), which are uniformly peraluminous and classed as S-type on the basis of their A/NK and ASI relations, form a compositional array that shows strong correlated variations between TiO2 and Fe2O3, and TiO2 or Fe2O3 with Co and Y. These reflect coupled variations in modal garnet and ilmenite and require the presence of up to 15–20 wt% of entrained peritectic garnet in the higher Y and HREE leucosomes. The leucosomes have REE patterns with normalised La/Sm of 10, negative Eu anomalies (Eu* < 0.8) and flat HREE at 6–20 times chrondrite, whereas leucogranites range to much lower HREE (1–5 times chondrite) with higher La/Sm and large positive Eu anomalies (Eu* = 1.4–3.4). Despite broadly similar major element compositions that lie within the granite minimum melt field in terms of felsic components, leucogranite Zr contents are highly variable, ranging down to 4–20 ppm in the lowest HREE and high Eu* cases, resulting in Zr saturation temperatures (544–647 °C) that are lower than any feasible melt. These geochemical features, coupled with covariations between Nb,-Ta and YYb, collectively support petrological and field observations that the leucosomes are mixtures between former melt and entrained peritectic garnet and ilmenite. The leucogranites are the products of melt extraction and migration on metres to several metres lengthscales. Leucogranite NbY, TaYb, Eu* and SrBa relationships demonstrate that their chemical diversity is principally controlled by variation in peritectic phase assemblage, i.e. proportion of entrained garnet, ilmenite and K-feldspar, with near-situ feldspar-dominated fractional crystallisation and accumulation coupled with the loss of fractionated melt being secondary controls. Age-corrected heat productions of the best-segregated leucogranites that most closely approach melt compositions are 2.7 ± 0.8 μWm−3, somewhat greater than the weighted average, 1.7 ± 0.5 μWm−3, of preserved migmatite, in-situ leucosomes and leucogranites of all types. Correction of the 550 Ma heat production to account for an estimated 30% of migration and loss of melt from this middle crustal migmatite complex yields a syn-orogenic heat production of c. 2.0 ± 0.6 μWm−3 which, though elevated compared with model middle-crust, is unlikely to have been high enough to have facilitated the near-UHT to UHT metamorphism in the Trivandrum Block without the addition of heat external to the orogenic crust itself.