Evolution of the melt source during protracted crustal anatexis: An example from the Bhutan Himalaya

Thomas Hopkinson,Clare J Warren,Sam Hammond,Christopher J Spencer,Nigel Harris,Randall R Parrish,Nick M.W Roberts

doi:10.1130/g47078.1

Thomas Hopkinson, Clare J Warren + Show 5 more

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https://doi.org/10.1130/g47078.1

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Abstract

Abstract The chemical compositions of magmatic zircon growth zones provide powerful insight into evolving magma compositions due to their ability to record both time and the local chemical environment. In situ U-Pb and Hf isotope analyses of zircon rims from Oligocene–Miocene leucogranites of the Bhutan Himalaya reveal, for the first time, an evolution in melt composition between 32 and 12 Ma. The data indicate a uniform melt source from 32 Ma to 17 Ma, and the progressive addition of an older source component to the melt from at least ca. 17 Ma. Age-corrected ɛHf ratios decrease from between −10 and −15 down to values as low as −23 by 12 Ma. Complementary whole-rock Nd isotope data corroborate the Hf data, with a progressive decrease in ɛNd(t) from ca. 18 to 12 Ma. Published zircon and whole-rock Nd data from different lithotectonic units in the Himalaya suggest a chemical distinction between the younger Greater Himalayan Series (GHS) and the older Lesser Himalayan Series (LHS). The time-dependent isotopic evolution shown in the leucogranites demonstrates a progressive increase in melt contribution from older lithologies, suggestive of increasing LHS involvement in Himalayan melting over time. The time-resolved data are consistent with LHS material being progressively accreted to the base of the GHS from ca. 17 Ma, facilitated by deformation along the Main Central thrust. From 17 Ma, decompression, which had triggered anatexis in the GHS since the Paleogene, enabled melting in older sources from the accreted LHS, now forming the lowermost hanging wall of the thrust.

Highlights

Crustal melting is a fundamental process both for chemical differentiation and for facilitating ductile deformation of the Earth’s continental crust
Given that the available melt-fertile source material in the Himalaya is restricted to the Lesser Himalayan Series (LHS) and Greater Himalayan Series (GHS), the results from this study provide clear evidence for a GHSsourced melt zone throughout the Oligocene, and subsequent tectonic or thermal evolution allowing some melting of the LHS during the early Miocene
In situ U-Pb and εHf LA-ICP-MS compositions of zircon rims from Himalayan leucogranites provide the first evidence for secular compositional change of melts generated by protracted anatexis during the earliest Oligocene to mid-Miocene

Summary

Introduction

Crustal melting is a fundamental process both for chemical differentiation and for facilitating ductile deformation of the Earth’s continental crust. Melting took place along the Himalaya orogen from at least 25 Ma to 9 Ma (Guo and Wilson, 2012), in southern Tibet, crustal melting of Eocene age has been reported (Aikman et al, 2012). Despite this extensive period of regional anatexis, no time-dependent change in the magmatic source has been recognized far. We observe a temporal evolution in the source region, and these data provide, for the first time, direct evidence for a time-dependent change in the mid-crustal material undergoing melting and decompression during Himalayan crustal thickening

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