Low-δ¹⁸O mantle-derived magma in Panjal Traps overprinted by hydrothermal alteration and Himalayan UHP metamorphism: Revealed by SIMS zircon analysis

Highlights

• Zirconsin Himalayan UHP eclogites formed from a subducted low-δ¹⁸O (hydrothermally altered) oceanic crustalsource.

• Their protoliths were further hydrothermally altered by high-T meteoric-waterinteraction before eclogitization.

• During the UHP metamorphism new low- δ18O minerals crystallized from the δ-18O-depleted precursors.

Abstract

We report two generations of low-δ¹⁸O zircons from the Himalayan eclogites and their host gneisses. In situ SIMS δ¹⁸O analyses on single zircon crystals (with known age and Hf isotope ratios) from two populations of chemically distinct zircons demonstrate a complex history: (1) an early low-δ¹⁸O mantle-derived magma, (2) followed by post-emplacement high-temperature meteoric-water alteration and finally (3) crystallization of new, low-δ¹⁸O minerals during the ultrahigh-pressure metamorphism. Magmatic zircon (269 Ma) in Group I eclogites yielded δ¹⁸O values from 1.9 to 4.6‰ VSMOW with an average value of 4.0 ± 0.2 (n = 35, the error is 2SD analytical precision and “n” represents number of analyzed spots), which is lower than the typical mantle values (5.3 ± 0.6, 2SD). In contrast, metamorphic zircons (45 Ma) in Group II eclogites preserve unusually low, negative δ¹⁸O values from − 3.9 to − 2.7‰ (average: − 3.4 ± 0.4, n = 35, 2SD). Zircons in felsic gneiss that surround Group II eclogites have inherited magmatic cores (ca. 260 Ma) with δ¹⁸O values of ca. 2.9‰, which decrease to − 0.1‰ in metamorphic (ca. 45 Ma) rims. These zircons preserve lower δ¹⁸O values than would be equilibrated with typical mantle. The low-δ¹⁸O values in magmatic zircons suggest that the mafic protolith to these eclogites formed from a hydrothermally altered subducted oceanic crust and the negative δ¹⁸O values in metamorphic zircons indicate hydrothermal alteration after crystallization of the mafic magmas but before growth of metamorphic zircons. This study reports evidence for melting of subducted low-δ¹⁸O ocean crust to form low-δ¹⁸O mantle-derived mafic magmas as previously proposed by Cartwright and Valley (1991, Geology 19, 578–581) for Proterozoic Scourie Dikes.

Introduction

The δ¹⁸O values (defined as [(¹⁸O/¹⁶O_sample ÷ ¹⁸O/¹⁶O_STD) − 1] × 1000 in which the STD is Vienna Standard Mean Ocean Water; VSMOW) of mantle-derived magmas (basalts and gabbro) exhibit a narrow range of ca. + 5.7 ± 0.3‰ (Eiler, 2001, Valley et al., 2005, Hoefs, 2015). The fairly homogenous δ¹⁸O composition of a majority of mantle-derived rocks results because most of the mantle is well mixed in δ¹⁸O and the oxygen isotope composition during magmatic fractionation remains almost the same. However, eclogite facies mantle nodules and silicate inclusions in diamond attest to mantle domains of subducted ocean crust with anomalous values of δ¹⁸O that have been preserved (Garlick et al., 1971, Schulze et al., 2013). Higher δ¹⁸O_Wole-rock values (> 6‰) are interpreted as upper oceanic crust that was altered by low temperature interaction with seawater and low δ¹⁸O values (~ 0 to 5‰) are attributed to high temperature alteration of lower oceanic crust. Melting tends to homogenize these extreme values, but rare low-δ¹⁸O mantle-derived magmas have been identified that are proposed to result from melting of subducted lower oceanic crust (Cartwright and Valley, 1991, Cartwright and Valley, 1992, Wei et al., 2002, Davies et al., 2015).

Low-δ¹⁸O values can form subsolidus in metamorphic rocks through high temperature hydrothermal alteration by surface (marine or meteoric) water (Valley, 1986, Criss and Taylor, 1986, Zheng et al., 2003). Understanding the timing and source of low δ¹⁸O values is crucial. However, it is difficult to determine the original magmatic source of metamorphic rocks with mixed protolith and complex evolutionary histories. Oxygen isotope ratios, combined with U–Pb age, and multi-isotope geochemistry, provide significant information regarding the nature of the source magma from which the rocks have crystallized and how they have been geodynamically evolved through time.

Zircon is a common accessory mineral in igneous, sedimentary, and metamorphic rocks, which provides a robust tool to unravel the history of Earth's crust and mantle. Zircons have been widely studied for U–Pb age dating, Hf isotopes, trace element geochemistry, and oxygen isotope ratios. Because of its chemical and physical inertness and resistance to alteration, crystalline zircon (i.e., not radiation damaged) can retain compositions from crystallization (Wasserburg et al., 1969, Compston et al., 1984, Gebauer, 1996, Liati and Gebauer, 1999, Belousova et al., 2010, Valley, 2003, Valley et al., 2005, Valley et al., 2015, Liu et al., 2006). Moreover, due to the extremely slow diffusion rate of oxygen in zircon (Watson and Cherniak, 1997, Zheng and Fu, 1998, Peck et al., 2003, Page et al., 2007, Bowman et al., 2011) it can preserve the original δ¹⁸O values of the source from which it crystallized. Thus, zircons can provide the key for understanding the evolutionary history of rocks. If zircons are zoned or have chemically distinct domains (Chen et al., 2011), then combined in situ U–Pb age, Hf isotope, and δ¹⁸O data in zircon provide critical information to trace back the geodynamic evolution from magmatic to post-magmatic, and subsequent metamorphic events.

In this study we report very low and negative δ¹⁸O values of magmatic and metamorphic zircons (with known U–Pb age and Hf isotope compositions; Rehman et al., 2013a, Rehman et al., 2016) from Himalayan eclogites and their host gneisses. The aim of this study is to determine the composition of pre-eclogite magmas from which these rocks formed and how they hydrothermally altered. Finally, what happened to their chemistry when these rocks were subjected to ultrahigh-pressure metamorphism at mantle depths (coesite-stability) and subsequent exhumation. These results yield significant insight into the evolution of the Indian plate before and after its breakup from Gondwana and the Eocene India-Asia collision.