Trace element budgets and (re-)distribution during subduction-zone ultrahigh pressure metamorphism: Evidence from Western Tianshan, China

Yuanyuan Xiao,Yaoling Niu,Huaikun Li,Huichu Wang,Xiaoming Liu,Jon Davidson

doi:10.1016/j.chemgeo.2013.12.005

Abstract

We have conducted an LA-ICP-MS in situ trace element study of garnet, epidote group minerals, phengitic muscovite and paragonite in rocks of basaltic and sedimentary protolith from an ultrahigh pressure metamorphic belt along Western Tianshan, China. The data are used to evaluate the capacity of these minerals for hosting incompatible elements in response to subduction-zone metamorphism (SZM). The results confirm existing studies in that the presence and stability of these minerals largely control the geochemical behaviors of elements during SZM.We found that redistribution of rare earth elements (REEs), Th and U into newly-formed minerals during progressive SZM precludes the release of these elements from the down-going ocean crust, which contradicts the common perception in models of slab-dehydration and flux-melting. This suggests that additional processes, such as the involvement of supercritical fluids or hydrous melts formed at depth are required to supply these elements to the mantle wedge for arc magmatism. In addition, the ready release of large ion lithophile elements (LILEs) by different minerals, and the high immobility of REEs in rocks of basaltic protolith indicate that the contribution of altered ocean crust after SZM may not be responsible for the correlated Sr–Nd (Hf) isotope systematics observed in oceanic basalts. That is, subducted ocean crust that has gone through SZM cannot be the major source material for ocean island basalts.

Highlights

Subduction zones are the most important tectonic environment on Earth in terms of the extent, magnitude, and diversity of mass exchange between the Earth's exterior and the deep mantle
We focus on the distributions of lithophile elements (LILEs) and rare earth elements (REEs) in different mineral assemblages of blueschistto eclogite-facies metamorphic rocks of basaltic and sedimentary protoliths from the ultrahigh pressure (UHP) metamorphic belt of Western Tianshan, China, in order to understand trace element behaviors in response to the mineral stability at different metamorphic stages along a specific P–T path
Epidote group minerals contain more than 95% of LREEs, Th, U, Pb and Sr, around 60% of MREEs and less than 30% of HREEs in the bulk rock

Summary

Introduction

Subduction zones are the most important tectonic environment on Earth in terms of the extent, magnitude, and diversity of mass exchange between the Earth's exterior and the deep mantle. Subduction zones provide a highly selective physical and chemical filter, conceptualized as the “subduction factory” (e.g., Tatsumi and Kogiso, 2003). The main process in the subduction factory is metamorphism. It is the subduction-zone metamorphism (SZM) that is considered to trigger subduction-zone magmatism (e.g., Tatsumi, 1986; Peacock, 1990; McCulloch and Gamble, 1991), while residual material passing through the SZM contributes to mantle chemical and isotopic heterogeneities

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Conclusion