Geochemical consequences of subduction zone metamorphism: Case studies of metamorphic rocks from Palaeozoic subduction zone complexes in Tianshan and Qilian Orogenic Belts, NW China

Abstract

Subduction-zone metamorphism (SZM) is considered to be a major geochemical process on Earth of both petrological and geodynamic significance that triggers the subduction-zone magmatism and contributes to the mantle compositional heterogeneity. To understand SZM and elemental responses to SZM, detailed petrological and geochemical studies were conducted on metamorphic rocks of basaltic and sedimentary protoliths from two orogenic belts, i.e., Western Tianshan and North Qilian Mountain, in NW China.
Based on the bulk-rock geochemistry of rocks from ultrahigh pressure metamorphic belt of Western Tianshan, different elemental mobility/immobility has been identified using the inter-elemental correlations.
Mineral compositions have also been analyzed for the same rocks. The significant elemental hosts are phengitic muscovite, paragonite, garnet, epidote group minerals, rutile and titanite. Together with detailed petrography and considering a series of plausible metamorphic reactions, we conclude that it is the presence and stability of these minerals that largely controls the geochemical behaviors of chemical elements during SZM.
In terms of both bulk-rock composition and mineral geochemistry for rocks from North Qilian Mountain, we conclude the same except the mobility of U, which may be attributed to the seafloor alteration rather than SZM.
The consistent immobility of U, Th and light rare earth elements (LREEs), like high field strength elements (HFSEs), during SZM indicates that the enrichment of these elements in arc magmas is not caused by simple dehydrated aqueous fluids. Therefore, the traditionally accepted fluid flux induced-melting needs reconsideration in order to explain the arc signature in melts produced through subduction-zone magmatism. In addition, the lack of Rb/Sr-Sm/Nd (or Lu/Hf) correlation in these and other metabasites world-wide is inconsistent with the observed first-order Sr-Nd (or Hf) isotope correlation in oceanic basalts. Hence, the subducted residual ocean crust cannot be the major source materials for oceanic basalts although it can contribute to mantle compositional heterogeneity.