Contrasting geochemical signatures of fluid-absent versus fluid-fluxed melting of muscovite in metasedimentary sources: The Himalayan leucogranites

Abstract

Most of the Himalayan Cenozoic leucogranites are products of partial melting of metapelite sources. In the Malashan-Gyirong area (southern Tibet), the geochemical compositions of leucogranites define two groups with distinct whole-rock major elements, large ion lithophile elements, rare earth elements, high field strength elements, and Sr and Hf isotope ratios. Based on published experimental results that define generalized melting reactions of metapelitic sources, we infer that these leucogranites are the products of two different types of crustal anatexis: fluid-fluxed melting and fluid-absent melting of muscovite in metasedimentary sources. As compared to the leucogranites derived from fluid-absent melting, those from fluid-fluxed melting have relatively higher Ca, Sr, Ba, Zr, Hf, Th, and light rare earth element concentrations, and Zr/Hf, Eu/Eu*, and Nd/Nd*, but lower Rb, Nb, Ta, and U concentrations, Rb/Sr and 87Sr/86Sr ratios, and εHf(t). The geochemical differences can be explained by melting behaviors of major (muscovite, feldspar) and accessory minerals (zircon and monazite) during different modes of crustal anatexis. The systematic elemental and isotopic signatures of different types of crustal anatexis and, in particular, the coupling of major and trace elements that results from common influences on rock-forming and accessory mineral behaviors provide tools with which to refine our understanding of the nature of crustal anatexis.

Tracking the timing and nature of protolith, metamorphism, and partial melting of tourmaline‐bearing migmatites by zircon U–Pb and Hf isotopic compositions in the Yuka terrane, North Qaidam UHP metamorphic belt

“…Partial melting is generally driven by dehydration reactions involving the breakdown of a hydrous phase such as micas (phengite, muscovite, and biotite), epidotes (allanite, epidote, and zoisite), and amphiboles (hornblende, glaucophane, and pargasite; Auzanneau et al, ; Brown, ; Hermann, Spandler, Hack, & Korsakov, ; Irifune, Ringwood, & Hibberson, ; Sawyer, Cesare, & Brown, ) or nominally anhydrous phases such as omphacite and garnet (Chen et al, ). Hydration melting, known as water‐fluxed melting, is also very important for crustal anatexis (Gao & Zeng, ; Gao, Zeng, & Asimow, ; Weinberg & Hasalová, ). Migmatites with leucosomes that formed under P‐T conditions below the hydrate phase solidus have generally been regarded as products of water‐fluxed melting (Berger, Burri, Alt‐Epping, & Engi, ; Hu et al, ; Mogk, ; Sawyer, ; Weinberg & Hasalová, ).…”

Hydrothermal ore deposits in collisional orogens

“…These leucogranites were emplaced in the Late Cenozoic [65] and contemporaneous with adjacent migmatites and granulites [66], indicating their petrogenetic link to rifting orogeny [17]. With respect to the tectonic architecture along the convergent continental boundary between India and Asia [18], the Himalayan orogen is composed of metasedimentary rocks derived from the subducting Indian continent, and the leucogranites herein were derived from partial melting of the metasedimentary rocks [67,68]. Therefore, the extraordinary enrichment of ore-forming elements in the S-type granites has no bearing on the chemical metasomatism of the mantle wedge during the Neo-Tethyan oceanic subduction in the Mesozoic.…”

Migmatites record multiple episodes of crustal anatexis and geochemical differentiation in the Sulu ultrahigh‐pressure metamorphic zone, eastern China

“…The geochemical differences in these melts can be explained by the melting behaviour of major and accessory minerals during different modes of crustal anatexis (e.g. Gao, Zeng, & Asimow, 2017;Knesel & Davidson, 2002;Weinberg & Hasalová, 2015). For example, melts derived from Bt-/Msdehydration reactions have high Rb/Sr ratios, whereas waterfluxed melting produces melts with low Rb/Sr ratios (e.g.…”