High Sr/Y Rocks Formed by the Fractional Crystallization of Water-Rich Magma: An Example from the Enclave-Bearing Suanjingzi Batholith in the Beishan Orogen, Nw China

Abstract

The petrogenesis of intermediate-silicic rocks with high Sr/Y ratios that are closely related to porphyry Cu-Au mineralization and TTGs is still uncertain. In this paper, we present new geochronological and geochemical data for granodiorites with high Sr/Y ratios (ranging from 30 to 157) and associated mafic microgranular enclaves (MMEs) from the Suanjingzi batholith in the Beishan orogen, NW China. The MMEs are dioritic and fine-grained, with mineral assemblages similar to those of their hosts, but they contain more amphibole. The similarities in ages (359 to 358 Ma) and Sr-Nd-O isotopes (87Sr/86Sr(i) = 0.7045 to 0.7057, εNd(t) = -0.2 to 0.9, and δ18O = 4.8 to 7.1 ‰) between the host granodiorites and MMEs indicate that they share a common magma source. Our geochemical studies and quantitative modelling imply that the upper crustal fractional crystallization of water-rich (H2O > 4 wt%) magma, represented by the associated MMEs, played a key role in the formation of the Suanjingzi and contemporaneous high Sr/Y intrusive rocks in the central Beishan. High H2O content promotes amphibole crystallization and delays plagioclase appearance during magma differentiation. The MMEs, which are the best available proxy for the parental magma of the Suanjingzi granitoids, exhibit high MgO (4.35–5.04 wt%) contents, with Mg# values (54–61), Cr contents (171–201 ppm), and Ni contents (57–93 ppm), are enriched in large ion lithophile elements, and are depleted in high field strength elements, suggesting that they were derived from partial melting of a metasomatized mantle wedge. This study emphasizes the role of fractional crystallization of water-rich magma in forming high Sr/Y intermediate-silicic rocks, implying that Sr/Y ratios cannot be used to quantify crustal thickness without careful consideration. Furthermore, we argue that Late Devonian–early Carboniferous magmatism in the central Beishan occurred in a subduction tectonic setting, which provides a stronger basis for the Palaeozoic tectonic evolution of the Beishan orogen.

Keywords: High Sr/Y, Water-rich magma, Fractional crystallization, Beishan, Central Asian Orogenic Belt