The Middle Triassic Meiwu Batholith, West Qinling, Central China: Implications for the Evolution of Compositional Diversity in a Composite Batholith

Abstract

An integrated study involving whole-rock and Sr–Nd–Hf isotope geochemistry and zircon geochronology and trace element combined with detailed field investigation was carried out for the composite Meiwu batholith in the West Qinling orogenic belt of central China to probe the origins of its compositional diversity and its emplacement history. The batholith is composed of quartz diorite, granodiorite and biotite granite, with abundant mafic magmatic enclaves and minor tonalitic enclaves in the granodiorite. The crystallization age of the batholith is ∼240–245 Ma. Geochemical and Sr–Nd–Hf isotopic data indicate that the magmas that formed the quartz diorite and the mafic enclaves were derived by partial melting of enriched lithosphere mantle, followed by variable degrees of hybridization with crustal magmas in deep crustal hot zones. These initially heterogeneous, hybrid magmas successively intruded into the upper crust and coalesced into a large magma chamber. Zircon trace element and Hf isotopic compositions suggest that the outer fine-grained part of the quartz diorite pluton crystallized from a less differentiated magma as a result of rapid cooling and thus preserved its initial heterogeneities, whereas the inner medium-grained part of the quartz diorite pluton crystallized from a convecting, isotopically homogeneous magma that had undergone advanced magmatic differentiation. The Sr–Nd–Hf isotope compositions of the mafic magmatic enclaves are strikingly similar to those of the host granodiorite, implying their isotopic equilibration. The tonalitic enclaves have high Sr/Y ratios and most probably represent magmas derived from partial melting of thickened mafic lower crust. The Sr–Nd–Hf isotope data suggest that the granodiorite and biotite granite were dominantly derived from isotopically heterogeneous crustal sources. However, the granodiorite also has relatively high Mg#, Cr, and Ni, indicating a minor contribution from a mantle source. The granodiorite was constructed incrementally from a number of discrete melt batches that were generated by partial melting of mafic lower crust under variable water fugacity. These melt batches did not assemble into a large magma chamber and thus preserved their source chemical features. The granodiorite magma was also replenished by mafic and high Sr/Y magmas, resulting in abundant and compositionally diverse magmatic enclaves. The biotite granite formed by the successive accumulation of discrete magma pulses generated by dehydration melting of mafic lower crust under water-absent conditions. These magma pulses coalesced into a small single magma chamber where they underwent fractional crystallization. The various rock types exhibit distinct geochemical variations, indicating that the Meiwu batholith was constructed from multiple injections of magma over a protracted period. Fractional crystallization, assimilation, magma mixing and/or mingling occurred during magma ascent and at the emplacement level. Distinct magma sources played a primary role in controlling the chemical diversity of the igneous bodies at pluton and batholith scale.