Compositional changes with incremental growth of the Quxu granite batholith, southern Tibet: Evidence from geochronology and geochemistry

Abstract

How to reconstruct the detailed processes and identify the key factors of incremental growth to form a large granite batholith is controversial. Such in-depth investigations along convergent plate boundaries are of great importance for better understanding the growth and reworking of continental crust. This work presents an integrated study of SIMS (secondary ion microscope) U-Pb geochronology, trace elements and oxygen isotopes of zircon, microstructure and chemistry of plagioclase, and whole-rock elemental geochemistry and Sr-Nd isotopes to decipher the temporal compositional change and incremental growth of the Quxu granitoid batholith in the Gangdese magmatic belt, southern Tibet. The Quxu host granitoids are mainly metaluminous and calc-alkaline to high-K calc-alkaline in geochemistry, whilst the melanocratic microgranular enclaves (MMEs) and mafic dykes vary from calc-alkaline to shoshonitic series. New SIMS zircon U-Pb data suggest a prolonged history (ca. 16.6 Myr) for the Quxu batholith accompanied by several episodes of intermediate–mafic magma replenishments. Zircon δ18O and whole-rock (87Sr/86Sr)i values fluctuate from ∼5.6‰ to ∼6.8‰ and from 0.7038 to 0.7085, respectively, while the whole-rock εNd(t) values show a fluctuated decrease from +5.2 to −4.5 with time. The calculated Ti-in-zircon temperature (574–903 °C) and oxygen fugacity (ΔFMQ –0.9 to ΔFMQ +3.2) of the granitoid magmas fluctuated as well. The youngest group of the host granitoids is distinguished by the most elevated Th/La (>1.5) and Th/Ce (>1.0) ratios and K2O contents (>3.0 wt%) with evolved isotopic compositions, implying pronounced incorporation of supracrustal materials into the source region. Microstructural and compositional features of zircon and plagioclase from the host granitoids indicate that the incremental assembly of Quxu batholith was foremost facilitated by multiple episodes of magma recharge with distinct compositions and sources. The temporal variations of magma condition and composition throughout the lifetime of the magma reservoir highlight the importance of determining the longevity of batholiths and clarifying the discrepancy of individual magma pulses before deciphering the genesis of composite batholith. The aforementioned Sr–Nd–O isotopes show good correlations with the fluctuating whole-rock (La/Yb)N (2.7–68.5) and zircon Eu/Eu* (0.02–1.26) ratios, which further affirms episodic syn-collisional crustal thickening and episodic input of supracrustal material in magma sources. Based on the spatiotemporal variations of geochemistry, we propose a zoned incremental growth model to better illuminate the detailed processes of the incremental amalgamation of the Quxu batholith.