Magma Recharge and Reactive Bulk Assimilation in Enclave-Bearing Granitoids, Tonglu, South China
Authors
Magmatic processes leading to granitoid formation are of relevance to the evolution of continental crust and its mineralization. A comprehensive study of field observations with whole-rock and in situ mineral chemical and isotopic compositions was carried out on granitoids, mafic microgranular enclaves (MME) and country-rock xenoliths (CRX) from Tonglu, South China to constrain the magmatic processes operating. Zircon U–Pb geochronology indicates that the MME and granitoids formed coevally at _130 Ma. Petrographic observations suggest that the MME are quenched mafic clots formed during incomplete magma mixing. The different zircon Hf isotopic compositions of the MME (eHf(t).-4_0 to -0_7) and the host granitoids (eHf(t).-8_1 to -1_7) indicate mingling between mafic and felsic magmas from different sources. The CRX are composed of fresh cores and darkrims. The pyroxene-rich fresh cores are depleted in Rb, Ba and K while the biotite-rich dark rims show obvious enrichments in Rb, Ba and K, indicating modification by hydrous K-rich felsic melts or fluids from the host magma. In contrast, some large CRX have embayed structures and are surrounded by several small, biotite-rich CRX, suggesting disaggregation and modification of large CRX into the host magma. The occurrence of abundant felsic magmatic veinlets in the CRX implies that they could have experienced brittle deformation in the cold shallow crust, which agrees with an emplacement depth of about 5 km estimated using Al-in hornblende geobarometry. The high 87Sr/86Sri (0_7129) and low eNd(t) (-10_2) values imply that these CRX were derived from the upper continental crust. All these features suggest a typical reactive bulk assimilation process. Microanalysis of 87Sr/86Sr ratios in plagioclase from the Tonglu granitoids vary over a large range (0_7073–0_7137) with complex rim-core-rim variations, which resulted from open-system processes. Given the variation in Sr isotopes, four types of plagioclase were identified. Type I plagioclase are homogeneous in terms of 87Sr/86Sr, suggesting normal crystal fractionation. Recharge of mafic magma injecting into felsic magma resulted in the core–mantle variations of type II plagioclases. Albitic cores with high 87Sr/86Sr (up to 0_7092) indicate felsic magma with highly radiogenic Sr (87Sr/86Sr>0_7092). Influx of mafic magma with less radiogenic Sr (87Sr/86Srdecrease in 87Sr/86Sr and an abrupt increase of An contents (_An30 to _An60) from core to mantle. Type III plagioclase are distinguished by lower 87Sr/86Sr ratios in the core (0_7084–0_.7086) and significantly more radiogenic Sr at the rim (0_7097–0_7112), which is attributed to the assimilation of the country rocks. Core-mantle-rim variations in type IV plagioclase not only record magma recharge events, but also crustal assimilation. Increasing An values and decreasing 87Sr/86Sr ratios (down to 0_7075) from core to mantle and significantly more radiogenic Sr (up to 0_7117) in the outer rim suggest that the recharge event took place prior to the assimilation of ancient crustal components. Recharge of hot mafic magma into a pre-existing magma chamber will not only increase the temperature of the magma in the chamber, but may also induce intense shattering of the brittle country-rocks, both of which could have enhanced the process of reactive bulk assimilation.
Fig. 1. (a) The location of the South China Block; (b) Simplified geological map of South China showing the distribution of Mesozoic granitoids and volcanic rocks (modified after Zhou et al. (2006) and Yang et al. (2012)); (c) Simplified geological map of the Tonglu granitoids (modified after the Geological map of Jiande sheet 1: 200 000).
