Reworked restite enclave: Petrographic and mineralogical constraints from the Tongshanling intrusion, Nanling Range, South China
Résumé
Microgranular enclaves, which can provide important petrogenetic indications for the host granitoids, are commonly observed in the Middle-Late Jurassic Cu-Pb-Zn-bearing granodiorites in the Nanling Range of South China. The origin of these Cu-Pb-Zn-bearing granodiorites is still controversial, with two different hypotheses: crust-mantle mixing and partial melting of the mafic lower crust, primarily based on geochemistry and geochronology. Detailed petrographic and mineralogical studies have been carried out on the Tongshanling granodiorite and its microgranular enclaves to provide new textural and compositional constraints on the petrogenesis. The microgranular enclaves have dioritic compositions with a mineralogy dominated by plagioclase, amphibole, and biotite. Abundant residual materials, such as mafic mineral clots, inherited and metamorphic zircon, and Ca-rich core plagioclase, occur in the enclaves and are in favor of a restite origin. The amphibole-rich clots are considered as vestiges of residual pyroxene-rich precursors from the source. Three different types of amphibole, i.e., magmatic, metamorphic, and magma reworked metamorphic amphibole, have been recognized in the granodiorite and its enclaves. The magmatic amphibole occurs as enclosed and idiomorphic isolated crystals in the granodiorite with Al and Si contents of 1.34–2.12 apfu (atoms per formula unit) and 6.25–6.88 apfu, respectively, and a ΣREE content of 307–764 ppm. The metamorphic amphibole, with a granoblastic triple-junction texture, occurs as aggregated crystals dominantly in the enclaves and a few in the granodiorite. It has actinolitic compositions (Al: 0.31–0.81 apfu, Si: 7.33–7.72 apfu) and distinctly lower contents of incompatible elements (ΣREE: 99–146 ppm). The magma reworked amphibole has intermediate compositions (Al: 0.81–1.59 apfu, Si: 6.71–7.35 apfu, ΣREE: 317–549 ppm) between the magmatic and metamorphic amphibole. The zonal amphibole-rich clots exhibit increasing Al and decreasing Si contents from interior amphibole to exterior amphibole and also from core to rim in amphibole grains, and the outer parts of enclave magmatic zircon have higher ThO2 + UO2 contents and lower Zr/Hf ratios than the inner parts, showing the process of magma reworking of the restite enclaves. Based on the textural and compositional evidence, these microgranular enclaves are thought to be reworked restite enclaves. This is also supported by thermobarometric calculation. A model illustrating the formation process of reworked restite enclave has been proposed. Combined with previous experimental studies of partial melting, the Tongshanling granodiorite is deduced to be derived from dehydration melting of amphibolite in the mafic lower crust. The fertile amphibolitic source is beneficial to the formation of Cu-Pb-Zn-bearing granodiorites in the Nanling Range.
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