Heterogeneous Conservation of Zircon Xenocrysts in Late Jurassic Granitic Intrusions within the Neoproterozoic Jiuling Batholith, South China: a Magma Chamber Growth Model in Deep Crustal Hot Zones

Southern China experienced extensive, episodic, Mesozoic granitic magmatism and W-Sn polymetallic mineralization, making this area an ideal natural laboratory for the study of granitic magmatism and related metallogenesis. The c. 2500 km(2) Neoproterozoic Jiuling batholith in southern China hosts Mesozoic intrusions, although the genetic relationship between these two episodes of granitic magmatism within the batholith remains unclear. Here, we present new petrographic, mineralogical, geochronological, and geochemical data for the Mesozoic Shiqiao intrusion and the Ganfang (GF)-Guyangzhai (GYZ) intrusive suite, and use these data to determine the petrogenetic history of the intrusions and assess whether they are genetically related to the Neoproterozoic host batholith. The Shiqiao intrusion crystallized at 148-147 Ma and is dominated by a granodiorite phase and contains scarce old zircons or zircon cores. The GF-GYZ intrusive suite is composed of the Ganfang and Guyangzhai intrusions, which crystallized at 147-144 and 147-146 Ma, respectively. They are dominated by two-mica and muscovite granite phases and contain abundant old (i.e. Neoproterozoic) zircon xenoliths or zircon cores. Numerous magmatic zircons within the GF-GYZ intrusive suite contain homogeneous dark domains visible under cathodoluminescence (CL) imaging, suggesting that they were generated during magmatic fluid-enriched later stages of magmatism. The Shiqiao granitoids define a distinct fractional crystallization trend from that of the GF-GYZ intrusive suite granitoids, suggesting different magmatic evolution. Simulations using the Rhyolite-MELTS and energy-constrained assimilation-fractional crystallization (EC-AFC) programs indicate fractional crystallization and concurrent assimilation during the formation of granites in the GF-GYZ intrusive suite. However, the first-stage Ganfang intrusion underwent different fractional crystallization and assimilation processes from the second-stage Guyangzhai intrusion, although they could be highly differentiated and possibly originate from the same source. Such highly differentiated residual magmas could be enriched in fluids and generate the dark-CL zircon domains in the granites. These processes lead to a model of magma chamber growth that can be used to explain the geochemical and petrographic variations of granitic plutons worldwide. In addition, various geochemical indicators of granite-related mineralization, the highly differentiated magmatic compositions and the high metallic element concentration of the sources all favor the formation of W-Sn mineralization, as evidenced by the presence of the world-class Dahutang W deposit within the northeastern part of the Neoproterozoic Jiuling batholith.