Zircon geochronology and Hf isotopic composition of Mesozoic magmatic rocks from Chizhou, the Lower Yangtze Region: Constraints on their relationship with Cu-Au mineralization

摘要

Z.rcon U-Pb ages and Hf isotop.c compositions of Mesozoic magmatic rocks from the Chizhou Area are systematically investigated to reveal the tectonic setting of magmatism and their relationship with Cu-Au min-erahzation in the Lower Yangtze River Belt southeastern China. The samples cover nearly all types of magmatic rocks in a 30x50 km~2 reg.on including 6 granite porphyries, 6 dacites and 4 granites. The zircon U-Pb geochronology yields a range of 151-124 Ma with granite porphyries rangi from 151 to 146 Ma dacites from 132 to 127 Ma and granites from 127 to 124 Ma, indicating two magmatic episodes of the late Jurassic and the early Cretaceous The earlier episode mainly formed small granite porphyries (generally <5 km) and is always associated with porphyry Cu-Au deposits. The later episode began with dacites and was then dominated by large granite intrusions (generally >10 km), which are barren in mineralization. The ore-barren dactes and the granites (131-124 Ma) are poor in inherited zircons. Zircons in these rocks yield a very large ε_(Hf) (t) variation of -20.8-0.4, suggesting a mixing between mantle-derived and crustal-denved magmas. By contrast, the ore-bearing porphyries (151-146 Ma) are rich in inherited zircons. The magmatic zircons have ε_(Hf)(t) values of -8.8-0.9, and the inherited ones yield U-Pb ages of 1156-811 Ma with ε_(Hf)(t) values of 2.5-11.5 The existence of quantitative inherited zircons indicates that the crustal rocks of 1156-811 Ma significantly contribute to the formation of the ore-bearing porphyries either being source or contamination. Since these inherited zircons are igneous as indicated by their oscillatory zonings. they may derive from components of the Grenvillian oceanic crust (ca. 1100-1000 Ma) i.e the Neoproterozoic magmatic rocks related to arc (970-890 Ma) and Nanhua rift (ca. 825 Ma). Recent studies reveal that the ore-baring porphyries of the Lower Yangtze River Belt have slab melt features and conclude that they could derive from partial melting of the Pacific oceanic crust. Our results provide another possibility for the origin of the ore-bearing porphyries: partial melting of Neoproterozoic crustal rocks that contain the Grenvillian oceanic crust fragment beneath the Yangtze Block. Such a new model can well explain the observations that are difficult to be explained by other models: e.g., the slab melt features with enriched Sr-Nd iso-topic composition of the ore-bearing porphyries, the west-east distribution of the Lower Yangtze River Belt.