Diagenetic and Metallogenic Responses to the Late Paleoproterozoic Breakup of the Columbia Supercontinent on the Western Margin of the Yangtze Block, SW China

The mafic intrusive rock groups (MIRGs) of diabase gabbro are found extensively in Dongchuan City, Yunnan Province, which acts as the western margin of the Yangtze Block, SW China. There is still some doubt about the genetic relationship between the diagenesis of mafic intrusive rocks and Cu-(Pb-Zn) polymetallic mineralization. In this study, petrology, petrogeochemistry, isotope geochemistry, and geochronology were employed to address this issue. The results show that the MIRGs in the Dongchuan district exhibit zircon U-Pb ages of 1767.3 +/- 8.5 Ma (MSWD = 0.87, n = 15), 1740 +/- 22 Ma (MSWD = 0.82, n = 10), and 1719 +/- 13 Ma (MSWD = 0.16, n = 21), indicating that they were emplaced during the late Paleoproterozoic. The MIRGs manifest high MgO contents (6.16 %-9.02 %, mean 6.97 %), high TiO2 contents (2.04 %-3.26 %, mean 2.44 %), and alkali enrichment (2.64 %-4.82 %, mean 3.95 %), suggesting the geochemical characteristics of slightly alkaline sodic high-titanium basalts. The MIRGs are rich in elements with large-ion lithophile elements (LILEs, such as K and Rb) and are not significantly depleted in high field strength elements (HFSEs, such as Ta, Nb, Zr, and Hf). Furthermore, the MIRGs show similar distribution patterns of trace and rare earth elements (REEs) to typical ocean island basalts (OIBs), suggesting that they were formed in an extensional tectonic setting. In-situ Lu-Hf isotopes of zircon indicate that the primitive magmas of the MIRGs resulted from the high-degree partial melting (about 20 %-25 %) of the primitive mantle. The sources of the MIRGs are considered the middle Paleoproterozoic depleted mantle or newly young crust accreted from the depleted mantle. Furthermore, the ages of the MIRGs agree well with those of significant non-orogenic magmatic activity on the western margin of the Yangtze Block and the breakup of the global Columbia supercontinent during the late Paleoproterozoic. Interestingly, the emplacement ages of the MIRGs coincide with the metallogenic ages (from 1760.2 +/- 51.6 Ma to 1806.8 +/- 63.9 Ma) of Cu polymetallic deposits in the area, implying that the large-scale magmatic activity during the late Paleoproterozoic might have provided critical ore-forming materials for Cu polymetallic mineralization. Furthermore, this study suggests a reclassification of local strata. In other words, the former Dongchuan Group in the LuoxueYinmin tectonic belt in the Dongchuan ore district should be reorganized into the Mesoarchean Yuanjiang Group, the Neoarchean Puduhe Group, and the Paleoproterozoic Yimen Group. Its strata may constitute the paleo-basement that predated the extension of the Kunyang intracontinental rift.