Chronology, Geochemistry and Geological Significance of Haizi Bimodal Intrusive Bodies in Wuding District, Central Yunnan

Abstract

The Haizi diabase-granite porphyry composite rock mass is located in the Wuding-Yanjing taphrogenic trough within the Paleoproterozoic Kangdian fault-uplift zone. According to field observations, the diabase is divided into two types: central facies and marginal facies; the granite porphyry directly contacts the central facies of diabase without transitional intermediate rock. In order to disclose its geological and petrological significances, this paper carried out the LA-ICP-MS zircon U-Pb dating and geochemical investigation on both masses of Haizi diabase and granite porphyry. The results showed that the 207 Pb/ 206 Pb weighted average age of granite porphyry is 1764 ± 18 Ma and diabase 1765 ± 5.4 Ma. The geochemical investigations revealed that the Haizi diabase is a kind of high-potassium alkaline basalt and the granite porphyry a kind of low-potassium rhyolite, both enriched with high field intensity elements and depleted in large-ion lithophile elements; there is obvious component intermittence between the diabase and the granite porphyry, both being bimodal and showing characteristics of continental rift valley magmatite. The in situ Hf isotope of dating zircon showed that mantle materials participated in the formation and emplacement of Haizi granite porphyry and crust materials participated in the formation of diabase. This meant that crustal remelting and accretion of new crust occurred in this region in the 1.7 Ga period. It can be therefore inferred that the Haizi bimodal intrusive rock came into being due to the breakup of Kunyang rift valley and rapid ascent of mantle materials in the 1.7 Ga period, which echoed the global Columbia supercontinent breakup and was the first petrological record of that breakup found at the southwest margin of Yangtze platform.