LA-ICP-MS zircon U-Pb age and geochemical signatures of the amphibolites in the Changning-Menglian suture zone and its geological implications
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摘要: 昌宁-孟连结合带牛井山地区发育以构造岩片和透镜体形式产出的斜长角闪岩，其对于认识和恢复昌宁-孟连结合带特提斯演化历史具有重要意义。本研究对牛井山蛇绿混杂岩带内的斜长角闪岩进行了系统的岩石学、锆石U-Pb定年、Hf同位素及全岩地球化学研究。锆石CL图像揭示斜长角闪岩锆石为岩浆成因锆石。锆石U-Pb定年结果为272±1.2 Ma（MSWD=1.1，n=21），代表斜长角闪岩原岩时代。岩石地球化学分析表明，斜长角闪岩SiO2含量为51.83%~52.6%，全碱（Na2O+K2O）含量为3.33%~4.16%，Na2O/K2O比值为5.8~19.8，属于低钾拉斑玄武岩系列。微量元素结果表明斜长角闪岩具有N-MORB的地球化学特征。原岩恢复研究揭示斜长角闪岩的原岩为272±1.2 Ma的N-MORB型辉长岩。斜长角闪岩锆石初始（176Hf/177Hf）i值为0.282906~0.282956，对应的εHf（t）为10.7~12.5；单阶段亏损地幔Hf模式年龄tDM1为416~499 Ma （平均值为466Ma），明显老于原岩结晶时代。岩石地球化学特征和锆石Hf模式年龄揭示昌宁-孟连特提斯洋在272 Ma时具有一个长期亏损的地幔，地幔年龄为早古生代416~499 Ma。结合该带存在早古生代洋壳残余及洋壳俯冲成因埃达克岩的事实，我们认为昌宁-孟连带是一个连续演化的原-古特提斯洋，晚古生代272 Ma时还存在洋中脊扩张并产生具有N-MORB性质的洋壳。Abstract: Some outcrops of amphibolites occur as tectonic slices and lenses in the Niujingshan area,and play an important role in deciphering the tectonic evolution of the Changning-Menglian Tethyan ocean in southwestern China. The petrological,whole-rock geochemical and zircon U-Pb and Hf isotopic data for the amphibolites from the Niujingshan area in the Changning-Menglian suture zone are reported in details in the present paper. The cathodoluminescence (CL) images reveal that the zircons extracted from the amphibolites are of magmatic origin. The LA-ICP-MS zircon U-Pb dating results for the amphibolites yield a weighted mean 206Pb/238U age of 272±1.2 Ma (MSWD=1.1,n=21),representing crystallization age of the protoliths of the amphibolites. The geochemical analyses show that the amphibolites have SiO2 contents ranging from 51.83% to 52.6%,relatively low total alkali (Na2O+K2O)contents ranging from 3.33% to 4.16%,and high Na2O/K2O ratios of 5.8-19.8. Major elements show the low-K tholeiitic magma series of the amphibolites,while the trace elements show the typical features of normal mid-oceanic ridge basalts (N-MORB)of the amphibolites. The protolith restoration study for the amphibolites indicates that the parent rocks of the amphibolites are gabbro with N-MORB affinity. The initial Hf isotope ratios (176Hf/177Hf)i of the zircons extracted from the amphibolites range from 0.282906 to 0.282956, with corresponding εHF(t)values of 10.7-12.5. These zircons have depleted mantle Hf model ages (tDMI)of 416-499 Ma,with an average of 466 Ma,obviously older than the timing (272 Ma)of the zircon growth. It is suggested from the whole-rock geochemical signatures and zircon U-Pb and Hf isotopic data that the Changning-Menglian Tethyan ocean has a long-term depleted mantle with the ages of 416-499 Ma during the Late Palaeozoic (272 Ma). In the light of the comprehensive study mentioned above and the presence of the Early Palaeozoic oceanic crust remnants and oceanic subduction-induced high-Mg adakites,we propose that the Changning-Menglian ocean is a Proto- and Palaeo-Tethyan ocean with a long and complicated evolutionary history,and the mid-oceanic ridge spreading once took place and resulted in the formation of N-MORB oceanic crust during the Late Palaeozoic (272 Ma).
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