沉积与特提斯地质2017，Vol.37Issue(4)：17-28,12.
昌宁-孟连结合带斜长角闪岩锆石U-Pb年龄、 地球化学特征及其地质意义
LA-ICP-MS zircon U-Pb age and geochemical signatures of the amphibolites in the Changning-Menglian suture zone and its geological implications
摘要
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 206 Pb / 238 U 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 (Na 2 O + K2 O)contents ranging from 3. 33% to 4. 16%,and high Na 2 O/ K2 O 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 (176 Hf/ 177 Hf) 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 (t DMI )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).关键词
斜长角闪岩/锆石U-Pb定年/原-古特提斯洋/昌宁-孟连结合带Key words
amphibolite/zircon U-Pb dating/Proto- and Palaeo-Tethyan ocean/Changning-Menglian suture zone分类
天文与地球科学引用本文复制引用
王冬兵,罗亮,唐渊,尹福光,王保弟,王立全..昌宁-孟连结合带斜长角闪岩锆石U-Pb年龄、 地球化学特征及其地质意义[J].沉积与特提斯地质,2017,37(4):17-28,12.基金项目
西南三江有色金属资源基地调查项目(编号:DD20160016)、国家自然科学基金项目(编号:41302157)资助 （编号:DD20160016）
