LA-ICP-MS zircon U-Pb age and geochemical signatures of the amphibolites in the Changning-Menglian suture zone and its geological implications

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 ε_HF(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).