Tectonic affinity and timing of two types of amphibolites within the Qinling Group, north Qinling orogenic belt
秦岭岩群中两类斜长角闪岩的性质和时代及其地质意义

There are two types of amphibolites within the Qinling Group, north Qinling orogenic belt. One is layer-liked or blocked amphibolites involved in marbles, another is the dikes of amphibolites intruding into mica-schist, indicative of significant differences of petrogenesis and time between two types of amphibolites. Geochemistry shows that the protolithes of two types of amphibolites are basaltic. The dikes of amphibolites were formed in a tectonic setting of the within-plate, and isotopes of strontium and neodymium indicate that magma of those dikes sourced from depleted mantle. SHRIMP U-Pb analyses for zircons show that the dikes were formed in the Late Ordovician, ca.449±11Ma, and 40Ar/39Ar dating results reveral that those dikes underwent amphibolite facies metamorphism at the end of the Carboniferous, ca.301.3±6.4Ma. In comparison, layer-liked or blocked amphibolites involved in marbles of the Qinling Group show the geochemical and isotopic affinity with oceanic island basalts, or seamount basalts. 40Ar/39Ar dating results show that this type of amphibolite underwent amphibolite facies metamorphism in the Late Permian, ca.258.1±5.7Ma, later about 40Ma than the dikes. Difference of tectonic affinities and timings, and similar metamorphism and ages suggest that two types of amphibolites in the Qinling Group are belonged to different tectonic blocks, and then assembled together to constructed a high-graded metamorphic terrane, that is the Qinling Group. In conclusion, this paper suggests that the dikes of amphibolites intruding into the mica-schist were products of crust extension or lithospheric delamination and thinning triggered by thermal contraction during the post Caledonian orogen in the north Qinling orogenic belt. While layer-liked or blocked amphibolite involved in marbles might be the assemblages of basic extrusive rocks and carbonate cap of the oceanic rises, such as the OIBs and seamounts, most likely. Those oceanic rises were allochthonous in the Middle Permian.