Origin, age and tectonic implications of metamafic rocks in the Kuanping Group of the Qinling orogenic belt

Geochemical analysis shows that: metabasic volcanic rocks (greenschist) and amphibolite in the Kuanping Group both have lower SiO2 (<53%) and MgO (Mg#=29-49) and their TiO2 contents are 0.87-2.03% and 0.92-2.12% respectively, indicating that their protoliths are basaltic rocks of the subalkaline .tholeiitic series. The chondrite-normalized REE patterns of greenschist and amphibolite in the Kuanping Group show no pronounced differentiation between LREE and HREE ((La/YbN)=0.82-1.42), with LREE showing depletion or slight enrichment. The REE patterns are flat, similar to the N-MORB-normalized patterns. The two types of metamafic rocks are slightly enriched in large-ion lithophile elements (LILE) (e.g., K, Rb, and Ba) and neither depleted nor enriched in high-field strength elements (HFSE) (e.g., Nb, Ta, Zr and Ti), indicative of a strong geochemical affinity to E-MORB. The two types of mafic rocks have relatively dispersed initial Sr ratios, but their initial Nd ratios (143Nd/144Nd)(t)) are relatively concentrated and homogeneous, being 0.511962 to 0.512192 and 0.512028 to 0.512157 respectively ? Nd(t) values of the two types of mafic rocks are both positive, +5.7 - +10.2 and +7.0 - +9.5 respectively, indicating that they were sourced from source regions slightly depleted in LREE and LILE. SHRIMP U-Pb zircon age analysis reveals that greenschist in the Kuanping Group formed in the late Neoproterozoic Ediacaran, at ~611±13 Ma. The older relict core zircons are most likely to be the record of the older crust component captured during magmatism. 40Ar/39Ar thermochronological analysis of biotite reveals that the protoliths of the Kuanping Group were metamorphosed in the Carboniferous Serpukhovian, at ~319.1±3.6 Ma. Conclusively, all those lines of evidence show that the two types of metamafic blocks in the Kuanping Group formed in a late Neoproterozoic passive continental-margin rift ocean basin and were derived from the rifting of the south passive continental margin of the North China block.