Petrogenesis and Geotectonic Significance of Early-Neoproterzoic Olivine-Gabbro within the Yangtze Craton: Constrains from the Mineral Composition, U-Pb Age and Hf Isotopes of Zircons

The olivine-gabbroic rocks located at the Huangling anticline within the Yangtze Craton are dated at circa 857-854 Ma by LA-ICP-MS method. The rocks belong to the sub-alkaline series and consist of pyroxene (35%-40%), plagioclase (40%-45%), olivine (8%-10%) and spinel (3%-5%). Olivine has Fo values of 73-83 that is classified as chrysolite. Pyroxene has relative low contents of FeO (6.60 wt.%-8.23 wt.%) but high CaO (20.23 wt.%-21.25 wt.%) contents, however, plagioclase has high Al2O3 (31.78 wt.%-32.37 wt.%), CaO (16.08 wt.%-16.25 wt.%) and An (79-80) values, but low Na2O contents (1.95 wt.%-2.11 wt.%). Spinel are magnesioferrite with characteristics of high contents of MgO (13.65 wt.%-13.68 wt.%), FeO (23.27 wt.%-23.40 wt.%) and Al2O3 (62.43 wt.%-62.74 wt.%). Chemical compositions of these minerals are similar to those of gabbro rocks that were formed in the post-orogeny environment. The olivine-gabbro samples have negative zircon epsilon (Hf) values (-16.57 +/- 0.47) that resemble the mafic rocks in the same region, indicating that they are derived from the extremely enriched mantle source. On the compilation of documented Neoproterozoic mafic rocks in the Yangtze Craton, it is proposed that the mantle in the northern Yangtze Craton has experienced different degrees enrichment during the Neoproterozoic.