Arc magmatic activity of Qingbaikou Period along the southeastern margin of Yangtze Block: Implications from the zircon U-Pb age and geochemical characteristics of Nb-enriched gabbro and high-Mg diorite in the Jinhua plutonic complex

In order to probe into the Neoproterozoic magmatic activity along the southeastern margin of Yangtze Block, the authors conducted petrographic, geochemical and chronologic studies of the first-discovered Nb-enriched gabbros (NEGs) and high-Mg diorites (HMDs) in the Jinhua plutonic complex, Zhejiang Province. The NEGs show relatively high P2O5, TiO2 and Nb values with (La/Nb)N less than 1.3, similar to features of Archean and Palaeozoic Nb-enriched basalts. The HMDs have relatively higher MgO content with Mg# higher than 62.5 and lower TiO2 content, and are enriched in Cr but depleted in Sr, Y and HREE, consistent with features of Miocene high-Mg andesites of Setouchi volcanic belt in Japan and Piip-type HMAs in west Aleutian. The enrichment of LILE (e.g., Rb, Ba, K and Pb) and depletion of HFSE (e.g., Nb, Ta and Ti) with data εNd(t)>+2.92 and ISr=0.70308~0.70474, probably indicate that the NEGs and the HMDs were formed by partial melting of mantle wedge peridotite with slab fluid metasomatism. The HMD yielded a weighted mean 206Pb/238U age of 848±10Ma (measured by LA-ICP-MS). Based on the regional geology and previous studies, the authors hold that obvious island arc magmatic activity still existed along the southeast margin of Yangtze Block at Ca.848Ma, and hence the Precambrian geotectonic evolution history of south China should be reconsidered.