矿床地球化学国家重点实验室知识库

Tectonic affinity of Grenvillian magmatism in the South China Block is important for understanding the role of the block in the reconstruction of the Rodinia supercontinent during the Mesoproterozoic. In the southwestern Yangtze Block, South China, late Mesoproterozoic metabasalts occur as layers in the Pudeng Formation of the Julin Group. Most metabasalts are highly metamorphosed to be amphibolites composed of dominantly plagioclase and amphibole with minor Fe–Ti oxides, titanite and apatite. Two metabasalt samples have identical zircon U–Pb ages of ∼1050 Ma. These basalts are tholeiitic in compositions and have high and variable TiO2 (1.1–3.2 wt.%) and low Mg# values (=Mg/(Mg + Fe2+); 35–60). In the primitive mantle-normalized diagram, they are characterized by enrichments in Th, Nb, Ta and light REE without significant Zr–Hf–Nb–Ta–Ti anomalies. The samples have εNd(t) values ranging from −1.4 to +4.0 and their zircon grains have εHf(t) values from −1.7 to +11.0. The low Mg# values and Cr (55.1–261 ppm) and Ni (13.5–76.5 ppm) contents suggest fractional crystallization of olivine and/or clinopyroxene, whereas minor negative Eu anomalies indicates slightly fractional crystallization of plagioclase. High positive εNd(t) and εHf(t) values suggest derivation ofthe parental magma from a depleted asthenospheric mantle source. On the other hand, variable εNd(t) and εHf(t) values and La/Sm and Nb/La ratios are consistent with significant crustal assimilation by the Paleoproterozoic rocks in the region. These samples have high Ti and Ti/V ratios (mostly >40), typical of within-plate basalts. In the Zr–Nb–Y and Th–Ta–Hf diagrams for tectonic discrimination,the samples with least crustal contamination are plotted in the ‘within-plate’ and ‘E-MORB’ fields, indicating an intra-continental rifting setting. In view of subsequent Neoproterozoic arc magmatic activities (950–730 Ma) in the region, we suggest that these basalts were likely produced at a rifting basin in a passive margin. Results presented here demonstrate that the Grenville-age magmatism in the southwestern Yangtze Block was not orogenic in origin, thus arguing against the existence of Grenvillian Orogen in the region that was traditionally used to place the South China Block in the central part of the Rodinia supercontinent.