Crustal-contaminated komatiitic basalts in Southern China: products of a Proterozoic mantle plume beneath the Yangtze Block

Abstract

Abundant mafic and ultramafic rocks including basalts, komatiitic basalts, and peridotites occur in the Proterozoic Sibao Group, northern Guangxi Province, China. Whereas the basalts are generally pillow lavas, the komatiitic basalts are typically spinifex-textured and, in a few cases, show pyroxene accumulation associated with NiCu(PGE) sulfide deposits. The peridotites occur in the lower portions of differentiated sills, which contain gabbro and diorite in their upper parts. The sills are believed to be co-magmatic with the komatiitic basalts. The spinifex rocks of the Jiepai and Hejia Flows have MgO ranging from 8.9 to 14.3 wt%. The Zhongkui Flow is highly fractionated to form a spinifex zone with lower MgO (5.3–5.9 wt%) and a cumulate zone with higher MgO (17.3–17.9 wt%). Overall the rocks have TiO 2=0.44–0.74 wt%. Relative to primitive mantle, they are enriched in Th and LREE, but exhibit negative Ti-, Nb-, and P-anomalies. These features are consistent with their formation from a crustally-contaminated komatiitic magma. During this process of crustal contamination, the magmas assimilated sulfur from sediments, which caused sulfide-segregation resulting in the formation of NiCu(PGE) sulfide deposits. The occurrence of the komatiitic basalts in the Sibao Group can be explained by the ascent of a mantle plume beneath a continental rift environment, and implies that the Yangtze Block may have had an Archean basement through which the Sibao komatiitic basalts erupted.