Late Tonian (ca. 785 Ma) subduction-related mafic-ultramafic cumulates in the West Cathaysia terrane, South China

Abstract

The Neoproterozoic tectonic setting of the West Cathaysia terrane and its relationship with the Yangtze Block are critical to understanding the formation and tectonic evolution of South China. A detailed study of late Tonian mafic-ultramafic cumulate slivers embedded in the Shenshan tectonic mélange along the northwest margin of West Cathaysia is presented here with the aim of elucidating its Neoproterozoic tectonic history. These partly-altered mafic-ultramafic rocks include olivine-bearing cumulates (olivine pyroxene hornblendite or olivine hornblende pyroxenite), pyroxene hornblendite, and gabbroic cumulates. The early crystallization of clinopyroxene and the abundance of hornblende indicate that they formed from hydrous magmas. The rocks have similar geochemical distribution patterns and show variable initial 87Sr/86Sr ratios of 0.702679 to 0.709742 and positive εNd(t) values of +2.10 to +4.52. The mineral compositions of the clinopyroxene and hornblende and their whole-rock extended normalized trace element patterns show features (e.g., negative Nb anomalies, LILE enrichment) typical of rocks with an arc affinity. The high calculated Mg# values (58–77, average 71) of the equilibrium magmas are close to those of primary mantle-derived basalts. We conclude that these mafic and ultramafic cumulates were derived from a subduction-related, hydrous, magnesian basalt in a subduction setting. The estimated crystallization temperature and pressure, based on the composition of the amphiboles, suggest that the minerals accumulated in magma chambers at middle to lower crustal depths. LA-ICP-MS U-Pb dating of zircon from the gabbroic cumulate indicates crystallization at ca. 785 Ma. A previous interpretation for West Cathaysia at this time involved a continental rift basin (the Nanhua basin) that straddled the southeastern margin of the Yangtze Block and the northwestern margin of West Cathaysia. In contrast, our results suggest that West Cathaysia was an arc at ca. 785 Ma, which supports the interpretation that the amalgamation between Yangtze and West Cathaysia did not occur in the Tonian, but more likely in the early Paleozoic.