Akaz greenschist, West Kunlun: Within-plate basalt contaminated by continental crust origin of the Akaz greenschist (metabasalt), West Kunlun: Accreted seamount or continental rift basalt?

Abstract

The Sinian-Early Cambrian Akaz greenschists (metavolcanic rocks) of West Kunlun, NW China was metamorphosed from mafic to andesitic rocks. They contain low to intermediate SiO 2 (42.3% - 64.7%), MgO (2.69% - 7.54%) and have wide range of TiO 2 (0.94% - 3.05%) and Fe 2O 3T(7.64% - 18.47%) contents, indicating a basaltic to andesitic protolith. These rocks possess high HFSE (e. g. Nb, Ta, Zr, Hf, Ti), REE (∑ REE = ∼ 407 μg/g) and Th (∼ 16 μg/g) contents, and are characterized by relatively high Ti/Y (183 - 673), Th/Yb (0.6 - 2.8) and low Hf/Ta (1.6 - 8.6) ratios. They are relatively LREE-enriched (La/Yb = 5.4 - 20), and except for two samples characterized by positive Nb-anomalies, most samples show variable negative Nb-anomalies (Nb/Nb* = ∼ 0.2) and exhibit transitional characteristics between within-plate and subduction-related basalts. The subduction-related characteristics preclude oceanic plateau, oceanic island and within-plate seamount basalts as the protolith; while their within-plate characteristics, together with the existence of positive Nb-anomaly samples, suggest that the metavolcanic rocks were not produced in destructive settings (island arc or active continental margin). In particular, their relatively high Gd/Yb ratios (1.4 - 3.2) distinguish clearly these rocks from island-arc tholeiites, and their relatively high Zr/Y (3.1 - 12), Ta/Yb (0.27 - 0.73) and relatively low Zr/Nb (mostly less than 12) are of strong continental within-plate characteristics, suggesting that the protolith of the greenschist was continental rift-related basalts probably contaminated by continental crust materials during their ascent. This indicates that the Akaz metavolcanic rocks were originally formed in a continental rift environment. The Akaz metavolcanic rocks, in combination with approximately coeval within-plate volcanic rocks in East Kunlun and North Qilian, suggest that the rifting of Tarim from Gondwana most likely occurred in Sinian or Early Cambrian. Based on this study and previous work, an amended model is proposed to interpret the Sinian to Early Paleozoic tectonic evolution of West Kunlun.