Nb-depleted, continental rift-related Akaz metavolcanic rocks (West Kunlun): Implication for the rifting of the Tarim Craton from Gondwana

Abstract

The Akaz metavolcanic rocks of the West Kunlun Mountains possess low to intermediate SiO2 (42.3-64.7 wt%) and MgO (2.69-7.54 wt%) and high TiO2 (0.94-3.05 wt%) and Fe2O3 T (7.64-18.47 wt%), indicating a basaltic to andesitic protolith. These rocks have high contents of Zr (89.6-470 ppm), Nb (10.0-40.3 ppm), Y (19.7-52.7 ppm), Th (0.86-15.96 ppm) and total REE (67.7-407 ppm), and are characterized by relatively high Ti/Y (183-649), Th/Yb (0.5-3.9), and low Hf/Ta (3.0-8.6) ratios. They are LREE-enriched (La/Yb = 5.4-20) and most have small negative Nb anomalies (Nb/Nb* = 0.20-1.16). These characteristics are transitional between within-plate and subduction-related basalts. The relatively higg Gd/Yb ratios (1.4-2.9) distinguish these rocks from island-arc tholeiites and the high Zr/Y (3-12), Ta/Yb (0.3-0.7) and low Zr/Nb (< 12) ratios strongly support a continental affinity. The protoliths for the Akaz metavolcanic rocks are interpreted to be continental rift basalts formed during rifting of the Tarim Craton from Gondwana. Stratigraphic and palaeontological data indicate that the rifting occurred in Sinian to Cambrian times, roughly contemporaneously with rifting in the East Kunlun and North Qilian orogenic belts farther to the east. © The Geological Society of London 2004.