Zhaheba potassic basalt, eastern Junggar (NW China): Geochemical characteristics and tectonic implications

Abstract

Zhaheba potassic basaltic rocks outcrop within the Early Devonian Tuorangekuduke Group, NE margin of the Junggar Basin. The rocks exhibit dual geochemical characteristics of both potassic rocks and high-Nb basalts. On one hand, the rocks contain relatively high K 2O(1.66% - 4.63%) and characterized by high K 2O/Na 2O ratios (0.36 - 1.65), high Sr (665 × 10 -6 - 906 × 10 -6), Ba (680 × 10 -6 - 1596 × 10 -6) and highly fractionated REE (La/Yb N>8), displaying characteristics of shoshonitic rocks. On the other hand, these rocks possess relatively high TiO 2 (∼1.7%), P 2O 5 (∼0.8%), Nb (21 × 10 -6 - 23 × 10 -6) and other HFSEs, which are quite analogous to those of high-Nb basalts. Although their high Nb contents, these rocks exhibit significant Nb-Ta depletion and show typical island-arc characteristics. The relatively high Nb/Yb (∼5.9), Nb/Ta (19 - 21), Zr/ Hf (46 - 51) ratios and K 2O contents suggest that these potassic basaltic rocks were derived from a metasomatized, phlogopite- and/or amphibole-bearing mantle source. Aqueous fluid from dehydration of subducting slab cannot be the metasomatic agent because of the weak ability of transporting HFSEs to the mantle wedge. These rocks possess high Ce/Th (47 - 50), Ce/Pb (13 - 19) and Ba/Th (341 - 777) ratios and do not show significant negative Ce anomaly, suggesting a negligible contribution from subducted sediments. We suggest that the metasomatic agent was probably the low-degree partial melt which, when interacted with the mantle wedge, could give rise to the relatively high K 2O and enrichment of HFSEs and LILEs. The Zhaheba potassic basalts were most likely produced in an island arc environment which, in combination with typical Nb-enriched basalts in the center area of the Junggar basin, suggests that hot-subduction scheme could frequently occur during the evolution of the Junggar plate, and that arc-originated terranes could dominate the basement of the Junggar basin.