GENESIS OF CARBONIFEROUS VOLCANIC ROCKS IN THE EASTERN JUNGGAR: CONSTRAINTS ON THE CLOSURE OF THE JUNGGAR OCEAN

Abstract

Zhaheba Carboniferous volcanic rocks mainly composed of basalts and basaltic andesites occur within Upper Carboniferous Batamayi Group, north of Zhaheba ophiolite in the eastern Junggar. These rocks have relatively high Mg# (64 - 73) values, moderate TiO2 (<0.85%), Al2O3 (13.75% - 16.88%), but low CaO (4.51% - 7.28%) and FeOT (5.82% - 7.71%) contents, showing characteristics of primitive magmas. These rocks have relatively high Na2 O/K2O ratios (>1) and alkali contents (Na2O + K2O) = 4.3% - 7.1%). They are enriched in LILEs but relatively depleted in HFSEs, with strong negative Nb-Ta anomalies, showing typical characteristics of subduction zone. These samples are slightly enriched in LREEs ((La/Yb)N = 1.0 - 2.1) and possess weak Eu and Ce anomalies (0.94 - 1.08, 1.00 - 1.06, respectively), without significant fractionations of HREEs (GdN/YbN = 1.1 - 1.3). Furthermore, they have high radioactive Sr (ISr = 0.705282 - 0.705420) and low radioactive Nd (εNd (t) = + 6.59 - + 7.58). These characteristics, along with their low contents of Th (<0.55 × 10-6) and Pb (<3.52 × 10-6) and high ratio of Ce/Pb (4 - 79), preclude the possibility for involvement of continental crust, suggesting that they were most likely produced in a subduction-related environment. Their high Ce/Th (18 - 32), Ce/Pb, Ba/Rb (21 - 76), Ba/ Th (>179), εNd (t) values and lack of obvious Ce anomalies indicate that there was no significant involvement of subducted sediment during their genesis. Moreover, their trace elements and Nd-Sr isotope compositions are consistent with their origination from a mantle wedge metasomatised by slab-derived fluid. Different from typical arc-related magmas, however, most Zhaheba samples show negative Pb anomalies and relatively high Ce/Pb ratios, which can probably be ascribed to Pb-poor fluid derived from an amphibolite-facies slab in high temperatures. These characteristics indicate that an intra-oceanic subduction environment existed in the eastern Junggar, and that the Junggar Ocean must close after the Late Carboniferous.