Carboniferous variation of crustal thickness and subduction angles in Eastern Tianshan, NW China: evidence from the petrogenesis of the magmatic rocks in the Aqishan–Yamansu Belt

Abstract

ABSTRACT New geochronological and geochemical data were presented for the Early Carboniferous basaltic to andesitic volcanic rocks and Late Carboniferous granitic intrusions in the Aqishan–Yamansu Belt of Eastern Tianshan, NW China. Zircon U–Pb dating indicates that the andesite and granite were emplaced at 330 Ma and 321 Ma, respectively. The andesitic rocks of the Yamansu Formation (FM) display subalkaline to calc-alkaline characteristics with moderate SiO2, relatively high Mg# values, and low Cr, Co and Ni, indicating an origin from mantle-derived melts. Their relatively low εNd(t) values (–0.02 ~ +0.61) and old TDM ages (TDM = 1.39–1.40 Ga) suggest a mantle source with addition of old crustal materials. Their high Rb/Y and Ba/La, low Nb/Y and Th/Yb ratios elucidate that the magma sources were probably metasomatized by slab-derived fluids. Besides, the high Th and low Ce/Th and Ba/Th ratios of the andesites suggest a contribution from subducted sediment–derived melts. Integrated with the depletion in high field strength elements and enrichment in light rare earth elements (LREEs), we suggest that the Yamansu andesites were formed by partial melting of relatively enriched mantle wedge metasomatized by both subducted sediment-derived melts and fluids. The basalts of the Tugutublak Formation (TF) have high MgO, Mg# (56–58) and εNd(t) (+5.9 ~ +6.3) values with enriched in LREEs and depleted in Nb–Ta–Ti anomalies, implying an arc setting. Their high Ba/Th, Ba/La and low Th/Ta, Th/Yb ratios suggest that the slab-derived fluids were involved. The basaltic andesites of the TF have higher SiO2, lower Mg# (48–50), Cr, Ni and εNd(t) (+1.21) values than the basalts of this formation, indicating that they were evolved from the basaltic magma. Therefore, the Carboniferous volcanic rocks in this study indicate that the Aqishan–Yamansu Belt was in a subduction background. The granites intruding into the TF show typical characteristics of I-type granite. Their highly positive εHf(t) values (+14 ~ +17) suggest a significant contribution from juvenile basaltic lower crust. Moreover, changes in the Dy/Yb and Ho/Yb ratios of Late Paleozoic felsic igneous rocks in the Aqishan–Yamansu Belt imply that the crust underwent four periods of thickening and thinning, which were likely triggered by the variation of subduction angles.