Devonian to carboniferous tectonic evolution of the Kangguer Ocean in the Eastern Tianshan, NW China: Insights from three episodes of granitoids

Abstract

Deciphering the tectonic evolution of the Kangguer Ocean is critical to understand the architecture of the Eastern Tianshan, NW China. Previous studies for the Kangguer Ocean mainly concentrated on final closure during the Latest Carboniferous to Permian. Three distinct episodes of granitoids have been recognized in the Kangguer belt: Late Devonian (ca. 360 Ma) normal I-type granites, Early Carboniferous (ca. 348 Ma) fractionated I-type granites, and Late Carboniferous (ca. 317–302 Ma) adakitic granites, attempting to provide constraints on the sources of these rocks, which further unravels the evolution of the Kangguer Ocean during this period. The Late Devonian granites exhibit calc-alkaline characteristics, with negligible Eu anomalies and low (K2O + Na2O)/CaO ratios, akin to I-type granites without significant fractional crystallization. They display high SiO2 (>70 wt.%), with low MgO (<0.83 wt.%) and Mg# (35–41) values, together with depleted Sr and Nd isotopic values ((87Sr/86Sr)i = 0.7041; εNd(t) = 5.48–6.38) and juvenile TDM2 values (596–671 Ma), suggesting that they were probably originated from the lower crustal-derived juvenile materials. The Early Carboniferous granites have similarly depleted Sr and Nd isotopic values to those of the Late Devonian samples, but display striking depletions in Eu and Sr and have lower Mg# (26–29) values and higher (K2O + Na2O)/CaO ratios, indicating that they also represent juvenile crustal-derived magmas, but experienced extensive crystal fractionation. The Late Carboniferous rocks exhibit high SiO2 (63.7–69.0 wt.%), Sr/Y (17.4–83.3), La/Yb (4.00–13.7), and Na2O/K2O (1.66–7.03) values, and low Y (4.46–12.2 ppm) and Yb (0.50–1.10 ppm) contents, indicative of adakitic affinity. The above geochemical features, together with Sr and Nd isotopic values comparable to slab-derived adakites and ophiolitic rocks in the Kangguer belt, implying they were likely formed by slab-derived magmas and mantle wedge peridotites. Coupled with regional geology, a northward subduction of the Kangguer oceanic slab was suggested to interpret the formation of the Late Devonian normal I-type granites. Subsequently, the southward rollback of the Kangguer Ocean caused the generation of the Early Carboniferous granites with further fractional crystallization. We infer that a northward flat subduction resulted in formation the Late Carboniferous adakitic rocks.