Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt, NW China: Constraints on the initiation of a magmatic arc in the southern Central Asian Orogenic Belt

Abstract

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt (ETOB) have been studied in order to constraint the initiation of a magmatic arc formed in this region. Zircon U–Pb dating indicates that two dioritic plutons in the northern ETOB were generated in the Late Ordovician (452±4Ma) and the Early Silurian (442±3Ma), respectively. Diorites from the two plutons are characterized by enrichments in large ion lithophile elements (LILE) and highly incompatible elements, with depletions in high field strength elements (HSFE) displaying typical geochemical features of a subduction-related origin. They have positive εNd(t) values (+5.08–+6.58), relatively young Nd model ages (TDM=0.71–1.08Ga), with Ta/Yb (0.05–0.09) and Nb/Ta ratios (12.06–15.19) similar to those of depleted mantle, suggesting a juvenile mantle origin. Their high Ba/La (13.3–35.9), low Th/Yb (0.72–2.02), and relatively low Ce/Th (4.57–14.7) and Ba/Th (47.8–235) ratios indicate that these diorites were probably produced by partial melting of a depleted mantle wedge metasomatized by both subducted sediment-derived melts and slab-derived aqueous fluids. Zircon U–Pb dating of a granitic pluton in the northern ETOB yielded a Late Ordovician intrusion age of 447±5Ma. Granites from this pluton show calc-alkaline compositions with geochemical characteristics of I-type granites. They also show positive εNd(t) values (+6.49–+6.95) and young Nd model ages (TDM=0.69–0.87Ga), indicating that the granites were most likely derived from juvenile lower crust. Our new dating results on the dioritic and granitic plutons suggest that arc-type magmatism in the northern ETOB began prior to or at the Late Ordovician (452–442Ma). In addition, north-dipping subduction of the Kangguertage oceanic lithosphere may account for the arc-type magmatism and the geodynamic process of the ETOB in the Early Paleozoic.