Geochemistry, zircon U–Pb ages and Lu–Hf isotopes of early Paleozoic plutons in the northwestern Chinese Tianshan: Petrogenesis and geological implications

Abstract

The northwestern Chinese Tianshan is a key part of the southern Central Asian Orogenic Belt. Voluminous arc-related igneous rocks in this region have recorded the opening and closure of several oceanic basins that may represent southern branches of the Paleo-Asian Ocean. We have conducted geochemical, geochronological and Hf-in-zircon isotopic studies on mafic and felsic intrusive rocks in the northwestern Chinese Tianshan to provide important clues for the evolution of the Junggar Ocean and to understand the early Paleozoic crustal growth in the Central Asian Orogenic Belt. The mafic plutons are composed of metagabbro (453.8±2.0Ma) and fine-grained diorite (461.1±2.0Ma). These rocks are mostly sub-alkaline in composition and belong to calc-alkaline series. The mafic rocks are enriched in LILE (such as Rb, Sr, Ba, Pb) with pronounced negative Nb–Ta–Ti anomalies. Depleted εHf (t) (−0.03–+8.32), high Ba/Th (>79) and Ba/La (>11) as well as low Th and Th/Yb suggest that these mafic rocks were likely generated by partial melting of slab fluid-metasomatized mantle peridotite. The felsic plutons consist of quartz diorite (463.1±2.0Ma, 462.4±2.0Ma), granodiorite (460.2±3.0Ma) and muscovite-bearing granite (448.9±6.0Ma) and show characteristics of calc-alkaline igneous rocks. These felsic rocks are also enriched in LILE and display HFSE depletion with obvious Nb–Ta–Ti troughs, showing typical geochemical features of arc-related magmas. The quartz diorite is characterized by slightly depleted juvenile εHf (t) values (−0.69–+4.52) and relatively young Hf crustal model ages (TDMC: 1.16–1.49Ga), indicating a relatively juvenile magmatic source. The occurrence of Neoproterozoic zircon xenocrysts suggests that the quartz diorite was produced, at least in part, by partial melting of Precambrian basement rocks with limited involvement of a juvenile mantle component. The granodiorite and muscovite-bearing granite both exhibit enriched εHf (t) values (−3.88–+0.11), relatively old TDMC ages (1.44–1.69Ga) and contain numerous early Paleozoic to Meoproterozoic inherited zircons (493–1012Ma). Their εHf (t) values plot on the evolution trend of the Neo- to Mesoproterozoic basement rocks and demonstrate an origin through partial melting of the Precambrian basement. Therefore, the petrogenesis of the mafic and felsic rocks unravels an arc-related tectonic setting in the early Paleozoic. The ages of these intrusive rocks suggest that the onset of a southward subduction of the Junggar Ocean beneath the Yili block predates the middle Ordovician. The subduction process produced extensive arc-related granitoid complexes and triggered the involvement of juvenile mantle components in producing new continental crust and probably played an important role in vertical crustal growth in the CAOB during the early Paleozoic. In combination with new data from adjacent regions, the Middle Ordovician arc-related magmatism in the northwestern Chinese Tianshan and Cambrian to early Ordovician arc-derived magmas in the southern West Junggar was likely produced by bidirectional subduction of the Junggar Ocean.