Phanerozoic granitoids in the central and eastern parts of Central Asia and their tectonic significance

Abstract

The widespread occurrence of voluminous granitoids is a typical characteristic of the Central Asian Orogenic Belt (CAOB) in the central and eastern parts of Central Asia. The granitoids occupy a total area more than 8 million km2, with three major area peaks at Cambro-Ordovician, Carboniferous-Permian, and Jurassic. Six zircon age peaks at ca. 500, 430–370, 330–300, 290–260, 210–170 and 145–110Ma.The early Paleozoic granitoids (540–440Ma, with age peak at ca. 500Ma) mainly occur in the northwestern CAOB, and are of calc-alkaline I-type. They probably formed in a subduction setting (500–480Ma) within the Paleo-Asian Ocean (PAO) and subsequent microcontinental-collision (ca. 460Ma). The middle Paleozoic granitoids (430–370Ma) predominately occur in the NW and southern CAOB, and are characterized by a high-K calc-alkaline affinity, suggesting an accretionary and/or a collisional setting. The late Paleozoic (350–260Ma) granitoids are widespread in the CAOB and formed in variable tectonic settings within an intra-continental regime or active continental margins in the PAO and in the evolving Mongol-Okhotsk Ocean (MOO). Carboniferous granitoids (350–300Ma) in the northern and northwestern CAOB exhibit A-type, alkaline features and formed in a post-accretionary settings, whereas granitoids in the southern CAOB show I-type, calc-alkaline features and were generated in an arc setting. Permian granitoids (290–270Ma) are predominantly of I-A, A-type with alkaline signatures, associated with coeval mafic magmatism and suggesting a post-collisional or intraplate setting, contemporaneous with a Large Igneous Province (LIP) in the Tarim Craton. Some 270–250Ma calc-alkaline granitoids also occur in the southernmost CAOB. Combined with 250–245Ma high Sr/Y granitoids, these rocks reflect final closure of the PAO during 270–250Ma.Early Mesozoic (mainly Triassic) granitoids occur mainly in the eastern CAOB or eastern Central Asia. The early-phase granitoids (250–230Ma) are of I- and S-types, whereas the mid- and late-phase granitoids (ca. 230–190Ma) are of A-type and transitional I-A type or highly fractionated I-type. All these granitoids occur in variable settings (intraplate, post-orogenic, and continental arc) related to three tectonics regimes, namely the Paleo-Asian Ocean (PAO), the Mongol-Okhotsk Ocean (MOO), and the Paleo-Pacific Ocean (PPO). Late Mesozoic granitoids mainly occur in eastern Central Asia and were emplaced during 6 major magmatic events at 190–180, 180–165, 165–145, 145–135, 135–100 and 100–60Ma. The 190–165Ma granitoids are of I-type, calc-alkaline/high-K calc-alkaline, and evolved along active margins related to the Mongol-Okhotsk and Paleo-Pacific ocean domains. The 145–120Ma granitoids are highly fractionated I-, transitional I-A or, A-types, with high-K calc-alkaline/shoshonitic signatures. The magmatic evolution from the Late Jurassic to Early Cretaceous coincided with a tectonic transition from contractional, thickened crust to extensional thinning. The 120–65Ma granitoid magmatism displays a southeastward younging trend, suggesting a roll-back of the Paleo-Pacific oceanic plate.The granitoids in the CAOB of eastern Central Asia show variable whole-rock Nd and Hf-in-zircon isotopic compositions, and most have juvenile signatures. In some regions (e.g., Altai-Junggar and Great Xing’an), the volume of juvenile and slightly juvenile crust (<1.0Ga) is much higher than that of other orogens (such as the Qinling in Central China. The proportion of juvenile vs. old crust may be a marker to identify a typical accretionary orogen from a collisional orogen.