Evolution, source and tectonic significance of Early Mesozoic granitoid magmatism in the Central Asian Orogenic Belt (central segment)

摘要

Numerous Early Mesozoic granitoids have been recognized from the central segment of the Central Asian Orogenic Belt (CAOB). They can be broadly classified into two groups according to zircon U-Pb ages: an early-stage group covering the time span from Early to Middle Triassic (250-230. Ma) and a late-stage group emplaced during Late Triassic to Early Jurassic (ca. 230-190. Ma). Early-stage (250-230. Ma) granitoids are mainly distributed in the western Central Mongolia-Erguna Belt (CMEB), the western Altai Belt (AB), the South Mongolia-Xing'an Belt (SMXB) and the Beishan-Inner Mongolia-Jilin Belt (BIJB). They consist mainly of quartz-diorites, granodiorites and monzogranites, mostly of I-type, with minor mafic intrusions, with some of them showing adakite-like signatures and some with S-type features. Late-stage (230-190. Ma) granitoids mainly occur in the North Mongolia-Transbaikalia Belt (NMTB), the eastern CMEB (Erguna massif) and the eastern Altai Belt (AB). They are predominately syenogranites, monzogranites and syenites, associated with many alkaline granites and mafic intrusions and are A-type and transitional I-A type or highly fractionated I-type granites.Whole-rock Sr-Nd and zircon Hf isotopic data have been compiled for regional isotopic mapping. The -_(Nd) (t) values show large variations from -7.0 to +7.4 and Nd model ages (T_(DM)) from 0.46Ga to 1.43Ga as well as the initial Sr isotopic ratios (Sr_i) from 0.7023 to 0.7174. The zircon ε_(Hf) (t) values vary from -4.6 to +15.3 and give two-stage Hf model ages (T_(DM2)) from 0.30Ga to 2.09Ga. The extremely large variations of whole-rock Sr-Nd and zircon Hf isotopes demonstrate strong isotopic heterogeneity of the source regions that are mainly dominated by juvenile components with significant old crustal participation. Furthermore, the late-stage granitoids in the NMTB, the CMEB and the AB generally have more negative ε_(Nd)(t) values and more variable zircon ε_(Hf)(t) values than those of the early-stage granitoids in the same belt, implying an increasing crustal signature from early- to late-stage in the assumed heterogeneous source regions within the same belt, which probably results from melting of shallower crust in parallel with a shift to more alkaline chemistry of the late granitoid magmas. By contrast, most late-stage granitoids in the SMXB and the BIJB have more positive εNd(t) values and homogeneous zircon εHf(t) values than those of the early-stage granitoids in the same belt, indicating more juvenile contribution to the source of the these granitoids.The generation of the Early Mesozoic granitoid magmas in the NMTB and the CMEB was dominated by the ongoing closure of the Mongol-Okhotsk Ocean and some was probably related to a mantle plume process. They were possibly derived from subducted materials melting or juvenile components with some probable contributions from ancient continental crust. Early Mesozoic granitoid magmas in the SMXB, the AB and the BIJB were generated in a post-on-orogenic setting after the closure of the Paleo-Asian Ocean and were the results of partial melting of crustal components in response to underplating of mantle-derived magmas, most likely linked to lithospheric thickening and delamination and asthenospheric upwelling. Early Mesozoic granitoid magmatism provides critical information on Mesozoic post-accretionary tectonic evolution of the Paleo-Asian Ocean and transitional tectonic regimes from Early Mesozoic subduction to Late Mesozoic closure of the Mongol-Okhotsk Ocean as well as post-accretionary continental growth.