Triassic Granitoids In Beishan-Inner Mongolia,China And Its Tectonic Implications

The Beishan-Inner Mongolia area is situated in the southernmost part of the Central Asian Orogenic Belt (CAOB) along the Solonker suture zone. The tectonic development of the Beishan-Inner Mongolia area was dominated by the Paleo-Asian geodynamics before the Mesozoic, wherea it was also strongly influenced by a far-field effect of subduction of the Paleo-Pacific Ocean to the east since the Mesozoic. The Triassic in southern CAOB is characterized by tectonic transition from the Paleo-Asian Ocean to the Paleo-Pacific Ocean. Regarding the CAOB Triassic granitoids, fundamental relationships about spatial and temporal variations, origin, isotopic evolution and tectonic setting as well as their tectonic significance as indicators of Phanerozoic crustal growth are poorly constrained in spite of the some published data.The Beishan-Inner Mongolian Triassic granitoids can be classified into two groups: early granitoids (251-238 Ma) show high Sr/Y ratios features with adakite-like signatures; late granitoids (225-217 Ma) have strong negative Eu anomalies, and are A-type or highly fractionated I-type granites. Early Dahuoluo pluton in the Beishan and the Shuangjing pluton in Inner Mongolia show high Sri values, negative d(t) values and weakly negative zircon f(t) values, indicating the contribution of the old crustal components. The other early granitoids exhibit low Sri values (0.7036-0.7043), positive d(t) values (+0.8-+5.8) and high zircon f(t) values (+7.3-+15.6), indicating the partial melting of the juvenile crust. The Jianshedong pluton and the Jianshexi pluton have mantle-like zircon δ18O values, indicating no fluid-rock interaction, whereas the other plutons display weakly zircon δ18O values (5.85-7.85%o), suggesting some inplut of supracrustal materials on or near the Earth’s surface. These early granitoids were derived from the partial melting of the thickened mafic lower crust in response to the compressional orogeny, and dominated by fractional crystallization of garnet, clinopyroxene, hornblende and plagioclase. However, the late granitoids show high Sri values (0.7049-0.7096), negative εNd(t) values (-0.9--2.8), old Nd model ages (0.89-1.13Ga) and variable zircon εNf(t) values (-1.3-+6.1) and old two-stage Hf model ages (TDm2= 0.87-1.34 Ga), indicating the partial melting of both juvenile and ancient crust. The lage granitoids were likely the result of fractional crystallization and crustl contamination for the granitic magma emplaced in a post-orogenic setting.In comparation with the others in the CAOB, the Triassic granitoids can be broadly classified into two groups according to zircon U-Pb ages:an early group covering the time span from Early to Middle Triassic (250-230 Ma) and a late-stage group emplaced during Late Triassic (ca.230-200 Ma). Early granitoids show adakite-like feature, and emplaced in a compassional setting, whereas late granitoids exhibit high-K calc-alkaline characteristics, and most are A-type signature. Triassic granitoids in circum-Mongol-Okhotsk belt were related to ongoing subduction of the Mongol-Okhotsk Ocean. Numerous Early-Middle Permian (290-260 Ma) high-K grnaitoids, alkaline granites, mantle-derived mafic intrusions and bimodal volcanic rocks represent the termination of the Late Paleozoic magmatism. Therefore, these Triassic granitoids in the Altai-Beishan-Inner Mongolia-Jilin were generated in a post-/non-orogenic setting after the closure of the Paleo-Asian Ocean and were transitional to the Paleo-Pacific tectonic regime.The Renjiayingzi pluton was dated as an early Cretaceous pluton that previous was considered as a Triassic pluton. This pluton was built up by an incremental growth mechanism of three small magma pulses, and was a result of the Late Mesozoic magmatism dominanted by the Paleo-Pacific geodynamics.