Petrogenesis And Tectonic Implications Of The Triassic Granites From The Mingshui Region In The Great Xing’an Range

The Great Xing’an Range, located in the eastern segment of the Central Asian Orogenic Belt(CAOB), is characterized by immense volumes of Phanerozoic granitic rocks which are previously thought to form in the Paleozoic. Recent dating indicates that these granitic rocks were emplaced in the Mesozoic, mainly during the Jurassic to Early Cretaceous. At present, the Mesozoic igneous rocks have attracted much attention and massive achievements have been obtained. However, in this context, the relevant special reports of the Triassic magmatic events in this region, especially those about isotopic and whole-rock geochemical studies are still very weak, which significantly hampers our understanding of petrogenesis and origin of these Triassic granitic rocks. Thus a further research is highly required.This paper presents geochronological, geochemical and zircon Hf-isotopic data for the Triassic Toudaohe granitoids in the Mingshui region of the central Great Xing’an Range, in order to determine their emplacement ages, petrogenesis and tectonic setting. The Toudaohe pluton is mainly composed of granodiorite and monzogranite. LA-ICP-MS zircon U-Pb data indicate that the Toudaohe pluton formed during the Middle Triassic time with the ages of 243-240 Ma. Geochemically, the granitic rocks are high in Si O2(62.39%~78.19%), and Al2O3(11.85%~17.17%), but low in total Fe2O3(0.56%~6.03%), Mg O(0.16%~3.36%),and Ca O(0.26%~2.94%), suggesting that they are high-K calc-alkaline I-type granites.These granitoids have low total REE content(ΣREE=75.5×10-6~149.49×10-6) with fractionated REE((La/Yb)N = 8.71~17.61), and no remarkable Eu anomalies. Moreover,they are enriched in large ion lithophile elements(e.g., Rb(50.40ppm~132.00ppm)、Th(4.71ppm~35.80ppm)、Pb(9.24ppm~29.60ppm)、U(1.08ppm~5.19ppm)), and depleted in high field strength elements(e.g., Nb(8.57ppm~30.80ppm)、Ta(0.44ppm~1.38ppm)).Zircon Hf isotope analyses show that these granitoids have high initial 176Hf/177 Hf ratios(0.282811~0.283227), positive εHf(t) values(6.55~21.28), and young Hf two-stage model ages of 595-1107 Ma. The above geochemical characteristics indicate the source materials of the Toudaohe pluton are juvenile igneous crust rocks that originated from depleted mantle during the Mesoproterozoic to Phanerozoic period. Considering the regional geological investigations and temporal–spatial distribution of the coeval igneous rocks, we propose that the Middle Triassic Toudaohe pluton was emplaced in a post-orogenic extensional environment related to the final closure of the Paleo-Asian Ocean.