Constraints on timing of the early-paleoproterozoic magmatism and crustal evolution of the Oulongbuluke microcontinent: U-Pb and Lu-Hf isotope systematics of zircons from Mohe granitic pluton

Abstract

The Olongbuluke micro-continent is composed of a two-fold basement with cover strata. The lower basement is the medium-to high-grade Delingha complex (DC) and the Dakendaban Group, the upper is the low-grade Wandonggou Group. Zircon LA-ICP-MS dating gives U-Pb isotope of 27 zircon grains from the Mohe granite of the DC, of which 26 zircon grains suffered various degree of Pb loss and defined a Concordia with a upper intercept age of 2470 +19/-18Ma. Hf isotope of 25 zircon grains was measured using LA-MC-ICP-MS technique, of which 23 grains crystallized from Mohe granite show Hf(2470Ma) = 0.28129-0.28140 (weighted average of 0.28134±0.00003). εHf = 2.94-6.95(weighted average of 4.58 + 0.54/-0.76), resident time for the felsic crustal reservoir TcDM=2.54-2.75 Ga with weighted average of 2.66 + 0.04/-0.02Ga. These data indicate that the Mohe granite pluton was derived from partial melting of the mantle, and the intrusion of the Mohe granite pluton implied a crustal accretion event occurred in Olongbuluke micro-continent at ca. 2.5 Ga.

欧龙布鲁克微陆块具典型的基底与盖层二元结构，基底自下而上由德令哈杂岩、达肯大坂岩群和万洞沟群三个岩石-构造单元组成。应用LA-ICP-MS测定了德令哈杂岩中的莫河花岗岩体的27个颗锆石的U-Pb同位索成分，其中26颗锆石发生不同程度的放射成因铅同位素丢失，其不一致线上交点年龄为2470+19/-18Ma。应用LA-MC-ICP-MS测定了25颗锆石的Hf同位索成分，其中岩浆结晶成因的23颗锆石的Hf(2470Ma)=0.28129-0.28140，平均值0.28134±0.00003；εHf值的变化范围2.94-6.95，加权平均值4.58+0.54/-0.76，长英质地壳存留年龄TcDM=2.54-2.75Ga，加权平均值2.66+0.04/-0.02Ga。以上数据将该花岗岩的形成年龄约束在2470Ma，其岩浆来源于地幔物质的部分熔融，指示欧龙布鲁克微陆块在-2.5Ga的地壳增生事件。