莫河花岗岩的锆石U-Pb和Lu-Hf同位素研究:柴北欧龙布鲁克微陆块始古元古代岩浆作用年龄和地壳演化约束

Onstraints 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

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同位素成分,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/-18 Ma.应用LA-MC-ICP-MS测定了25颗锆石的Hf同位素成分,其中岩浆结晶成因的23颗锆石的Hf（2470 Ma）=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/-0.02 Ga.以上数据将该花岗岩的形成年龄约束在2470 Ma,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～2.5 Ga的地壳增生事件.

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 Hf1= 0.28129 ～ 0.28140 （weighted average of 0.28134 ± 0.00003 ） , БHf=2.94～ 6.95 （ weighted average of 4.58 ＋ 0.54/-0.76 ） , resident time for the felsic crustal reservoir T^CDM= 2.54～ 2.75Ga 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. 2.5Ga.