北山北带新元古代A型花岗岩: Rodinia超大陆裂解早期的地质响应

北山造山带位于中亚造山带南缘, 带内存在多个前寒武纪微陆块, 对这些前寒武纪微陆块的正确认识, 是造山带构造格架划分及构造演化过程研究的关键。本文选取北山北带明水地块小孤梁片麻状正长花岗岩进了岩石学、元素地球化学、锆石U-Pb年代学和Hf同位素分析研究。结果显示, 小孤梁片麻状正长花岗岩的LA-ICP-MS锆石U-Pb年龄为784±2.7Ma, 揭示北山北带存在新元古代基底岩石; 全岩样品具有高SiO₂(72.22%~74.06%)、富碱(7.50%~8.33%)、高Zr+Y+Nb+Ce含量(567.6×10^-6~644.7×10^-6)和10000×Ga/Al值(2.71~2.81), 低Al₂O₃(12.39%~12.95%)、MgO(0.40%~0.54%)、CaO(0.91%~1.38%)的特征; 微量元素富集Rb、K、Th、U, 亏损Sr、Nb、Ta、P、Ti; 稀土元素总量较高, 轻稀土富集, 重稀土亏损, 具有明显的负Eu异常; 锆饱和温度计算小孤梁片麻状正长花岗岩的结晶温度为824~859℃; 以上这些岩石地球化学特征与A型花岗岩一致。锆石ε_Hf(t)值介于-19.55~-13.81之间, 均为负值, 单阶段模式年龄(t_DM1)为1.56~1.33Ga, 两阶段模式年龄(t_DM2)为2.02~1.65Ga, 表明其岩浆源区可能为古元古代地壳物质的部分熔融。构造环境判别显示其为A1型花岗岩, 属于大陆裂谷岩浆活动的产物。结合区域地质资料和前人研究成果, 认为小孤梁片麻状正长花岗岩可能是北山地区最早响应Rodinia超大陆裂解的花岗岩浆记录。

Neoproterozoic A-type granites in northern Beishan Orogenic Belt: Early response of the Rodinia supercontinent break-up

The Beishan Orogenic Belt is located in the southern margin of the Central Asian orogenic belt (CAOB) and there are several Precambrian microcontinents in the belt. A correct understanding of these Precambrian microcontinents is the key to the study of structural framework division and tectonic evolution process of the orogenic belt. In this paper, the Xiaoguliang gneissic syenogranite from the Mingshui block in the northern belt of Beishan is selected for petrology, zircon U-Pb chronology, element geochemistry and Hf isotope analysis. The results show that the LA-ICP-MS zircon U-Pb age of the Xiaoguliang gneissic syenogranite is 784±2.7Ma, revealing the existence of Neoproterozoic basement rocks in the northern belt of Beishan. The whole rock samples are high in SiO2 (72.22%~74.06%), ALK (7.50%~8.33%), Zr+Y+Nb+Ce (567.6×10-6~644.7×10-6) and 10000×Ga/Al (2.71~2.81), low in Al2O3 (12.39%~12.95%), MgO (0.40%~0.54%) and CaO (0.91%~1.38%). The trace elements are rich in Rb, K, Th and U and poor in Sr, Nb, Ta, P and Ti. All samples are enriched in LREE and depleted in HREE with obvious negative Eu anomaly. Calculated by zirconium saturation temperature, the crystallization temperature of Xiaoguliang gneissic syenogranite is 824~859℃. Combined with the geochemical characteristics above, the Xiaoguliang gneissic syenogranites should be defined as A-type granite. The zircon εHf(t) values ranges from -19.55 to -13.81, which are all negative values. tDM1 model ages from 1.56 to 1.33Ga and tDM2 model ages from 2.02 to 1.65Ga, indicating that there magma source may be originated from Paleoproterozoic crust partial melting. A1-type granites showed in the diagrams of tectonic setting discrimination imply that the Xiaoguliang gneissic syenogranites were most likely the products of continental rifting magmatism. Combined with regional geological data and previous research results, the Xiaoguliang gneissic syenogranite may be the earliest granitic record in response to the break-up of the Rodinia supercontinent in the Beishan Orogenic Belt.