铜陵矿集区深部发现126Ma的正长花岗岩:来自3000m科学钻探的证据

铜陵矿集区3000 m科学钻揭示,舒家店地区在1775 m以下存在红色的正长花岗岩,从该深度继续钻进约700 m后仍为这种岩石,而这种岩石在地表或其他钻孔中从未见到过。研究表明,正长花岗岩以富硅、富碱,富含高场强元素如Zr、Nb、Y和轻稀土元素(Eu除外)和较低的Mg O、Ba和Sr的含量为特征,全岩的10000Ga/Al比值为2.6,Zr+Nb+Ce+Y350×10-6,稀土元素配分模型具有明显的Eu负异常,微量元素原始地幔标准化曲线具有明显的Ba、Sr、P、Ti亏损,表现出A型花岗岩的地球化学属性。锆石LA-MC-ICP-MS U-Pb定年结果表明,该A型花岗岩的年龄为126 Ma,与长江中下游地区其他A型花岗岩的时代一致,属于长江中下游地区三期岩浆活动当中的第二期岩浆活动的产物。研究表明,长江中下游地区3次岩浆作用与岩石圈的多次拆沉作用密切相关,而且每一次拆沉作用都会使壳幔附近的深位岩浆房向地壳浅部迁移,而A型花岗岩则是迁移到浅部的深位岩浆房诱发的地壳部分熔融形成的岩浆,沿断裂带侵位而形成,也标志着本区的构造背景在126 Ma时处于拉张的高峰期。

The discovery of the syenogranite with an age of 126 Ma in the depth of Tongling: Evidence from 3000 meters scientific drilling

Tongling Scientific Drilling revealed that there exists a red syenogranite at 1775 meters below the surface in Shujiadian,Tongling,which has never been seen at the surface or in other drill holes of Tongling area before,and drilling about 700 meters still intersects the rock. Studies show that the syenogranite is characterized by higher content of SiO₂ and Na₂O+K₂O, and lower content of MgO, Ba and Sr, enrichment of HFSE (high field strength element) such as Zr, Nb, Y and LREE (light rare earth element) except Eu. The rock has a ratio of 10000Ga/Al>2.6,the total Zr+Nb+Ce+Y>350×10^-6, obvious Eu negative anomaly in the chondritenormalized REE patterns and depletion of Ba, Sr, P and Ti in the primitive mantle-normalized trace element diagram, showing Atype granite property. Zircon LA-MC-ICP-MS dating yielded a U-Pb age of 126 Ma for the A-type granite, which is consistent with the age of the other A-type granites in the middle and lower reaches of the Yangtze River (MLRYR), and belongs to the product of the second episode in the three episodes of the magmatism in the area. Researches indicate that the three episodes of magmatism in the MLRYR region had a closed relation to the multiple lithosphere delamination. With the continuing delamination, the deep magma chamber near the crust-mantle migrated to the shallow part, causing partial melting of the crust to form A-type granite when the magma was emplaced along the deep fault. It also marked a peak pull-apart in the tectonic background of the MLRYR region.