内蒙古东乌旗晚石炭世角闪石岩锆石U-Pb年龄、地球化学及其构造意义

Zircon U-Pb dating and geochemistry of the Late Carboniferous hornblendite in Dong Ujimqi Inner Mongolia and its tectonic significance

为研究兴蒙造山带晚古生代地幔性质和造山演化特征,对东乌旗巴彦都兰新识别出的角闪石岩进行了岩相学、角闪石电子探针分析、锆石LA-ICP-MS U-Pb年代学、锆石Hf同位素和岩石地球化学分析。结果表明,东乌旗角闪石岩主要由角闪石(85%~90%)、少量斜长石(5%)和磁铁矿(<5%)等组成。获得的角闪石岩测年样品的锆石U-Pb年龄为310±1 Ma,表明岩体形成于晚石炭世。6件样品的地球化学数据显示,SiO2含量为48.66%~52.50%,MgO含量11.53%~13.10%,Al2O3含量为10.11%~12.18%,Na2O+K2O含量2.21%~3.13%,小于3.5%,m/f比值为0.36~0.45,Mg#值为70.27~74.54,TFeO值为7.77~10.14;稀土元素球粒陨石标准化配分模式表现为轻稀土相对富集的右倾型,岩石富集大离子亲石元素Rb、K等,相对亏损高场强元素Nb、Ta、Ti。电子探针分析结果显示角闪石岩中角闪石属于钙质角闪石,具有幔源角闪石的特点。角闪石岩中锆石εHf(t)值为7.47~15.54,单阶段地壳模式年龄(TDM1)为847~332 Ma,表明其源区主要为亏损地幔。结合区域地质演化,认为东乌旗角闪石岩可能形成于古亚洲洋俯冲背景下,这一发现为兴蒙造山带晚古生代地幔构造岩浆演化提供了岩石证据。

This study undertookelectron microprobe, zirconU-Pb dating and petrographic, geochemical analyses onthehornblendite firstly recognized in the Bayandulan of Dong Ujimqi to understand the characteristics of the late Paleozoic mantle nature and structure. In these rocks, mainrockformingminerals are hornblende, someplagioclase and magnetite.The dating results indicate that the hornblendite formed in the Late Carboniferous 310±1 Ma. The geochemical data of the samples show SiO2(48.66%~52.50%), MgO(11.53%~13.10%), Al2O3(10.11%~12.18%), Na2O+K2O(2.21%~3.13%, <3.5%, m/f(0.36~0.45), Mg#(70.27~74.54), TFeO(7.77%~10.14%). The standardized distribution pattern of REE chondrites is right-leaning with relatively enriched LREE.The rocks are enriched in LILE(Rb, K), and depleted in HFSE(Nb, Ta, Ti), and there are obvious negative anomalies of Nb, Ta and Ti. Electron probe analyses results show that hornblende belongs to calcic amphibole and has the characteristics of mantle hornblende. The εHf(t) values of zircons from the hornblendite vary between 7.47~15.54, and their Hf one-stage model ages vary from 847 to 332 Ma, implying that their magma source was derived from a depleted mantle.Combined with regional tectonic evolution, the hornblendite formed in the subduction setting of the Paleo-Asian Ocean.This discovery provides rock evidence for the late Paleozoic mantle tectonomagmatic evolution.