大别山北缘定远组双峰式火山岩-新元古代晚期裂解事件记录  

Bimodal Volcanic Rocks of Dingyuan Formation on the Northern Margin of Dabie Belt: A Witness of Late Neoproterozoic Rifting Event

作　　者：许雅雯 李承东[1] 赵利刚 孙烜烨 许腾 滕雪明 Xu Yawen;Li Chengdong;Zhao Ligang;Sun Xuanye;Xu Teng;Teng Xueming(Tianjin Center of China Geological Survey,Tianjin 300170,China)

机构地区：[1]中国地质调查局天津地质调查中心,天津300170

出　　处：《地球科学》2021年第8期2732-2750,共19页Earth Science

基　　金：国家自然科学基金项目(No.41272065);中国地质调查局项目(Nos.DD20160043,DD20190371)。

摘　　要：为了了解大别山北缘构造属性,对定远组地层组成、形成时代及地球化学特征进行调查与研究.野外调查表明,定远组主要由一套变火山岩及云母片岩、云母石英片岩、浅粒岩、板岩等组成,其中变火山岩包括变玄武岩与变流纹质火山岩,并构成典型的双峰式火山岩建造;此外,还含有早古生代构造地层单位.运用LA-ICP-MS对酸性火山岩锆石进行U-Pb定年,获得725.7±1.4 Ma、736.6±5.4 Ma的年龄,形成时代为新元古代,不是前人认为的早古生代.变玄武岩分为低Ti(TiO2=1.19%)和高Ti(TiO2平均含量为3.11%)两种类型.低Ti玄武岩稀土总量较低(低于N-MORB),岩浆来自亏损的地幔源区.高Ti玄武岩又可以分为两种类型,一类富集Nb、Ta等元素,不相容元素的比值接近大陆裂谷玄武岩;另一种类型亏损Nb、Ta、Th、U等元素,岩浆可能来源于被下地壳或蚀变大洋地壳改造的地幔,其Th/Ta为1.6,与大陆裂谷玄武岩相当.总之,变玄武岩地球化学特征具有很大差别,是地幔源区不均一的反映.变酸性火山岩富集大离子亲石元素Rb、Ba、Th、U、K,亏损Nb、Ta、P、Ti等元素,锆石Hf同位素分析显示主体εHf(t)值为-3.0~-10,二阶段Hf模式年龄TDM2(Hf)为1630~2258 Ma,揭示其来源于古老地壳的部分熔融.定远组新元古代双峰式火山岩形成于大陆裂谷环境,并非岛弧构造背景.定远组双峰式火山岩及广泛分布的同时代岩浆岩,揭示了扬子陆块北缘在新元古代(800~611 Ma)时期一次重要的大陆边缘裂解-岩浆事件,是Rodinia超级古大陆裂解作用深部地球动力学的地表响应.In order to understand the tectonic properties of the northern margin of Dabie belt, the stratum, age and geochemical characteristics of Dingyuan Formation were investigated and studied. Field survey shows that the Dingyuan Formation is composed of a set of metavolcanic rocks, mica schist, mica quartz schist, leptite and slate, etc.. The metavolcanics are mainly metabasalts and meta-rhyolitic volcanics which constitute typical bimodal volcanic formation. In addition, it contains Early Paleozoic tectono-stratigraphic unit. The LA-ICP-MS U-Pb dating of zircon from acidic volcanic rocks yields the ages of 725.7±1.4 Ma and 736.6±5.4 Ma, indicating the Neoproterozoic rather than the Early Paleozoic ages. The metabasalts are divided into two types: low Ti(TiO2=1.19%)and high Ti(TiO2=3.11% on average). The total amount of rare earth elements of low Ti basalts is lower than N-MORB type and the magma came from the depleted mantle source area. The high Ti basalt can be further divided into two types. One type is rich in Nb and Ta, and the ratio of incompatible elements is close to that of continental rift basalt. The other type of loss-Nb, Ta, Th, U and other elements, the magma probably came from the modified mantle by the lower crust or altered oceanic crust. Its Th/Ta is 1.6, which is close to the Th/Ta ratio of the continental rift basalt. In a word, the geochemical characteristics of the metamorphic basalts are very different, which reflects the heterogeneity of the source area of the mantle. Large ionic lithophile elements Rb, Ba, Th, U and K are enriched in acidic volcanic rocks, depletion Nb, Ta, P, Ti and other elements, the value of εHf(t) is-3.0 to-10 by zircon Hf isotope analysis, two-stage Hf mode age(TDM2(Hf)) is 1 630-2 258 Ma, it reveals that it came from a partial melting of the ancient crust. The Neoproterozoic bimodal volcanic rocks of the Dingyuan Formation was formed in the continental rift environment, not in the background of island arc structure. The bimodal volcanic rocks and the widely distribu

关 键 词：定远组 双峰式火山岩 新元古代裂解 大别山北缘 地质年代学 地球化学 

分 类 号：P581[天文地球—岩石学] P597[天文地球—地质学]