| 扬子克拉通北缘新元古代A型花岗岩的发现及大地构造意义 |
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| 引用本文： | 曹正琦,蔡逸涛,曾佐勋,胡正祥,陈静,蒋幸福,孙政权,吴波,刘成新,郭盼.扬子克拉通北缘新元古代A型花岗岩的发现及大地构造意义[J].地球科学,2017,42(6):957-943. |
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| 作者姓名： | 曹正琦 蔡逸涛 曾佐勋 胡正祥 陈静 蒋幸福 孙政权 吴波 刘成新 郭盼 |
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| 作者单位： | 1.中国地质大学地球科学学院，湖北武汉430074 |
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| 基金项目： | 国家自然科学青年基金项目41402075国家自然科学基金重点项目41230206-1中国地质调查局项目“大悟等4幅1:5万区域地质矿产调查”12120114048301 |
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| 摘 要： | 对大磊山片麻状花岗岩的研究可以为新元古代罗迪尼亚超大陆在扬子克拉通北缘裂解提供约束.在详细野外地质调查和岩相学工作基础上，对该片麻状花岗岩进行了系统的锆石U-Pb定年、全岩地球化学和锆石原位Lu-Hf同位素分析，研究发现，这些片麻状花岗岩富硅(SiO2=73.18%~77.40%)、碱(Na2O+K2O=8.07%~8.70%)、贫铝(Al2O3=12.11%~13.92%)和镁(MgO=0.10%~0.34%)，富集LILE、Ga、Rb、Th，Zr和Hf元素具明显正异常，Nb、Sr、P、Ti等元素具明显负异常，表现出后造山A型花岗岩(A2型)的特征.这些花岗岩中，锆石均具典型的震荡环带，Th/U比值均大于0.5，为岩浆成因；两个样品的LA-ICP-MS(laser ablation inductively coupled plasma mass spectrometry)锆石的谐和分别是:801.3±3.0 Ma(MSWD=0.62，n=21) 和796.1±6.3 Ma(MSWD=1.70，n=15)，其中一个样品继承锆石谐和年龄为845.0±12.0 Ma(MSWD=1.30，n=6)，表示他们是新元古代岩浆结晶产物.锆石εHf(t)值变化于-7.5~+8.0，正εHf(t)值对应的亏损地幔单阶段模式年龄(tDM1)为1 242~1 059 Ma，负εHf(t)值对应的地壳两阶段模式年龄(tDM2)为1 636~1 981 Ma，显示该地区存在的最古老物质是古元古代(老至1 981 Ma).这些数据结果表明形成大磊山A型花岗岩的初始物质主要为元古代古老地壳物质，暗示该岩浆源自于壳幔混合作用，幔源端元可能源自于伸展拉张背景下地幔岩浆上涌.结合区域研究成果认为，该A型花岗岩的形成与罗迪尼亚超大陆聚合后-裂解中的陆缘弧后拉张环境所引起的深部古元古代地壳拉张垮塌有关.
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| 关 键 词： | 扬子克拉通北缘 大磊山地区 A型花岗岩 锆石U-Pb定年 Lu-Hf同位素 岩石学 |
| 收稿时间： | 2016-12-10 |
Discovery of Neoproterozoic A-Type Granite in Northern Yangtze Craton and Its Tectonic Significance |
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| Abstract: | Researches on the Daleishan gneissic granites can constrain Neoproterozoic Rodinia supercontinent breakup in the northern margin of Yangtze craton. In this paper, zircon U-Pb dating, in-situ Lu-Hf isotope and whole-rock geochemical compositions analyses are presented for the Daleishan gneissic granites in the northern margin of the Yangtze craton. The results show that the Daleishan gneissic granites have high contents of SiO2 (73.18%-77.40%) and alkali (8.07%-8.70%), but low contents of Al2O3 (12.11%-13.92%) and MgO (0.10%-0.34%), and display enrichment in Ga, Rb, Th, Zr, Hf but depletion in Nb, Sr, P, Ti, which indicates the post-orogenic A-type affinity. Zircons selected from two Daleishan gneissic granitic samples show typical oscillatory zone structure with high ratios of Th/U (greater than 0.5), exhibiting the magmatic zircon genesis. LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) zircon U-Pb data from the two samples yield Neoproterozoic crystallization ages of 801.3±3.0 Ma (MSWD=0.62) and 796.1±6.3 Ma (MSWD=1.70), respectively, which are interpreted to represent the intrusion time. In-situ Hf isotopic compositions show that the two gneissic granitic samples have wide range of εHf(t) values that are indicative of a heterogeneity property. εHf(t) values of these zircon ranges from -7.5 to +8.0, with the positive εHf(t) values, coinciding with the single phase model age (tDM1) of the depleted mantle of (1 242-1 059 Ma), and the negative εHf(t) values coinciding with two-phase model age (tDM2) of the crustal of (1 636-1 981 Ma), suggesting that the oldest existing material in the region belongs to Paleoproterozoic (as old as 1 981 Ma). These data show that the source materials of the Daleishan A-type granites were usually derived from Palaeoproterozoic ancient crust. The magmatic source of the granites was originated from the crust-mantle migmatization, and the mantle composition could have been generated from the upwelling of the mantle magma in the extensional background. Combining with the pre-existing regional publications, it is proposed that the Daleishan A-type granite was formed at the crustal extensional background in the continental back-arc setting associated with the assembly-break up process of the Rodinia supercontinent. |
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