| 赣西北小九宫-沙店白垩纪花岗岩的岩石成因及构造意义 |
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| 引用本文： | 王艳,马昌前,王连训,刘园园.赣西北小九宫-沙店白垩纪花岗岩的岩石成因及构造意义[J].地球科学,2020,45(4):1115-1135. |
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| 作者姓名： | 王艳 马昌前 王连训 刘园园 |
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| 作者单位： | 1.广东省有色地质勘查院, 广东广州 510080 |
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| 基金项目： | 中央高校基本科研业务费专项资金资助项目CUGCJ1711中央高校基本科研业务费专项资金引智项目DL20180059 |
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| 摘 要： | 赣西北-湘东北地区出露较多晚中生代花岗岩，并与中、下扬子地区晚中生代花岗岩组成一条NEE向岩浆岩带.通过对赣西北小九宫和沙店花岗岩进行系统的岩石学、年代学、元素地球化学、全岩Sr-Nd同位素研究，探讨其岩石成因及其构造意义.小九宫和沙店花岗岩的岩石类型主要为中粗粒斑状黑云二长花岗岩，LA-ICP-MS锆石U-Pb定年结果表明，其形成年龄分别为124±1 Ma和125±1 Ma，均为燕山晚期花岗岩.小九宫和沙店花岗岩均具有高钾、钙碱性、贫铁镁的含量特征，主要为弱过铝质花岗岩.两岩体具有相似的稀土元素分布型式和微量元素特征，表现为轻稀土富集的右倾型式，具有明显的Eu负异常（Eu/Eu*=0.17~0.50），富集K、Rb、Th、U等大离子亲石元素，亏损Ba、Sr、P和高场强元素Nb、Ta、Ti等.主量、微量元素地球化学特征显示，两岩体为高钾钙碱性Ⅰ型花岗岩.小九宫和沙店花岗岩的εNd（t）值分别为-8.06~-6.20、-6.51~-6.08，两阶段模式年龄（TDM2）分别为1.42~1.57 Ga、1.42~1.45 Ga.Sr-Nd同位素组成和地球化学特征表明，两岩体主要来源于下地壳源区，其源岩可能为中元古代中性-基性火成岩.小九宫和沙店花岗岩体通过岩墙扩张作用方式被动侵位，结合区域构造背景，认为两岩体形成于伸展构造环境.地幔物质上涌可能为下地壳火成岩部分熔融提供持续的热源，岩浆在上升过程中经历明显的分离结晶作用.岩体中含有少量微粒包体及围岩捕虏体，反映岩浆在上升过程中可能受到较弱的混染作用.区域对比表明，从燕山早期到燕山晚期，赣西北-湘东北地区花岗质岩浆源区存在压力变小的趋势，可能反映了地壳的明显减薄.
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| 关 键 词： | 锆石U-Pb年代学 地球化学 Sr-Nd同位素 Ⅰ型花岗岩 岩石成因 构造意义 岩石学 |
| 收稿时间： | 2019-05-14 |
Petrogenesis and Tectonic Implications of the Cretaceous Granites from Xiaojiugong-Shadian,Northwest Jiangxi Province |
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| Abstract: | Many Late Mesozoic granites occur in northwest Jiangxi Province and northeast Hunan Province, which constitute a NEE-trending magmatic belt together with the Late Mesozoic granites in the Middle and Lower Yangtze regions. In this paper, it presents the petrogenesis and tectonic implications, based on the systematic petrographical, zircon U-Pb geochronological, geochemical and isotopic analyses of two representative samples from the Xiaojiugong and Shadian granites. The Xiaojiugong and Shadian intrusions principally consist of medium to coarse-grained porphyritic biotite monzogranite. LA-ICP-MS zircon U-Pb dating yields magma crystallization ages of 124±1 Ma for the Xiaojiugong intrusion and 125±1 Ma for the Shadian intrusion. The two intrusions emplaced in the period of Late Yanshanian. The Xiaojiugong and Shadian granites are all characterized by high-K, calc-alkaline and low contents of Fe2O3T and MgO, indicating they are mainly weakly peraluminous granites. They show similar distribution patterns of REE and trace elements, i.e. enrichment in LREE and large-ion lithophile elements of K, Rb, Th and U, depletion in Ba, Sr, P and high field strength elements of Nb, Ta and Ti, and pronounced negative Eu anomalies (Eu/Eu*=0.17-0.50). Major and trace elements show that the Xiaojiugong and Shadian granites are high-K calc-alkaline Ⅰ-type granites. The Xiaojiugong granites have εNd(t) values of -8.06 to -6.20 and two-stage model ages (TDM2) of 1.42-1.57 Ga. The Shadian granites have εNd(t) values of -6.51 to -6.08 and the corresponding TDM2 of 1.42-1.45 Ga. The Sr-Nd isotopic composition and geochemical characteristics of the Xiaojiugong and Shadian granites suggest the main origin of lower crust. Their source might be the Mesoproterozoic neutral-basic igneous rocks. The Xiaojiugong and Shadian granites passively emplaced through expansion of the dikes. Combined with the regional tectonic background, the granites formed in extensional environment, and upwelling mantle probably provided a continuous heat source for the partial melting of lower crustal igneous rocks. The magma experienced a significant process of fractional crystallization during upwelling. A handful of microgranular enclaves and surrounding rock xenoliths in the granites reflect that the granitic magma might have been weekly contaminated when rising. Regional comparison shows that, from Early Yanshanian to Late Yanshanian, the pressure of the granitic magma source decreased significantly in the northwest Jiangxi and northeast Hunan. This might indicate that the crust had undergone significant thinning in the period of Late Mesozoic. |
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