川西甲基卡二云母花岗岩与伟晶岩脉地球化学特征及其地质意义
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中国地质调查局地质调查项目(DD20160074);四川省科技厅重点研发项目(2018SZ0276);中国地质调查局地质调查子项目(121201004000150017-93,12120113049500)


Geochemical characteristics of two-mica granite and granite pegmatite in Jiajika area, western Sichuan, and their geological implications
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    摘要:

    对川西甲基卡地区二云母花岗岩及伟晶岩脉的岩石学特征、地球化学特征进行了分析,探讨了二云母花岗岩物质来源、构造背景及其与伟晶岩脉的成因联系。分析结果表明,甲基卡二云母花岗岩的SiO2含量在73.93%~75.06%之间;全碱含量7.90%~8.36%,钾质含量较高,属高钾钙碱性系列岩石;Al2O3含量为14.24%~14.77%,A/CNK=1.14~1.24,具强过铝质S型花岗岩特征;ΣREE=31.18×10-6~41.67×10-6,LREE/HREE=4.15~6.41,δEu=0.46~0.70;CaO/Na2O=0.07~0.12(<0.3),指示其物源可能是含砂屑物质极少的泥质岩;Al2O3/TiO2=133.1~279.8,比值较高,表明甲基卡二云母花岗岩属高压低温型后碰撞强过铝质岩石。花岗伟晶岩脉SiO2含量在72.59%~80.91%之间,全碱含量5.26%~10.60%,Al2O3含量11.79%~17.64%,σ=0.74~3.80,A/CNK=0.98~2.38,ΣREE=4.03×10-6~8.29×10-6,LREE/HREE=2.61~10.40,δEu=0.18~0.68,与岩体差异明显,主、微量元素含量变化也较大。甲基卡二云母花岗岩与伟晶岩脉有密切的成因联系。岩浆不混溶作用可能是形成甲基卡含矿伟晶岩脉的关键因素,花岗质岩浆在上升过程中通过不混溶作用分离出富含挥发分的伟晶岩熔体,在运移或侵位过程中可能交代围岩矿物而使稀有金属元素进一步富集。二云母花岗岩浆和伟晶岩熔体性质的不同导致稀土元素和Th、Sr、Ti、Y、Rb、N等微量元素在两者中含量有明显差异。伟晶岩熔体在运移或就位过程中所经的路径不同和周围环境的差异导致不同位置产出的伟晶岩的元素含量有较大的变化。与二云母花岗岩相比,花岗伟晶岩的形成演化具有一定的跳跃性。

    Abstract:

    This paper discusses the petrogenesis of the two-mica granite and its genetic relationship with pegmatite in Jiajika area, west Sichuan Province, by detailed field investigation and laboratory analysis. The whole rock analytical results show that SiO2 content of two-mica granite is 73.93%~75.06%, and total alkali is 7.90%~8.36% with high concentration of K, suggesting high-K calc-alkaline series; the content of Al2O3 is 14.24%~14.77%, and A/CNK=1.14~1.24, implying strong peraluminous S type granite; ΣREE=31.18×10-6~41.67×10-6,LREE/HREE=4.15~6.41,δEu=0.46~0.70, and CaO/Na2O=0.07~0.12(<0.3),indicating that its source might have been pelite with rare content of psammite, and high ratio of Al2O3/TiO2 (133.1~279.8) implies that it is high-pressure-low-temperature post-collision granite. The SiO2 content of the granite pegmatite is 72.59%~80.91%, the total alkali is 5.26%~10.60%, Al2O3 is 11.79%~17.64%, σ=0.74~3.80; A/CNK=0.98~2.38; ΣREE=4.03×10-6~8.29×10-6, LREE/HREE=2.61~10.40, and δEu=0.18~0.68. There are considerable differences between two-mica granite and granite pegmatite in the aspect of the content of major and trace elements. And there is close genetic relationship between granite and pegmatite in the area. Magma immiscibility might be the key factor for the formation of (ore-bearing) granite pegmatite. The pegmatite melt riched in volatile components might be separated during the upwelling of granite magma and metasomatized minerals in wall rock to further concentrate rare metal elements during its migration. The difference between two-mica granite and pegmatite might cause the apparent differences in REEs and elements such as Th, Sr, Ti, Y, Rb and Nb. The different characteristics of pathways and environments that the pegmatite melt passed and emplaced might have resulted in element concentration variance in different sites. In comparison with two-mica granite, there existed a certain degree of jumping characteristic during the formation and evolution of the pegmatite in Jiajika area.

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李名则,秦宇龙,李峥,等, 2018. 川西甲基卡二云母花岗岩与伟晶岩脉地球化学特征及其地质意义[J]. 岩石矿物学杂志, 37(3):366~378.
LI Ming-ze, QIN Yu-long, LI Zheng, et al, 2018. Geochemical characteristics of two-mica granite and granite pegmatite in Jiajika area, western Sichuan, and their geological implications[J]. Acta Petrologica et Mineralogica, 37(3): 366~378.

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  • 收稿日期:2017-05-23
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  • 在线发布日期: 2018-05-25
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