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

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    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,徐云峰,武文辉,刘伟,叶亚康,周雄,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, XU Yun-feng, WU Wen-hui, LIU Wei, YE Ya-kang, ZHOU Xiong,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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  • Received:May 23,2017
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  • Online: May 25,2018
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