再论南岭侏罗纪“铝质”A型花岗岩的成因及其对古地温线的制约  

PETROGENESIS OF THE JURASSIC ALUMINOUS A-TYPE GRANITES IN THE NANLING AREA,SOUTH CHINA AND ITS CONSTRAINT ON PALEO-GEOTHERM

作　　者：汪洋[1] 

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083

出　　处：《大地构造与成矿学》2008年第3期365-381,共17页Geotectonica et Metallogenia

基　　金：国家自然科学基金(批准号:40572128;40376013;40104003)资助成果

摘　　要：对南岭地区侏罗纪4个典型“铝质”A型花岗岩岩基——柯树北、寨背、西山和南昆山的成因分析表明：柯树北、寨背岩基中的低分异花岗岩SiO2-70％，A／CNK〈1．1，CaO≥1％，高Zr、Ba含量，是下地壳部分熔融产物；而SiO2含量较高者由低分异花岗岩岩浆通过分离结晶演化而来。西山花岗质火山-侵入杂岩也是下地壳部分熔融产物。南昆山花岗岩为高硅花岗岩，贫Zr、低Ba、Sr和Eu／Eu值，但具有高的Nb、Ga、REE含量和Ga／Al比值，在Whalen等（1987）图解中地球化学参数落在A型花岗岩区域内。碱性玄武岩浆分离结晶的成岩模式无法解释南昆山岩基较大的体积、均一的成分和低的Nb／Ta比值。详细的成岩分析表明，南昆山花岗岩可能是先期侵入的（幔源）碱性正长岩在富水和相对低温低压条件下发生部分熔融的产物。由这些“铝质”A型花岗岩的熔融温压条件估算得出热流值达到80～95mWm。的南岭地区侏罗纪古地温线。由古地温线推算出的岩石圈厚度45～75km。南岭侏罗纪高热流背景及其对应的花岗质岩浆活动可能与后碰撞造山阶段岩石圈地幔拆沉或被“热侵蚀”有关，但并不一定意味着岩石圈伸展的大地构造环境。The petrogenetic analysis on four typical Jurassic‘aluminous＇ A-type granite batholiths, i.e. Keshubei, Zhaibei, Xishan, and Nankunshan in the Nanling Area, South China shows that, the least-fractionated granites in the former two batholiths, which have characteristics of SiO2 ≈70% , A/CNK 〈 1.1, CaO≥1% , and high Zr and Ba contents, were generated by partial melting of the lower crust; meanwhile, the fractionated granites evolved from the least-fractionated ones by fractional crystallization. The Xishan volcanic-intrusive granitic complex was also generated by partial melting of the lower crust. The Nankunshan batholith belongs to high silica granite, and has low contents of Zr, Ba, Sr and low Eu/Eu value, but high contents of Nb, Ga, REE and high Ga/Al ratio. In Whalen et al. （1987） discrimination diagrams, the samples of the Nankunshan granite plot within the A-type granite field. However, the petrogenetic model requiring fractional crystallization of alkali basaltic magma cannot explain the large volume, homogeneous composition, and lower Nb/Ta ratio of the Nankunshan batholith. The detailed analysis suggests that Nankunshan granite was generated by partial melting of the previous intruded （mantlederived） alkali syenite under H2O-rich and lower P-T conditions. The paleo-geotherms of 80 -95mWm-2 heat flow were estimated from melting pressures and temperatures of these Jurassic ＇aluminous＇ A-type granites. The lithospheric thickness of 45 - 75kin was inferred from paleo-geotherms. The high heat flow and related granitic magmatism in the Jurassic Nanling Area might be due to the foundering or ＇thermal erosion＇ of lithospheric mantle during post-collisional stage of orogeny, but might not imply a tectonic setting of lithospheric extension.

关 键 词：A型花岗岩 岩石成因 地温线 侏罗纪 南岭 华南 

分 类 号：P581[天文地球—岩石学]