佛冈铝质A型花岗岩的地球化学及其形成环境初探

GEOCHEMISTRY AND TECTONIC SETTING OF THE FUGANG ALUMINOUS A-TYPE GRANITE, GUANGDONG PROVINCE,CHINA—A PRELIMINARY STUDY

对广东佛冈岩体的初步研究表明 ,这一以往一直被认为是典型S型花岗岩的大岩基 ,其主体 (燕山第三期 )应为铝质A型花岗岩。岩石富SiO2 [W(SiO2 )为 69 2 1 %～ 77 72 % ]、准铝质 (A/NKC =0 97～ 1 0 6)。FeO /MgO比值较高 (平均 1 0 6) ,高于S型和I型花岗岩。在微量元素组成上 ,岩体明显亏损Sr、Ba、Ti、P、Eu,富Ga和高场强元素Nb,Ta,Zr等 ,与我国东南沿海A型花岗岩和世界典型A型花岗岩微量元素分布型式相似。全岩Rb Sr等时线年龄为 1 5 8± 1 7Ma,(87Sr/86 Sr) i为 0 71 5 6。钕模式年龄为 1 2 8～ 1 64Ga。一般认为A型花岗岩来源于I型花岗岩或大量抽提了S型花岗岩物质后的地壳源区 ,本研究表明 ,佛冈岩体可能直接来源于地壳物质的部分熔融 ,但不排除有地幔物质的加入。佛冈铝质A型花岗岩 南昆山A型花岗岩 恶鸡脑碱性正长岩这一造山后至非造山的构造岩浆组合的确定 ,揭示了区内乃至整个中国东南大陆岩石圈碰撞造山运动的结束和大规模的拉张伸展运动应始于晚侏罗世 ,至白垩纪造山带山根构造完全消失、拉张

The massive Fugang granitic pluton located in the northern part of Guangdong Province, on the conjunction of the NE Guangzhou-Conghua fault and the EW Fugang-Fengshun transcurrent fault. The exposure area is measured at 5000 km 2. Petrochemically, the pluton is aluminous or sub-aluminous, rich in SiO 2 (69.21 % -77.72 %, averaging 74.99 %), and is characterized by high FeO */MgO ratio (with an average of 10.6), which is different from the ordinary S- and I-type granites. The granite is rich in Ga and Nb, Ta, Zr and other HFSE (high field strength elements), and depleted in Sr, Ba, P, Eu, which can well be analogued to the known A-type granites in the Southeast China coastal area and other parts of the world. The whole-rock Rb-Sr isochron age of the pluton is 158±17 Ma (MSWD=2.79),with an initial 87Sr/ 86Sr ratio of 0.7156. The Nd isotopic model (T 2DM) ages range from 1.28 to 1.64 Ga, a little bit lower than those of the metamorphic basement and Mesozoic granitoids in the area. It can be deduced that the granite was probably derived from crustal material. In regard to the major and trace element data, the Fugang granite belongs to the post-collisional A-type granite. It is commonly accepted that the A-type granite was derived from the I-type sources or crustal sources depleted in S-type melts. The preliminary investigation of the Fugang granite shows that the A-type granite can be derived directly from the typical S-type sources. The petrogenesis model requires further investigation. The association of the Fugang aluminous A-type granite(columbite and olivine bearing), Nakunshan A-type granite and Erjinao alkali-syenite occurred along the Enping-Xinfeng fault and it is very important for interpreting the regional tectonic evolution. It is suggested that there would have occurred large-scale rifting in Southeast China during the Mesozoic starting from Late Jurassic. The A-type granite-alkali-syenite association is probably an important clue to trace the shift of dominated tectonic settings from NE squeezing to EW extension.