贵池岩体的锆石定年和地球化学:岩石成因和深部过程

摘要

贵池地区的贵池岩体和茅坦岩体是下扬子地区两个重要的A型花岗岩岩体.野外岩相学观察将贵池岩体分为中心相和边缘相.在前人工作的基础上,对其进行了锆石U-Pb定年以及元素、同位素地球化学研究.两个岩体的SiO2和K2O含量高,MgO、Al2O3、CaO、P2O5、MnO、TiO2含量低；K2O/Na2O比值、FeOT/MgO比值高,碱铝比(NK/A)在0.88 ～0.93之间,铝过饱和度(A/CNK)在1.02 ～1.10之间.同时,这些岩体稀土含量较高,轻稀土富集,具有明显的Eu负异常,大离子亲石元素Ba、Sr明显负异常以及很低的Cr、Ni含量.初始87Sr/86Sr (t)比值介于0.7065 ～0.7111之间,εNd(t)较为集中,为-6.7 ～-7.3.其中,贵池岩体边缘相和茅坦岩体SiO2含量相对较高,全碱含量相对较低,稀土含量相对较低,TAS图上位于亚碱性系列,U -Pb定年结果分别为124.6±2.0Ma和127.7±1.8Ma;贵池岩体中心相SiO2含量相对较低,全碱含量相对较高,TAS图上位于碱性系列,U-Pb定年结果显示了两期岩浆活动,分别为145.4±3.2Ma和123.0±3.7Ma.综合分析表明,贵池岩体边缘相和茅坦岩体源于中上地壳的部分熔融；贵池岩体中心相是两期岩浆混合的产物,分别是早期来自富集岩石圈地幔结晶分异的中酸性碱性岩浆和晚期形成贵池岩体边缘相的A型花岗岩浆.结合长江中下游地区高碱钙碱性侵入岩和火山岩的研究成果,表明127 ～123Ma是长江中下游地区地壳伸展最强烈的阶段.%The Guichi area in Anhui Province is located in fold belt of the Lower Yangtze region. There occur two A-type granite pluton, e. g. Guichi pluton and Maotan pluton. The former can be dividing into central face and marginal face by field observation. Based on present studies, we conduct zircon U-Pb dating, major, trace element and isotopic analysis on these rocks. The intrusive rocks in Guichi area are characterized by high SiO2, K2O and low MgO, A12O3, CaO, P2O5, MnO and TiO2 contents, with high ratios of K2O/Na2O and FeO/MgO. NK/A ratios are ranging from 0. 99 to 1. 04 and A/CNK from 0. 94 to 0. 98. These rocks have high 2 REE values and show evident Eu, Ba and Sr anomalies as well as LREE-enrichment features. They have initial 87Sr/86Sr(t) valuesrnof 0. 7065 ~0. 7111 and εNd(t) values of -6. 7~7. 3. The Maotan pluton and marginal face of Guichi pluton with relatively highrnSiO2 and low total alkali contents are the sub-alkaline series on TAS diagram. U-Pb dating of them are 124. 6 ±2. 0Ma and 127. 7 ± 1. 8 Ma. While the central face of Guichi pluton with relatively low SiO2 and high total alkali contents is the alkaline series on TAS diagram with U-Pb dating results showing two-stage magmatism, e. g. 145. 4 ±3. 2Ma and 123. 0 ±3. 7Ma. Integrated studies suggest that the marginal face of Guichi pluton and Maotan pluton were generated by partial melting of the middle and upper crust. However, the central face of Guichi pluton was product by mixing of two stage magmas. The early stage magma was formed by fractional crystallization of metasomatized enriched lithospheric mantle and the later was A-type granite magma which formed marginal face of Guichi pluton. Together with other accuracy dating data of volcanic rocks and intermediate-acid high-K calc-alkalic plutons in the Lower Yangtze region, it is implied that 127Ma to 123Ma is the peak period of extension.