郯庐断裂带南段两侧花岗岩地球化学特征:对大别造山带中-下地壳流动的限定

Geochemical features of granites from two sides of the southern segment of the Tan-Lu fault zone: Evidence for middle-lower crust flow of the Dabie orogenic belt

大别造山带形成于印支期华南、华北板块陆-陆碰撞过程中。大别造山带造山后发生了山根减薄,其可能的动力学机制为岩石圈拆沉或中-下地壳的塑性流动。为了检验大别造山带是否在造山后发生过中-下地壳流动,本次工作从对比郯庐断裂带南段两侧大别造山带及洪镇、月山地区花岗岩地球化学特征入手,对同样位于郯庐断裂带南段东侧的徐桥岩体开展地球化学研究,为大别造山带造山后是否经历过中-下地壳流动提供证据。本次工作对徐桥岩体的研究表明,该岩体侵位时间为127Ma左右;具有较高的Si O_2和全碱含量;明显富集Rb、Th、U、Pb等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE);稀土元素配分图解中表现为轻稀土富集和重稀土亏损的右倾配分模式,具有较弱的Eu负异常。徐桥岩体的地球化学特征与造山带东侧的月山岩体类似,且与下扬子地区沿江地区侵入岩基本一致。但同样位于造山带东侧的洪镇岩体,其地球化学特征与月山、徐桥岩体明显不同,而与大别造山带一致的地球化学特征,说明两者可能具有相似的源区。基于以上分析,笔者推断洪镇花岗岩的物质来源可能为大别造山带加厚的地壳。而大别造山带东侧出现造山带地壳物质的现象表明造山带在造山后发生过地壳的塑形流动。由于大别造山带中-下地壳物质穿过郯庐断裂带南段,表明郯庐断裂带造山后大规模左行平移事件应发生于大别造山带地壳流动变形之前。

The Dabie orogenic belt（ DOB） was formed by continent-continent collision between the North China Block（ NCB） and the South China Block（ SCB）. Crust thinning occurred in the DOB during the post-orogenic period,and the possible mechanism is lithospheric delamination or middle-lower crust flow. To examine whether middle-lower crust flow happened or not in the DOB,we compared geochemical features of granites from two sides of the southern segment of the Tan-Lu fault zone,such as granites in the DOB and Hongzhen pluton,Yueshan pluton and Xuqiao pluton in the Lower Yangtze region east side of the DOB. If geochemical features of granites in the Hongzhen area are the same as those in the DOB,a conclusion should be drawn that the middle-lower crust of the DOB flow towards SE. Zircon U-Pb dating show that Xuqiao pluton occurred at 127 Ma. The granites at Xuqiao show high contents of Si O_2 and total alkali,and are enriched in LILEs（ e. g. Rb,Th,U,Pb） but depleted in HFSEs（ e. g. Nb,Ta,Ti）. They are characterized by rightward heeling chondrite-normalized REE patterns,with enrichment of LREEs and depletion of HREEs as well as weakly negative Eu anomalies. The Xuqiao pluton shows similar Sr-Nd isotope features as the Yueshan pluton and is accordant with granites in the Lower Yangtze region. However,the Hongzhen pluton,located east side of the DOB belt near the Xuqiao and Yueshan pluton,show distinct different Sr-Nd isotope features with granites in the Xuqiao and Yueshan pluton,while exhibits the same Sr-Nd-Pb isotope compositions as that in the DOB. Therefore,the protolith source of the Hongzhen granites may be the thicken middle-lower crust of the DOB. During thinning of crust of the DOB,the thicken crust flow towards SE,resulting in crust thinning in the DOB and thickening in the Hongzhen area. Subsequently,partial melting of the thicken crust in the Hongzhen area resulted in the Hongzhen granites,which is also supported by the Hongzhen granites body intruded later than crust thinning in the region of the DOB. Because the thicken middle-lower crust of the DOB traversed the Tan-Lu fault zone,post-orogenic large-scale sinistral movement along the TanLu fault zone should happen before middle-lower crust flow in the DOB.