蚌埠隆起区石榴辉石岩变质P-T轨迹的构建

The P-T path of garnet pyroxenite in metamorphic complex from Bengbu uplift

蚌埠隆起区位于华北克拉通东南缘,胶—辽—吉造山带的最南端,主体由五河杂岩组成。前人对该地区的研究主要集中于同位素年代学和变质温压条件研究,其中变质P-T条件研究结果差异较大,以压力变化最为显著,对峰期变质P-T条件缺乏统一认识。本文对蚌埠隆起区石榴辉石岩进行了大量的岩相学、矿物化学成分分析,表明该岩石记录了3期变质作用,其中S-M1和S-M2的矿物组合类似为Grt+Cpx+Opx+Amp+Pl+Ilm,S-M3的矿物组合为Cpx+Amp+Pl+Grt (极边窄带)。结合变质温压条件分析和锆石U-Pb年代学分析,本次主要取得以下几点认识:1)石榴辉石岩WS047-1中记录的3期变质作用,温压条件分别为T-M1 = 616 ℃~647 ℃、P-M1 = 1.03~1.08 GPa,T-M2 = 721 ℃~837 ℃、P-M2 = 1.11~1.29 GPa和T-M3 = 531 ℃~607 ℃、P-M3 = 0.81~0.91 GPa,经历了由较高压力的角闪岩相→中-低麻粒岩相→角闪岩相的变质过程;2)据变质温压条件分析知,蚌埠隆起区具有顺时针的P-T轨迹特征,S-M1→S-M2和S-M2→S-M3分别为近等压升温和近等压降温的缓慢过程;3)石榴辉石岩锆石U-Pb年代学结果主要分为4组:1 839±13 Ma、1 925±31 Ma、2 041±55 Ma和2 762±14 Ma,其中峰值变质年代为1.93~1.84 Ga;4)结合温压条件和锆石U-Pb年代学分析结果,本文认为蚌埠隆起区的P-T轨迹与弗朗西斯科型俯冲或大陆碰撞环境的P-T轨迹较为类似,其应与1.93~1.84 Ga华北克拉通东、西陆块的碰撞拼合及胶—辽—吉造山带形成时限基本吻合。本次研究为深入理解华北克拉通的构造演化特征和蚌埠隆起区的变质作用及演化,提供了大量可靠的科学资料。

Bengbu uplift tectonic is located in the southeastern margin of North China Craton (NCC),the southernmost of Jiao-Liao-Ji orogenic belt,and its mainly composed of Wuhe complex. Previous studies in this area mainly focused on isotope chronology and metamorphic P-T conditions. The metamorphic P-T conditions were quite different,with the most significant change in pressure and there was a lack of unified understanding of the P-T conditions of peak metamorphism. In this paper,a large amount of petrographic and mineral chemical composition analysis of garnet pyroxenite in Bengbu uplift has been carried out,indicating that this rock has record 3 metamorphic stages,in which the mineral combinayion of S-M1 and S-M2 is similar to Grt+Cpx+Opx+Amp+Pl+Ilm and the mineral combinayion of S-M3 is Cpx+Amp+Pl+Grt (extremely narrow band). Taking metamorphic P-T conditions and zircon U-Pb chronology result into consideration,the following conclusions are drawn: 1) The garnet pyroxenite sample WS047-1 recorded three metamorphic stages,temperature and pressure conditions respectively: T-M1 = 616 ℃~647 ℃ and P-M1 = 1.03~1.08 GPa,T-M2 = 721 ℃~837 ℃ and P-M2 = 1.11~1.29 GPa,T-M3 = 531 ℃~607 ℃ and P-M3 = 0.81~ 0.91 GPa. It was showing a recycle from high-pressure amphibolite facies,granulite facies to amphibolite facies. 2) According to the analysis of metamorphic P-T conditions,Bengbu uplift has a clockwise P-T paths,and S-M1→S-M2 and S-M2→S-M3 are revealed a slowly process from near isobaric heating to isobaric cooling. 3) The U-Pb geochronology of zircon can be divided into 4 groups: 1 839±13 Ma,1 925±31 Ma,2 041±55 Ma and 2 762±14 Ma,and the peak metamorphic zircon age is 1.93~1.84 Ga. 4) Combined with the study of P-T conditions and chronology,the paper draws a conclusion that Bengbu uplift is similar to that of Francisco-type subduction or continental collision environment,which should be consistent with the collision and combination of the east and west blocks of North China Craton from 1.93 Ga to 1.84 Ga and the formation time of the Jiao-Liao-Ji belt. This study provides a large amount of reliable scientific data for further understanding the structural evolution characteristics of the North China Craton and the metamorphism and evolution of Bengbu uplift.