辽东半岛辽河群变泥质岩变质变形研究:对古元古代造山作用及折返过程的启示

前寒武纪造山带解析是地球科学研究的重点。胶-辽-吉造山带作为华北克拉通最具代表性的造山带之一,是研究前寒武纪造山过程和物质折返的理想场所。本文以辽东半岛辽河群为研究对象,以变质变形为研究手段,系统解析胶-辽-吉造山带的造山及折返过程。含夕线石榴黑云片麻岩(样品18TZH49)位于造山带核心部分,其岩相学观察和相平衡模拟表明,其峰期和峰后变质阶段矿物组合分别为Sil+Grt+Pl+Bi+Qz+Ilm+Melt和St+Grt+Pl+Bi+Chl+Qz+Ilm,变质温压条件为720~780℃/5.9~6.9kbar和540~565℃/3~4.1kbar,从峰期到峰后记录了一个降温降压的顺时针P-T演化轨迹。锆石UPb年代学研究表明其退变质年龄为1851Ma。结合南、北辽河群其他变泥质岩研究工作,提出造山带北部北辽河群泥质岩经历低角闪岩相变质作用,以典型的巴罗式变质带为特征;造山带南部南辽河群经历角闪岩相至麻粒岩相变质作用,退变质特征明显。从构造角度来说,北辽河群受向北逆冲推覆构造影响,显示向龙岗基底的运动学特征;造山带核心部位的南、北辽河群直立褶皱发育,指示地壳强烈收缩过程;南辽河群受向南逆冲推覆构造影响,显示向南侧狼林陆块的运动学特征。变质变形及年代学共同指示,辽河群经历1.95~1.90Ga造山作用,构造增厚作用使不同类型岩石相互叠置并发生不同程度变质作用。随后,~1.85Ga发生造山后伸展作用,不同变质等级岩片剥露于地表。

A metamorphic and deformational study of meta-pelites in the Liaohe Group located at Liaodong Peninsula: Significance to process of Paleoproterozoic orogenesis and exhumation

It is a key point for earth science to study the Precambrian orogenic belt.The Jiao-Liao-Ji Orogenic Belt(JLJOB),as one of the most representative orogenic belts in the North China Craton,is an ideal site for understanding the Precambrian orogenesis and exhumation process.In this study,combined with studies on metamorphism and deformation of the Liaohe Group in the Liaodong Peninsula,we systematically analysis the process of Paleoproterozoic orogenesis and exhumation of the JLJOB.Sillimanite-bearing garnet biotite gneiss(Sample 18 TZH49)is located at the center of orogenic belt.The results of petrological analysis and pseudosection modelling suggest that its peak and post-peak mineral assemblages are characterized by Sil+Grt+Pl+Bi+Qz+Ilm+Melt and St+Grt+Pl+Bi+Chl+Qz+Ilm,and metamorphic P-T conditions are 720~780℃/5.9~6.9 kbar and 540~565℃/3~4.1 kbar,respectively,which indicates a clockwise P-T path for decrease of temperature and pressure.Zircon U-Pb dating suggests its retrograde age is at 1851 Ma.According to other metapelites researches of the South and North Liaohe groups,we put forward that the North Liaohe Group,located at the northern orogenic belt,has experienced low-amphibolite facies metamorphism,which is charactered by typical Barrovian metamorphic zone;the South Liaohe Group,belonging to the southern orogenic belt,has undergone amphibolitegranulite facies metamorphism which shows obvious retrograde feature.From the standpoint of structure,the North Liaohe Group is influenced by northward thrust nappe structure,indicating a movement to the Longgang Block;the upright fold develops in the core of the orogenic belt which is also a boundary of the South and North Liaohe groups,showing a strong contraction of crust;the South Liaohe Group is affected by southward thrust nappe structure,revealing a movement to the Langrim Block.The metamorphic and deformational characteristics commonly suggest that the Liaohe Group has experienced orogenesis at 1.95~1.90 Ga.Structural thickening makes many different rock units superimpose each other and undergo various metamorphism.Subsequently,post-orogenic extension at~1.85 Ga results in that rocks of different metamorphic grades are exposed to the earth surface.