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华北克拉通南缘太华杂岩组成及演化

     

摘要

太华杂岩位于华北克拉通南部,其组成复杂,记录了几乎所有早前寒武纪各阶段重要的地质事件;此外,由于其所处特殊地理位置,研究太华杂岩对于华北克拉通早前寒武纪地壳形成和演化、构造单元划分和基底拼合等都具有举足轻重的科学价值.本文综合已有的岩石学、变质作用、地球化学以及同位素年代学等诸多研究工作,得到以下阶段性结论和认识:1)将鲁山地区太华划分为以深成侵入岩为主的片麻岩系和以变质沉积-火山岩为主的表壳岩系;前者形成于中太古代晚期-新太古代早期,后者形成于古元古代晚期.而小秦岭地区太华杂岩中变质深成侵入岩形成时间跨度较大,为中太古代晚期-古元古代早期;而其上覆的火山-沉积岩可与鲁山太华杂岩的表壳岩类比,形成时间亦为古元古代晚期.2)中太古代-新太古代(2.91~2.50Ga)为华北克拉通南部大陆最主要的地壳形成时期.提出太华杂岩在太古宙经历了两期明显的地壳生长时间,一期发生在2.85~2.70Ga,以鲁山太华片麻岩系中的深成侵入岩和斜长角闪岩为代表;另一期发生在~2.50Ga,以小秦岭华山和崤山地区太华杂岩中各类花岗质岩石为代表.3)太华杂岩在所谓的全球陆壳生长“沉寂期(2.45 ~2.20Ga)”岩浆活动异常发育,推测这一时期的岩石形成于古元古代俯冲-汇聚环境,可能是与华北克拉通南部太古宙陆块和其他陆块汇聚-碰撞相关.4)太华杂岩在古元古代晚期普遍遭受了强烈的变质和变形,其变质程度主体为高角闪岩相,局部可达麻粒岩相,且记录了包含近等温降压退变质片段的顺时针变质作用P-T轨迹,经历了一个漫长的变质演化过程(1.97~1.80Ga),变质作用的时限跨度可达150Myr.5)提出华北克拉通南部曾经为一个统一基底,称之为“南部太古宙地块”,此地块形成时间为新太古代末期(~2.5Ga).该古老陆块经历了如下5个构造-演化阶段:(1)冥古宙-始太古代初始陆核形成;(2)中太古代-新太古代陆壳快速生长;(3)古元古代早期(~2.3Ga)岩浆活动异常活跃;(4)古元古代(2.30 ~1.97Ga)陆内拉伸-破裂;和(5)古元古代末期(1.97~1.80Ga)陆块最终拼合.%The Taihua Complex is widely exposed in the southern North China Craton (NCC) and records almost all the important geological events in the Early Precambrian,thus it is vital important to reveal the generation and evolution of continental crust of the NCC as well as the earth.Moreover,due to its important geographic location,which makes the Taihua Complex serve as an excellent area for investigating a variety of models for the tectonic subdivision and amalgamation of the NCC.In this paper,we present a synthetic overview of the Taihua Complex based on available stratigraphic,structural,geochemical,metamorphic and geochronologic data,and draw the following conclusions:1) the Taihua Complex in Lushan area can be divided into two major lithological units along the Dangze River,namely gneisses series and supracrustal rocks;The former is composed of TTG gneisses and amphibolites,and were formed in the period of the Late Mesoarchean to Early Neoarchean,whereas the formation time of the supracrustal rocks can be limited during the Late Palaeoproterozoic.However,the gneisses series of the Taihua Complex in Xiaoqinling area was formed in a large time span from Late Mesoarchean to Early Palaeoproterozoic,whereas the supracrustal rocks were coeval with those in the Lushan area.2) The Mesoarchean to Early Neoarchean (2.91 ~2.50Ga)was the major period of continental crust growth in the southern NCC,the Taihua Complex has undergone two marked episodes of continental crust growth during the Archean time.The earlier occurred at 2.85 ~2.70Ga,and formed TTG gneisses and amphibolites in Lushan area;the later happened at ~2.50Ga,and formed all types of Late Neoarchean granitoid rocks in Huashan and Xiaoshan areas.3) Available isotope data suggest that the Early Paleoproterozoic magmatic rocks are widespread in the Taihua Complex during the crustal age gap,those rocks were likely generated in an Andean type continental margin arc setting;and probably due to the Early Paleoproterozoic subduction-collision between the southern block with other microcontinents.4) The Taihua Complex has suffered strong metamorphism and deformation,and experienced high amphibolite to granulite facies metamorphism characterized by clockwise P-T paths including ITD process,and record long-lived Palaeoproterozoic metamorphic event may define a slow exhumation process,at least lasted 150Myr.5) All the complexes in the southern NCC were welded together to form a coherent ancient terrane at the end of the Neoarchean,we named it the "Southern Archean Block".It has experienced five major Precambrian geological events:(1) formation of Hadean-Paleoarchean initial continental nucleus;(2) major continental crust growth during the period of Mesoarchean to Early Neoarchean;(3) hyperactive magmatism between 2.45Ga and 2.20Ga during the global crustal age gap;(4) Paleoproterozoic (2.31 ~1.97Ga) rifting event;(5) final subduction-accretion-collision process to form the NCC.

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