摘要
北祁连山和柴北缘是典型的早古生代大陆造山带,分别发育有北祁连山大洋型俯冲缝合带和柴北缘大陆型俯冲碰撞带。作为早古生代大洋冷俯冲的典型代表,北祁连山经历了从新元古代-寒武纪大洋扩张、奥陶纪俯冲和闭合及早泥盆世隆升造山的过程。高压变质岩变质年龄为490～440Ma,证明古祁连洋经历了至少50m.y.的俯冲过程。柴北缘超高压变质带是大陆深俯冲的结果,岩石学、地球化学和同位素年代学表明,柴北缘超高压变质带中榴辉岩的原岩分别来自洋壳和陆壳两种环境。高压/超高压变质的蛇绿岩原岩的年龄为517±11Ma,与祁连山蛇绿岩年龄一致。榴辉岩早期的变质年龄为443～473Ma,与祁连山高压变质年龄一致,代表大洋地壳俯冲的时代,而柯石英片麻岩和石榴橄榄岩所限定的超高压变质时代为420～426Ma,代表大陆俯冲的年龄。从大洋俯冲结束到大陆俯冲最大深度的转换时间最少需要20m.y.。自420Ma起,俯冲的大洋岩石圈与跟随俯冲的大陆岩石圈断离,大陆地壳开始折返,发生隆升和造山。北祁连山和柴北缘两个不同类型的高压-超高压变质带反映了早古生代从大洋俯冲到大陆俯冲、隆升折返的造山过程。
Two end-member subduction zones of the Early Paleozoic are preserved in the Qilian-Qaidam mountain systems at the northern edge of the Tibetan Plateau. One is the North Qilian oceanic-type suture zone and the other is the North Qaidam continental-type subduction zone. The North Qilian suture zone records a series of temporal events from seafloor spreading in the Neo-Proterozoic/ Cambrian, to the cold oceanic subduction in the Ordovician, and to the strong mountain building in the Early Devonian. Metamorphic ages of the high-pressure rocks range from 490 to 440Ma, suggesting that the Qilian seafloor subduction continued for at least 50m. y. The North Qaidam UHP belt is the product of continental subduction. The petrology, geochemistry and geochronology reveal that protoliths of the eclogites are of both continental and oceanic origins. The HP-UHP metamorphosed ophiolite assemblage of 517±11 Ma is coeval with the ophiolite suites in the North Qilian suture zone. The early stage of eclogite-facies metamorphism took place at 443～ 473Ma, the same age as that of the HP rocks in the North Qilian oceanic suture zone, representing the early stage of seafloor subduction. Coesite-bearing zircons from the metapelite and diamond-bearing zircons from garnet peridotites constrain the UHP metamorphic ages to be ca. 420～426Ma, which represents the timing of continental deep subduction and is ～20m. y. younger than the early stage of the Qilian seafloor subduction. At ～420Ma, the UHPM rocks started to exhume with strong orogenic movement in the North Qilian-Qaidam region. These two end-member subduction belts illustrate a complete mountain-building progression from seafloor spreading/subduction to continental subduction, to continental collision, and to the ultimate uplift with the accompanied exhumation in the Early Paleozoic.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2009年第9期2067-2077,共11页
Acta Petrologica Sinica
基金
国家自然科学基金(40825007,40821002,40773012)
国家基础科学发展规划项目(2009CB825007)
全国博士学位论文作者专项资金项目(200531)资助
