“钉合岩体”与新疆北部主要缝合带的形成时限

Timing of major suture zones in North Xinjiang,China: Constraints from stitching plutons

本文介绍了钉合岩体的概念,强调钉合岩体在造山带研究中具有重要的大地构造意义。在科迪勒拉增生造山带中,钉合岩体是在增生事件之后形成的;而在喜马拉雅碰撞造山带中,钉合岩体是在碰撞事件之后形成的。因此可以区分后增生和后碰撞两类钉合岩体,它们可以为限定增生或碰撞事件的时间上限提供年代学约束。特别是在缝合带被钉合岩体侵入的情况下,缝合带的形成时限(即增生或碰撞事件的起止时间)可以根据缝合带中最年轻的蛇绿岩质岩石和最老的钉合岩体给予严格限定。应用这种方法,能够限定新疆北部的主要缝合带(如额尔齐斯-斋桑缝合带、北天山缝合带和南天山缝合带等,东准噶尔和西准噶尔的蛇绿岩带)最晚是在晚石炭世形成的。西准噶尔增生杂岩也是在在晚石炭世形成的,但当时是否存在洋壳俯冲还需要进一步研究。目前的资料显示,虽然新疆北部各主要缝合带的形成时限存在一定差异,但没有三叠纪形成的缝合带。特别是南天山缝合带内发育的钉合岩体不但有效地限定了缝合带的时间上限,而且还从地质上约束了受到质疑的晚二叠世放射虫化石的可靠性和高压-超高压变质岩中三叠纪锆石U-Pb年龄的解释的合理性。

This study introduces the concept of stitching pluton because a stitching pluton can provide an upper-age bound for the timing of an accretionary or a collisional event in orogens,with an emphasis on the geochronological constraints from the stitching plutons crosscutting major suture zones on the timing of accretionary and collisional events in North Xinjiang. A combination of the oldest stitching plutons and the youngest ophiolitic rocks within the major suture zones in North Xinjiang may tightly constrain the timing of these major suture zones. The Irtysh-Zaysan suture zone between the Altai terrane in the north and the Zharma-Saur magmatic arc in the south,the North Tian Shan suture zone between the Junggar terrane in the north and the Kazakhstan-Yili terrane in the south,and the South Tian Shan suture zone between the Kazakhstan-Yili terrane in the north and the Tarim craton in the south were formed during the Late Carboniferous,whereas the other ophiolite zones in East Junggar and West Junggar during the Early Carboniferous or earlier. The western Junggar accretionary complex was commonly crosscut by many Late Carboniferous stitching plutons,but it is unclear that these plutons were generated by either subduction-related or post-collisional magmatism during the Late Carboniferous so that more work needs to be done in the future in order to eliminate the uncertainty. The pre-existing data are in support of Late Paleozoic orogens, without evidence for Triassic collision,in North Xinjiang. Particularly,the stitching plutons within the South Tian Shan provide an upper-age bound for the timing of the South Tian Shan suture zone and strong evidence against the suspected Late Permian radiolarians from the Baleigong ophiolite,and they also help re-interpret the Triassic zircon U-Pb ages from the high and ultrahigh pressure metamorphic rocks within the suture zone.