青藏高原北缘早古生代板块构造演化和大陆深俯冲

Early Paleozoic plate-tectonic evolution and deep continental subduction on the northern margin of the Qinghai-Tibet Plateau

青藏高原北缘北祁连山和柴达木盆地北缘分别发育有2类不同的高压和超高压变质带。北祁连山为典型的环太平洋型俯冲带,榴辉岩锆石的SHRIMP年龄为464Ma±5Ma。而柴北缘为典型的大陆俯冲型变质带,片麻岩锆石中的柯石英证明柴北缘是一超高压变质带。石榴橄榄岩中的超硅石榴子石和橄榄石中的钛铁矿等出溶反映其形成深度大于200km。柴北缘榴辉岩的原岩具有MORB和OIB的特征,并与北祁连山榴辉岩的年龄相同,代表早古生代祁连洋俯冲变质的时代。而含柯石英片麻岩的锆石和石榴橄榄岩超高压变质年龄均为423Ma,代表大陆地壳深俯冲发生的时间。北祁连山和柴北缘很可能是一个俯冲带从大洋俯冲到大陆碰撞的不同阶段的产物。

Two different types of high-pressure(HP)-ultrahigh-pressure(UHP)metamorphic belts are developed in the North Qilian Mountains and on the north Qaidam on the northern margin of the Qinghai-Tibet Plateau. The North Qilian Mountains are a typical circum-Pacific type subduction zone and zircons from eclogite give a SHRIMP U-Pb age of 464±5 Ma; whereas the north Qaidam is a typical continental subduction type metamorphic belt and coesite in zircon from gneiss demonstrates that the northern margin of the Qaidam basin is an UHP metamorphic belt. Super-silicic garnet and ilmenite exsolution in olivine in garnet peridotite reflect that it formed at >200 km depth. The protolith of eclogite in the north Qaidam shows the features of MORB and OIB and has the same age as eclogite of the North Qilian Mountain, representing the age of metamorphism due to subduction of the Early Paleozoic Qilian Ocean. The zircon age of coesite-bearing gneiss and the age of UHP metamorphism of garnet peridotite are both 423 Ma, representing the timing of continental deep subduction. The authors think that the North Qilian Mountain and the north Qaidam are very likely to be the product in different stages from oceanic subduction to continental collision of a subduction zone.