1.State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi'2.'3.an,710069;4.China
内容提要 秦岭和祁连造山带在华北地块西南部的陇山地区交汇,陇山地区是揭示华北地块、北秦岭造山带与北祁连造山带时空联系的关键区域。前人在陇山地区确定的新街中元古代石英闪长岩在岩石类型、形成时代和形成环境等方面都存在疑问,本文对这套岩石和其中包裹的黑云二长片麻岩进行了岩石学、地球化学和独居石、锆石U-Pb年代学研究。这套岩石被重新定名为新街斜长角闪片岩,根据地球化学对其进行的原岩恢复表明其原岩是一套基性岩浆岩,锆石U-Pb测年给出原岩的形成时代为433.15 ± 3.42 Ma(中志留世),变质锆石和独居石U-Pb 测年给出变质年龄427.9 ± 1.46 Ma,原岩在形成后不久就受到变质作用改造,这是新街斜长角闪片岩在形成与演化历史上的一个重要特征。结合区域地质资料分析表明,晚奥陶世—中志留世,陇山地区在北祁连洋东段向北俯冲的背景下整体处于强烈的弧后伸展构造环境,发育大规模的基性—酸性岩浆活动,新街斜长角闪片岩的原岩就是在这种伸展背景下形成的基性岩浆岩。原岩经历由同期岩浆作用伴生的热液交代作用而成为斜长角闪片岩。被包裹在斜长角闪片岩之中的黑云二长片麻岩的碎屑锆石年龄与陇山地区南部葫芦河群(S1H)碎屑锆石年龄具有很好的一致性,其物源区与鄂尔多斯地块和北秦岭造山带都有比较好的亲缘性。本文的研究结果表明,陇山地区在晚奥陶—中志留世期间由弧后伸展作用引发的岩浆作用规模大,持续时间长,在此背景下的岩浆作用、变质作用与沉积作用具有很好的时空联系。
The Qinling-Qilian Junction intersects at the Long Shan Tectonic Belt in the southwestern part of the North China Block, and the Longshan tectonic belt is the key area to reveal the spatio-temporal relationship between North China block, North Qinling orogenic belt and North Qilian orogenic belt. A set of Mesoproterozoic quartz diorite in Xinjie determined by predecessors has doubts in terms of rock type, formation age and formation environment. In this paper, petrology, geochemistry and U-Pb chronology of monazite and zircon have been studied for this set of rocks and biotite monzonite gneisses inclusions. This set of rocks has been renamed as Xinjie plagioclase amphibolite schist. According to geochemistry, the protolith is a set of basic magmatic rocks. Zircon U-Pb dating gives the formation age of protolith is 433.15 ±3.42Ma (Middle Silurian).Metamorphic zircon and monazite U-Pb dating gives metamorphic age of 427.9 ±1.46Ma. The protolith was metamorphic shortly after its formation. This is an important feature of Xinjie plagioclase amphibolite schist in the history of formation and evolution. Combined with the analysis of regional geological data, it is shown that in the late Ordovician-middle Silurian, Longshan area was in a strong back-arc extensional tectonic environment under the background of northward subduction in the eastern part of North Qilian. Large-scale basic-acid magmatic activity developed in Longshan area under this extensional background. The protolith of Xinjjie plagioclase amphibolite schist is the basic magmatic rock formed under this extensional background. The protolith experienced hydrothermal metasomatism associated with magmatism in the same period and became plagioclase amphibolite schist. The detrital zircon age of biotite monzonitic gneiss enclosed in plagioclase hornblende schist is in good agreement with the detrital zircon age of Huluhe Group(S1H) in southern Longshan area, and its provenance area is closely related to Ordos block and North Qinling orogenic belt. The results of this paper show that the magmatism caused by back-arc extension in Longshan area during the late Ordovician to middle Silurian is large in scale and lasts for a long time, and there is a good time-space relationship between magmatism, metamorphism and sedimentation in this background.