新疆榆树沟麻粒岩-橄榄岩地体：南天山北缘出露地表的古生代大陆壳-幔过渡带

The Yushugou Granulite-Peridotite Terrane as a Paleozoic Continental Crust-Mantle Transition Zone Exposed at the Northern Margin of the Southern Tianshan(Xinjiang)

对于大陆造山带内部的超基性岩体,除了蛇绿岩成因之外还有其他或许更为合理的解释。例如,位于新疆托克逊库米什地区南天山北缘的榆树沟麻粒岩和橄榄岩分别代表早古生代时塔里木板块北部大陆地壳莫霍面附近的下地壳底部与上地幔顶部的岩石,由于应变的局部化,被剪切分割成两个透镜状冲断岩片,顺着切割整个地壳直达上地幔顶部的韧性剪切带被挤出,最终呈固态挤侵到中地壳内绿片岩-低角闪岩相的变沉积岩(构造片岩、长英质糜棱岩)之中。最后,两个构造透镜体被剪切叠合到一起。榆树沟麻粒岩-橄榄岩地体为研究塔里木板块北部大陆地壳古莫霍面及其附近的岩石流变学与地球物理性质提供了一个十分难得的研究基地,因为大陆古莫霍面很少直接出露地表。榆树沟麻粒岩-橄榄岩组合的地体的大地构造意义与蛇绿岩所代表的截然不同。

The Yushugou massif in the Kumishi region at the nouthern margin of the southern Tianshan（Xinjiang） consists of extensively deformed peridotites（lherzolite, harzburgite and wehrlite） associated with foliated and lineated mafic composition-dominated granulites, together carried up and emplaced into greenschist-lower amphibolitefacies metasediments at 380–420 Ma. The granulite-peridotite units are interpreted as tectonic slices of continental mantle and lower crustal materials immediately adjacent the Moho. The high pressure metamorphic slices were extruded by tectonic compression from a detachment in the crust-mantle boundary up into the greenschist-lower amphibolite-facies midde crust along intracontinental thrust shear zones. Such compressional uplifts of the crust-mantle transition zone were similar to the Ivrea-Verbano zone in the southern Alps of Italy and the Jijal-Patan-Chilas seuqence in the Kohistan arc of Pakistan. The boundary between mafic granulites and peridotites, which represents an exposed example of the Moho, is a high strain zone： in the mantle rocks olivine defomed by diffusion-accommodated grain bounadry sliding while pyroxenes deformed by disloaction creep, and in the lower crustal mafic granulites both pyroxenes and plagioclase deformed by dislocation creep. Thus, the Yushugou massif provides important clues to the nature and seismic reflecitivity of the continental Moho.