Several mantle peridotite massifs crop out along the northern margin of the South Qilian belt, but their origin remains controversial. The Xigou peridotite massif is the largest one within the Lajishan ophiolite belt and mainly consists of lherzolite, harzburgite, and serpentinized dunite, in association with anorthosite, amphibolite, dolerite dikes, and pillow basalt. Lherzolites show typical features of relatively fertile abyssal peridotites, with iron-rich olivine, Al-spinel, and aluminous pyroxene. Harzburgites and serpentinized dunites are characterized by U-shaped rare earth element patterns and low Pd-group platinum-group elements resembling the geochemical features of suprasubduction peridotites. Spinel Cr#’s ( Cr / ( Cr + Al ) ) range from 0.38 in harzburgite to 0.75 in dunite. The progressive increase in spinel Cr#’s is related to changing melt compositions from mid-ocean ridge basalt (MORB) like to boninitic during subduction initiation. The Xigou suprasubduction peridotites, MORB-like mafic rocks, and boninites probably constitute an incomplete forearc crust-mantle section. The abyssal lherzolite preserved in the forearc region might represent older oceanic lithosphere or younger fertile mantle residue along a trench–fracture zone intersection with a transform fault. Therefore, we suggest that the intraoceanic subduction of the Proto-Tethyan Ocean may have begun no later than 535 Ma, before the development of the Lajishan intraoceanic arc-trench system.

中文翻译：

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拉脊山蛇绿岩带中的深海和超俯冲橄榄岩：原始特提斯洋初始俯冲的意义

南祁连带北缘出露多个地幔橄榄岩地块，但其成因尚存争议。西沟橄榄岩地块是拉矶山蛇绿岩带内最大的地块，主要由辉长岩、斜方辉石和蛇纹石化纯英岩组成，并伴有斜长岩、角闪岩、辉绿岩脉和枕状玄武岩。橄榄石显示出相对肥沃的深海橄榄岩的典型特征，具有富含铁的橄榄石、铝尖晶石和铝辉石。Harzburgites 和蛇纹石化单质岩的特征是 U 形稀土元素模式和低 Pd 族铂族元素，类似于超俯冲橄榄岩的地球化学特征。尖晶石 Cr# (Cr / ( Cr + Al ) ) 的范围从 0.38 的方辉石到 0.75 的纯晶石。尖晶石 Cr#'s 的逐渐增加与在俯冲开始期间从洋中脊玄武岩 (MORB) 到博尼特岩的熔体成分变化有关。西沟超俯冲橄榄岩、类MORB基性岩和博长岩可能构成了不完整的弧前壳幔剖面。保存在弧前区的深海二镁橄榄石可能代表了沿海沟-断裂带与转换断层相交的较老的海洋岩石圈或较年轻的肥沃地幔残留物。因此，我们认为原特提斯洋的洋内俯冲可能不晚于535 Ma，也就是拉积山洋内弧形海沟系统发育之前。和boninitites可能构成一个不完整的弧前壳幔剖面。保存在弧前区的深海二镁橄榄石可能代表了沿海沟-断裂带与转换断层相交的较老的海洋岩石圈或较年轻的肥沃地幔残留物。因此，我们认为原特提斯洋的洋内俯冲可能不迟于535 Ma，也就是拉积山洋内弧形海沟系统发育之前。和boninitites可能构成一个不完整的弧前壳幔剖面。保存在弧前区的深海二镁橄榄石可能代表了沿海沟-断裂带与转换断层相交的较老的海洋岩石圈或较年轻的肥沃地幔残留物。因此，我们认为原特提斯洋的洋内俯冲可能不迟于535 Ma，也就是拉积山洋内弧形海沟系统发育之前。