北山造山带白云山蛇绿岩地幔橄榄岩成因及形成环境

白云山蛇绿岩是北山造山带保存最好的蛇绿岩之一，其起源存在较大的争议。该蛇绿岩主要由橄榄岩、辉长岩以及基性火山岩等组成。橄榄岩主体为方辉橄榄岩以及少量二辉橄榄岩。这些岩石均发生不同程度的蛇纹石化以及强烈的变形变质作用。研究选取9件橄榄岩样品进行了全岩主量元素分析及单矿物电子探针分析，结果显示岩石均来自地幔。岩石中的尖晶石和辉石组分用来限定白云山蛇绿岩的成因及形成环境。样品包含了高Cr#（46.7~68.1）的尖晶石以及低Al2O3（0.78%~2.94%）含量的单斜辉石。矿物化学特征显示，地幔橄榄岩熔融程度为23%~36%，表明白云山地幔橄榄岩可能经历了多个阶段。矿物学及全岩地球化学特征表明，白云山蛇绿岩形成于洋中脊（MOR）环境，后受到俯冲带（SSZ）环境的改造。

Petrogenesis and Geotectonic Setting of Mantle Peridotites from the Baiyunshan Ophiolite in Beishan Orogen

The Baiyunshan ophiolite is one of the most extensive tracts of oceanic crust preserved in Beishan orogen, but its origin has remained controversial. This massif consists of peridotite, gabbro, and volcanic rocks. The peridotites are mostly harzburgite with a small amount of lherzolite. Most of these bodies are suffered various degrees of serpentinization and also have been highly deformed and metamorphosed. 9 peridotites have been collected from the Baiyunshan ophiolite massif for carrying out a study of major element composition and mineral chemistry. The mineral and bulk-rock compositions show that these peridotites belonged to mantle rocks. The compositions of the spinel cores and clinopyroxenes were used to deduce the petrogenesis and tectonic environments for the formation of the Baiyunshan ophiolite massif. These samples contain high Cr# spinel (46.7-68.1) and low Al2O3 (0.78%-2.94%) clinopyroxene. The degree of partial melting of mantle peridotite is 23%-36%, indicating that Baiyunshan mantle peridotite may experience a multi-stage process. The mineralogy and geochemistry of the Baiyunshan peridotites suggest that they were formed originally at a MOR setting and were later modified by SSZ melts.