Formation of ultramafic cumulate rocks at the arc root serves as a key process to understanding the deep evolution of arc magma and crustal growth. However, the arc root cumulates are easy to delaminate due to their high density, making limited field exposure. Here, we conducted comprehensive petrographic observations and geochemical analysis of typical ultramafic rocks (peridotites and pyroxenites) in an ophiolitic mélange from the central Beishan orogenic belt in the southern Central Asian Orogenic Belt (CAOB). Petrographic evidence shows that these rocks exhibit characteristic adcumulate and mesocumulate textures with intercumulus minerals such as olivine, orthopyroxene, and clinopyroxene. The mineral characteristics of ultramafic rocks, including olivine and orthopyroxene with lower NiO (0.10–0.33 wt% and 0.03–0.09 wt%, respectively) and Mg# (81–87 and 86–88, respectively), clinopyroxene with higher CaO (19.0–24.7 wt%), and spinel with lower Mg# (0–49), are distinct from those of mantle rocks but resemble cumulates. Both whole rock and calculated equilibrium melt exhibit the compositions of island arc magma characterized by enrichment in large ion lithophile elements (e.g., Cs, U, Pb, and Sr) and depletion in high field strength elements (e.g., Nb, Ta, Zr, Hf, and Ti). The covariation of high Mg# and low SiO in whole rocks is akin to typical arc root cumulates from classic island arcs (e.g., Kohistan and Talkeetna). The positive zircon ε(t) values (+13 to +16) and calculated crystallization pressure of 0.9–1.1 GPa (∼30–40 km) support the formation of ultramafic cumulates in a juvenile island arc setting. These findings imply that most ultramafic rocks in the central Beishan orogenic belt are arc root cumulates instead of the mantle portion of an ophiolitic mélange. Fractional crystallization models suggest that arc root cumulate formation may have influenced the evolution of intermediate-felsic rocks in this area, and that the differentiation of hydrous arc magma played a crucial role in large-scale crustal growth in the southern CAOB during the early Paleozoic.

中文翻译：

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中亚造山带南部深弧根积累的形成及其对地壳生长的影响

弧根超镁铁质堆积岩的形成是了解弧岩浆深层演化和地壳生长的关键过程。然而，弧根堆积体由于密度高，容易分层，现场暴露受到限制。在这里，我们对中亚造山带南部北山造山带中部蛇绿混杂岩中的典型超镁铁岩（橄榄岩和辉石岩）进行了全面的岩相观察和地球化学分析。岩相学证据表明，这些岩石表现出特有的堆积和中堆积结构，其中含有橄榄石、斜方辉石和单斜辉石等积云矿物。超镁铁岩的矿物特征，包括含较低 NiO 的橄榄石和斜方辉石（分别为 0.10-0.33 wt% 和 0.03-0.09 wt%）和 Mg#（分别为 81-87 和 86-88），含较高 CaO 的单斜辉石（19.0 –24.7 wt%) 和镁含量较低的尖晶石 (0–49)，与地幔岩石不同，但类似于堆积物。整个岩石和计算的平衡熔体都表现出岛弧岩浆的成分，其特征是富集大离子亲石元素（例如，Cs、U、Pb和Sr）和贫化高场强元素（例如，Nb、Ta、Zr、铪和钛）。整个岩石中高 Mg# 和低 SiO 的共变类似于经典岛弧（例如科希斯坦和塔尔基特纳）的典型弧根累积。正锆石 ε(t) 值（+13 至 +16）和计算出的 0.9–1.1 GPa (∼30–40 km) 结晶压力支持在新生岛弧环境中形成超镁铁质堆积物。这些发现表明，北山造山带中部的大多数超镁铁质岩石是弧根堆积物，而不是蛇绿混杂岩的地幔部分。分次结晶模型表明，弧根堆积物的形成可能影响了该地区中长英质岩石的演化，含水弧岩浆的分异对中亚造山带南部早古生代地壳的大规模生长起到了至关重要的作用。