Abstract Kohistan, an intra-oceanic arc, formed due to the India-Asia collision-related tectonics. The arc is comprised of mafic-ultramafic and crustal lithologies. Chilas Complex, one of the major mafic-ultramafic body that lies at the base of the Kohistan arc, is dominantly composed of massive gabbronorites with local ultramafic-mafic intrusive bodies. The ultramafic rocks were considered as the products of fractional crystallization whereas gabbronorites were reported to have formed at the root zone of the magma chamber either from melts or due to fractional crystallization, based on major and trace element data. To understand the petrogenesis and origin of the Chilas gabbros, we conducted platinum group elements (PGEs) geochemistry combined with whole-rock major and trace element compositions. Geochemical data of the Chilas gabbros show tholeiite series, likely derived from a depleted lithospheric mantle source via peridotite melts. Presence of hornblende and relatively higher Al/Ti ratios of clinopyroxene in the Chilas gabbros suggest hydrous magma, ascribed to mantle wedge metasomatism by slab-derived fluids. Poor correlations between MgO vs. Ni, Cr, and V, Ni vs. Cr, Dy/Yb vs. SiO2, Sr/Y vs. Eu/Eu* and relatively low TiO2 indicate a minimal role of fractional crystallization. Low contents of LOI (0–3.24 wt%), Ce/Ce* (0.92–1.01), and positive relation of La, Ta, and Hf with Zr strongly support the insignificant impact of hydrothermal alteration. Enrichment of PGEs, such as Pt (0.63–13.75 ppb), Pd (0.43–13.87 ppb), and lower Pd/Ir ratios (0.76–40.04) suggest low degree partial melting of the parent magma. Higher Pd, Pd/Ir ratios compared to Pt and Pt/Pd, combined with enrichment of PPGE over IPGE, suggest the participation of slab-dehydrated fluids and mantle metasomatism however Cu/Ir vs. Ni/Pd, V-Pd/Ir and La-Pd/Ir ratios point towards olivine fractionation in the magma. Lower Cu/Pd ratios, relationships of Ru and Pt against Cr, and Os diminution corroborate S-undersaturation in parental magma. The PGEs contents and Pd/Pt ratios attest depleted mantle source for their production. In view of Cu/Zr and Cu/Pd ratios, it is substantiated that the sulphide segregation cannot be a promising reason for PGEs depletion. Therefore, we infer PGEs depletions in parental magma are due to PGEs-depleted mantle source, though sulfide retention in mantle source cannot be eliminated. The geochemical fingerprints in tectonic diagrams and negative anomalies of Nb and Zr in the studied samples provide credible facts that favour arc magmatism. Based on major and trace element data and PGEs values, we consider that the Chilas gabbros were formed from a lithospheric mantle that was metasomatized by fluids generated at the mantle wedge and derived from mafic melts in an arc setting. Geochemical data and their plotting on various tectonic discrimination diagrams indicate Chilas Complex as a remnant of the Neo-Tethyan oceanic arc system.

中文翻译：

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巴基斯坦东北部科希斯坦岛弧白垩纪 Chilas 辉长岩的岩石成因、铂族元素地球化学和地球动力学演化

摘要 Kohistan 是一个洋内弧，由印度-亚洲碰撞相关构造形成。该弧由镁铁质-超镁铁质和地壳岩性组成。Chilas 杂岩体是位于科希斯坦弧底部的主要镁铁质-超镁铁质体之一，主要由块状辉长岩和局部超镁铁质-镁铁质侵入体组成。超镁铁质岩石被认为是分步结晶的产物，而据报道，根据主量和微量元素数据，辉长岩是在岩浆房根部由熔体或分步结晶形成的。为了了解 Chilas 辉长岩的成因和成因，我们结合全岩主要和微量元素组成进行了铂族元素 (PGEs) 地球化学。Chilas辉长岩的地球化学数据显示拉斑岩系列，可能来自于通过橄榄岩熔体的耗尽岩石圈地幔源。Chilas辉长岩中角闪石的存在和相对较高的单斜辉石Al/Ti比率表明含水岩浆，归因于板片衍生流体的地幔楔交代作用。MgO 与 Ni、Cr 和 V、Ni 与 Cr、Dy/Yb 与 SiO2、Sr/Y 与 Eu/Eu* 以及相对较低的 TiO2 之间的相关性较差，表明分级结晶的作用很小。低含量的 LOI (0–3.24 wt%)、Ce/Ce* (0.92–1.01) 以及 La、Ta 和 Hf 与 Zr 的正相关性强烈支持热液蚀变的微不足道影响。PGEs 的富集，如 Pt (0.63–13.75 ppb)、Pd (0.43–13.87 ppb) 和较低的 Pd/Ir 比率 (0.76–40.04) 表明母岩浆的低度部分熔融。更高的钯，Pd/Ir 比与 Pt 和 Pt/Pd 相比，结合 PPGE 对 IPGE 的富集，表明板片脱水流体和地幔交代的参与，但是 Cu/Ir 与 Ni/Pd、V-Pd/Ir 和 La-Pd /Ir 比率指向岩浆中的橄榄石分馏。较低的 Cu/Pd 比率、Ru 和 Pt 与 Cr 的关系以及 Os 减少证实了母岩浆中的 S 欠饱和。PGEs 含量和 Pd/Pt 比证明了它们产生的贫化地幔来源。鉴于 Cu/Zr 和 Cu/Pd 比率，证实硫化物偏析不能成为 PGEs 耗尽的有希望的原因。因此，我们推断母体岩浆中 PGEs 的消耗是由于 PGEs 耗尽的地幔源，尽管无法消除地幔源中的硫化物滞留。构造图中的地球化学指纹和研究样品中 Nb 和 Zr 的负异常提供了有利于弧岩浆作用的可信事实。基于主量和微量元素数据以及 PGEs 值，我们认为 Chilas 辉长岩是由岩石圈地幔形成的，该地幔被地幔楔处产生的流体交代，并在弧形环境中源自基性熔体。地球化学数据及其在各种构造判别图上的绘制表明 Chilas 杂岩是新特提斯洋弧系统的残余。我们认为 Chilas 辉长岩是由岩石圈地幔形成的，该地幔被地幔楔处产生的流体交代，并衍生自弧形环境中的基性熔体。地球化学数据及其在各种构造判别图上的绘制表明 Chilas 杂岩是新特提斯洋弧系统的残余。我们认为 Chilas 辉长岩是由岩石圈地幔形成的，该地幔被地幔楔处产生的流体交代，并衍生自弧形环境中的基性熔体。地球化学数据及其在各种构造判别图上的绘制表明 Chilas 杂岩是新特提斯洋弧系统的残余。