Abstract Chromite deposits hosted in a layered ultramafic–mafic intrusion in the Archean Chithradurga greenstone belt are part of a suture that divides the Western and Central Dharwar craton blocks in southern India. Serpentinised ultramafic rocks including dunite, peridotite, and phlogopite-rich clinopyroxenite occur with massive chromitite. The olivine compositions show two distinct groups, one with Fo ~91–92 olivines, in the massive chromitite and another group with olivine Fo values of ~95–96, in the serpentinised dunite. The chromite occurs as massive chromitite and as an accessory phase in dunite. The former are characterized by high Cr# (Cr/(Cr + Al) × 100 = 67–70) and moderate to high Mg# (Mg/(Mg + Fe) = 52–66); The latter have a larger variation in Cr which is likely related to sub-solidus re-equilibration of the chromite during the serpentinization event. In the massive chromitites, sub-solidus re-equilibration of chromite is less significant or absent. Bulk-rock major and trace element analyses of the peridotites reveal a composition less magnesian than depleted mantle rocks, with low TiO2, Na2O, CaO and high Cr, Ni content, a general depletion in Sr and enrichment in Th, U. The chromitites contain varied PGE abundances (ΣPGE = 14–167 ppb) and IPGEs (Ir, Os, Ru) as well as enriched primitive-mantle normalized PGE patterns (PdN/IrN = 0.004–0.241). These features suggest the chromitites are cumulates formed from a highly magnesian parental magma with komatiitic affinity, derived by large degrees of partial melting of a refractory mantle protolith, which underwent crustal contamination. The Re Os isotope data demonstrate a minimum Re-depletion model age (TRD) of 2895 ± 24 Ma for the serpentinized dunite (MY11/3), and a more robust age of 3120 ± 12 Ma for two chromitite samples, which we interpret as the best estimate of the magmatic age of this ultramafic intrusion. Abundant zircon grains also occur within these ultramafic rocks with U Pb ages spanning from ca. 2.9 Ga to ca. 2.4 Ga, with a dominant age population in the range 2.8–2.6 Ga. We interpret the upper intercept age at ca. 2.9 Ga to represent the initial crystallization of zircon in the ultramafic rocks, potentially as a result of melt infiltration and metasomatism associated with regional granitoid crust formation and related metasomatic activity. The 2.5–2.4 Ga zircon ages reflect a late-stage high-grade metamorphic overprint.

中文翻译：

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来自印度南部西达尔瓦克拉通的太古代层状侵入体的铬铁矿

摘要 位于太古宙 Chithradurga 绿岩带的层状超镁铁质-镁铁质侵入体中的铬铁矿矿床是分隔印度南部达尔瓦克拉通块西部和中部的缝合线的一部分。蛇纹石化超镁铁质岩石，包括纯英岩、橄榄岩和富含金云母的单斜辉石岩，与块状铬铁矿一起出现。橄榄石成分显示出两个不同的组，一个在块状铬铁矿中具有约 91-92 的橄榄石，另一组在蛇纹石化纯质中具有约 95-96 的橄榄石 Fo 值。铬铁矿以块状铬铁矿的形式出现，并作为纯长岩中的副相出现。前者的特点是高Cr#(Cr/(Cr+Al)×100=67-70)和中高Mg#(Mg/(Mg+Fe)=52-66)；后者具有较大的 Cr 变化，这可能与蛇纹石化事件期间铬铁矿的亚固相线再平衡有关。在块状铬铁矿中，铬铁矿的亚固相线再平衡不太显着或不存在。橄榄岩的块状岩石主量元素和微量元素分析表明，其成分比贫化地幔岩的镁质低，TiO2、Na2O、CaO 含量低，Cr、Ni 含量高，Sr 普遍贫化，Th、U 富集。铬铁矿含有不同的 PGE 丰度（ΣPGE = 14-167 ppb）和 IPGE（Ir、Os、Ru）以及丰富的原始地幔归一化 PGE 模式（PdN/IrN = 0.004-0.241）。这些特征表明铬铁矿是由高镁质母岩浆形成的堆积物，具有科马提质亲和性，源自难熔地幔原岩的大量部分熔融，经历了地壳污染。Re Os 同位素数据表明蛇纹石化纯质 (MY11/3) 的最小再消耗模型年龄 (TRD) 为 2895 ± 24 Ma，而两个铬铁矿样品的更稳定年龄为 3120 ± 12 Ma，我们将其解释为对这种超镁铁质侵入岩浆年龄的最佳估计。这些超镁铁质岩石中也出现丰富的锆石颗粒，U Pb 年龄跨越约 10 年。2.9 Ga 至约 2.4 Ga，主要年龄人口在 2.8-2.6 Ga。我们将截距年龄上限解释为大约。2.9 Ga 代表超基性岩中锆石的初始结晶，可能是与区域花岗岩类地壳形成和相关交代活动相关的熔体渗透和交代作用的结果。2.5-2.4 Ga 锆石年龄反映了晚期高品位变质叠印。