Major, trace and platinum-group element geochemistry of mesoarchean komatiites from the banasandra greenstone belt, Western Dharwar craton (southern India): implications for multi-stage plume magmatism in generating compositionally diverse komatiites

The early Archean Al-undepleted komatiites (AUK) from two locations (Kunikenahalli, Kodihalli) of the Banasandra greenstone belt in the Western Dharwar Craton are studied for understanding their petrogenesis. The field distribution, mineralogy and geochemistry indicate their cogenetic nature. The samples are mainly cumulates (MgO: 37–45 wt%), now completely serpentinized, while the spinifex zone is obscure. Using geochemical modelling, the MgO content of the primary undifferentiated magma is estimated to be nearly 25 wt%. The depleted LREE values from this study, and the positive ε_Nd values from previous studies suggest that the komatiites are derived from a depleted mantle source. Trace-element and PGE geochemistry eliminate the possibility of crustal contamination and sulphide-saturation in these komatiites after emplacement. Modelling results estimated in this study suggest ~45% melting of a deeper mantle (~6 GPa) to generate the komatiites. Fractionation of olivine (Fo₉₄) from the parental melt and their accumulation formed the mesocumulates in Kodihalli, and the more fractionated orthocumulates in Kunikenahalli. The compositional spectrum from AUK through Al-depleted (ADK) to Al-enriched (AEK) varieties in the Banasandra greenstone belt reported in this study and others, are proposed to be a product of melting of a single mantle plume at different depths.

对西达瓦尔克拉通（Western Dharwar Craton）巴纳桑德拉绿岩带（Banasandra greenstone belt）两处产地（库尼肯纳哈利、科迪哈利）产出的早太古代Al未亏损科马提岩（Al-undepleted komatiites, AUK）开展岩石成因研究。野外展布、矿物学与地球化学特征均指示其具有同成因属性。样品主体为堆晶岩（MgO含量介于37~45 wt%），现已完全蛇纹石化，而鬣刺结构带已模糊难辨。通过地球化学模拟估算，该类科马提岩对应的原始未分异岩浆MgO含量约为25 wt%。本研究获得的轻稀土元素（LREE）亏损特征，结合前人报道的正ε_Nd同位素值，指示该科马提岩起源于亏损地幔源区。微量元素与铂族元素（PGE）地球化学特征排除了该科马提岩侵位后发生地壳混染与硫化物饱和的可能性。本研究的模拟结果显示，约45%的深部地幔（压力约6 GPa）熔融作用可形成该类科马提岩。科迪哈利地区的中堆晶岩由原生熔体结晶析出的镁橄榄石（Fo₉₄）经堆积作用形成，而库尼肯纳哈利地区则发育分异程度更高的正堆晶岩。本研究与已有研究均在巴纳桑德拉绿岩带中发现了从Al未亏损科马提岩（AUK）经Al亏损科马提岩（Al-depleted komatiites, ADK）至Al富集科马提岩（Al-enriched komatiites, AEK）的组分序列，该序列被认为是单一地幔柱在不同深度发生熔融的产物。