Mesoproterozoic lamproite dykes occurring in the Banganapalle Lamproite Field of southern India show extensive hydrothermal alteration, but preserve fresh spinel, apatite and rutile in the groundmass. Spinels belong to three genetic populations. Spinels of the first population, which form crystal cores with overgrowth rims of later spinels, are Al-rich chromites derived from disaggregated mantle peridotite. Spinels of the second population include spongy-textured grains and alteration rims of titanian magnesian aluminous chromites that formed by metasomatic interactions between mantle wall-rocks and precursor lamproite melts before their entrainment into the erupting lamproite magma. Spinels that crystallised directly from the lamproite magma constitute the third population and show five distinct compositional subtypes (spinel-IIIa to IIIe), which represent discrete stages of crystal growth. First stage magmatic spinel (spinel-IIIa) includes continuously zoned macrocrysts of magnesian aluminous chromite, which formed together with Al–Cr-rich phlogopite macrocrysts from an earlier pulse of lamproite magma at mantle depth. Crystallisation of spinel during the other four identified stages occurred during magma emplacement at crustal levels. Titanian magnesian chromites (spinel-IIIb) form either discrete crystals or overgrowth rims on spinel-IIIa cores. Further generations of overgrowth rims comprise titanian magnesian aluminous chromite (spinel-IIIc), magnetite with ulvöspinel component (spinel-IIId) and lastly pure magnetite (spinel-IIIe). Abrupt changes of the compositions between successive zones of magmatic spinel indicate either a hiatus in the crystallisation history or co-crystallisation of other groundmass phases, or possibly magma mixing. This study highlights how different textural and compositional populations of spinel provide important insights into the complex evolution of lamproite magmas including clues to elusive precursor metasomatic events that affect cratonic mantle lithosphere.

中文翻译：

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印度 Dharwar 克拉通 Banganapalle 橄榄石-白榴石菱镁矿脉的多阶段岩浆历史：来自尖晶石成分区带的证据

发生在印度南部 Banganapalle Lamproite 油田的中元古代菱镁矿脉显示出广泛的热液蚀变，但在地块中保留了新鲜的尖晶石、磷灰石和金红石。尖晶石属于三个遗传种群。第一类尖晶石形成晶核，带有后期尖晶石的过度生长边缘，是源自分解的地幔橄榄岩的富含铝的铬铁矿。第二类尖晶石包括海绵状纹理颗粒和钛酸镁铝铬铁矿蚀变边缘，它们是由地幔围岩和前体菱镁矿熔体在夹带进入喷发的菱镁矿岩浆之前交代相互作用形成的。直接从菱镁矿岩浆结晶的尖晶石构成第三类群，并显示出五种不同的成分亚型（尖晶石 IIIa 到 IIIe），代表晶体生长的离散阶段。第一阶段岩浆尖晶石（尖晶石-IIIa）包括连续分带的镁铝铬铁矿大晶，它们与地幔深度较早的菱镁矿岩浆脉冲形成的富含铝铬的金云母大晶一起形成。其他四个确定阶段的尖晶石结晶发生在地壳水平的岩浆侵位期间。Titanian 镁铬铁矿 (尖晶石-IIIb) 在尖晶石-IIIa 核上形成离散晶体或过度生长的边缘。再几代的过度生长边缘包括钛镁铝铬铁矿（尖晶石-IIIc）、具有超尖晶石成分的磁铁矿（尖晶石-IIId）和最后的纯磁铁矿（尖晶石-IIIe）。岩浆尖晶石连续带之间成分的突然变化表明结晶历史或其他基质相的共结晶的中断，或者可能是岩浆混合。这项研究强调了尖晶石的不同质地和成分群如何为斑铜矿岩浆的复杂演化提供重要见解，包括影响克拉通地幔岩石圈的难以捉摸的前体交代事件的线索。