Abstract Mantle-derived magmas at the base of the lower crust exerted a key control on late Archean cratonization in many continents. Since well-preserved, complete lower crustal section is rarely exposed, direct studies on the genetic link between mantle-derived magmas and cratonic lower crustal stabilization are inadequate. Cratonic lower crustal section is well-preserved in the southern margin of the Dharwar Craton (southern India), with a number of late Archean anorthositic-gabbroic complexes. Among these complexes, the Sittampundi anorthosite complex (SAC) consists of white- and dark-anorthosite (>60 vol.%), gabbros, and ultramafic rocks. In this study, SIMS zircon U-Pb dating of the anorthosite revealed a minimum emplacement age of 2522 ± 12 Ma, similar to the chromite Os model ages (2528–2563 Ma) of the anorthosite-hosted chromitite. In-situ plagioclase (87Sr/86Sr)i ratios (0.70079–0.70100) of the dark anorthosite and the chromite γOs (T) values (-0.2 to -0.4) of the chromitite suggest that the SAC was derived from a depleted mantle source. From the dark to white anorthosite, the (87Sr/86Sr)i ratios increase while the An contents decrease, suggesting crustal assimilation occurred during fractionation. Similarly, the mantle-like zircon δ18O values and relatively-wide eHf(T) (-2.1 to +8.4) range of the SAC anorthosite suggest that the parental magma had assimilated the ancient mafic lower crust. Emplacement ages of the SAC and published ages of the mafic/felsic granulites and charnockites altogether indicate that the anorthosites were formed during the Dharwar cratonization, and that the mantle-derived magma underplating may have led to extensive lower crustal melting. We argued that during underplating, high-density olivine-pyroxene cumulates (from fractionation of the mantle-derived magma) and partial-melting residues (in the overlying lower crust) mostly sank back to the underlying mantle. In contrast, the lower-density plagioclase and minor amphibole remained in the lower crust to form anorthositic-gabbroic sills. The magmas underplating and subsequent lower-crustal melting have likely made the cratonic lower crust more refractory and buoyant, which facilitated cratonization.

中文翻译：

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印度南部 ~2.5 Ga Sittampundi 斜长岩复合体的形成：对 Dharwar Craton 下地壳稳定性的影响

摘要 下地壳底部地幔源岩浆对许多大陆晚太古代克拉通化具有关键控制作用。由于保存完好的完整下地壳剖面很少出露，对地幔源岩浆与克拉通下地壳稳定之间的成因联系的直接研究是不够的。达瓦尔克拉通（印度南部）南缘克拉通下地壳剖面保存完好，有许多晚太古代斜长岩-辉长岩杂岩。在这些复合体中，Sittampundi 斜长岩复合体 (SAC) 由白色和深色斜长岩 (>60 vol.%)、辉长岩和超镁铁质岩组成。在这项研究中，该斜长岩的 SIMS 锆石 U-Pb 定年显示最小侵位年龄为 2522 ± 12 Ma，与以斜长岩为主体的铬铁矿的铬铁矿 Os 模型年龄（2528-2563 Ma）相似。暗斜长岩的原位斜长石 (87Sr/86Sr)i 比值 (0.70079–0.70100) 和铬铁矿的铬铁矿 γOs (T) 值 (-0.2 到 -0.4) 表明 SAC 来自贫化的地幔源。从深色到白色斜长岩，(87Sr/86Sr)i 比值增加，而 An 含量减少，表明分馏过程中发生了地壳同化作用。类似地，SAC 斜长岩的类地幔锆石 δ18O 值和相对较宽的 eHf(T)（-2.1 至 +8.4）范围表明母岩浆已经同化了古代基性下地壳。SAC 的侵位年龄和已公布的镁铁质/长英质麻粒岩和卡诺岩的年龄共同表明斜长岩是在 Dharwar 克拉通化过程中形成的，并且地幔来源的岩浆底侵可能导致了广泛的下地壳熔融。我们认为，在底镀过程中，高密度橄榄石辉石堆积物（来自地幔来源的岩浆的分馏）和部分熔融残留物（在上覆的下地壳中）大部分沉回到下面的地幔中。相比之下，密度较低的斜长石和小角闪石留在下地壳中，形成斜长岩-辉长岩窗台。岩浆底侵作用和随后的下地壳熔化可能使克拉通下地壳更难熔且更具浮力，从而促进了克拉通化。密度较低的斜长石和小角闪石留在下地壳中形成斜长岩-辉长岩窗台。岩浆底侵作用和随后的下地壳熔化可能使克拉通下地壳更难熔且更具浮力，从而促进了克拉通化。密度较低的斜长石和小角闪石留在下地壳中形成斜长岩-辉长岩窗台。岩浆底侵作用和随后的下地壳熔化可能使克拉通下地壳更难熔且更具浮力，从而促进了克拉通化。