Formation of ~2.5 Ga Sittampundi anorthosite complex in southern India: Implications to lower crustal stabilization of the Dharwar Craton
Formation of ~2.5 Ga Sittampundi anorthosite complex in southern India: Implications to lower crustal stabilization of the Dharwar Craton
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印度南部 ~2.5 Ga Sittampundi 斜长岩杂岩的形成：对达尔瓦尔克拉通下地壳稳定的影响
DOI：
10.1016/j.precamres.2020.106012
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发表时间：
2020-11
影响因子：
3.8
通讯作者：
Zhai Ming-Guo
中科院分区：
文献类型：
--
作者：
He Hai-Long;Wang Yu-Quan;George P. M.;Sajeev K.;Guo Jing-Hui;Lai Chun-Kit;Zhai Ming-Guo
下地壳底部的地幔岩浆对许多大陆的晚太古代克拉通化发挥了关键控制作用。由于保存完好、完整的下地壳剖面很少暴露，因此对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还不够。达尔瓦克拉通南缘（印度南部）克拉通下地壳剖面保存完好，发育有许多晚太古代斜长岩杂岩。在这些杂岩中，Sittampundi 斜长岩杂岩 (SAC) 由白色和深色斜长岩 (>60 vol%)、辉长岩和超镁铁岩组成。在这项研究中，斜长岩的 SIMS 锆石 U-Pb 测年显示，最小侵位年龄为 2522 ± 12 Ma，与斜长岩托管铬铁矿的铬铁矿 Os 模型年龄 (2528–2563 Ma) 相似。暗色斜长岩的原位斜长石 (87Sr/86Sr) 比值 (0.70079–0.70100) 和铬铁矿的铬铁矿 γOs(T) 值 (-0.2 至 -0.4) 表明 SAC 源自耗尽的地幔源。从深色斜长石到白色斜长石，(87Sr/86Sr)比值增加，而An含量减少，表明分馏过程中发生了地壳同化。类似地，SAC斜长岩的类幔锆石δ18O值和相对较宽的εHf(T)（-2.1至+8.4）范围表明母体岩浆已经同化了古老的镁铁质下地壳。 SAC 的侵位年龄和已发表的镁铁质/长英质麻粒岩和角长岩年龄共同表明斜长岩是在达尔瓦尔克拉通化期间形成的，并且地幔源岩浆底侵可能导致了广泛的下地壳熔融。我们认为，在底侵期间，高密度橄榄石-辉石累积（来自地幔岩浆的分馏）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。相反，密度较低的斜长石和少量角闪石则留在下地壳中，形成斜长岩-辉长岩基岩。岩浆底侵和随后的下地壳熔化可能使克拉通下地壳更加耐火和浮力，从而促进了克拉通化。
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)iratios (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)iratios increase while the An contents decrease, suggesting crustal assimilation occurred during fractionation. Similarly, the mantle-like zircon δ18O values and relatively-wide εHf(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.
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