印度Dharwar克拉通~2.5 Ga Sittampundi斜长岩的成因及其对克拉通化过程的制约

Crust-mantle interaction exerted a major control on the continental cratonization in the end Archean, including exchange of materials and heat flows. Underplating of mantle-derived magmas to the base of the lower crust would have influenced the architecture and composition of the lower crust. Among the mafic intrusions at the base of the cratonic lower crust, anorthosite is a very important and special rock unit. Since well-preserved complete lower crustal section is rarely exposed, studies on these anorthosites are largely inadequate, especially concerning their role in the cratonic lower crustal stabilization. In the southern margin of the Dharwar craton in India, a typical cratonic lower crustal section is exposed. Anorthosite is well-developed at the bottom of the lowermost crust. We have already conducted preliminary field geological studies and lab analyses, which indicate that the anorthosites (~2.5 Ga) were formed during the time of cratonization. Therefore, the proposed study area is an ideal research target. We propose to study the best developed anorthosite in the region. Using techniques such as petrology, mineral geochemical and isotopic microanalyses, we aim to unravel the magmatic evolution features of the anorthosite. High-precision U-Pb dating on zircon, sphene and monazite, and their comparison with the published geochronological studies on the granulites and charnockites would elucidate the age and genetic relationship between the mantle-derived magmatism (that formed the anorthosite) and the magmatism/metamorphism associated with the cratonic lower crustal stabilization. This would enhance our understanding on the processes and mechanisms of continental cratonization.

壳幔相互作用是太古宙末克拉通化的重要机制之一，包括物质和能量的交换。幔源岩浆底侵会影响下地壳的物质组成和结构。斜长岩是克拉通最下部地壳重要和有特色的岩石，由于完整的克拉通下地壳剖面出露较少，斜长岩的研究较为匮乏，特别是对克拉通下地壳稳定化的作用关系尚不明确。印度Dharwar克拉通南缘出露了典型的克拉通下地壳剖面，其底部发育成规模的斜长岩，是研究克拉通下地壳的理想对象。申请人已对该区斜长岩进行了野外考察和室内研究，确定斜长岩的年龄为~2.5 Ga，正是该区域克拉通化的时代，为项目执行奠定了基础。项目将对斜长岩开展岩石学、矿物微区元素和同位素地球化学研究，揭示该斜长岩的岩浆演化特征；进行高精度的锆石、榍石、独居石年代学研究，并与区域上各类麻粒岩、紫苏花岗岩进行年代学对比，厘清形成斜长岩的幔源岩浆与下地壳稳定化过程中相关的岩浆作用和变质作用的年代格架和成因关系，深化对克拉通化过程的认识。

幔源岩浆底侵到大陆地壳底部对太古宙末期的大陆地壳克拉通化具有重要意义。由于保存完好的克拉通下地壳（尤其是最底部）极少出露。极大的限制了我们对幔源的岩浆底侵与克拉通化之间的成因关系的认识。印度Dharwar克拉通南缘出露了完整的克拉通下地壳剖面，其底部出露大量的辉长-斜长岩体，以Sittampundi斜长岩体出露最为完好。Sittampundi斜长岩体主要由白色和暗色斜长岩（>60 vol.%），辉长岩和少量的超镁铁质岩石。斜长岩中锆石SIMS U-Pb年龄显示，其侵入年龄为2522 ± 12 Ma，与铬铁矿Os同位素模式年龄一致（2528–2563 Ma）。暗色斜长岩中斜长石具有低的（87Sr/86Sr）i 值（0.70079–0.70100），结合铬铁矿与亏损地幔相似的γOs（T）值（-0.2 to -0.4），指示Sittampundi斜长岩源区为亏损地幔。从暗色斜长岩到白色斜长岩，斜长石的（87Sr/86Sr）i 值升高，同时伴随着An值降低，说明在岩浆结晶分异过程中发生了地壳混染。同时，斜长岩中锆石显示地幔的δ18O值和相对较大的εHf（T）变化范围（-2.1 to +8.4），说明混染的地壳成分为基性下地壳。以Sittampundi斜长岩体为代表的的幔源岩浆年龄与区域上紫苏花岗岩，长英质麻粒岩的成岩年龄一致，说明幔源岩浆底侵诱发了下地壳的大规模部分熔融。在幔源研究底侵过程中，高密度的幔源岩浆结晶分异形成的超镁铁质堆晶和下地壳部分熔融残余，在重力作用下折返会上地幔顶部。而低密度的斜长石和少量的角闪石则保留在下地壳底部，形成下地壳最底部的辉长-斜长岩体。幔源岩浆底侵和伴生的下地壳部分熔融，使得下地壳变得干化和具有浮力，能够稳定存在于地幔之上，最终完成克拉通化。