The geochemistry of Archean anorthosite-bearing layered intrusions has major implications for the thermal and chemical state of the Archean crust/mantle system, as originally posited by Bowen (1917) as “the anorthosite problem” and expanded on by Ashwal (1993). Debates have centred on the nature of the parental magmas, emplacement mechanisms and geodynamic settings of Archean anorthosites, many of which have megacrystic textures. In this review, we have compiled whole-rock major and trace element and Nd isotope geochemical data from Archean anorthosite-bearing layered intrusions worldwide to address the outstanding questions outlined above regarding the petrogenesis of anorthosites. Archean anorthosite-bearing layered intrusions were not significantly affected by hydrothermal alteration and were derived from depleted mantle sources and most (85%) were emplaced in oceanic settings. Some intrusions were intruded in continental settings or ocean-continent transition zones, reflecting the emergence of continents in the Paleoarchean into the Neoarchean. Based on their petrography and major and trace element geochemistry, Archean anorthosite-bearing layered intrusions mostly crystallised from hydrous Ca- and Al-rich tholeiitic magmas that fractionated from more primitive tholeiitic parental magmas. Archean layered intrusions formed by shallow- and deep-level fractional crystallisation of tholeiitic magmas and predominantly formed in back-arc suprasubduction zone and volcanic arc settings. Archean anorthosite-bearing layered intrusions started forming at ca. 3850 Ma, most of them representing relicts of dismembered Archean subduction-related ophiolites. Modern-style plate tectonic processes have operated at least since the earliest Archean and were the predominant contributor to Archean crustal growth.
Discussion(0)
No comments yet. Be the first to comment.