Authors / CoAuthors
Champion, D.C. | Doublier, M.P. | Smithies, R.H.
Abstract
Large geochronological and geochemical data sets for the Paleo- to Mesoarchean Pilbara and Meso- to Neoarchean Yilgarn cratons, Western Australia, show that both cratons exhibit similar evolutionary trends in felsic magmatism, providing important constraints on Archean tectonics. The most obvious trend is a transition from sodic magmatismthe ubiquitous tonalite-trondhjemite-granodiorite (TTG) series with their high pressure (high-Al) signatureto potassic magmatism. In the Pilbara craton this transition is marked by two periods of potassic magmatism separated by 50 Myr. In the Yilgarn, the transition is mostly diachronous with potassic magmatism broadly younging to the west, except for one terrane where potassic magmatism begins ~40 Ma earlier. The change from sodic to potassic magmatism is, in part, a continuation of trends observable within the sodic granites themselves, which become more LILE-enriched with decreasing age. It is also evident in both cratons that magmatism derived from basaltic precursors is not confined to high-pressure formation of High-Al TTGs but includes lower pressure variants. The latter include low-Al TTGs (significant in the Pilbara Craton), and a group with high-HFSE and low- to moderate LILE-contents typical of A-type magmas. In the Yilgarn Craton such rocks form a locally common, often bimodal, association, representing formation at high-temperature and low-pressure. They are not often recognised as belonging to the sodic magmatic group but clearly reflect a magmatic pathway that starts with a largely mafic protolith, albeit at lower pressures and, unlike the low-Al TTGs, higher temperatures. Another shared trend is the appearance of a diverse group of rocks not unlike those seen in modern-day convergent tectonic settings. These comprise high-Mg diorites (or sanukitoids) (and related rocks), boninite-like rocks, `calc-alkaline basalts and andesites, calc-alkaline lamprophyres, but also syenites and monzonites. These rocks appear well after the first appearance of high- (and low-) Al TTGs and are most abundant just prior to major onset of potassic magmatism. In both cratons they are largely confined to younger linear geological terranes or marginal to/within the larger generally older terranes, and this, along with their enriched geochemistry permits the interpretation that they tap enriched mantle along crustal scale structures. Such rocks form a significant local component but overall are not abundant. The trends documented above are evident in many Archean terranes. The simplest way to explain the variation in the TTGs (high- and low-pressure variants) and the trends from sodic to potassic magmatism is via progressive reworking (maturation) of existing continental crust (for crustal-derived magmatism) and increasing involvement of felsic crust (for non-crustal magmatism). The chemical and isotopic evidence suggests a role for both mechanisms. It is, however, clear that crustal reworking played an early and persistent role in the compositional evolution of both the Pilbara and Yilgarn cratons (and probably Archean cratons in general), suggesting that models advocating a switch from slab-derived TTGs to crustal-derived potassic magmas are too simplistic. The appearance of magmas with an arc-like signature suggests that proto-subduction-like tectonic processes operated, at least intermittently, but not necessarily that they dominated Archean crustal evolution and crust formation.
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nonGeographicDataset
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83218
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- External PublicationAbstract
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- WA
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2015-01-01T00:00:00
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[-36.0, -20.0, 114.0, 126.0]
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