The Archean Fortescue large igneous province: A result of komatiite contamination by a distinct Eo-Paleoarchean crust
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The ca. 2770–2690 Ma Fortescue Group of the Pilbara Craton in Western Australia represents one of the oldest known large igneous provinces (LIP) on Earth. Existing and new data show that the Fortescue LIP lavas have relatively evolved Nd isotopic compositions and are enriched in highly-incompatible elements. This is exemplified by mafic lavas of the 2775 Ma Mt Roe, 2740 Ma Kylena, 2715 Ma Bunjinah-Maddina and 2690 Ma Jeerinah Formations, with Th/Ti ca. 25, Th 13–5 ppm and eNd i around -4 (High-Th suite). Basalts and rare Al-depleted komatiites of the 2724–2715 Ma Pyradie Formation show more primitive compositions, with Th/Ti ca. 4, Th ca. 3–0.5 ppm and eNd i of 0 to -1 (Intermediate-Th suite). In this paper, we show that: (1) Parent magmas to all Fortescue LIP lavas were probably komatiitic; (2) Fortescue lavas contain 3.5–3.3 Ga xenocrystic zircons that correlate with the known age of Pilbara crust; and (3) the widespread enriched compositions in the Fortescue lavas require contributions to the ultramafic parent magmas from highly-enriched 3.7–3.5 Ga continental crust of the type available in the underlying Pilbara Craton. These results indicate that enriched chemical/isotopic compositions of the Fortescue mafic lavas result from large-scale crustal assimilation rather than enriched mantle/lithospheric sources. This implies that komatiite magmas were emplaced through Eo- to Paleoarchean enriched crust for ca. 90 Ma, via either (1) a long-lived source of high-temperature mantle melts such as a mantle plume; or (2) multiple mantle plumes, possibly represented by the 2775–2763 Ma Mt Roe and 2724–2715 Ma Tumbiana-Pyradie-Maddina-Bunjinah basalts. This study demonstrates that mafic magmas, with arc-like geochemical character and intermediate to felsic continental crustal compositions, can form via contamination of plume magmas by existing (local) crustal reservoirs, and the employment of modified lithospheric sources or enriched mantle is not required.
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