Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
8912582 | Precambrian Research | 2018 | 64 Pages |
Abstract
The ca. 2770-2690â¯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 2775â¯Ma Mt Roe, 2740â¯Ma Kylena, 2715â¯Ma Bunjinah-Maddina and 2690â¯Ma Jeerinah Formations, with Th/Ti ca. 25, Th 13-5â¯ppm and εNdi around â4 (High-Th suite). Basalts and rare Al-depleted komatiites of the 2724-2715â¯Ma Pyradie Formation show more primitive compositions, with Th/Ti ca. 4, Th ca. 3-0.5â¯ppm and εNdi of 0 to â1 (Intermediate-Th suite). In this paper, we show that: (1) Parent magmas to all Fortescue LIP lavas were probably komatiitic; (2) Fortescue lavas contain 3.5-3.3â¯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 3.7-3.5â¯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. 90â¯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 2775-2763â¯Ma Mt Roe and 2724-2715â¯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.
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geochemistry and Petrology
Authors
David R. Mole, Stephen J. Barnes, Zhuosen Yao, Alistair J.R. White, R. Maas, Christopher L. Kirkland,