Geochemistry and origin of Archean volcanic rocks from the Upper Keewatin assemblage (ca 2.7 Ga), Lake of the Woods Greenstone Belt, Western Wabigoon Subprovince, Superior Province, Canada |
| |
| Authors: | Osamu Ujike Alan M Goodwin and Tomoyuki Shibata |
| |
| Institution: | Department of Earth Sciences, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan (email: ),;Department of Geology, Earth Sciences Center, University of Toronto, Toronto, M5S 3B1, Canada and;Institute for Geothermal Sciences, Kyoto University, Beppu, Japan |
| |
| Abstract: | Abstract Volcanic rocks from the Upper Keewatin assemblage ( ca 2720 Ma) were geochemically classified into five groups; komatiites, tholeiitic rocks having near-flat primitive mantle-normalized abundance patterns, Nb-enriched basalts and andesites (NEBA) plus normal calc-alkaline (NCA) rocks, adakites and shoshonites. The adakites having La/Yb]N >30 and <30 were probably derived from felsic magmas formed by partial melting of a subducted slab at relatively greater and smaller depths, respectively. Ascending adakite magmas, by interaction with the overlying mantle wedge, decreased in Al2O3 / Y ratio and selectively lost high-field strength elements, thereby forming mantle sources for both NEBA + NCA and shoshonite magmas. Under the influence of a mantle plume, the source of komatiites, the NEBA + NCA magmas were generated from that part of the mantle wedge metasomatized by adakite magmas having La / Yb]N <30, and tholeiitic magmas from unmetasomatized part of the same mantle wedge. Magmas of both adakites having La / Yb]N >30 and shoshonites were generated in a normal Archean Arc system setting. |
| |
| Keywords: | adakite arc volcanism Archean greenstone belt mantle plume Nb-enriched basalts and andesites shoshonite |
|
|
