Trace-element geochemistry of metabasaltic rocks from the Yukon-Tanana Upland and implications for the origin of tectonic assemblages in east-central Alaska

摘要

We present major- and trace- elementgeochemical data for 27 amphibolites and six greenstones fromthree structural packages in the Yukon-Tanana Upland of east-central Alaska: the Lake George assemblage (LG) of Devono--Mississippian augen gneiss, quartz-mica schist, quartzite, andamphibolite; the Taylor Mountain assemblage (TM) of maficschist and gneiss, marble, quartzite, and metachert; and theSeventymile terrane of greenstone, serpentinized peridotite, andMississippian to Late Triassic metasedimentary rocks. Most LGamphibolites have relatively high Nb, TiO_2, Zr, and light rareearth element contents, indicative of an alkalic to tholeiitic,within-plate basalt origin. The within-plate affinities of the LGamphibolites suggest that their basaltic parent magmasdeveloped in an extensional setting and support a correlation ofthese metamorphosed continental-margin rocks with lessmetamorphosed counterparts across the Tintina fault in theSelwyn Basin of the Canadian Cordillera. TM amphiboliteshave a tholeutic or caic-alkalic composition, low normalizedabundances of Nb and Ta relative to Th and La, and Ti/V valuesof <20, all indicative of a volcanic-arc origin. Limited resultsfrom Seventymile greenstones indicate a tholeiitic or calc-alkalic composition and intermediate to high Ti/V values (27-48), consistent with either a within-plate or an ocean-floor basaltorigin. Y-La-Nb proportions in both TM and Seventymilemetabasalts indicate the proximity of the arc and marginal basinto continental crust. The arc geochemistry of TM amphibolitesis consistent with a model in which the TM assemblage includesarc rocks generated above a west-dipping subduction zoneoutboard of the North American continental margin in mid-Paleozoic through Triassic time. The ocean-floor or within-platebasalt geochemistry of the Seventymile greenstones supports thecorrelation of the Seventymile terrane with the Slide Mountainterrane in Canada and the hypothesis that these oceanic rocksoriginated in a basin between the continental margin and an arc to the west.