Geochemistry and tectonic setting of basaltic volcanism, northern Coast Mountains, southeastern Alaska

Abstract

Subgreenschist to amphibolite facies metavolcanic rocks found along the western flank of the Coast Plutonic Complex (western metamorphic belt) between Berners Bayand Endicott Arm, southeastern Alaska, are divided into three distinct sequences on the basis of age and the nature of interbedded metasedimentary rocks. These sequences crop out in linear belts, from west to east: the Gravina belt of Jurassic-Cretaceous age, the western Taku terrane of Permian to Triassic age, and the eastern Taku-YukonTanana terrane of largely unknown age. Major, minor, trace element, and relict phenocryst chemistry are used to locate boundaries between the sequences and to provide clues to their origin. Variations in large ion lithophile element (LILE) and high field strength element (HFSE) abundances indicate that Gravina and eastern Taku magmas were derived through subduction-related processes. Pyroxene phenocryst compositions, LILE, light rare earth element, and HFSE enrichments in Gravina belt rocks are suggestive of high-K calc-alkaline to shoshonitic magmatism. Amphibolite facies metavolcanic rocks in the eastern Taku terrane are inferred to be island arc tholeiites. Western Taku metavolcanic rocks show little or no evidence of subduction-related genesis and resemble within-plate, possibly plume-related, basaltic magmas.Tectonic settings and magma sources inferred from metabasalt geochemistry and the associated stratigraphy of the three sequences are compatible with derivation of all the volcanic rocks in the western metamorphic belt and the Wrangellia terrane from a single Permian to Cretaceous arc complex. Initiation of arc tholeiite volcanism in the eastern Taku terrane possibly occurred during or prior to the Permian. Correlation of western Taku and Wrangellia rocks suggests a Triassic link between the Alexander and at least part of the Taku terrane, and nonsubduction-related basaltic volcanism prior to the Late Jurassic. Later rifting and/or changes in subduction geometry may have resulted in eruption of Gravina arc lavas into a marine basin, possibly floored by western Taku-Wrangellia rocks.

Mesozoic and Cenozoic tectonic growth of southern Alaska: A sedimentary basin perspective

“…Instead, source materials responsible for the production of magmas are limited to the depleted mantle wedge above the downgoing slab Ϯ juvenile lithosphere (Snyder and Hart, 2005, this volume). Correlative volcanic rocks in southeastern Alaska yield similar lithologic and geochemical traits (Berg et al, 1972;Rubin et al, 1990;McClelland et al, 1992a;Stowell et al, 2000). The Chisana arc was an important sediment source based on compositional and detrital geochronologic data from the Kahiltna, Nutzotin, and Matanuska Valley-Talkeetna Mountains basins.…”

Sedimentology, U-Pb detrital geochronology, and Hf isotopic analyses from Mississippian–Permian stratigraphy of the Mystic subterrane, Farewell terrane, Alaska

“…Pennsylvanian igneous rocks from the western margin of Stikinia exhibit enriched mid-oceanridge basalt (E-MORB) geochemical signatures (Logan, 2004) and are interpreted to have developed within an intra-arc or back-arc rift setting. Permian igneous rocks have been reported from the Taku terrane in southeast Alaska (Gehrels, 2002) and exhibit E-MORB to enriched withinplate geochemical signatures (Stowell et al, 2000). Geochemical analyses from the Klinkit Group show that volcanic rocks have very minor crustal inheritance and reflect primitive (e Nd : +6.7 to +7.4) isotopic signatures .…”

Geochemical and Stratigraphic Analysis of the Chisana Formation, Wrangellia Terrane, Eastern Alaska: Insights Into Early Cretaceous Magmatism and Tectonics Along the Northern Cordilleran Margin

“…19 whole rock geochemical analyses of lavas from the Chisana Formation with no stratigraphic context (Barker et al, 1994), lavas from the Gravina belt (Rubin & Saleeby, 1991;Stowell et al, 2000) and sparse tuffs in the Dezadeash Formation (Lowey, 2011).…”

“…Previous studies of the Jurassic–Cretaceous basins exposed along the inboard margin of the Insular terranes have focused chiefly on the sedimentary records. Late Jurassic–Early Cretaceous volcanic rocks are absent in the Kahiltna basin, limited to sparse tuffs in the Dezadeash basin, and intensely deformed/altered in the Gravina basin (Haeussler, 1992; Hampton et al, 2010; Kalbas et al, 2007; Lowey, 2011; Manuszak et al, 2007; McClelland et al, 1992; Stowell et al, 2000; Yokelson et al, 2015). Thus, >2 km thick volcanic successions represented by the Chisana Formation in the Nutzotin basin provide a unique opportunity to evaluate the nature of volcanism along the inboard margin of Wrangellia.…”