Generation of felsic rocks of bimodal volcanic suites from thinned and rifted continental margins: Geochemical and Nd, Sr, Pb-isotopic evidence from Haida Gwaii, British Columbia, Canada

摘要

AbstractThe compositionally bimodal volcanic rocks of the Eocene–Miocene Masset Formation from Queen Charlotte basin, Haida Gwaii, British Columbia, Canada, underlie an area greater than 5000km2where their exposed sections are up to 1.6km thick. The suite of mafic and felsic rocks (dacites and rhyolites) that erupted closely spaced in time, in both submarine and subaerial conditions, was associated with significant crustal extension and thin continental crust (~19–24km thick), with volcanism persisting for ~35Ma (from 46 to 11Ma). Predominant mafic types (mafic:felsic ~2:1) are moderately enriched mid-ocean-ridge-like basalts that were derived by a partial melting of a heterogeneous spinel peridotite source. Felsic rocks are plagioclase-phyric, two pyroxene-bearing, mainly peraluminous types which have Nd, Pb and Sr isotopic compositions overlapping those of basalts including high positive ?Nd(t)values (up to >+6). The chondrite-normalized REE patterns show light REE enrichment but flat heavy REE along with a variable negative Eu anomaly. Mineralogy, major and trace elements, Nd–Sr–Pb isotopic data and model calculations using MELTS are consistent with a derivation of felsic rocks from the basalts by fractional crystallization. The intercalation of basaltic and felsic rocks suggests the existence of separate, simultaneously active plumbing and feeder systems and relatively stable magma chamber(s) to generate large volumes of differentiated felsic magmas by fractional crystallization. The Masset rocks provide an example for the generation of felsic magmas of bimodal volcanic suites during rifting along a thinned continental margin.Graphical abstractDisplay OmittedHighlights?Masset Formation (46 to 11Ma) is a bimodal volcanic suite at Haida Gwaii, British Columbia.?Intraplate tholeiitic basalts and intercalated silicic rocks have overlapping isotopic values.?Felsic rocks were derived by fractional crystallization of a mafic parental magma.?Masset Formation formed during rifting along a thinned continental margin.]]>