Abstract
Understanding the formation and evolution of Precambrian greenstone belts is hampered by gaps in the rock record and the uncertainty of the tectonic regime that was operating at the time. Thus identifying a modern analogue of a Precambrian greenstone belt can be problematic. In this paper we present geological, geochemical and petrological evidence outlining the case for Haida Gwaii (British Columbia, Canada) as a modern example of a greenstone belt. Haida Gwaii is comprised of two rift-related volcano-sedimentary sequences. The older (Early Triassic) Karmutsen volcanic sequence consists of subaqueous ultramafic-mafic volcanic rocks that are capped by marine carbonate and siliciclastic rocks. The younger (Paleogene) Masset bimodal volcanic sequence consists of tholeiitic and calc-alkaline basalt along with calc-alkaline silicic volcanic and intrusive rocks that are capped by epiclastic sandstones. The Karmutsen and Masset volcanic rocks have indistinguishable Sr-Nd-Pb isotopes demonstrating they were derived from a similar mantle source. Some of the Masset calc-alkaline rocks are compositionally similar to magnesian andesites (SiO2 = 56–64 wt%; Mg# = 0.50–0.64) that are typical of subduction-related Archean greenstone belts. We show that the calc-alkaline signature observed in the bimodal sequence of the Masset Formation is likely due to fractional crystallization of a tholeiitic parental magma under relatively oxidizing (ΔFMQ + 0.7) conditions indicating that a calc-alkaline signature is not prima facie evidence of a subduction setting. Given the geological and geochemical evidence, Haida Gwaii represents one of the best analogues of a modern subduction-unrelated Archean greenstone belt.
Highlights
Beyond the volcanic nature of Archean greenstone belts their origin remains one of the most debated issues in the solid earth sciences[1,2,3]
The bimodal Masset Formation unconformably lies on top of the Karmutsen basalt and Mesozoic sedimentary rocks that belong to Wrangellia
The composition of the volcanic rocks and the stratigraphy of the island broadly match that of an archetypical Archean greenstone belt[2,3,41]
Summary
Greenstone belts are linear to curvilinear belt-like structures that are typically 10–25 km wide, 100–300 km long, 5 km to 30 km thick, and comprised of a lower volcanic unit and an upper sedimentary unit[1,2,3]. The volcanic sequences of an archetypical[1,2] greenstone belt are comprised of subaqueous and subaerial volcanic and volcaniclastic rocks whereas the upper unit is predominately comprised of sedimentary rocks (Fig. 2). The lower portion of the volcanic unit consists of subaqueous ultramafic (komatiite) to mafic (tholeiite, boninite) volcanic rocks with minor felsic tuffs. The upper portions of the lower unit consist of a bimodal sequence of tholeiitic flows and calc-alkaline silicic (andesite to rhyolite) volcanic rocks[1,49,50]. In some cases the mafic-ultramafic sequences are separated by thin layers of calc-alkaline rocks at time intervals ranging from 3 to 30 million years[33]. Sedimentary depositional gaps exist between volcanic episodes that range in duration from 2 to 27 million years[51]. Many sedimentary rocks of the upper unit consist of chemically precipitated rocks such as banded iron formation, cherts and jaspers but the uppermost sedimentary rocks consist of terrigenous sediments (shales, pelitic sandstones, conglomerates, quartzites)
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