Shellnutt J Gregory, Dostal Jaroslav
National Taiwan Normal University, Department of Earth Sciences, 88 Tingzhou Road Section 4, Taipei, 11677, Taiwan.
Saint Mary's University, Department of Geology, 923 Robie Street, Halifax, NS, B3H 3C3, Canada.
Sci Rep. 2019 Mar 1;9(1):3251. doi: 10.1038/s41598-019-39818-7.
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 (SiO = 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.
前寒武纪绿岩带的形成和演化研究因岩石记录的缺失以及当时所运行的构造体制的不确定性而受到阻碍。因此，确定前寒武纪绿岩带的现代类似物可能存在问题。在本文中，我们展示了地质、地球化学和岩石学证据，概述了加拿大不列颠哥伦比亚省海达瓜伊作为绿岩带现代实例的情况。海达瓜伊由两个与裂谷相关的火山 - 沉积序列组成。较老的（早三叠世）卡穆特森火山序列由水下超镁铁质 - 镁铁质火山岩组成，其上覆盖着海相碳酸盐岩和硅质碎屑岩。较年轻的（古近纪）马西特双峰火山序列由拉斑玄武岩和钙碱性玄武岩以及钙碱性硅质火山岩和侵入岩组成，其上覆盖着浅成碎屑砂岩。卡穆特森和马西特火山岩具有难以区分的锶 - 钕 - 铅同位素，表明它们源自相似的地幔源区。马西特的一些钙碱性岩石在成分上与俯冲相关的太古代绿岩带典型的镁质安山岩（SiO = 56 - 64 wt%；Mg# = 0.50 - 0.64）相似。我们表明，在马西特组双峰序列中观察到的钙碱性特征可能是由于拉斑玄武质母岩浆在相对氧化（ΔFMQ + 0.7）条件下的分离结晶作用，这表明钙碱性特征并非俯冲环境的初步证据。鉴于地质和地球化学证据，海达瓜伊代表了现代与俯冲无关的太古代绿岩带的最佳类似物之一。
