麻豆传媒

Detailed petrological and geochemical studies in the White Rock Formation, a predominantly volcanic unit of Lower Paleozoic age near Yarmouth, Nova Scotia, indicate that this sequence was affected (probably in Devonian times) by regional metamorphism (cf. Winkler, 1974) under conditions of low pressure (<3.5 kilobar) and low-medium grade (480^o-580^oC). It is apparent that mineral-chemical equilibrium was not generally attained, and "onion shell-type" compositional zones retained by amphibole and plagioclase phases are interpreted as attesting to prograde metamorphism from greenschist-amphibolite transition facies to lower-amphibole facies (c.f. Turner, 1968)

Conspicuously zoned epidosite lenses, common in metaigneous rocks of this area, were probably formed by "metamorphic diffusion metasomatism" (c.f. Korzhinskii, 1970). Their detailed geochemical investigation suggests that while the alkalies, the alkaline earths, Co and Fe were extensively mobilized during the formation of the epidosite lenses, the elements P, Ti, Cr, Sc, Zr, Hf, Nb, Ta, Y, Ree, Th and Mn were relatively "immobile".

The "immobile" behaviour of a number of minor and trace elements permits us to apply empirical and theoretical discriminant tests and to speculate on the pre-metamorphic petrochemistry and possible tectonic environment of the igneous rocks. It is suggested that the White Rock volcanic suite represents two series, one mildly alkaline basalt-trachyte series, which is strongly differentiated, and a less voluminous olivine tholeiite-rhyolite series. The "parental" magmas for these two series possibly evolved at different times from a non-depleted mantle source and involved fractionation of garnet.

The White Rock Formation appears to have been erupted through a sialic basement and in a tectonic regime akin to "failed" continental rifting and/or a distant back-arc situation related to early Paleozoic subduction.

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Supervisor:聽 Gunter Muecke