Journal of Petrology | Volume 17 | Number 4 | Pages 522-545 | 1976
© Oxford University Press 1976
research-article |
Geochemical Variation in the Greenstones of S.W. England
Department of Geology, University of Keele Staffordshire, England ST5 5BG
Received October 16, 1975; Revised December 22, 1975; ABSTRACT
The characteristic rocks of the Upper Palaeozoic greenstones of S.W. England are intrusive dolerites and extrusive basic pillow lavas with minor intermediate volcanics and ultrabasics (picrites). Pyroclastics are represented by keratophyric and basic tuffs.
The intrusive greenstones show varying degrees of alteration (spilitization) from a primary ilmenite-plagioclase-clinopyroxene±olivine assemblage to a hydrous low-grade spilite (or meta-dolerite) assemblage composed of variable proportions of albite, chlorite, epidote, calcite, and amphibole.
Based on the distribution of elements little affected by secondary processes (Ti, P, Y, Nb, and Zn), the intrusive greenstones contain representatives of both the alkali olivine basalt and tholeiitic basalt magma series. Magmatic differentiation is generally minimal with the Devonian alkali basalt greenstones being principally basaltic, while some of the Carboniferous alkali basalt greenstones tend towards mugearitic compositions. No intrusive acid differentiates have yet been reported.
Apart from differences of magma type and minor differentiation, low-grade alteration or ‘spilitization’ has also governed the geochemical variation seen in the greenstones. Spilitization caused (a) local redistribution of principally Ca (forming epidote-rich and calcite-rich patches) and Mg (forming chlorite-rich patches), together with their respective coherent trace elements, and (b) the variable, but often limited, loss of Ca, Sr, K, Rb, and Ba from many bodies, together with a gain in Na and H2O. Progressive hydration, however, caused a decrease in the oxidation ratio—a feature found to be common in many spilitic suites and mainly governed by the relative distribution of chlorite versus epidote.
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