Geochemical Variation in the Greenstones of S.W. England

The characteristic rocks of the Upper Palaeozoic greenstonesof S.W. England are intrusive dolerites and extrusive basicpillow lavas with minor intermediate volcanics and ultrabasics(picrites). Pyroclastics are represented by keratophyric andbasic tuffs. The intrusive greenstones show varying degrees of alteration(spilitization) from a primary ilmenite-plagioclase-clinopyroxene?olivineassemblage 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 secondaryprocesses (Ti, P, Y, Nb, and Zn), the intrusive greenstonescontain representatives of both the alkali olivine basalt andtholeiitic basalt magma series. Magmatic differentiation isgenerally minimal with the Devonian alkali basalt greenstonesbeing principally basaltic, while some of the Carboniferousalkali 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 alsogoverned the geochemical variation seen in the greenstones.Spilitization caused (a) local redistribution of principallyCa (forming epidote-rich and calcite-rich patches) and Mg (formingchlorite-rich patches), together with their respective coherenttrace elements, and (b) the variable, but often limited, lossof Ca, Sr, K, Rb, and Ba from many bodies, together with a gainin Na and H₂O. Progressive hydration, however, caused a decreasein the oxidation ratio—a feature found to be common inmany spilitic suites and mainly governed by the relative distributionof chlorite versus epidote.     

Metasomatic Hornfelses of the Land's End Aureole at Tater-du, Cornwall

At Tater-du, in the south of the Penwith peninsula, Cornwall,occurs a small isolated part of the Land's End aureole whichhas not been described in detail before. The aureole rocks hereare mostly banded amphibolitic hornfelses, some of which areof metasomatic origin. The metasomatic hornfelses can be divided into (1) Fe-Mg hornfelsescontaining anthophyllite, cummingtonite, and cordierite; (2)Ca hornfelses containing diopside, grossularite, epidote minerals,and axinite; and (3) K hornfelses containing a very high proportionof biotite. All three types are considered to have been derivedfrom hornblende hornfelses (‘greenstone-hornfelses’)of basic intrusive origin. Data are presented indicating their originally intrusive natureand similarity to other metasomatic hornfelses in the aureoleat Kenidjack and Botallack (Tilley, 1935). The Fe-Mg and Cahornfelses are considered to have resulted from the internalmigration of the Ca ion during metamorphism and the K hornfelsesfrom the allochemical addition of K from the granite.