The petrogenesis of the calc-alkaline borrowdale volcanic group, Northern England

Abstract

The Borrowdale Volcanic Group constitutes a major part of the Ordovician succession in the English Lake District. It comprises a suite of lavas, tuffs and ignimbrites with a maximum measured thickness of 5 km. The rocks of the Lake District are folded into a broad anticline which results in the Borrowdale Volcanics being exposed in two main outcrops, one north and the other south of a central core of older Skiddaw Slates. The northern outcrop consists almost entirely of basalts, basaltic andesites and occasional rhyolites, with very few intermediate members. In contrast, the southern outcrop is composed largely of andesites and dacites. Garnet phenocrysts, absent in the northern outcrop, are relatively abundant in the volcanic rocks of the southern outcrop. Analyses of 229 samples of lavas, ignimbrites and associated intrusives are presented together with electron microprobe analyses of selected garnet and augite phenocrysts. The southern outcrop volcanics are of calc-alkaline affinities, whereas those of the northern outcrop are transitional in character between tholeiitic and calc-alkaline. The use of La/Y ratios is shown to be particularly effective in distinguishing between members of the two suites. Detailed analytical studies on the garnet phenocrysts, especially La and Y abundances, show that crystal fractionation of garnet phenocrysts is incompatible with the geochemistry of their host rocks. It is concluded that the southern outcrop magmas evolved by some process other than crystal fractionation. A partial-melting hypothesis is proposed as an alternative, the melt being stored at depth (possibly at the crust mantle interface) long enough for garnet to nucleate, and then transferred rapidly to the surface. In contrast, the northern outcrop lavas are highly porphyritic and present abundant evidence of crystal fractionation. It is suggested that these rocks evolved by the fractionation of a basalt or basaltic and site parent under relatively dry conditions at shallow depth. The Borrowdale Volcanics are compared with the volcanic rocks of modern Island arcs. In particular the southward transition of magna type from tholeiitic to calc-alkaline compares with similar transitions occurring across modern island arcs. It is concluded that the Borrowdale Volcanics were erupted in an ancient Island arc at the margin of a contracting, proto-Atlantic ocean. This hypothesis is consistent with current models for the evolution of the Caledonian Appalachian orogen. The Borrowdale magmas were probably derived by the partial melting of basaltic oceanic crust carried down into the mantle on descending lithosphere plates. In the case of the southern outcrop rocks the magma- was not affected by subsequent ciystal fractionation, whereas the northern outcrop magma has undergone considerable modification by this process. Finally- the partial melting of oceanic crust is examined in the light of recent experimental studies. It is suggested that island arc tholeiitic magmas are generated at shallow depth by reactions involving amphibole breakdown. Calc-alkaline magmas are produced at greater depths by the partial melting of wet eclogite, and between these two extremes a continuum of transitional magma types could be generated.