Petrology and Geochemistry of the Rhobell Volcanic Complex: Amphibole-Dominated Fractionation at an Early Ordovician Arc Volcano in North Wales

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Abstract
The Rhobell Volcanic Complex is a remnant of a late Tremadoc, dominantly calc-alkaline, arc volcano. It is the only substantially preserved representative in the southern British Caledonides of an early phase of Ordovician ensialic arc volcanism which followed the onset of southeasterly subduction of Iapetus oceanic lithosphere beneath the northern margin of Gondwanaland. The complex includes extrusive basalts and associated breccias, known as the Rhobell Volcanic Group, which rest unconformably on folded, uplifted and eroded rift-basin sediments of the earlier passive margin of the Iapetus Ocean. Amongst the basalts are relatively primitive pargasite-bearing varieties which contain cognate cumulate blocks, dominantly of pargasite but also with calcic clinopyroxene, Ti-magnetite, and (rarely) plagioclase. Basaltic rocks also occur in an associated feeder-sheet intrusion complex, and as numerous minor sills and dykes. In the intrusion complex, basaltic sheets are cut by microdiorites and scarce microtonalites. The compositional range in the volcanic complex, from low-SiO2 basalts to microtonalites (SiO2 45–66 wt. per cent), is attributable to fractional crystallization, early stages of which were dominated by removal of pargasite at (water-undersaturated) pressures close to 10 kb, within the mantle. The parental magma was derived by hydrous partial melting of a supra-subduction zone mantle wedge. Trace-element patterns indicate that the mantle was slightly depleted relative to the putative primordial composition (Ta/Yb = 0·1), prior to metasomatism by components from the subduction zone. Textural variations in cumulate blocks, and various phenocryst forms in basalts, are interpreted as indicating that the erupted magmas came from a thermally and compositionally stratified magma chamber with associated layered crystal accumulations, and that materials from initially separate layers were mixed prior to eruption.

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