Petrology and Geochemistry of the Rhobell Volcanic Complex: Amphibole-Dominated Fractionation at an Early Ordovician Arc Volcano in North Wales |
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| Authors: | KOKELAAR PETER |
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| Institution: | Department of Environmental Studies, University of Ulster Newtownabbey, Co. Antrim, BT37 0QB, Northern Ireland, U.K. |
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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 substantiallypreserved representative in the southern British Caledonidesof an early phase of Ordovician ensialic arc volcanism whichfollowed the onset of southeasterly subduction of Iapetus oceaniclithosphere beneath the northern margin of Gondwanaland. Thecomplex includes extrusive basalts and associated breccias,known as the Rhobell Volcanic Group, which rest unconformablyon folded, uplifted and eroded rift-basin sediments of the earlierpassive margin of the Iapetus Ocean. Amongst the basalts arerelatively primitive pargasite-bearing varieties which containcognate cumulate blocks, dominantly of pargasite but also withcalcic clinopyroxene, Ti-magnetite, and (rarely) plagioclase.Basaltic rocks also occur in an associated feeder-sheet intrusioncomplex, and as numerous minor sills and dykes. In the intrusioncomplex, basaltic sheets are cut by microdiorites and scarcemicrotonalites. The compositional range in the volcanic complex,from low-SiO2 basalts to microtonalites (SiO2 45–66 wt.per cent), is attributable to fractional crystallization, earlystages of which were dominated by removal of pargasite at (water-undersaturated)pressures close to 10 kb, within the mantle. The parental magmawas derived by hydrous partial melting of a supra-subductionzone mantle wedge. Trace-element patterns indicate that themantle was slightly depleted relative to the putative primordialcomposition (Ta/Yb = 0?1), prior to metasomatism by componentsfrom the subduction zone. Textural variations in cumulate blocks,and various phenocryst forms in basalts, are interpreted asindicating that the erupted magmas came from a thermally andcompositionally stratified magma chamber with associated layeredcrystal accumulations, and that materials from initially separatelayers were mixed prior to eruption. |
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