Lower Palaeozoic convergent plate margin volcanism on Bømlo, southwest Norwegian Caledonides: geochemistry and petrogenesis

• Published in: Geological magazine 1986-03, Vol.123 (2), p.123-142
• Main Authors: Furnes, Harald, Brekke, Harald, Nordås, Jan, Hertogen, Jan
• Format: Article
• Language: English
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Soil and rock geochemistry
• ISSN: 0016-7568; 1469-5081
• DOI: 10.1017/S0016756800029782

Major and trace element analyses of a Lower Palaeozoic metavolcanic sequence of convergent plate type from Bømlo, southwest Norwegian Caledonides, are presented and discussed. This sequence ranges in age from the Upper Cambrian through the Lower Silurian. Petrogenetic models for the lavas in terms of partial melting and crystal fractionation are discussed. Two models are presented for the metabasalts in order to explain their different trace element abundances and ratios: (1) REE modelling, assuming a mantle source with REE abundances twice chondritic, suggests progressively more varied sources with time. Thus the metabasalts from the oldest (Upper Cambrian–Lower Ordovician) Geitung Unit of primitive island arc type, and those of the mid-Ordovician Siggjo Complex of ‘Basin and Range’ type can be modelled in terms of high (around 25%) and moderate (around 5%) degrees of partial melting of spinel lherzolite, respectively. The metabasalts of the post-Ashgillian Vikafjord Group of typical continental flood basalts are compatible with moderate (c. 5–10%) degrees of partial melting of spinel- and garnet-lherzolite sources. The supposed Lower Silurian Langevåg Group of calc-alkaline ‘Andean’ type metabasalts, grading into alkaline to tholeiitic metabasalts of early marginal basin (youngest) character, require low (