Petrological and Geochemical Variations within the Tal y Fan Intrusion: a Study of Element Mobility During Low-Grade Metamorphism with Implications for Petrotectonic Modelling
The Tal y Fan Intrusion is an altered olivine dolerite sheet emplaced into a coeval sequence of subaqueous volcanic rocks of Caradoc (Ordovician) age in NE Snowdonia, Wales. Primary mineral and chemical variations across the 110 m thick sheet suggest that the magma was drawn from a zoned magma chamber, although the intrusion consolidated predominantly as a single cooling unit. An horizon of ferrodolerite resulted from in situ fractionation. Secondary mineral assemblages are indicative of the prehnite-pumpellyite and prehnite-actinolite fades, suggesting metamorphic alteration conditions of approximately 310°C and 1-85 kb. Major elemental variation largely reflects primary mineral variations across the intrusion, although Ca, Al, and Na show limited mobility in the outermost 4-5 m, related to breakdown of plagioclase feldspar during metamorphism. The LIL elements Rb, Sr, K, and Ba were highly mobile, particularly in the marginal zones, whereas Th, in addition to the incompatible elements Zr, Y, Ti, P, Nb, Ta, Hf, and the REE, was immobile even in the marginal zones. Accordingly petrotectonic modelling based on discriminant diagrams using these immobile elements is considered most reliable. The Tal y Fan Intrusion has characteristics transitional between N-type and E-type MORB, similar to tholeiitic within plate basalts. In contrast with other Ordovician volcanic sequences of the Welsh Basin, no subduction component is identified in the Tal y Fan magma, the LIL element enrichment observed being related to alteration