Aspects of low-grade metamorphism in northern Snowdonia, Wales

Abstract

The effects of low-grade metamorphism have been examined in the Lower Palaeozoic rocks of northern Snowdonia.
A variety of techniques have been utilised including:- bulk rock geochemistry, petrography and microprobe determinations of secondary phase chemistry in the widespread meta-dolerite intrusions; illite crystallinity determination and white mica bO measurement in the argillaceous meta-sediments; and fluid inclusion analysis in vein quartz.
Meta-dolerite mineral assemblages and phase compositions indicate an increase in grade southwards with dominantly prehnite-pumpellyite facies assemblages in the northern, and lower greenschist facies assemblages in the central and southern, Conwy Valley. Meta-dolerite assemblages are less diagnostic in central northern Snowdonia.
The phyllosilicates reveal a sharp metamorphic break within northern Snowdonia with relatively high illite crystallinity (epizone) and bO in the western Conwy Valley and relatively low illite crystallinity (anchizone) and bO throughout the rest of northern Snowdonia. The boundary between the two areas is sharp and can be correlated, over parts of its length, with major Lower Palaeozoic faults.
Fluid inclusions contain relatively low salinity, C02-poor, liquidvapour, aqueous solutions. Mean filling temperatures range from 157 - 194°C with a slight increase from north to south.
Pre- and syn-deformation metamorphic effects have been recognised in northern Snowdonia. Combination of data from different techniques indicates pre-deformation temperatures and pressures ranging from approximately 290°C and 1.4Kb in central northern Snowdonia to 325 - 345°C and 2Kb in the western Conwy Valley, marking the transition from the prehnite-pumpellyite to lower greenschist facies of low-grade metamorphism. These figures indicate a low pressure facies series of metamorphism. The metamorphic effects of Caledonian deformation are continued recrystallisation in both the meta-dolerites and metasediments and further, minor, quartz veining.