A white mica crystallinity study of the Berwyn Hills, North Wales |
| |
| Authors: | M A AWAN N H WOODCOCK |
| |
| Institution: | Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK |
| |
| Abstract: | Abstract White mica crystallinity studies have been carried out on 90 samples of mudrocks, six of spotted slate, and five of accretionary lapilli tuff from the area around the Berwyn Hills, North Wales. Strain was measured for some of the spotted slate and tuff samples. The metamorphic grade increases from southeast to northwest, with values of the Kübler index varying from 0.64 to 0.20Δ2θ. Metamorphic zonal boundaries follow the strike of bedding and cleavage, but crystallinity values increase into stratigraphically younger rocks on the northwest side of the Berwyn Dome. This effect is attributed mainly to a rapid increase in the thickness of synmetamorphic overburden to the northwest, comprising exposed Silurian turbidites and inferred Lower Devonian non-marine sediments. Strain variations have a more local influence on crystallinity, and lateral variations in the contemporary geothermal gradient cannot be ruled out. However, only with unrealistically high gradients would the need for a thick Lower Devonian component to the overburden be removed. This reasoning implies that the metamorphic peak was coeval with the Acadian (late Caledonian) event, rather than with an early diastathermal event. |
| |
| Keywords: | Berwyn Dome Caledonian metamorphism Welsh Basin white mica crystallinity |
|
|
