The Petrogenesis Of The Penmaenmawr Intrusion, North Wales

Durham, John (2004). The Petrogenesis Of The Penmaenmawr Intrusion, North Wales. MPhil thesis The Open University.



The Penmaenmawr Intrusion forms a prominent hill on the north Wales coast and consists of a very fine- to medium-grained microdiorite. Previous studies have considered the Penmaenmawr Intrusion to be related to the extrusive rocks of the Llewelyn Volcanic Group, that were erupting in early Caradoc times in a back arc basin associated with subduction due to the closure of the lapetus Ocean to the northwest.

The objective of this study was to examine the mineralogical and geochemical variations within the intrusion in order to investigate its petrogenesis. Seventy samples were collected for petrological examination and geochemical analysis and the results were used to model possible magmatic processes.

This study proposes that the Penmaenmawr Intrusion was the result of two emplacements of magma from an underlying, already fractionated and layered, magma chamber. Initial injections of magma took place along the southern boundary and in the east of the present intrusion resulting in the very finegrained rocks at the margins of the intrusion. The second, larger emplacement, with a slightly more basic composition forms the main body of the intrusion. Local mixing between the two batches of magma took place along their common boundary blurring their junctions and some localised mixing also took place during the transport and emplacement of the second batch of magma, which served to obscure the evidence for the original compositional layering of the underlying magma chamber.

Following emplacement, the Penmaenmawr Intrusion was variably altered forming a range of secondary minerals. This study proposes that the alteration is mainly due to the ingress of hydrothermal fluids along faults and shear planes during two alteration episodes. The first associated with emplacement where hot fluids, originating from the cooling magma, caused changes to the original minerals and the second associated with sub-greenschist facies metamorphism during the Acadian regional metamorphic event of early to mid Devonian times.

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