The Geochemistry of Granitoid Intrusions in Llyn, North Wales.

Jones, Gladys May (1989). The Geochemistry of Granitoid Intrusions in Llyn, North Wales. MPhil thesis The Open University.



The Llyn area of North Wales has late Precambrian and Lower Pa1aeozoic sedimentary and volcanic formations into which are emplaced about twenty intrusions. These vary in composition from microtonalite to microgranite and are c1osely associated with Caradocian volcanic rocks which include andesitic and basaltic lavas.
Major and trace element analyses show that the intrusions can be divided in to five groups; a peralkaline group with high Zr/Nb ratios and four intermediate groups. Petrogenetic modelling using the high field strength elements Mb and Zr suggests that the intrusions could resuIt from 50% fractional crystallisation of plagioclase and/or clino/orthopyroxene and/or hornblende from basalt magmas such as LL142 or LL143. This is consistent with the lavas and the intrusions both being of Ordovician age.
The most likely setting for the lavas and the intrusions is a complex marginal basin environment dominated by E—W extension.

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