The geochemistry and petrogenesis of the Lower Paleozoic granitoids of North Wales
The Lower Paleozoic rocks of North Wales are penetrated by numerous bodies of dolerite and intermediate-acid material which are closely associated with pillow basalts, acid and basic tuffs and ignimbrites. Igneous activity occurred throughout the Ordovician although in Snowdonia and Lleyn it reached a climax during the Caradoc stage. The present study has considered the granitoids of Snowdonia and Lleyn which are intruded as stocks, laccoliths and high level plugs (<5 km2) into clastic and volcaniclastic sediments. A substantial body of new chemical data has been obtained for over thirty intrusions. Twenty-four major and trace elements were determined on each sample by X-ray fluorescence techniques and the rare earth abundances of forty-five selected samples were measured using a radiochemical neutron activation procedure. Three distinctly different types of granitic material are recognised and their possible evolutionary mechanisms are discussed in detail. On the basis of the geochemistries of the various sampled intrusions it is concluded that they were all ultimately derived from the mantle by partial melting. One group originated from the remobilisation of calc-alkaline plutons formed by hydrous melting of the mantle wedge above a destructive plate margin. The other two groups, it is argued, evolved from tholeiitic basaltic magma by low-pressure fractional crystallisation. The goochemical affinity between some Caradocian andesitic material from Lleyn and the tonalitic-granitic intrusions of the same area has provided good evidence against a view that the latter were emplaced late in the Caledonian cycle (Silurian-Devonian). A paleo-tectonic reconstruction of the North Wales region is presented and it is claimed that the area evolved as a volcano-tectonic rift zone, a precursor of a back-arc basin.