New geochemical data for the Carrock, Threlkeld, Ennerdale, Shap, Skiddaw and Eskdale granites of the Lake District are presented and discussed with particular reference to the metasomatism and petrogenesis of the intrusions.
The Caledonian granites of the Lake District have more associated hydrothermal activity and mineralization than their equivalents north of the Iapetus suture in Scotland. Pervasive high-temperature metasomatism which affects the Shap, Skiddaw and Eskdale intrusions is accompanied by remobilization of the large ionic lithophile elements (K, Rb, Sr, U) and Li and B, although high field strength elements, including the rare earth elements, remain unaffected except near greisen and mineral veins. The Threlkeld intrusion appears to have suffered loss of a Ba and Sr during a low-temperature event. However, there is no consistent relationship between granite composition, hydrothermal activity and mineralization which could be used to support the granite cupola model for mineralization in the province. Rather, the episodic mineralization which affects the Lake District may be related more to the broad geothermal field consequent upon the emplacement of the deeper Lake District batholith, identified from geophysical data.
Despite the variable metasomatic perturbations of their primary chemistry, three distinct groups of intrusions can be recognized on geochemical grounds. The Carrock granophyre is a product of tholeiitic fractionation and the Threlkeld and Ennerdale bodies are more typical calc-alkaline intrusions, whereas the Eskdale, Shap and Skiddaw plutons show a progressive evolution towards more complex geochemical patterns, suggesting decreasing hornblende but increasing plagioclase control of fractionating processes. The culmination of this trend may have been the emplacement of a large, high heat production granite beneath the Lake District at the end of the Caledonian orogeny. The suggestion made on geophysical grounds that the Eskdale intrusion is a cupola of the Lake District batholith cannot be simply reconciled with geochemical modelling. None of the Lake District granites has geochemical or isotopic characteristics which are convincingly S-type (i.e. formed by partial melting of a sedimentary protolith), and a model of subcrustal magmagenesis beneath an evolving arc or continental margin appears to be more appropriate.