The geochemistry, metasomatism and petrogenesis of the granites of the English Lake District

Abstract

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 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.

Granitoid plutonism in the Caledonian orogen of Europe

“…The following is a very general summary of the age and composition variations over the segment based on a at University of Manchester Library on April 13, 2015 http://sp.lyellcollection.org/ Downloaded from geochemical database of more than a thousand samples representative of the province and considerable isotopic information mainly from the work of A. N. Halliday and co-workers. An alternative interpretation of the Chappell-White classification applied to the Caledonian granitoids of Britain has been presented by Plant and co-workers (Plant et al 1983, Fowler 1984, Atherton & Plant 1985, O'Brien et al 1985 in which their assignments are often at variance with those presented here.…”

“…The main period of granitoid magmatism was Silurian to early Devonian in which the Eskdale, Ennerdale, Skiddaw and Shap bodies were emplaced. These appear to have high primary 180 values, although strontium isotope initial ratios are not very high (see compilation by O'Brien et al 1985) and U-Pb, common lead and neodymium isotope studies strongly favour a crustal source (see Hampton & Taylor 1983). This suggests a rather young source which has gone through a sedimentary cycle (immature S-type).…”

The geochemistry and significance of sills within the Ordovician Borrowdale Volcanic Group around Black Combe, SW English Lake District

“…In view of the known mobility of Na and K (and Rb, Sr and Ba) in the BVG (Millward et al 1978;O'Brien et al 1985), the total alkalis v. silica diagram (e.g. Le Maitre et al 1989) must be regarded as unreliable for classifying rocks of this history (Section 4).…”

The volcanogenetic significance of garnet-bearing minor intrusions within the Borrowdale Volcanic Group Eskdale area, Cumbria

“…Bi-element plots relating compositions of Airy's Bridge volcanics and garnetiferous intrusions. Threlkeld microgranite from Caunt (1984) and O'Brien et al (1985); Armboth dyke from J. G. Fitton (unpub. Ph.D. thesis, Univ.…”