Crustal contamination and the granite problem in the British Tertiary Volcanic Province

Abstract Isotopic studies on the British Tertiary Volcanic Province have shown that the granites contain both mantle-derived and crustal components, while basic rocks display pronounced crustal contamination. In order to study contamination mechanisms in greater detail, isotopic and chemical variations in Skye plateau lavas and Cuillins Complex intrusive units were examined as a function of time. In both cases degrees of crustal contamination generally fall with time, which, it is argued, reflects the progressive melting and ‘sweating out' of Lewisian acid gneissic sheets from the more refractory intermediate basement during repeated intrusion and contamination of sheet-like magma reservoirs. Long lived developing magma chambers do not fit the data well. Skye granites had basic precursors that suffered similar types of contamination in the lower crust, but tended to form longer lived developing magma chambers in the upper crust. Nevertheless, Skye granites display similar crustal Pb contents to associated basic intrusions, and are argued not to be the products of crustal anatexis, but the differentiates of Preshal Mhor basic magmas contaminated by ca. 10% with large degree partial melts of Lewisian acid gneisses.