Thermal aspects of the origin of Hebridean Tertiary acid magmas. II. Experimental melting behaviour of the granites at 1 kbar PH20

Abstract

AbstractExperimental melting equilibria at 1 kbar PH2O are reported for three early Tertiary granites from Skye and one from Rhum, together with a quartz monzonite from Ardnamurchan. The Hebridean Province leucogranites show cotectic melting behaviour, whilst associated ‘primitive’ (mostly adamellitic) acid plutons have silicate liquidi ∼ 925 °C and plagioclase liquidi ∼ 875 °C. At such high temperatures the morefusible crustal rock types of the region would be almost completely molten. If the Tertiary ‘primitive’ acid magmas were simple crustal melts, they would inherit the incompatible element ratios of their sialic progenitors. This is demonstrably not so. Minor element ratios in these plutons are consistent with extensive fractional crystallization during the evolution of their magmas. But it is, in most individual instances, extremely difficult to specify, either by thermal or geochemical arguments, whether (1) fractional crystallization followed sialic contamination of basic magmas or (2) mixing occurred between fractionation residua and sialic melts. Both the minor element ratios and cotectic melting behaviour of the Hebridean Province leucogranites are consistent with the view that they are products of extreme fractional crystallization of (sial contaminated) more-basic magmas. Nevertheless, it is clear—when the sequence and timing of emplacement of acid magmas within individual intrusive centres is considered—that very complex polycyclic combinations of fusion, fractionation, and re-fusion were involved in the evolution of these granites.The solidi of the samples studied experimentally, together with published data, show a progressive rise in temperature as the compositions of the rocks diverge from the minimum melting composition in NaAlSi3O8-KAlSi3O8-SiO2 at 1 kbarH2O. This relationship is important when considering the likely nature of sialic contamination of basic to intermediate continental magmatic suites fed through reservoir systems which are swarms of dykes and, or, sills. Because salic low-solidus crustal rock types also show near-cotectic melting behaviour, the composition of the contaminant may be strongly biased towards these (usually minor) crustal units, rather than the ‘bulk’ or average crustal composition.