Thermal aspects of the origin of Hebridean Tertiary acid magmas. I. An experimental study of partial fusion of Lewisian gneisses and Torridonian sediments

Abstract

ABSTRACT. Thermal aspects of the conflicting hypotheses that the acid magmas of the British Tertiary Igneous Province were generated by either fractional crystallization of basic magmas or partial fusion of country rocks around basic intrusions are investigated by combining petrological and experimental data. Sparse large xenoliths (up to 12 m 2 outcrops) of Lewisian gneiss occur in the ferrodiorite member of the Marscoite Suite in the Western Redhills Eocene intrusive complex of Skye. Most of the blocks are of granodioritic-tonalitic gneiss which is very similar to the grey gneisses of the northern and southern regions of the mainland Lewisian. One is a two-pyroxene basic granulite, closely resembling in petrography and composition the granulite-facies basic gneisses of the central region of the Scottish mainland Lewisian. During Tertiary thermal metamorphism the xenoliths recrystallized patchily to pyroxene hornfelses and up to 509/oo of melting occurred in any parts of blocks that were rich in alkali feldspar and quartz. Microprobe analyses of intergrown Ca-rich and Ca-poor pyroxenes in a hornfeised ultramafic mica pyroxenite lens within a leucogneiss block give a calculated temperature of 950 ~ for the Tertiary metamorphism. Melting experiments at 1 kb PH2o were carried out on leucocratic and mesocratic Lewisian gneisses (from the xenoliths), Torridonian arkose and shale. Extensive melting of the leucogneiss and arkose, to peraluminous liquids, occurs within a few degrees above their solidus (715 ~ The silicate liquidus of the leucogneiss is at 930 ~ Microprobe analyses of the glass products of melting experiments (mostly of leucogneiss) show that liquid composition trends during progressive melting are consistent with theoretical equilibria in the synthetic system Ab-Or An-Qz-H20. Partial fusion in the Skye Lewisian gneiss xenoliths took place at a water vapour pressure of a few hundred bars, probably under water-undersaturated conditions.

Field evidence for turbulence during flow of basalt magma through conduits from southwest Mull

“…Thus lithologies with substantial normative fractions of alkali feldspar and quartz, such as granite, pegmatite, aplite, arkose and Ca-poor pelite, might readily melt adjacent to a turbulent basic magma, whilst adjacent units of more refractory rock types would remain subsolidus and only dissolve by relatively slow chemical reaction. The above features are observed in the outcrops illustrated by Figures 2 and 3, which show evidence for heating of the schist to its solidus, which is slightly above the minimum melting temperature (at equivalent P HlO ) in the synthetic system NaAlSi 3 O 8 -KAlSi 3 O 8 -SiO 2 (Thompson, 1981). The chilled margins of the sheet at these outcrops are also very poorly developed.…”

“…This implies that the margin would have reached a temperature of about 650 °C, if it is assumed that the basaltic magma is unlikely to have been significantly superheated. Pelites begin to melt at the same or very slightly higher temperatures than the minimum in the synthetic system NaAlSi 3 O 8 -KAlSi 3 O 8 -SiO 2 ( Thompson, 1981), which is approximately 950 °C at 1 atm. This temperature would be reduced to 800 °C in the unlikely event of the wall rock being water-saturated.…”

Early mafic magmatism and crustal anatexis on the Isle of Rum: evidence from the Am Màm intrusion breccia

“…1), were much lower than that needed to begin melting (cf. Thompson, 1981). The lack of correspondence of metamorphic grade in the gneiss block with the isograds in the main aureole suggests that this fault-bounded block of Lewisian gneiss was metamorphosed during an early high-temperature event.…”