Thermal aspects of the origin of Hebridean Tertiary acid magmas. II. Experimental melting behaviour of the granites at 1 kbar P_H20

Abstract

ABSTRACT. Experimental melting equilibria at 1 kbar PH2O are reported for three early Tertiary granites from Skye and one from Rhum, together with a quartz rnonzonite from Ardnamurchan. The Hebridean Province leucogranites show cotectic melting behaviour, whilst associated 'primitive' (mostly adamellitic) acid plutons have silicate liquidi N 925 ~ and plagiodase liquidi ~ 875 ~ 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 NaA1Si30 sKA1Si3Os SiO2 at 1 kbarH2 o. 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, (~ Copyright the Mineralogical Society

Temperature, Pressure, and Composition of the Mantle Source Region of Late Cenozoic Basalts in Hainan Island, SE Asia: a Consequence of a Young Thermal Mantle Plume close to Subduction Zones?

“…In the primitive mantle-normalized multi-element patterns, except for four drill samples (ZK05-25.4, ZK05-36.5, ZK03-24.4, and ZK03-29.1), all samples show Thompson (1983) and Thompson et al (2001). Arrows point in the direction of decreasing temperature.…”

Elemental and Sr?Nd isotopic systematics of the early Mesozoic volcanic sequence in southern Jiangxi Province, South China: petrogenesis and tectonic implications

“…The results show that 5-30% partial melting followed by $30-50% fractionation might be the most likely process to generate the group 3 rhyolites could then induce the melting of the underplating mafic rocks hybridized with crustal materials to form the group 3 A-type magma at shallower levels. Experimental work has demonstrated that the mineral assemblage of A-type magma can only be produced in high temperature environments (>900°C) with low H 2 O contents (Clements and Vielzeuf 1987;Thompson 1983). This suggests that the group 3 rocks represent highly fractionated products from high-temperature and relatively anhydrous magmas.…”

“…An EMII-type source is generally characterized by 87 Sr/ 86 Sr ratios in excess of 0.7065 and intermediate 143 Nd/ 144 Nd ratios (Hart 1988;Weaver 1991). It has lower Ba/Nb and Ba/La ratios, but higher Th/La and Zr/Nb ratios, than those of an EMI-type mantle source (Hart 1988;Thompson 1983;Palacz and Saunders 1986;Weaver 1991). The group 1 rocks have 87 Sr/ 86 Sr ratio of 0.70602 $ 0.70812, Nd (t) value=À1.63 to +0.11, Nb/La=0.64-0.99 and Zr/Nb=4.43-9.07, displaying an affinity to an EMII-like source (Fig.…”

The Centre 3 layered gabbro intrusion, Ardnamurchan, NW Scotland

“…The geochemical work of Walsh (1975) and Walsh & Henderson (1977) supports this conclusion to some extent, as they suggested that the tonalite was derived from country-rock contamination rather than fractional crystallization of the gabbros. Thompson (1983) carried out melting experiments on a sample of the inner 'quartz monzonite' part of the intrusion (3h on Fig. 1) at 1 kbar H 2 O and showed that its liquidus temperature was in excess of 1020 • C. This led him to conclude that the petrogenesis of the quartz monzonite must have involved a combination of both crustal contamination and fractional crystallization.…”

“…1) has been accorded some significance as the most areally extensive intermediate (in composition) intrusion in the British Palaeogene Igneous Province (e.g. Richey & Thomas, 1930; Thompson, 1983). Contact information with the surrounding quartz gabbro and laminated gabbro is poor, but along its southern margin a steeply outward-dipping (79°SE) intrusive contact toward the laminated gabbro is observed (NM 47523 68048) (fig.…”