The Glen Tilt diorite: crystallization, petrogenesis and relation to granitic rocks

Abstract

Synopsis The Glen Tilt diorite forms a significant component of a late Caledonian, calc-alkaline, predominantly granitic intrusive complex on the north-west side of the Loch Tay Fault. It intrudes Grampian Group and lowermost Appin Group Dalradian metasedimentary rocks. The diorite is massive and contains common to abundant hornblende, variable amounts of biotite and clinopyroxene and late-stage intergranular quartz and K-feldspar. Opaque oxide and titanite ar e minor components. Associated granitic rocks comprise biotite granodiorite/granite and muscovite-bearing leucogranite. Dominantly north-easterly trending porphyritic microdiorite dykes are common in the area and cut elements of the granitic plutons. Petrographical, mineralogical and chemical variation in the diorite is the result of crystal fractionation and accumulation processes. Comparison with experimental studies together with thermobarometric estimates suggests the body crystallized at 2–4 kbar and under hydrous conditions; early clinopyroxene exhibits a reaction relationship with later crystallizing hornblende. Pressure estimates are consistent with phase assemblages developed in contact metamorphosed pelitic rocks. Bulk rock and mineralogical chemical comparison suggest the porphyritic microdiorite minor intrusions are co-magmatic with the main diorite. Field relations and petrographical observations indicate that biotite granodiorite/granite is emplaced early in the intrusive sequence and that localized melt remobilization and back-veining accompanied diorite emplacement. Leucogranite probably occurs later in the intrusive sequence although the complex as a whole was probably emplaced in a short time span. The chemical compositions of the main plutonic elements suggest they are not directly related by differentiation of a common magma but more likely represent contribution s from differing mantle and crustal sources. The complex as a whole probably represented the site of a magma chamber that fed coeval Siluro–Devonian volcanism, the ascent of magma probably being focused along the Loch Tay Fault and an intersecting NW trending structure.