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

The Glen Tilt diorite forms a significant component of a late Caledonian, c alc-alkaline, predominantly granitic intrusive complex on the north-west si de of the Loch Tay Fault. It intrudes Grampian Group and lowermost Appin Gr oup Dalradian metasedimentary rocks. The diorite is massive and contains co mmon to abundant hornblende, variable amounts of biotite and clinopyroxene and late-stage intergranular quartz and K-feldspar. Opaque oxide and titani te are minor components. Associated granitic rocks comprise biotite granodi orite/granite and muscovite-bearing leucogranite. Dominantly north-easterly trending porphyritic microdiorite dykes are common in the area and cut ele ments 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 clinopyr oxene exhibits a reaction relationship with later crystallizing hornblende. Pressure estimates are consistent with phase assemblages developed in cont act metamorphosed pelitic rocks. Bulk rock and mineralogical chemical compa rison suggest the porphyritic microdiorite minor intrusions are co-magmatic with the main diorite. Field relations and petrographical observations indicate that biotite grano diorite/granite is emplaced early in the intrusive sequence and that locali zed melt remobilization and back-veining accompanied diorite emplacement. L eucogranite probably occurs later in the intrusive sequence although the co mplex as a whole was probably emplaced in a short time span. The chemical c ompositions of the main plutonic elements suggest they are not directly rel ated by differentiation of a common magma but more likely represent contrib utions from differing mantle and crustal sources. The complex as a whole pr obably represented the site of a magma chamber that fed coeval Siluro-Devon ian volcanism, the ascent of magma probably being focused along the Loch Ta y Fault and an intersecting NW trending structure.