The structure and petrology of the eastern part of the central Igneous Complex of Arran

Ashford, John H (1959) The structure and petrology of the eastern part of the central Igneous Complex of Arran. PhD thesis, University of Glasgow.

Abstract

The central Igneous Complex of Arran is one of the Tertiary volcanic centres of the West coast of Scotland. The thesis describes the structure and petrology of the eastern part of the Complex, and includes a detailed account of the area of Glen Dubh. Three main divisions of the Complex are recognised; the volcanic caldera of Ard Bheinn, in the West; the granite and agglomerate masses of the eastern part on A'Chruach and the Tir Dubh; and the instructions of diorite, dolerite and gabbro on the eastern margin on the Complex in Glen Dubh and Glen Ormidale. The complex is emplaced in rocks of Old Red Sandstone, Carboniferous and Permian age and chalk occur. The country rocks of the Glen Dubh area are Old Red Sandstone sediments, bleached and partly recrystallised by thermal metamorphism. A sinous fault divides the Glen Dubh area, and forms, to the North, the boundary of the Complex. In Glen Dubh, the inner area is mainly of dolerite forming an irregular margin to the gabbro. A dyke of felsite occurs along the fault for a short distance and to the West in the Innner Area, there is a thick sill-like body of felsite breccia. Veins of breccia extend into the over-overlying dolerite. In two places the brecciation of the dolerite above the felsite is so intense as to form small vents. In the Outer area Glen Dubh, there is a layered sequence. Sediments of Old Red Sandstone age were intruded by a concordant gabbro and dolerite body (the base is not exposed) and subsequently, before cooling of the basic rocks, a thin sheet of microgranite was intruded along the contact. Reaction between the microgranite and basic rocks resulted in the formation of a variable suite of diorites- finegrained diorites formed as a result of the alternation of solid gabbro or dolerite, mainly by diffusion of granite material, and diorites formed by contamination of the microgranite, which intrude the other rocks in places. Alternation of the gabbros is by the introduction of granitic material. No occurrence of assimilation of sedimentary material has been found, although small-scale assimilation of quartzite fragments in fine hornblende diorite occurs in one locality.The igneous history determined for the Glen Dubh area is correlated with that of the rest of the Central Complex thus. The isolated areas of diorite around the northern edge of the Complex are equivalent to the diorites of Glen Dubh. The rocks of Glen Dubh area are interpreted as being relics of the early stages of the igneous period which culminated in the formation of the vents and caldera of Ard Bheinn. The equivalent rocks in the Ard Bheinn areas have been removed durin the volcanic episode, or are present only as small isolated remnants.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Geology, Petrology
• Date of Award: 1959
• Depositing User: Enlighten Team
• Unique ID: glathesis:1959-72700
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 05 Jun 2019 14:01
• Last Modified: 05 Jun 2019 14:26
• URI: https://theses.gla.ac.uk/id/eprint/72700