The ascent and emplacement of granitic magma: The northern Arran granite

Abstract

This thesis is a study of the mechanisms by which granitic magmas rise through the crust to be empiaced at a level above their source, with particular reference to diapirisni. and how these mechanisms may be analysed by combined structural and petrological studies. The Northern Arran granite is used as an example of how this problem may be approached. The Northern Arran granite is a two component granite of Tertiary age intruded into structurally heterogeneous upper crustal rocks under regional tension. A synform concentric to the granite, synchronous with the development of a narrow thermal aureole, records the vertical ascent of a single body of magma with a hemispherical upper surface. Post ascent, radial expansion of this body, indicated by flattening strains parallel to its surface and superimposed on the concentric synform records a change in shape of the pluton. This was permitted by the reactivation of an existing fault which the pluton intersected during its ascent. Petrological studies of the outer coarse unit of the northern granite indicate that it is a single body of magma derived by differentiation of a crustally contaminated basaltic source. Theoretical modelling of the crystallisation of the coarse granite shows that textural and chemical variations, are consistent with solidification by sidewall crystallisation (liquid fractionation) but not fractional crystallisation. The inner (younger) fine granite is also a single body of magma derived from the same or a similar source as the coarse granite. The sharp undeformed contacts between the coarse and fine granites and the presence of internal sheets in the fine granite parallel to its contacts with the coarse granite are consistent with emplacement of the fine granite as a series of pulses which filled a propagating ring dyke fracture within the coarse granite. Theoretical modelling of the ascent of the coarse granite using the Hot Stokes equation indicates that bouyancy driven ascent aided by a reduction in wall rock viscosity controlled by the rate of heat loss of from the granite is a viable ascent mechanism. The patterns of strain in the aureole of the Northern Arran granite result from the ascent and emplacement of a single diapiric body. They provide examples of the types of structure which may be used to recognise and distinguish between diapiric ascent and radial expansion. This has important implications for the study of ballooning diapirs. The reactivation of an existing fault system during emplacement suggests that existing crustal structure can influence the final geometry of an intrusive body. It is shown that the complete evolution of the Northern Arran granite can be determined using a combination of structural and petrological data. Structural data provides constraints on the later stages of ascent and the emplacement of granitic plutons. Petrological data can be used to constrain the origin, early stages of ascent and the crystallisation of a magma body.