Processes of granite emplacement: NW Ireland and SE Brazil

Abstract

This thesis is a study of some of the processes which operate at mid crustal levels (10- 20km) during the emplacement of granitoid magmas, with particular reference to the process of in-situ expansion and the association between magma emplacement and orogen evolution. A number of plutons were studied to assess this the Ardara pluton from Donegal, NW Ireland, together with the Atibaia, Morro Azul, Imbiricu and Itapeti plutons from the Rio Paraiba do Sul shear belt (RPSSB), Sao Paulo state, SE Brazil. The emplacement of the Ardara pluton has been the subject of studies by many authors since the 1950's. It has been interpreted as a diapir ascending along a thrust, a granite balloon which forcefully created more than 60% of its space and, most recently, as a set of nested diapirs which forcefully created only 30% of their own space. Field mapping together with the determination of finite strain within the pluton, shortening estimates, shear sense determinations, petrographic deformation fabrics and computer modelling indicates that: i) the pluton displays a concentrically increasing finite strain and it expanded in-situ from a central 'injection point', having possibly ascended along a shear zone-related conduit; ii) the country rocks partitioned intense strains into the 500m closest to the pluton, a deformation feature which is shown to be consistent with a power-law wallrock theology; Hi) earlier granite pulses and the country rock were 'shouldered aside' to the east and west, expansion of the pluton, which was preferentially in a northwards direction; iv) that the intrusion related shortening preserved within the wallrocks was approximately equivalent to the strain preserved within pluton and the space requirements for the pluton are essentially met by preserved forceful emplacement-related strains; and v) if the pluton magma is considered as having ascended through dyke-like conduits emplacement could occur in a minimum of ~4000yrs, whereas magma ascent as a Hot Stokes diapir would require hundreds of thousands to millions of years for complete emplacement. Finally, the Ardara pluton does not conform to any of the established criteria for diapir-like ascent of a magma body and the data demonstrate it to be a testable example of a pluton which expanded entirely in-situ. The Rio Paraiba do Sul shear belt (RPSSB) is a dextral transpressional segment of the Late Precambrian Brasiliano-Pan African orogenic belt. The studied plutons were emplaced in the latter stages of this orogen in close spatial association with one of the many northeast-southwest trending, sub-vertical, continental-scale shear zones. All the granites show a similar emplacement and deformation history despite their different petrographic features and isotopic ages. This history consists of an early shallowly dipping, low angle fabric, in the country rocks, associated with crustal thickening, the later development of dominantly dextral sub-vertical shear zones, followed by the emplacement of the studied plutons and finally overprinting by a pervasive dextral plane strain and late-stage discrete mylonitic shears. Field examination demonstrates that: i) each of the plutons preserves an internally homogeneous emplacement-related finite strain, weak magmatic fabric, magma sheets and weakly deformed wallrocks; ii) magmatic shear sense determinations and wallrock deformation fabrics indicate that, during emplacement, the RPSSB was extending (at least at the emplacement level) in an east-west direction creating sinistral dilatational pull-aparts along the major shear zones, into which granitic magma was preferentially emplaced; and Hi) within this generally extensional context there was a component of long axis perpendicular in-situ forceful expansion, this created no more than 20% of the width of any one granite. Applying a simple pull-apart extension model suggests that pluton emplacement was associated with approximately 40% regional extension sub-parallel to the former orogenic convergence direction. This emplacement, as a result of sheeting through dyke-like conduits, requires thousands of years, rather than the millions of years required for ascent and emplacement of a Hot Stokes diapir. Interpreting these results suggests that the intruded granitoids are associated with a mid-crustal component of orogen perpendicular extension I collapse during the latter stages of the Brasiliano orogeny. Such extension I collapse could have been initiated by: a cessation in continental convergence or; by the delamination of a thickened thermal boundary layer, a process which has been suggested to be the driving force behind orogenic collapse in younger orogenic belts. These examples demonstrate that: i) 'forceful' balloon-like emplacement of granitic magma can occur; ii) there are very close inter-relationships between granite emplacement and orogenic dynamics; and Hi) that 'space' for granitoid magma can be created by a combination of forceful and dilatational and fault-related mechanisms.