Multiple sheeting as a mechanism of pluton construction: the main Donegal granite, NW. Ireland

Abstract

This study is a detailed investigation concerning the construction of granite plutons by the incremental emplacement of granitic sheets. The modem consensus is that sheeted plutons are often controlled by tectonic structures such shear zones. The Main Donegal Granite (MDG), NW Ireland forms the basis to this study. This pluton is the largest presently exposed member of the Caledonian Donegal Batholith (~405 Ma). Field evidence from this highly deformed pluton, attest to emplacement along the long-axis of a sinistral transcurrent shear zone. The presence of long and persistent xenolith "trains" within the pluton has been taken as evidence of an overall sheeted structure; however detailed maps have not been available to test this hypothesis. Two earlier members of the Donegal Batholith, the Ardara and Thorr plutons, whilst having their main outcrops outside the MDG, also occur as xenoliths within the main body. It can be demonstrated in a number of critical situations that these xenoliths are commonly more deformed than the host MDG facies. Furthermore the presence of original country rock contacts implies these xenoliths were originally in situ. These features imply that the shear zone was active prior to the emplacement of the MDG, with it controlling the emplacement of substantial parts of these earlier plutons. Further evidence from the study of parts of the petrographically similar and younger Trawenagh Bay Granite implies the sinistral shear zone was still operational after the majority of the MDG had crystallised. New, detailed (scale 1:250) and reconnaissance mapping of the MDG, reveals its hitherto unrecognised heterogeneity. At least seven major plutonic zones or packages have been identified. All these units have an NE -SW elongate form parallel to the long axis of the pluton and are often, but not always, separated by extensive "raft-trains" of country rock and older plutons. The major packages in the central regions of the pluton are often complex and are composed of three main granitoid phases, ranging in composition form early granodiorites and tonalites to latest porphyritic and to lesser extent equigranular, monzogranites. The early granodiorite and tonalite sheets are now only preserved as xenolithic rafts within the later monzogranites. The broad range in composition/chemistry together allied with field observations implies a complex intrusion history, with these granitoid packages representing sites of long-standing intrusion within the pluton. In contrast, towards the more marginal areas of the pluton there are large units of monzogranite which are characterised by general homogeneity, but in reality are believed to consist of relatively small compostionally similar sheets. On all scales, either meta-sediments, older plutonic material, or early MDG facies are found to lie along the boundaries of younger intrusive units. This implies the pluton is primarily sheeted in character and that the "raft-trains" are partially disrupted, in situ roof material which has been wedged apart during the intrusion of the sheets. The appearance of sheets within the field is dependent on the rheology of the material into which the granitic material was intruded into, i.e. to what extent has the host was crystallised. The degree of crystallisation in the host is related to how fast later sheets were being intruded, i.e. the rate of emplacement. The field relationships, in the central regions of the pluton, between the granodiorites tonalites and the later monzogranites, are interpreted as representing zones of episodic-to slow emplacement, where earlier phases had become essentially competent by the time later units were intruded (i.e. capable of fracture). These earlier phases may be preserved as angular rafts within later sheets. At moderate emplacement rates earlier sheets may still be crystallising but sufficiently viscous to prevent mixing, except at their immediate boundaries with transitional contacts developing. The more homogeneous zones are believed to be related to rapid emplacement with original contacts between pulses being destroyed at the level of emplacement due to homogenisation of pulses which had similar viscosities and hence allowed mixing. The emplacement of granitic melts within active shear zones can lead to the development of a self- perpetuating situation, where melts in a shear zone will enhance deformation rates and cause greater displacements subsequently allowing more melt to enter the shear zone promoting even greater displacement rates. This process is only halted when melts within the source regions are drained; hence the rate of pluton construction and appearance of sheets within plutons is ultimately related to how fast granitic melts are being generated within the source regions.