Evidence of Fluid Phase Behaviour and Controls in the Intrusive Complex and Its Aureole

The recognition of multiple magmatic events and pre-existing deformation zones in metamorphic rocks as illustrated by CL signatures and numerical modelling: examples from the Ballachulish contact aureole, Scotland

“…Remnant small grains are recorded as Signature 2, which is a result of initial grain boundary migration in the presence of fluid I. An intrusion-derived small amount of fluid I is consistent with the fact that (a) fluid I signature is only seen in the direct vicinity of the intrusion, (b) only small amounts of CL activators were present at the time of initial GBM and (c) surrounding country rocks were relatively anhydrous due to pre-intrusive regional metamorphism (Harte et al 1999). Alternating bands, bright rims, healed fractures and undulating bands (Signatures 3, 4, 6, and 7, respectively) were produced during the infiltration of fluid II.…”

“…Since there is a clear spatial relationship between Signature 5 and proximity to the intrusion, possible sources of this fluid are (a) hydrothermal/meteoritic fluid linked to temperature perturbations close to the intrusion, (b) magmatic, but cooled fluid connected to an additional third igneous event and (c) a late-stage cooled rest fluid associated with the second magma event. Following Harte et al (1999) significant hydrothermal circulation in the aureole is unlikely and a third magmatic event is not documented. Therefore, we suggest that fluid III was derived from late-stage fluid originating from the cooling second magma.…”

“…Rocks are assumed to have been largely dehydrated. Therefore, during contact metamorphism, little or no fluid derived from metamorphic reactions occurring within the sequence is expected (Harte et al 1999). Holness and Watt (2001) followed up the initial microstructural work of Lind (1996), performing a cathodoluminescence study focusing on recrystallisation and fluid flow in the Appin Quartzite within 210 m of the contact, which resulted in the recognition of five generations of quartz.…”

“…Buntebarth (1991) conducted theoretical thermal calculations based on detailed mineralogical studies by Pattison andHarte (1985, 1991), indicating that rocks in the entire contact aureole experienced temperatures above c. 550°C for about 500 ka, while those close to the intrusion were above c. 660°C for about 270 ka with a maximum temperature of 750-800°C at the contact. Stable isotopes suggest that, during magma emplacement, no open crack systems allowing circulation of meteoric water were maintained (Harte et al 1999;Holness and Watt 2001). Rocks are assumed to have been largely dehydrated.…”

“…We defined the bulk rock density (q bulk ) and whether heat transfer was conductive only or included convection via fluid advection. We chose the conductive only option as (a) the country rocks are assumed to have been largely dehydrated due to a pre-intrusion metamorphic event (Harte et al 1999;Holness and Watt 2001) and (b) calculations allowing conduction and convection of a lowpermeability material showed that the effect of convection was negligible. In accordance to the composition of intrusive rocks seen in the field (cf.…”