Partial melting of the Appin Quartzite driven by fracture-controlled H2O infiltration in the aureole of the Ballachulish Igneous Complex, Scottish Highlands |
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| Authors: | Marian B Holness John D Clemens |
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| Institution: | (1) Department of Earth Sciences, University of Cambridge, Downing St, Cambridge, CB2 3EQ, UK, GB;(2) School of Geological Sciences, CEESR, Kingston University, Penrhyn Rd, Kingston-upon-Thames, Surrey, KT1 2EE, UK, GB |
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| Abstract: | The Ballachulish Igneous Complex consists of an outer quartz diorite and an inner granite, emplaced at about 300 MPa, initially
at 1000 to 1050 °C. The contact aureole (0.5–2 km wide) occurs in metapelites and metapsammites plus minor graphitic slates,
carbonate rocks and metaquartzites. A textural examination of the arkosic Appin Quartzite, which was previously believed to
have melted only within a few metres of the intrusion, demonstrates that partial melting occurred up to 500 m away from the
vertical eastern contact. Coupling petrographic observations with Qtz-Ab-Or-H2O phase relations, we determined both the amounts of actual melt and the maximum possible amounts of melt in the samples.
Melting efficiency was everywhere less than 100% and decreased with distance from the intrusion. Though perhaps not the only
possible source of fluid throughout the aureole, simple models demonstrate that H2O evolution from the pluton would have been volumetrically sufficient and persisted long enough to account for the observed
partial melting. A time-integrated fluid flux of 7000 kg/m2 from the pluton is necessary to account for the observed amounts of partial melt in the Appin Quartzite. From its inefficiency,
we infer that infiltration of the Appin Quartzite cannot have occurred along interconnected grain-edge channels. Rather, it
was controlled by hydraulic fracturing, with fracture density determining melting efficiency. Bulk-rock permeability is calculated
to be 10−20 m2, an order of magnitude lower than that necessary to permit pervasive flow of all the fluid exsolving from the pluton. There
is little difference between the calculated time-integrated fluid flux through the Appin Quartzite on the eastern flank and
an estimate of the infiltrating flux through the pelitic Leven Schist on the western flank in the time interval during which
both rock types were above their solidus temperature, despite differences in their equilibrium quartz-H2O dihedral angles at temperatures immediately below the solidus, and differences in the attitude of the contact. The rates
of H2O expulsion from the cooling pluton are consistent with highly efficient fracture-dominated flow, allowing insufficient time
for textural equilibration.
Received: 26 March 1998 / Accepted: 8 March 1999 |
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