Fluid flow and Al transport during quartz-kyanite vein formation, Unst, Shetland Islands, Scotland

Abstract

Quartz-kyanite veins, adjacent alteration selvages and surrounding 'precursor' wall rocks in the Dalradian Saxa Vord Pelite of Unst in the Shetland Islands (Scotland) were investigated to constrain the geochemical alteration and mobility of Al associated with channelized metamorphic fluid infiltration during the Caledonian Orogeny. Thirty-eight samples of veins, selvages and precursors were collected, examined using the petrographic microscope and electron microprobe, and geochemically analysed. With increasing grade, typical precursor mineral assemblages include, but are not limited to, chlorite+chloritoid, chlorite+chloritoid+kyanite, chlorite+chloritoid+staurolite and garnet+staurolite+kyanite+chloritoid. These assemblages coexist with quartz, white mica (muscovite, paragonite, margarite), and Fe-Ti oxides. The mineral assemblage of the selvages does not change noticeably with metamorphic grade, and consists of chloritoid, kyanite, chlorite, quartz, white mica and Fe-Ti oxides. Pseudosections for selvage and precursor bulk compositions indicate that the observed mineral assemblages were stable at regional metamorphic conditions of 550–600 °C and 0.8–1.1 GPa. A mass balance analysis was performed to assess the nature and magnitude of geochemical alteration that produced the selvages adjacent to the veins. On average, selvages lost about −26% mass relative to precursors. Mass losses of Na, K, Ca, Rb, Sr, Cs, Ba and volatiles were −30 to −60% and resulted from the destruction of white mica. Si was depleted from most selvages and transported locally to adjacent veins; average selvage Si losses were about −50%. Y and rare earth elements were added due to the growth of monazite in cracks cutting apatite. The mass balance analysis also suggests some addition of Ti occurred, consistent with the presence of rutile and hematite-ilmenite solid solutions in veins. No major losses of Al from selvages were observed, but Al was added in some cases. Consequently, the Al needed to precipitate vein kyanite was not derived locally from the selvages. Veins more than an order of magnitude thicker than those typically observed in the field would be necessary to accommodate the Na and K lost from the selvages during alteration. Therefore, regional transport of Na and K out of the local rock system is inferred. In addition, to account for the observed abundances of kyanite in the veins, large fluid-rock ratios (102–103 m3fluid m−3rock) and time-integrated fluid fluxes in excess of ∼104 m3fluid m−2rock are required owing to the small concentrations of Al in aqueous fluids. It is concluded that the quartz-kyanite veins and their selvages were produced by regional-scale advective mass transfer by means of focused fluid flow along a thrust fault zone. The results of this study provide field evidence for considerable Al mass transport at greenschist to amphibolite facies metamorphic conditions, possibly as a result of elevated concentrations of Al in metamorphic fluids due to alkali-Al silicate complexing at high pressures.

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