Relationships between dyke intrusion, quartz veining and fluids in the Easdale phyllite, SW Scotland.

Abstract

The Easdale phyllite at Easdale, Scotland comprises; pyritic dark phyllite (Qtz + ms + chl + ab dol) and calc-phyllite, (Qtz + ms + chl + dol ab). It was deformed by the regional polyphase Caledonian D1-D3 ductile deformation and a weaker D4 phase, that comprised essentially minor crenulation and normal faulting. The S2 foliation dominates, and was accompanied by chlorite-zone, green schist facies metamorphism and formation of Caledonian quartz veins. Intrusion of Tertiary mafic dykes, has given rise to modifications of Caledonian vein quartz microstructure and the characteristics of trapped fluid inclusions. Three groups of veins: (1) Caledonian quartz + chlorite + muscovite veins (VI), and (2) quartz (chlorite + muscovite) veins (V2), mostly concordant with regional Caledonian S2 foliation and cut by mafic dykes, and (3) post-Caledonian dolomite + calcite quartz veins parallel to sub-parallel to the dykes, occur. By petrographic analysis in a series of traverses (0-150cm) away from dykes of varying width (up to 5m) the veins are shown to contain medium (0.5-2mm) to coarse (>2mm) recrystallized equigranular quartz that becomes more equant nearer to the dykes. Close to dykes the quartz shows a bimodal grain-size distribution and linear fluid inclusion arrays that define healed micro-cracks. Two fluid inclusion populations (Types A & B) are defined based on their nature and occurrence. These are: Type A; isolated inclusions (Al) in clean quartz cores, and inclusion clusters (A2) along grain boundaries representing the initial fluid inclusion populations in the veins. (2) Type B; mostly short, straight intracrystalline trails that represent the secondary fluid inclusion populations. Both populations are aqueous fluids of very low salinity (1.65-5wt% NaCl equivalent). They are dominantly 5-10m in size and inclusions ^O!!! are uncommon. The Type A inclusions re-equilibrated at approximately 250C (219-298C) and Type B at 150C (116-200C) under 0.19-2.17kb hydrostatic pressure and have experienced systematic post-entrapment modifications with increasing proximity to dykes. Intrusion-related static recrystallization occurs and the zone affected varies systematically with dyke thickness. The mean distance of vein overheating to cause fluid inclusion modification is about 5cm from the vein-dyke contacts for thin dykes (<lm thick) and 20-30 cm near thick dykes (>lm thick). Of Type A inclusions >12m diameter, 80-100% decrepitated between 0-30cm in veins intercepted by <lm wide dykes, because overheating was high (>100C), cause overpressuring >1.0kband induce decrepitation of inclusions in the zone. For veins intruded by 2m and 5m thick dykes the width of the zone is 0-70cm and 0-160cm respectively. This zone represents the maximum width of the veins significantly affected by heat-induced recrystallization and modification of fluid inclusions. It is established that fluid inclusion leakage is systematic and quantifiable in response to heat-induced overpressure due to dyke intrusion. Large- (>12m) and medium-size (5-12m) inclusions leaked and deformed from irregular and ellipsoidal to polygonal forms close to the vein-dyke contact. The leakage produces a decrease in density and degree of fill of early-formed fluid inclusions. This produces a spread of homogenization temperature (Th) values to higher T, such that careful interpretation is needed to avoid serious underestimate of trapping pressures (Pt). This has important implication for the interpretation of metamorphic and mineralisation events.

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