The origin and importance of A12Si0s - bearing quartz veins in metamorphic rocks with particular reference to the Amphibolite Facies

Metamorphosed pelitic rocks from the Eastern Dalradian (Scotland) contain Al₂SiO₅-bearing quartz veins. The veins formed during amphibolite-grade regional metamorphism. Veins occur in both Barrovian and Buchan-type terranes. Staurolite-grade pelitic schists in the Buchan region (Whitehills and Boyndie Bay) contain numerous pegmatitic quartz-and alusite-biotite veins. Optically strain-free quartz inclusions located within vein andalusite contain primary CO₂-CH₄/N₂ fluid inclusions (Type Aqz). Several generations of secondary H₂O-NaCl (Type B) fluid inclusions have also been identified in vein quartz. A number of Type Aqz inclusions have densities appropriate to the peak-metamorphic conditions calculated for the surrounding host rock and appear to indicate a CO₂-rich fluid was present during vein genesis. Post-entrapment modification however may have occurred during uplift as the mean Type Aqz isochore lies outside peak P-T conditions calculated, possibly suggesting some compositional modification. Kyanite-grade pelitic schists in the Sandend and Glen Esk areas (Barrovian-terranes) host pegmatitic kyanite-quartz veins. Pegmatitic kyanite crystals contain CO₂-bearing fluid inclusions (Type AkY); however, densities calculated lie outside the stability field of kyanite indicating either the Type Aky inclusions have been subjected to post-entrapment modification or the inclusions formed during the later stages of uplift. A source for the CO₂-rich fluid is enigmatic. The possible sources of CO₂ identified in Type A fluid inclusions are: 1) decarbonation reactions; 2) oxidation of carbonaceous or graphitic matter; and 3) deep-seated origin, ultimately from degassing of the mantle. Calc-silicate horizons and graphite-bearing schists are found within the Eastern Dalradian providing a source for both 1 and 2, with the gabbros of the 'Younger Basics', emplaced between the regional D₁ and D₃ events, providing a source for 3. The Portsoy Gabbro (component of the 'Younger Basics') appears to have altered the ¹³C isotope value of carbonate rocks structurally above the gabbro and may therefore be the source for the CO₂-CH₄/N₂ (Type Aqz) inclusions identified in veins from the Buchan-type terrane. Metamorphosed pelitic rocks of the Betic Cordillera (southern Spain) contain Al₂SiO₅-bearing quartz veins. The veins commonly contain all three Al₂SiO₅ polymorphs. Two vein types are recognised based on mineralogy. Fluid inclusion analysis shows that vein-forming fluids were predominantly CO₂-H₂O mixtures produced during devolatilisation of the local graphite-bearing pelitic host rocks. Spectacular post-entrapment modification textures attest to the rapid uplift path proposed for the Al₂SiO₅-bearing quartz veins and host rocks.