An oxygen and carbon isotopic study of multiple episodes of fluid flow in the Dalradian and Highland Border Complex, Stonehaven, Scotland

The carbon and oxygen systematics of rocks north of Stonehaven, Scotland we re studied to provide new constraints on the nature and timing of metamorph ic and post-metamorphic fluid infiltration. Carbon anti oxygen isotopic dat a were collected from carbonated spilites and carbonate-bearing veins in th e Highland Boundary Fault, from Dalradian metacarbonate layers and pervasiv ely carbonated schists; and from three generations of carbonate-bearing vei ns, and a quartz porphyry dyke within the Dalradian. Isotopic evidence for syn-metamorphic fluid infiltration in the Dalradian is preserved in quartz- calcite veins in the upper chlorite zone and in staurolite zone metacarbona tes. delta(18)O values for coexisting quartz (delta(18)O(quaratz)=13.6 part s per thousand and calcite (delta(18)O(calcite)=11.6 parts per thousand) in a vein from the upper chlorite zone are consistent with precipitation of b oth minerals from a single fluid at c, 375 degrees C, Oxygen isotopic data from staurolite zone metacarbonate layers (delta(18)O(calcite)=10.4 to 12.9 parts per thousand) suggest major isotopic exchange with an external fluid equilibrated with a quartzo-feldspathic reservoir, such as the Dalradian m etasediments, at metamorphic temperatures. Low, negative carbon isotope dat a from these metacarbonate layers (delta(13)C(calcite)= - 12.4 to - 17.7 pa rts per thousand) may indicate a component of oxidized organic material in the infiltrating fluid. Post-metamorphic fluids derived from or equilibrate d with a high-temperature silicate reservoir such as felsic igneous rocks w ere responsible for precipitation of post-metamorphic vein carbonate, carbo nation of pelitic schists, and alteration of metamorphic feldspar porphyrob lasts from Barrow's famous spotted chloritoid schists in an area geographic ally associated with Several carbonated quartz porphyry dykes. Semi: veins in the biotite and garnet zones have been conduits for multiple generations of fluid flow. Quartz from this generation of veins preserves a metamorphi c isotopic signature (delta(18)O(quartz)=11.2 to 13.2 parts per thousand) w hereas dolomite and calcite are considered to be post-metamorphic based on textural evidence and isotopic data indicating extreme oxygen isotopic dise quilibrium between vein quartz and carbonate minerals (delta(18)O(dolomite) =23.7 to 24.2 parts per thousand, delta(18)O(calcite)=25.3 parts per thousa nd, delta(18)O(quartz)=11.2 to 13.2 parts per thousand). Variably depleted carbon isotopes from carbonate minerals in these veins (delta(13)C(dolomite ) = - 8.1 to - 13.4 parts per thousand, delta(13)C(calcite)= - 12.2 parts p er thousand) may reflect input of oxidized organic material in the minerali zing fluid. Isotopic data from carbonate veins and variably carbonated spil ites from the Highland Boundary Fault are consistent with hydrothermal alte ration of basalts by seawater, which had its carbon isotope systematics mod ified by addition of magmatic CO2 and/or oxidation of organic carbon. Such alteration may have occurred on the seafloor or during emplacement in the H ighland Boundary Fault.