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

Abstract

The carbon and oxygen systematics of rocks north of Stonehaven, Scotland were studied to provide new constraints on the nature and timing of metamorphic and post‐metamorphic fluid infiltration. Carbon and oxygen isotopic data were collected from carbonated spilites and carbonate‐bearing veins in the Highland Boundary Fault, from Dalradian metacarbonate layers and pervasively carbonated schists, and from three generations of carbonate‐bearing veins, 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 metacarbonates. δ 18 O values for coexisting quartz (δ 18 O quartz = 13.6‰) and calcite (δ 18 O calcite = 11.6‰) in a vein from the upper chlorite zone are consistent with precipitation of both minerals from a single fluid at c. 375°C. Oxygen isotopic data from staurolite zone metacarbonate layers (δ 18 O calcite = 10.4 to 12.9‰) suggest major isotopic exchange with an external fluid equilibrated with a quartzo‐feldspathic reservoir, such as the Dalradian metasediments, at metamorphic temperatures. Low, negative carbon isotope data from these metacarbonate layers (δ 13 C calcite = −12.4 to −17.7‰) may indicate a component of oxidized organic material in the infiltrating fluid. Post‐metamorphic fluids derived from or equilibrated with a high‐temperature silicate reservoir such as felsic igneous rocks were responsible for precipitation of post‐metamorphic vein carbonate, carbonation of pelitic schists, and alteration of metamorphic feldspar porphyroblasts from Barrow’s famous spotted chloritoid schists in an area geographically associated with several carbonated quartz porphyry dykes. Some veins in the biotite and garnet zones have been conduits for multiple generations of fluid flow. Quartz from this generation of veins preserves a metamorphic isotopic signature (δ 18 O quartz = 11.2 to 13.2‰) whereas dolomite and calcite are considered to be post‐metamorphic based on textural evidence and isotopic data indicating extreme oxygen isotopic disequilibrium between vein quartz and carbonate minerals (δ 18 O dolomite = 23.7 to 24.2‰, δ 18 O calcite = 25.3‰, δ 18 O quartz = 11.2 to 13.2‰). Variably depleted carbon isotopes from carbonate minerals in these veins (δ 13 C dolomite = −8.1 to −13.4‰, δ 13 C calcite = −12.2‰) may reflect input of oxidized organic material in the mineralizing fluid. Isotopic data from carbonate veins and variably carbonated spilites from the Highland Boundary Fault are consistent with hydrothermal alteration of basalts by seawater, which had its carbon isotope systematics modified by addition of magmatic CO 2 and/or oxidation of organic carbon. Such alteration may have occurred on the seafloor or during emplacement in the Highland Boundary Fault.

Palaeomagnetic dating of fluid‐flow events in dolomitized rocks along the Highland Boundary Fault, central Scotland

“…If these samples are discarded, then the average δ 13 C value is −2.33 ± 1.39‰. The stable isotope values are close to the range of values reported by Masters et al . (2000) for Dalradian‐hosted dolomite veins in north‐eastern Scotland that were derived from post‐metamorphic, low‐temperature hydrothermal fluids.…”

Mobility of gold during metamorphism of the Dalradian in Scotland

“…Multiple fluid flow events have affected the Dalradian metamorphic rocks including pre metamorphic spilitisation (Graham, 1976;Skelton et al, 2010), metasomatism during prograde metamorphism (Ague, 1997;Masters and Ague, 2005;Masters et al, 2000), syn metamorphic carbonation (Skelton et al, 1995;Pitcairn et al, 2010b;Arghe et al, 2011) and post-metamorphic quartz-carbonate-sulphide veining (Anderson et al, 2004;Baron and Parnell, 2005;Craw and Chamberlain, 1996). Spilitisation and carbonation events are observed in the SW Highlands.…”

An online method combining a Gasbench II with continuous flow isotope ratio mass spectrometry to determine the content and isotopic compositions of minor amounts of carbonate in silicate rocks

“…Small and even trace amounts of carbonate often exist in silicate rocks, such as shale, basalt, eclogite and gneiss, as fine grains, cement or veinlets 8–12. These carbonates can provide important information on the protolith nature, formation temperature and fluid source in geological processes 13–16.…”