Origin of vein-graphite derived from metamorphic fluids in Moine (Glenfinnan Group) rocks, NW Scotland
A. J. Wright
N. J. F. Blamey
J. Conliffe
A. Costanzo
J. Parnell
10.6084/m9.figshare.3457574
https://geolsoc.figshare.com/articles/journal_contribution/Origin_of_vein-graphite_derived_from_metamorphic_fluids_in_Moine_Glenfinnan_Group_rocks_NW_Scotland/3457574
<p>Proterozoic metasediments at Glen Strathfarrar, Inverness-shire, host a deposit of vein-graphite that is unique within the
Moinian stratigraphic sequence. Carbon isotopic analysis, Raman microspectroscopy, fluid inclusion studies and volatile gas
analysis were used to constrain the origin of the vein-graphite. Carbon isotopic analysis shows that the carbon is heavier
than that expected for sedimentary material (δ<sup>13</sup>C value of −14.35‰), suggesting that fractionation has taken place since sedimentation. Graphitization of sedimentary carbon
in the Glenfinnan Group pelitic gneiss occurred in response to changes in both temperature and pressure arising from a series
of regional metamorphic events. Scavenging of carbon by metamorphic fluids generated during amphibolite-grade metamorphism
associated with the youngest, possibly Caledonian, event is the most probable source of the vein-graphite at Glen Strathfarrar.
Similarities in gas composition with quartz veins occurring in Dalradian rocks show that carbonaceous sedimentary sequences
consistently generate fluids rich in CO<sub>2</sub> and N<sub>2</sub>, making volatile gas analysis a valuable tool in determining the history of metamorphic fluids. The occurrence of crystalline
graphite in the pelitic gneiss at Glen Strathfarrar makes it highly likely that the carbon-rich fluids have been generated
from within the Glenfinnan Group sediments rather than from Lewisian rocks at depth. Despite the assertion that graphite is
uncommon in Moine rocks, the high ammonium content of biotite in these metasediments and the consistently high N<sub>2</sub> values of fluid inclusions within the siliceous schist and quartz veins suggest that, prior to metamorphism, the sediments
contained more organic matter than their present composition suggests.
</p>
2016-06-22 10:59:13
Glenfinnan Group pelitic gneiss
Glenfinnan Group sediments
NW Scotland Proterozoic metasediments
N 2 values
fluid inclusion studies
quartz veins
Glen Strathfarrar
Dalradian rocks show
CO
Moinian stratigraphic sequence
gas analysis
Geology