Changes in stable isotope ratios of metapelites and marbles during regional metamorphism, Mount Lofty Ranges, South Australia: implications for crustal scale fluid flow

Abstract

The Mount Lofty Ranges comprises interlayered marbles, metapsammites, and metapelites that underwent regional metamorphism during the Delamarian Orogeny at 470–515 Ma. Peak metamorphic conditions increased from lowermost biotite grade (∼350–400°C) to migmatite grade (∼700°C) over 50–55 km parallel to the lithological strike of the rocks. With increasing metamorphic grade, δ18O values of “normal” metapelites decrease from 14–16‰ to as low as 9.0‰, while δ18O values of calcite in “normal” marbles decrease from 22–24‰ to as low as 13.2‰ These isotopic changes are far greater than can be accounted for by devolatilisation, implying widespread fluid-rock interaction. Contact metamorphism appears not to have affected the terrain, suggesting that fluid flow occurred during regional metamorphism. Down-temperature fluid flow from synmetamorphic granite plutons (δ18O=8.4–8.6‰) that occur at the highest metamorphic grades is unlikely to explain the resetting of oxygen isotopes because: (a) there is a paucity of skarns at granite-metasediment contacts; (b) the marbles generally do not contain low-XCO2 mineral assemblages; (c) there is insufficient granite to provide the required volumes of water; (d) the marbles and metapelites retain a several permil difference in δ18O values, even at high metamorphic grades. The oxygen isotope resetting may be accounted for by along-strike up-temperature fluid flow during regional metamorphism with time-integrated fluid fluxes of up to 5x109 moles/m2 (∼105 m3/m2). If fluid flow occurred over 105–106 years, estimated intrinsic permeabilities are 10-20 to 10-16m2. Variations in δ18O at individual outcrops suggest that time-integrated fluid fluxes and intrinsic permeabilities may locally have varied by at least an order of magnitude. A general increase in XCO2 values of marble assemblages with metamorphic grade is also consistent with the up-temperature fluid-flow model. Fluids in the metapelites may have been derived from these rocks by devolatilisation at low metamorphic grades; however, fluids in the marbles were probably derived in part from the surrounding siliceous rocks. The marble-metapelite boundaries preserve steep gradients in both δ18O and XCO2 values, suggesting that across-strike fluid fluxes were much lower than those parallel to strike. Up-temperature fluid flow may also have formed orthoamphibole rocks and caused melting of the metapelites at high grades.