Element redistribution and mobility during upper crustal metamorphism of metasedimentary rocks: an example from the eastern Mount Lofty Ranges, South Australia

Abstract

We present a detailed study on element mobility during prograde metamorphism of metasedimentary rocks of the eastern Mt. Lofty Ranges, South Australia. Mineral and bulk rock compositions were monitored across a regional metamorphic gradient from ≈350–400 °C to migmatite grade (≈650–700 °C) at ≈0.3–0.5 GPa, where pervasive up-temperature fluid flow during metamorphism has been proposed previously. Major and most trace elements (including rare earth elements) are isochemical during metamorphism as they are effectively redistributed into newly formed major and/or accessory minerals. Monazite or allanite and xenotime control the whole rock concentration of rare earth elements (REEs), whereas apatite and titanite are minor REE hosts. The only non-volatile elements that are demonstrably mobilized by metamorphic fluids are Zn, Pb, Ag, Cs, Sb, Bi and As, whose concentrations decreased with increasing metamorphic grade. Depletion of Zn, Sb and Pb was progressive with increasing temperature in staurolite-absent psammopelites, with losses of ≈80 % of the original Zn and >80 % of the protolithic Sb and ≈50 % of the original Pb from the rocks from high-grade metamorphic zones. Pronounced depletion of As and Cs occurs at the greenschist/amphibolite facies boundary and the transition to migmatite grade, respectively, while Ag and Bi contents decrease between 500 and 550 °C where >50 % of the original Ag and Bi is lost. While for most elements, unmetamorphosed sedimentary sequences can be considered chemical equivalents of metasedimentary rocks occupying deeper crust levels, in some cases, such as the extensive flow of Cl-rich fluid documented here, metals such as Zn, Pb and Ag may be stripped and may serve as a metal source for orebody formation. The decrease of As, Bi and Sb contents during prograde metamorphism might be a more universal feature that is linked with sulphide phase transitions.