Syn-tectonic hematite growth in Paleoproterozoic Stirling Range “red beds”, Albany-Fraser Orogen, Australia: Evidence for oxidation during late-stage orogenic uplift

Abstract

The timing of hematite growth in ancient red-colored sedimentary rocks (“red beds”) is controversial. While most of the hematite is considered to have formed at or near the land surface shortly after deposition, paleomagnetic evidence suggests that some hematite in ancient red beds formed long after deposition during orogenic fluid flow. Metamorphosed red sandstones and shales in the Stirling Range Formation, Western Australia, were deposited in a shallow marine environment between 2.0 and 1.8 billion years ago. The sandstones, which preserve an assemblage of discoidal and trace-like fossils, are stained red by abundant fine-grained hematite crystals. The hematite is aligned in a spaced cleavage defined by elongate laths of muscovite and chlorite, and its occurrence as inclusions in ∼1.2 Ga metamorphic monazite indicates that the oxide formed during the 1.2–1.15 Ga Albany-Fraser orogeny, >600 million years after deposition. Thermometry of two distinct populations of chlorite, an earlier Fe-chlorite (Chl1) and a later Mg-chlorite (Chl2), indicates a cooling trend from ∼370 °C (Chl1) to ∼290 °C (Chl2), consistent with evidence for rapid late-stage uplift elsewhere in the orogen. The results indicate that most of the hematite in the Stirling “red beds” formed during syn- to late-orogenic oxidation of previously more reduced marine sediments. The migration of oxidizing fluids during orogenesis, possibly related to rapid uplift and gravity-driven influx of surface water, could explain the origin of other post-orogenic “red beds”.