Using Multiple Sulfur Isotope Signatures to Delineate Terrane Boundaries and Investigate Crustal Formation Mechanisms during the Paleoproterozoic

Abstract:

Proterozoic cratonic margins are structurally and magmatically complex areas of the Earth’s crust, having undergone one or more orogenic cycles. Syn- to late-orogenic plutonic suites have a profound effect on the stabilization of cratonic margins by enabling the development of a refractory and buoyant subcontinental mantle and/or lower crust from which they were derived. In addition, these intrusive bodies act as isotopic tracers to the underlying lithospheric keel and its intricate and often obscured architecture.

In situ multiple sulfur isotope systematics are a robust and powerful tool for fingerprinting spatially and temporally anomalous signatures found in the crust. When combined, δ³⁴S and Δ³³S have the potential to link source environment and age of a sulfur-bearing mineral. Here, we investigate the multiple sulfur isotopic signatures of two syn- to late-orogenic supersuites that form a large component of the Paleoproterozoic Capricorn Orogen of Western Australia: the ca. 1820–1775 Ma Moorarie Supersuite and the ca. 1680–1620 Ma Durlacher Supersuite.

Results from secondary ion mass spectrometry demonstrate that the magmatic pyrite from the Moorarie Supersuite yields two differing signatures with δ³⁴S and Δ³³S equal to: 1) 3.1–4.8; ~0.00, and 2) 5.1–8.4; 0.1–0.24. This dichotomy is spatially associated with unexposed seismic blocks defined by Johnson et al., (2013).

Multiple sulfur isotope systematics also lend insight into the poorly understood formation mechanisms and sources of syn- to late-orogenic plutonic suites. The Durlacher Supersuite does not preserve sample-to-sample variation (δ³⁴S=5.0–8.3; Δ³³S=0.0), indicating that it crystallized from a widespread and homogeneous source. This is in direct contrast to the systematic δ³⁴S and Δ³³S variation preserved in the Moorarie Supersuite, a feature that we attribute to smaller batch melting of localized and isotopically separate blocks. Lastly, the Durlacher Supersuite contains anomalous within-sample +Δ³³S that we interpret to reflect the entrainment of surrounding Archean crust during diapiric ascent and emplacement. This study is part of a greater effort to investigate how anomalous sulfur isotopic signatures can be transported from metallowell-endowed Archean cratons and fractionated into younger orogenic belts.