Vestiges of earliest crust
: crustal evolution in the Narryer Terrane, Yilgarn Craton

  • Leanne G Staddon

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Provenance studies and our understanding of the early Earth are dominated by the mineral zircon. However, zircons are derived dominantly from felsic lithologies, which are thought to be a volumetrically minor component of Hadean and Archean crust. This thesis focuses on detrital chromites, derived solely from mafic and ultramafic protoliths, within metasediments at Jack Hills, in the Narryer Terrane, Yilgarn Craton, Western Australia. Detrital zircons found within the same rocks pre-date the rock record by up to 350 Myr, but their petrogenesis remains , with conflicting hypotheses on the generation, evolution and destruction of Hadean and crust, and the geodynamic regime under which these processes operated. Detrital chromites show significant evidence of metamorphism, including elevated ZnO and MnO and lowered Mg#s of ≤30. The correlation of ZnO content with chromite modal proportions and the fit of most heavily modified chromites to the shape of an Fe-Mg exchange isopleth provides strong evidence chromites underwent metamorphic re-equilibration in the host metasediments. This event, likely at ca. 2650 Ma, also replaced primary mineral assemblages, which are now dominated by the same metamorphic assemblages as the host metasediments. However, despite meter-scale variability Mg#, and wt. % ZnO and MnO, Cr# are consistent across all samples and regardless of rounding shape, indicating a single, magmatically dynamic igneous source of detrital chromites from Jack Hills. When compared to chromites from known tectonic settings a komatiitic origin can be excluded: the source of detrital chromites is inferred to a layered intrusion. Bulk chromite samples yield unradiogenic 187Os/188Os isotopic compositions of 0.10412 to 0.11443, resulting in rhenium-depletion ages (TRDs) of 1849 Ma to 3323 Ma. Three analyses possessed ‘true’ TRDs (Re analytically indistinguishable from 0), indicating Re loss of chromites in both the Archean and Proterozoic. When filtered for magmatic chromite compositions (187Re/188Os < 0.1) five Re-Os model ages (TMAs) yield a weighted mean of 3528±34 Ma (2se, MSWD=1.3). Two younger model ages at ~3000 Ma are also present, likely representing Re-loss from ~3530 Ma chromites, or metamorphic re-equilibration. The 3730 Ma Manfred Complex yields a chondritic Osi, indicating it may not represent the source of detrital chromites. Zircons yield typical Jack Hills 207Pb206Pb age distributions, with a dominant peak at ca. 3380 Ma and more minor peaks at ~3440 Ma, ~3490 Ma, ~3520 Ma to 3540 Ma, and ~4000-4100 Ma. Oscillatory zoned zircons possess subchondritic ԐHf(t)CHUR, and plot on a shallow array indicative reworking of crust with a 176Lu/177hf ratio of >0.02. These values are indicative of crust with a mafic composition: this study observes no evidence to suggest zircons are derived from reworking of felsic crust, as has been suggested in numerous recent publications. Detrital chromite Re-Os TMAs of ~3530 Ma overlap with two minor 207Pb206Pb age distribution peaks, that are composed of zircons that possess subchondritic ԐHf(t)CHUR compositions, indicating a temporal link between the mafic and felsic components of the Jack Hills detrital records. This is a hallmark of a stagnant lid regime rather than modern-style plate tectonics. However, uncertainties on aspects of Re-Os TMAs, such as the potential interaction of the chromite protolith with reservoirs of super- or sub-chondritic compositions, suggests geodynamic conclusions should be taken tentatively.
Date of Award1 Oct 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorIan J Parkinson (Supervisor) & Tim R Elliott (Supervisor)

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