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Geochemical, oxygen, and neodymium isotope compositions of metasediments from the Abitibi greenstone belt and Pontiac Subprovince, Canada: Evidence for ancient crust and Archean terrane juxtaposition

Authors
Journal
Geochimica et Cosmochimica Acta
0016-7037
Publisher
Elsevier
Publication Date
Volume
57
Issue
3
Identifiers
DOI: 10.1016/0016-7037(93)90375-7
Disciplines
  • Chemistry
  • Earth Science

Abstract

Abstract The Abitibi greenstone belt (AGB) and Pontiac Subprovince (PS) in the southwestern Superior Province are adjacent greenstone-plutonic and metasedimentary-dominated terranes, respectively, separated by a major fault zone. Metasediments from these two contrasting terranes are compared in terms of major- and trace-element and O- and Nd-isotope compositions, and detrital zircon ages. The following two compositional populations of metasediments are present in the low-grade Abitibi southern volcanic zone: 1. (1) a mafic-element-enriched population (MEP) characterized by flat, depleted REE patterns; enhanced Mg, Cr, Co, Ni, and Sc; low-incompatible-element contents; and minor or absent normalized negative troughs at Nb, Ta, and Ti; and 2. (2) a low-mafic-element population (LMEP) featuring LREE-enriched patterns; enhanced Rb, Cs, Ba, Th, and U contents; and pronounced normalized negative troughs at Nb, Ta, and Ti. These geochemical features are interpreted to indicate that the MEP sediments were derived from an ultramafic- and mafic-dominated oceanic provenance, whereas the LMEP sediments represent mixtures of mafic and felsic arc source rocks. The PS metasediments are essentially indistinguishable from Abitibi LMEP on the basis of majorelement and transition metal abundances, suggesting comparable types of source rocks and degrees of maturity, but are distinct in terms of some trace elements and O-isotope compositions. The Pontiac metasediments are depleted in 18O and enriched in Cs, Ba, Pb, Th, U, Nb, Ta, Hf, Zr, and total REE and also have higher ratios of Rb K , Cs Rb , Ba Rb , Ta Nb , Th La , and Ba La relative to the Abitibi LMEP. Two subtypes of REE patterns have been identified in PS metasediments. Both have moderately fractionated distributions, with La Yb n = 3.9–17 and 7.8–13.6, respectively; but the first has minor Eu anomalies ( Eu Eu ∗ = 0.89–1.16 ), whereas the second features pronounced negative anomalies ( Eu Eu ∗ = 0.52–0.81 ) coupled with high concentrations of incompatible elements such as K, Rb, Th, U, Nb, and Ta. The first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks, whereas the Eu-depleted type has geochemical features that are typical of post-Archean sediments or Archean K-rich granites and volcanic equivalents. Abitibi metasediments have ϵ Nd (2.7 Ga) values of −0.71 to +1.83, corresponding to model T CHUR = 2.82−2.55 Ga, with the majority around 2.65 Ga, close to the depositional age of ~2.7 Ga. In contrast, PS metasediments are characterized by ϵ Nd (2.7 Ga) = +0.95 to −3.51, corresponding to T dhur = 2.7 to ~3.1 Ga, consistent with a detrital zircon age spectrum of ~2.69 Ga to ~3.1 Ga. Pontiac Subprovince and Abitibi metasediments have contrasting provenances, suggesting their separate, initial geodynamic settings and later tectonic juxtaposition. Major and trace elements, O- and Nd-isotopic compositions, and detrital zircon ages collectively suggest that some of the PS metasediments were derived mainly from juvenile rocks; whereas others require various degrees of contribution from older (3.0–3.5 Ga) evolved crust mixed with juvenile (~2.7 Ga) rocks. Older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components. If the PS represents in part an accretionary prism to the Abitibi southern volcanic zone, then oblique subduction may have been involved, which allowed the Pontiac Subprovince to juxtapose or sample exotic source rocks.

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