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Strongly Peraluminous Granites across the Archean–Proterozoic Transition

Bucholz, Claire E. and Spencer, Christopher J. (2019) Strongly Peraluminous Granites across the Archean–Proterozoic Transition. Journal of Petrology, 60 (7). pp. 1299-1348. ISSN 0022-3530. doi:10.1093/petrology/egz033.

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Strongly peraluminous granites (SPGs) form through the partial melting of metasedimentary rocks and therefore represent archives of the influence of assimilation of sedimentary rocks on the petrology and chemistry of igneous rocks. With the aim of understanding how variations in sedimentary rock characteristics across the Archean–Proterozoic transition might have influenced the igneous rock record, we compiled and compared whole-rock chemistry, mineral chemistry, and isotope data from Archean and Paleo- to Mesoproterozoic SPGs. This time period was chosen as the Archean–Proterozoic transition broadly coincides with the stabilization of continents, the rise of subaerial weathering, and the Great Oxidation Event (GOE), all of which left an imprint on the sedimentary rock record. Our compilation of SPGs is founded on a detailed literature review of the regional geology, geochronology, and inferred origins of the SPGs, which suggest derivation from metasedimentary source material. Although Archean and Proterozoic SPGs are similar in terms of mineralogy or major-element composition owing to their compositions as near-minimum melts in the peraluminous haplogranite system, we discuss several features of their mineral and whole-rock chemistry. First, we review a previous analysis of Archean and Proterozoic SPGs biotite and whole-rock compositions indicating that Archean SPGs, on average, are more reduced than Proterozoic SPGs. This observation suggests that Proterozoic SPGs were derived from metasedimentary sources that on average had more oxidized bulk redox states relative to their Archean counterparts, which could reflect an increase in atmospheric O₂ levels and more oxidized sedimentary source rocks after the GOE. Second, based on an analysis of Al₂O₃/TiO₂ whole-rock ratios and zircon saturation temperatures, we conclude that Archean and Proterozoic SPGs formed through partial melting of metasedimentary rocks over a similar range of melting temperatures, with both ‘high-’ and ‘low-’temperature SPGs being observed across all ages. This observation suggests that the thermo-tectonic processes resulting in the heating and melting of metasedimentary rocks (e.g. crustal thickening or underplating of mafic magmas) occurred during generation of both the Archean and Proterozoic SPGs. Third, bulk-rock CaO/Na₂O, Rb/Sr, and Rb/Ba ratios indicate that Archean and Proterozoic SPGs were derived from partial melting of both clay-rich (i.e. pelites) and clay-poor (i.e. greywackes) source regions that are locality specific, but not defined by age. This observation, although based on a relatively limited dataset, indicates that the source regions of Archean and Proterozoic SPGs were similar in terms of sediment maturity (i.e. clay component). Last, existing oxygen isotope data for quartz, zircon, and whole-rocks from Proterozoic SPGs show higher values than those of Archean SPGs, suggesting that bulk sedimentary ¹⁸O/¹⁶O ratios increased across the Archean–Proterozoic boundary. The existing geochemical datasets for Archean and Proterozoic SPGs, however, are limited in size and further work on these rocks is required. Future work must include detailed field studies, petrology, geochronology, and constraints on sedimentary source ages to fully interpret the chemistry of this uniquely useful suite of granites.

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Bucholz, Claire E.0000-0001-5332-8278
Additional Information:© 2019 The Author(s). Published by Oxford University Press. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Received February 11, 2019; Accepted June 3, 2019; Published: 07 June 2019. During the preparation of this paper conversations about the details of local Archean and Proterozoic geology were conducted with numerous people, including F. Breaks, Y. Pan, P. Weiblen, T. Boerboom, C. Frost, R. Frost, D. Bagdonas, H. Marschall, P. Nabelek, I. Annesley, and L. Anderson. We thank these people for sharing their knowledge of these unique rocks. S. Newall is thanked for assistance with sample preparation for whole-rock XRF analyses. C. Frost and I. Bindeman provided thoughtful and constructive reviews that improved this paper. We also thank M. Holness for the suggestion to write such a review and her infinite patience while we put this paper together.
Subject Keywords:Archean–Proterozoic transition; Great Oxidation Event; peraluminous granites; partial melting; metasedimentary rock; assimilation
Issue or Number:7
Record Number:CaltechAUTHORS:20191114-112103999
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Official Citation:Claire E Bucholz, Christopher J Spencer, Strongly Peraluminous Granites across the Archean–Proterozoic Transition, Journal of Petrology, Volume 60, Issue 7, July 2019, Pages 1299–1348,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99840
Deposited By: Tony Diaz
Deposited On:14 Nov 2019 21:21
Last Modified:16 Nov 2021 17:49

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