Abstract

The tectonomagmatic history of the early Paleoproterozoic (Siderian) global tectonomagmatic lull (TML; ca. 2.4–2.2 Ga) is poorly known, limiting understanding of lithospheric processes during the Archean to Proterozoic transition. In this study, we identify and describe an unusual ca. 2.4–2.3 Ga postcollisional A-type granite suite associated with orogenic collapse of a continental collision in the middle of the TML. The postcollisional A-type granitoids consist of monzogranite to tonalite intrusions of the North Shore plutonic suite, emplaced during the middle and later stages of the ca. 2.5–2.3 Ga Arrowsmith orogeny, which represents a long-lived accretionary orogen on the northwestern margin of the Archean Rae craton in northern Canada. New laser ablation (LA)-ICP-MS U-Pb zircon age data indicate that the North Shore plutonic suite represents protracted granitic magmatism over ca. 70 My. The new U-Pb ages combined with previous and new LA-MC-ICP-MS Hf isotope and secondary-ion mass spectrometry O isotope zircon data reveal compositional heterogeneities not previously recorded by bulk isotopic methods and require changes to the classification of the granitoids and their role in the Arrowsmith orogeny. Zircon εHf values of the North Shore plutons are consistent with a mixture of juvenile and older crustal components in their sources, but their nonenriched zircon δ18O values are inconsistent with a large supracrustal melt component, precluding an S-type granite parentage. Instead, their trace-element proportions in discrimination diagrams and zircon δ18O values show A-type granite affinities, which are often missed in studies of ancient, poorly defined orogens. Aluminous compositions of the A-type granites are compatible with emplacement in a continent-continent collision, following postcollisional slab break-off and delamination. Postcollisional emplacement of the A-type granites is consistent with other geologic evidence suggesting that subduction beneath the Rae craton margin ceased by 2.4 Ga.

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