Was the Grawler Craton a Continental Margin During the NeoArchean Through PaleoProterozoic

SHRIMP U-Pb zircon age constraints on the tectonics of the Neoarchean to early Paleoproterozoic transition within the Mulgathing Complex, Gawler Craton, South Australia

Authors:

Reid et al

Abstract:

The Mulgathing Complex within the Gawler Craton, South Australia, preserves evidence for magmatism, sedimentation and metamorphism spanning the transition between the Neoarchean and Paleoproterozoic (ca. 2555–2410 Ma). Prior to this study, limited data were available to constrain the timing of these tectonothermal events. We report SHRIMP zircon U-Pb dating of metamorphosed sedimentary and magmatic rocks from the Mulgathing Complex that shows metasedimentary gneisses (Christie Gneiss) have maximum depositional ages ca. 2480 Ma, in contrast to previous studies that have suggested deposition had occurred ca. 2510 Ma. The oldest metamorphic zircons in our data are ca. 2465 Ma, thus indicating there was a time interval of less than 15 Myr between the cessation of sedimentation and the occurrence of high-grade metamorphism. Metamorphic zircons have a range of ages, from ca. 2465 to ca. 2415 Ma, consistent with a period of ∼50 Myr during which high-grade metamorphism occurred. Mafic and felsic intrusions have ages that range from ca. 2520 Ma to 2460 Ma, indicating magmatism occurred during sedimentation and continued during the early stages of metamorphism and deformation of these rocks. The presence of bimodal magmatism with continental arc affinity (Devils Playground Volcanics) together with mafic intrusions showing temporal overlap with sedimentation within the Mulgathing Complex suggests that the overall tectonic regime prior to deformation likely involved lithospheric extension, possibly related to a continental magmatic arc. The Mulgathing Complex shows similarities in the nature and timing of these tectonothermal processes with several other Neoarchean - Paleoproterozoic terranes, in particular the Terra Adelie Craton in Antarctica, the Sask Craton, Canada, and regions within the North China Craton and the North Australian Craton. It is possible that similarities in process could imply along-strike relationships between these terranes, for example as laterally continuous terranes at the margin of a former continental domain (or domains). Basement rocks with ages of ca. 3250–3150 Ma, ca. 2950–2800 Ma and 2600–2550 Ma within the Gawler Craton, as inferred from magmatic and inherited zircons, are temporally equivalent to some of the main basement units within the Pilbara Craton, and basement rocks in both the Gawler and Pilbara cratons are overlain by sedimentary rocks at ca. 2480 Ma that include banded iron formations. It is possible to envisage the Gawler Craton as representing an extended continental margin adjacent to a larger, more intact continental domain, such as the Pilbara.