Non-arc setting for “Cadomian” magmatism in Iran and Anatolia

Abstract

Establishing the type and position of plate boundaries is crucial for paleogeographic reconstructions. The northern margin of Gondwana (NMG) is inferred to have been a convergent, Andean-style, plate margin for much the late Neoproterozoic to early Paleozoic (i.e., the Avalonian-Cadomian Orogen), based largely on the presence of igneous rocks with arc-related geochemical affinities. However, a major segment of the margin that fringed the Arabian Plate includes bimodal Ediacaran–middle Cambrian (ca. 600–500 Ma) igneous rocks, more typical of continental rift settings, which has led to ambiguous and contradictory interpretations of magma tectonogenesis. Here, we employ an alternative approach to investigate the tectonic setting of the NMG by studying the evolution of sedimentary basins that developed in Iran, Anatolia, and the Arabian Plate simultaneously with the ca. 600–500 Ma magmatism. The Ediacaran–middle Cambrian successions in this segment of the NMG consist of laterally continuous siliciclastic and carbonate sequences, which have been broadly correlated across the region. The consistent northward and eastward paleocurrent directions and decrease in clastic sediment grain-size from proximal (the Arabian Plate) to distal (Iran and Anatolia) successions suggest a northward and eastward deepening basin with relatively flat topography. The new detrital zircon (n = 2870) and apatite (n = 1178) U-Pb ages from the Ediacaran–middle Cambrian siliciclastic strata of Iran are mostly older than 600 Ma. Detrital apatite trace element compositions indicate that most grains are sourced from I-type granitoids and mafic igneous rocks, low- and high-grade metamorphic rocks, with a minority from ultramafic rocks. Together with published detrital zircon U-Pb age data from correlative strata in Anatolia and the Arabian Plate, these data suggest the pre-existing Arabian-Nubian Shield as the main source. Importantly, detrital zircon and apatite grains with ages < 600 Ma are rare in these strata, suggesting that they received little input from contemporaneous Ediacaran–middle Cambrian (ca. 600–500 Ma) igneous rocks. We suggest that the very small sediment contribution from the ca. 600–500 Ma igneous rocks argues against the tectonic model that considers the development of a large Andean-style magmatic arc at this segment of the NMG. Integrating these constraints from the sedimentary record with geochemical data from the ca. 600–500 Ma igneous rocks in Iran and Anatolia favors an extensional continental margin setting that may be related to escape tectonics or post-collisional relaxation during Ediacaran–Cambrian following Gondwana assembly.