Links Between the Ediacaran Zircon Geochemistry Anomaly and Extension-Related Magmatism in Gondwana

Abstract:

The secular record of zircon geochemistry is particularly well documented, and among a variety of global datasets that include ~10⁴ zircons, zircons with U–Pb ages that span from roughly 600 to 550 Ma, and which are centred on ca. 580 Ma, form a group of distinctive composition. This group has been most extensively documented in detrital zircon datasets from Australia, Zealandia, and Africa. Anomalous characteristics of zircons in this age interval are particularly wide ranges of Hf isotope ratio, O isotope ratio, and the concentration ratios of HREE to LREE and U. The unusually wide ranges of these parameters are indicative of global middle to late Ediacaran magmatism that involved both “juvenile,” mantle-derived sources, as well as extensive crustal assimilation/melting that is notably different to much of the remainder of the zircon geochemistry record. The zircon compositional anomaly seems clearly related to “crustal reworking” of some fashion, although proposed varieties of Ediacaran crustal reworking include processes as diverse as glacial erosion of upper continental crust (with subsequent subduction and melting of this detritus), subduction erosion of lower continental crust, and/or peak crustal thickening and partial melting during Gondwana-forming orogenesis.

In Phanerozoic sedimentary rocks of eastern Australia and northern Africa, the major peak of detrital rutile U–Pb cooling ages overlaps with the period of unusual zircon composition. This correspondence suggests that in parts of Gondwana, magmatism associated with the anomalous Ediacaran zircons occurred in a tectonic environment of extensional exhumation, perhaps related to lithospheric delamination during orogenic collapse in the late stages of the Gondwana-forming orogens. The defining characteristic of the Ediacaran zircon geochemistry anomaly (the particularly wide compositional span from depleted mantle to very old continental crust) may be fundamentally the signal of a significant period of upwelling asthenosphere and resultant crustal melting in portions of Gondwana.