The enigma of crustal zircons in upper-mantle rocks: Clues from the Tumut ophiolite, southeast Australia

Abstract

Artículo de publicación ISIWe suggest a new explanation for the presence of crustally derived zircons in the upper-mantle rocks of ophiolitic complexes, as an alternative to subduction-related models. Integrated isotopic (U-Pb, Hf, and O isotopes) and trace-element data for zircons from the Tumut ophiolitic complex (southeast Australia) indicate that these grains are related to granitic magmatism and were introduced into the mantle rocks after their emplacement into the crust. These observations emphasize that a clear understanding of the origin of individual zircon populations and their relationship to the host rock is essential to interpretations of the tectonic history of upper-mantle rocks and the dynamics of crust-mantle interactions

Oxygen isotope and trace element geochemistry of zircons from porphyry copper system: Implications for Late Triassic metallogenesis within the Yidun Terrane, southeastern Tibetan Plateau

“…The PDL zircons are generally characteristic of slight Eu anomalies, whereas RL and XWC zircons show distinct negative Eu anomalies (our unpublished data; Fig. 3), suggesting that the former probably crystallized before plagioclase (Belousova et al, 2015) or formed in reduced environment that was relatively undepleted in Eu 2+ (Cavosie et al, 2006). The higher zircon Ce 4+ /Ce 3+ ratios suggest that the Eu anomaly was controlled by the absence of plagioclase fractionation rather than a reduced depositional environment (see Fig.…”

Genesis and Multi-Episodic Alteration of Zircon-Bearing Chromitites from the Ayios Stefanos Mine, Othris Massif, Greece: Assessment of an Unconventional Hypothesis on the Origin of Zircon in Ophiolitic Chromitites

“…In the absence of any isotopic data for these zircons it is not possible to provide a direct answer to this question; our textural data cannot rule out either hypothesis at this stage of study. A quite similar scenario has been proposed to explain the origin of zircons in the chromitites of the Tumut ophiolite complex in southeast Australia [27] and the Cedrolina chromitites in Brazil [28]. The zircons we studied bear some resemblance to those found in the Early Jurassic Finero alpine chromitites (western Alps) wherein zircon genesis was ascribed to direct crystallization from the melt that formed the Finero chromitites during the Early Jurassic [81,82]; those anhedral to subhedral zircons occur in interstitial positions between Cr-spinel and olivine and/or orthopyroxene [65], forming aggregates of up to four crystals [64].…”

“…Though the gabbroic intrusions are missing from the lenticular chromitite, they crop out within a radius of less than 20 m away from this ore body and crack propagation in the mantle is known to provide a robust pathway for the transport of metasomatic fluids and melts over long distances [27,79]. Moreover, all zircons display textural and compositional similarities implying that they share a mutual history.…”

“…Even in this case our zircon analyses bear similarity to those from mafic to intermediate and felsic sources. Most of them are characterized by low HfO 2 /Y 2 O 3 ratios and low Hf (<1 wt %) contents likewise early to late magmatic zircons [74] from mafic settings (Figure 13c) [27]. Alternatively, it can be said that these zircon analyses define a fractionation trend toward progressively more depleted-in-Hf compositions.…”

“…Three main scenarios have been proposed to explain the 'paradoxical' finding of Zr-rich phases in ophiolitic chromitites: (i) incorporation of ancient zircons in the sub-oceanic mantle by ascending fluids/melts as a result of deep subduction [22]; (ii) precipitation of Zr-dominant phases by rich-in-fluid melts related to mantle metasomatism (e.g., [64,65]) and (iii) introduction of zircon in chromitites through (a network of) intergranular fluids representing late fractions of igneous intrusions in the upper mantle [27,28].…”