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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - CIAO (Central Iran Amphibole-bearing Ophiolite)

Teaser

Summary

In subduction systems, there is an intermediate mantle zone in the overriding plate, located between the wedge and the neighbouring back arc. It constitutes one of the most compositionally and physically complex environments of the entire Upper Mantle. This vigorous site is the arena of various important processes that shape our planet, including magmatism and the formation of new seafloor at spreading centers. However, our knowledge of this peculiar geodynamic setting is very limited. A better understanding of its petrological features and chemical-physical processes will greatly improve our knowledge. This may eventually lead to better understanding of geological processes that have a significant impact on mankind, e.g., volcanic eruptions.
Within CIAO, we conducted a throughout petrological and geochemical study on the amphibole-bearing peridotites of the Nain ophiolite (Iran) (Figure 1). Petrological studies gave strong lines of evidence that these derive from a back-arc setting and provide thus an almost unique opportunity to investigate the properties of the intermediate mantle zone. Second, we conducted also an analysis of the volcanic rocks from Nain and the neighbouring Ashin ophiolites. These allowed us to model the mantle sources composition and its chemical heterogeneity.
The objectives that we aim within CIAO are manifold. Among them, we aim at shedding light at a geologically compelling subduction zone in Iran. The study is also intended to constraint the mass transfer between two colliding plates, ultimately leading to a better understanding of the input/output budget of subduction systems. Our most ambitious objective though lies in a throughout, and to the authors knowledge first-time, characterization of the magmatic processes in the peculiar intermediate mantle zone of an arc/back arc, through an unprecedented direct study of its broadly accessible peridotites. In a broad setting, CIAO is intended to opens a tremendous and novel perspective on the relationship between important planet shaping processes, namely metasomatism and mantle melting.

Work performed

During this project, we have conducted an extensive field work on the Nain and Ashin Ophiolite complexes. The collected samples have then undergone a throughout chemical analysis. We have then interpreted this bulk of raw data, from which the following main conclusions were derived.
Our studies have shown that the amphibole (pargasite) bearing peridotites of Nain are mainly spinel- and plagioclase-lherzolite in composition. Structural and chemical characteristics of these lherzolites (Figure 2a) have clearly shown that the plagioclase lherzolites are formed from spinel lherzolites by a series of metamorphic reactions (recrystallization). The metamorphosed origin of the plagioclase lherzolites allows us to conclude important facts about the origin of these peridotites, and consequently about the tectonic setting of the pre-existing Nain basin. Recrystallization features recorded in Nain plagioclase lherzolites suggest that these peridotites have subcontinental origin. This therefore suggests that the Nain basin was rifted at the boarder of the Central-East Iranian microplate (CEIM) through asymmetrical passive extension.
Pargasites occurring in the lherzolites indicate a refractory chemistry (Figure 2b). These minerals are fairly depleted in incompatible elements. The refractory chemistry of the pargasite is unexpected and refutes our primary hypotheses, relating their origin to the addition of subduction related fluids. The pargasites have thus likely been formed by addition of low-density H2O-rich fluids that were originally depleted in incompatible elements (Figure 2b).
Careful observations have indicated that the pyroxenes and pargasites in spinel lherzolites of Nain show an Eu positive anomaly (Figure 2c, d). Although slight, the observed anomaly is remarkable, mainly because Eu is an incompatible element in mantle pyroxenes during melting. We know that Eu is the only REE that can exist in the divalent and trivalent states under magmatic conditions. However, its behavioural dependence to redox condition have usually been overlooked or underestimated, especially in processes that take place in the Upper Mantle. Here we have related unusual concentration of Eu in the pyroxenes to the oxidized condition of the studied lherzolites.
Petrographical and chemical data has shown that the volcanic rocks from Nain and Ashin ophiolites are basalts, ferrobasalts and basaltic andesites in composition. Two samples from the Nain ophiolite are characterized by N-MORB normalized incompatible element patterns. These samples are chemically similar to the calc-alkaline basalts (CAB) and are interpreted to originate from a cordilleran-type volcanic arc setting. All other samples from the Nain and Ashin display a wide range of chemical composition. These overall chemical features resemble those of island arc tholeiites (IAT). The depletion in incompatible elements, in comparison to N-MORB, suggests that these rocks were derived from mantle sources that underwent enrichment in subduction-derived chemical components prior melting. Our data confirms that the Nain and Ashin ophiolites were formed in a subduction-related tectonic setting during the Early Cretaceous. The chemistry of the studied rocks is compatible with transition zone either from forearc to arc or from arc to backarc setting.

Final results

So far, few studies have documented spinel to plagioclase facies transition processes in mantle peridotites. Our studies on amphibole bearing peridotites of Nain is one of the few studies that documents this transitions by careful describing structural and chemical changes that occur in peridotites in response to the facies exchange from spinel to plagioclase. We reported amphiboles with an unusual depleted chemistry and we have shown, via modelling, that this refractoriness was not the result of post-crystallization processes as it is generally inferred. Rather, it is likely related to the originally depleted chemistry of the metasomatizing agents. We have also drawn some attention to the behaviour of Eu in the upper mantle minerals and the influence of redox condition on the behaviour of Eu in magmatic systems.
However we confidently can say that the most important result of our study is our finding about the origin of the studied lherzolites, and consequently, the Nain basin setting. For the very first time, subcontinental peridotites are reporting from Central Iran ophiolites. Consequently, for the first time passive extension is used to explain the origin of the narrow oceanic basin that once existed in Central Iran, at Mesozoic time.
Our studies on volcanics of Nain and Ashin ophiolites show that the Nain Ocean was likely an arc-forearc basin and not a back-arc, as suggested by their IAT to CAB geochemical composition. This study highlighted a very close similarity between the studied volcanic rocks and those of the Sabzevar ophiolites, as well as, significant differences with volcanics from the southern ophiolites (e.g., Baft ophiolites). This is surprising and in contrast to the initial hypothesis this project rested upon, where the Nain and Ashin ophiolites are considered fragments of the southern oceanic branch. Our results are thus remarkable and important as they provide new robust constrains for a new tectonic reconstruction of the whole Central Iran area (Figure 3).