Geochronologie der HP/LT-Gesteine und Amphibolite der Hajiabad-Esfandagheh-Region, Sanandaj-Sirjan Zone, Iran
Zusammenfassung der Projektergebnisse
The focus of this study is on the Hajiabad-Esfandagheh region in the Zagros Orogenic Belt. In order to better integrate the geology of the study area into geodynamic reconstructions of the Zagros orogen, improved and robust age constraints are necessary. For this purpose we have applied Rb–Sr and U–Pb geochronology for dating of magmatic and metamorphic rocks. Understanding of the ages recorded by the Zagros blueschists and associated lower grade metamorphic rocks plays a key role for in-depth understanding of the metamorphic events that are associated with convergence between Arabia and Eurasia. Rb–Sr analyses of four glaucophane schists from the Seghina unit and of three greenschists to amphibolite facies rocks from the Ashin unit revealed variable degrees of scatter. Isochron relationships between all data points are not or only poorly defined. It is obvious from this data set that not all phases isotopically equilibrated at the same time or that subsequent disturbance affected the isotope system. Nevertheless it had been possible to extract geologically meaningful ages from subsets of the complete data. In variable degrees, all samples show grain-size dependent age trends with the youngest ages recorded by the size fraction representing the smallest analysed grains. P–T estimates indicate that recrystallization of phengitic mica took place close to or below the commonly assumed closure temperature of ~500 °C for the Rb– Sr system. Thus significant diffusive loss of radiogenic Sr during cooling is unlikely. The grain-size dependent younging trend most likely is related to deformation. We interpret the youngest ages as reliable age estimates for final ductile deformation, as also suggested by Angiboust et al. (2016). Taken together new and published Rb–Sr and Ar–Ar white mica ages indicate that the Seghina unit experienced blueschist-facies P–T conditions between 68 to 85 Ma. Furthermore, the good agreement of both isotopic age groups suggests the absence of a significant excess Ar component that might have compromised the accuracy of the Ar–Ar data. The new Rb–Sr results provide indications for a yet poorly constrained >85 Ma highpressure metamorphic history. Unambiguous linking of apparent ages for presumed HP rocks of the Ashin unit with blueschist facies P–T conditions (ca. 93–106 Ma) has not yet been demonstrated. It remains unclear why there is no difference in metamorphic ages between amphibolite- and presumed blueschist facies rocks. Rb–Sr ages of two chlorite-epidote-actinolite schists indicate that greenschistfacies P–T conditions had already been attained around 125–130 Ma. These are the oldest apparent ages as yet reported for samples from the coloured mélange (Ashin unit). With our U–Pb study we aimed to take the initiative in building a geochronological database that can serve as a framework for unravelling the tectonostratigraphic relationships across the larger study area. However, due to the geological complexity and the range in ages this data set is yet too small and only of a reconnaissance nature. The new U–Pb zircon ages suggest that remnants of different micro–continental blocks are present in the study area. Four groups of protolith ages can be distinguished that are related to Paleoproterozoic, Ediacaran (Cadomian), Carboniferous and Jurassic magmatic and metamorphic events. Especially noteworthy are U–Pb zircon ages of ca. 1.7–1.9 Ga for a pegmatite from an exotic crustal fragment, because rocks of Paleoproterozoic age have yet not been documented in Iran.
