NEW TIME CONSTRAINTS FOR HP METAMORPHISM AND EXHUMATION OF MELANGE ROCKS FROM THE SISTAN SUTURE ZONE, EASTERN IRAN

The Sistan Suture Zone in the eastern part of Iran documents the closure of a Neotethyan oceanic basin due to convergence between the Central Iranian and Afghan microcontinents [1]. This Cretaceous–Tertiary orogenic belt (ca. 500 x 100 km²) exposes a well-preserved subduction zone complex, which includes a tectonic mélange consisting of disrupted meta-ophiolitic rocks within a low-grade matrix of chlorite-talc schists, mica schists and greenschists. Large slabs of ultramafic rocks and meta-gabbros record low-grade metamorphism, but some smaller metabasic blocks (<200 m) were affected by high-pressure and/or epidote amphibolite facies conditions. Understanding of the petrological and geochronological record of these rocks plays a key role in unraveling the geodynamic evolution of the Sistan ocean. Radiolaria in pelagic cherts from the mélange document that the Sistan ocean existed already in Early Cretaceous time (Aptian to Albian) [4], but the closure history is yet poorly constrained.

Metamorphic conditions recorded by samples from different HP blocks vary considerably. P–T estimates for the eclogite and blueschist-facies stages indicate pressures of ca.1.5–2.4 GPa at temperatures of ca. 450–650 °C, whereas pressure conditions of ca. 0.5–0.7 GPa at temperatures of 520–590 °C were reported for epidote amphibolite facies rocks [2]. Previously published Ar–Ar data for a phengite-paragonite mixture from an eclogite and paragonite from a blueschist indicated apparent ages of 122.8 ± 2.2 Ma and 139 ±19 Ma [3]. Phengite and barroisite from amphibolites yielded Ar–Ar ages of 124 ± 13 Ma, 116 ± 19 Ma and 124 ± 10 Ma, respectively [2]. All Ar–Ar ages were interpreted as cooling ages that date post-epidote amphibolite facies stages of the exhumation path, implying that both HP metamorphism and the epidote–amphibolite-facies overprint occurred prior to 125 Ma. However, caution is warranted in assigning geological significance to this data set, because the presence of excess Ar leading to geologically meaningless dates was documented for many HP and UHP occurrences.

In order to evaluate the importance of the Ar–Ar data and to obtain a more detailed picture of the metamorphic evolution, we use a multimethod geochronological approach (Rb–Sr, Sm–Nd, Lu–Hf, U–Pb). Samples representing HP blocks and matrix were collected in the Ratuk Complex near Gazik, Gurchang and Sulabest within a NNW-SSE trending belt spanning a distance of ca. 95 km that exposes the major occurrences of HP rocks and epidote amphibolites. First Rb–Sr results (internal mineral isochrons comprising several grain size fractions of white mica, ± epidote, omphacite, glaucophane) for 5 samples indicate a regional consistent age pattern (86.1 ± 1.0 Ma, 86.6 ± 0.6 Ma, 84.3 ± 4.7 Ma, 84.7 ± 0.7 Ma, 85.7 ± 0.7 Ma) with considerably younger ages than indicated by Ar–Ar geochronology. This observation is consistent with assuming variable amounts of extraneous argon in the white mica, and suggests that the Ar–Ar data do not provide reliable ages. The Rb–Sr ages are in accordance with field and biostratigraphic observations and document subduction of the Sistan ocean in Late Cretaceous time. Additional Sm-Nd and Lu–Hf studies (work in progress) will help to understand the exact meaning of the age difference and to further constrain the time of HP metamorphism in the study area.

Textural evidence indicates that epidote amphibolites represent completely overprinted HP rocks. Understanding the geological significance of white mica dates of multiply metamorphosed rocks is often not free of ambiguity, due to incomplete resetting or mixing of different growth generations. To avoid such complexities, the age of the overprint is best determined by dating of texturally well-constrained, newly formed phases. Phengite and omphacite of eclogite sample 5611 from an outcrop near Sulabest yielded an 85.7 ± 0.7 Ma age for the HP stage. In the same sample, the lower pressure overprint is recorded by the occurrence of epidote, biotite and plagioclase that formed as breakdown products of garnet and omphacite ± phengite. Rb–Sr dating of the retrograde mineral assemblage provided an apparent age of 78.3 ± 0.8 Ma, indicating the time of cooling below ca. 300–350 °C.

The new metamorphic ages for HP rocks from the Ratuk Complex do not provide support for previously published Ar–Ar data. The results obtained so far underline the necessity to apply several geochronometers for unravelling the age information of HP rocks.

[1] Fotoohi Rad et al. (2005), Lithos 84: 1–24; [2 ] Fotoohi Rad et al. (2009), Geological Journal 44: 104–116; [3] Tirrul et al. (1982), Geological Society of America Bulletin 94: 134–150. [4] Babazadeh & De Wever (2004), Geodiversitas 26: 185–206.