In Central Iran, the mixed siliciclastic-carbonate Nakhlak Group of Triassic age is commonly seen to have a Cimmerian affinity, although it shows considerable resemblances with the Triassic Aghdarband Group in far northeastern Iran, east of Kopeh-Dagh area, with Eurasian affinity. The Nakhlak Group is composed of the Alam (Late Olenekian-Anisian), Baqoroq (Late Anisian-?Early Ladinian), and Ashin (Ladinian-?Early Carnian) formations consisting mainly of volcanoclastic sandstone and shale and fossiliferous limestone. The Baqoroq Formation contains also metamorphic detritus. Sandstone petrofacies reflect the detrital evolution from active volcanism to growing orogen and again active volcanism. Textural and modal analyses of volcanic lithic fragments from the Alam Formation reflect the eruption style and magma composition of a felsic to intermediate syn-sedimentary arc activity. The
The closure of Palaeotethys that led to the collision of the Cimmerian blocks with the southern Eurasian margin causing the Eo-Cimmerian orogeny during the Early Mesozoic is still controversially discussed. The Triassic Nakhlak Group in Central Iran is a key sedimentary succession for better understanding the closure of Palaeotethys and the Eo-Cimmerian orogeny in the Middle East. The Nakhlak Group is composed of the Alam (Olenekian to Middle Anisian), Baqoroq (?Upper Anisian to Middle Ladinian) and Ashin (Upper Ladinian to ?Carnian) formations, which consist mainly of volcaniclastic sandstones, mixed siliciclastic conglomerates, and marine carbonates. Here we present for the first time detrital zircon U-Pb ages from the Nakhlak Group to unravel its provenance and constrain its palaeotectonic position within the Palaeotethyan realm. Most detrital zircons from the Nakhlak Group are euhedral and subhedral with Permian-Triassic ages (ca. 280-240 Ma) suggesting sediment supply from Permian-Triassic magmatic rocks of the Silk Road Arc. Minor zircon populations show pre-Permian Palaeozoic ages, with age peaks at ca. 320 Ma and 480 Ma, which are probably derived from the basement on which the magmatic arc developed. Neoproterozoic-latest Mesoproterozoic (ca. 550-1100 Ma) and Palaeoproterozoic (ca. 1800-2200 Ma) zircon grains are anhedral (rounded). The latter are prominent in the upper Baqoroq Formation (Middle Ladinian) suggesting recycling of older sedimentary rocks. Sandstone petrography points toward an additional metamorphic provenance for this formation. This short-lived provenance change can be explained by tectonic uplift in the source area that led to erosion of metamorphosed rocks with a northeast Gondwanan affinity. It clearly indicates that northeast Gondwana-derived continental fragments likely belonging to the Cimmerian blocks already arrived at the southern Eurasian margin in pre-Late Triassic time. Current palaeotectonic models of the closure of Palaeotethys and the Eo-Cimmerian orogeny in the Middle East during the Triassic may need to be revised.
a b s t r a c tThe Ching-dar syncline is located to the west of the city of Birjand, in the east of Iran. The ca. 500 m thick studied section at the eastern flank of the syncline contains a sequence of almost continuous shallowmarine limestones that exhibit no major sedimentary breaks or evidence for volcanic activity. Skeletal grains consist of large benthic foraminifera and green algae whereas non-skeletal grains are mostly peloids and intraclasts. They were deposited on a shallow-marine carbonate ramp. The limestones have undergone extensive diagenetic processes with varying intensities, the most important of which are micritization, cementation, compaction (chemical and mechanical), internal filling and stylolitization. Chemical analysis of the limestone samples revealed high calcium and low magnesium content. Major and minor element values were used to determine the original carbonate mineralogy of these limestones. Petrographic evidence and elemental values indicate that calcite was the original carbonate mineral in the limestones of the Ching-dar syncline. The elemental composition of the Ching-dar carbonates also demonstrates that they have stabilized in a meteoric phreatic environment. Variation of Sr/ Ca vs. Mn values suggests that diagenetic alteration occurred in an open geochemical system.
Detrital chrome spinel (Cr-spinel) is for the first time described from the Nakhlak Group, which is a distinctive Triassic (Late Olenekian-?Early Carnian) sedimentary succession in Central Iran. Previously published data from the Nakhlak Group suggest deposition along an active margin during the Triassic and Eurasian affinity. Presently, the Nakhlak Group is located in the northwestern region of the Central-East Iranian Microcontinent, a major segment of the Cimmerian block. The Nakhlak region is believed to have been dislocated from its prior position at the Turan Plate from the southern Eurasian active margin after deposition in Triassic time. Almost all of the analysed detrital Cr-spinels have high Cr-numbers (0.6-0.85), variable Mg-numbers (0.33-0.78), and low Fe 3+ (<0.25), Al2O3 (<20 wt%) and TiO2 (<0.4 wt%) contents, suggesting a magmatic source for the Crspinels formed both within oceanic (mainly harzburgitic) mantle and in a supra-subduction zone (SSZ) tectonic setting. The data suggest that mafic-ultramafic rock assemblages with SSZ signatures were generated in the Paleo-Tethyan realm before their obduction as an ophiolite. By comparing data from the present study with results from previous work on the Triassic in Central Iran it is evident that Paleo-Tethyan ophiolites of intra-oceanic island-arc (IOIA) origin supplied detrital material to the Nakhlak Group at the southern margin of Eurasia during pre-to syn-Eocimmerian tectonics in the Triassic.
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