Geochronology and geochemistry of subducted Cadomian continental basement in central Iran: Decompressional anatexis along the Jurassic Neotethys margin
Abstract
Late Neoproterozoic-Early Cambrian calc-alkaline granitoids are ubiquitous in the continental basement of Iran and indicate formation within a Cadomian arc system at the northern margin of Gondwana. A basement complex comprising mainly mica schist, paragneisses, and metagranite along with metabasite and rare pegmatite is exposed in the Zayanderud region north of Shahrekord located in the hinterland of the Zagros mountain range. This complex is unique in the Neotethyan realm because it includes eclogites with Jurassic metamorphic ages implying involvement of continental crust at the onset of subduction. Ion microprobe U
Pb zircon dating along with trace element and oxygen isotope analyses for metagranites define two zircon age clusters of ca. 552 and 565 Ma confirming connection with the other Ediacaran age basement arc plutons in the belt. Zircon geochronology for pegmatite, by contrast, yielded a concordant age population averaging 176.5 ± 3.3 (2σ) Ma. Zircon crystals from the pegmatite also have unusually low rare earth element (REE) abundances with sharp increases towards the heavy REE. Along with an absence of a negative Eu anomaly, this indicates a high-grade metamorphic origin of zircon crystallizing from a pegmatite which was formed by melting of mica schist and possibly amphibole eclogite during decompression where incipient garnet breakdown released Zr and HREE to form zircon, and LREE were retained in stable apatite and titanite. Corresponding 40Ar/39Ar phengite dates from the pegmatite and the mica schist country-rock are overlapping with or only slightly postdate the UPb zircon ages, indicating rapid cooling after reaching maximum metamorphic pressure in the Early Jurassic. The Zayanderud basement complex is thus potentially a rare example of deep burial of continental crust and rapid exhumation due to buoyant escape during the incipient stages of subduction, well before the ultimate closing of the Neotethys ocean basin between Arabia and Eurasia in the mid-Tertiary.
Introduction
The increasing availability of UPb zircon age data for Iran and the neighboring terranes now reveals that the late Neoproterozoic was an episode of immense continental growth in the region by addition of magma from the mantle during the final stages of Gondwana supercontinent assembly (Condie, 2014). This major crust formation event is linked to extensive and complex magmatic arc development which led to intrusion of Ediacaran-Early Cambrian (ca. 635 Ma to 520 Ma) calc-alkaline granitoids all along the northern periphery of Gondwana (Stern, 1994). Recent UPb zircon geochronology and compositional data for granitoids from most structural zones of the Iranian basement have confirmed their origin in Cadomian arc systems which are also observed in various terranes from southern Europe to Turkey and farther east (Stern, 1994; Hassanzadeh et al., 2008; Moghadam et al., 2016). Late Paleozoic rifting and continental rupture progressing to a passive continental margin was followed by closure of the resulting Neotethys ocean during mid-Tertiary times (Hassanzadeh and Wernicke, 2016 and references therein; Pirouz et al., 2017). The subsequent collision between Arabian and Eurasian continental plates exposed high-grade metamorphic rocks in the Zayanderud metamorphic complex (ZMC) located in the hinterland of the Zagros mountain chain, which are unique in providing excellent opportunities for studying the transition from a passive margin to an Andean-type subduction zone. Two major uncertainties related to the ZMC are the ages of protolith formation and the timing of peak metamorphism. Protolith ages have been presumed to be Precambrian based on inferred cross-cutting by late Neoproterozoic metagranites (Malek-Mahmoudi et al., 2017). Therefore, determination of the magmatic age of the metagranites is key for constraining the unknown timing of protolith formation. The existing age data of these metagranites is highly variable from Ediacaran (Nutman et al., 2014; Davoudian et al., 2016) to Middle Jurassic (Hosseini and Ahmadi, 2016), indicating multiphase plutonism in the region. More recently, however, the veracity of Jurassic dates has been challenged by attributing middle Cambrian magmatic ages to the metagranites (Badr et al., 2018). 40Ar/39Ar phengite dates from different lithologies in the ZMC mostly indicate Middle Jurassic cooling, but there is ambiguity regarding the timing of deep burial of ZMC eclogites and their subsequent thermal history. To resolve these unsettled ambiguities regarding the Jurassic evolution of the ZMC, we present 40Ar/39Ar phengite dates for a newly discovered anatectic pegmatite and its host mica schist along with UPb ages, trace element, and oxygen isotopic compositions for zircon from the anatectic pegmatite. Jurassic zircon ages for the anatectic pegmatite indicate that peak metamorphism postdates eclogite formation, which suggests that a passive continental margin assemblage of metasediments and local metabasites in the ZMC experienced rapid burial, metamorphism, and exhumation during subduction initiation of the Neotethys Ocean.
Section snippets
Tectonic setting
The NW-trending Sanandaj-Sirjan Zone (SSZ) is situated to the northeast of the Main Zagros Thrust (MZT) in Iran (Fig. 1). The pre-Permian substrate of the SSZ is analogous to Arabia-Nubia (Berberian and King, 1981) and includes calc-alkaline granitoid plutons with crystallization ages of ca. 600–500 Ma (e.g., Ramezani and Tucker, 2003). Those plutons developed along a continental arc along the northern Gondwana active margin during the late Neoproterozoic Cadomian orogeny (Hassanzadeh et al.,
Geology of the studied area
The studied basement block crops out along the Upper Zayanderud River, ~50 km north of the MZT (Fig. 1, Fig. 2). Outcrops occur around the Zayanderud dam lake, near the town of Chadegan, and extend downstream from the dam for several tens of km (Fig. 1, Fig. 2). Covering an area of ~800 km2, the ZMC consists of preferentially erodible schistose rocks and is largely concealed underneath Neogene lacustrine sediments. Consequently, the ZMC rocks are best exposed along the walls of the deeply
Samples and analyses
Four ZMC metagranites, two anatectic pegmatites, and their host mica schist have been selected for compositional and geochronological analyses. Locations and petrographic summaries of the studied samples are presented in Fig. 2 and Supplementary Table S1. For UPb zircon age determinations, three protomylonitic, one weakly- to non-porphyroclastic metagranite, and one sample from an anatectic pegmatite pocket were selected from different locations in the ZMC (Fig. 3). Zircon crystals from two
Whole rock chemistry
Whole rock powders of three least altered protomylonitic to mylonitic granitoids were characterized by major and trace element analyses using the ICP-AES VARIAN Vista MPX instrument at University of Potsdam, Germany. Data for major and trace element abundances are presented in Supplementary Table S2.
Microprobe analysis
White mica from pegmatite CH14–2 and its host mica schist CH14–3 were analyzed by electron microprobe using the five spectrometer JEOL JXA-8200 at University of Potsdam operating at 15 kV and 15 nA
Granitoid geochemistry
Silica and alkali oxide compositions of the studied rocks range from 70.4–71.4 wt% (SiO2), 2.7–3.9 wt% (Na2O), and 2.4–4.8 wt% K2O (Supplementary Table S2). The samples plot in the granite field, near the granodiorite border in a total alkali vs. silica (TAS) diagram and show subalkaline affinity (Fig. 5A). Their magmatic arc character is pronounced as shown in the Rb vs (Y + Nb) discrimination diagram of Pearce et al. (1984) (Fig. 5B). There is some overlap of the data with the “Within plate
Magmatic age of basement granitoids
The first geological map of the region attributed a Precambrian age to the metamorphic complex including its metagranites based on identifying a nonconformable contact between the Permian sediments and the metamorphic rocks (Zahedi, 1976). In a subsequent revision, a Jurassic age was proposed for the granitic protoliths based on an erroneous Jurassic age attributed to the protoliths of the metamorphic host rocks for these granites (Ghasemi et al., 1996). This age dispute has continued even
Conclusions
UPb zircon geochronology resolves persistent controversies regarding the protolith age of the voluminous metagranite association in central Iran and sheds light on a previously undetected Early Jurassic partial melting event in the ZMC basement-related metamorphic-plutonic complex of the Zagros hinterland north of Shahrekord. The dominant zircon population yields an age of 567.5 ± 5.5 Ma for all protomylonitic and mylonitic granitoids. This confirms the presence of Ediacaran granitic intrusions
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This article is based largely on the lead author's dissertation at the Shahid Beheshti University (SBU), Tehran, Iran. Comments on an earlier version by Janet Harvey improved the English. The quality of the presentation was greatly improved by constructive reviews from Bob Stern, Jean-Pierre Burg and two anonymous reviewers, and editorial guidance from Taras Gerya. Discussions with Brian Wernicke on involvement of passive margin in subduction initiation have been helpful. Support from the SBU,
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