|
Discussion |
Department of Earth Sciences, University of Durham, Durham DH1 3LE, UK
*Corresponding author (e-mail: m.b.allen{at}durham.ac.uk)
Mazhari et al. (2009) provide valuable geochemical and geochronological data for plutonic rocks from the Piranshar massif. This is one of a set of plutons within the Sanandaj–Sirjan zone in Iran, SW of the Urumieh–Dokhtar magmatic zone. This comment is not about the new data of Mazhari et al. (2009), but refers to their interpretation that the c. 41 Ma (Middle Eocene) intrusions in the Piranshahr massif post-date the initial collision of the Arabian and Eurasian plates by 10–20 Ma. I argue that (1) these magmatic rocks are not reliable markers of a tectonic setting post-dating initial collision, and (2) there are abundant geological data that suggest initial collision was later, most plausibly in the Late Eocene (c. 35 Ma).
The main argument presented for a tectonic setting post-dating initial collision is the A2-type chemistry of the felsic rocks within the massif. By comparison with other A2-type granitoids reviewed by Eby (1990, 1992) it is concluded that the intrusions were emplaced some 10–20 Ma after initial collision (i.e. Palaeocene to Early Eocene, 50–60 Ma). Mazhari et al. (2009) also refer to papers that independently suggested a Palaeocene–Eocene age for initial collision (Numan 2001; Ghasemi & Talbot 2006). However, it is clear that A-type granitoids occur in different tectonic settings, and are not always post-collisional. Eby (1990) noted this in his original review, referring to granitoids intruded in suprasubduction-zone settings, and commenting that his classification was based on geochemical and mineralogical criteria (i.e. not tectonic settings). He later (Eby 1992) specified A2-types as ‘magmas derived from continental crust or underplated crust that has been through a cycle of continent–continent collision or island-arc magmatism'. This is not an unambiguous pointer to a post-initial collision setting. Many A-type granitoids have now been described from subduction-zone settings (i.e. before collision). Examples range from Neoproterozoic granitoids in South China (Zhao et al. 2008) to the Pliocene of Patagonia (Espinoza et al. 2008) and include the specific A2-type identified in the Piranshahr massif.
If the Piranshahr plutons post-date initial collision, it follows that the voluminous Eocene magmatism across SW Asia must share this tectonic setting. This is hard to believe. There are now several papers that describe the arc or back-arc chemistry of these rocks (e.g. Vincent et al. 2005; Omrani et al. 2008). All of these magmatic rocks are plausibly interpreted as generated in a suprasubduction-zone setting, during northerly subduction of Neo-Tethyan oceanic crust, and before the initial continental collision with Arabia.
It is notable how much of the Palaeogene magmatism and associated sedimentation across SW Asia is specifically Middle Eocene in age (Ramezani & Tucker 2003; Vincent et al. 2005; Verdel et al. 2007); this seems to have been the most productive time for magmatism across the active continental margin (McQuarrie et al. 2003), an age consistent with the new data of Mazhari et al. (2009) for the Piranshahr intrusions. As Mazhari et al. (2009) note, A2-type granitoids commonly occur in extensional settings. Such extension is inconsistent with the collisional setting they infer, however, given that there is no evidence for ‘orogenic collapse'. Many of the Middle Eocene volcanic rocks of Iran and adjacent countries are interbedded with marine limestones or turbidites, implying that extension took place within normal thickness or thinned crust.
If the Middle Eocene was a time of rapid back-arc extension predating collision, when did initial collision actually occur? Mazhari et al. (2009) draw attention to this long-running debate: in recent years age ranges have been published from the Late Cretaceous (Mohajjel & Ferguson 2000) to the Late Miocene (McQuarrie et al. 2003; Guest et al. 2006). Allen & Armstrong (2008) reviewed evidence from both sides of the original Arabia–Eurasia suture, and concluded that initial collision took place in the Late Eocene (c. 35 Ma). A critical part of the argument is a marked reduction in magmatism between the Eocene and the Oligocene, consistent with the end of subduction under the Eurasian plate. Other datasets include evidence for initial deformation, uplift and terrestrial sedimentation at about this time, from both sides of the suture (e.g. Hessami et al. 2001; Agard et al. 2005; Vincent et al. 2007).
Reliable determination of the timing of initial collision is not a parochial matter. The correct age provides a marker that gives context to all tectonic, magmatic and sedimentary events in the collision zone, which now covers an area of c. 3 x 106 km2. Given the roughly constant plate convergence since 56 Ma described by McQuarrie et al. (2003), the time of initial collision is a baseline for understanding how much plate convergence has since been accommodated in the continental lithosphere. There is an enormous implication for the tectonics, depending on whether the collision is 10 Ma old (which implies c. 220 km convergence at the longitude of central Iran), 35 Ma old (implies 770 km) or 55 Ma (implies >1000 km). Collision also reorganized palaeoceanography on a global scale, cutting the Tethyan connection between the Indian and Atlantic oceans, and so removing an intermediate-latitude ocean gateway. The timing of this event therefore has potential climatic implications on a global scale, with a possible connection to the pronounced cooling at the Eocene–Oligocene transition (Allen & Armstrong 2008).
Scientific editing by
Allan Collins
 |
References |
|---|
 Top References |
|---|
Agard, P., Omrani, J. Jolivet, L. & Mouthereau, F. 2005. Convergence history across Zagros (Iran): constraints from collisional and earlier deformation. International Journal of Earth Sciences, 94, 401–419.[CrossRef][Web of Science][GeoRef]
Allen, M.B. & Armstrong, H.A. 2008. Arabia–Eurasia collision and the forcing of mid-Cenozoic global cooling. Palaeogeography, Palaeoclimatology, Palaeoecology, 265, 52–58.[CrossRef][GeoRef]
Eby, G.N. 1990. The A-type granitoids—a review of their occurrence and chemical characteristics and speculations on their petrogenesis. Lithos, 26, 115–134.[CrossRef][Web of Science][GeoRef]
Eby, G.N. 1992. Chemical subdivision of the A-type granitoids—petrogenetic and tectonic implications. Geology, 20, 641–644.
Espinoza, F., Morata, D. Polve, M. et al. 2008. Bimodal back-arc alkaline magmatism after ridge subduction: Pliocene felsic rocks from Central Patagonia (47°S). Lithos, 101, 191–217.[CrossRef][Web of Science]
Ghasemi, A. & Talbot, C.J. 2006. A new tectonic scenario for the Sanandaj–Sirjan Zone (Iran). Journal of Asian Earth Sciences, 26, 683–693.[CrossRef][Web of Science][GeoRef]
Guest, B., Stockli, D.F. Grove, M. Axen, G.J. Lam, P.S. & Hassanzadeh, J. 2006. Thermal histories from the central Alborz Mountains, northern Iran: Implications for the spatial and temporal distribution of deformation in northern Iran. Geological Society of America Bulletin, 118, 1507–1521.[CrossRef][Web of Science]
Hessami, K., Koyi, H.A. Talbot, C.J. Tabasi, H. & Shabanian, E. 2001. Progressive unconformities within an evolving foreland fold–thrust belt, Zagros Mountains. Journal of the Geological Society, London, 158, 969–981.
Mazhari, S.A., Bea, F. Amini, S. et al. 2009. The Eocene bimodal Piranshahr massif of the Sanadaj–Sirjan Zone, West Iran: a marker of the end of collision in the Zagros orogen. Journal of the Geological Society, London, 166, 53–69.
McQuarrie, N., Stock, J.M. Verdel, C. & Wernicke, B. 2003. Cenozoic evolution of Neotethys and implications for the causes of plate motions. Geophysical Research Letters, 30.[CrossRef]
Mohajjel, M. & Fergusson, C.L. 2000. Dextral transpression in Late Cretaceous continental collision, Sanandaj–Sirjan Zone, western Iran. Journal of Structural Geology, 22, 1125–1139.[CrossRef][Web of Science][GeoRef]
Numan, N.M.S. 2001. Discussion on ‘Dextral transpression in Late Cretaceous continental collision, Sanandaj–Sirjan Zone, western Iran' [Journal of Structural Geology, 22(8) (2000), 1125–1139]. Journal of Structural Geology, 23, 2033–2034.[CrossRef][Web of Science][GeoRef]
Omrani, J., Agard, P. Whitechurch, H. Benoit, M. Prouteau, G. & Jolivet, L. 2008. Arc-magmatism and subduction history beneath the Zagros Mountains, Iran: A new report of adakites and geodynamic consequences. Lithos, 106, 380–398.[GeoRef]
Ramezani, J. & Tucker, R.D. 2003. The Saghand region, central Iran: U–Pb geochronology, petrogenesis and implications for Gondwana tectonics. American Journal of Science, 303, 622–665.
Verdel, C., Wernicke, B.P. Ramezani, J. Hassanzadeh, J. Renne, P.R. & Spell, T.L. 2007. Geology and thermochronology of Tertiary Cordilleran-style metamorphic core complexes in the Saghand region of central Iran. Geological Society of America Bulletin, 119, 961–977.
Vincent, S.J., Allen, M.B. Ismail-Zadeh, A.D. Flecker, R. Foland, K.A. & Simmons, M.D. 2005. Insights from the Talysh of Azerbaijan into the Paleogene evolution of the South Caspian region. Geological Society of America Bulletin, 117, 1513–1533.
Vincent, S.J., Morton, A.C. Carter, A. Gibbs, S. & Barabadze, T.G. 2007. Oligocene uplift of the Western Greater Caucasus: an effect of initial Arabia–Eurasia collision. Terra Nova, 19, 160–166.[CrossRef][Web of Science][GeoRef]
Zhao, X.F., Zhou, M.F. Li, J.W. & Wu, F.Y. 2008. Association of Neoproterozoic A- and I-type granites in South China: Implications for generation of A-type granites in a subduction-related environment. Chemical Geology, 257, 1–15.[CrossRef][Web of Science][GeoRef]
| ||||||||||||||||||||||||||||||||||||||||||||||
