Metamorphic stages in mountain belts during a Wilson cycle: A case study in the central Sanandaj–Sirjan zone (Zagros Mountains, Iran)

Farzaneh Shakerardakani, Franz Neubauer, Manfred Bernroider, Fritz Finger, Christoph Hauzenberger, Johann Genser, Michael Waitzinger, Behzad Monfaredi
2021 Geoscience Frontiers  
22 Polymetamorphic units are important constituents of continent-continent collisional orogens, 23 and rift metamorphic assemblages are often overprinted by subsequent metamorphism during 24 subduction and collision. This study reports the metamorphic conditions and evolution of the 25 Dorud-Azna metamorphic units in the central part of the Sanandaj-Sirjan zone (SSZ), Iran. 26 Here, new geothermobarometry results are integrated with 40 Ar/ 39 Ar mineral and Th-U-Pb 27 monazite and thorite ages
more » ... o provide new insight of polyphase metamorphism in the two 28 different basement units of the SSZ, the lower Galeh-Doz orthogneiss and higher Amphibolite-29 Metagabbro units. In the Amphibolite-Metagabbro unit, staurolite micaschist underwent a 30 prograde P-T evolution from 640 ± 20 °C/6.2 ± 0.8 kbar in garnet cores (M1) to 680 ± 20 °C/7.2 31 ± 1.0 kbar in garnet rims (M2). Three Th-U-Pb monazite ages of 306  5 Ma, 322  28 Ma 32 and 336  39 Ma from the garnet-micaschists testify the Carboniferous age of M1 33 metamorphism. In the same unit, the metagabbro records P-T conditions of 4.0 ± 0.8 kbar and 34 580 ± 50 °C in the (magmatic) amphibole core (Late Carboniferous intrusion) to 7.5 ± 0.7 kbar 35 and 700 ± 20 °C in the amphibole rim indicating a prograde P-T path during subsequent burial 36 (M1). New 40 Ar/ 39 Ar dating of white mica from the staurolite micaschist yielded a staircase 37 pattern ranging from 36 ± 12 Ma to 170 ± 2 Ma. This implies polymetamorphism with a 38 minimum Late Jurassic cooling age through the Ar retention temperature of ca. 425 ± 25 °C 39 after M2 metamorphism and a Paleogene low-grade metamorphic overprint (M3), while 40 40 Ar/ 39 Ar white mica dating of garnet micaschist yielded a plateau age of 137.84 ± 0.65 Ma. 41 We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and 42 is likely between 180 and 200 Ma. Furthermore, it is overprinted at about 36 Ma under 43 retrogressive low-grade M3 metamorphism (at temperatures of ~ 350-240 °C) during final 44 shortening and exhumation. In the underlying Galeh-Doz unit, the Panafrican granitic 45 orthogneiss intruded at P-T conditions of 3.2 ± 4 kbar and 700 ± 20 °C, then it was 46 3 metamorphosed and deformed at 600 ± 50 °C and 2.0 ± 0.8 kbar (metamorphic stage M1) prior 47 to Late Carboniferous intrusion of mafic dykes. 40 Ar/ 39 Ar dating of amphibole from the Galeh-48 Doz orthogneiss gave plateau-like steps between 260 and 270 Ma, representing the age of 49 cooling through ca. 500 °C after the M1 metamorphic event. Interestingly, the results of this 50 study demonstrate polyphase metamorphic histories in both the Galeh-Doz orthogneiss and 51 Amphibolite-Metagabbro units at different P-T conditions and final thick-skinned Paleogene 52 emplacement of these units over the underlying low-grade metamorphic June Complex. Our 53 findings suggest that both units are affected by high-T/low-P Late Carboniferous orogenic 54 metamorphism along with the bimodal magmatism, as result of rifting. We propose that the 55 Early Jurassic amphibolite-grade M2 metamorphism of the SSZ is correlated with the initia l 56 subduction of the Neotethyan Ocean. Eventually, the investigated units reflect various stages 57 of a Wilson cycle, from rifting to initiation of the subduction in final plate collision. 58 59 62 63 65 high-pressure metamorphic relics are overprinted by collision-related Barrovian-type 66 metamorphism (e.g., Bousquet et al., 2008; Smye et al., 2011). Major metamorphic zones in 67 mountain belts result from several geodynamic processes such as the subduction and obduction 68 of oceanic lithosphere (e.g., Monié and Agard, 2009), subduction and subsequent exhumatio n 69 of the lower plate passive continental margin during collision and by collisional re-equilibra tio n 70 processes, e.g. by slab delamination or slab break-off (e.g., von Blanckenburg and Davis, 1995; 71 4 Mahéo et al., 2002; Keskin, 2003; Ustaszewski et al., 2010; Agard et al., 2011; Vissers et al., 72 2015). These metamorphic zones exposed in the interior of the orogen are exhumed by a number 73 of late-stage tectonic processes including horizontal and/or vertical extrusion, channel flow and 74 thrusting over marginal lower grade metamorphic or unmetamorphic rock successions (e.g., 75 Monié and Agard, 2009). A good example for all these processes is represented by the 76 Austroalpine unit of European Eastern Alps with Cretaceous-aged eclogite facies 77 metamorphism (including the type locality of eclogites, Haüy, 1822; Miller, 1990), which is 78 overprinted by retrogressive amphibolite facies metamorphism (e.g., Thöni and Jagoutz, 1992; 79 Janák et al., 2015) putting the question on the nature of heat sources for this specific case as 80 well as in general for units with a similar metamorphic evolution (Stüwe, 1998, Willingsho fer 81 et al., 1999). Detailed investigations revealed an even older stage of metamorphism, namely 82 rift-related temperature-dominated metamorphism due to lithospheric extension at the 83 beginning of the Wilson-cycle (Gaidies et al., 2008; Schuster and Stüwe, 2008; Thöni and 84 Miller, 2009). Basement metamorphic rocks from old orogens experienced polymetamorp his m 85 in subduction-to-collision orogens and young active margins. In the case of the Austroalp ine 86 unit of Eastern Alps, this older metamorphism is mostly Variscan (Neubauer et al., 1999). 87 Consequently, for understanding the tectonothermal evolution of orogens, the differe nt 88 metamorphic stages must be recognized, and one of the most useful methods to determine the 89 P-T-t history of metamorphic terranes is petrochronological investigations (e.g., Kohn, 2016; 90 Engi et al., 2017). Sirjan zone (SSZ) extends over 1500 km, with a width of 150-200 km, is located at the 95 northeastern margin of the Zagros fold-thrust belt and contains the metamorphic core of the 96 Zagros mountain belt (Fig. 1). The SSZ is a polyphase metamorphic terrane that experienced 97 several episodes of poorly dated deformation and associated metamorphism between Early 98 Jurassic and early Late Cretaceous. The basement comprises of Panafrican granites and is 99 mainly affected by magmatism during Middle-Late Jurassic times and Early Jurassic high-100 pressure metamorphism (Mohajjel and Fergusson, 2000; Mohajjel et al., 2003; Hassanzadeh et 101 al., 2008; Davoudian et al., 2016; Fergusson et al., 2016; Hassanzadeh and Wernicke, 2016). 102 The metamorphic grade within the SSZ ranges from greenschist to amphibolite facies 103 conditions. In one location an eclogite crops out that is Jurassic in age (Davoudian et al., 2016). 104 However, the tectono-metamorphic evolution and the ultimate driver of metamorphism remain 105 enigmatic, and despite extensive previous geochronological work, the age relationships across 106 the SSZ are still poorly resolved. Recent observations show polyphase Mesozoic 107 metamorphism in Permian to Early Mesozoic cover successions (Monfaredi et al., 2020).
doi:10.1016/j.gsf.2021.101272 fatcat:yaupd5ikcjbhxni6o4p34hy4ge