Geochemistry and petrogenesis of the Late Cretaceous Haji‐Abad ophiolite (Outer Zagros Ophiolite Belt, Iran): implications for geodynamics of the Bitlis–Zagros suture zone

Abstract

The Haji‐Abad ophiolite in SW Iran (Outer Zagros Ophiolite Belt) is a remnant of the Late Cretaceous supra‐subduction zone ophiolites along the Bitlis–Zagros suture zone of southern Tethys. These ophiolites are coeval in age with the Late Cretaceous peri‐Arabian ophiolite belt including the Troodos (Cyprus), Kizildag (Turkey), Baer‐Bassit (Syria) and Semail (Oman) in the eastern Mediterranean region, as well as other Late Cretaceous Zagros ophiolites. Mantle tectonites constitute the main lithology of the Haji‐Abad ophiolite and are mostly lherzolites, depleted harzburgite with widespread residual and foliated/discordant dunite lenses. Podiform chromitites are common and are typically enveloped by thin dunitic haloes. Harzburgitic spinels are geochemically characterized by low and/or high Cr number, showing tendency to plot both in depleted abyssal and fore‐arc peridotites fields. Lherzolites are less refractory with slightly higher bulk REE contents and characterized by 7–12% partial melting of a spinel lherzolitic source whereas depleted harzburgites have very low abundances of REE and represented by more than 17% partial melting. The Haji‐Abad ophiolite crustal sequences are characterized by ultramafic cumulates and volcanic rocks. The volcanic rocks comprise pillow lavas and massive lava flows with basaltic to more‐evolved dacitic composition. The geochemistry and petrology of the Haji‐Abad volcanic rocks show a magmatic progression from early‐erupted E‐MORB‐type pillow lavas to late‐stages boninitic lavas. The E‐MORB‐type lavas have LREE‐enriched patterns without (or with slight) depletion in Nb–Ta. Boninitic lavas are highly depleted in bulk REEs and are represented by strong LREE‐depleted patterns and Nb–Ta negative anomalies. Tonalitic and plagiogranitic intrusions of small size, with calc‐alkaline signature, are common in the ophiolite complex. The Late Cretaceous Tethyan ophiolites like those at the Troodos, eastern Mediterranean, Oman and Zagros show similar ages and geochemical signatures, suggesting widespread supra‐subduction zone magmatism in all Neotethyan ophiolites during the Late Cretaceous. The geochemical patterns of the Haji‐Abad ophiolites as well as those of other Late Cretaceous Tethyan ophiolites, reflect a fore‐arc tectonic setting for the generation of the magmatic rocks in the southern branch of Neotethys during the Late Cretaceous. Copyright © 2012 John Wiley & Sons, Ltd.

Structure and metamorphism of a subducted seamount (Zagros suture, Southern Iran)

“…Given its structural position below subducted units (oceanic and offscraped from the slab or as part of a forearc, i.e., the Seghin and Ashin units, respectively; Angiboust et al, 2016), it can be concluded that Siah Kuh was located outboard of Eurasia, as part of the lower plate with respect to the Eurasian subduction zone. The presence of boninites (Moghadam et al, 2013) suggests that the Siah Kuh seamount was also in a supra-subduction position with respect to a second, more southerly subduction (which is well documented by regional studies; e.g., Searle and Cox, 1999;Agard et al, 2006Agard et al, , 2007. Due to its structural position below the Ashin unit and the Seghin blueschists, the Siah Kuh unit most likely underwent subduction and metamorphism after them.…”

“…We consider hypothesis 3 to be the most likely, because isostatic equilibration of the lithosphere under the load of a seamount is very fast (<1 m.y., Campbell, 1986) and expected to be of the order of several kilometers (e.g., Hawaii, Watts and ten Brink, 1989). The boninites reported in Siah Kuh (Moghadam et al, 2013) also point to an intra-oceanic (and supra-subduction) setting.…”

“…The Siah Kuh unit has no characteristics of a mélange and is an almost continuous piece of oceanic lithosphere with serpentinite, gabbro, basalt, and associated hyaloclastite, pelagic, and platform sediments (Sabzehei, 1974), and it is likely a seamount (Angiboust et al, 2016;Bonnet et al, 2019). The report of boninitic lavas (Moghadam et al, 2013) suggests formation of Siah Kuh above an intra-oceanic subduction zone. Traces of incipient metamorphism have been recognized in the Siah Kuh unit, in particular with the occurrence of lawsonite and aragonite (Sabzehei, 1974;Angiboust et al, 2016).…”

“…The exposed thickness of the Siah Kuh unit is, however, much less than what is observed in the largest seamounts on Earth (e.g., Hawaii or the Canary islands), where the base lies at ~4-5 km depth and the summit reaches a few km above sea level. The Siah Kuh seamount was, however, not a mid-ocean seamount (with alkaline geochemical signatures) but more likely formed in a supra-subduction environment as an arc volcano (Moghadam et al, 2013). Due to upper-plate bending, the base of arc volcanoes is often situated at shallower depths, ~1.5-3 km (e.g., the Izu-Bonin-Mariana arc, somewhat shallower in the Tonga-Kermadec arc), which is more compatible with the top of the >1.5-km-thick Siah Kuh unit reaching or being close to the surface.…”

Geochemistry of platinum-group elements and mineral composition in chromitites and associated rocks from the Abdasht ultramafic complex, Kerman, Southeastern Iran

“…Boninites are common in nearly all of the Late Cretaceous Neotethyan ophiolites as well as the Zagros Inner and Outer Belt ophiolites. These rocks are essentially found in fore-arc environments such as in the IBM fore-arc region (Shafaii Moghadam et al, 2013). (Bridges et al, 1995).…”