Iranshahr blueschist: subduction of the inner Makran oceanic crust

Abstract

The Makran accretionary prism in SE Iran and SW Pakistan is one of the most extensive subduction accretions on Earth. It is characterized by intense folding, thrust faulting and dislocation of the Cenozoic units that consist of sedimentary, igneous and metamorphic rocks. Rock units forming the northern Makran ophiolites are amalgamated as a mélange. Metamorphic rocks, including greenschist, amphibolite and blueschist, resulted from metamorphism of mafic rocks and serpentinites. In spite of the geodynamic significance of blueschist in this area, it has been rarely studied. Peak metamorphic phases of the northern Makran mafic blueschist in the Iranshahr area are glaucophane, phengite, quartz±omphacite+epidote. Post peak minerals are chlorite, albite and calcic amphibole. Blueschist facies metasedimentary rocks contain garnet, phengite, albite and epidote in the matrix and as inclusions in glaucophane. The calculated P–T pseudosection for a representative metabasic glaucophane schist yields peak pressure and temperature of 11.5–15 kbar at 400–510 °C. These rocks experienced retrograde metamorphism from blueschist to greenschist facies (350–450 °C and 7–8 kbar) during exhumation. A back arc basin was formed due to northward subduction of Neotethys under Eurasia (Lut block). Exhumation of the high‐pressure metamorphic rocks in northern Makran occurred contemporarily with subduction. Several reverse faults played an important role in exhumation of the ophiolitic and HP‐LT rocks. The presence of serpentinite shows the possible role of a serpentinite diapir for exhumation of the blueschist. A tectonic model is proposed here for metamorphism and exhumation of oceanic crust and accretionary sedimentary rocks of the Makran area. Vast accretion of subducted materials caused southward migration of the shore.

Formation and preservation of fresh lawsonite: Geothermobarometry of the North Makran Blueschists, southeast Iran

“…Finally, stage 4 records significant exhumation and cooling to reach an intermediate metamorphic gradient (~8°C/km). This late history is comparable, albeit a bit cooler, to that of another blueschist occurrence to the east of the study area, where glaucophane+garnet+epidote+phengite-bearing metabasic rocks have been overprinted by greenschist facies minerals (Omrani, Moazzen, Oberh€ ansli, & Moslempour, 2017).…”

Early Cretaceous Plume–Ridge Interaction Recorded in the Band-e-Zeyarat Ophiolite (North Makran, Iran): New Constraints from Petrological, Mineral Chemistry, and Geochronological Data

“…The Deyader Metamorphic Complex is exclusively cropped out in the eastern sector of the North Makran domain (Figure 1c). It is made up by meta-limestones, metavolcanics, and meta-gabbros affected by Late Cretaceous high pressure-low temperature (HP-LT) metamorphism [15,47,57,58].…”

Deciphering the metamorphic evolution of the Pulo do Lobo metasedimentary domain (SW Iberian Variscides)

“…According to this classical interpretation, features typical of modern subduction systems should be expected, such as high-pressure metamorphic gradient remnants of partial subduction-exhumation in an accretionary wedge (e.g., Platt, 1986;Ernst, 2005) or slices of oceanic slab-derived lithologies (varied mid-ocean ridge meta-igneous lithologies and also deep ocean bottom metasediments). Thus, recent works on the Makran accretionary prism (Omrani et al, 2017) and the subduction system of Japan (Endo and Wallis, 2017) describe an accretionary mélange complex composed of pelagic sedimentary rocks, ophiolites, greenschists, amphibolites, and blueschists with high-pressure minerals such as lawsonite and glaucophane. However, most of the geological data concerning the Pulo do Lobo domain do not support such an interpretation (see Sect.…”