Serpentinised peridotites from Sikhoran mafic-ultramafic complex (Southern Iran): Petrological and geochemical constraints on serpentinization processes and tectonic setting

Maghdour-Mashhour R., Heidari M., Esmaeily D.

Faculty of Geology, College of Science, University of Tehran, Iran

samer.mashour@gmail.com

The Sikhoran mafic-ultramafic complex (SMUC), located in south of Iran, mainly consists of two main lithological units: 1) foliated harzburgite and dunitic units; 2) magmatic ultramafic and gabbroic layered cumulates. Textural and chemical evolution of Serpentinites from tectonite harzburgite and dunites have been investigated to better understand the serpentinization processes and emplacement of this complex.

The alteration assemblage produced by serpentinization has been determined by XRD and petrographic studies: lizardite, chrysotile, antigorite, brucite, with subordinate amounts of syn-serpentinization magnetite, carbonates, chromium spinel and chlorite. Antigorite blades with flame texture form after the lizardite and brucite intergrowth and commonly grow into the relict olivine crystals. Textures are mainly mesh and hourglass and sometimes, interlocking, and penetrating; indicating a transition from pseudomorphic to non-pseudomorphic textures (classification of O’Hanley and Wicks 1995 [5]).

Kinking in clinochlores reflects a post-serpentinization deformation while, Undulose extinction, kinking, foliation, and deformation lamellae in bastites, olivine and platy serpentines indicate pre-serpentinization plastic flow of peridotite at high temperature, typical of the mantle rocks [7]. Plastic deformation and the formation of mylonites (composed of almost antigorite), on the other hand, might be caused in response to tectonic stresses at the Arabian-Central Iran plate boundary. Continued stress, diapirism at shallow depths and uplift distribute more brittle deformation, cutting earlier ductile structures. The presence of carbonate reflects the presence of a CO2-bearing fluid in the late stage of the serpentinization process in the Sikhoran.

The presence of chrysotile as cross-fiber vienlets cutting across the other serpentine minerals indicates a late genesis as a result of the activity of meteoric waters, activated by low-grade regional metamorphism. The hydrothermal fluids caused serpentinization and mobilized Fe and the other elements from the ferromagnesian minerals in the initial peridotite and precipitated magnetite on decrease of temperature. Thus, the magnetite veins, found in serpentinized harzburgite of the SMUC are considered to be epigenetic deposits formed along with the serpentinization process during and after the emplacement of the complex.

Sikhoran serpentine chemistry shows high and variable Mg #, from 0.81 to 0.83, and a highly restricted range of SiO2 (38.8-41.7 wt.%). They exhibit a marked depletion in TiO2 (< 0.01), K2O (0.01-0.1), Na2O (< 0.1), Al2O3 (0.1-1.3) and CaO (0.3-0.9 wt.%). They are very poor in Rb (0.2-0.5 ppm), Y (=10), Nb (<1) and to lesser extent in Sr (10–50), Cu(7-200) and Ba (10-30 ppm) as well . The serpentinites contain appreciable amounts of compatible trace elements; 1000–2000 ppm Cr, 1520–2270 ppm Ni and 15–42 ppm V, which are relatively immobile during alteration.

MgO/SiO2 (0.98–1.17) vs. Al2O3/SiO2 (0.0–0.03) ratios of the harzburgites along with depletion in CaO, Al2O3, SiO2 and enrichment of Cr and Ni are consistent with a depleted mantle harzburgite or dunite protolith.

Samples from Sikhoran serpentinites fall within Himalayan field of Mg/Si versus Al/Si diagram [3] which means that they are refractory peridotite residues after partial melting of mantle wedge with low Al/Si. Based on the whole rock Yb–V bivariate, the melt extraction from the primitive mantle is in excess of 15% up to 25%.

The analyzed serpentinites are plotted within the field of metamorphic peridotites in diagram of Coleman [1]. They exhibit low concentrations of Al2O3 (0.1–1.3 wt.%), similar to active margin peridotites, compared with those from other tectoic settings from Floyd [2]. Most of the analyses plot within the supra-subduction zone ophiolite field on the Cr versus TiO2 diagram of Pearce et al. [6]. The Sikhoran serpentinites fall within the oceanic array in MgO/SiO2 –Al2O3/SiO2 field of Niu [4] plot. Oceanic lithospheric peridotites originated in supra-subduction zone (SSZ) settings.

The serpentinite textures and mineralogy suggest that early low-temperature serpentinization at 50-400°C by hydration of peridotite (sub-greenschist facies) produced widespread lizardite±brucite pseudomorphic serpentinites. Late stage recrystallization of peridotite and hydration of peridotite at 250-600°C produced massive antigoritic serpentinites (greenschist facies) under higher pressure, most probably during their tectonic emplacement onto the continental margin. Later-stage serpentinization results in formation of magnetite, primarily from the breakdown of ferroan brucite. Furthermore they represent a fragment of oceanic lithosphere from a supra-subduction zone that has been formed in a fore-arc or back-arc environment, which were thrust over the continental margins during the collisional stage of Arabian plate and Central Iran. The ascent of slab-derived fluids could be due to dehydration reactions and sediment pore-water expelled from subducted sediments from Neotethyan oceanic lithosphere, resulting in the serpentinization of upper mantle wedge causing serpentinite diapirs. During diapirism, Sikhoran serpentinite rises vertically due to differences in density between the serpentinite and the overlying rocks

References:

1. Coleman, R.G., 1977. Ophiolites, Ancient Oceanic Lithosphere? Springer, Berlin.

2. Floyd, P.A., Kelling, G., Gocken, S.L. and Gocken N., 1991. Geochemistry and tectonic environment of basaltic rocks from the Miss ophiolitic, south Turkey. Chem. Geol., 89, 263-80.

3. Hattori, K.H., Guillot, S., 2007. Geochemical character of serpentinites associated with high- to ultrahighpressure metamorphic rocks in the Alps, Cuba, and the Himalayas: Recycling of elements in subduction zones. Geochem. Geophys. Geosys., 8(9), Q09010.

4. Niu, Y., 2004. Bulk-rock major and trace element compositions of abyssal peridotites: implications for mantle melting, melt extraction and post-melting processes beneath mid-ocean ridges. J. Petrol. 45, 2423–2458.

5. O’Hanley and Wicks, 1995, Conditions of formation of lizardite, chrysotile and antigorite, Cassiar, British Columbia. Can. Mineral., 33, 753-773

6. Pearce JA, Harris NBW, Tindle AG, 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrol., 69, 33–47

7. Suhr, G., 1993, Evaluation of upper mantle microstructures in the Table Mountain massif (Bay of Islands ophiolite): Journal of Structural Geology, 15, 1273-1292.

U-Pb age and whole rock geochemistry of Herris A-type granitoid, NW Iran

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