Origin of the pleonaste-bearing mafic–ultramafic rocks from the Armutlu peninsula, NW Turkey

Al-rich spinels were rarely reported compared to Cr-spinels, which were mostly observed in ophiolitic rocks. Pleonaste [(Mg, Fe2+)Al2 O4 ], which is an Al-rich spinel, was observed in the ophiolitic mafic–ultramafic rocks tectonically located in the Early Cretaceous accretionary complex, at the eastern part of the Armutlu peninsula, NW Turkey.

Turkish Journal of Earth Sciences http://journals.tubitak.gov.tr/earth/ Research Article Turkish J Earth Sci (2018) 27: 167-190 © TÜBİTAK doi:10.3906/yer-1711-13 Origin of the pleonaste-bearing mafic–ultramafic rocks from the Armutlu peninsula, NW Turkey Mutlu ƯZKAN, Ưmer Faruk ÇELİK* Department of Geological Engineering, Faculty of Engineering, Kocaeli University, İzmit, Kocaeli, Turkey Received: 14.11.2017 Accepted/Published Online: 20.03.2018 Final Version: 17.05.2018 Abstract: Al-rich spinels were rarely reported compared to Cr-spinels, which were mostly observed in ophiolitic rocks Pleonaste [(Mg, Fe2+)Al2O4], which is an Al-rich spinel, was observed in the ophiolitic mafic–ultramafic rocks tectonically located in the Early Cretaceous accretionary complex, at the eastern part of the Armutlu peninsula, NW Turkey The ophiolitic mafic–ultramafic rocks have cumulate character, and most of them are represented by peridotite and pyroxenite Pleonaste was observed in pyroxenites and gabbros of the ophiolitic rocks Pyroxenites consist mainly of clinopyroxene + orthopyroxene + amphibole ± olivine + spinel Gabbros are composed of clinopyroxene + orthopyroxene + amphibole + plagioclase + spinel Pleonaste in these rocks lies parallel to the magmatic layers and is distinguished by its emerald greenish color under the microscope Pleonastes have high Al2O3 (59.65–62.24 wt.%) and low Cr2O3 (0.05–1.32 wt.%) contents with Mg# and Fe3+# ranging from 54.23 to 59.77 and 3.83 to 4.28, respectively Petrographical observations and the pressure–temperature (P-T) pseudosection modelling suggest that pleonaste in the mafic–ultramafic rocks from the study area crystallized during magmatic processes Presence of amphibole and Ca-rich (An % 85–88) plagioclase in these rocks suggests that the ophiolitic rocks, located in the Early Cretaceous accretionary complex at the eastern part of Armutlu peninsula, formed from an arcrelated hydrous magma source Key words: Cumulate, ophiolite, pseudosection models, pyroxenite, spinel Introduction Spinel-group minerals (general formula: AB2O4) are important geological tools to understand the petrogenetic properties and geodynamic environment of the rocks in which they occur in a wide compositional range (e.g., Dick and Bullen, 1984; Arai et al., 2011) Spinel (MgAl2O4) and hercynite (Fe2+Al2O4) solid-solution series are the aluminum-rich (Al-rich) two end-members of spinelgroup minerals They rarely occur as pure end-members in nature and if the substitution ratio of divalent Mg2+ and Fe2+ in these end-members is from to 1, the mineral is called pleonaste [(Mg0.25–0.75Fe0.25–075)Al2O4] (Deer et al., 1992) In the present study Al-rich spinel is used as a general name for spinel, hercynite, and pleonaste The Al-rich spinel solid solution series have been observed in the amphibolite–granulite facies basic–ultrabasic and pelitic rocks, the basic–ultrabasic magmatic rocks, and the mantle xenoliths in volcanic rocks (e.g., Evans and Frost, 1975; Babu et al., 1997; Ho et al., 2000; Bucher and Frey, 2002; Topuz et al., 2004; Amortegui et al., 2011; Daczko et al., 2012; Rodriguez et al., 2012; Gargiulo et al., 2013) Diverse origins, such as magmatic, metamorphic, and * Correspondence: celikfrk@gmail.com metasomatic, have been proposed for the Al-rich spinel occurrences in these rocks (e.g., Evans and Frost, 1975; Della-Pasqua et al., 1995; Claeson, 1998; Franz and Wirth, 2000; Berger et al., 2010; Daczko et al., 2012; Gargiulo et al., 2013) On the other hand, spinel-group minerals in the ophiolite-related mafic–ultramafic rocks are commonly represented by chromite, magnesium chromite, and chrome-spinel (e.g., Dick and Bullen, 1984; Arai, 1994) These types of spinels have commonly high Cr2O3 (>10 wt.%) and low to moderate Al2O3 ( 10 and were observed in mantle and cumulate rocks of many well-known ophiolites (e.g., Kızıldağ, Eldivan, Refahiye, Troodos, and Semail ophiolites) (e.g., Uysal et al., 2012; Topuz et al., 2013; Chen et al., 2015; Rollinson and Adetunji, 2015; Çelik et al., 2018) Consequently, our investigated spinel-group minerals, which are Al-rich (~60 wt.%) and Cr-poor (1 cm) pyroxenites Cumulate nature of the ultramafic rocks in the field is defined by the alternation of fine-grained and black-colored peridotites with relatively coarse-grained and green-colored pyroxenites (Figure 3b) Cr-spinels in dunites and Al-rich spinel (pleonaste) in 169 ƯZKAN and ÇELİK / Turkish J Earth Sci Figure (a) Field view of pyroxenites and gabbros showing magmatic rhythmic layering (b) Close up view of peridotite and pyroxenite showing rhythmic layering (c) Cr-spinels in dunite (d) Pleonaste along the magmatic layers of pyroxenite (e) Close view of the magmatic layers of gabbro and pyroxenite 170 ÖZKAN and ÇELİK / Turkish J Earth Sci pyroxenites lie parallel to the magmatic banding (Figures 3c and 3d) Plagioclase and/or pyroxene-rich parts in the mafic and ultramafic cumulates, with thicknesses varying from a few millimeters to three meters, can be observed in some locations (Figures 3a and 3e) Depending on the increase in plagioclase, the rock passes from wehrlite and pyroxenite to gabbros (Figures 3a and 3e) The gabbro layers together with pyroxenites often show rhythmic alternation Mineralogy and petrography Ultramafic cumulate rocks are defined as dunite, wehrlite, websterite, and pyroxenite based on petrographical observations Wehrlites, showing generally adcumulate and mesocumulate textures, consist mainly of olivine + clinopyroxene + orthopyroxene ± spinel ± magnetite minerals (Figure 4a) Serpentine, talc, chlorite, and calcite were observed as secondary minerals Olivine in serpentinized dunite and wehrlite is found as rounded crystals (Figure 4a) Spinel-group minerals show colors of dark brown to light brown in dunites and of light brown or greenish brown in wehrlites (Figure 4a) Pyroxenites consist mainly of clinopyroxene ± orthopyroxene ± olivine ± amphibole ± plagioclase ± spinel ± magnetite minerals They are commonly coarse-grained and show orthocumulate and adcumulate textures (Figures 4b–4e) In some pyroxenites (e.g., M-041) amphibole, plagioclase, and opaque minerals, which make up about 2% of the mineral assemblage, lie parallel to magmatic banding Olivine in the pyroxenites occurred both as intercumulus minerals among pyroxenes and as inclusions in clinopyroxenes (Figures 4b and 4c) Clinopyroxene inclusions and exsolution lamellas are seen in orthopyroxenes of the coarse-grained websterites (e.g., M-043 and M-045) Amphibole (~10 modal %) showing brown to light brown pleochroism is observed in some websterites (Figure 4d) In some pyroxenites (e.g., M-113), amphibole abundance reaches up to 15 modal % The amphiboles in the pyroxenites extend discontinuously but parallel to the magmatic layering Cumulate gabbros show orthocumulate, adcumulate, mesocumulate, and heteradcumulate textures They consist mainly of plagioclase ± clinopyroxene ± orthopyroxene ± amphibole ± spinel ± magnetite ± ilmenite (Figures 4f–4h) Although gabbros are commonly fresh, some of them show alteration with secondary minerals, such as epidote, chlorite, and clay (Figure 4g) Amphibole is rarely observed in gabbros, where it is coarse grained (Figure 4h) Spinel minerals with pleonaste composition are evident by green to dark green (emerald green) colors (Figures 5a–5d) Pleonaste is usually observed as skeletonshaped xenomorphic crystals with dimensions up to mm and extends parallel to the magmatic layering (Figure 5b) While the modal abundance of pleonaste is 20% in the clinopyroxenites, it is about 1% or 2% in gabbros and olivine bearing clinopyroxenites Pleonaste is mostly observed in between clinopyroxene and amphibole minerals and it contains clinopyroxene inclusions (Figures 5c and 5d) This indicates that the pleonastes represent the postcumulus phase and crystallize after the formation of clinopyroxene Analytical methods Mineral compositions were determined at the IGG-CNR Padova (Italy) on a Cameca SX50 electron microprobe (EPMA) using ZAF online data reduction and matrix correction procedures The beam current for analyzing amphibole and plagioclase was set at 15 nA, whereas pyroxenes and oxides were analyzed with a beam current of 20 nA The accelerating voltage was 15 kV in all cases Repeated analyses of standards indicate relative analytical uncertainties of about 1% for major and 5% for minor elements Mafic and ultramafic rocks were analyzed for major and trace elements by XRF and ICP-MS (PerkinElmer Elan DRC-e model), respectively, at the Kocaeli University Analytical Geochemistry Laboratory in Kocaeli, Turkey Major oxides were measured on pressed powder pellets, using a SKAYRAY EDX3600B model XRF spectrometer About 0.2 g of rock powder was fused with 1.4 g of LiBO2 and dissolved in 50 mL of 5% HNO3 for trace element and rare earth element (REE) analyses Loss on ignition (LOI) was determined by heating a separate aliquot of rock powder at 900 °C for ˃2 h Mineral chemistry Electron microprobe analyses were performed on olivine, orthopyroxene clinopyroxene, spinel-group minerals, amphibole and plagioclase of wehrlite (M-101), websterite (M-105), clinopyroxenite (M-113), and gabbro (M-006) The light brown spinels in wehrlites are Cr-spinel in composition and their Cr# and Mg# (100 × Mg/[Mg + Fe2+]) range from 8.87 to 22.23 and from 62.43 to 75.69, respectively Cr-spinels have low TiO2 (1.50 (Na+K)A0.50 iv Al>Fe 3+ Si 5.5 Si 6.5 pargasite Mg/(Mg+Fe2+) 1.0 gabbro (M-006) websterite (M-105) clinopyroxenite (M-113) ferropargasite Diagram parameter CaB>1.50 (Na+K)A>0.50 iv Al