RESEARCH Geochemistry and geochronology of the Troodos ophiolite: An SSZ ophiolite generated by subduction initiation and an extended episode of ridge subduction? Soichi Osozawa1,*, Ryuichi Shinjo2, Ching-Hua Lo3, Bor-ming Jahn3, Nguyen Hoang4, Minoru Sasaki5, Ken’ichi Ishikawa6, Harumasa Kano7, Hiroyuki Hoshi8, Costas Xenophontos9, and John Wakabayashi10 1DEPARTMENT OF EARTH SCIENCES, GRADUATE SCHOOL OF SCIENCE, TOHOKU UNIVERSITY, SENDAI 980-8578, JAPAN 2DEPARTMENT OF PHYSICS AND EARTH SCIENCES, UNIVERSITY OF THE RYUKYUS, NISHIHARA 903-0213, JAPAN 3DEPARTMENT OF GEOSCIENCES, NATIONAL TAIWAN UNIVERSITY, TAIPEI 10699, TAIWAN 4FORMER: GEOLOGICAL SURVEY OF JAPAN, ADVANCED INDUSTRIAL SCIENCE AND TECHNOLOGY (AIST), TSUKUBA 305-8567, JAPAN 5DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, GRADUATE SCHOOL SCIENCE AND TECHNOLOGY, HIROSAKI UNIVERSITY, HIROSAKI 036-8561, JAPAN 6CENTER FOR THE ADVANCEMENT OF HIGHER EDUCATION, TOHOKU UNIVERSITY, SENDAI 980-8576, JAPAN 7TOHOKU UNIVERSITY MUSEUM, SENDAI 980-8578, JAPAN 8DEPARTMENT OF EARTH SCIENCES, GRADUATE SCHOOL OF EDUCATION, AICHI UNIVERSITY OF EDUCATION, KARITA 448-8542, JAPAN 9FORMER: CYPRUS GEOLOGICAL SURVEY, NICOSIA 1415, CYPRUS 10DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, CALIFORNIA STATE UNIVERSITY, FRESNO, FRESNO, CALIFORNIA 93740, USA ABSTRACT New trace-element, radiogenic isotopic, and geochronologic data from the Troodos ophiolite, considered in concert with the large body of previously published data, give new insight into the tectonic history of this storied ophiolite, as well as demonstrating the variability of suprasubduction-zone ophiolites, and differences between them and commonly used modern analogs. Similar to earlier studies, we fi nd that island-arc tholeiite of the lower pillow lava sequence erupted fi rst, followed by boninite. We further divide boninitic rocks into boninite making up the upper pillow lava sequence, and depleted boninites that we consider late infi ll lavas. We obtained an Ar-Ar age from arc tholeiite of 90.6 ± 1.2 Ma, comparable to U-Pb ages from ophiolite plagiogranites. New biostratigraphic data indicate that most of the basal pelagic sedimentary rocks that conformably overlie the boninitic rocks are ca. 75 Ma. This suggests that voluminous eruption of boninitic rocks persisted until ca. 75 Ma. Limited eruption of boninitic lavas may have continued until 55.5 ± 0.9 Ma, based on the Ar-Ar age we obtained. The duration of arc magmatism at Troodos (at least 16 m.y., with some activity perhaps extending 35 m.y.) without the devel- opment of a mature arc edifi ce greatly exceeds that of other well-studied suprasubduction-zone ophiolites. We propose that Troodos was formed over a newly formed subduction zone, similar to many proposed models, but that the extended period of magmatism (boninitic) resulted from a prolonged period of ridge subduction. LITHOSPHERE; v. 4; no. 6; p. 497–510 | Published online 14 November 2012 doi: 10.1130/L205.1 INTRODUCTION zone magmatism in the generation of many erated, given that LILE enrichment is a funda- ophiolites (e.g., Shervais, 2001; Pearce, 2003; mental characteristic of arc lavas. Analysis of Research on the Troodos ophiolite of Cyprus Nicolas and Boudier, 2003; Pearce and Robin- fresh volcanic glass of pillow and dike margins, has strongly infl uenced the evolution of the son, 2010). however, can largely surmount this diffi culty, as ophiolite concept and ideas on the signifi cance The interpretation of suprasubduction-zone evidenced by the extensive major-element data of on-land sheets of oceanic lithosphere in oro- generation of many ophiolites derives primarily set of Pearce and Robinson (2010), and trace- genic belts (e.g., Moores and Vine, 1971; Dilek, from geochemical affi nities of ophiolite lavas element and isotopic analyses (e.g., Rauten- 2003; Robinson et al., 2003). Accordingly, pet- and dikes. Since Miyashiro (1975) initially schlein et al., 1985), which demonstrate an arc rologic and geochemical studies of the Troodos proposed an island-arc setting for the Troodos origin for the lavas. ophiolite have also fi gured prominently in the ophiolite generation based on its major-element Although previous studies have fi rmly estab- wide acceptance of the role of suprasubduction- compositions, many subsequent studies, cited in lished the suprasubduction-zone origin of the this paper, have identifi ed island-arc characteris- Troodos ophiolite, further exploration of the tics, based on minor- and trace-element compo- geochemistry and geochronology of the lavas sitions and isotopic ratios. offers additional insight into details of mag- *E-mail: [email protected]. A potential problem in some of the early matic evolution of the Troodos ophiolite and Editor’s note: This article is part of a special issue ti- geochemical studies is the mobility of large other suprasubduction-zone ophiolites as well tled “Initiation and Termination of Subduction: Rock Re- ion lithophile elements (LILEs) during altera- as providing a means by which to better com- cord, Geodynamic Models, Modern Plate Boundaries,” edited by John Shervais and John Wakabaya shi. The tion, which may result in enrichment of these pare the geochemistry of Troodos rocks to other full issue can be found at http://lithosphere.gsapubs elements in nonarc volcanic rocks, and an erro- ophiolites and modern seafl oor rocks (particu- .org/content/4/6.toc. neous interpretation of such rocks as arc gen- larly those in arc environments). LITHOSPHEREFor permission to| Volumecopy, contact 4 | Number [email protected] 6 | www.gsapubs.org | © 2012 Geological Society of America 497 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/6/497/3039274/497.pdf by guest on 25 September 2021 OSOZAWA ET AL. infill lava (depleted boninite) 106(th) 28(th) 15(bn) 103(bn) upper pillow lava sequence (boninite) dip and strike of lava 26(th) dip and strike of dike 104bn 21th 27th lower pillow lava sequence (tholeiite) Ar-Ar 3(bn) Vuoni sheeted dike complex Skouriotissa mine Akamas peninsula 5th gabbro 157(bn) CR Koronia Mitsero mine ultramafic rocks Akaki River sheeted dike complex KambiaAnaliondas Troulli Margi MR 18MR MC 35°N 44CRMR Troodos 99dbn MR 55(th) MR 111CR 88dbn MR Arakapas 125CR Ayios Nocolaos MR MR47MR Ayios Mamas MR N Kalavasos mine Pafos 144bn MR 139CR 137CR 152dbn 119dbn 132CR island of Cyprus Limassol Ar-Ar 138bn Troodos ophiolite 130bn Lefkara F. Nicosia 20 km R Late Campanian radiolarians 21th geochemical site C: chert, M: mudstone th: tholeiite, bn: boninite, dbn: depleted boninite (Perapedhi F.) 100 km 33°E (th) indicated by major elements only Figure 1. Geological map of the Troodos ophiolite, after Geological Survey Department Cyprus (1995) and Gass et al. (1994). There are 11 sample loca- tions, but 12 total samples (location 152 has f1 glass and f2 rock samples), with the sample numbers in the larger font size. Stars are sample localities of sediments (sample numbers in smaller font size), all of which contain late Campanian radiolarians. To exploit the improvements in technology GEOLOGIC SETTING al., 1994). Ultramafi c rocks, gabbro, and sheeted and methodology and gain better insight into dikes also crop out in the Limassol Forest com- the tectonic settings of Troodos magmatism, The Troodos ophiolite extends 100 km in plex, but tectonism has severely disrupted the we analyzed trace elements of the 12 freshest E-W and 30 km in N-S dimensions (Geological original tectonomagmatic relationships. The samples from throughout the ophiolite (Fig. 1), Survey Department Cyprus, 1995). The Troodos Troulli inlier is isolated from the ophiolite, but it and also analyzed the Nd, Sr, and Pb isotope massif forms a gentle dome structure elongated is clearly an eastern extension of the main mas- ratios of fi ve of these samples. Whereas our E-W (Fig. 1). Because of the superposition of sif. Another isolated massif with the same lithol- samples span a greater geographical extent of this domal structure and erosion, the structurally ogy as the main massif crops out on the Akamas the Troodos ophiolite than the previous stud- lowest ultramafi c rocks constitute the center of Peninsula (Fig. 1). ies by Rautenschlein et al. (1985), Cameron the Troodos massif and the highest elevations at The Geological Survey Department Cyprus (1985), and others, our sample set is small Mount Olympus (1952 m altitude). The ultra- (1995) divided the pillow lavas into (1) a basal compared to the 137 samples of glass analyzed mafi c rocks are fl anked by gabbro associated group, (2) lower pillow lavas, (3) upper pillow by Pearce and Robinson (2010), although their with a small amount of plagiogranite, an exten- lavas, mostly in the axial sequence, and (4) lavas study presented major-element data only. sive sheeted dike complex, pillow lava, and and volcanic breccias in the Arakapas sequence In addition to geochemical data, we present sediments, in ascending structural-stratigraphic whose stratigraphic relationship with the other radiolarian biostratigraphic data, some paleo- order (Fig. 1). three groups is not clear. The basal group lavas magnetic data, and Ar-Ar age analyses of two The Troodos ophiolite has been subdivided display greenschist-facies hydrothermal meta- fresh volcanic glasses that directly date tholeiite into the northern main massif and its axial morphism (Geological Survey Department and boninite eruption in the Troodos ophiol- sequence, including Mt. Olympus, and the Cyprus, 1995) and are easily distinguished from ite. The combination of new geochemical