Evolution of volcanism and magmatism during initial arc stage ...

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Evolution of volcanism and magmatism during initial arc stage: constraints on the tectonic setting of the Oman Ophiolite YUKI KUSANO1*, MAIKA HAYASHI2, YOSHIKO ADACHI3, SUSUMU UMINO1 & SUMIO MIYASHITA4 1

Department of Earth Sciences, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan

2

Foster Electric Company, Limited, 1-1-109 Tsutsujigaoka, Akishima, Tokyo, 196-0012, Japan 3

Centre for Transdisciplinary Research, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata, 950-2181, Japan 4

Institute of Science and Technology, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata, 950-2181, Japan *Corresponding author (e-mail: [email protected]) Abstract: Based on detailed stratigraphy, petrology and geochemistry, the initial arc magmatism of the Oman Ophiolite consisting of tholeiitic lavas followed by boninite flows and tephras is studied in the Wadi Bidi area, northern Oman Mountains. An 1110-m-thick V2 sequence is divided into the lower 970 m (LV2) and upper 140 m (UV2) thick subsequences by a 1.0-mthick sedimentary layer. Pahoehoe flows dominate in the lower part of the LV2, while the upper part consists mainly of sheet flows with sparse interbedded pelagic sediments and a cylindrical plug. In addition to the presence of a feeder conduit, the flow-dominant lithofacies with a few thin sedimentary interbeds in the LV2 indicates that the study area was the centre of a volcano grown in a short period. The UV2 is composed of boninite sheet flows overlain by a 2.0-m-thick pyroclastic fall deposit. A small amount of boninite lavas at the end of the V2 sequence overlain by thick pelagic sediments suggests that the subduction-related arc volcanism was short lived and terminated long before the ophiolite obduction. Supplementary material: Locations, mode of occurrence, phenocryst assemblages and bulk-rock major and trace element compositions of lavas in the Wadi Bidi area are available at http://www. geolsoc.org.uk/SUP18684.

How oceanic lithosphere starts subduction and how juvenile arcs form on oceanic crust and evolve to matured arcs are principal issues in the understanding of the evolution of subduction zones. The Izu – Bonin–Mariana (IBM) Forearc preserves the early arc volcanic history since the beginning of the Pacific Plate subduction along the eastern margin of the Philippine Sea Plate at 50 Ma (e.g. Stern 2004). Boninite has been regarded as the first volcanic product of the juvenile arc whereas mid-ocean ridge basalt (MORB) -like lavas, referred to as forearc basalt (FAB), predate the boninite volcanism (Reagan et al. 2010). The FAB volcanism was caused by a short-lived spreading of the hanging wall lithosphere of the Philippine Sea Plate induced by the sinking footwall lithosphere of the Pacific Plate. The FAB was followed by transitional arc tholeiitic lavas, which are in turn unconformably overlain by boninite lava with a 2 myr interval estimated by zircon U –Pb ages (Ishizuka et al. 2011; Kanayama et al. 2012). On the other

hand, modern boninite volcanism is occurring on the Tonga Ridge which meets the NE Lau Spreading Centre (e.g. Falloon et al. 1989; Danyushevsky et al. 1995). The occurrence of boninite volcanoes adjacent to the Lau back-arc spreading axis represents the ongoing formation of a juvenile arc of boninite on the oceanic crust driven by heating from the Samoan mantle plume and Lau Spreading Centre. However, the Tonga Arc initiated at 45 Ma did not include boninite among the first arc magmatism; the modern boninite volcanism results from resumption of an old subduction zone (Bloomer et al. 1995; Gurnis et al. 2004). The appearance of boninite in a lava sequence is therefore one of the keys to understand the incipient stage of arc magmatism. The Oman Ophiolite is one of the best places to understand the tectonomagmatic processes during the transition from a spreading centre to a juvenile arc formation. Undisturbed complete lava stratigraphy is well exposed in the northern Oman Ophiolite,

From: Rollinson, H. R., Searle, M. P., Abbasi, I. A., Al-Lazki, A. & Al Kindi, M. H. (eds) 2014. Tectonic Evolution of the Oman Mountains. Geological Society, London, Special Publications, 392, 177–193. http://dx.doi.org/10.1144/SP392.9 # The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

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Y. KUSANO ET AL.

Downloaded from http://sp.lyellcollection.org/ by guest on March 27, 2014 EVOLUTION OF INITIAL ARC VOLCANISM

which is divided into V1 (Geotimes), V2 (Alley) and V3 (Salahi) sequences (Alabaster et al. 1980, 1982; Ernewein et al. 1988; Kusano et al. 2012; Umino 2012) in ascending order. Clinopyroxene-phyric Unit is partly intercalated with the Alley Unit and their cogenetic dykes intrude into the sheeted dykes, V1 and V2 lavas (Alabaster et al. 1980; Umino et al. 1990). The palaeoceanic lithosphere is dated from 100 to 96.4– 95.5 Ma by U –Pb ages of zircon in plagiogranites and upper-level gabbro and radiolarian fossils overlying the V1 sequence (Tilton et al. 1981; Tippit et al. 1981; Rioux et al. 2012). Radiolarian-bearing pelagic sediments overlying the V2 lavas are of early Turonian (93.5– 92.1 Ma) age (Kurihara & Hara 2012) and the interval between the V1 and V2 is estimated to be ,2 Ma. An 40Ar/39Ar dating on metamorphic hornblende from the metamorphic sole also suggests a similar age interval between the two episodes of magmatism (Hacker et al. 1996). The V1 sequence was formed on a spreading ridge, while the V2 sequence was formed above a subduction zone or in an arc-like (oceanic detachment) setting (e.g. Alabaster et al. 1982; Nicolas 1989). The above dating suggests that the V2 magmatism had launched near the spreading axis. The V2 sequence therefore records the magmatic evolutionary processes during the development of the mantle wedge; however, detailed volcanostratigraphy of the V2 sequence is poorly understood. Ishikawa et al. (2005) proposed a model of generation of V2 magmas through the progress of oceanic thrusting, where the addition of slab fluids to the mantle wedge gradually changed the melt composition from tholeiitic to boninitic. Boninite magmas rose into the mantle wedge as a porous flow (Suetake & Takazawa 2012) and formed dykes into the overlying ophiolite members from layered gabbros through the sheeted dykes and the late intrusive plutons and into the uppermost extrusive rocks (Umino et al. 1990; Ishikawa et al. 2002; Adachi & Miyashita 2003). In Wadi Jizi, boninite lava flows are interbedded with the V2 basalt flows (Ishikawa et al. 2002). This paper presents the first thorough description of the whole V2 sequence around the junction of Wadi Bidi and Hilti, and describes detailed stratigraphic variations of lithology and the geochemistry of V2 extrusive rocks including a boninite lava succession. Based on these results, the formation processes of the V2 sequence are discussed.

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Geology Geology and division of the V2 sequence The V1 and V2 extrusive rocks in Wadi Bidi area are gently folded with a SE-plunging anticline, generally striking NE –SW and dipping 20 –308E, and attaining c. 2500 m in total thickness (Fig. 1b). The sheeted dyke complex striking north –south to NW –SE and dipping 60 –908W changes into the V1 extrusive rocks through a 20-m-thick transition zone, which is partially lost by faulting. The 935m-thick V1 and 1110-m-thick V2 sequences are separated by a 0.5-m-thick metalliferous sedimentary layer (umber). The V2 sequence conformably overlies the V1 sequence, while in Wadi Ahin 4 km to the south of the study area, eroded V1 lava is covered with 20-m-thick conglomerate containing V1 pillows and further overlain by the V2 lava (Umino et al. 1990). The V2 sequence is structurally concordant with the overlying V3 flow in most parts of the study area and is covered with a 6-m-thick sedimentary layer (umber and pelagic sediments), which in turn is unconformably overlain by the V3 lavas (Umino 2012). Although a 135-mthick interval comprising 14% of the V2 sequence are hidden by gravels, the entire V2 sequence is almost continuously exposed in this area. The V2 sequence is divided into two subunits by an intercalated 1.0-m-thick umber. We hereafter call these subunits the lower V2 (LV2) and the upper V2 sequence (UV2). The UV2 is present only in the east of the study area, where it is truncated by the base of the V3 flow along the northern boundary. Stratigraphic levels are indicated by the height in metres above the base (m.a.b.) of the V2 sequence (Fig. 1c). Based on lava morphology, three facies were identified in the V2 sequence: pillow, pahoehoe and sheet flows. Methods of observation and terminology of lava morphology follow that of Kusano et al. (2012).

Lower V2 sequence The lower V2 sequence (LV2) is 970 m thick and consists mainly of lobate sheet flows with a subordinate amount of pahoehoe lobes and a few pillow flows. Pahoehoe flows are abundant in the lower part and attain 274 m in total thickness (34% of the entire LV2 sequence). The pahoehoe lobes have erosive coarse-grained cores surrounded by

Fig. 1. Location and geology of the study area along the junction of Wadi Bidi and Hilti: (a) locality of the study area; (b) geological map; and (c) columnar section of the V2 sequence along the bold lines in (b). Broken lines indicate boundaries of each sequence. The numbers denote the height in metres above the boundary between the V1 and V2 sequence (m.a.b.) and those in brackets are the heights of sedimentary interbeds. Phenocryst assemblages are shown on the right. Bold and thin lines represent phenocryst and groundmass crystals. Ol, olivine; Pl, plagioclase; Opx, orthopyroxene; Cpx, clinopyroxene; Spl, spinel.


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hard fine-grained crust (Fig. 2a). Compound pahoehoe flows consisting of numerous pahoehoe lobes have a minimum thickness of 3 m. Lobate and flat sheet flows comprise 62% of the LV2. Some lobate sheet flows branch into numerous pillow lobes from underneath the base of massive lava. Lobate flows show dome-like uplifts on the upper surface and taper laterally to blunt margins while a few flat sheet flows have flat smooth tops and almost constant thicknesses (Fig. 2b, c). Pillow flows are as thick as 31 m, comprising 4% of the LV2. Pillow flows are of ‘bulbous’ pillows without showing clear alignments of lobes. Boundaries between individual flows are defined by the abrupt change in flow morphologies and/or structures. Around 270 m.a.b., a large tumulus 20 m in thickness is present among pahoehoe flows. A cylindrical plug 5–7 m in diameter at 760 m.a.b. shows a concentric cross-section of alternating layers of hard finer-grained and soft coarser-grained lava (Fig. 2d). Lapill-tuff is interbedded with sheet flows at 830 m.a.b., composed of 36% scoriae 0.5 –3 cm in diameter and 43% angular lithic clasts with chilled margin 0.5–6 cm in diameter. Although lithic clasts are affected by secondary alteration and most scoriae are palagonitized, the relatively fresh glass of scoriae show 54 –60 wt% of SiO2, 0.4– 0.5 wt% of TiO2 and 7–11 wt% of MgO. A 0.2– 1.2-m-wide fissure vent is present at 890 m.a.b. and is filled by welded angular lithic bombs 10 –20 cm across and scoriae ,2 mm in diameter. The vent consists of a few dyke segments arranged en echelon that extends about 100 m north–south and dips 788E as a whole. Thin umber lenses occur at 927 and 930 m.a.b. The uppermost LV2 consisting of lobate sheet flows is overlain by an 1.0-m-thick umber layer which can be traced more than 1.8 km to south.

Upper V2 sequence The upper V2 sequence (UV2) has a thickness of 140 m and consists of lobate sheet flows intercalated with six discontinuous umber layers between 970 and 1000 m.a.b. A lobate sheet has a gently curved upper surface with well-developed columnar joints perpendicular to the surface with coarse-grained erosive cores due to weathering (Fig. 2e). Varioles less than 1 cm and rarely up to 3 cm in diameter are present in the chilled margin of the lobate sheet. A 2.0-m-thick lapill-tuff occurs at the uppermost level of the UV2. It consists of 58% angular scoriae 0.5– 3 cm in diameter with 10– 30 vol% vesicles, 2% clinopyroxene crystals c. 2 mm long and 40% grey to red altered aphanitic lithic clasts c. 5 mm in diameter cemented by calcite (Fig. 2f). The scoriae are of poorly vesiculated transparent to yellow-brown glass shards and vesicular green scoriae with spherical vesicles 0.3–0.5 mm in

diameter rimmed by chlorite. Lithic angular clasts have curved convex outlines 0.08– 0.16 mm in width and 0.5 mm in length and lenticular vesicles (,5 vol%) aligned subparallel to each other. Some clasts have a chilled margin of glass now replaced by chlorite. The lapill-tuff is finely stratified and well sorted without imbrication or crossstratification and laterally changes into tuff at a distance of 100 m. The UV2 is terminated by the appearance of a 0.5-m-thick umber layer and succeeding 6-m-thick pelagic sediments, which are overlain by the V3 flow.

Petrography Plagioclase, olivine and pyroxenes occur as discrete crystals or glomerocrysts (Fig. 3a). Although V2 lavas underwent intensive low-temperature alteration, primary phenocrysts are generally preserved. Plagioclase usually occurs as glomerocrysts with or without mafic minerals which are otherwise discrete crystals. Most 1.5–2.5-mm-long plagioclases have a skeletal or spongy core and are altered to albite, saussurite or calcite. Some plagioclase cores are replaced by clay minerals rimmed by albite. Hexagonal olivine phenocrysts, 0.5–3.0 mm in length, are replaced by carbonate and chlorite, sometimes accompanied by clay minerals in the rim and cleavages, and by clay minerals. Orthopyroxene 0.5–0.8 mm in length is also altered to talc, zoisite or clay minerals. Because the distinction of olivine and orthopyroxene pseudomorphs is sometimes ambiguous, phenocrystic orthopyroxene contents could be underestimated. Clinopyroxene 0.5 –2.0 mm in diameter is replaced by calcite in several samples. Clinopyroxene shows a weak sector or compositional zoning. Both clinopyroxene and orthopyroxene commonly occur as prismatic groundmass minerals through the V2 sequence and often show parallel intergrowths (Fig. 3b). Chrome-spinel and magnetite are commonly enclosed by olivine and orthopyroxene and rarely occur as inclusions in plagioclase and as discrete skeletal crystals. Based on the assemblage of phenocrysts the V2 volcanic rocks are divided into aphyric, olivine –clinopyroxene (Ol–Cpx), olivine –orthopyroxene– clinopyroxene (Ol–Opx –Cpx), olivine – plagioclase –clinopyroxene (Ol–Pl –Cpx), Ol– Pl– Opx –Cpx and Pl–Cpx-phyric types. The LV2 rocks are aphyric to sparsely phyric (less than 3 vol%) depending on the amount of plagioclase, whereas the UV2 rocks have 5–10 vol% phenocrysts mainly of large olivine (3 mm in length) (Fig. 3c). Orthopyroxene phenocrysts are common and rarely coexist with clinopyroxene phenocrysts. Bladed clinopyroxene crystals in chilled margins (Fig. 3d) and branching clinopyroxene crystals


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Fig. 2. Representative lava morphology and lithofacies. (a) Pahoehoe flows with eroded core enclosed by thick crust (hammer is 0.33 m). (b) Lobate and flat sheet flows at 511 m.a.b. Gentle surface of a lobate sheet flow is overlain by 1.8-m-thick flat sheet flows and five flat sheet flows have well-developed columnar joints. (c) Lobate sheet flows at 630 m.a.b. underlain by pillow lava, which is in turn underlain by a 3.2-m-thick sheet flow. Green coarse-grained massive cores of the sheet flow are sandwiched by columnar jointed lava crusts. (d) Cylindrical plug in the V2 section. The plug has an elliptical section 7 m NE–SW and 5 m NW –SE and sticks out more than 3 m in height. Concentric layers of hard fine-grained and erosive coarse-grained lava suggest repetition of magma surges through the conduit for a prolonged period. (e) Sheet flows with a gently curved upper surface at 1080 m.a.b. Quenched crusts show columnar joints perpendicular to the surface and coarse-grained cores are eroded by weathering. (f) Pyroclastic rock at the top of V2. Red volcanic clasts are embedded in the tuffaceous matrix.

in pyroclastic scoriae occur in the groundmass glass. Plagioclase phenocrysts are rare in the UV2 rocks; tabular plagioclase crystals are well

developed in the groundmass of differentiated samples, however. A summary of petrography is provided in Figure 1.


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Fig. 3. Microphotographs of V2 lava samples. (a) Pl –Ol–Cpx glomerocrysts in LV2 (crossed polar). (b) Orthopyroxene and clinopyroxene are ubiquitous in the LV2 groundmass (opened polar). (c) Altered olivine with partings giving a jigsaw-puzzle-like appearance in UV2 (opened polar). (d) Quench texture of an UV2 sample showing bladed clinopyroxene microphenocrysts set in the groundmass glass (opened polar). Ol, olivine; Pl, plagioclase; Opx, orthopyroxene; Cpx, clinopyroxene; Ves, vesicle.

Hyalo-ophitic and intersertal textures are well developed in the groundmass and a variolitic texture frequently develops near the surface of pahoehoe flows. Vesicularity of V2 samples is 10– 15 vol% (in the range 3–30 vol%) and hemispherical to irregular-shaped vesicles are filled with zeolite, chlorite, calcite and prehnite.

Bulk chemistry Whole-rock major elements, Ni, Y, Zr, Cr and V contents of 66 samples were analysed by X-ray fluorescence (XRF; RIX3000, Rigaku Denki) at Niigata University. The analytical method is described by Takahashi & Shuto (1997). Other trace elements of 40 samples were analysed by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 7500a) after Roser et al. (2000) at Niigata University. Most V2 samples have 2.2– 9.9 wt% of loss on ignition (LOI) except for seven samples containing 11.2–22.7 wt% (Fig. 4). Most LV2 samples have less than 10 wt% LOI, and do not show a

clear correlation with concentrations of major element oxides. Although UV2 samples show a negative correlation between CaO and Na2O, which is ascribed to pervasive albitization of plagioclase, those with more than 10 wt% LOI show a drastic increase in CaO with decreasing Na2O. These five samples have .20 wt% CaO, which is most likely raised by the abundant secondary carbonate filling vesicles and replacing olivine. The abundance of the secondary carbonate would also be responsible for the decrease in SiO2 with increasing LOI. In addition to these, two LV2 samples have extremely high LOI .20 wt% and plot away from other LV2 samples on any LOI– oxides plots. Samples .10 wt% LOI are therefore significantly affected by secondary alteration and excluded from the following chemical considerations. Large-ion lithophile (LIL) elements such as Rb, Ba and Li and Sr are generally highly mobile during ocean floor weathering and low-temperature hydrothermal alteration (e.g. Thompson 1973). These elements are highly dispersive, suggesting intensive


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SiO2 (wt%)

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modification of the original concentrations during the secondary processes. On the other hand, highfield strength (HFS) elements such as Nb and Zr, and Cr are generally immobile against such alteration processes (e.g. Thompson 1973). In the following sections, we discuss the petrological and geochemical characteristics of the extrusive rocks on the basis of these immobile elements. Zr in LV2 samples ranges from 25 to 80 ppm, while Zr in UV2 samples ranges from 9 to 15 ppm. With the increase in Zr, LV2 samples increase in TiO2, P2O5 and Y and decrease in Ni and Cr (Fig. 5). The LV2 samples are plotted in and near the compositional fields of the Alley Unit (Alabaster et al. 1982; Lippard et al. 1986) and V2 sequences (Beurrier et al. 1989; Godard et al. 2003; Kusano et al. 2012) and are slightly depleted compared to the latter. While UV2 samples are low and limited in Zr and TiO2 variations, compatible elements such as Ni and Cr are much higher and show larger ranges than the LV2. Compatible element abundances of the UV2 are at most about five times higher than those of the LV2. The UV2 samples are plotted in the field of the Clinopyroxenephyric Unit (Alabaster et al. 1982; Lippard et al. 1986) and partially overlap the V2 field of Godard et al. (2003) (Fig. 5). LV2 samples have 8.6– 15 chondrite (Sun & McDonough 1989) normalized Yb content (YbN) and increased CeN/YbN ratios from 0.4 to 0.7 (Fig. 6a). Most LV2 samples are plotted in the compositional fields of the Alley Unit and V2 sequence. They show light rare earth element (LREE) depleted patterns with 0.5 –0.8 LaN/SmN ratios except for two samples with enriched LaN/SmN ratios of 1.3 and 1.4 (Figs 6b & 7a). The LREE-depleted patterns are similar to that of low-Pb andesite of Ishikawa et al. (2005). On the other hand, UV2 samples have low heavy REEs (YbN of 5.1–7.5) and U-shaped patterns with middle REE depletion and light REE enrichment, characteristic of boninite (Fig. 7c). Weak negative Eu anomalies in the UV2 are slightly different from the boninite of Ishikawa et al. (2005). High LaN/SmN ratios of the UV2 are distinct from those of the Clinopyroxene-phyric Unit (Fig. 6b). LV2 samples have a slightly positive Zr anomaly on a multi-element spider diagram, suggesting different sources or degrees of melting from the UV2 magmas (Fig. 7b, d). Lower Nb/Ta (10– 12) and Hf/Ta (6.5–7.5) ratios are observed for UV2 samples than for LV2 (11–17 and 16 –26, respectively; Fig. 6c, d).

Fig. 4. SiO2, MgO and CaO contents plotted against LOI. Note the correlations of SiO2 and CaO with LOI .10%, indicative of modification by secondary alteration.


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Fig. 5. TiO2, P2O5, Ni, Cr, Y and Zr/Y ratios plotted against Zr. TiO2 and P2O5 are based on an anhydrous basis. LCB, low-Ca boninite; HCB, high-Ca boninite. Pre-boninite volcanic rocks from the IBM Arc (Bonin forearc, Mariana forearc and John Beach Volcanics) are plotted after Wood et al. (1982), Reagan et al. (2010), Ishizuka et al. (2011) and Kanayama et al. (2012). V2 (encircled by broken line) after Ernewein et al. (1988) and Godard et al. (2003, 2006); Alley (open field) and Cpx-phyric Unit (shaded field) after Alabaster et al. (1982) and Lippard et al. (1986); LCB and HCB after Ishikawa et al. (2002), Cameron (1985), Falloon & Crawford (1991) and Wood et al. (1982) are shown for comparison.

Zr/Y ratios and Zr contents of LV2 are lower than 3.4 and 112, respectively, which is an arc signature (Pearce & Norry 1979). They also have Ti/V ,20, typical of island-arc tholeiite (IAT) (Shervais 1982).

Chemical stratigraphy The bulk-rock compositions shown above are examined in terms of the lithological stratigraphy. The lithological column and geochemical


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Fig. 6. Chondrite-normalized (a) Ce/Yb and (b) La/Sm ratios plotted against chondrite-normalized Yb contents of V2 lava in the northern Oman Ophiolite; also shown are (c) Nb/Ta –Yb and (d) Hf/Ta– Yb. The chondrite value is after Sun & McDonough (1989). FAB compositions and fields of V2, Alley and Cpx-phyric units are taken from the same sources as in Figure 5.

variations around the Wadi Bidi are illustrated in Figure 8. The LV2 sequence shows overall smooth profiles containing weak wavy variations (0.6– 0.9 wt% TiO2; 32–65 ppm Zr; 25 –51 ppm Ni) except for five spikes. LV2 samples are generally homogeneous as shown in tight Zr/Y ratios. The wavy variations are divided into five compositional cycles where they exhibit differentiation trends upwards; these are intervals of the base 200 m.a.b. (Cycle 1), 200 –440 m.a.b. (Cycle 2), 440 –660 m.a.b. (Cycle 3), 660 –800 m.a.b. (Cycle 4) and 900 –970 m.a.b. (Cycle 5). Inflexion points in the weak wavy variations are well correlated with flow boundaries between pahoehoe and sheet flows. Five compositional spikes occur around the base or the top of each flow boundary. Ni and Cr have spikes at 2, 527 and 952 m.a.b. The sample from 591 m.a.b. forms negative anomalies in incompatible elements. Positive spikes in LaN/SmN ratios appear at 2 and 928 m.a.b.

The 1.0-m-thick umber layer marks the sharp compositional gap between the LV2 and UV2. The UV2 samples have an extremely low and narrow range of incompatible elements, and higher and broader range of compatible element profiles (Zr: 9–15 ppm; Cr: 399–874 ppm) than those of the LV2 samples. These large ranges of compatible elements such as Ni and Cr in the UV2 show zig-zag profiles. LaN/SmN ratios of the UV2 are relatively higher than those of the LV2 bounded by the umber layer.

Discussion Tholeiitic magmatism as the first product at subduction initiation The relatively homogeneous 970-m-thick LV2 succession suggests that the V2 lavas around Wadi Bidi formed part of a subaqueous volcano.


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0.1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

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Fig. 7. Chondrite-normalized REE patterns and N-MORB-normalized multi-element diagrams for (a, b) LV2 and (c, d) UV2 sequences. The chondrite and N-MORB values are after Sun & McDonough (1989). Broken lines are low-Pb andesite and boninite from the Oman Ophiolite after Ishikawa et al. (2005). Shaded areas in (b) and (d) represent compositional ranges of LV2.

Pahoehoe flows dominant in the lower LV2 are preferentially emplaced on gentle slopes of ,58, while sheet flows prevailed in the upper section indicate emplacement on a subhorizontal seafloor (Umino et al. 2002, 2008). The change in lava morphology from pahoehoe to sheet flows upsection suggests the change in volcanic edifice from a small lowangle shield to a larger flat-topped seamount like those on slow-spreading mid-ocean ridges and submarine rift zones on Hawaiian volcanoes (Clague et al. 2000). Lack of sedimentary interbeds from the base to 910 m.a.b. indicates a continuous growth of the main volcanic edifice in a relatively short period. Several sedimentary layers and lenses at 910 –970 m.a.b. suggest that the volcanism was intermittent towards the end of the LV2. At least two volcanic centres are inferred by the presence of the cylindrical conduit in the south (Fig. 2d) and fissure vent in the middle of the study area which have similar petrological characters to the LV2. Alternating pahoehoe and sheet flows and thin sedimentary interbeds upsection suggest that the eruption lasted for a relatively long period with repeated magma surges, which is consistent with the formation of a concentrically layered cylindrical conduit.

The majority of LV2 samples with olivine, plagioclase and pyroxene phenocrysts show a homogeneous IAT character in incompatible element compositions and their wide Zr range can be ascribed to fractional crystallization of basaltic andesite (Fig. 5). The appearance of high Ni and Cr samples at the base of each compositional cycle followed by gradual decreases in Ni and Cr upsection suggest renewal of magma composition followed by fractionation of olivine and clinopyroxene during a magmatic cycle. Similar cyclic variations of lava compositions were observed during an eruptive episode of Kilauea Volcano, Hawaii and are ascribed to variable degrees of phenocrystic olivine fractionation during ascent from depths due to temporal variations of magma supply rate (Garcia et al. 1996). This model may be applicable to the compositional cycles in LV2 in response to the fluctuation of magma supply rate. The LaN/SmN ratio plotted in the V2 compositional field by previous studies (Alabaster et al. 1982; Lippard et al. 1986; Ernewein et al. 1988; Godard et al. 2003, 2006; Fig. 6) shows an upwards increase in a compositional cycle, which is consistent with fractionation of phenocrystic phases. Samples at 2 and 928 m.a.b. have higher


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Fig. 8. Stratigraphic variations of selected bulk-rock TiO2, Zr, Ni, Cr, Zr/Y, Ti/V, LaN/SmN and Nb/Ta. Note that the labels of Ni and Cr contents for UV2 and LV2 samples are shown on the top and bottom of the diagram, respectively. Black and grey broken lines indicate the boundaries between LV2 and UV2, and compositional cycles of LV2. Arrows indicate compositional trends in individual cycles.

LaN/SmN ratios and LREE (La, Pr, Nd and Sm) values than the other LV2, and CeN/YbN ratios which are within the range of the other LV2 samples due to a negative Ce anomaly (Fig. 7a). Such a LREE pattern may be explained by LREE-rich and Ce-poor fluid addition during hydrothermal alteration in Mariana arc lavas (Woodhead 1989; Elliott et al. 1997). The negative Ce anomaly is considered to be derived from the characteristics of subducted sediment since geochemical compositions of sedimentary rocks in the Oman Ophiolite also show a negative Ce anomaly (Robertson & Fleet 1986) and both samples at 2 and 928 m.a.b. are within ,0.5 m from the underlying sedimentary lenses. With the exception of Ce, LREEs are mobile under oxidized conditions through hydrothermal interactions with the sediments, releasing a LREE-rich and Ce-poor fluid that overprinted

the overlying LV2 lava compositions. The basal LV2 sample at 2 m.a.b. may have been highly affected by the umber-like sediment and strongly increased in LREEs besides Ce. A significant La enrichment and a negative Ce anomaly of the UV2 sample at 986 m.a.b. may also be due to hydrothermal interaction with the underlying sediments. Since the orthopyroxene phenocrysts are even contained as the groundmass mineral, they are rare in the compositional cycle 1 and common in cycles 2 –5, the V2 sequence experienced a sudden change in magma composition at the beginning of cycle 2. The lowermost 150-m-thick V2 sequence (V2 type I, low-Ti magmatism; Godard et al. 2006) in Salahi area 4 km north of Wadi Bidi, corresponding to compositional cycle 1, has a similar compositional trend to LV2 (Figs 5 & 6). The V2 type I lava lacks sedimentary fluid components in Nd


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and Pb isotope data, derived from remelting of depleted MORB mantle after extraction of Geotimes magma (Godard et al. 2006). This low-Ti magmatism and subsequent abundant orthopyroxene-bearing LV2 indicate that subduction-related compositional changes occurred during the LV2 magmatism.

Boninite volcanism in the Oman Ophiolite The pyroclastic deposit at 830 m.a.b. consisting of scoriae .53 wt% SiO2, ,0.5 wt% TiO2 and .8 wt% MgO indicates concomitant activity of the early boninitic magma within the upper LV2 sequence. However, most products of the boninite volcanism are represented by the UV2 sequence that extruded over LV2 after a repose period indicated by a 1.0-m-thick umber and pelagic sediments and produced at least 140 m of thick lava piles. The UV2 volcanism is characterized by intermittent extrusions of thin flows separated by short quiescent periods represented by sedimentary layers and lenses, followed by continual extrusions of 100-mthick flows in total that formed a main part of the UV2 sequence. The pyroclastic fall deposit (Fig. 2f) on the top of the UV2 sequence includes a significant proportion of aphanitic lithic clasts and finely vesiculated glass shards in addition to vesicular scoriae, suggesting that the erupting magma was not uniformly vesiculated but bubbles were unevenly distributed within the erupting magma column. Convexly curved surfaces of aphanitic clasts with occasional chilled margins resemble those of spatter fed by lava fountaining. These occurrences suggest the disruption of rapidly cooled less-vesiculated lava exposed at the head of the erupting magma column by surges of highly vesiculated magma which bubbled beneath the extruding lava surface. Mild submarine lava fountaining was observed on a boninite seamount in the NE Lau Spreading Centre (Clague et al. 2009). Finely stratified well-sorted scoriae and lithic clasts without evidence of deposition under flow and lateral change in grain size into tuff indicate the lapill-tuff was a fall deposit over a subhorizontal seafloor adjacent to the source vent. The formation of a thick pyroclastic deposit in the uppermost UV2 sequence suggests that shallowing water depth with the growth of the volcanic edifice allowed vesiculation and mild lava fountaining of the UV2 magma. In spite of the concurrent activity of the tholeiitic and boninitic magmas during the transition from the LV2 to UV2, the lack of evidence of magma mixing between the two magma types such as linear bulk compositional trends and coexistence of disequilibrium mineral compositions and textures suggests that the LV2 and UV2 magmas were supplied from discrete magma plumbing systems.

Although the whole-rock major element compositions have been modified by secondary alteration, most UV2 samples show a similar mineralogy and bulk trace element compositions to the previously reported boninite dykes and flows such as slightly LREE-enriched spoon-shaped REE patterns (Umino et al. 1990; Ishikawa et al. 2002; Adachi & Miyashita 2003; Kusano et al. 2012; Fig. 7c). According to the definition of boninite, boninites in the Oman Ophiolite belong to high-Ca boninite similar to those in the Troodos Ophiolite and from the north Tonga Trench (Crawford et al. 1989; Falloon et al. 1989; Ishikawa et al. 2002). Although plagioclase phenocrysts are rare in the UV2 the common groundmass plagioclase in the UV2 flows and boninite dykes (Ishikawa et al. 2002), as well as relatively water-deficient conditions for high-Ca boninite magmas estimated by experimental studies, suggest that high-Ca boninites would crystallize plagioclase earlier in the crystallization path than low-Ca boninites (van der Laan et al. 1989). The negative Eu anomalies in the UV2 boninite are in accordance with a small amount of plagioclase fractionation. Weak Eu anomalies are also recognized for the differentiates of the boninite series rocks from the Ogasawara Islands (e.g. Kanayama et al. 2012). The UV2 sequence of 140-m-thick sheet flows shows highly variable Ni and Cr abundances (Fig. 8). The broad Ni and Cr variations are correlated with the modal amounts of olivine and clinopyroxene phenocrysts, with c. 20% olivine phenocrysts up to 3 mm in diameter in the high-Ni and -Cr rocks. On the other hand, abundances and ratios of incompatible elements of the UV2 lavas vary only within limited ranges, suggesting that they experienced limited fractionation in a single magmatic episode. Despite similar depletion of heavy REE, higher LREE abundances of UV2 samples than the average boninite to the north of Wadi Jizi (Ishikawa et al. 2005) suggest variations in mixing ratios and compositions of slab-derived fluid and partial melt (Fig. 7). Spoon-shaped REE patterns recognized in V2 type II (Godard et al. 2003) exhibit no Eu anomaly and higher REE abundance than the UV2 and other boninites. The V2 type II REE patterns are parallel to the UV2 and well explained by fractionation from the UV2 rocks without Eu anomaly (Figs 6 & 7). Their limited distribution (Godard et al. 2003) may be correlatable to the boninitic UV2 rather than arc tholeiitic LV2. The UV2 is distinguished from the Clinopyroxene-phyric Unit (Alabaster et al. 1980, 1982; Lippard et al. 1986) by the presence of orthopyroxene phenocrysts in the former. Moreover, all UV2 rocks show spoonshaped REE patterns while the Clinopyroxenephyric Unit includes some LREE-depleted patterns (Fig. 6b).


The boninite magmas are generated by flux melting of the harzburgite residue during the ascent of slab-derived fluids which decreased in amount from Wadi Rajmi to the south of Wadi Fizh (Ishikawa et al. 2002; Suetake & Takazawa 2012; Takazawa 2012). Based on the field evidence, boninite dykes intruded into the crustal section (wehrlite, gabbros, sheeted dykes and V1 sequence) including the late intrusive plutons genetically linked to the LV2 lavas (Smewing 1981; Umino et al. 1990; Adachi & Miyashita 2003; Yamasaki et al. 2006). Around Wadis Zabin and Rajmi, the east –west-striking boninite dykes are rooted in the late intrusive plutonic bodies to the west (Umino et al. 1990). The presence of UV2 magma in the upper V2 sequence indicates that boninite is the last product during the late intrusive magmatism. As the boninite pyroclasts are interbedded with the tholeiitic flows in the LV2 in the study area, boninite flows are intercalated with arc tholeiitic lavas in the V2 sequence in Wadi Jizi (Ishikawa et al. 2002). This indicates that the boninite volcanism started before the cessation of the LV2 tholeiitic volcanism and subsequently took over the final phase of the V2 arc magmatism (Umino et al. 1990; Ishikawa et al. 2002).

Magmatic evolution during V2 Lower Zr/Y, Ti/V and CeN/YbN ratios of UV2 lavas compared to LV2 lavas indicate a more depleted source mantle. Lower Hf/Ta ratios in UV2 compared to LV2 indicate that LREE –Nb– Ta-rich and middle REE–Zr –Hf-depleted components are required for the UV2 (Figs 6d & 7d). These geochemical characteristics are most likely explained by the severely depleted source mantle metasomatized by a slab-derived fluid + partial melt. Progressive increase in the degree of slabderived fluids to the wedge mantle has been proposed for the V2 sequence in Wadi Jizi by Ishikawa et al. (2002) and for the Tonga Arc wedge mantle (Danyushevsky et al. 1995). Because both Nb and Ta are additional components to the UV2 and their bulk distribution coefficients are DNb , DTa (e.g. Green 1995), the UV2 has lower Nb/Ta ratios than the LV2. Similar compositional change during V2 magmatism can be seen in V2 type I and II (Godard et al. 2003). Remelting of the depleted mantle residue after extraction of V1 magma by the introduction of slab-derived fluid/melt should have produced V2 magmas with lower Nb/Ta ratios.

Ephemeral subduction zone environment The LV2 rocks in the study area are composed of basaltic andesite but the V2 sequence from other

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areas range from basalt to rhyolite (Alabaster et al. 1982; Lippard et al. 1986; Umino et al. 1990). We propose that there were many volcanic centres along the juvenile intraoceanic subduction zone, which varied in magma compositions and eruptive styles from centre to centre. The absence of V2 extrusive rocks in Wadi ath Thuqbah 25 km north of the study area corresponds to a blank area between V2 volcanoes, where the V1 sequence was directly covered with the Suhaylah Formation (late Cenomanian–early Turonian, 94.8–92.1 Ma) (Tippit et al. 1981; Hara & Kurihara 2012). Radiolarianbearing pelagic sediments overlying the V2 sequence are dated as early Turonian (93.5 –92.1 Ma; Kurihara & Hara 2012), indicating a long volcanic cessation during the V2 stage in Wadi ath Thuqbah. The sporadic occurrence of volcanoes is also indicated by numerous focal centres of cone sheets rooted in high-level gabbro intrusions, showing the presence of discrete magma sources for the V2 volcanic centres (Alabaster et al. 1980; Lippard et al. 1986; Umino et al. 1990; Adachi & Miyashita 2003; Godard et al. 2003; Yamasaki et al. 2006) as for the Quaternary arc volcanoes. Tholeiitic activity of the LV2 with the onset of subduction stage resembles the Eocene FAB in the earliest IBM Arc (e.g. Reagan et al. 2010; Ishizuka et al. 2011). Compositional characters of the FAB from DSDP Sites 458 and 459 in the Mariana and Bonin forearcs largely overlap with the Alley and V2 fields. The LV2 fields are similar to those of the Mariana FABs (Fig. 5). The Bonin Forearc lavas show higher concentrations in incompatible elements and depletion in LREEs and Nb than the Mariana Forearc lavas, which are interpreted by the increasing contributions of water-rich slab-derived fluids from south to north (Reagan et al. 2010). Likewise, differences in the concentrations of major and trace elements within the V2 arc tholeiites are also ascribed to regional variations along the palaeo-subduction zone. The presence of orthopyroxene-bearing LV2 differs from the aphyric FAB formed by forearc spreading. The John Beach Volcanics of Chichijima Island, proto-FAB which predates boninite, show higher Hf/Ta ratios than the V2 and FABs (Fig. 6d). The high Hf/Ta ratios of the John Beach Volcanics are a result of transition from FAB to boninites (Kanayama et al. 2012). The LV2 therefore represents infant arc tholeiitic magmatism with a few subduction-related components, unlike FABs on forearc spreading and the John Beach Volcanics which have a strong arc signature. Considering the 3-Ma-long period for the 500m-thick Ogasawara boninite series strata, comprising pillow and sheet flows and pyroclastic and hyaloclastic deposits that vary in composition from boninite through andesite to dacite and rhyolite


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(Umino & Nakano 2007; Ishizuka et al. 2011; Kanayama et al. 2012), the 140-m-thick UV2 magmatism would have lasted for less than the 3 Ma suggested by the fossil ages (Hara & Kurihara 2012; Kurihara & Hara 2012). The sporadic and limited distributions and small scales in magnitude of boninite volcanism at the end of the V2 sequence suggests that the V2 sequence was an initial infant arc magmatism formed above the rapidly cooling wedge mantle following the intraoceanic detachment. Generation of the UV2 boninite magma requires an introduction of slab-derived fluid and melt into the shallow and hot mantle beneath the overriding oceanic lithosphere onto the oceanic plate, both of which were on the adjacent limbs of the spreading ridge when the intraoceanic detachment took place (Ishikawa et al. 2002, 2005). Flux melting of the lithospheric mantle induced by fluid and melt liberated from the subducted lithosphere was only possible while the hanging wall lithospheric mantle was sufficiently hot for the first several million years from the beginning of the intraoceanic detachment (Ishikawa et al. 2005). The buoyant hot lithosphere went beneath the base of the overriding lithosphere, which hindered development of the asthenospheric counter flow within the mantle wedge (Nicolas 1989) and explains why the V2 magmatism did not develop into a matured arc like the IBM arc– trench system. On the contrary, the IBM arc originated as an oceanic arc by subduction of the aged (.100 Ma) and cold Pacific Plate, which lead to the circulation of asthenospheric flow in the mantle wedge and the development of a sustained arc – trench system (Ishizuka et al. 2006; Kanayama et al. 2012).

Conclusions This is the first thorough description of the whole V2 sequence around the confluence of Wadi Bidi and Hilti, the northern Oman Ophiolite, and shows a detailed stratigraphic variation in terms of lithofacies, petrology and geochemistry. The 1110m-thick V2 sequence consists of the 970-m-thick lower (LV2) and 140-m-thick upper (UV2) subsequences. The LV2 occupies the majority of the V2 sequence. The lower LV2 consists of pahoehoe flows and the upper part consists of sheet flows. A few pillow flows occur at c. 650 m.a.b. A cylindrical magma conduit 5–7 m in diameter is preserved at 760 m.a.b. Based on the occurrence of these lava flows with a trivial amount of volcaniclastic rocks, the LV2 represents the proximal facies of a flat-topped submarine volcano formed over a short period. The LV2 samples have IAT

characters based on their bulk rock chemistry, and can be divided into five compositional cycles through the geochemical stratigraphy. The gradual changes in the compositional cycles suggest that fluctuations in magma supply are responsible for the variable degrees of fractionation of olivine and clinopyroxene during the eruption of the LV2 sequence. The LV2 is terminated by a 1.0-m-thick umber and overlain by UV2 consisting of lobate sheet flows intercalated with several sedimentary lenses. The top of the UV2 is covered with a 2-m-thick pyroclastic fall deposit. The UV2 rocks generally have abundant olivine and clinopyroxene phenocrysts of length c. 3 mm. In addition to these, the depleted and spoon-shaped REE patterns indicate that the UV2 lavas are high-Ca boninites, similar to those reported from Wadi Jizi. Narrow ranges of incompatible element variations and wide variations in compatible elements correlated with olivine phenocryst contents suggest a small amount of olivine fractionation. The higher LaN/SmN and lower Hf/Ta ratios of the UV2 indicate a more depleted source mantle and a larger contribution of slab-derived fluids for the UV2 than for the LV2 magmas. The V2 sequence records the volcanic products in a short-lived arc-like (subduction) setting. The tholeiitic LV2 magma formed upon the initiation of the oceanic detachment and subsequent subduction and dehydration of the descending slab. As the subduction proceeded, the LV2 magma progressively changed its composition and crystallized orthopyroxene phenocrysts through the later compositional cycles. With the progress of subduction, the leading edge of the subducting lithosphere became sufficiently hot and partially melted to supply LREE and Ta to the severely depleted wedge mantle, producing the boninite magma to extrude as the UV2 sequence. The boninite magmatism was initiated at the end of the V2 before the cessation of the LV2 volcanism. The limited distribution and magnitude of boninite volcanism indicate the short-lived wedge mantle following the oceanic detachment. We thank H. Rollinson and other editors for publishing on this theme. We also thank H. Al Azri, S. Hamaed Al-Busaidi, D. A’shaikh (Ministry of Commerce and Industry of Oman) and the Japanese Embassy in Oman for their kind support during field surveys. We are indebted to S. Arai and K. Kanayama (Kanazawa University), E. Takazawa, N. Fujibayashi, T. Kurihara and K. Hara (Niigata University) for fruitful discussions. Thoughtful reviews by anonymous reviewers greatly improved the manuscript. This study was supported by Sasagawa Scientific Research Grant from The Japan Science Society (21– 711M), a Research Grant from The Niigata University 2010, 2011, Monkasho National

Downloaded from http://sp.lyellcollection.org/ by guest on March 27, 2014 EVOLUTION OF INITIAL ARC VOLCANISM University Corporations ‘Ocean Floor Dynamics as deduced from Ophiolites’ (No. 812000009), JSPS Institutional Program for Young Researcher Overseas Visits ‘Structure and formation of the oceanic crust-mantle system: Preparation for Mohole’ and KAKENHI Grant No. 23540530.

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