Contribution of slab-derived fluid and sedimentary melt in the incipient arc magmas with development of the paleo-arc in the Oman Ophiolite
Introduction
Knowledge of the processes operating in the deep mantle and the subducted slab are essential for a full understanding of the material and thermal evolution of the sub-arc mantle and the development of arc systems. Arc magmas are considered to receive recycled materials from the slab but the most part of subducted slab is recovered into the mantle. Therefore, material balance between the arc magmas and subducted slab is still uncertain. Some ophiolites, such as those in Oman and the United Arab Emirates, are favorable targets for research in that the entire sequence of volcanic and magmatic products before and after the initiation of subduction. They are well preserved and exposed on land, including huge masses of sub-arc mantle peridotites with underlying metamorphic rocks. The metamorphic rocks beneath ophiolite nappes are called metamorphic sole (e.g. Boudier et al., 1988).
The metamorphic rock-derived fluids are considered to be related to the arc magmatism (Ishikawa et al., 2005). The V2 extrusive rocks which unconformably overlie the V1 extrusive rocks have strong arc signatures and include boninite (Umino et al., 1990, Ishikawa et al., 2002, Gilgen et al., 2014, Kusano et al., 2014). Ishikawa et al. (2005) demonstrated that the trace-element characteristics of the V2 arc tholeiite and boninite could be explained by partial melting of the depleted source mantle with the introduction of hydrous fluids in equilibrium with amphibolite in the metamorphic sole. On the other hand, granitic dikes are found in the mantle section which have sedimentary feature for their origin (Haase et al., 2015, Rollinson, 2015). Since these dikes may be formed at high-temperature condition (Rollinson, 2015) similar to the metamorphic rocks, the metamorphic rocks are possibly the source of the granitic dikes, in other words, the remnants of the subducted slab. In the northern Oman Ophiolite, the pathways of these slab-derived fluids and melts are preserved as 5–10 km wide zones of highly refractory harzburgite which run subparallel to each other at a space of ca. 5 km and extend from below the Moho all the way down to the base of the mantle peridotite massifs just above the metamorphic sole (Kanke and Takazawa, 2014). These zones are interpreted to be the channels of V2 arc magmas that passed through and interacted with the peridotites in the mantle wedge. Therefore, the Oman Ophiolite can provide invaluable information on the magmatic processes in the mantle wedge involving the recycle system of subducted slab in the infant Oman arc, which is inaccessible in the present arc system. However, contribution of sediment melt to the V2 magmas is not well constrained by the previous studies.
In addition to the composition of the source mantle, the composition of arc magmas is largely varied by the composition of slab-derived fluids, which are best evaluated by analyzing mobile elements such as large ion lithophile element (LILE) liberated through the dehydration and partial melting of the subducted slab. To better understand the temporal and spatial evolution of the incipient arc and the mantle wedge, understanding of the genetic relationships between the arc magmas and the subducted slab is necessary. However, these LILE are susceptible to low-temperature alteration and weathering, which prevail in the extrusive rocks of the Oman Ophiolite.
The V2 arc magmatism ended after a short period (age ranges of 96–94 Ma; Hacker et al., 1996) with the extrusion of high-Ca and low-Si boninite. Based on the stratigraphy and geochemistry, Stern et al. (2012) applied the model of Izu–Bonin–Mariana (IBM) proto-arc development to ophiolites and proposed that ophiolites originate in the region of forearc extension that followed subduction initiation and resulted in the formation of a new arc–trench system. Although the Oman Ophiolite bears some resemblance in terms of magmatic history to the IBM proto-arc (Ishizuka et al., 2011), the incipient arc magmatism in the Oman Ophiolite significantly differs in that: 1) it started with the production of arc tholeiite and ended with boninite, and was followed by intraoceanic volcanism that produced alkali basalt (Alabaster et al., 1982, Umino et al., 1990, Umino, 2012, Kusano et al., 2014); 2) it was short lived and lasted for only < 3 m.y. (96–94 Ma; Hacker et al., 1996); and 3) the subduction initiated between young and buoyant oceanic plates including the ridge axis (Nicolas, 1989, Umino et al., 1990, Ishikawa et al., 2002, Kusano et al., 2014). It is currently uncertain whether the lowermost V1 stage rocks were formed at a mid-ocean ridge or represent oceanic lithosphere generated at a backarc or a forearc spreading axis, as the subtle arc-like signature of the V1 rocks is obscured by pervasive alteration and weathering of the volcanic products (e.g. Pearce et al., 1984, Nicolas, 1989, Moores et al., 2000, Stern et al., 2012, MacLeod et al., 2013). The incipient arc V2 magmatism was emplaced in this such an oceanic basement as plutonic bodies and on as the extrusive sequences (Umino et al., 1990, Ishikawa et al., 2002, Nagaishi, 2008, Yamazaki, 2013, Kusano et al., 2014).
We have recovered fresh volcanic glasses from the V1 and V2 volcanic sequences extending over ~ 80 km in the northern Oman Ophiolite. By analyzing these fresh volcanic glasses, we were able to obtain data on the pristine major- and trace-element compositions, including LILEs, of the V1 and V2 volcanic rocks. On the basis of these geochemical data and the Hf and Nd isotopic compositions of the bulk V1 and V2 rocks, we present a geochemical model of the V2 magma genesis and discuss the evolutionary processes of the intraoceanic subduction zone recorded in the Oman Ophiolite.
Section snippets
Geological background of the volcanic glass samples
The lowermost V1 unit was extruded and emplaced on and near the spreading axis at 98–96 Ma (Einaudi et al., 2003, Kusano et al., 2012, Rioux et al., 2012) and is comagmatic with the underlying sheeted dike complex and gabbros (e.g. Alabaster et al., 1982, Lippard et al., 1986). The V1 sequence consists mainly of lava flows with minor pyroclastic and hyaloclastic breccias, including pillow flows, pahoehoe and lobate flows, and sheet flows, with a few massive lavas (Kusano et al., 2012). The lower
Geochemistry
The major elements of 21 volcanic glass samples were analyzed using a JEOL JXA-8800 electron probe microanalyzer (EPMA) at Kanazawa University, Japan, with an accelerating voltage of 15 kV and a beam current of 12 nA. Counting time was 20 s for the peak and 10 s for the background using a broad beam of 20 μm in diameter, following the procedures of Noguchi et al. (2004) to correct the time-depending Na-loss. Corrections were made using the ZAF method. Analysis are monitored by laboratory-standard
Implications for the Indian MORB mantle as the source of the V1 magma
The fresh V1 glasses have relatively higher CaO and lower Na2O concentrations than the bulk V1 (Fig. 4), indicating that the latter have been significantly modified by low-temperature hydrothermal alteration, consistent with the widespread occurrence of the greenschist-facies mineral assemblage of albite, chlorite, actinolite, and epidote (e.g. Lippard et al., 1986). Also, comparison of the altered rinds and fresh cores of glasses clearly demonstrates LILE-enrichment in the former relative to
Conclusions
Major- and trace-element compositions of fresh glasses, and the bulk Nd and Hf isotopic compositions of the V1, LV2 (mainly arc tholeiite), and UV2 (boninite) flows and pyroclastic rocks are reported. The V1 samples have Indian MORB-like major-element and trace-element values, and Nd and Hf isotope compositions. The V2 volcanic glasses show arc signatures with lower HFSE, MREE, and HREE, and higher LILE abundances than the V1 glasses. Higher concentrations of B, Pb, and LILE can be ascribed to
Acknowledgements
We thank Salim Al-Ibrahim, Mahommed Al-Battashi, Mohamed Al-Araimi (Ministry of Commerce and Industry of Oman), and the Japanese Embassy in Oman for their kind support during field surveys. We also thank K. Kanayama and R. Otsuka for their support during field surveys. We are indebted to N. Tsuchiya, E. Takazawa, T. Kurihara, and K. Hara for fruitful discussions. We appreciate the assistance of A. Tamura and Y. Soda with EPMA and LA–ICP–MS analyses at Kanazawa University. This study was
References (79)
- et al.
Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)
Chem. Geol.
(1996) - et al.
Shearzones, thrusts and related magmatism in the Oman ophiolite: initiation of thrusting on an oceanic ridge
Tectonophysics
(1988) - et al.
The Lu–Hf and Sm–Nd isotopic composition of CHUR: constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets
Earth Planet. Sci. Lett.
(2008) - et al.
The oxidation state of Fe in MORB glasses and the oxygen fugacity of the upper mantle
Earth Planet. Sci. Lett.
(2011) - et al.
A MORB source for low-Ti magmatism in the Semail ophiolite
Chem. Geol.
(2006) - et al.
Hf–Nd isotope and trace element constraints on subduction inputs at island arcs: Limitations of Hf anomalies as sediment input indicators
Earth Planet. Sci. Lett.
(2011) - et al.
Trace element characteristics of the fluid liberated from amphibolite-facies slab: inference from the metamorphic sole beneath the Oman ophiolite and implication for boninite genesis
Earth Planet. Sci. Lett.
(2005) - et al.
Fluid-mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes
Earth Planet. Sci. Lett.
(2003) - et al.
The timescales of subduction initiation and subsequent evolution of an oceanic island arc
Earth Planet. Sci. Lett.
(2011) - et al.
Reference materials in geochemistry and environmental research and the GeoReM database
Chem. Geol.
(2008)