Elsevier

Lithos

Volume 106, Issues 1–2, November 2008, Pages 137-154
Lithos

Chromian spinels in mafic–ultramafic mantle dykes: Evidence for a two-stage melt production during the evolution of the Oman ophiolite

https://doi.org/10.1016/j.lithos.2008.07.001Get rights and content

Abstract

This paper describes a comprehensive study of the chromian spinels present in mafic–ultramafic dykes cropping out along the mantle section of the Oman ophiolite. We studied about 1100 samples in thin section and with the electron microprobe. Chromian spinel is almost ubiquitous in primitive dykes (high-Mg# troctolites and pyroxenites) and less common in more differentiated ones (olivine gabbros and gabbronorites). The Cr#, TiO2 content, and other compositional parameters are well correlated to the nature and composition of the co-genetic silicate assemblage. Chromian spinel composition contributes to establish that the mantle dykes of Oman are more or less evolved cumulates that crystallised from two main types of primary melts: tholeitic melt similar to Mid-Ocean Ridge basalts (MORB; 0.45 < Cr# < 0.63; 0.3 < Mg# < 0.6; TiO2 up to2 wt.%), and more silicic melts issued from a highly depleted mantle source, similar to boninitic–andesitic melts that preferentially form in subduction zone settings (0.35 < Cr# < 0.80; 0.1 < Mg# < 0.7; TiO2 up to 0.2 wt.%). The chromian spinel composition presents a higher variability than the associated silicates and allows us to further unravel the petrological evolution and segmentation of the Oman ophiolite. The composition of chromian spinel in mantle dykes and in the spatially related residual harzbugites display well correlated variations at the scale of the Oman ophiolite. This shows that these two lithologies share a common magmatic history, even if, strictly speaking, they cannot be related through direct parent–daughter relationships. The Cr# is on average higher, and the TiO2 lower in the NW than in the SE, consistent with an increasing influence of “marginal” magmatic processes in the NW, while the southeastern area has petrological characteristics closer to those of an “open” ocean. In this southeastern part, compositional variations of the chromian spinel are correlated to structural characteristics related to the spreading history: the distribution of the Cr# around a former mantle diapir cropping out in the Maqsad area is concentric, with the highest values in the centre of this structure, consistent with higher degrees of partial melting of the mantle. On the other hand, unusually low Cr# – diagnostic of a low melting degree – in a ridge-parallel band of about 20 km to the northeast of the Maqsad area can be related either to temporal variations in the partial melting degree or to off-axis magmatic activity.

Introduction

Chromian spinel has a general formula ([Mg,Fe2+][Cr,Al,Fe3+]2O4) where Cr concentration is appreciable. Mn, Zn, V and Ti are also usually present as minor components. This mineral is commonly found in mantle-derived peridotites where it is considered to be a powerful petrogenetic indicator, its composition being highly dependant on the degree and conditions of partial melting and of magma/rock interactions (Irvine, 1965, Irvine, 1967, Arai, 1992). The atomic ratio MgMg+Fe2+(=Mg#) of spinel in equilibrium with olivine depends on the equilibrium temperatures (Irvine, 1967, Jackson, 1969, Evans and Frost, 1975). The chromium number (Cr#=CrCr+Alatomicratio) is especially important for understanding the genesis of magmas as it indicates the degree of partial melting or the degree of depletion of the source (Dick and Bullen, 1984, Arai, 1994). Chromian spinels in peridotites are traditionally used as indicators of the degree of partial melting for tectonic discrimination between Mid-Ocean Ridge (MOR) and Supra-Subduction Zone (SSZ) settings (Dick and Bullen, 1984, Parkinson and Pearce, 1998, Pearce et al., 2000, Arai et al., 2006).

Interpreting the composition of chromian spinel in plutonic rocks is more complicated: it depends on many parameters including the nature of the parent melts, the conditions of crystallisation (T, P, fO2), the composition of coexisting phases and subsolidus processes like diffusion and recrystallisation. In the present paper, we examine chromian spinel composition in dykes, sensu lato – i.e. all kinds of former melt migration features – cropping out in the mantle section of the Oman ophiolite. In a previous study (Python and Ceuleneer, 2003), we presented detailed description of these dykes at the scale of the entire Oman ophiolite except for the characteristics of their oxide minerals, including especially chromian spinels. Accordingly, the general philosophy of our approach in the present paper is quite simple: we use the constraints from this previous study (field relations, distribution at the scale of the entire ophiolite, nature and composition of the silicates) to understand which kind of petrogentic information can be deduced from the study of chromian spinel. A key question is to what extent can the variability in the composition of “plutonic” chromian spinel be used to distinguish different magma series, and hence different tectonic settings. In the case of Oman, this question has still not been resolved in spite of extensive studies. The ophiolite has been interpreted as a fragment of oceanic lithosphere generated along a Mid-Ocean Ridge by researchers who mapped the plutonic and mantle section of the ophiolite (Reinhardt, 1969, Coleman, 1981), or alternatively as a fragment of a marginal basin (some kind of immature island arc) by those who focused their investigations on the volcanic section (Pearce et al., 1981, Alabaster, 1982). The comprehensive dataset about chromian spinel we present in this paper will, hopefuly, help to progress in tectonic discrimination based on the petrological characteristics of plutonic rocks.

Section snippets

Geological settings and previous work on mantle dykes

The Oman ophiolite is one of the largest ophiolites from the now disappeared Tethys Ocean. It was obducted on to theArabian margin in Maestrichtian times (see review in Nicolas, 1989). As it was not included in the Alpine collision belt, its internal structures are remarkably well preserved, although it was slightly dismembered during its final obduction and, more recently, during the Miocene uplift of the Oman mountains.

The large-scale structures of the Oman ophiolite (mostly the attitude of

Field relationships and general petrographic characteristics

On the basis of petrographic observations (modal compositions and textures), thirty six petrographical types were distinguished among the 1200 samples we collected for this study. The mineral composition of each sample, determined with the electron microprobe, was used as independent evidence to relate these dykes to two different differentiation series (Python and Ceuleneer, 2003). Thirty of the thirty six petrographic types can be grouped into four main families corresponding to primitive and

Chemical characteristics of the spinels

Mineral compositions of each dyke was analysed by electron microprobe (the Cameca SX-50 of Toulouse University for the large majority of the samples, the JEOL 8800 of the Kanazawa University was used for a few of them). With both instruments, a classical analysis program, with an acceleration voltage of 20 kV, a probe current of 20 nA and a beam diameter of 3 µm was used. Counting times varied from 10–20 s on the peak, and from 5 to 10 s on the background. The chemical characteristics of the

Discussion

Our previous study of Oman mantle dykes focused on field and petrographic characteristics, and on the chemical composition of silicate minerals. These data allowed us to distinguish four main petrographical families and to group them into two magmatic series(MORB and depleted, see Python and Ceuleneer, 2003, and Table 1). The chemical characteristics of the chromian spinels largely fits with this previous classification: chromian spinel has, on average, lower Cr# and higher TiO2 concentrations

Conclusions

Most Oman mantle dykes contain 0.5 to 2 vol.% of chromian spinel. The composition of this minor phase confirms the results from our previous study focused on the silicate minerals, i.e. troctolites and olivine gabbros are derived from MORB-like melts while pyroxenites and gabbronorites are derived from depleted, high-Mg andesites. The composition of chromian spinel presents a higher variability than that of the associated silicates and is useful in determining the petrogenesis of the mantle

Acknowledgements

We are grateful to Fabienne de Parseval, Anne-Marie Roquet, Raphaël Peyron and Jean-François Ména at the thin section manufacturing and to Philippe de Parseval at the microprobe service for their help at the Toulouse University. Marc Monnereau, Camille Ceuleneer and Hilal al Azri were of great help for our field work, and Akihiro Tamura for every day support and discussion at the Kanazawa University. We really appreciated the great review work of Sarah Dare and Hugh Rollinson, they considerably

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