The significance of metasomatic amphibole veins in the genesis of the intraplate lavas in NW Syria

George S.-K. Ma¹, John Malpas¹, Costas Xenophontos¹ & Gavin H.-N. Chan²

¹Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, georgema@graduate.hku.hk ; jgmalpas@hku.hk ; cosxen@spidernet.com.cy

²Department of Earth Sciences, Oxford University, Parks Road, Oxford, OX1 3PR, UK, gavin.chan@earth.ox.ac.uk

This webpage is a summary of: Ma, G. S.-K., Malpas, J., Xenophontos, C. & Chan, G. H.-N. (2011). Petrogenesis of latest Miocene–Quaternary continental intraplate volcanism along the northern Dead Sea Fault System (Al Ghab–Homs Volcanic Field), western Syria: evidence for lithosphere–asthenosphere interaction. Journal of Petrology, 52, 401-430.

Introduction

Basalts from scattered Neogene intraplate volcanic fields along the northern Dead Sea Fault System in northwestern Syria (termed N-DSFS lavas; Figure 1) are similar to many volcanic rocks of continental intraplate settings worldwide — low melt volume, ranging in composition from basanitic to tholeiitic (transitional), no clear age progression, and no associated domal uplift. No plausible relationship with a mantle plume can be determined. We have assembled geochemical data of this region previously published by various authors (Abdel-Rahman & Nassar, 2004; Lustrino & Sharkov, 2006; Krienitz et al., 2009) and those of our own, and consider the origin of these basalts using knowledge gained from recent partial melting experiments by others (e.g., Pilet et al., 2008).

Figure 1. Distribution of Cenozoic volcanic rocks in northwestern Syria and northern Lebanon, with major structural features redrawn after Brew et al. (2001). EAF, East Anatolian Fault; NAF, North Anatolian Fault; DSFS, Dead Sea Fault System.

The volcanic rocks we study erupted around Homs (~6–4 Ma) and in the northern Al Ghab depression (4–1 Ma), and are associated with a shield volcano and numerous small cinder cones respectively.

Geochemical signatures of the involvement of amphibole in the source region

Comparing the most primitive N-DSFS lavas (8.1-11.7 wt% MgO on anhydrous basis) with isobaric experimental partial melts (Figure 2) suggests that the lavas have an intimate relationship with amphibole-rich lithologies during partial melting.

Figure 2. CIPW normative diopside, olivine, hypersthene, nepheline+leucite and quartz variations in the relatively primitive, least crustally contaminated N-DSFS samples (MgO ≥ 8 wt%, Ce/Pb ≥ 21 and Ba/Nb ≤ 10). Also shown are the melt compositions in a range of partial melting experiments on various sources, including: phlogopite-bearing peridotite, carbonated peridotite (KLB-1), hydrous peridotite (KLB-1), silica-deficient pyroxenite (MIX1G), silica-excess (G2 and JB-1) pyroxenite, hornblendite (AG4), clinopyroxene-hornblendite (AG7) and sandwiched hornblendite–depleted peridotite (DMM1) which are referenced in detail in Ma et al. (2011). Arrows show partial melting trends, at different pressures, for anhydrous peridotite (KLB-1 and similar compositions) based on the compilation of Thompson et al. (2005), and trends for CO₂-bearing peridotite (Dasgupta et al., 2007).

In general, the compositions of the N-DSFS lavas lie close to the trends of anhydrous partial melts generated by progressive melting of peridotite at 1 and 1.5 GPa, with the majority of the lavas being Si-undersaturated and falling on the (Ne+Lc)–Di–Ol plane. What interested us most is that the Si-undersaturated lavas are “deflected” towards the compositional field defined by melts of hornblendite, in which some indeed fall.

Other potential mantle lithologies/components do not seem to have a significant role in the genesis of the N-DSFS lavas:

Figure 3. (a) Ti₁₀, (b) (La/Yb)_N, (c) (Sm/Zr)_N, (d) (K/La)_N and (e) (Rb/Ba)_N vs Si₁₀ for the relatively primitive N-DSFS samples. Ti₁₀ and Si₁₀ are the estimated concentrations of TiO₂ and SiO₂ respectively at 10 wt% MgO after correction for crystal fractionation. Subscript N denotes normalization by primitive mantle values.

Together, these facts suggest that hornblendite or similar metasomatic amphibole-rich veins are by far the most plausible components in the source for the genesis of these Si-undersaturated magmas.

It is found that within the range of Si₁₀, the Ti₁₀, La/Yb, K/La (K-anomaly), Rb/Ba (Rb-anomaly) and, to a lesser extent, Sm/Zr (Zr-anomaly) of the N-DSFS lavas vary systematically (Figure 3). These correlations are explained by mixing or interaction between at least two source components — a metasomatic component in the form of an amphibole-rich lithology (high in Ti, La/Yb and Sm/Zr, and low in Si, Rb/Ba and K/La) and a peridotitic component. This hypothesis is tested by calculating the expected melt compositions derived by melting a hornblendite and a perditotite. The modelling results reveal that the compositions of nearly all of the N-DSFS lavas lie between calculated partial melting curves of a garnet-bearing hornblendite and a garnet-bearing peridotite, and suggest that the N-DSFS lavas can be generated by mixing (or other kinds of interaction) between melts generated by the two source lithologies (Figure 4).

Figure 4. Primitive mantle-normalized (a) (K/La)_N, (b) (Dy/Yb)_N, (c) (Sm/Zr)_N and (d) (Rb/Ba)_N vs (La/Yb)_N for the relatively primitive, least crustally contaminated N-DSFS lavas. The continuous curves show non-modal batch melting models for a lherzolite (with a primitive mantle composition) and a hornblendite metasomatic vein (AG4 of Pilet et al., 2008) as starting materials, from 1 to 16.8% and from 25 to 60% melting respectively. The kink in the metasomatic vein melting curve results from complete consumption of amphibole at 47.5% melting; beyond this point, melting mode of the residual ‘‘pyroxenite’’ vein is assumed to be modal for simplicity. It should be noted that most basalt samples fall in the yellow-shaded fields, which define the possible melt compositions from 0 to 100% mixing of small-degree (F = 2–10%) lherzolite melts and large-degree (F = 30–45%) metasomatic vein melts. For clarity in the diagram, the degrees of melting and mixing are not always displayed but the latter can be evaluated from the representative mixing curve (dashed) between a 6% lherzolite melt and a 40% vein melt (numbers along the dashed curve denote the proportion of lherzolite melt in the hybrid melt). Italic numbers along the continuous curves are the degrees of partial melting. Despite some scatter, samples of the Al Ghab group (mainly basanites) are characterized by higher (La/Yb)_N, (Dy/Yb)_N and (Sm/Zr)_N, and lower (K/La)_N and (Rb/Ba)_N, suggesting greater influence of metasomatic veins during their petrogenesis. Positions of the terms basanite, alkali basalt (AB) and tholeiitic basalt (TB) along the top of (a) and (b) denote the approximate (La/Yb)_N ranges of each type. Other parameters are given in Table 6 of Ma et al. (2011). F, degree of partial melting.

Indeed, when the Si₁₀, which is a proxy of the metasomatic component of the lavas, are compared with the isotopic composition, it appears that the elemental correlations observed in Figures 3 and 4 cannot simply be a two-component binary mixing — three components are needed to explain the Si₁₀–isotope variations, and these are components interpreted to represent the metasomatic veins, lithospheric wall-rock peridotite which hosts the metasomatic veins and asthenospheric peridotite mantle (for details of derivation see Ma et al., 2011).

Geodynamic considerations

There is increasing dominance of basanites over alkali (and tholeiitic) basalts from south (Homs; latest Miocene–Early Pliocene) to north (Al Ghab; Late Pliocene–Quaternary) over time, and this dominance is accompanied by a significant decrease in the overall volume of magma erupted (Figure 1). These characteristics may therefore be more consistent with a diminished thermal perturbation (a rifting model is excluded considering the absence of large-scale lithosphere extension in the region) below the Al Ghab volcanic field during Late Pliocene to Quaternary times. These considerations lead to the following model, in which a lithosphere previously metasomatized to contain amphibole-rich veins is involved.

In Syria, the observation that the northward migration of the volcanism was contemporaneous with the propagation of the DSFS and accompanied by an increase in Si-undersaturation in magma chemistry and a decrease in magma volume appears to be consistent with a channelled flow model. When asthenospheric upwelling mantle material (small-scale upper mantle diapirs or passive upwelling as a result of slab break-off beneath eastern Anatolia) was first emplaced beneath the Homs region in the latest Miocene, it was at its hottest and itself melted adiabatically to contribute rising magmas (this give rise to the asthenospheric mantle component). Heat advection and conduction caused the lithosphere to melt; interactions involving wall-rock assimilation (metasomatic veins and lithospheric peridotite assimilated by the ascending, hot asthenospheric melts) or melt hybridization (melts formed separately at first and mixed during ascent through the crust) produced a spectrum of magmas from basanite to tholeiitic basalt. Northward propagation of the DSFS diverted the partially molten asthenospheric material, which might have been continuously rising beneath the Homs region, to the Al Ghab region in the Late Pliocene. This material was probably cooler than it had been, and was unable to penetrate the Al Ghab lithosphere. The amount of thermal perturbation by conduction was probably just sufficient to trigger melting of the lowest solidus materials within the base of the lithosphere, the amphibole-rich metasomatic veins, which interacted with their wall-rock lithospheric peridotite to produce a spectrum of basanites and alkali basalts.

Conclusions

Among the least contaminated N-DSFS lavas, the Si-undersaturated lavas (basanites) are characterized by low SiO₂, K/La and Rb/Ba, and high TiO₂, Sm/Zr and Dy/Yb. These features are consistent with magma derivation largely from garnet-bearing hornblendite metasomatic veins, plus modest interactions with the surrounding wall-rock peridotite within the lithospheric mantle. The increasing extent of interaction and hybridization of such veins or their melted products with the wall-rock peridotite and rising asthenospheric magmas generated the more Si-rich N-DSFS lavas (alkali and tholeiitic basalts).

The observation that the younger Al Ghab group is dominated by basanites and the older Homs group by alkali and tholeiitic basalts reveals a significant control on magma compositions by hornblendite metasomatic veins and peridotitic mantle, respectively, during magma genesis. The temporal, spatial and compositional variations of the N-DSFS basalts can be explained as a consequence of the northward propagation of the N-DSFS during the Late Cenozoic and consequent channelling at the base of the lithosphere of upwelling asthenospheric mantle from the Homs region.

Post-publication notes

The latest S-velocity studies (after the publication of Ma et al., 2011) provide independent geophysical evidence for widespread low-velocity seismic structures, which can be related to thermal anomalies or melt/volatile-rich zones, beneath much of Arabia and east Africa. Chang & Van der Lee (2011) demonstrate three independent mantle diapirs, beneath Jordan, Kenya and Afar, extending down to at least mid mantle depths (~1400 km). According to their model, the volcanism in NW Syria that we have studied may be associated with a deeply-rooted (lower mantle), larger low-velocity structure (diapir) beneath Jordan, Israel and southern Syria, which becomes flattened above ~400 km and flows northwards (and also southwards) to eastern Anatolia.

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