Petrogenesis of crustal wehrlites in the Oman ophiolite: Experiments and natural rocks

Abstract

[1] In the Wadi Haymiliyah of the Oman ophiolite (Haylayn block), discordant wehrlite bodies ranging in size from tens to hundreds of meters intrude the lower crust at different levels. We combined investigations on natural wehrlites from the Wadi Haymiliyah section with an experimental study on the phase relations in a wehrlitic system in order to constrain the petrogenesis of the crustal wehrlites of the Oman ophiolite. Secondary ion mass spectrometry analyses of clinopyroxenes from different wehrlite bodies imply that the clinopyroxenes were crystallized from tholeiitic, mid-ocean ridge (MORB)-type melts. The presence of primary magmatic amphiboles in some wehrlites suggests a formation under hydrous conditions. Significantly enhanced 87 Sr/ 86 Sr isotope ratios of separates from these amphiboles imply that the source of the corresponding magmatic fluids was either seawater or subduction zone-related. The experiments revealed that under wet conditions at relatively low temperatures, a MORB magma has the potential to produce wehrlite in the ocean crust by accumulation of early olivine and clinopyroxene. These show typically high Mg# which is a consequence of the oxidizing effect of the prevailing high aH 2 O. First plagioclases crystallizing after clinopyroxene under wet conditions are high in An content, in contrast to the corresponding dry system. Trace element compositions of clinopyroxenes of those wehrlites from the Moho transition zone are too depleted in HREE to be in equilibrium with present-day MORB, implying a genetic relation to the V2 lavas of the Oman ophiolite, which are interpreted to be the result of fluidenhanced melting of previously depleted mantle. We present a model on the petrogenesis of the crustal wehrlites in an upper mantle wedge above an initial, shallow subduction zone at the beginning of the intraoceanic thrusting.

A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

“…The high mg-numbers of the chromite-hosted Fe^Mg silicates are consistent with high water activity (a H2O ¼1) in the crystallizing melt (e.g. Koepke et al, 2009) or its crystallization directly from a high-temperature aqueous fluid (e.g. Python et al, 2007).…”

Architecture of the Oman–UAE ophiolite: evidence for a multi-phase magmatic history

“…Petrographical analysis shows that the wehrlites have a different mineral crystallisation sequence to that expected for MORB, with early crystallisation of pyroxenes which indicates more hydrous melts (Yamasaki et al 2006;Koepke et al 2009), typical of magmas formed above subduction zones. Some previous workers (e.g.…”

Crustal recycling processes in generating the early Cretaceous Fangcheng basalts, North China Craton: New constraints from mineral chemistry, oxygen isotopes of olivine and whole-rock geochemistry

“…Olivine phenocrysts with very high Fo (up to 94) were also reported in the arc basalts in Solomon Islands (Kamenetsky et al, 2006;Rohrbach et al, 2005). Recent experimental studies on hydrous basaltic magmas also support the fractional crystallization origin of such high-MgO mineral assemblage (Feig et al, 2006;Gaetani et al, 1993;Koepke et al, 2009). Therefore an alternative interpretation is that the high-Fo olivine phenocrysts from the Fangcheng basalts may have crystallized from a hydrous primary magma.…”

“…Open field represents clinopyroxene data from anhydrous experiments (0-15 kbar) and from mafic rocks in non-subduction-related settings (Gaetani et al, 1993 and references therein). Grey field represents cumulus clinopyroxene from ophiolites and subduction-related ultramafic igneous complexes (Gaetani et al, 1993;Gahlan et al, 2012;Koepke et al, 2009). Clinopyroxene megacrysts from the late Cretaceous and Cenozoic alkali basalts of the NCC are from Liu et al (1992) and Yan and Chen (2005).…”

“…Olivine with high forsterite (Fo =~92) content from the Fangcheng basalts has been interpreted as xenocrysts from the refractory lithosphere mantle beneath NCC despite the absence of 'true' mantle xenoliths such as spinel-and/or garnet-facies peridotites in the basalts (Zhang, 2005). On the other hand, mafic-ultramafic rocks that are mainly composed of olivine and clinopyroxene are reported to have formed from hydrous magmas at crust-level chambers, similar to arcrelated ophiolitic suites in Oman and Egypt (e.g., Gahlan et al, 2012;Koepke et al, 2009;Koga et al, 2001) and Alaska-type ultramafic-mafic complexes in Alaska and New Zealand (e.g., Li et al, 2012;Spandler et al, 2003). Olivine phenocrysts with very high Fo (up to 94) were also reported in the arc basalts in Solomon Islands (Kamenetsky et al, 2006;Rohrbach et al, 2005).…”