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Western Dinaride mantle peridotites: Krivaja-Konjuh case study

The western Dinaride peridotites in the Eastern Mediterranean are a part of the Alpine-Himalaya suture zone of the former Tethys ocean that separated Gondwana and Eurasia during the Mesozoic. The belt is characterized by lherzolites and poorly developed igneous suites. In comparison to the Alpine or Greek peridotites, the Bosnian and Serbian bodies are not as well characterized. The Krivaja-Konjuh massif in Bosnia is one of the largest complexes within this belt (~ 250 km2). It consists of a more fertile part western part (Krivaja) and a more depleted and altered eastern part (Konjuh).The complex is thrusted onto an ophiolite melánge of Middle to Late Jurrasic age. Parts of the southern and northern margins of the complex consist of a metamoprhic sole of granulitic to greenschist facies. K-Ar dating of amphibolites from the metamorphic sole gave age of 157 Ma. The present geotectonic interpretation of Krivaja-Konjuh is that it represents a supra-subduction setting (Lugović et al., 1991, 2006). In order to gain better insight into mantle processes predating partial subduction and obduction in this region, we examined the Krivaja subunit in more detail. This unit consists of peridotite, troctolite and gabbro, with possibly associated minor basalt. The dominant peridotite type in the Krivaja is spinel lherzolite with locally occurring plagioclase reaction rims around coarse spinel. The peridotites are deformed throughout the massif with textures ranging from relatively coarse-porphyroclastic to mylonitic. The peridotites are relatively fertile with olivine magnesium numbers (Mg#) ranging from 89 to 91. Samples with mylonitic textures from the southwestern margin of the masiff have spinel chromium numbers (Cr#) of 12 as well as high Na2O content in their clinopyroxenes (1.2-1.8 wt. %). By comparison, samples with coarse-grained textures have spinels with Cr# around 55. Near the transition from peridotite to troctolite both spinels coexist with high and low Cr#. Low Cr# spinels are interstitial and fine-grained. High Cr# spinels from this transition zone are very coarse and rimmed with olivine and plagioclase, separating them from surrounding orthopyroxenes. The overall shape of this assemblage recalls a former garnet. Such samples with two generations of spinel could provide evidence for refertilization in the spinel stability field. In a plot of Al2O3 in clinopyroxenes versus Cr# in spinel, our data from the Krivaja falls near that of a slow spreading ridge (Morishita et al., 2009). The mantle rocks transition into troctolites that in turn transition into olivine-rich gabbros and gabbros. The troctolites consist of more than 90% olivine and clinopyroxene, plagioclase, and spinel. Spinel occurs both as inclusions in olivine and also associated with interstitial minerals, plagioclase and clinopyroxene. These interstitial minerals show a striking resemblance to melt textures we observe in experimentally produced samples. Several factors suggest that this collection of peridotites spans the continent-ocean transition, including the fertile nature of the peridotites and the range of Na2O concentrations at a given Cr2O3 concentration in the clinopyroxenes (Müntener et al., 2010). This implies that this suite of samples provides a unique window on the transition from subcontinental to oceanic mantle.

Western Dinarides; mantle peridotites; Krivaja-Konjuh

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