Naslov
Origin of the ophiolite-related Al-(Mg) metamorphites – case study of saphirine and corundum amphibolites from the Central Dinaridic Ophiolite Zone (CDOB, NE Bosnia and Herzegovina)

Autori
Šegvić, Branimir ; Altherr, Rainer

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Skup
88th Annual meeting of the German Mineralogical Society

Mjesto i datum
Münster, Njemačka, 2010

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
no ; key ; words

Sažetak
Two rare amphibolite parageneses consisting of Am, Pl, Spr, and Spl (paragenesis 1), and Am, Pl, Crn, and Spl (paragenesis 2) were reported within the suite of metamorphic sole rocks that are associated to the Krivaja-Konjuh ophiolite complex (KKOC) in NE Bosnia and Herzegovina. It is the biggest Tethyan ophiolite complex of the CDOB, hosting diverse metamorphic rocks from greenschist to granulite and eclogite facies. These rocks are found in forms of separated zones of up to 10 km long and several hundred metres in width (Operta, 2004 ; Šegvić, 2010). The sapphirine and corundum rich amphibolites present a small portion of the KKOC metamorphites, dominated by the porphyroblastic Grt-Di and Grt-Di-Hy amphibolites as well as Di-Hbl and Pl-Grt-Di gneisses. In this study, we put an accent to the protolith ambiguities and an amphibolite formation mechanism, since the occurrence of sapphirine and corundum amphibolites is relatively rare, often being explained by a material exchange of the Al-rich rocks and ultrabasic rocks (e.g. Nicollet, 1986). Sapphirine amphibolite is featured by granoblastic to poikiloblastic, partly preserved relict ophitic texture, defined by coarse-grained amphibole (~0.25 mm). Often, blasts of Cr-spinel are rimmed with a corona composed of sapphirine and Al-rich spinel. Corundum amphibolite is characterised by medium-size amphibole and corundum blasts (~0.15 mm), defining the rock’s granoblastic to porphyroblastic texture. Textural characteristics, primarily marked by coarse euhedral pargasite crystalloblasts devoid of subgrain development and tapered mechanical twinning of basic plagioclase (~81.3 An) are usually considered to indicate the high temperature metamorphic conditions. The TAS diagram classifies the analysed rocks as basalts implying their tholeiitic affinity. They are silica-undersaturated (~40 wt%), rich in Al2O3 (~17-27 wt%) and MgO (>10 wt%), and low in TiO2 (0.06-0.09 wt%). Generally, their REE normalisation patterns are MORB-like, having significantly lower compositional levels (0.4-2 x chondrite). Elevated Mg# (~87), high abundances of incompatible trace elements (35- 153 ppm Ni, 30-63 ppm Co, 41-57 ppm V), low abundances of high field strength elements (HFSE), coupled with low light rare earth elements concentration (LILE) suggest that the protolith of the analysed metamorphites must have been an igneous cumulate, most probably of troctolitic origin rather than crystallised from the ‘normal’ tholeiitic melts. Phase chemistry characteristics, like high Al2O3 and K2O contents in amphibole (~17 and 0.69 wt%), corroborate an extreme heat influx having affected the protolith. Geothermobarometric calculations (sapphirine-spinel and amphibole- plagioclase pairs) provided peak metamorphic conditions of 0.90-1.10 GPa and ~880 °C. A P-T- t clockwise path is inferred. Comparing the petrological implications with the actual geotectonic models of the Dinaric Tethyan oceanic evolution in the Jurassic/Cretaceous period, the analysed metamorphic rocks are considered to represent a pile of a metacumulate sequence that experienced a HT-MP metamorphism as a metamorphic sole associated to near-ridge thrusting processes. As well, there are strong indications suggesting that during metamorphosis some of the mantle material was assimilated. It is evidenced in the preservation of igneous Cr-spinel in sapphirine and amphibole, along with a formation of calcite ‘blebs’ in amphibole. A sapphirine blastosis is linked to spinel destabilization induced by a change of chemical potentials.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

POVEZANOST RADA