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Evidence of short-living intraoceanic subduction in the Central Dinarides, Konjuh ophiolite complex (Bosnia-Herzegovina)

In the Dinarides ophiolites occur in two belts. The western belt, called Central Dinaric Ophiolite Belt (CDOB), is dominated by lherzolites and shows poorly developed igneous suites. The eastern belt corresponds to the Inner Dinaric Ophiolite Belt (IDOB), being better known as the Vardar Ophiolite Zone. This belt is characterized by harzburgite and dunite and has well developed igneous suites. The IDOB is segmented in two zones: the W zone stretches toward WNW meeting the CDOB in the Sava Zone. The E zone extends to the N and continues to the Apuseni ophiolites. The Krivaja-Konjuh ophiolite complex is 2 km thick, covers 250 km2 and represents the largest ophiolite complex in the CDOB. The complex is thrusted onto the Middle-Upper Jurassic ophiolite melange and is oversteped by Berriasian basinal Pogari Formation. It is segmented in two structural subunits: the western one consists of amalgamed blocks (Krivaja subunit) and the eastern represents the coherent synform (Konjuh subunit). The mantle peridotite, underlain by granulite to grenschist facies metamorphic sole, is predominant lithology. K-Ar data on amphiboles from the metamorphic sole amphibolites yielded age of 157 Ma. Mafic isotropic rocks and ultramafic cumulates are poorly developed. In the SE part of Konjuh subunit, pillow lavas are thrusted onto the unit which comprises massive dolerite, isotropic gabbroic rocks, ultramafic cumulates and tectonite peridotites in the top-bottom sequence. The lower part of the unit is occasionally intersected by thin mafic dikes. This rock pile was interpreted as completely preserved MORB-type ophiolite complex. However, our new field and geochemical data on the ophiolitic rocks from the SE part of Konjuh subunit, combined with the data for its N part, brought in different geotectonic interpretation of this rock association. In the pillow lavas and massive dolerites, clinopyroxene composition reveals MORB-like signatures whereas in the intersecting dikes it shows the composition akin to the clinopyroxene from volcanic arc settings. The intersecting dikes are composed of plagioclase (An73.0-22.3), tschermakite-magnesiohornblende, olivine (Fo58.1-59.8), orthopyroxene (Mg# = 55.5-69.6) which is typical assmblage of mafic rocks from the suprasubduction settings. In the mantle peridotites spinel shows composition [Mg# = 47.2-64.9 ; Cr# = 34.7-53.2] similar to the spinel compositions in the transitional harzburgites from the recent arc– basin systems represented by the South Sendwichs and Mariana Trough. However, plagioclase (An86.1-96.8) and/or pargasite metasomatism has introduced apparently lherzolitic major element compositions to the Konjuh massif transitional harzburgites. This could be the reason for classification of majority of Konjuh peridotites as lherzolites in the past. All minerals from the Konjuh transitional metasomatized harzburgites show significantly more depleted compositions when compared to the composition of equivalent minerals from the Krivaja-Konjuh lherzolites repoted in existing literature. Chondrite-normalized REE patterns of referent rocks from the SE Konjuh ophiolite complex reveal diverse geochemical signatures. Pillow lava is LREE-enriched [(La/Yb)cn = 9.7] and shows OIB-like primitive mantle-normalized multi-element pattern [(Nb/Y)mn = 9.3] with high negative Pb- and Sr-anomalies These features are typical of ridge basalts erupted close to ocean islands. Massive dolerites have N-MORB REE patterns [(La/Yb)cn = 0.54-0.58] but show slight negative anomaly of all HFS elements suggesting formation in the back-arc ridge setting. Intersecting dikes show slightly LREE depleted to flat REE patterns [(La/Yb)cn = 0.61-0.95], significant negative Ta-Nb anomaly and slight negative anomaly of other all HFS elements. Boninite dike displays rather flat REE pattern at the 2-4 times chondrite concentration. The sample is typically high Mg (Mg# = 85.7) and low Ti and Zr (0.21 wt.% TiO2, wt.%, 5.3 ppm Zr) with other HFSE concentrations under the detection limit. The geochemical signatures clearly decipher that dike rocks intersecting the Konjuh ophiolite complex were generated in a suprasubduction setting. Mantle peridotite shows U-shaped REE pattern at normalized concentration level typical for metasomatized transitional harzburgite. Strong positive Eu-anomaly is consistent with plagioclase metasomatism which caused lherzolitic major element composition (2.4-2.5 wt.% Al2O3, 2.3-2.6 wt.% CaO). MORB-type ophiolite complex of the SE Konjuh was exclusively generated at the ridge, most likely in a back arc setting. The exact age of beginning of steady state crust production is not known yet. Near ridge lithospheric detachment, with the production of metamorphic sole, took place around 157 Ma ago (Oxfordian) indicating the incipient shortening in the Krivaja-Konjuh oceanic domain. Thereupon, the Konjuh lithospheric segment has been overridden by Krivaja segment forming a kind of primitive submerged intraoceanic arc existed till the Late Tithonian when the Krivaja-Konjuh unit was finally obducted onto Adriatic platform. During the Oxfordian-Tithonian period the overriden MORB-lithospheric plate, i.e. Konjuh ophiolite complex, was modified by extensive metasomatism of its mantle wedge. Minor build in of crust by IAT- and boninite-type dikes occurred concomitantly. Berriasian Pogari overstep complex containing fragments of the Jurassic ophiolitic rocks and Late Variscan granites clearly indicates Late Jurassic extinction of the Krivaja-Konjuh oceanic domain. Some oceanic segments of the Central Dinarides, like for the instance the Kozara domain, produced oceanic crust younger than 156 Ma. Those domains finally closed during the Late Cretaceous/Paleocene.

Central Dinarides; Bosnia-Herzegovina; Konjuh ophiolite complex; MORB-crust; intraoceanic subduction

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