Naslov
Mantle harzburgites (serpentinites) from Gornje Orešje (Medvednica Mts., Croatia)

Autori
Lugović, Boško ; Slovenec, Damir

Vrsta, podvrsta i kategorija rada
Poglavlja u knjigama, znanstveni

Knjiga
Excursion Guide, Joint Meeting of Croatian and Hungarian Geological Societies on Geology of the Zagorje-Mid-Transdanubian Zone, 22.-24.10.2004

Urednik/ci
Halamić, Josip

Izdavač
Hrvatski geološki institut

Grad
Zagreb

Godina
2004

Raspon stranica
40-45

ISBN
000

Ključne riječi
Medvednica Mts., mantle harzburgites, back arc setting

Sažetak
Medvednica Mts. is located at the southwesternmost termination of the Zagorje-Mid-Transdanubian shear zone. In the central part of the mountains the Middle Jurasic to Hauterivian ophiolite mélange contains fragments of undisturbed ophiolite sequence comprising ultramafic cumulates, gabbros and basalts along with cross-cutting diabase dikes. The mantle peridotites were found as blocks incorporated in the Upper Cretaceous-Paleogene tectonic mélange apart from the base of the undisturbed ophiolite sequence. Mantle peridotites with metamorphic fabric crop out near the village of Gornje Orešje on the northeasternmost slopes of the Medvednica Mts. as two separated blocks exposed at surface of total 0.4 km2. From the S and SW the peridotites are transgressively overlain by sedimentary succession comprising Upper Cretaceous polimictic conglomerates. Both units are unconformatly covered by Upper Badenian-Sarmatian conglomerates, limestones and marls. The contact with Campanian-Maastrichtian rudist limestones is tectonic. An Upper Cretaceous crystallization age of peridotites was assumed and the recent position was explained by the peridotites “ diapiric upwelling” happened between Eocene and Helvetian. Fresh peridotites are rare, mostly severly altered to serpentinites without olivine remnants. Relic mineral assemblage consists of olivine (Mg# = 89.9– 92.1 ; 0.34– 0.45 wt.% NiO), Al-Cr enstatite (Mg# = 90.0– 91.4 ; Al2O3 = 2.03– 4.59 ; Cr2O3 = 0.54– 1.06 ; CaO = 0.87– 1.80), minor Al-Cr diopside (Mg# = 90.3– 92.8 ; Al2O3 = 2, 58– 6.22 wt% ; Cr2O3 = 0.78– 1.53 wt%) and accessory opâque spinel (Mg# = 56.1– 66.6 ; Cr# = 0.309– 0.471). Both pyroxenes show complementary exsolution lamellae. The samples show porphyroclastic texture with large ductile deformed orthopyroxene porphyroclasts set in moderate schistous matrix composed mostly of hrysotile/lizardite and magnetite with minor chlorite, quartz, talk, prehnite, and zeolite. Plagioclase (An95) was identified only in the fresh sample GO-B. Kink-bending of orthopyroxene porphyroclasts and cross-cutting by veinlets of dolomite are common. Two pyroxenes geothermometer of and olivine-spinel geothermometer gave consistent temperature results. The calculated average temperatures for coexisting orthopyroxene and clinopyroxene for the samples GO-1 and GO-5 are 1021 &plusmn ; ; 26°C and 1004 &plusmn ; ; 19°C, respectively, and 972 &plusmn ; ; 17°C for plagioclase-bearing peridotite GO-B. Calculated temperatures for coexisting olivine and spinel in the sample GO-B show the range of temperatures from 810°C to 600°C. In the diagram Al2O3– CaO– MgO constructed to classify the rock of a ophiolite suite the analysed rocks correspond to tectonite peridotites. The intensity of alterations range from moderate (H2O = 6.7 wt.% ; sample GO-B) to severe (H2O = 9.6– 10.9 wt.% ; samples GO-1 and GO-5). The concentrations of Al2O3 (1.7-2.4wt.%) and CaO (1.1-1.5 wt.%) expressed on volatile-free basis are consistent with moderately depleted peridotites, i.e. with the rocks of harzburgite to transitional composition. Low Mg# [= 100*MgO/(MgO + FeOtotal)] of 87.0-89.5 suggest significant loss of MgO during serpentinization. Chondrite normalized REE patterns in altered peridotite samples and show strongly depleted LREE (La, Ce and Pr under detection limit), and MREE-HREE segments of the patterns have steep positive slope (Smcn/Lucn = 0.17). On the contrary, fresh sample has U-shaped REE pattern with HREE profile similar to altered peridotites indicating melt/fluid supply to mantle wedge from subducted slabs. Concerning their bulk rock geochemical and textural characteristics harzburgites from Gornje Orešje correspond to Alpine-type peridotites with transitional characteristics between suprasubduction zone peridotite and MORB-type peridotites. Medvednica Mts. harzburgites show REE patterns apparently akin to harzburgites and depleted lherzolites from the South Sandwich arc-basin system, especially with peridotites from trench-fracture zone intersection. Consequently, a marginal basin setting was inferred as their original geotectonic setting. Chemical compositions of spinel and orthopyroxene were used to compare between the peridotites of Medvednica Mts., Centaral Dinaridic Ophiolite Belt (CDOB), Vardar Zone (i.e. Inner Dinaridic Ophiolite Belt - IDOB), and those reported from active back arc basins. According to spinel chemistry the Medvednica Mts. harzburgites belong to Type I peridotites and are transitional between CDOB lherzolites and IDOB harzburgites. Relationship between Al2O3 in orthopyroxene and Cr/(Cr+Al) ratio in spinel suggests that the coexisting minerals were equilibrated in the depleted segment of abyssal peridotites close to the field of fore-arc peridotites from southern Mariana. The CDOB peridotites (fertile lherzolites) plot beyond the expected field of abyssal peridotites and the IDOB peridotites (harzburgites and dunites), repeatedly believed to have formed in a back-arc setting, are stretched in the field of fore-arc peridotites. Since the overall chemical compositions of analysed rocks are highly correlated with recent back-arc peridotites from southern Mariana Trough we envisage a back arc setting (inboard in an intra-oceanic arc ?) as the most plausible environment for the Medvednica Mts. harzburgites.

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Engleski

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Geologija

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