Petrogenesis of peridotites and pyroxenites in the area of Banovina, Croatia
| Autor: | Bilić, Šime |
|---|---|
| Přispěvatelé: | Garašić, Vesnica, Tomljenović, Bruno, Slovenec, Damir, Balen, Dražen |
| Jazyk: | chorvatština |
| Rok vydání: | 2021 |
| Předmět: | |
| Popis: | U ovom radu istraženi su peridotiti i pirokseniti na području Banovine. Ove stijene su formirane kao dijelovi nekadašnjeg Zemljinog plašta te predstavljaju baznu jedinicu ofiolita i dokaz su postojanja oceanskog prostora, a izdignuti su u gornje dijelove kore intenzivnim tektonskim pokretima koji su se počeli odvijati za vrijeme jure, prije oko 150 milijuna godina. Peridotiti i pirokseniti Banovine dio su Centralnog dinaridskog ofiolitnog pojasa (CDOB = „Central Dinaridic Ophiolite Belt“) koji se proteže od Banovine prema jugoistoku preko Bosne i Hercegovine u Srbiju. Ovaj pojas ukazuje na nekadašnji oceanski prostor Neotethysa, a rekonstrukcija geodinamskog razvoja takvog prostora unutar šire regionalne slike vrlo je kompleksna. Napravljena su detaljna terenska istraživanja te petrografske i geokemijske analize peridotita i piroksenita. Utvrđeno je da se na prostoru Banovine po strukturnim i kemijskim značajkama razlikuju dvije vrste peridotita koje i geografski pripadaju različitim lokalitetima te se mogu svrstati u dva pojasa, sjeverni i južni. Unutar sjevernog pojasa (S-pojas) prevladavaju serpentinitne breče i serpentinizirani spinelski lercoliti, koji se na terenu prepoznaju po strukturama karakterističnim za ofiolitni melanž. Geokemijske karakteristike peridotita S-pojasa ukazuju na porijeklo iz suboceanskog plašta koji je prolazio kroz procese taljenja na području srednje oceanskih hrptova, zbog čega su klasificirani kao ofiolitni peridotiti. Južni pojas (J-pojas) sadrži spinelske lercolite, dunite i piroksenite koji se izmjenjuju unutar prostorno vrlo ograničenih prostora. Geokemijske karakteristike peridotita J-pojasa ukazuju na subkontinentalno porijeklo te su najvjerojatnije formirane tijekom faze početnog rifta gdje su se kao dijelovi kontinentalnog plašta izdizali u gornje dijelove kore, tijekom čega su prošli kroz vrlo nizak stupanj taljenja. Zbog navedenih značajki peridotiti J-pojasa klasificirani su kao orogeni peridotiti. Pirokseniti koji se nalaze unutar J-pojasa pokazuju drugačije petrografske i geokemijske karakteristike od peridotita. Oni su najvjerojatnije nastali kao zasebna parageneza kristalizacijom iz taljevina zasad nepoznatog porijekla koje su se probijale kroz plašt. Zaključno, ovaj rad je pokazao kako peridotiti i pirokseniti Banovine, a samim time i CDOB-a nose zapis triju različitih faza evolucije oceanskog prostora Neotethysa: a) rane faze početnog rifta i otvaranja oceana (peridotiti i moguće pirokseniti J-pojasa); b) kasnije faze već razvijenog oceanskog prostora (peridotiti S-pojasa) i c) faze zatvaranja oceana koja je evidentna iz struktura tipičnih za ofiolitni melanž, ali i prisutnosti amfibolita koji se nalaze u kontaktu s peridotitima, a koji su nastali kao posljedica obdukcije. In this paper, peridotites and pyroxenites in the Banovina area were investigated. These rocks were formed as parts of the former Earth's mantle and represent the base unit of ophiolites which indicates the existence of the ocean. They were ascended to the upper crust by intense tectonic processes that began about 150 million years ago. The peridotites and pyroxenites of the Banovina are part of the Central Dinaridic Ophiolite Belt (CDOB) that extends from the Banovina to the southeast through Bosnia and Herzegovina to Serbia. This belt indicates the former oceanic space, and the reconstruction of the development of such space within the wider regional picture is very complex. In this paper, detailed field research and petrographic and geochemical analyzes of peridotite and pyroxenite were performed. Field research has shown that ultramafics in the Banovina appear in two geographically different belts, the northern (S-belt) and the southern (J-belt). The S-belt contains mostly serpentinite breccias and serpentinized lherzolites. The J-belt contains larger masses of peridotites which consist predominantly of spinel lherzolites, and subordinate dunites and pyroxenites. Petrographic analyzes showed that spinel lherzolites and pyroxenites from Jbelt are significantly richer in clinopyroxene than spinel lherzolites from S-belt. In addition, petrographic analyzes showed large differences in the rock textures of the S- and J-belts. Within the S-belt, protogranular to porphyroblastic structures predominate, while within the J-belt, equigranular to porphyroclastic structures predominate. Pyroxenite structures show distinctly porphyroclastic structures. Chemical analyzes have shown that Jband spinel lherzolites have a higher content of Al2O3, CaO, Na2O, REE, and other trace elements than S-belt spinel lherzolites. J-belt pyroxenites have extremely high amounts of these elements, while J-belt dunites have a lower content of same elements, but higher amount of MgO. oxides of the main elements and a very low content of REE and trace elements. Analyzes of the major elements in all minerals show a higher Mg-number in spinel lherzolites from the S-belt. Clinopyroxenes and orthopyroxenes of all J-belt ultramafics generally have a higher content of Al2O3, Na2O, REE, and trace elements. The Cr2O3 content is higher in clinopyroxenes and orthopyroxenes from the S-belt and dunites from the J-belt than in spinel lherzolites from the J-belt. Both pyroxenes show zonal structure. In clinopyroxene, the content of SiO2, CaO, and TiO2 increases in the rim direction, and Al2O3, FeO, and Na2O increase in the core direction. In orthopyroxene, the content of SiO2, MgO, and FeO increases from the core to the rim, and the content of Al2O3 and CaO in the same direction decreases. Analyzes of the oxides of the main elements in spinel showed that S-belt spinel lherzolites have a significantly higher Cr2O3 content, while spinels from spinel lherzolites and pyroxenites from the J-belt have a significantly higher Al2O3 content. Part of the spinels of S-belt spinel lherzolites shows a pronounced zonation with Cr2O3 enrichment towards the rims, which is a consequence of the secondary alteration and formation of garnet along the edges of the spinel. Isotopic analyzes gave significantly higher εNd and 87Sr/86Sr values in S-belt spinel lherzolite compared to those in J-belt pyroxenite. The geochemical characteristics of S-belt peridotites indicate their origin from a suboceanic mantle, more than 70 km deep, which underwent melting processes (up to 15% partial melting) in the area of mid-ocean ridges and are classified as ophiolitic peridotites. The southern belt (J-belt) contains spinel lherzolites, dunites, and pyroxenites that alternate within very limited space. The geochemical characteristics of the J-belt peridotites indicate a subcontinental origin and were most likely formed during the initial rifting phase where they ascended as parts of the continental mantle to the upper crust, during which they went through a very small degree of melting (up to 5% partial melting). Peridotites from the J-belt are classified as orogenic peridotites. Pyroxenites, which are located within the J-belt, show different petrographic and geochemical characteristics and were most likely formed as a separate paragenesis by crystallization from melts of unknown but possibly already depleted source. Dunites show different geochemical characteristics and may have been formed by different geological multiple processes. The diverse lithology of ultramafics (spinel lherzolites, dunites, and pyroxenites) in the limited space of the J-belt indicates the very heterogeneous nature of the subcontinental mantle. In conclusion, this paper showed how the peridotites and pyroxenites of the Banovina, and thus the CDOBs, record three different phases of ocean evolution, the early phase of the initial rift and opening of the ocean (J-belt peridotites and pyroxenites), later phase of the already developed ocean (S-belt peridotites), and also the phase of ocean closure which is evident from the structures of melánge, but also from the presence of amphibolite contact formed as a result of obduction. |
| Databáze: | OpenAIRE |
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