The South Apuseni Mountains (western Romania) are located in the inner zone of the Carpathian belt. This area is characterized by a complex assemblage of nappes, where Jurassic igneous associations are well represented. New geological and geochemical data on these igneous associations point out the occurrence of Middle Jurassic ophiolites overlain by Late Jurassic calc-alkaline volcanic rocks. The ophiolite sequence is characterized by: (1) an intrusive section mainly represented by small gabbroic bodies showing both layered and isotropic textures, as well as scarce ultramafic cumulates, melagabbros, gabbronorites, and ferrogabbros; (2) a basaltic sheeted dyke complex; (3) a volcanic sequence including massive and pillow-lava basalts and rare pillow breccias; and (4) very rare Callovian-Oxfordian radiolarian cherts. Although chemically variable, gabbros show a clear high-Ti magmatic affinity. Basaltic rocks display N-MORB-normalized incompatible element patterns consistent with the compositions of present-day mid-ocean ridge basalts. Their high-Ti magmatic affinity is testified by the chondrite-normalized REE patterns, which are rather flat or slightly enriched in light-REE. All these features suggest that the South Apuseni Mountains ophiolites were generated in a mid-ocean ridge setting. Evidence of ductile deformation and synkinematic metamorphism are lacking, suggesting that ophiolites were dismembered in multiple slices at shallow structural levels during the orogenic tectonic phases. Calc-alkaline rocks are represented by massive lava flows including basalts, basaltic andesites, andesites, dacites, and rhyolites, as well as by some granitoid complexes intruded into the ophiolitic sequence. Volcanics show highly porphyritic textures, with clinopyroxene, orthopyroxene, hornblende, and plagioclase phenocrysts, and incompatible element compositions characterized by Ta-Nb, P, and Ti depletion, as well as by Rb-Ba-Th and La-Ce enrichment. These rocks also show marked light REE enrichments commonly interpreted as a consequence of mantle source enrichment by subduction-derived components. Ophiolites represent the remnants of an oceanic basin, whereas calc-alkaline rocks represent a magmatic island arc setting developed over the previously formed oceanic lithosphere. The geological and petrological data suggest that the South Apuseni Mountains ophiolites can be correlated with the MOR-type ophiolites of the Vardar zone. Their present-day location in the inner zone of the Carpathian belt is interpreted as occurring during the tectonic escape of Adria-related microplates during the Late Paleogene - Early Neogene.

Geodynamic implications of Jurassic ophiolites associated with island-arc volcanics, South Apuseni Mountains, Western Romania

MARRONI, MICHELE;PANDOLFI, LUCA;
2002

Abstract

The South Apuseni Mountains (western Romania) are located in the inner zone of the Carpathian belt. This area is characterized by a complex assemblage of nappes, where Jurassic igneous associations are well represented. New geological and geochemical data on these igneous associations point out the occurrence of Middle Jurassic ophiolites overlain by Late Jurassic calc-alkaline volcanic rocks. The ophiolite sequence is characterized by: (1) an intrusive section mainly represented by small gabbroic bodies showing both layered and isotropic textures, as well as scarce ultramafic cumulates, melagabbros, gabbronorites, and ferrogabbros; (2) a basaltic sheeted dyke complex; (3) a volcanic sequence including massive and pillow-lava basalts and rare pillow breccias; and (4) very rare Callovian-Oxfordian radiolarian cherts. Although chemically variable, gabbros show a clear high-Ti magmatic affinity. Basaltic rocks display N-MORB-normalized incompatible element patterns consistent with the compositions of present-day mid-ocean ridge basalts. Their high-Ti magmatic affinity is testified by the chondrite-normalized REE patterns, which are rather flat or slightly enriched in light-REE. All these features suggest that the South Apuseni Mountains ophiolites were generated in a mid-ocean ridge setting. Evidence of ductile deformation and synkinematic metamorphism are lacking, suggesting that ophiolites were dismembered in multiple slices at shallow structural levels during the orogenic tectonic phases. Calc-alkaline rocks are represented by massive lava flows including basalts, basaltic andesites, andesites, dacites, and rhyolites, as well as by some granitoid complexes intruded into the ophiolitic sequence. Volcanics show highly porphyritic textures, with clinopyroxene, orthopyroxene, hornblende, and plagioclase phenocrysts, and incompatible element compositions characterized by Ta-Nb, P, and Ti depletion, as well as by Rb-Ba-Th and La-Ce enrichment. These rocks also show marked light REE enrichments commonly interpreted as a consequence of mantle source enrichment by subduction-derived components. Ophiolites represent the remnants of an oceanic basin, whereas calc-alkaline rocks represent a magmatic island arc setting developed over the previously formed oceanic lithosphere. The geological and petrological data suggest that the South Apuseni Mountains ophiolites can be correlated with the MOR-type ophiolites of the Vardar zone. Their present-day location in the inner zone of the Carpathian belt is interpreted as occurring during the tectonic escape of Adria-related microplates during the Late Paleogene - Early Neogene.
Bortolotti, V.; Nicolae, I.; Marroni, Michele; Pandolfi, Luca; Principi, G.; Saccani, E.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/204331
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