Ophiolitic mélanges associated with ophiolitic sequences are widespread in the Mirdita-Subpelagonian zone (Albanide-Hellenide orogenic belt) and consist of tectono-sedimentary “block in matrix-type” mélanges. Volcanic and subvolcanic basaltic rocks included in the main mélange units are studied in this paper with the aim of assessing their chemistry and petrogenesis, as well as their original tectonic setting of formation. Basaltic rocks incorporated in these mélanges include: (1) Triassic transitional to alkaline within-plate basalts (WPBs); (2) Triassic normal (N-MORB) and enriched (E-MORB) mid-ocean ridge basalts; (3) Jurassic N-MORBs; (4) Jurassic basalts with geochemical characteristics intermediate between MORB and island arc tholeiites (MORB/IAT); (5) Jurassic boninitic rocks. These rocks record different igneous activities, which are related to the geodynamic and mantle evolution through time in the Mirdita-Subpelagonian sector of the Tethys. Mélange units formed mainly through sedimentary processes are characterised by the prevalence of materials derived from the SSZ environments, whereas in mélange units where tectonic processes prevail, the oceanic materials predominate. By contrast, no compositional distinction between structurally similar mélange units is observed, suggesting that they may be regarded as a unique mélange belt extending from the Hellenides to the Albanides, whose formation was largely dominated by the mechanisms of incorporation of the different materials. Most of the basaltic rocks surfacing in the MOR and SSZ Albanide-Hellenide ophiolites are incorporated in mélanges. However, basalts with island arc tholeiitic affinity, although they are volumetrically the most abundant ophiolitic rock types, have not been found in mélanges so far. This implies that the rocks forming the main part of the intra-oceanic arc do not seem to have contributed to the mélange formation, whereas rocks presumably formed in the forearc region are largely represented in sedimentary-dominated mélanges. In addition, Triassic E-MORBs, N-MORBs, and WPBs included in many mélanges are not presently found in the ophiolitic sequences. Nonetheless, they testify the existence throughout the Albanide-Hellenide belt of an oceanic basin since the Middle Triassic.
Petrogenesis and tectono-magmatic significance of volcanic and subvolcanic rocks in the Albanide-Hellenide ophiolitic mélanges
SACCANI, Emilio;
2005
Abstract
Ophiolitic mélanges associated with ophiolitic sequences are widespread in the Mirdita-Subpelagonian zone (Albanide-Hellenide orogenic belt) and consist of tectono-sedimentary “block in matrix-type” mélanges. Volcanic and subvolcanic basaltic rocks included in the main mélange units are studied in this paper with the aim of assessing their chemistry and petrogenesis, as well as their original tectonic setting of formation. Basaltic rocks incorporated in these mélanges include: (1) Triassic transitional to alkaline within-plate basalts (WPBs); (2) Triassic normal (N-MORB) and enriched (E-MORB) mid-ocean ridge basalts; (3) Jurassic N-MORBs; (4) Jurassic basalts with geochemical characteristics intermediate between MORB and island arc tholeiites (MORB/IAT); (5) Jurassic boninitic rocks. These rocks record different igneous activities, which are related to the geodynamic and mantle evolution through time in the Mirdita-Subpelagonian sector of the Tethys. Mélange units formed mainly through sedimentary processes are characterised by the prevalence of materials derived from the SSZ environments, whereas in mélange units where tectonic processes prevail, the oceanic materials predominate. By contrast, no compositional distinction between structurally similar mélange units is observed, suggesting that they may be regarded as a unique mélange belt extending from the Hellenides to the Albanides, whose formation was largely dominated by the mechanisms of incorporation of the different materials. Most of the basaltic rocks surfacing in the MOR and SSZ Albanide-Hellenide ophiolites are incorporated in mélanges. However, basalts with island arc tholeiitic affinity, although they are volumetrically the most abundant ophiolitic rock types, have not been found in mélanges so far. This implies that the rocks forming the main part of the intra-oceanic arc do not seem to have contributed to the mélange formation, whereas rocks presumably formed in the forearc region are largely represented in sedimentary-dominated mélanges. In addition, Triassic E-MORBs, N-MORBs, and WPBs included in many mélanges are not presently found in the ophiolitic sequences. Nonetheless, they testify the existence throughout the Albanide-Hellenide belt of an oceanic basin since the Middle Triassic.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
