Petrology and metamorphism of amphibolites with corundum from Prijepolje, Dinaridic ophiolite belt (Southwestern Serbia)

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Milovanovic, Dragan۱; Eric, Suzana۱; Seke, Lajos۲
Holding Date 04 October 2008

Amphibolites with corundum (AmC) occur as isolated masses, up to 100m thick, along the southern edge of the Zlatibor ultramafic massif (SW Serbia), showing tectonic contacts with surrounding rocks. These rocks are a part of the metamorphic basement of peridotite of Zlatibor i.e. belong to Mesozoic Dinaridic ophiolite belt. AmC show coarse-grained equigranular nematoblastic and granoblastic textures, consisting of edenitic hornblende, anorthite (partly transformed into secondary prehnite), clinopyroxene (diopside) and corundum (with high content of Cr). These rocks are crosscuted by thin, secondary veins of prehnite and zeolite (thomsonite). The most abundant mineral is edenitic hornblende (up to 40% vol.) which defines the foliation of rock. Plagioclase is anorthite, mostly prehnitized, clinopyroxene is diopside, poor in TiO2 (<0.2 wt%) and Cr2O3 (<0.6 wt%). Corundum occurs as pinkish porphyroblasts, up to 5 mm in size, sometimes shows prismatic crystal shapes. The chemical composition of AmC (high mg-number, low SiO2/Al2O3 values, REE pattern, positive Eu anomalies) is consistent with cumulate gabbroic protoliths, indicating an ocean crustal origin. Corundum was formed by metamorphism and/or partial melting of olivine gabbro protolith by reaction: Spinel+anorthite=corundum+diopside, with sufficiently high Al2O3 content, e.g. a troctolitic protolith. This reaction needs spinel as one of the reactants, which may be formed by reaction olivine+plagioclase=orthopyroxene+clinopyroxene+spinel. AmC mineral composition records the P-T conditions at or just below the solidus and show no evidence for migmatization due to partial melting. The maximum P-T condition still remains open. The most common mineral assemblage in the upper amphibolite grade metabasites is Hbl+Pl±Qtz, with or without either Cpx or garnet, so reactions involving the replacement of Hbl+Pl±Qtz by combinations of Opx, Cpx, Grt+corundum is the main task of this study. Based on the Al and Si isopleths of edenitic hornblende pressures in AmC range from 6 to 1,2 GPa. Geothermobarometric estimations on edenitic hornblende, pyroxene, anorthite and corundum yielded following metamorphic conditions in AmC: P>0.8 GPa and T> 8000C. AmC are cross-cuted by veins filled by prehnite and zeolite. This paragenesis is related with retrograde events. After cooling at shallow levels olivin gabbro cumulates, together with surrounding peridotites undergone subduction and metamorphose under granulite-eclogite facies. Metamorphism is most probably related with Jurassic subduction during formation of Dinaridic ophiolite belt. AmC arrived from deeper part of the subductioned zone. During these processes, ultramafic rocks have undergone analogous changes, deformations in solid rocks, creation of banded textures, etc. During uplift and tectonical emplacement (closing of ocean space) the AmC was dismembered, forming fragments which were included in the ophiolite melange.