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Metamorphic sole amphibolites from the Dinaride Ophiolite Zone (DOZ) and Sava-Vardar Zone (SVZ) of Dinarides

The Dinaridic ophiolites occur in the DOZ, genetically related to MORB setting, and in the SVZ, genetically related to BARB environment. Varieties of amphibolites and related crystalline rocks are frequently associated with the Dinaridic ophiolites both in the DOZ and SVZ. In the central and northwestern Dinarides, two compositionally and genetically different groups of amphibolite bearing metamorphic sole can be distinguished: (1) the amphibolites originated from ultramafic-mafic cumulates and (2) the amphibolites genetically related to diabase-dolerite sheeted complex. Radiometric ages of 174ą14 to 157.7ą4 Ma were measured on hormblende concentrates from higher graded amphibolites (PAMIĆ et al. 2001). 1. Amphibolites originated from cumulate rocks Within the DOZ, in tectonically less disturbed areas, the amphibolites build up the metamorphic sole of larger ultramafic massifs and occur as fragments in ophiolite mélange. The amphibolites have granoblastic, porphyroblastic and nematoblastic textures of various grain size. The structure is most commonly shown in foliation, rarely in layering. The most important varieties of these amphibolites are: (1) bimineralic (plagioclase + amphibole) amphibolite, (2) diopside-amphibole schist, (3) garnet-amphibole schist, (4) garnet-diopside-amphibole schist, (5) rare monomineralic amphibole schist and (6) extremely rare corundum-amphibole (edenite-pargasite) schist. Predominant amphiboles range from pargasite, edenite, and their isomorphic mixture through transitional varieties enriched in tschermakite to “common” hornblende (Fig. 1 A, B). Only some of them contain up to 10% of glaucophane molecule. Plagioclase varies from anorthite to oligoclase, and garnet also shows variations of the main end-members: pyrope 59-22%, almandine 49-28%, grossularite 23-16% and andradite 7-3%. By contrast, the chemical composition of clinopyroxene is fairly monotonous ; in the highest grade amphibolites it is diopside with admixtures of hedenbergite and jadeite (Di62-56Hd31-24Jd20-10) – PAMIĆ et al. (1973). Subordinate minerals of the amphibolites are hypersthene, corundum and clinozoisite or epidote and biotite(?). Mutual variation of the rock-forming minerals shows a regular pattern. Putting aside any rock terminology, the mineral assemblages of amphibolites can be grouped as follows (PAMIĆ et al. 1973): a) Pargasite-edenite amphibole + jadeitic diopside (omphacite) + anorthite–bytownite ą garnet ą corundum. Calculated P-T conditions for these highest-grade sole amphibolites are in range between 670 to 750 &ordm ; ; C and 740 to 930 MPa, respectively, indicating transition from granulite to amphibolite facies P-T conditions. b) Transitional greenish pargasite-edenite rich hornblende + labradorite–calcic andesine ą diopside ą pyrope rich garnet ą hypersthene. Calculated P-T conditions are 670ą50 &ordm ; ; C and average pressure of 720ą100 MPa. c) Brownish tschermakite rich hornblende ą andesine ą diopside ą almandine rich garnet. This paragenesis belongs to the amphibolite facies. d) “Normal” hornblende ą oligoclase ą clinozoisite ą almandine rich garnet (c. 30-45 %) which belong to epidote-amphibolite facies. 2. Amphibolites related to diabase-dolerite sheeted complex These rocks are also very common, both in the DOZ and the SVZ. The amphibolites are granoblastic to nematoblastic ; relict blastoophitic and ophitic textures are common. They are commonly distinctly foliated but never layered. As a rule, they are bimineralic and composed of amphibole and plagioclase with subordinate zoisite or epidote. Predominant amphibole is commonly actinolitic hornblende and magnesian hornblende with low AlVI and generally low Ti, suggesting low-pressure conditions ; in some places amphiboles are slightly to moderately transformed into chlorite. Plagioclase ranges in composition from An46 to An21, but relict transitional dolerite-amphibolite varieties contain more calcic plagioclase (MAJER & LUGOVIĆ 1985). As the protolithic diabase-dolerites, these amphibolites are cross-cut by veins filled by laumontite, stilbite, epidote, albite, chlorite, calcite, tremolite and quartz (PAMIĆ et al. 1979). This paragenesis, together with the occurrence of secondary actinolite, chlorite and sodic plagioclase indicate that the amphibolites were influenced by a retrograde greenschist event as did the surrounding ophiolites. The P-T values obtained on hornblende-plagioclase pairs from the northwestern SVZ are in a range between 600-650 &ordm ; ; C and 520-250 MPa. This result fits quite well with P-T values of 580-645 &ordm ; ; C and 3.5-5.4 kbar calculated for the surrounding metasedimentary metamorphic sole around adjacent ultramafic body (MAJER 1984). The generation of the amphibolite metamorphic soles was related to the subduction of oceanic crust, its part composed of cumulate rocks produced high-grade eclogite(?)-granulite-amphibolite facies sequence whereas its part composed of sheeted diabases produced lower-grade amphibolites. With the initiation of obduction processes, the exhumation of higher grade amphibolites started under changed geothermal gradient giving rise to gradual metamorphic zonation of the metamorphic sole amphibolites.

metamorphic sole; amphibolite; Dinaride Ophiolite Zone; Sava-Vardar Zone

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