Nagy, Géza

Sadek Ghabrial, Doris

Árkai, Péter

English

The Szarvaskő ophiolite-like Jurassic complex (southwestern Bükk Mountains, NE Hungary) shows Alpine metamorphic crystallizations related to two distinct events. The younger (Cretaceous) regional event was formerly evidenced by ÁRKAI (1983). The new result presented here prove an earlier hydrothermal (ocean-floor) metamorphic event resulted from the interaction of seawater-derived fluids with the still hot oceanic crust. Metamorphic grade increases downwards from the prehnite-pumpellyite or zeolite facies (metabasalts), through the greenschist facies (metadiabases), to the greenschist/amphibolite transitional and amphibolite facies (metagabbros). In metagabbros, the scarcity of secondary (metamorphic) clinopyroxene and Mg-Fe amphibole, and the compositional changes of the green metamorphic amphibole from hornblende to actinolitic hornblende suggest that the metamorphic crystallization occurred at temperatures decreasing from about 700 to 400°C. The metamorphic clinopyroxene is always poor in its Al content reflecting subsolidus re-equilibration of magmatic clinopyroxene during ocean-floor hydrothermal metamorphism. However, the metamorphic clinopyroxene reported from other ophiolitic gabbro occurrences is much lower in Al and Ti contents (MÉVEL et al., 1978; GIRARDEAU and MÉVEL, 1982; MÉVEL, 1988; GILLIS, 1995) than that of the Szarvask ő complex . Th e metamorphic amphibole formed in metadiabases is rather actinolitic in composition, and coexists with albite, chlorite and titanite, indicating temperature below 400°C. The phases formed in metabasalts during hydrothermal metamorphism were modified by subsequent Alpine regional prehnite-pumpellyite facies metamorphism. The younger, low temperature regional (dynamothermal) metamorphism produced prehnitepumpellyite facies assemblages in meta-igneous rocks, and late diagenetic to low temperature anchizonal alteration in the surrounding sedimentary rocks, as stated earlier by ÁRKAI (1983). The compositional variation of Ca-Al silicate minerals (i.e. prehnite, pumpellyite and epidote) could not be taken as indicative of metamorphic conditions because of their high variance occurrences. However, a large variation in Fe content of these minerals was commonly recorded, and appears to be depend largely on the fluid composition, mode of occurrence and/or on the extent of rock alteration, but is weakly related to bulk rock chemistry. In the low variance assemblages, the average Xpe3+ values of coexisting prehnite, pumpellyite and epidote are 0.003, 0.137 and 0.008, respectively. This contrasts with the results of CHO et al. (1986), who found that the Xp£3+ values most probable increase in order of prehnite, through pumpellyite to epidote

Sadek Ghabrial Doris

Árkai Péter

Nagy Géza

University of Szeged

Alpine polyphase metamorphism of the ophiolitic Szarvaskő complex, Bükk Mountains, Hungary

http://acta.bibl.u-szeged.hu/24837/1/mineralogica_037_099-128.pdf

Alpine polyphase metamorphism of the ophiolitic Szarvaskő complex, Bükk Mountains, Hungary

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University of Szeged