Nagy, Géza

Sadek Ghabrial, Doris

Árkai, Péter

English

Plagiogranite forms cross-cutting dikes, veins with sharp boundaries within the MORB-like Jurassic basic-ultrabasic complex (Szarvaskő complex) of the southwestern Bükk Mountains, NE-Hungary. Earlier penological and geochemical interpretation suggests that the basic magmas became contaminated by terrigenous sediments in a shallow magma chamber (DOWNES et aL 1990), thus producing atypical, incomplete ophiolitic sequence. In order to characterise the formation of the plagiogranite in question, literature data on oceanic granitic rocks are critically reviewed. The Szarvaskő plagiogranite consists mainly of quartz, plagioclase, biotite ± garnet. Its major element bulk chemistry is similar to that of the oceanic plagiogranites (high Si, moderate Al, low total Fe+Mg and extremely low K contents), crystallised from late differentiated basic melt. The garnet being present in strongly varying proportions in granite is usually accumulated near or at the contact with the metaclastic enclaves. The garnet is almandine-rich, and displays slight chemical zoning of changing character. Most frequently, the Fe content increases and the Ca content decreases from the centres towards therimsofthe grains. Its chemistry corresponds to that of the garnet found in metapelites or is plotted near to the field of the igneous silicic rocks in the diagrams used for discrimination. As the investigated metaclastic enclaves are represented by biotite-plagioclase hornfels, and garnet never forms contact metamorphic rim around the enclaves, but it is always dispersed in the granitic melt, the formation of the garnet is most probably a consequence of local assimilation of the metaclastic rocks at a deeper level. The magmatic minerals are extensively altered: the plagioclase is completely albitised, while the biotite is totally altered to chlorite, epidote, prehnite and titanite. The secondary mineral assemblage consists of quartz, albite, chlorite, epidote, prehnite, calcite, titanite, white mica (sericite) and smectite, while pumpellyite is lacking. Illite and chlorite "ciystallinity" indices indicate anchizonal metamorphic circumstances, agreeing fairly well with the temperature data (270-350° C, in average, 298° C) obtained by the chlorite-AlIV  geothermometer of CATHELINEAU (1988). These data refer to metamorphic conditions corresponding either to the pumpellyite-prehnitequartz or to the prehnite-actinolite facies. Earlier petrological data of ARKAI (1983) prove Cretaceous regional (dynamothermal) metamorphism in the surrounding formations. In order to get evidence on eventual preceding (ocean-floor hydrothermal or burial) metamorphic effects, detailed study of the whole ophiolite-like sequence is required

Sadek Ghabrial Doris

Árkai Péter

Nagy Géza

University of Szeged

Magmatic features and metamorphism of plagiogranite associated with a Jurassic MORB-Iike basic-ultrabasic complex, Bükk Mountains, Hungary

http://acta.bibl.u-szeged.hu/24816/1/mineralogica_035_041-069.pdf

Magmatic features and metamorphism of plagiogranite associated with a Jurassic MORB-Iike basic-ultrabasic complex, Bükk Mountains, Hungary

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University of Szeged