Petrology and geochemistry of Mesozoic igneous rocks, Bükk Mountains, Hungary

Abstract

Basic and ultrabasic igneous rocks of the Jurassic Szarvaskő complex (southwestern Bükk Mountains, NE Hungary) have previously been interpreted as dismembered fragments of a Mesozoic ophiolitic sequence comprising mainly gabbroic sills intruded into deep-water terrigenous shales. Abundant extrusive pillow basalts, layered ultramafics and plagiogranite bodies are also present. Gabbros and basalts showing slight LREE depletion ( Ce Yb N =0.84–0.93 ) and very slight negative Eu anomalies (0.92–0.98) are considered to be close to liquid compositions, while some gabbros with the lowest overall REE abundances and positive Eu anomalies ( Eu Eu ∗ = 1.14–1.29 ) are regarded as plagioclase+clinopyroxene cumulates. The REE characteristics of the most primitive rocks are very similar to those of N-type MORB. Differentiated magmas show negative Eu anomalies and increasing REE enrichment, consistent with fractionation of plagioclase and clinopyroxene. In most rocks from the Szarvaskő complex, strong enrichment is seen in Ba, Rb and K, compared with the values for MORB. Age-corrected ϵSr t -values of the Szarvaskő complex range from −0.2 (basic rocks) to + 36.4 (evolved rocks). The ϵNd t -values vary between +6.4 (basic rocks) and +3.3 (evolved rocks). The variation in ϵNd t is probably not due to alteration or metamorphism, since simple REE patterns indicate REE immobility, and is considered to be caused by contamination during magmatic fractionation. The high and variable ϵSr t -values could be due to transfer of radiogenic Sr from the surrounding sediments during very low-grade regional Alpine metamorphism. It is suggested that the basic magmas of the complex were probably originally MORB-like, but during evolution in a shallow magma chamber, they became contaminated by terrigenous sediments. The nearby Triassic Darnó Hill complex is much more strongly altered, does not show the same depletion in LREE, and has lower ϵNd t -values. It is considered to have been formed in a different tectonic setting, possibly a continental margin or oceanic island.