Original paper

Origin of syenitic dike by flowage differentiation (Modra massif, Male Karpaty Mts., Western Carpathians)

Broska, Igor; Zahradník, Leonard; Petrík, Igor

Abstract

Dikes occurring in the area of the Dolinkovský vrch Hill (Malé Karpaty Mts.) are, according to geochemical data, derivates of the Middle Carboniferous Modra I-type granitoid massif. They intrude both metapelitic and carbonatic wall rocks. Dikes in the metapelites and metabasites (X.0 meters in width) in the centres show syenogranitic to syenitic composition. Earlier authors considered them to be products of K-metasomatism, but in the present paper a process of K-feldspars compaction due to flowage differentiation is proposed. The Ba contents in the K-feldspars show a bell shape distribution indicating magmatic origin and pointing to fractionation of K-feldspar from the melt. The abundant apatite in syenite enclosed mainly in interstitial quartz, is interpreted as a crystallizing phase in a Ca, Si and P rich boundary layer, adjacent to K-feldspars. The fractionation of amphibole present in some dikes may have increased the A/CNK ratio to values exceeding 1.05 sufficient for the delay in apatite crystallization and enabling the growth of P in the system. In the following compaction, a quartz-albite melt was expelled, leaving a cumulate to various degrees enriched in K-feldspar and apatite. The dikes cutting the limestones show a wide contact aureola of calc-silicate hornfelses, suggesting an extensive fluid interaction between magma and wall rock. The maximum pressure estimated from Al-in-hornblende thermobarometer is less than 150 MPa.

Keywords

granitoidflowage differentiationsyeniteapatitemale karpaty mountains