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On the occurrence and significance of biopyribole from mafic subvolcanic rocks from the External Dinarides

Triassic magmatism in the Dinarides has been related to the formation of the Tethyan Mesozoic Adriatic-Dinaridic carbonate platform(s) and its(their) subsequent disintegration. Sporadically occurring subvolcanic rocks emerge along the major fault zones found at the rims of the platform(s). One of such occurrences has been reported in north Dalmatia (Konj Hill, Croatia) representing a hypabyssal dyke made of basaltic dolerite in its core which laterally evolves into the medium- grained diabase. Rocks’ mineral paragenesis include altered plagioclase and K-feldspar, clinopyroxene, tschermakite and pargasite amphibole, Fe-Ti oxide and apatite. Deuteric phases encompass tremolite/actinolite, fine- grained muscovite, chlorite, prehnite, and pumpellyite. Of particular importance is a complete topotactic replacement of primary amphibole by micaceous phases. The amphibole to mica mechanism is complex and leads to the formation of biopyribolic structures. These are defined by restructuration of amphibole double-chain I-beams to form alternating double and triple chains, pure triple chains, and lastly continuous sheets of silicate tetrahedra (i.e., phyllosilicate structure). Our preliminary data shows that such formed mica is phengitic muscovite which is defined by the increased Mg and Fe contents along the Tschermak substitution vector. This Mg-Fe muscovite is characterized by a reduced dimensional difference between the larger trans-oriented M1 site and smaller cis-oriented M2 octahedral site. Diagnostic peaks related to the 2M2 polytype may be explained by such space reduction. Formation of Fe-Mg muscovite which is essentially controlled by the Tschermak exchange reaction which requires lower values of aK+ and/or neutral pH. Such conditions were likely buffered by igneous host rocks whereby liberated Na+ likely vacated the system. All this suggests high fluid activity and element diffusion during alteration event. The crystallization of phengitic mica required at least low-temperature hydrothermal conditions (200-300°C). This further implies that the studied rocks were impacted by a large-scale hydrothermal volcanism characteristic for the ocean-floor setting.

biopyribole ; amphibole ; micaceous phases ; subvolcanic rocks ; External Dinarides

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