The origin of post-magmatic Ca-Al minerals in granitediorite mingling zones: The Tatra granitoid intrusion, Western Carpathians
Gawęda, Aleksandra; Włodyka, Roman
published: Nov 1, 2012
ArtNo. ESP154019001003, Price: 29.00 €
Secoundary mineral assemblages from thin contact zones of mafic and felsic rock portions in the polygenetic granitoid pluton of Western Tatra and High Tatra Mountains are described. The growth of these assemblages was stimulated by the presence of mafic (dopritic) rocks, represented by both mafic precursors and mafic micrograncular enclaves. The mafic rocks acted as the sources of elements necessary for Ca-Al silicates and actinolite crystallization, while the physical contact of the two types of provided a path for fluid circulation and cation exchange. In case of the Western Tatra mingling zone, the mineral assemblage consists of actinolite + garnet (G1 G2) + titanite (Ttn1 and Ttn2) + epidote + chlorite + muscovite. In the High Tatra Mts. mingling zones the secondary mineral assemblages comprises garnet (G2) + epidote + chloride + muscovite + magnetite. Actinolite grew in two stages. The first stage involved crystallization from high temperature hydrothermal fluids, separated from the magma, at relatively high oxygen fugacity. The second stage was a result of crystallization from a subsolidus fluid phase at lower temperature and oxygen fugacities. Different models of secondary Ca-garnet growth (G1 and G2) can be applied for the two described examples of secondary alteration. The crystallization of G1 garnet started at many centres and involved the independent development of discrete crystals. Widespread chlorite, postdating the garnet formation, preserves the Mg/(Mg + Fe) ratio of parent biotite. In the High Tatra granitoid, G2 garnet lenses are products of early biotite alternations that grew simultaneously with chlorite and epidote in free spaces and secondary magnetite governing the Fe budget. The lack of titanite in the latter assemblage may reflect poorly developed biotite chloritization restricted to the vicinity of garnet and epidote lenses. The growth of secondary minerals occurred in two stages: first stage under the increasing oxygen fugacity at pressures of ca 3.5 kbar and temperature from 485 °C to 377 °C, and the second stage under conditions of decreasing oxygen fugacity, high silica activity in hydrothermal fluid and at temperature falling from 380 °C to 300-220 °C.