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Browsing by Subject "two-pyroxene thermometry"

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  • Ruhanen, Hanna-Kaisa (2022)
    The 1.64 Ga Ahvenisto complex in the southeastern Finland, is an anorthosite-mangerite-charnokite-granite (AMCG) suite, which has been discovered to host conspicuous magma interaction structures formed in magma mixing and mingling. These structures form areas where the consequences of mingling are found as monzodioritic pillows and granitic veins forming net-veined complexes. Mixing of monzodioritic and hornblende granitic magmas has produced hybrid rocks with intermediate composition. The main goals of the study were to report the major element mineral chemistry of the main minerals (plagioclase, alkali-feldspar, amphibole, and pyroxene) from the hybrid rocks, hornblende granite, and monzodiorite and to test the suitability of geothermobarometric methods for the rock types related to the different magma interaction structures and to evaluate crystallization pressures and temperatures of the pertinent magmas. The magma interaction structures of the Ahvenisto complex are recognized in the mineral chemistry of the mingling and mixing related rocks. The mineral chemistry confirms that the hornblende-granite represents the most primitive granitic phase in the mixing set and the composition of the minerals change as the mixing proceeds towards the final hybrid rock. The mineral chemistry from the monzodiorites reveals that the monzodiorites from different areas of the Ahvenisto complex have distinguished features and they can be separated by their composition. Hornblende-plagioclase thermobarometers and two-pyroxene thermometers were used, and the results were compared to the previous studies related to the emplacement conditions of the Ahvenisto complex. The hornblende-plagioclase thermobarometers suggest that the crystallization of the mixing related rocks took place at ~790–860°C. For the temperature results, pressures of 100 MPa and 300 MPa were used based on earlier studies and a conclusion is drawn that the pressure has had only minor impact on the temperature. The two-pyroxene thermometers show similar regional difference for the crystallization temperatures of the monzodioritic rocks as recognized in the mineral chemistry. The results from the two-pyroxene thermometers show wider variation in the temperatures, which leaves some uncertainty on the liability of the results. The variation is mainly explained by the mineral chemistry of the mafic silicate minerals in the Ahvenisto complex rocks being very iron-rich and outside the calibration standards of the geothermobarometers.