Abstracts Travel
Program Organizing committee

  A-type granitoid magmas formed by melting of tonalite in presence of the H2O-CO2-chloride fluids at the middle-crustal conditions

 O.G. Safonov

Institute of Experimental Mineralogy, Russian Academy of Science, Chernogolovka, Russia.


The A-type (anorogenic) granitoids are a specific variety of ferroan granitic rocks distinct from other granitoids by their low Al2O3 and CaO, high K2O+Na2O contents, elevated F and Cl. Melting of the Archean TTGs in the continental basement is invoked to explain origin of these granites. However, neither fluid-absent nor hydrous melting of TTGs at pressures up to 15 kbar can fully satisfy the required parameters of the melts. The influx of K and Na-rich fluids seems to be necessary for the formation of the A-type granitoid complexes. In order to inspect this suggestion, the experiments on melting of a biotite-hornblende tonalite gneiss in presence of the H2O-CO2-(K, Na)Cl fluids at 5.5 kbar and 750 and 800OC were performed. At 800ОС, the increase of the chloride/(H2O+CO2) ratio in the fluids results in formation of the rhyolitic melts with Al2O3 < 13.5 wt. %, CaO < 2 wt. %, K2O+Na2O > 7 wt. %, FeO/(FeO+MgO) > 0.8, K2O/Na2O > 1, and 0.2-0.6 wt. % Cl. The melt composition clearly correlates with the coexisting mineral assemblages. Stabilization of clinopyroxene and alkali feldspar at the high chloride/(H2O+CO2) ratio in the fluids corresponds to the decrease of CaO and Al2O3 contents in the melts, while increase of the Cl content in these melts promotes the FeO/(FeO+MgO) ratio. High chloride/(H2O+CO2) ratios in the fluids provokes formation of the aegirine component of clinopyroxenes and shifts compositions of amphiboles toward winchite-barroasite. Stability of pyroxenes at the moderate chloride/(H2O+CO2) ratios in the fluids reflects low aH2O in the melts. These characteristics are similar to the A-type granitoids. At 750OC, the H2O-CO2-(K, Na)Cl fluids produce trachytic melts, modeling syenitic members of the A-type granitoid complexes. The experiments prove the model for formation of these complexes by crustal anatexis in presence of the aqueous-carbonic-salt fluids fluxing in the extensional environments.