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 800^OC were performed. At 800^ОС, 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 a_H2O in the melts. These characteristics are similar to the A-type granitoids. At 750^OC, 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.