Mineral chemistry and thermobarometery of high temperature xenoliths in the contact of rhyodacitic rocks, Astaneh area (Southwest Arak)

In Astaneh area (southwest Arak), in the contact with rhyodacitic subvolcanic rocks xenoliths with pyroxene- hornfels mineralogical composition have formed. These rocks have mineral paragenesis including corundum + spinel + andalusite + K-feldspar + plagioclase+ muscovite + biotite and chlorite with granoblastic, lpydoporphyroblastic, porphyrolpydoblastic, symplectite and poikiloblastic textures. In this complex, reaction texture (symplectite) of spinel + plagioclase+ muscovite, is replaced andalusite rim.  Petrography evidences show that the reaction texture of the andalusite rim has been created under conditions of partial melting. Geological and mineralogical evidence show that the pyrometamorphism process has been responsible as an indicator in this mineral complex. The field study, petrography evidence and thermobarometery (temperature 718 oC and pressure 2.8 kbar range) of these rocks show that the andalusite stability in the study xenolithe to the corundum-spinel zone is due to rapid incalescence rate of these rocks and overstepping. The heat diffusion from rhyodacitic subvolcanic is caused pyrimetamorphism process. So, the temperature period imposed and incalescence rate (geotherm gradient 60 oC/Km) on these xenoliths has been low and high, respectively.