In West Sangilen (South-East Tuva, Russia), there are outcrops of metamorphic and magmatic complexes of early Caledonides, which are related to the period of long-term collisional and post-collisional events in the northwestern edge of the Tuva-Mongolian massif. The evolution of orogenic structures in West Sangilen is an example of the collapse of folded structures in case of changes in tectonic regimes from compression and transpression (collision period) to intra-and marginal continental transform-shear extension (post-collision period). Numerous geologic features give evidence of changes in the kinematics and characteristics of deformations, as well as in the conditions of metamorphism and magmatism in the study region. However, thinning of the crust during the collapse of the collisional orogenic structure has not been supported by any direct data. Indicators of such events are the complexes of combined dykes, which are abundant in West Sangilen, especially in the area between the Erzin and Naryn rivers and on the right bank of the Erzin river. The most representative object is a combined basite-granite dyke at the foot of the Tavit-Dag mountain. Its position is controlled by the strike-slip fault system. The thermochronological analysis of mingling rocks shows different ages of the closure of isotope systems: 494.8±5.4 Ma (U/Pb, zircon, basites), 489.7±7 Ma (U/Pb, zircon, granitoids), 471.2±1.9 Ma (Ar/Ar , amphibole, basites), and 462.5±1.0 Ma (Ar/Ar, biotite, basites). Taking into account the parameters of the closure of isotope systems (~800-900 °C, zircon, U/Pb; ~500 °C, amphibole, Ar/Ar; ~300 °C, biotite, Ar/Ar), the cooling curve of the mingling dyke is estimated. It corresponds to lowering of the temperature by 600 °C (900 °С  500 °С  300 °C) in the period from 500 (494.8±5.4) Ma to 461 (462.5±1.0) Ma. It is shown that the recent thermal events did not affect the mingling dyke located on the Tavit-Dag site. The sequential changes in the age of the closure of isotope systems are indicative of thinning of the crust in the study region GEODYNAMICS & TECTONOPHYSICS

A model for the thermodynamic properties of rhombohedral oxide solid solutions in the system Fe 2 O 3 -FeTiO 3 -MgTiO 3 -MnTiO 3 (containing minor amounts of Al 2 O 3 ) is presented. The model accounts for temperature and compositionally dependent long-range cation-order and the related high to low symmetry structural phase transition. The model is calibrated from the cation-ordering data of Harrison and others (2000; Harrison and Redfern, 2001) and experimental data on Fe ؉2 Ti N (Fe ؉3 ) 2 exchange between rhombohedral oxide and spinel from Lattard and others (2005) and Evans and others (2006). Successful calibration require introduction of an energetic contribution attributed to short-range cation-order, which reduces the configurational entropy of the solid solution. The resultant thermodynamic model for the rhombohedral oxides is internally consistent with the model for spinel solid solutions of Sack and Ghiorso (1991a, 1991b) and with the endmember thermodynamic properties database of Berman (1988); a new model equation for the isobaric heat capacity of ulvo ¨spinel (cubic Fe 2 TiO 4 ) is proposed and values of the enthalpy of formation, ؊1490.417 kJ/mol, and third law entropy, 184.199 J/K-mol, at 298.15 K and 10 5 Pa are recommended.The new model forms the basis of a revised FeTi-oxide geothermometer/oxygen barometer, which is applied to a newly compiled dataset of natural two oxide pairs from silicic volcanic rocks. Results are compared to previous formulations with the general conclusion that the new model gives a better estimate of oxidation state for magmas that equilibrated under conditions more oxidizing than the nickel-nickel oxide buffer. Estimates of oxygen fugacity are fairly insensitive to analytical uncertainties in oxide compositions. By contrast, temperature estimates are especially sensitive to analytical error and to the abundances of "minor" constituents. Application of the geothermometer to oxide pairs that grew under conditions where the rhombohedral phase was cation disordered (that is high temperature or at oxygen fugacities greater by about one log 10 unit than the nickel-nickel oxide buffer) results in an uncertainty due solely to analytical error of at least 50°C and sometimes as high as 100 °C. Temperature estimates from the new geothermometer can be made using either the Fe ؉2 Ti N (Fe ؉3 ) 2 exchange or Fe ؉2 N Mg exchange between the two oxides. Comparison of the two temperature estimates provides a means of evaluating the internal consistency of coexisting oxide compositions and assessing the extent of disequilibrium. Temperatures calculated from the new model are found to be consistent with experimental phase relations for the stability of cummingtonite in silicic volcanics. Other petrologic constraints on derived temperatures are examined including limits on the width of the miscibility gap and the development of self-reversed remanent magnetization in the rhombohedral series.Software that implements the new thermodynamic model and the two-oxide geothermometer/oxygen baromete...

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