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The Ab Ask mineral springs are located 85 km northeast of Tehran, in the southern range of the Damavand volcano. Deposits of these calcareous springs are mainly precipitated as travertine. Petrographical, mineralogical, and stable isotope studies were conducted on different types of travertine to determine their genesis and factors that govern carbonate precipitation. Based on sedimentation consequence and lithofacies these travertines are categorized as first type (fresh travertine), second type (fissure-ridge, dam, and cascade), and third type (laminated) travertines, illustrating a specific condition of formation, deposition and diagenesis. Combined XRD and microscopic investigations show that the Ab Ask travertines are is composed of about 95 % calcite and a minor amount of quartz along with iron oxide impurities. The origin and transport of springs water from which travertine was precipitated are elucidated by 13 C and 18 O isotopic studies of the travertines.δ 18 O and δ 13 C values of travertines increase (from -13.0 to -6.3‰ VPDB and from 6 to 9.8‰ VPDB, respectively) with increasing distance from the spring orifice. This significant increase is attributed to temperature decrease, rapid degassing of CO 2 , and biological activities. It seems that CO 2 content of these fluids may have originated from limestone decarbonation. Based on the integrated petrographic and stable isotope study, the Ab Ask travertines can could be thermogenic in origin.

Handling Editor: M. Santosh Variations in the pressures and temperatures of metamorphism reflect the complexity of collisional orogenic systems. In the north-west Sanandaj-Sirjan Zone of western Iran, migmatites in the Boroujerd area record earlier dynamothermal metamorphism followed by the development of a contact metamorphic aureole induced by the Jurassic Boroujerd Batholith. Integrated micro-textural observations, mineral compositions, and the results of phase equilibrium modelling indicate that peak metamorphic temperatures reached 640-660 C in andalusite schists and~700 C in cordierite-sillimanite migmatites. The pressure does not exceed 3.5 kbar, implying a maximum depth of~13 km. A hairpin clockwise P-T path in the Boroujerd region was inferred from mineral relations (e.g., replacement of andalusite 1 by sillimanite in the migmatites in the heating stage, which was followed by a retrograde episode with growth of andalusite 2 ) and P-T pseudosections. Metamorphism of the Boroujerd area was coeval with voluminous intermediate-mafic magmatism, which provided the heat necessary for partial melting. This study demonstrates that metamorphic sequences of the north-west Sanandaj-Sirjan Zone developed under similar metamorphic conditions in the Lower to Middle Jurassic and record an extensional back-arc setting related to the subduction of Neo-Tethys.

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