Study of temperature and pressure of amphibolite formation in the Posht-e-Badam ophiolite (Northeast of Yazd Province)

Amphibolites are one of the major rock units in the Posht-e-Badam ophiolite that are formed by metamorphism of basalts. There are tow types of amphibolite in the Posht -e- Badam ophiolite: 1) amphibolites with intermediate plagioclase (first type), amphibole, plagioclase, quartz, sphene, ilmenite, apatite, sericite and epidote, 2) amphibolites with high content of anorthite (second type), amphibole, plagioclase, quartz, and muscovite, with mior amounts of garnet, ilmenite, sphene, prehnite, chlorite and leucoxene. Amphibole barometry and amphibole - plagioclase thermobarometry conclude 6.16 to 7.79 kbar and 650 to 738 ◦c (amphibolites facies) for the first type of amphibolites and 4.47 to 9.31 kbar and 930 to 1566 ◦c (granulite and higher P - T codition) for the second type. The proposed high temperature for the second type of amphibolites can attributes to the prehnitization of some plagioclases (M1) before metamorphism of the amphibolite facies (M2). High anorthite plagioclases are formed by metamorphism of prehnite. Presence of prehnite, chlorite, epidote, sericite and leucoxene indicates occurrence of a retrograde metamorphism in the greenschist facies (M3) which follows the amphibolites facies metamorphism. It can be concluded that this Paleozoic ophiolite has suffered three phases of metamorphism M1 (EarlyCimmerian), M2 (Middle Cimmerian) and M3 (LateCimmerian) in the Mesozoic.