Contact metamorphism in tbe Shahindezh MetamorphicCore (SMC}-NW Iran; PT conditions and microstructuralevidence for partial melting of metapelites

Contact metamorphism in the Shahindezh Metamorphic Core
(SMC), which is occurred due to intrusion of Pichaghchi granodiorite during
Laramian orogeny (Upper Cretaceous), is studied here. The well deve loped
mineral parageneses at the aureo le is quatz + plagioclase + biotite +
cordierite ± chlorite ± K-feldspar ± muscovite ± andalusite ± sillimanite ±
garnet. Pressure and temperature of the contact metamorphism at the SMC
for andalusite + cord ierite -bearing assemblages (the most common
paragenesis at the aureole) are estimated -3.5 ± 1.5 kbar and 500 ± 50°C
using THERMOCALC computer program. Also the exact pos ition of the
muscovi te + biot ite breakdown reaction at the PT space, is calculated.
Temperature for muscovite-biotite breakdown was about 650 ± 40°C at 3.5
kbar. The calculated position of the reaction curve, is in good agreement
with publ ished experi mental data. The metape lites at the north-eastern part
of the SMC contain hi gh-temperature assemblage quartz + biotite + garnet +
sillimanite + plagioc lase ± cordierite ± K-fe ldspar. The mineral assemblages
and reaction textures in the aureole rocks record temperature about <750 °C
at the si llimanite -cordierite zone and pressure - 4 kbar, detennined using
petrogenetic grids. In the partially molten rocks garnet porphyroblasts are
associated with coarse-grained quartz. Cord ierites occurred both at the rims
of res idual garnets crystallized during regional metamorphism and in the
matrix. All cordierites are altered to pennite. Fibrolite inc lusions in the
matrix cordierites are common.