Polymetamorphic normal-, reverse-zoned, and unzoned garnets from the Darakht-Bid aureole, Mashhad, Iran

Abstract

Thermal metamorphism and later retrogression of low metamorphic grade garnet-bearing pelites has produced diverse patterns of garnet zoning. In the narrow thermal aureole, fibrolite, staurolite and biotite are present and commonly are retrogressed to assemblages containing sericite, chlorite and chloritoid. In the thermal aureole, garnet contains inclusions of quartz, biotite, fibrolite, ilmenite, chlorite and muscovite and underwent relatively rapid growth from pre-existing low to medium grade regionally metamorphosed rocks. Garnet was armoured from breakdown reactions by fibrolite which nucleated on garnet. The grain size of poikiloblastic garnet, the volume of zones of inclusions in the garnet and the size of the inclusions all decrease with increasing distance from the contact. In the thermal aureole, normal compositional zoning is common and rare reversezoned garnets probably result from the partitioning of Mn from ilmenite during thermal metamorphism. Garnet grains in the thermal aureole have a peripheral enrichment in Mn in the outermost 200 μm as a result of diffusion of Fe from garnet into the matrix during incipient retrogression. Coarse retrograde garnet in schists is unzoned and richer in Fe than the normal- and reverse-zoned prograde garnet in hornfelses suggesting that relatively large scale local diffusion in retrograde schists was operative during retrogression but not effective enough to completely remove the relic prograde zoned garnets in the hornfelses.