Origin of garnet in aplite and pegmatite from Khajeh Morad in northeastern Iran: A major, trace element, and oxygen isotope approach

• Garnet in granitic aplite–pegmatite, granite and metapelite differs in composition.
• Garnet in aplite–pegmatite has MnO > 12 wt.%, CaO < ~ 2 wt.%, δ18O ~ 10.3‰.
• Mn in garnet decreases core-to-rim by aH2O increase and Mn decrease of the melt.
• Y and HREE in core-to-rim of garnet decrease by increasing fluid content in the melt.
• δ18O values of garnet indicate crystallization of fractionated S-type monzogranitic melt.

Triassic monzogranites and granodiorites of the Khajeh Morad region in northeastern Iran are cut by two types of garnet-bearing intrusive veins: (1) aplite and (2) granitic pegmatite. The former is composed of quartz, feldspar, muscovite, with minor garnet, biotite, and ilmenite. The latter contains quartz, plagioclase (± quartz and muscovite inclusions), alkali feldspar, and muscovite, with minor amounts of garnet, tourmaline, beryl, columbite, and ilmenite. Garnet in both rock types has MnO > 12 wt.% and CaO < ~ 2 wt.% with spessartine-rich cores, and a core-to-rim increase in Fe, Mg, and Ca. Garnet cores are enriched in Y, REE, Zr, Nb, Ta, Hf, and U. The Y, HREE, and Mn concentrations show strong positive correlations in both types of garnet associations and decrease from core-to-rim. These core-to-rim elemental variations can be explained by increasing fluid content and H2O activity in magma, together with decreasing Mn contents of an evolved host melt. Aplite and pegmatite garnet δ18O values are nearly identical (~ 10.3‰, n = 7, SD = 0.09) and are similar to magmatic garnets in granitoids elsewhere. On the basis of calculated δ18O values for magma (~ 12.5 and 12.6‰) and quartz (~ 13.6‰, n = 7, SD = 0.08) as well as the major and trace element characteristics, we suggest that the Khajeh Morad garnets crystallized from a variably fractionated S-type monzogranitic magma.