Garnet-bearing rhyolite from Deh-Salm area, Lut block, Eastern Iran: anatexis of deep crustal rocks

摘要

Rhyolite from the Deh Salm area, Lut block, Iran, is a 10-km-long flow unit that crops out as a 6-m thick sub-horizontal sheet (<5 deg dip) underlain by agglomerate and overlain by crystal lithic tuff. The ryholite contains phenocrysts of garnet, plagioclase and amphibole phases residing in a cryptocrystalline groundmass consisting of plagioclase, K-feldspar and quartz. Euhedral garnet phases range in size from 0.2 to 3.0 mm, have inclusions of ilmenite, rutile, apatite and zircon, and show no reaction/ resorption micro textures. Electron microprobe analysis reveals that the garnet is homogeneous, lacks substantial chemical zoning and is dominantly almandine with subordinate pyrope, grossular, spessartine and andradite. The absence of inclusions of metamorphic minerals within garnet as well as the lack of included metamorphic rock fragments in the host rhyolite rule out a xenocrystic origin for the garnet. The garnet phases have low CaO content and variable MnO concentration, resembling those formed from S-type magma. The host rhyolite contains quartz, K-feldspar, and amphibole, has normative corundum, and is peraluminous. Inclusions within garnet as well as the chemistry of the garnet and the host rhyolite suggest that the garnet is a phenocryst phase crystallized from a deeprooted (>25 km) crustal-derived peraluminous magma. It is likely that the peraluminous feature of the magma, inherited from partial melting of metapelitic source rocks, has favoured crystallization of garnet. Upon rising to higher levels, the garnet phases were in equilibrium with the host magma and thus were preserved as relatively large euhedral crystals in the Deh-Salm rhyolite.