Garnet Xenocrysts Research Articles

Upper Mantle and Crustal Fragments in the Ithaca Kimberlites

Xenocrysts and xenoliths from three Ithaca kimberlite localities are consistent with derivation from mantle depths of less than 150 km. Mantle mineral assemblages reconstructed from xenocrysts and rare 2-4 mineral fragments include spinel and garnet peridotites, and garnet pyroxenites. The Cr-rich garnets, pyroxenes, and spinels and the subcalcic diopsides commonly found in mantle xenoliths from other kimberlites are apparently absent. Secondary pargasitic amphibole attributed to mantle metasomatism is found associated with pyrope garnet. Al-rich pyroxenes and spinels suggest that Al-spinel lherzolites of the type commonly found in alkali basalts, but rarely found in kimberlites, occur. Some of the spinels are the most aluminous ever reported from kimberlites. These Al-spinel lherzolites suggest either a fossil high geothermal gradient or a very undepleted mantle in the Ithaca region. Fragments of presumed crustal origin include a suite of Mg-rich minerals which may belong to a granulite carbonate assemblage, small mafic syenite xenoliths, and clinopyroxene and garnet xenocrysts similar to minerals found in basic to intermediate composition granulites. The Ithaca basement is probably an extension of the Grenville metamorphic terrain, and the lower crust may be in the granulite facies. The Ithaca kimberlites are kimberlites on the basis of their whole rock compositions and mineral components, yet they lack the deep mantle fragments found in other kimberlites. The Ithaca kimberlites may have been finally emplaced by explosive eruptions from depths of less than 150 km.

Garnet xenocrysts in the Mashhad granite, NE Iran

Garnet-bearing tonalite is present within 300 m of the contact with garnet-rich hornfels. It is suggested that the almandine xenocrysts in the tonalite are of accidental origin and represent residuals from the assimilation of garnet-rich hornfels by tonalite. Contaminated tonalite is enriched in Al2O3 and FeO and depleted in SiO2 and biotite and amphibole from the garnet-bearing tonalite are richer in Fe and Al than from the centre of the pluton. The garnet xenocrysts are unzoned and derived from normally-zoned porphyroblasts in the hornfels. Ca and Mg have partitioned from the tonalite into garnet whereas Fe has partitioned from garnet to biotite and amphibole and Mn has partitioned from garnet to ilmenite.

Intrusive and polymeta morphic rocks of the darakht-bid area, near Mashhad, Iran

Mesozoic tonalites were emplaced in the axial zones of F2 folds in low to medium grade metamorphic rocks. Tonalite has assimilated Fe-rich pelite and, at the contact, tonalite contains refractory garnet xenocrysts. Digestion of the xenocrysts to biotite-quartz-plagioclase assemblages has produced a decrease in amphibole, K-feldspar and garnet and an increase in biotite und quartz from the margins towards the centre of the tonalite mass. The magma probably intruded hot regionally metamorphosed rocks at high pressures. In the schistose thermal aureole, almandine porphyroblasts decrease in size and abundance away from the contact. Garnet grew from pre-existing nuclei in the regional metamorphic rocks and was armoured from reaction by quartz-sillimanite sheaths. In the aureole, chloritoid and staurolite are confined to rocks with a restricted bulk rock composition, sillimanite is present in rocks with a high iron content and andalusite is common in more aluminous pelites.

Experimental study of the stability of cordierite and garnet in pelitic compositions at high pressures and temperatures

In pelitic rocks, under conditions of low f O 2 and low f H 2 O, the stability of the mineral pair cordierite-garnet is limited by five univariant reactions. In sequence from high pressure and low temperature to high temperature and low pressure these are: cordierite+garnet⇄ hypersthene+sillimanite+quartz, cordierite+garnet⇄ hypersthene+sapphirine+quartz, cordierite+garnet⇄ hypersthene+spinel+quartz and cordierite+garnet⇄ olivine+spinel +quartz. In this sequence of reactions the Mg/Mg+Fe2+ ratio of all ferro-magnesian minerals involved decreases continuously from the first reaction to the fifth. The five univariant boundaries delimit a wide P-T range over which cordierite and garnet may coexist. Two divariant equilibria in which the Mg/Mg+ Fe2+ ratio of the coexisting phases are uniquely determined by pressure and temperature have been studied in detail. P-T-X grids for the reactions cordierite⇄ garnet+sillimanite+quartz and cordierite+hypersthene⇄ garnet+quartz are used to obtain pressure-temperature estimates for several high grade metamorphic areas. The results suggest temperatures of formation of 700–850° C and load pressures of 5–10 kb. In rare occasions temperatures of 950–1000° C appear to have been reached during granulite metamorphism. On the basis of melting experiments in pelitic compositions it is suggested that Ca-poor garnet xenocrysts found in calc-alkaline magmas derive from admixed pelitic rocks and did not equilibrate with the calc-alkaline magma.