PHOSPHATIC GARNET FROM METAPELITIC SCHISTS OF WESTERN MASSACHUSETTS AND EASTERN GREECE

The substitution of P for Si in garnet is well known from HP/UHP and mantle settings, including: the Kimi Complex of the Rhodope (Greece), 0.33 wt% P₂O₅ (Mposkos and Kostopoulis, 2001); the northern Rif (Spain), 0.12 wt%; and mantle eclogites, 0.13 wt% (Haggerty et al. 1994). The two localities with the highest reported P in garnet are the microdiamond-bearing metapelitic schists of the Rhodope, and the Goshen Dome of western MA where P in garnet reaches 0.24 wt% P₂O₅ but where free carbon is not found. Because of similarities in P in garnet among samples from the Rhodope and western MA localities, garnet compositions, precipitate suites and textures were compared.

Garnet compositions and profiles are similar across the sample suite. Some XGrs20-25 cores have broad mantles of XGrs2. Mg# ranges from 0.13-0.30. Former presence of P throughout each garnet is indicated by abundant apatite precipitates, even where intrinsic P is low (0.03 wt%). The highest P remaining in the garnet structure is found where XGrs is lowest and precipitates are few (0.24wt% P₂O₅ - western MA; 0.39 wt% - Sidironero (Rhodope)). Concentric distribution of intrinsic P was confirmed by WDS mapping.

Apatite and rutile are found in co-precipitation textures. Apatite typically occurs as anhedral to subhedral crystals distributed among oriented rutile needles, but in one MA specimen, apatite prisms 10µm dia. in three crystallographic orientations are found together with multiply-twinned, space-filling structures of rutile. Balanced equations for precipitation of these phases discretely are problematic, mainly due to site occupancy considerations. Co-precipitation of apatite (drawing Ca [VIII] and P [IV]), and rutile (Ti [VI]) allows equations that approach site balance.

Precipitates in garnet containing HP/UHP substituents P and Ti likely record processes during decompression. Morphological differences between the MA and Rhodope examples may reflect differences in their cooling histories. If the bulk compositions of the metapelitic schists are similar in all important respects, then similarities in extreme P content, major element compositions, profiles, and precipitate suites in garnet imply similarity of P-T history among the Western MA and Rhodope localities.