Bell-shape garnet zoning development during eclogitization of the corona textures

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Larikova, Tatiana
Holding Date 04 October 2008

Eclogite facies rocks with the reaction textures were studied in the Marun-Keu complex, the Polar Urals, Russia (Udovkina 1971, Molina et al. 2000). The P-T metamorphic path was outlined for the Marun-Keu eclogites: the prograde stage (up to 670-700C and 17-18 kbars); and the retrograde amphibolization and Cpx-Pl symplectites development at T=640-670C and P=11-13 kbars.
Typical corona texture consist of orthopyroxene, clinopyroxene, and garnet layers between primary olivine and plagioclase. In the unusual corona sequence with double garnet layers, they are separated by a symplectite layer, made of omphacite, hornblende, and kyanite. Both garnet layers contain tiny kyanite/corundum inclusions; the outer garnet corona is extremely Ca-rich (up to Grs83). The total bulk composition of the symplectite layer is very close to transitional one of both garnet coronas (with small difference in Fe and Na). The width of the continuous garnet corona is similar to the double one; so during the diffusion-controlled corona growth, distance of the components’ mobility was probably restricted; and it is likely, that both garnet coronas formed simultaneously. Later the space between two garnet layers was overgrown by the newly formed garnet.
Garnet zoning in the "typical" corona texture as well as in the corona with the double garnet layer, is the following: Mg/(Mg+Fe) is almost constant in whole garnet corona; Mg content decreases towards plagioclase, Ca - increases. Coronas coexist with corundum+omphacite+zoisite assemblage in pseudomorphs after plagioclase. During the corona textures development the former plagioclase was filled with the garnet inward from the rim. When the whole plagioclase is overgrown with the garnet, a "bell-shape" zoning is formed, where the Ca-rich and Mg-poor "garnet grain" core was formed at the latest stage. With the consequent transformation of the coronite into the eclogite, the garnet zoning would become smooth. Small omphacite inclusions were found inside this garnet. Thus, a typical "prograde bell-shape garnet core-to-rim" zoning with increasing of Mg, and decreasing of Ca content in the Marun-Keu eclogites has been developed from the rim inward.
During the retrograde amphibolization, coronas of clinopyroxene-plagioclase symplectites replaced the relicts of omphacite. Cpx-Pl symplectites consist of three zones: fine symplectites (~1 mkm in width) closer to relict omphacite; intermediate ones, and outer big symplectites - up to 20-30 mkm in width. Total square amount of plagioclase and clinopyroxene was calculated using the BSE images; the ratio Pl:Cpx (37-41 : 63-59) is similar in all three types of the symplectites. That may indicate that the size/width of the symplectites depends not on their composition or P-T conditions, but on the kinetics during the omphacite breakdown.
Author thanks the RFBR 06-05-65291, and SFB-574 for the financial support.