REACTION TEXTURES IN RETROGRADE ECLOGITES: MECHANISMS AND MASS BALANCE AS A FUNCTION OF DECOMPRESSION AND COOLING RATES

The presence of eclogites in convergent orogens is a key criterion for inferring the former presence of a subduction zone. ‘Retrograde eclogites' are equally important in Archean, Proterozoic, and early Phanerozoic orogens; intergrowths of Cpx (Di-Hd ss) and sodic Pl are usually inferred to have formed by ‘exsolution' of an originally omphacitic Cpx (Omp) during decompression and cooling. High resolution BSE imaging and X-ray mapping of eclogites from the Appalachian Blue Ridge (ABR; low-P eclogite and transitional amphibolite-eclogite [14-18 kbar] assemblages) and UHP Norwegian Caledonides (~30 kbar) indicate the net breakdown reaction is broadly similar in each setting (Omp + H₂O => Di-Hd + Pl + Hbl ± Qtz), although the reaction mechanism differs. Low-Na Omp in the ABR breaks down via an ‘exsolution-like' process yielding crystallographically-controlled Pl + Hbl + Qtz lamellae within Cpx or grain boundary nucleation. Chemical zoning is well developed in Omp interiors adjacent to Pl+Hbl lamellae and at grain boundaries in slowly decompressed and cooled Omp (0.1 kb/ and 5-10 ^oC/m.y.). Nucleation of lamellae and diffusion-controlled coarsening is consistent with low degrees of overstepping of the breakdown reaction. UHP Caledonide Omp (I) breaks down to complex, fine-grained, grain boundary symplectic intergrowths of Omp II + Pl + Hbl; Omp I is unzoned. Lack of chemical zoning in Omp I adjacent to symplectite, and symplectite-forming reactions in UHP settings, are associated with rapid uplift rates (cm a^-1), steep (3-6.5 kb/ and 50^oC/m.y.) decompression paths, and disequilibrium on a micron scale. The textures develop during significant pressure overstepping of the breakdown reaction, but only modest cooling. In both cases the reaction is not isochemical and not simply a combination of Jd + Qtz = Ab and CaTs + Qtz = An reactions. Area scans via EPMA indicate the breakdown assemblage is enriched in Al (and in Na in ABR) and depleted in Ca, Fe, and Mg, relative to the original Omp; the degree of open system behavior is greatest in ABR Cpx. Al can be accounted for by breakdown of garnet rims and loss of Di-Hd components. Reintegration of breakdown assemblages in ABR (and other occurrences worldwide) to obtain an original peak high-P Cpx composition will yield erroneous compositions, and erroneously high peak P.