COMPARISON OF TWO CONTRASTING ECLOGITES FROM THE MOTAGUA FAULT ZONE, GUATEMALA: SOUTHERN LAWSONITE ECLOGITE VERSUS NORTHERN ZOISITE ECLOGITE

Two contrasting eclogite occurrences in the Motagua fault zone (MFZ) record quite different metamorphic evolutions. Lawsonite eclogites and garnet-lawsonite blueschists occur as blocks in antigorite serpentinites of the southern MFZ (SMFZ). Many lawsonite eclogites contain Grt (Alm_54-71Grs_18-31Pyr_2-17Sps_<16) + Omp [w/ exsolved quartz rods] (up to 53 mol% Jd) + Lws + Rt + Qtz +/- Gln +/- Phe as peak-stage assemblage; the peak-stage lawsonite contains inclusions of omphacite and rare precursor pumpellyite. Some newly investigated jadeite-bearing lawsonite-eclogites contain Jd (Jd_62-78Aug_17-24Ae_3-17) + Grt [megacrysts] (Alm_55-76Grs_17-28Pyr_1-10Sps_<22) + Lws + Rt + Phe (3.6 Si p.f.u.) + Qtz as peak-stage assemblage; coexisting pairs of retrograde Jd (Jd_75-90Aug_10-16Ae_3-11) + Omp (Jd_44-51Aug_37-46Ae_8-14) occur as recrystallized products of eclogitic impure jadeite. Textural and geothermobarometrical evidences indicate that the SMFZ eclogites experienced unusually low-T prograde eclogite-facies metamorphism (T=300-450 °C and P=2.0-2.5 GPa) and subsequent decompression and simultaneous cooling. In contrast, amphibolitized paragonite-bearing zoisite eclogites occur as blocks in antigorite serpentinites (lherzolite origin) of the northern MFZ (NMFZ). They contain Grt + Omp [w/ exsolved quartz rods] (Jd_30-51Aug_48-65Ae_<6) + Zo + Phe (3.5 Si p.f.u.) + Pg + Rt + Qtz as peak-stage assemblage. In particular, garnet porphyroblasts display two discontinuous stages of garnet growth; earlier garnet with inclusions of Omp + Rt contains up to 6.7 wt% MgO (Alm_46-52Grs_25-31Pyr_18-26Sps_<4), whereas later stage garnet rim containing inclusions of Hbl + Ttn is less magnesian (Alm_45-53Grs_27-31Prp_14-20Sps_<4). Hornblende (0.2-0.3 Na[B-site]; 1.0 ^[4]Al p.f.u.) replacing omphacite is overgrown by minor latest stage glaucophane (1.8-1.9 Na[B-site] p.f.u.). Compared with the SMFZ eclogites, the NMFZ eclogites were formed at significantly higher-T condition (T=600-650 °C and P=2.0-2.3 GPa). Moreover, the NMFZ eclogites were overprinted by amphibolite-facies mineral assemblage with new growth of garnet during decompression, and then overprinted by blueschist-facies minerals. Geochronological data of these two contrasting eclogite-facies metamorphism remain to be determined; they may have formed at discrete subduction system of oceanic lithosphere.