SERPENTINITES OF THE MOTAGUA FAULT ZONE MELANGES, GUATEMALA: AN ASSESSMENT
Much serpentinite from north and south of the MFZ is sheared and recrystallized to antigorite schist, which lacks textural or mineralogical relics of peridotite. Magnetite is the most common accessory mineral (but less so in foliated rocks), and in order of relative abundance, magnesite and/or dolomite, talc, chlorite, chromite and secondary diopside. Only trace lizardite and no chrysotile is found. Mn-ilmenite, Ni-sulfides, FeNi alloys, and andradite are rarest. Less-tectonized or -altered samples contain relict pyroxene grains, pseudomorphs of exsolved Opx and Cpx grains (bastite with relict Al-rich Cpx: to 8 wt% Al2O3). Textures, minerals and bulk compositions suggest harzburgite was the most common protolith, followed by lherzolite and rare dunite; however, the abundances of Mg-Si-Al in the serpentinites are controlled by antigorite modes and compositions, complicated by fluid-produced talc and carbonate minerals.
Relatively unserpentinized samples retain Ol + Cpx + Sp ± Opx. Some with high modal Cpx indicate lherzolite protoliths. However, Cpx is LREE depleted with MORB-like Sr-Nd patterns, indicating significant melt extraction. The Cpx may have formed by refertilization of harzburgite with fluids derived from depleted mantle. Pyroxene thermometry yields T = 900-1000ºC at P = 10-20 kbar. These rocks also display coexisting Atg-Ol pairs. Their Fe-Mn-rich Ol grain boundaries indicate local hydrous equilibration at 500-600 ºC.
Antigorite serpentinite is consistent with mantle wedge hydration above a subduction channel. However, variations of modes of talc, carbonate, chlorite, and magnetite show important influences of subduction zone fluids in modifying peridotite compositions. Relic phases remain the best indicators of protolith identity. These argue for complex, reprocessed mantle peridotites for both mélange belts.