Serpentinites of the Guatemala Suture Zone: Varieties, Origin, and Compositional Modification

Abstract

Serpentinite belts in central Guatemala are exemplars of multiple emplacement systems involving accretion, collisional, and oblique-slip tectonics along a plate boundary. The Guatemala Suture Zone straddles the Motagua River (and Suture Zone) along most of the river’s length. To the north lie the Baja Verapaz, Sierra de Santa Cruz, and smaller Juan de Paz ophiolites, while to the south are slivers of the ophiolitic El Tambor volcano-sedimentary sequence. Between the ophiolite units are two exhumed serpentinite mélanges, the North Motagua Mélange, north of, and the South Motagua Mélange, south of the Motagua River. Ophiolite units host lizardite/chrysotile (Lz/Ctl) serpentinites, while mélanges are dominated by antigorite serpentinites. Hundreds of serpentinites are documented from several belts, and phase assemblages, serpentine mineral identification, and compositional trends in serpentine composition are reported. These provide insights into petrological and tectonic processes associated with serpentinite origins. First, both mélanges, traditionally interpreted as exhumed serpentinized mantle wedge plus tectonic blocks, are mostly antigoritites but are interleaved by tectonics with slivers of Lz/Ctl serpentinites. Second, all serpentines have compositions that reflect the replaced peridotite minerals with serpentine minerals that provide inferences about serpentinizing fluids. Serpentine compositions can be either supersilicic or subsilicic without evidence of extra phases, arguing for interlayer intergrowths in the serpentine. Magnetite abundance and antigorite composition Mg/(Mg+Fe) (Mg#*) covary inversely in antigoritites, but whole rock Mg#* is essentially constant. Rather than changing fO2 over time to produce and then remove magnetite to explain the different abundances of magnetite, formation of magnetite-poor antigoritites at higher T (>>400°C, but <650°C) than the others is a simpler explanation. Finally, brucite, an expected product of peridotite serpentinization, is essentially absent in these rocks; excess MgO is present as carbonate or talc, implying addition of CO2 and SiO2 by the serpentinizing fluids. Admixture of two types of serpentinites in mélanges is interpreted as a complex tectonic process either sampling mantle wedge with overriding obducted seafloor or fault juxtaposition of slivers of mantle wedge and ophiolitic/slab origin.