ORIGIN OF METATRONDJHEMITE BLOCKS WITHIN SUBDUCTION-INTERFACE COMPLEXES: INSIGHTS FROM THE NORTH MOTAGUA MELANGE (GUATEMALA)

Trondhjemite and tonalite are felsic intrusive rocks commonly associated with modern oceanic crust and ophiolites. Mainly composed of sodic feldspar and quartz, they are commonly described as oceanic plagiogranites. Their genesis has been linked either to fractional crystallization of mantle melt in mid-ocean ridges or partial melting of hydrated mafic crust in supra-subduction settings.

Plagiogranites in the Guatemala Suture Zone (GSZ) occur as (i) dikes and discrete intrusions cutting metabasalts and metasedimentary sequences of the El Tambor Complex in the southern section of the GSZ; (ii) discrete bodies intruding serpentinites, gabbros, and basalt sequences in the northern Baja Verapaz and Sierra de Santa Cruz obducted ophiolites; and (iii) as enigmatic, meter-sized, highly deformed and metamorphosed blocks within a subduction interface-related serpentinite matrix complex known as the North Motagua Mélange (NMM). While plagiogranites of (i) and (ii) can be associated with mid-ocean ridges or oceanic arcs, the origin of (iii) is unknown.

Preliminary field and petrological observations indicate that most samples are trondjhemitic in composition. The Baja Verapaz and El Tambor samples are primarily comprised of plagioclase and quartz with minor amphibole and display no to greenschist-facies metamorphic textures. In contrast, the trondhjemite blocks within the NMM underwent varying degrees of deformation and metamorphism. In addition to the quartz + plagioclase primary assemblage, samples contain sillimanite, phengite, amphibole, and garnet. Incipient to well-developed foliation is revealed by subparallel sillimanite + mica + amphibole bands and rotated garnet porphyroblasts. Dynamic recrystallization of matrix quartz and feldspar indicate deformation temperatures above 500°C.

The NMM metatrondjhemites occur in close proximity to garnet ± pyroxene ± rutile ± titanite-amphibolites and jadeitite blocks. Some of these garnet-amphibolites display anatectic features or restite textures. It is unclear whether metatrondjhemites represent subducted MOR plagiogranite suites, tectonically eroded supra-subduction zone plagiogranite slivers, or products of slab partial melting subsequently incorporated in the subduction interface mélange. Thus, this study aims to elucidate the origin of these blocks.