CONTRASTING PROTOLITH GEOCHEMICAL SIGNATURES OF JURASSIC PACIFIC-RELATED METABASITES NORTH AND SOUTH OF THE CHORTÍS BLOCK (GUATEMALA AND NICARAGUA)

The NW edge of the modern Caribbean Plate is a jig-saw puzzle of displaced continental slivers, tectonically sandwiched between highly deformed oceanic crust assemblages, and as assigned to the Chortís Block. The oceanic crust assemblages consist of slices of metavolcanic sequences, partially serpentinized ultramafic bodies and serpentinite-matrix mélanges within metabasite blocks including high-pressure–low-temperature (HP–LT) rocks. At the northern margin of the Chortís Block these assemblages have been grouped as the El Tambor Complex (ETC) and the South Motagua Mélange (SMM), and in the south as the Siuna Serpentinite Mélange (SSM). The ETC and the SMM contain slivers of Jurassic Pacific-related radiolarites in original sedimentary contact with metabasalts. HP–LT blocks from the SMM and SSM have yielded Late Jurassic metamorphic peak and Early Cretaceous exhumation ages suggesting a similar, if not a contemporary origin.

New geochemical data from these metabasites show major contrasts in signature and geotectonic setting. Low to medium grade spilite, greenschist, and amphibolite facies metabasites as well as HP–LT blocks show trace elements patterns that resemble those of (1) MORB, OIB, and BABB, and (2) typical IAT and CAB with large enrichments in the most fluid mobile elements (Rb, Ba, K and U), strong depletion of Nb and moderate depletion of Th and Ti.

Typical arc lava signatures in HP–LT blocks are expected due to the metasomatic overprint in the subduction channel. However, such large variations in tectono-magmatic origin of the other metabasites evidence a tectonic scenario that includes an island arc setting encountering a back-arc basin, as well as oceanic islands and normal oceanic crust. The latter scenario can be compared to the modern tectonic setting of the Izu-Bonin-Mariana (IBM) arc system and suggest the occurrence of a paleo-Pacific intra-oceanic island arc system accreted or collided into the continental active margin of the Americas.