Mesozoic Oceanic Terranes of Southern Central America-Geology, Geochesmistry and Geodynamics.

Abstract

The Northwestern edge of the modem Caribbean Plate, located in central Middle America (S-Guatemala to N-Costa Rica), is characterized by a puzzle of oceanic and continental terranes that belonged originally to the Pacific facade of North America. South of the Motagua Fault Zone, the actual northern strike slip boundary of the Caribbean Plate, three continental slivers (Copan, Chortis s. str. and Patuca) are sandwiched between two complex suture zones that contain HP/LT mafic and ultramafic oceanic rocks: The Motagua Melanges to the North, extensively studied in the last ten years and the newly defined Mesquito Composite Oceanic Terrane (MCOT) to the South. No modern geological data were available for the oceanic terrane located in the southern part of the so called continental “Chortis Block”. Classically, the southern limit of this block with the Caribbean Large Igneous Province (CLIP) was placed at a hypothetical fault line connecting the main E-W fault in the Santa Elena Peninsula (N-Costa Rica) with the Hess Escarpment. However, our study in eastern Nicaragua and northwestern Costa Rica evidences an extensive assemblage of oceanic upper mantle and crustal rocks outcropping between the Chortis/Patuca continental blocks and the CLIP. They comprise collided and accreted exotic terranes of Pacific origin recording a polyphased tectonic history. We distinguish: 1The MCOT that comprises a Late Triassic to Early Cretaceous puzzle of oceanic crust and are-derived rocks set in a serpentinite matrix, and 2The Manzanillo and Nicoya Terranes that are made of Cretaceous plateau-like rocks associated with oceanic sediments older than the CLIP. This study has been focused on the rocks of the MCOT. The MCOT comprises the southern half of the former “Chortis Block” and is defined by 4 corner localities characterized by ultramafic and mafic oceanic rocks of Late Triassic, Jurassic and Early Cretaceous age: 1 The Siuna Serpentinite Melange (NE-Nicaragua), 2The El Castillo Melange (Nicaragua-Costa Rica border), 3DSDP Legs 67 and 84 (Guatemala fore-are basin), and 4The Santa Elena Peridiotite (NWCosta Rica). The Siuna Serpentinite Melange (SSM) is a HP/LT subduction zone melange set in a serpentinite matrix that contains Oceanie crust and arcrelated greenschist to blueschist/eclogite facies metamafic and metasedimentary blocks. Middle Jurassic (Rajocian-Bathonian) radiolarites are found in original sedimentary contact with arc-derived greenstones. Late Jurassic black detrital chert possibly formed in a marginal (fore-arc?) basin shortly before subduction. A phregnite 40Ar/39Ar -cooling age dates the exhumation of the high pressure rocks as 139 Ma. The El Castillo Melange Mesozoic oceANic TeRRANes oF soUTHeRN ceNTRAL AMeRicA-GeoLoGY, GeocHesMisTRY AND GeoDYNAMics