Eclogites and related metamorphism in the North America-Caribbean plate boundary: An example from the Motagua fault zone, Guatemala (Invited)

Active volcanic arcs and strike-slip fault systems characterize the present-day Caribbean Plate margins. The northern boundary of the Caribbean plate in Guatemala is the Motagua fault zone (MFZ). Along the MFZ in central and eastern Guatemala, eclogite- and jadeitite-bearing serpentinite-matrix mélange are exposed stretching ∼200 km on either side of the Motagua-Polochic fault system. The MFZ eclogites and related high-pressure metamorphic rocks represent key evidence for testing existing competing Caribbean plate tectonic models (e.g., Harlow et al., 2008, GSA Abs. Prog. 40:452). The presence of lawsonite-eclogite and jadeite-eclogite with lawsonite-blueschist-facies overprinting in the south of the MFZ (Tsujimori et al., 2006, GSA Spec. Paper 403:147-68) indicates that the geotherm in the paleo-subduction zone is very low. Protoliths for these high-pressure rocks consist of uppermost oceanic crust including MORB-origin greenstones and cherts with minor trench turbidites. Epidote-paragonite-eclogites of layered gabbroic origin occur in the north of the MFZ. Although some N-MFZ eclogites record amphibolite-facies hydration and new mineral growth, eclogitic garnets preserve lawsonite-pseudomorph and glaucophane, suggesting a cold subduction zone. The lack of ubiquitous lawsonite in northern eclogites may be attributed to due to water under-saturated conditions. Considering recent geochronology that implies a coeval eclogite-facies metamorphism at ∼130 Ma (Bruekener et al., 2009, EPSL284:228-35) and petrologic similarities of host serpentinized spinel-lherzolite on either side of the MFZ, we can tentatively reconstruct the succession of a fragmented eclogite-facies metamorphosed oceanic crust. However, the correct interpretation of the paleo-subduction record and further understanding of a cold subduction-zone process at paleo-Caribbean plate margin requires a more comprehensive approach to focus on in-situ geochemical and isotopic analyses of prograde metamorphic minerals. In this contribution new insights into the metamorphic evolution of the MFZ eclogites are presented.