HR: 08:30h
AN: T11D-03    [Abstracts]
TI: The Siuna Serpentinite Melange: An Early Cretaceous Subduction/Accretion of a Jurassic Arc
AU: * Flores, K
EM: Kennet.FloresReyes@unil.ch
AF: Institute of Geology and Paleontology, Anthropole, University of Lausanne, Lausanne, CH- 1015, Switzerland
AU: Baumgartner, P O
EM: Peter.Baumgartner@unil.ch
AF: Institute of Geology and Paleontology, Anthropole, University of Lausanne, Lausanne, CH- 1015, Switzerland
AU: Skora, S
EM: Susanne.Skora@unil.ch
AF: Institute of Mineralogy and Geochemistry, Anthropole, University of Lausanne, Lausanne, CH-1015, Switzerland
AU: Baumgartner, L
EM: Lukas.Baumgartner@unil.ch
AF: Institute of Mineralogy and Geochemistry, Anthropole, University of Lausanne, Lausanne, CH-1015, Switzerland
AU: Muntener, O
AF: Institute of Mineralogy and Geochemistry, Anthropole, University of Lausanne, Lausanne, CH-1015, Switzerland
AU: Cosca, M
EM: Michael.Cosca@unil.ch
AF: Institute of Mineralogy and Geochemistry, Anthropole, University of Lausanne, Lausanne, CH-1015, Switzerland
AU: Cruz, D
EM: danielcruzni@gmail.com
AF: CIGEO, Universidad Nacional Autónoma de Nicaragua, Managua, A-131, Nicaragua
AB: The Siuna Serpentinite Mélange (SSM), located in northeastern Nicaragua, is a small tectonic window of a large assemblage, the Mesquito Oceanic Terranes (MOT). The SSM was subducted during the Early Cretaceous at the border of continental Chortis Block (sensu stricto). The occurrence of the SSM is in conflict with the current plate tectonic schemes. The pre-Albian, N-S striking subduction-related mélange of SSM zone is exposed SW of the town of Siuna. It consists of metamorphosed, Ca-depleted ultramafic rocks. Relict Cr-rich spinel (Cr # _0.57-_0.79) in serpentinites and chromite pods indicates a high degree of melting. The serpentinite matrix contains blocks of sedimentary and igneous origin. Late Bajocian-early Bathonian (169-167 Ma) red- ribbon-bedded radiolarites are in sedimentary contact with greenstones (calc-alkaline metandesites) and volcanic arenites. Middle Oxfordian to late Kimmeridgian/early Tithonian (159-151/148 Ma) black radiolarian-rich cherts, minor shales and siliceous mudstones blocks were found, as well as quartzites, metasandstones and riebeckite-rich metaturbidites. Furthermore, various metamafic rocks including gabbros can be found. This indicates an oceanic setting for the mélange blocks. The metamafic rocks of the mélange exhibit assemblages corresponding to different metamorphic conditions. They range from typical greenschist and amphibolite facies assemblages to high pressure barroisite bearing greenschists. Possible blueschist to eclogite facies conditions are indicated by mica schist with silica rich phengites yielding 139.2 ± 0.4 Ma ⁴⁰ Ar/³⁹Ar geochronology and blocks containing garnet with inclusions of aegirine/omphacite. Hence, the SSM resembles typical subduction zone mélanges (e.g. Franciscan, Motagua). It exposes parts of a major zone of oceanic terranes of pacific origin, the MOT, that are placed between the Chortis Block and the Caribbean Large Igneous Province (CLIP). The overlap sequence documents Aptian/Albian deep water turbiditic sequences overlain by shallow water limestones (Atima formation) associated with calc-alkaline andesitic lavas. The presence of oceanic remnants in NE Nicaragua (SSM) may radically change the current concepts of plate boundaries of the western Caribbean plate: 1-The SSM contain the oldest HP-LT suite of the circum-Caribbean metamorphic terranes, and 2- The boundary between the Paleozoic-Mesozoic Chortis Block and the oceanic terranes associated to the CLIP has been commonly placed as far south as the Costa Rica/Nicaragua border. A suture zone was interpreted to be aligned with the EW-trending Santa Elena Fault and the Hess Escarpment, based on outcrops of serpentinites that are found along the San Juan River (Costa Rica/Nicaragua border). However, newly discovered outcrops of serpentinites and oceanic sediments far north of the San Juan River suggests that Tertiary and Quaternary volcanism of Nicaragua may hide an extensive collage of oceanic terranes that we call Mesquito Oceanic Terranes.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3660 Metamorphic petrology
DE: 5475 Tectonics (8149)
DE: 8104 Continental margins: convergent
DE: 8140 Ophiolites (3042)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting