HR: 0800h
AN: U51A-07    [Abstracts]
TI: Petrographic and Microstructural Characterization of Greenschist Facies Rocks in Northern Nicaragua
AU: * Blandino, B
EM: estilpnomelana@yahoo.com
AF: CIGEO, Colonia Miguel Bonilla Casa #165 Apdo. A-131, Managua, Nicaragua
AU: Martens, U
EM: umartens@pangea.stanford.edu
AF: Dept. Geological & Environmental Sciences, 450 Serra Mall, Bldg 320, Stanford, CA 94305, United States
AU: Valle, M
EM: mvalle@cigeo.edu.ni
AF: CIGEO, Colonia Miguel Bonilla Casa #165 Apdo. A-131, Managua, Nicaragua
AB: Low-grade basement rocks mapped south of the Guayape fault in the Macuelizo and San Fernando areas of Nueva Segovia in Nicaragua include quartz-chlorite-muscovite +/- andalusite schist, metalimestone, quartzite, metaconglomerate, and minor graphite-bearing schist. Two bands of blue amphibole-bearing schist were mapped adjacent to the Ocotal-Macuelizo road. A second localized metamorphic event at higher grade formed garnet-biotite-bearing hornfels that defines an aureole around the intrusive Dipilto batholith. Microstructurally the schists show up to three phases of deformation. A first foliation defined by phylosilicate schistosity is deflected by cleavage domains spaced between 1mm and 2 cm. Opaque content in cleavage domains is high, and quartz is relatively scarce, suggesting that pressure solution was an important mechanism for the formation of the second foliation. Some samples showed crenulation within the cleavage domains, probably due to a third deformation phase. Quartz in schist invariably shows signs of intracrystalline deformation as strong undulatory extinction and deformation lamellae. Fine grained aggregates of recrystallized quartz are common in some of the samples. Mapped metaconglomerates contain stretched pebbles with aspect ratio of ca.7:1. In Telpaneca, greenschists with structures resembling deformed pillows were found associated with chlorite-muscovite schists, which suggest an oceanic component in the metamorphic basement. Reconnaissance of low-grade rocks north of the Guayape fault in Olancho, Honduras, revealed that the most abundant rock type is likewise quartz-muscovite chlorite schist and quartzite, with minor greenschist. The later contained albite + epidote + chlorite + actinolite, indicative of similar greenschist facies conditions. Microstructurally the schist is strongly microfolded, and quartz rods define a strong lineation. Neither prototith type, metamorphic facies, nor ductile deformation features point to the existence of two disparate metamorphic units on either side of the Guayape fault.
DE: 3625 Petrography, microstructures, and textures
DE: 8030 Microstructures
DE: 8110 Continental tectonics: general (0905)
SC: Union [U]
MN: 2007 Joint Assembly