Physicochemical conditions and timing of rodingite formation: evidence from rodingite-hosted fluid inclusions in the JM Asbestos mine, Asbestos, Québec

Fluid inclusions and geological relationships indicate that rodingite formation in the Asbestos ophiolite, Québec, occurred in two, or possibly three, separate episodes during thrusting of the ophiolite onto the Laurentian margin, and that it involved three fluids. The first episode of rodingitization, which affected diorite, occurred at temperatures of between 290 and 360°C and pressures of 2.5 to 4.5 kbar, and the second episode, which affected granite and slate, occurred at temperatures of between 325 and 400°C and pressures less than 3 kbar. The fluids responsible for these episodes of alteration were moderately to strongly saline (~1.5 to 6.3 m eq. NaCl), rich in divalent cations and contained appreciable methane. A possible third episode of alteration is suggested by primary fluid inclusions in vesuvianite-rich bodies and secondary inclusions in other types of rodingite, with significantly lower trapping temperatures, salinity and methane content. The association of the aqueous fluids with hydrocarbon-rich fluids containing CH4 and higher order alkanes, but no CO2, suggests strongly that the former originated from the serpentinites. The similarities in the composition of the fluids in all rock types indicate that the ophiolite had already been thrust onto the slates when rodingitization occurred

t~ROS1ON ET DEPO'I'S PRODUITS PAR UN EVI~NEMENT CONVULSIF, SAN CARLOS, COLOMBIE, LE 21 SEPTEMBRE 1990

Le bassin supdricur de la riviC:re San Carlos, situd sur le flanc est de la Cordill~re Centrale de Colombie, a dtd aft'ectd par un << ~vdnemertt convulsif ,, le 2I septembre 1990. Une averse de 208 ram. ~,ans 6.quiva/enl historique dans ia zone, a cau.,,6 lu mort de 20 personnes et d'importants ddgfits. Plus de cent couldcs boueuses se sont produites sur les versants les plus 61evds et les plus inclinds, quelle que soil la nature du sol. Dans les zones moyennes du bassin Its lits des torrents ont did dlurgis et approfondis p~u" le creusement d'anciens ddp6ts torrentiels et de versants et de la granodiorite mdtdorisde. Une grande quantit6, de matdriau a dt~. drod~e, y compris des blocs d'un diametre atteignant 8 m. Les d@6ts montrent une diffdrenciation classique : les blocs accumules sur la pattie haute, les sables sur le cours moyen, et les argiles ont dr6. transport6es jusqu'au rdservoir de Punchin'a, environ 20 km en aval. Une couche de cendres volcaniques qui recouvre les ddp6ts torrentiels anciens montre des associations mindralogiques qui pet, vent etre corrdldes avec d'autres cendres trouvdes plus 5, I'est, dont I'&ge radiomdtrique cst de I0.500 ans BP enviro

Subducted seamounts in an eclogite-facies ophiolite sequence: the Andean Raspas Complex, SW Ecuador

The metamorphic Raspas Complex of southwest Ecuador consists of high-pressure mafic, ultramafic, and sedimentary rocks. The Lu–Hf ages of a blueschist, a metapelite, and an eclogite overlap at around 130 Ma and date high-pressure garnet growth. Peak metamorphic conditions in the eclogites reached 1.8 GPa at 600"C, corresponding to a maximum burial depth of *60 km. The geochemical signatures of the eclogites suggest that their protoliths were typical mid-ocean ridge basalts (MORB), whereas the blueschists exhibit seamount-like characteristics, and the eclogite-facies peridotites seem to represent depleted, MORB-source mantle. That these rocks were subjected to similar peak PT conditions contemporaneously suggests that they were subducted together as an essentially complete section within the slab. We suggest that this section became dismembered from the slab during burial at great depth—perhaps as a consequence of scraping off the seamounts. The spatially close association of MORB-type eclogite, seamount-type blueschist, serpentinized peridotite, and metasediments points to an exhumed high-pressure ophiolite sequence

Shallow seismogenic zone detected from an offshore-onshore temporary seismic network in the Esmeraldas area (northern Ecuador)

Geochemistry, Geophysics, Geosystems, v. 5, p. Q02009, 2004. http://dx.doi.org/10.1029/2003GC000561International audienceFor a given site, many factors control the seismic risk. Earthquake magnitude, hypocentral distance, rupture mechanism, site effects and site vulnerability are among the most important. This article deals with one of these factors: the depth of the seismogenic zone, in the northern Ecuadorian subduction system, beneath a highly vulnerable site, the city of Esmeraldas and its industrial complex, the Ecuadorian oil refinery and shipping terminal. To address this problem, we analyzed data from a three weeks passive seismological experiment, conducted in the spring of 1998, using 13 Ocean Bottom Seismometers and 10 portable land-stations. A preliminary interpretation of wide-angle data obtained in the fall of 2000, in the Manta area, 100 km South of the study area, unambiguously indicates the presence of a velocity inversion in the Ecuadorian margin velocity structure. This velocity inversion is characterized by a shadow-zone of ∼1 s on the record-sections, and is interpreted as the result of a velocity contrast between the upper plate structure and the sedimentary and basaltic layer II of the subducted oceanic Nazca plate. One-dimensional velocity models are deduced from these wide-angle data and are used for earthquake location in the Esmeraldas area. This highly improved the hypocentral parameter determinations. The updip limit of the seismogenic zone is found at a depth of ∼12 km, 35 km eastward of the trench, and the depth of the seismogenic zone below the Esmeraldas city is found at ∼20 km. This shallow depth of the seismogenic zone dramatically increases the seismic hazard of the area