Jean-Philippe - PDF document

ESCOBAR(~), Michel MONZIER(~*~), Jo
ESCOBAR(~), Michel MONZIER(~*~), Jo CO~'TEN(~) and Jean-Philippe EISSEN(~) (l) 1-6596, Quito, Ecuador robin@orstom.ecx.ec) (2) Instituto Geofisico, Escuela Nacional A.P. 17-01-2759, Quito, Ecuador (e-mail @instgeof.ecx.ec) (3) Dtpartement des Sciences Terre, CNRS-URA 1278, Universitt de Bretagne Occidentale, 29285 Brest, France (e-mail JoCotten@univ-brest.fr) (4) ORSTOM Centre de Brest, 29 280 Plouzant, France (e-mail eissen@ifremer.fr) eruptive dynamics, Plinian activity, andesites, adakites, Ecuador. Ecuador, Mojanda volcano, located Interandean Depression the western Cordillera from the Cordillera Real. little information (Hall 1977) available about this edifice which was previously thought orientation and convergence other deposits indicate the existence two volcanic the Mojanda the Fuya Fuya (Fig. 1). This volcano experienced essentially effusive edifice (Mojanda 4000 m elevation, suggests Si02). Scoria flow deposits (56-58% Si02) containing bombs vitric bloc

ks, found flanks (unit MII-2). deposits
ks, found flanks (unit MII-2). deposits suggest eruptive episodes open-conduit regime. Dense juvenile blocks also indicate andesitic domes emplaced summit area were destroyed strong explosive A 300 unit of basic andesite breccias forming the Mojanda (unit breccias, bearing abundant quenched vitric A sequence Plinian and phreatoplinian deposits (ash lapilli beds) of basaltic products have ejected during major eruption of eruptions responsible for the formation the small summit caldera. Mafic cross the caldera N-S orientation with this the western flank thick andesitic lavas and lava domes extruded, forming edifice. Thick sequences of Plinian dome collapse deposits are related stage of development. The interpretation numerous sections on the (block and ash flow block rich lahars, Plinian ashfalls extensive pumice fall deposits), clearly indicate that, during period, the activity explosive, constantly related acid magmatism from gas-rich shallow chamber and Six major Plinian deposits interbedded a

ctivity and emitted during long magmatic
ctivity and emitted during long magmatically and volcanically cycles are observed. Each cycle began voluminous Plinian eruption resulting thick rhyolite pumice deposit (70-71% Si02), dome construction and minor andtor two main rhyolite pumice deposits, and R2, at the base each sequence, away as Quito. The lower pumice R1, Fuya Fuya divided into layers. The progressively enriched (Si02 bombs and deposits R1 represent cataclysmal Plinian eruptions, probably responsible for large vents summit area. The intermediate construction stage Fuya-Fuya is represented Bartolo cone, formed acid andesite lava flows covering remnants of the Mount St Helens collapse event responsible for of the major volume of this extrusions. The collapse also affected the western flank I1 edifice (Fig.1). Voluminous dacitic pyroclastic flows, overlying the avalanche avalanche event. Late Pleistocene complex consisting flows and Fuya-Fuya avalanche caldera. eruptive activity is mainly represented to the the drainage p

revailing winds. Unglaciated Colangal
revailing winds. Unglaciated Colangal Panecillo domes are last extrusions trace elements have made other the whole volcanic complex. Both Mojanda and Fuya-Fuya volcanic centres show remarkably distinct chemical characteristics on diagrams major as trace elements. suites lie medium-K calc-alkaline field, clearly more K-enriched the Fuya Fuya series. The Ca, and consequently less silicic. discriminant diagram SrIY figure 2, rocks from Fuya-Fuya fit the adakite field, while shows characteristics of the normal continental andesite-dacite calc-alkaline rocks. diagram, rocks Cushnirumi volcano, greatly destroyed erosion (Fig. l), also appear as adakites (Monzier et al, this volume). Two major closely associated volcanic centres previously considered volcano. Although completely contemporaneous apart, these centres developed different histories and eruptive styles. caldera collapse. which later turned explosive dynamics characterized magma-water interactions. of this ended with phreatoplinian eruptio