Cenozoic Alkalibasaltic Toultrabasic Volcanism in the Uppermost Magdalena Valley,Southernhuil Adepartment , Colombia
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Cenozoic alkalibasaltic toultrabasic volcanism in the uppermost Magdalena Valley,SouthernHuil aDepartment , Colombia SALOMON B. KROONENBERG HANS PICHLER HANS DIEDERIX3 •ABSTRACT The volcanic edifices inth e SanAgusti'n ,Sa nJos é deIsnos ,L a Argentina, Oporapa -Sa n Roque and Acevedo areasi nth e southern part of the Huila Department mainly consist of lavasan dpyroclastics , which according to chemical analyses vary in composition betwen alkali olivine basalt and mafitic ne- phelinite. Two volcanoes emitted tholeiitic andesites. Chemical composition indicates an upper mantle origin for the alkalibasaltic magma. Geochronological and morphological data suggest thisvolcanis m to haveculminate d inth e Pliocene or Pleistocene.Th e position of this newvolcani c province to the East of, and parallel to the main andesitic volcanic chain of the Central Cordillera suggeststha t the fracturezo nes enabling the ascent of the magma from the mantle are somehow related to the subductionprocess . RESUMEN- Los edificios volcânicos dela szona sd eSa nAgusti'n , SanJos é de Isnos, La Argentina, Oporapa- San Roque y Acevedo en el Sur del Huila consisten principalmente de lavas y piroclésticos que segûn anâlisis qui'micosvan'a n en composición entre basaltos alcalinos olivfnicos y nefelinitas mafi'ticas. Dos volcanes emitieron andesitas tolei'ticas. La composición qufmica indica que el magma alcalibasàlticos e originó en el manto superior. Ûataciones isotópicas y caracterfsticas morfológicassugiere n quee lvolca nismo alcalibasàltico culminô en el Plioceno oPleistoceno . La extension deest e nueva provinciavolcâni - ca el Este del cinturón volcânico andesi'tico de la Cordillera Central y paralelo al mismo, hace supone r que lasfractura s que permitieron el ascenso delmagm a desde el manto estânrelacionada s enalgun ama - neraco n el proceso del asubducciôn . INTRODUCTION g. in the Ruiz - Tourna complex (JARAMILLO, 1980), the Puracé volcano (KURODA & PARIS, 1978; Fig. 1) and Cenozoic volcanism in the Colombia Andes takes other volcanoes in SW Colombia (MURCIA & MARIN, place from the Miocene onward from vents in the highest 1981). Rhyolitic obsidian was reported from nearth e Sota- part of the Central Cordillera, the Cauca-Patia depression rä volcano (MANJARRES & NICHOLLS, 1958) and large and in the southernmost part of the Western Cordillera.An volumes of rhyolitic ignfmbrites were deposited in the area desitic and dacitic compositions predominates, as shown e. presentely under study, the Uppermost Magdalena Valley, 1. Direccion actual: Agricultural University. Department of Soil Science and Geology - Duivendaal 10, P.O. Box 37 - 6700. Wageningen, Holanda. 2. Mineralogisch-Petrographisches Institut d. Universität Tübingen, Wilhelmstrasse 56, D-7400 Tüblngen-1, Rep. Fed. Alemania. GEOLOGIA NORANDINA, 5. Mayo, 1982 3. CIAF. Apartado Aéreo 53754, Bogota • Colombia. possibly from the Cutanga Caldera (KROONENBERG, eta/., The occurrence of basaltic rocks in the areawa s first 1981). Although the westernmost volcanoes developed on noted by BERGT (1899) through pétrographie study of oceanic crust and the volcanoes in the Central Cordillera on statues of the San Agustfn archeological area and of some continental crust, no clear W-E trends in major element pebbles collected by Stübel in 1862. TELLO & geochemistry of the constituing volcanic rocks have been HERNANDEZ (1976) were the first to recognize the detected so far (MURCIA & MARIN, 1981). The slight Si, presence of volcanic cones in the same area;the y described K and Rb enrichment from south to north, noted by the constituent rocks as andesites. KROONENBERG,et al. Murcia & Marfn (1981) is counterbalanced by the occu (1981) first established the clearly basaltic character of rrence of rhyolites in the south and their absence in the these volcanoes and found many eruption centres. The north. present study results from photointerpretation and field work by the first and the third author while teachin post In this paper a new volcanic province will bedescri graduate photogeology courses at the Centro Interameri- bed to the east of the Central Cordillera, which differs sig cano de Fotointerpretación, Bogota, Colombia, comple nificantly from the provinces known up to now, not only in mented with petrography by the first and the second the much more continentwards location of its vents (Fig.1), author, and chemical analyses by the second author in the but also in its much more basic composition and different Mineralogisch-Petrologisches Institut of Tübingen University. •ruption style. Western Germany. VALLC DI LAS PAPAS AChontillal FIG. 1. Location of the alkalibasaltic volcanic province of the Uppermost Magdalena Valley. Lenged: I.AIkalibasaltic and related rock». 2. Rhyo- litic ignimbrites. 3. Active stratovolcano. 4. Extinct stratovolcano. 5. Small cone. 6. Main border faults of the Magdalena BJ sin; ticks point to upthrust direction.S k 433: Location of analysed sample. 20 GEOLOGICAL SETTING deeply eroded,an dweatherin g commonly reachesdepth s of over 10 m. Only occasionally fresh rock is encountered in The geology of the uppermost Magdalena Valley road cuts. In a few instances remains of a crater can be was recentely describedb y KROONENBERG& DIEDERIX discerned,no t exceeding 400 m in diamètre. (1982). The main morphostructural elements are the Cen tral Cordillera, the Magdalena Basm and the EasternCordi The Horqueta volcano hasa lessweathere d appear llera. Both Cordilleras areunderlai n by Precambrian-Jurassic ance, and may represent one of the younger edifices in the crystalline rocks,a sshow n by extensive outcrops in upthrus- area.A roadcut in this volcano shows alternating layer of la ted blocks in the area. In the southernmost part of the area pilli and small bombs.Th e crater rim of the Alto de los Ido- the two cordilleras converge to form the Central - Eastern los is an important archeological site; basalts were used for Cordillera, but the individual components are still recog tombstonesan d statues here,a swel l asignimbrite s (BERGT, nizable far south.Th e Central Cordillera hasbee na positive 1899; TELLO & HERNADEZ, 1976). feature since the Caledonian Orogeny. Shallow - marine Cretaceous sediments and, after a period of uplift. Tertiary molasse deposits were laid down to the east of it. (2) La Argentina area The beginning of Cenozoic volcanism is evidenced The second most important area is situated in the by volcaniclastic components n the molasse deposits of the La Plata Basin at 30 km SW from the town of La Plata. Miocene Honda Formation and the Mio-Pliocene Gigante Three volcanic centres have been found here. The Meren- Formation, aswel l asb y thin dacitic pumice flows in the la berg volcano at 2500 m has a swampy caldera 1,5 km in tter formation that were dated by VAN HOUTEN (1976) diamètre, the rim of which only projects 100 m above the at 8.5 ± 0.4 m.y. surrounding ignimbrite plain. Andesitic lava flows have been emitted mainly towards the notheast. The El Pensil Cretaceous and Tertiary deposits were folded and volcano at 2000 m, showsa lo wcrate r rim breached towards faulted along high angle reverse faults at the end of the Plio the north by ashor t lava flow. Another of andesitic compo cene during a major orogenic event, during which the Eas sition was issued towards the east, filling avalle y eroded in tern Cordillera was uplifted. The orogenic paroxysm at the the Ignimbrites. Two small cones have been found between end of Pliocene was accompanied by adramati c increase of the Merenberg and El Pensil volcanoes,thu s defining a short volcanism in the Central Cordillera. Hundreds of metres of NW lineament. These forms are much fresher than the ba torrential volcaniclastic conglomerates, lahar - type andesi- saltic volcanoes of San Agustfn - San José de Isnos. The tic volcanicbreccia san drhyoliti c ignimbriteswere laid down third volcano. El Morro, isa perfectl y fresh scoria cone abo on both sides of the Central Cordillera.Th e increase of mu- ut 100 m in height at about 1800 m abovese alevel .A road- dflows as compared with older deposits may testify to the cut shows alternating layerso f fresh lapilli and small bombs. uplift of the Cemffal Cordillera to above the snow line and The El Morro cone is situated on top of the easternandesi of the beginning of Pleistocene glaciation.Th eoldes t posto- tic lava flow of the El Pensil Volcano. rogenic volcaniclastic terrace deposits at Paicol in the same area were dated by VAN HOUTEN (1976) at 3.7 ± 0.2 ma. (3) Oporapa -Sa n Roque area. BASALTIC-ULTRABASIC VOLCANOES OF THE UPPER MAGDALENA VALLEY A deeply weathered volcanic cone,simila r to those of San José de Isnos,wa sfoun d at 2000 m on the southern (1) San Agusti'n -San José de Isnosare a (Fiy.1 ) side of the Serrani'a de las Minas, near San Roque,Munici - TELLO & HERNANDEZ (1976, see also pio de Oporapa. No fresh rock outcrops have been found, HERNANDEZ & TELLO, 1978) identified five volcanic but columnar joint blocks of basaltic lavas are found along cones in this area, and eight more were found by the slopes and in the streams that drain the area. A basalt KROONENBERG, et a/., 1981. All volcanoes are small pebble from the Q. El Guayabo,wa sdescribe d petrographi- conical edifices, not exceeding 1-2 km in diamètre and cally by BERAT, (1899). A second cone further west was protruding about 100-150 m above the surrounding ignim- identified in the aerial photographs. brite plain at 1750 m above sea level. They are usually 21 (4) Acevedoare a SK 38 from San José de Isnos (not analysed) con sists of olivine phenocrysts and augite (micro) Between Acevedo and San Adolfo in the SuazaVa phenocrysts only, set in a brown glassy ground- lley at the western flank of the Eastern Cordillera a thick mass without any feldspar. Apatite is the com basaltic lava valley fill has been found at an altitude of monest accessory in both alkali basalts and ultra- 1700-1200 m above sea level.