Eruptive processes and caldera formation in a nested downsagcollapse caldera: Cerro Panizos, central Andes Mountains

Journal o f H)lcanology and Geothermal Research, 56 ( 1993 ) 221-252

!21

Elsevier Science Publishers B.V., A m s t e r d a m

Eruptive processes and caldera formation in a nested downsagcollapse caldera: Cerro Panizos, central Andes Mountains Michael H.

Oft 1

Department O/Geo/ogical &'ieHces, L:mversiO, o/ ('alUiwma. .~,'cmlaBart,ar~. ( I ~J_~I06 ~

1

(Received February 20, 19t)2: revised version accepted Deccmbcr 20, 1~)93 )

ABSTRACT Fhe Cerro Panizos ignimbrite center, in the central Andes Mountains. produced two ignimbrile shecls and man~ lav~ flows. The ignimbrite of Quebrada Cienago was erupted at 7.9 Ma, and effusive eruptions continued unnl lhe tx~o cooling units of the crystal-rich dacilic Cerro Panizos lgnimbrite were emplaced at 6.7 Ma. The lov, cr unit has no lalerally contin uous flow breaks and was erupted from a single vent or small cluster of vcnts with limited fluctuation in dischargc, l_ithi~ fragments reach significant concentrations ( > 5%) onl} in the uppermosl two meters o[ this cooling unit, where [he\ document vent-wall collapse or the opening of a new vent. The upper cooling unit contains many tlox~ unfis with ~ariations in welding, thicknesses, and lithic fragment concentrations, implying an unsteady eruption column, lhe opening of many x ents, and probable caldera collapse. Triangulation of anisotropy of magnetic susceptibility tlov,-direclion measurcmenls locate a single vent for the lower cooling unit, whereas the upper cooling unit had man,v \ cnts wilhin lhc present domt. cluster. A 15-km diameter topographic depression, marked by inward dips of 4-8 at the cooling uml contact, is centered on the vent area of the lower cooling unit. The depression is interpreted as a downsag caldera t0rmcd during emplacemem of the lower cooling unit. Collapse began late in the eruption of the lower cooling unit and continued through the emplace mcnt of the upper cooling unit. Resurgent magmatism occurred as lava flows that inundated the caldcra area. X ring ol dacite domes, erupted until at least 6.1 Ma, in the northern halt of the downsag caldera traces the margin of a collapsv caldera associated with the upper cooling unit. Maximum caldera subsidence ( 353 km ~I is not enough to accounl for lhc erupted volume (652 km 3 DRE minimum ). Fhe ash-poor, crystal-rich nature (up to 50% crystals in the pumice, 75% in the matrix ) of the ('erro Panizos p) roclasti< flows resulted in poor retention of gases, and there is liltle evidcncc of fluidization. The high cr}stal contcn|, hov<~e~ t:avored modified grain-flow processes, in which several panicle support mechanisms combine with grain-dispersive torces It is postulated thai the flows were initially parlially fluidized, but rapidl? lost their gases and grain-flow processes grew it relative importance. Grain-flow processes destroyed evidence of fiuidization belbrc deposition.

Introduction Much of our present understanding of calderas and caldera-forming processes is based on studies of Tertiary ignimbrite centers in the western United States (e.g., Williams, 1942; Smith and Bailey, 1966, 1968; Lipman, 1984; Fridrich et al., 1991 ), where the collapse caldera model is the most widely used paradigm. Investigations of calderas in other regions have *Present address: D e p a r t m e n t o f Geology, PO Box 4099, Northern Arizona University, Flagstaff, AZ 86011, USA.

0377-0273/93/$06.00

shown, however, that calderas are complex and variable systems and that several different types exist. Collapse calderas involve pistonlike foundering of roof rock into the magma chamber, forming a large caldera at the surface (Smith and Bailey, 1968: Lipman, 1984). Trap-door calderas (Rytuba and McKec, 1984: Lipman and Fridrich, 1990) differ from collapse calderas in that the down-dropped block subsides unevenly, with one side folding downward like a hinge. Vertically-elongate ,lunnel cameras form when a moderately largevolume eruption leads to piecemeal collapse of

Of) 1993 Elsevier Science Publishers B.V. All rights reserxed.

222 the country rock into the magma chamber (Aramaki, 1984; Kamata, 1989; Scandone, 1990). Walker (1984) describes downsag calderas, in which downward warping of the caldera surface forms a large-diameter depression, with no ring faults at the surface. Ignimbrite shields are sources of large volumes of ignimbrite that have experienced little or no collapse (Baker, 1981 ). Several large eruptive centers in the central Andes were identified as ignimbrite shields based on Landsat imagery (Baker, 1981 ). Ignimbrite shields are enigmatic structures, for crustal accommodation of the evacuation of > 100 k m 3 of magma without associated subsidence has not been explained. Cerro Purico, a proposed ignimbrite shield in northern Chile (Baker, 1981 ), has been studied from a petrological standpoint (Hawkesworth et al., 1982; Francis et al., 1984), but its structure has not been documented. Cerro Purico has a central depression and may be a downsag caldera (S.L. de Silva, pers. commun., 1989). This paper describes the eruption dynamics, including vent migration and caldera formation, of Cerro Panizos, a proposed ignimbrite shield in the central Andes Mountains (Fig. 1; Baker, 1981 ). I find that the unusual morphology of the Cerro Panizos ignimbrite center is caused by a downsag caldera with a collapse caldera nested in its northern half. Subsequent volcanism filled the caldera, resulting in positive topography. The center does not meet Baker's ( 1981 ) definition of an ignimbrite shield, but the limited subsidence cannot account for the large volume of erupted magma.

Andes volcanism The Andes mountain range extends more than 4500 km along the west coast of South America. Volcanism has occurred along its entire length during Cenozoic time, but is presently concentrated in four active sections: the northern, central, southern and austral volcanic zones (Thorpe, 1984).

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Fig. 1. Map of the Central Volcanic Zone of the ~kndes Mountains. Modified from De Silva ( 1989a 1.

The central volcanic zone (CVZ) is marked by the 300-km-wide by 2000-km-long Altip l a n o - P u n a plateau, which lies between 15 °S and 27°S and has an average elevation of nearly 4000 m (Fig. 1 ). The CVZ consists of a chain of active strato-volcanoes and an extensive region of large calderas and associated ignimbrites that lie in a N-S-trending zone about 50-150 km east of the active Andean volcanic arc. The ignimbrites of the southern CVZ (21 °S to 24°S) range in age from about 25 Ma to 1 Ma, with the greatest volume erupted during "ignimbrite flareups" in Late Miocene and Pliocene times (Kussmaul et al., 1975; Baker and Francis, 1978; Grant et al., 1979; Bonh o m m e et al., 1988; de Silva, 1989a,b; Lavenu et al., 1989). These "flare-ups" typically produced large-volume crystal-rich dacites. Rhyolitic ignimbrites and andesites are volumetrically minor (de Silva, 1989a,b).

Cerro Panizos Cerro Panizos is a Late Miocene volcanic center in the southern CVZ at 22 ° 15'S, 66 °45'

ERUP FIVE PROCESSES AN[) CALDERA FORMATION: CERRO PANIZOS, CENTRAL ANDES

W (Fig. 1 ). Previous workers (Kussmaul et al., 1977; Friedman and Heiken, 1977; Turner, 1978; Baker, 1981; Coira and Mazzoni, 1986; Coira et al., 1987; Oft et al., 1988, 1989; Ort, 1992 ) have identified it as the source of a largevolume dacitic ignimbrite. The Cerro Panizos Ignimbrite forms a 40-km diameter shield-like plateau around a 10-15-km diameter cluster of dacitic lava domes (Fig. 2). The ignimbrite surface dips 1-3 ° radially outward from the dome cluster. The Cerro Panizos Ignimbrite overlies three other exposed ignimbrites and underlies dacite lava flows and domes. The arid climate of the region has resulted in landforms that are youthful and deeply incised locally by small streams. Turner (1978) placed all Cenozoic ignimbrites of the Cerro Panizos area in the Lower Pleistocene Lipiyoc Formation and the lava flows and domes in the contemporaneous Vicufiahuasi Formation. Whole-rock samples of dislal deposits of Cerro Panizos have been K / Ar dated at 9.7_+0.4 Ma (Kussmaul et al., 1975), 8.49_+0.2 Ma (Aquater, 1979), and 9.4 Ma (Kretzschmar, pers. commun., quoted in Baker, 1981 ).

Terminology Most volcanological terms used in this work are those of Fisher and Schmincke (1984), but several modifications are made to clarify discussion. The juvenile material in the Cerro Panizos Ignimbrite is crystal-rich and vesiclepoor (Fig. 3 ). A few exposures at the base and top of the ignimbrite contain juvenile material with up to 25% vesicles, considerably less than in typical rhyolitic pumice (Fisher and Schmincke, 1984; Houghton and Wilson, 1989). Juvenile material in moderately welded rocks is almost devoid of vesicles. The terms "cognate lithic fragment" and "block" are not appropriate for the juvenile clasts at Cerro Panizos because the clasts have glassy matri-

22 3

ces, have the same mineral assemblage as the ignimbrite matrix, and went through limited vesiculation in an eruption conduit and column. The term "block" also has size significance (Fisher, 1961: Schmid, 1981 ). Because of the greater similarity of the juvenile material to pumice, the terms "pumice" and "pumice fragment" are used for the juvenile clasts at Cerro Panizos. Welding in crystal-rich pyroclastic rocks is also distinct from that of typical ignimbrites. Ratt6 and Steven (1967) propose that a rock is densely welded when its pumice porosity is less than 10%. This is not conveniently applied to the pumice at Cerro Panizos because the most vesicular juvenile material expanded only about twice that amount. At Cerro Panizos, pumice fragments with 20% vesicles commonly have height/length ratios of 1/1 to 1/ 2.5 and occur in poorly indurated host rocks. Rocks that have a partially fused matrix, slightly deformed glass shards in thin seclion, and vesicularity above 5%, contain pumice fragments with height/length ratios between 1/ 2 and 1/3. Pumice height/length ratios of 1/3 to 1/5 typify strongly indurated rocks in which the pumice has virtually no vesicularity, pumice glass is dark, and shards are deformed. Height/length ratios of 1,/5 and less are associated with rocks with dark gray glass and no vesicularity; biotite crystals in such pumice fragments are commonly broken over neighboring plagioclase crystals, as over a fulcrum. Pumice fragments more elongate than about 1/ 8 do not occur, because the crystals are in contact and do not allow further deformation of the pumice. Height/length ratios are used to define degree of welding at Cerro Panizos in the following manner: 1/1 to l / 2 . 5 = n o n welded, 1/2 to 1/3=incipiently welded, 1.,/3 to 1/ 5 = moderately welded, and < 1/ 5 = densely welded. These ratios are not greatly different from those used by Peterson (1979) for the early Miocene Apache Leap Tuff of east-central Arizona.

224

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Fig. 2. (a) Geologic map of Cerro Panizos. Black lbrms are shallow lakes. (b) Map showing geographic localities discussed in text. (c) Landsat TM image of the Cerro Panizos area, using a linear stretch of band 4, prepared by Peter Francis at the Lunar and Planelary Institute Image Processing Facility. a, b, and c are of same area.

ERUPTIVEPR()CESSES.~,ND('ALDERAFORMATION:('ERP,()P\NIZOS.('[NTRM.XNI)ES

)25

Fig. 2. (Continued).

Deposits of Cerro Panizos Pre-Cerro Panizos rocks'

CENTIMETERS

Fig. 3. Photo of typical crystal-rich, vesicle-poor dacitic pumice fragment of Cerro Panizos Ignimbritc.

The Cerro Panizos volcanic center is constructed on a thick sequence of Paleozoic arcrelated sedimentary rocks (Acoite Formation) and Tertiary volcanic and volcaniclastic rocks (Pefia Colorada Formation!. Paleozoic rocks in the Cerro Panizos region strike northsouth with steep westward dips (Turner, 1978 ). A Tertiary normal fault cuts these rocks southeast of Cerro Panizos (Turner, 1~)76, 1978). The pre-Cerro Panizos Tertiary rocks consist of poorly indurated volcaniclastic sandstone, breccia, and debris-flow deposits, interlayered with ignimbrites and lava flows.

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Fig. 4. S t r a t i g r a p h i c c o l u m n s f r o m 1 1 section localities. L o c a t i o n s s h o w n in Figure 2c. N u m b e r s refer to s a m p l e localities t b r K / A t a n d 4°Ar/39Ar d a t i n g ( T a b l e I ).

Crystalline basement rocks are not exposed within 100 k m of Cerro Panizos. Granodioritic and tonalitic xenoliths occur in the Cerro Panizos ignimbrite, but they show no structural fabric or metamorphic overprint, and may be cognate. Two Miocene ignimbrites underlie the Cerro Panizos volcanic sequence. A densely welded dacite tuff exposed in Quebrada ("canyon"; hereafter abbreviated "Q." ) Quefioal (Figs. 2c, 4) has a m i n i m u m thickness of 40 m. It is rich in plagioclase and biotite, and contains abundant orthopyroxene and minor quartz. Its appearance on Landsat TM images suggests that the ignimbrite of Q. Quefioal correlates with the Cerro Corutu ignimbrite center to the southwest. A non-welded tuff in Q. Cusi Cusi (Figs. 2c, 4 ) is 40 m thick, rich in quartz, biotite, and plagioclase, and contains sanidine and no orthopyroxene. Above these ignimbrites are volcaniclastic rocks of mixed provenance, which vary in thickness and texture around the plateau ( Fig.

4). Some deposits are well-sorted and crossbedded, whereas others are massive and unsorted, and are interpreted as fluvial sandstones and debris-flow deposits, respectively. Sparse paleocurrent indicators suggest that they were deposited by streams draining a paleotopographic high to the west. Nearly all contain a pumice variety similar to that of the tuff of Cusi Cusi, and many contain Paleozoic lithic fragments. The volcaniclastic sequence in Q. Cusi Cusi (Figs. 2c, 4 ) is covered by 40 c m of stratified pumice lapilli, with which no pyroclastic flow deposit has been correlated. Above this is 30 m of high-angle cross-bedded, very well-sorted, biotite-rich feldspathic arenite, interpreted as eolian sandstone derived from a local volcanic source.

Ignimbrite of Q. Cienago The volcaniclastic rocks are overlain by a biotite quartz dacite ignimbrite in Q. Cienago,

ERUPTIVE PROCESSES AND CALDERA FORMATION: (?ERR() PANIZOS, ( ENTRAL ~,NDES

Q. Pupusayo, and Q. Cusi Cusi (Figs. 2c, 4), herein termed the ignimbrite of Q. Cienago. Its base is marked by two Plinian fallout layers, above which lie two pumice flow deposits. Both pumice flows are thicker and more strongly welded to the north. In some sections, gentle channels and minor re-working occur at the top of the ignimbrite. The ignimbrite of Q. Cienago is probably the oldest ignimbrite of the Cerro Panizos magmatic system, based on similarities in composition (Ort, 1991) and age (see below). Dacite lava flows were erupted in the Cerro Limitayoc area before the emplacement of the Cerro Panizos Ignimbrite, but their stratigraphic relationship to the ignimbrite of Q. Cienago is unknown.

Cerro Panizos lgnimbrite The climactic ignimbrite at Cerro Panizos, herein named the Cerro Panizos Ignimbrite with a type locality in Q. Cusi Cusi 1 km west of the edge of the ignimbrite plateau, consists of a simple cooling unit overlain by a compound cooling unit, separated by bedded pyroclastic deposits. The Cerro Panizos Ignimbrite is crystal-rich and poor in lithic fragments. The thicknesses of the cooling units vary, but are thicker close to the central domes. The lower cooling unit is 160 m thick within 2 km of the plateau edge; its base is not exposed closer to the domes (Figs. 2c, 4). The upper cooling unit varies from 0-50 m thick at the plateau edge to over 100 m near the domes.

Lower cooling unit The base of the Cerro Panizos Ignimbrite is marked by about 1 m of crystal-rich, moderately well-sorted lapillistone. Bubble-wall shards are common, many crystals are broken, and the deposit is laminated. Its mineral assemblage consists of plagioclase (90%), biotite (9%), hypersthene ( < 1%) and rare epidote. Quartz, c o m m o n in the overlying sequence, is notably absent. Crystals are 5-75% of the rock, varying between layers. This fall-

]2"]

out layer is exposed only in Q. Cusi Cusi, possibly due to generally poor exposures at the base of the ignimbrite. The ignimbrite grades from non-welded at its lower contact to densely welded 1-2 m above (Fig. 4). The overlying densely welded zone is 50-100 m thick and grades upward into 10-30 m of moderately to non-welded ignimbrite that contains vapor phase minerals. Above this is a 10-50-m columnar-jointed section of moderately to incipiently welded ignimbrite marked by vapor-phase crystallization. The upper surface of this cooling unit forms a large semi-circular topographic depression south of the dome complex. The pumice of the lower cooling unit contains 10-15% each of 1-2.5 m m crystals of biotite, plagioclase, and quartz, _+minor orthopyroxene. Vesicles form < 20% of the pumice. Some zones contain more or larger pumice fragments, suggestive of separate flow pulses, but no distinct flow contacts occur. Matrix crystal contents range from 40% to 75% by volume, and lithic fragments are less than 1% of the rock. The sequence of non-welded to densely welded to non-welded tuff. with vapor-phase crystallization at the top, is typical of a simple ignimbrite cooling unit (Smith, 1960). The uppermost two meters of the lower cooling unit are characterized by a pumice concentration zone that contains two types of pumice, as well as orbs and fragments of fresh tonalite. One pumice type is light-colored and typical of the Cerro Panizos Ignimbrite (Fig. 3 ). The other is a dark gray crystal-rich dacite, with fine-grained plagioclase (25%) and biotite (25%) phenocrysts, and quartz oikacrvsts ( 30% ) that enclose the smaller plagioclase and biotite crystals. The orbs are oval-shaped and have concentric rings of coarse-grained, radially oriented plagioclase and bronzite crystals alternating with rings of fine-grained biotite and ilmenite (Ort, 1992). The tonalite fragments contain fresh biotite, plagioclase and quartz. Fragments of Paleozoic and Tertiary

228 volcanic rocks constitute up to 7% of the ignimbrite here. In addition to the unusual pumice and lithic fragments, 30-cm diameter pieces of welded ignimbrite from the lower part of the cooling unit occur at this level and throughout the upper cooling unit. Ort ( 1992 ) interprets this layer to indicate eruption of magma from a water-rich cupola at the top of the magma body.

Upper cooling unit Within about 6 km of the central dome complex, the lower cooling unit is overlain by a l 5-m section of 1-20-cm beds of fines-poor material and 10-30-cm massive layers with inversely graded bases. The thin-bedded deposits, interpreted as surge deposits, consist of laterally continuous, accretionary lapilli-rich ash and crystal fallout layers and strata of laterally variable thicknesses containing flat-lying biotite, lineated plagioclase, and small ( < 1 cm ), rounded pumice fragments (Fig. 5a ). The massive layers are pyroclastic flow deposits containing < 3 cm diameter crystal-rich pumice in a matrix of plagioclase, quartz, and biotite crystals and ash (Fig. 5b). Bubble-wall shards are found in both types of deposits. The deposits vary in different locations, so that they are dominated by thin-bedded surge deposits in Q. Cuevas, but by 10-30-cm-thick layers of pyroclastic flow deposits in Q. Buenos Aires (Fig. 2c). In an unnamed quebrada 1 km north of Q. Cuevas, the deposits are mostly from pyroclastic flows, showing the high degree of variability over a short distance. Rounded, poorly lithified Tertiary sandstone boulders up to 2 m in diameter occur at the base of this unit. Wide, low-angle channels cut into the lower cooling unit at Q. Cuevas were carved by surges (rip-up blocks characterize the lower surge deposits ), and do not represent a significant time break. The zone of vapor-phase crystallization at the top of the lower cooling unit continues through this stratified section and into the overlying ignimbrite. A compound ignimbrite cooling unit ("up-

MH.ORT per cooling unit") overlies the bedded deposits or lies directly on the lower cooling unit, The rock varies from friable and non-welded to densely welded vitrophyre, and total thickness varies from 30 m to 165 m. Individual flow units radiate from the central domes, with none traceable for more than a 90 ° arc. Thin, moderately well-sorted pumice-fall layers occur at various levels throughout the upper cooling unit. The pumice in the upper cooling unit is similar to that of the lower cooling unit, except that it contains < 1-5% hypersthene. Many outcrops have two types of pumice that vary solely in their degree of welding, with one type non-welded to incipiently welded and the other moderately to densely welded. With no difference in composition or the mass of overlying rock, the two pumice types appear to have had different temperatures or volatile contents at the time of emplacement. The upper cooling unit is notably richer in lithic fragments (up to 10% by volume) than the lower cooling unit. The thickness, degree of welding, percentage and size of pumice and lithic clasts, and types of lithic clasts of the upper cooling unit vary widely around the ignimbrite center and are described in detail below.

Post-ignimbrite volcanism The upper cooling unit is capped by lava flows in the area near the dome cluster (Figs. 2, 4). A lava platform forms a broad plateau on the ignimbrite surface. The platform lava flows are biotite-hypersthene dacite with about 3% hypersthene, < 1% biotite, and 5% plagioclase. No vents have been found for these flows. The flows filled the depression south of the central domes and formed a t 0-km-long lava flow with weak pahoehoe features to their north. Proximal platform flows are overlain by a 10-15-km diameter cluster of lava domes. The domes are mono- and poly-genetic and are biotite-hypersthene dacite containing up to 20% crystals. A single biotite-hypersthene dacite

ERUPTIVE PROCESSES-kND('ALDER&FORMATION: ('ERR() P~,NIZOS,('ENI RM ',NI)ES

~:_

Fig. 5. (a) Photograph o f layered surge and Fallout deposits at base of upper cooling unil in Q. ( ' u e \ a s . Note pinch-andswell bedding and laminae. (b) Photograph of t h i n - b e d d e d massive p)roclastic flow deposits in can,.on 1 km norlh oi'Q. ('uevas. Scale is 60 cm.

230

M,H. ORT

lava flow from a source high on Cerro Limitayoc overlies the ignimbrite at Q. Hornillos (Figs. 2c, 4). Lava flows from Cerro Limitayoc also underlie this section, so Cerro Limitayoc was active both before and after the Cerro Panizos Ignimbrite was emplaced.

Timing of volcanism New isotopic dates of the rocks in the Cerro Panizos area were determined using 4°Ar/39Ar stepwise heating techniques and conventional K / A r techniques (Fig. 4, Table 1 ). 4°Ar/3°Ar analysis and age determinations were done at the U.S. Geological Survey in Reston, Virginia, and K / A r dates were obtained at the U.S. Geological Survey in Menlo Park, California. Methods used for the 4°Ar/agAr dates are described in Richard et al. (1990). Two wholerock samples were analyzed by conventional K / A r methods. Gases were extracted by RF induction heating, and Ar was purified using Ti and Cu getters, after introduction of an 3SAr spike into the gas. Isotopic composition of the Ar was determined on a Nier mass spectrome-

ter and K20 concentrations were determined by flame mass spectrometry.

Discrepancies between dates The discrepancy between the new dates and those published previously for the Cerro Panizos Ignimbrite (9.7-8.5 Ma; Kussmaul et al.. 1975; Aquater, 1979; Baker, 1981 ) may reflect the presence of xenolithic material in the K / Ar samples. All K / A r dates, including those reported here, were made on material from whole rock samples. Sample contamination by xenocrysts from biotite- and potassium-rich country rock can explain these old ages. The 4°Ar/39Ar dates were obtained on biotites separated from pumice samples, in order to minimize contamination problems. The upper compound cooling unit of the Cerro Panizos Ignimbrite yields a slightly older age than the lower cooling unit, probably because a mechanical malfunction resulted in rapid heating of the sample and produced a total fusion age. Slight alteration of biotite in other Cerro Panizos samples caused early heating steps to yield higher apparent ages, so total fusion is likely to

TABLE 1 K / A r and S9Ar/4°Ar data from analyses of Cerro Panizos area rocks

Whole rock K/Ar dates Sample

Rock type ~

Location

K%

4°Ar rad%

Age and error 2

AMO-51D AMO-26A

Lava LCI

Co. La Ramada, 5200 m Q. Cusi Cusi, 4150 m

4.03 3.79

36.23 4.52

6.12 _+0.07 9.09 _+0,79

4°Ar/sgAr pumice biotite dates Sample

Rock type ~and Location

K/Ca

% 39Ar

Total gas age (Ma)

Plateau age (Ma) ± l a

P87-833 AMO-164 AMO-22E 5 AMO-20B 6

UCI, Rio Khuchu Mayu, 4670 m LCI, Q. Cuevas, 4420 m Ig. ofQ. Cienago, Q. Cusi Cusi, 4050 m T u f f o f Cusi Cusi, Q. Cusi Cusi, 3970 m

107.3 54.5 43.1 32.0

31.9 33.6 24.0 10.8

6.78 6.70 8.04 12.19

6.80_+ 0.023 6.71 _+0.04 7.87 ± 0.59 12.43 _+0.08

Notes J LCI = Lower cooling unit, UCI = Upper Cooling Unit, Cerro Panizos Ignimbrite. 2 Mechanical problem caused rapid heating of sample. One step yielded 96% of Ar, so date is total fusion. 3 K / A r errors calculated based on an empirical function relating the coefficient of variation in the age to percent radiogenic argon (Tabor et al., 1985 ).

ERISP11VE PROCESSES AND CALDERA FORMATION: CERRO PANIZOS, CENTRAL ~.NDES

produce a slightly older age. A 6.7 Ma age is preferred for the Cerro Panizos Ignimbrite, using the better date for the lower cooling unit. The volcanic sequence of the Cerro Panizos area began at 12.4 Ma with the eruption of the ignimbrite of Cusi Cusi. No age constraint is available for the ignimbrite of Q. Quefioal. The ignimbrite of Q. Cienago was emplaced at 7.9 Ma. Intermittent volcanism at Cerro Limitayoc occurred around this time and continued until some time after the eruption of the Cerro Panizos Ignimbrite at 6.7 Ma. The 6.1 Ma lava flow at Cerro La R a m a d a indicates that dome growth continued for at least 600,000 years after the climactic eruption. Lateral variation of volcanic units

Lateral and vertical variations in the upper cooling unit of the Cerro Panizos Ignimbrite are significant. One major problem in correlating the lateral variations is that upper cooling unit flows are distributed radially, forming wedge-shaped sectors in plan view. With many flow units overlapping each other, correlation between different quebradas (canyons) is difficult.

Lithic fragments The Cerro Panizos Ignimbrite generally contains < < 1% lithic fragments. They are concentrated (up to 10% of the rock) in the uppermost 2 m of the lower cooling unit and at the bases of flow units in the upper cooling unit. There is a weak decrease in lithic fragment sizes away from the central domes. The largest lithic fragment, a 2-m rounded Tertiary sandstone block, occurs in a thin proximal pyroclasticflow deposit at the base of the upper cooling unit near Q. Cuevas (Fig. 2c). Typical maxim u m lithic fragment sizes are 8-10 cm near the domes, and decrease to several centimeters 10 km from the domes. The two main types oflithic fragments in the Cerro Panizos Ignimbrite, Tertiary volcanic

231

and Paleozoic argillaceous and volcaniclastic rocks, show little systematic vertical or lateral variation in their relative abundance. Tonalite fragments occur only in the upper three meters of the lower cooling unit and in the upper cooling unit, where they are concentrated (although << 1%) in some flow units. Welded-tuff fragments occur only in the lithic concentration zones. Orbs occur only in the upper three meters of the lower cooling unit (Ort, 1992 ).

Pumice Diameters of pumice clasts (average of five largest pieces in each outcrop) range from 80 cm to <1 cm, with a weak decrease in sizes away from the central dome complex. Pumiceand lithic-concentration zones indicate that separate flow pulses occurred during emplacement of the lower cooling unit, but m a x i m u m pumice sizes do not vary much within any one section. Pumice concentration zones several meters thick and containing up to 50% pumice (to 1 m in diameter near the domes) occur near the tops of each cooling unit. The concentrations are in sections that otherwise show no vertical gradient. Both pumice concentration zones can be traced to the edge of the ignimbrite plateau, and pumice in them decrease in size outward from the dome cluster.

Thickness The distribution and thickness of the Cerro Panizos Ignimbrite are strongly affected by topography. The ignimbrite covers and drowns much of the topography on which it was deposited, as is the case with many large ignimbrite sheets (Fisher and Schmincke, 1984). The lower cooling unit is especially thick in several paleo-valleys but thins steadily from the domes over smooth topography. The upper cooling unit has a more complex distribution pattern. It forms a 5-10-m-thick deposit on the steep north side of Cerro Limi-

232

MH. ORT

ERIIP IIVE PROCESSES ,X.ND('ALDERA FORMATION: CERRO PANIZOS, ('ENTR~I -\NDES

tayoc (Figs. 2c, 4, 6a) but is many tens of meters thick across the quebrada. Elsewhere in the south, the upper cooling unit is confined within a depression described below (Fig. 6b), where it is at least 100 m thick. It thins and ends abruptly, forming 20-30-m-thick escarpments at the edge of the depression. The confinement to the depression is probably depositional, as no remnant outcrops occur outside the depression in this area. The upper cooling unit extends eastward from the domes, gradually thinning before ending abruptly at the eroded plateau edge ( Fig. 2 ). Steep 30-50-m-high scarps define the southern and northern margins of this eastern extension. I interpret the escarpments as depositional because no outliers of the upper cooling unit occur beyond them. In the west and north, the distal ends of individual flow units form steeply descending fronts that roughly parallel the edge of the dome cluster.

WeMing The degree of welding in the Cerro Panizos Ignimbrite varies laterally. The lower cooling unit has a densely welded interior along the entire east side of the plateau. At Q. Hornillos (Figs. 2c, 4 ), however, it is moderately welded in its core, even though the section is as thick as many to the east. A large lateral variation in the degree of welding of the upper c o m p o u n d cooling unit is at least in part due to the presence of different flow units (Fig. 6c). In the south and southeast, the cooling unit is densely to moderately welded in its lower 70 m, but is mostly nonwelded in its upper part. In the northeast and north, it is non-welded to moderately welded

233

in its lower 50-80 m, but this is covered by 2030 m of densely welded tuff with fiamme. In the north, this is overlain by 95 m of nonwelded tuff. Thus, the base tends to be more welded in the south, while the top is more welded in the north.

Sur[ace morphology o/the proximal deposits The largest domes in the central dome complex have a distinct ring-like distribution, with minor lava flows in the center (Fig. 2). The appearance is similar to that of ring domes along the borders of collapse calderas elsewhere (e.g., Smith and Bailey, 1968 ). Near the southern lava domes, the ignimbrite forms a depression with inward radial dips toward Cerro Crucesnioc (Figs. 2c, 6b, 7a). The depression is best seen at the contact between the lower and upper cooling units where dips are 4-8=, with a distinct "hinge line" where dips change from inward to outward. This hinge line curves in a semi-circle of constant radius from near Cerro Limitayoc to the head of Q. Pupusayo, where it disappears under the overlying domes. The contact between the upper cooling unit and lava platform dips 1-2 ° toward Cerro Crucesnioc.

Studies of anisotropy of magnetic susceptibility It is commonly difficult to determine flow directions, and, hence, vent locations, for pyroclastic flows. Methods used include the orientations of pumice and lithic fragments, glass shards, and crystals (Suzuki and Ui, 1982, 1988; Caress, 1986; Potter and Oberthal, 1987; Ui et al., 1989), orientation of logs (Froggatt

Fig. 6. ( a ) Photograph of upper cooling unit deposits on the side o f C e r r o Limitayoc. Flow unit contacts parallel the dome surface. ( b ) Photograph of upper cooling unit near Q. Garcia. The contact between the cooling units dips 6 here, and the steep front of the deposit (left in p h o t o ) indicates where tile flow was confined within a depression in the surface of the lower cooling unit. (c) Photograph of typical section in the upper cooling unit. Note inversely graded bases, lithic fragment concentrations, and pumice concentrations, indicating discrete flow units. Scale is 2 m, also note person's head in lower left corner.

234

M.H.ORT

(a)

r

I

66045 '

LOWER COOLING UNIT ORIENTATION OF K 1 AXES

@

79/ ling(

e

(b)

Fig. 7. Maps of distribution and orientation ofAMS Kt axes and the distribution of the ignimbrite from which they were sampled. AMS samples were taken from sites at the center of the bars. Large numbers indicate sample number (see Table l ); small numbers are angle of dip of attitudes of the contact between the lower and upper cooling units. Tv = Pre-Cerro Panizos Tertiary volcaniclastic rocks; PC= Pre-ignimbrite lava dome. See text for discussion of anomalous orientations. a) Lower cooling unit, Cerro Panizos Ignimbrite. Hinge line indicates where dips of cooling unit contact change from inward to outward. Note how the dips of the bedding and the AMS directions radiate around the same area (indicated by shaded area). (b) Upper cooling unit, Cerro Panizos Ignimbrite. A large range of vent locations (shaded area) is needed to explain the AMS directions.

ERUPTIVE PROCESSES AND CALDERA FORMATION: CERRO PANIZOS, CENTRAL ANDES

et al., 1981; Potter and Oberthal, 1987), imbrication of pumice and lithic fragments (Kamata and Mimura, 1983), and asymmetrical depositional ramps (Suzuki and Ui, 1982, 1988). These methods give geologically reasonable paleo-flow directions, but are slow and tedious, and so restrict the number of samples and sites that can be analyzed. Elston and Smith (1970) and Ui et al. (1989) emphasize the need for data from a large number of widely separated sites in order to recognize local topographic effects. Recent work has demonstrated the utility of anisotropy of magnetic susceptibility (AMS) in the determination of paleo-flow directions in ignimbrites (Ellwood, 1982; Incoronato et al., 1983; Knight et al., 1986; Wolffet al., 1989; Froggatt and LaMarche, 1989; MacDonald and Palmer, 1990; Palmer et al., 1991; Seaman et al., 1991; Hillhouse and Wells, 1991 ). It is a relatively fast method, conducive to analyzing samples from many sites, and can discern the relatively weak lineations in ignimbrites with a reasonable degree of precision. Magnetic susceptibility indicates the response of a material to an applied magnetic field. When a rock sample is placed in a magnetic field, it acquires an induced magnetic moment. The magnetic susceptibility per unit volume (K) is the ratio of the magnitude of the induced moment to the applied magnetic field. K is commonly given as susceptibility per unit volume in SI units. If a material is anisotropic, K is a second-order tensor, such that K= ( Kl + K2 + K3 ) / 3, where K1, K2, and K3 are the orthogonal maximum, intermediate, and minimum susceptibility magnitudes and azimuths, respectively. The three elements of the susceptibility tensor form the axes of the susceptibility ellipsoid (Nye, 1969). The determination of paleo-flow directions by AMS uses the orientation of the long axis (K~) to indicate the azimuth of flow. Other symbols used in describing anisotropy of magnetic susceptibility are the degree of anisotropy ( P = K~/K~ ),

235

the degree of lineation (L=K~/K2), and the degree of foliation ( F = K2/K3 ).

AMS and ignimbrites Rees (1965), Ellwood (1982, 1984), and Knight et al. (1986) have shown that: ( 1 ) K, axes in deposits from turbulent and laminar flows are aligned parallel to flow directions; ( 2 ) K~ axes are aligned perpendicular to the major compression direction; and (3) K3 axes are perpendicular to the plane of foliation of the rock and parallel to the axis of maximum compression. Thus, in a non-rotated ignimbrite that has undergone some compaction, the K1 axis should be sub-horizontal and parallel to flow directions and the K3 axis nearly vertical. Empirical evidence suggests AMS K~ axes define flow directions in pyroclastic deposits. Studies by Knight et al. (1986), Knight and Walker (1988), MacDonald and Palmer (1990) and Seaman et al. ( 1991 ) compare AMS data with data obtained using field and petrographic methods and conclude that AMS is accurate and considerably quicker. Wolff et al. ( 1989 ) demonstrate that the heterogeneous character of ignimbrites does not seriously affect the AMS fabrics, and that AMS provides a reasonable indication of flow directions in pyroclastic rocks. The distribution of magnetic grains produces the anisotropic magnetic fabric observed in ignimbrites, but it is not known whether inequant magnetic minerals are aligned by flow processes or if equant magnetic grains are distributed among aligned nonmagnetic grains to produce an anisotropic magnetic fabric (Knight et al., 1986; Borradaile, 1988; Wolffet al., 1989; and Schlinger et al., 1991 ).

Paleornagnetisrn methods Paleomagnetic cores were taken from 4-7 hand samples from each site, oriented using

324.6 274.4 0.1 14.8 191.7 313.3

311.0 355,9; 300.3 328.7 95. I 358.9 309.2 132.1 31.9 181.4: 37.2 215.8 33.2 194.6 242.3 45.9 269.7 292.4

Trend

14.5 2.8 12.5 11.6: 0.1 10.1

10.4: 9.9 ° 14.2 8.5 14.1 14.3 15.1: 5.2 21.0 12.0 1t.8 15.2 9,0 9.1 9.0: 7.7 23.2 9.9

Plunge

M a x i m u m (KI) eigenvector

219.9264.4 31.7' 59.7 3.0 91.0 39.4 42.0 : 127.4 91.5 306.5 311.1 124.0: 285.1 152.4: 320.9 0.5: 21.6"

Trend

8.6 11.9 14.8 ~ 3.0 3.9 4.7

4.9 5.8 6.8 4.3 8.7 6.7 9.3 1.3 13.0 6.6 1.8 0.7 2.4 2.5 1.1 8.7 3.3: 0.5

Plunge

Intermed. (K2) eigenvector

0.6015 2 3 2 . 9 I).8271 4.9 0.7964 2 6 5 . O 0.7790 284.00.6779 t 0 2 . 3 : 0.8935 4 3 . 5

0.6963 0.6135 0.9588 0.7557 0.7763 0~8488 0.7272 0.8684 0.8452 0.6107 0.7879 0,5901 0.6585 0.9054 0.6935 0.5893 0.6353 0.7712

K~ eigenvalue

104.7: 156.5 145.4 175.6 242.8 205.5 161.2 305.2 243.7 350.4 202.9 18.4 216.5 23.4 = 50.1 158.8 = 101.7 109.3:

Trend

71_8 78.7 70.6 77.0 87.8 77.1

78.7 79.1 74.1 80.0 72.9 73.2 79.3 84.4 65.2 76.2 79.5 86.4 79.8 80.6 81,6 79.5: 68.0 77.4:

Plunge

M i n i m u m (K3) eigenvector

0.5826 104.5: 0.8242 166.1 0.8101 125.7: 0.7842 190.7 0.6788 291.7: 0.8944 156.0

0.6943 0.6160 0.9280 0.7520 0.7800 0.8500 0.7132 0.8664 0.8491 0.6169 0.7808 0.5101 0.6544 0.9161 0.6931 0.5968 0.6333 0.7708

/£2 eigenvalue

0.9293 0.9849 0.9481 0.9875 0.9878 0.9519

0.9968 0.9894 0.9635 0.9915 0.9905 0.9889 0.9646 0.9961 0.9913 0.9585 0.9835 0.8762 0.9858 0.9787 0.9930 0.9328 0.9916 0.9667

K3 eigenvalue

*Normed principal susceptibility is susceptibility o f each principal axis divided by mean susceptibility. P = K ~ / K 3. b L = K , / K , , ~ F = K 2 / K ~ . LCU = lower cooling unit. I I(;U = upper cooling unit.

LCU UCU UCU UCU UCU LCU

lg.

LCU LCU LCU UCU UCU UCU UCU UCU U'CU UCU LCU UCL' LCU UCU UCU UCU LCU Cienago

P87-10 (8) I'87-11 (6) P87-14 (8) P87-16 (7) P87-18 (6) I787-35 (9) P87-36 (4) P87-43 (8) P87-45 (7) P87-48 (10) P87-58(8) P87-73 (8) P87-79 (9) P87-80 (6) P87-82 (11 P87-83(11 P87-84 (7) P87-85(11

P87-87(9) P87-91 (8) P87-94(10) P87-98(8) P89-218(9) P89-221 (5)

Stratigraphic unit

Site ( ~ samples analyzed )

S u m m a r y of AMS site d a t a from Cerro Panizos

TABLE 2

785 3178 836 1166 1879 1193

1929 1062 1052 596 1835 441 1208 1767 1461 759 489 1024 213 454 613 1165 1492 418

k=mean suscept, ( × 10 . 6 S1 )

1.0173 1.0202 1.0206 1.0266 1.0108 1.010l

1.0304 1.0145 1.0308 1.0103 1.0t46 1.0251 !.0225 1.0174 1.0272 1.0182 1.0156 1.0255 1.0181 1.0206 1.0339 1.0117 1.0193 1.0201

K~

1.0092 1.0087 1.0073 1.0180 1.0057 1.0024

1.0238 1.0111 1.0206 1.0078 1.0048 1.0205 0.9997 1.0100 1.0159 1.0124 1.0120 1.0935 1.0118 1.0136 1.0225 1.0055 1.0151 1.0157

K2

0.9735 0.9711 0.9721 0.9554 0.9835 0.9875

0.9458 0.9744 0.9487 0.9819 0.9807 0.9545 0.9881 0.9699 0.9569 0.9694 0.9725 0.9651 0.9702 0.9658 0.9435 0.9828 0.9655 0.9636

K3

Normedprincipal susceptibility*

Lb

Fc

1.045 1.051 1.050 1.074 1.028 1.023

1.089 1.041 1.087 1.029 1.035 1.074 1.025 1.052 1.074 1.051 1.044 1.063 1.049 1.057 1.096 1.029 1.056 1.059

1.0080 1.0110 1.0133 1.0085 1.0051 1.0078

1.0064 1.0033 1.0101 1.0026 1.0118 1.0046 t.0128 1.0098 1.0110 i.0058 1.0036 1.0161 1.0060 1.0070 1.0111 1.0063 1.0043 1.0050

1.037 1.039 1.036 1.066 1.023 1.015

1.083 1.038 1.076 1.027 1.025 1.069 1.012 1.041 1.062 1.045 1.041 1.046 1.043 1.050 1.084 1.023 1.051 1.054

(Degree (Degree (Degree anisolineatbliatropy ) tion ) tion )

pa

I'J

ER[~PTIVE PROCESSES AN[) CALDERA FORMATION: C E R R O PANIZ()S. ('EN FR \1. -\NI)ES

a l u m i n u m rings glued to the rock and a magnetic compass. Cores were extracted from oriented samples in two ways. One m e t h o d used a clamp and a l u m i n u m plates to hold the samples and rings horizontal, so they were drilled perpendicular to the rings. Additional cores were extracted from samples after setting them in plaster. Comparisons show no systematic orientation errors between cores drilled using the two methods on the same sample, nor between cores drilled from the same sample but oriented by separate rings. Cores were analyzed on a Kappabridge KLY2 using a KLY-2.1 pickup unit with a sensitivity of 4 × 10 8 SI. Some data were gathered using a Kappabridge at University of Texas, Arlington, but the majority of the analyses were performed at University of California, Santa Barbara. Replicate analyses in the two laboratories agree well. Sample data were collected and reduced using the ANISO 17 program. Site data were evaluated using Bingham statistical methods and plotted on equal area stereonet diagrams using the Stereonet program.

Cerro Panizos A M S data Five to eleven cores from each of twenty-four sites were analyzed ( Fig. 7, Table 2 ). Care was taken to avoid areas of slumping or post-depositional deformation. Microprobe analyses of oxide phases in the Cerro Panizos Ignimbrite show them to be members of the hematite-ilmenite solid solution series. No magnetite crystals were observed. Thermal demagnetization experiments show a rapid loss in remanence between 500°C and 550°C, consistent with a low-Ti-magnetite magnetic carrier. A similar situation, where magnetite is the probable magnetic carrier but hematite and ilmenite are the microscopically visible oxide phases, is described by Seaman et al. ( 1991 ). Total remanence is c o m m o n l y single component, with minor secondary magnetizations removed by early stages of AF demagnetization.

~ 37

Mean susceptibilities range from 3 × 10- ~ to 2X 10 a SI (Table 2). P varies from 1.023 to 1.096, indicating a significant degree of anisotropy. The AMS ellipsoids from the Cerro Panizos ignimbrites are oblate ( Fig. 8 ), with a strong sub-horizontal planar fabric ( F = 1.0031.11 ), and a weak to moderate lineation within that plane ( L = 1.003-1.016; Table 2 ). Statistical data indicate that the AMS axes are generally well defined (Table 2). The eigenvectors are the K~, K~, and K~ axes determined from all the samples of each site. The eigenvalues indicate the statistical quality, with 1.000 indicating all samples have the same vector and 0.333 a random distribution of vectors. K~ eigenvalues are higher than 0.93, indicating a strong foliation. K~ and K_~eigenvalues are mostly between 0,6 and 0.9, implying that the lineations are moderately strong and statistically significant. This study finds no correlation between low values of K and poor clustering of principal axes down to 2 × 1 0 ~ SI (Table 2). Seaman el al. ( 1991 ) found that samples with K < 1-2 × 10 ~ S1 yield unrealistic flow lineations. This disagreement may be due to differences in analytical precision or to factors in the rocks themselves.

Orientations ~!/susceptihility axes A single site in the Cienago Ignimbrite in Q. Cienago yields an east-west KL direction, consistent with a postulated vent area in the northern d o m e cluster. The paleomagnetic sites in the lower cooling unit of the Cerro Panizos Ignimbrite give K~ directions that are more or less radial about an area in the southern part of the central d o m e cluster (Fig. 7a). Three sites have significantly different orientations. P87-11, a very densely welded rock, P87-84, a densely welded rock, and P87-87, an incipiently welded vapor-phase altered rock, have very poor clusters of Kl axes, reflected in their low eigenvalues (Table 2; Fig. 9a). Alteration is minor in these samples, and Wolffet al. (1989) assert that secondary rain-

238

M.H.ORT 1.12

.

i

•

I

.

1.10

'~ i

•

i

•

i

'

PROLATE 1.08 ,_= v

1.02

1.00 1.00

: ".'ii: "

. . . . . . . . . . 1.02 1.04

~. . . . 1.06 F

..

'

" " 1.08

' 1.10

1.12

(Kin t/Kmi n I

Fig. 8. L, the degree of lineation, vs. F, the degree of foliation, for Cerro Panizos AMS samples. Nearly all samples have an oblate magnetic fabric, indicating that the foliation is better developed than the lineation.

erals should yield flow-parallel AMS Kt axes anyway. The orientation of P87-87 is explained if the particles were aligned perpendicular to the original flow direction by particle traction or rolling. Such an alignment is supported by the orientation of P87-87 at 90 ° to the expected direction and by the large variations in axis orientations, consistent with transport by rolling and/or saltation. Orientation of plagioclase laths parallel to K~ directions is visible in cores from this site. Another possible cause of the orientation of the P87-87 maximum axis is that the site may have been at the edge of a flow lobe, where the flow was expanding sideways, orthogonal to the main flow direction. K] axes from sites in the upper compound cooling unit of the Cerro Panizos Ignimbrite center on many points in a broad area of the central dome cluster. The eigenvalues of the sites in the upper cooling unit are, in general, higher than those in the lower cooling unit (Table 2 ), but three sites have eigenvalues distinctly lower than the others. The stereonet diagram ofP87-48 (Fig. 9b) shows that the Kt axes cluster near the eigenvector, with three Kt axes plotting at nearly right angles. Both P8773 and P87-83 have two clusters at right angles to each other that contain both K] and Kz axes (Fig. 9c). This phenomenon, also reported for

other ignimbrites (Ellwood, 1982; Knight et at., 1986), may be due to some particles aligning perpendicular to flow by processes of traction or rolling. These three samples, along with the three samples from the lower cooling unit with large scatter and anomalous directions, have eigenvalues below 0.65. There is no correlation between low eigenvalues and low K. The causes of the within-site scatter are not fully understood, but I do not use any sites with eigenvalues less than 0.65 in locating vents, as they appear to yield anomalous results.

Implications of AMS for flow directions To use flow directions for identifying vent locations, the influence of other factors, such as local topography and whether particles align parallel or perpendicular to flow, must be considered (Froggatt et al., 1981 ). The lower part of the thick lower cooling unit filled in the paleo-topographic irregularities in the area, so subsequent flows were little affected by local landforms. At Cerro Panizos, AMS data from most sites indicate flow directions from the general source area (the central dome cluster), suggesting that Kl axes parallel flow directions. The closer a site is to the vent, the better it will constrain the vent location, assuming equal data quality (MacDonald and Palmer, t 990). Five samples in the upper cooling unit, P87• 18, P87-43, P87-80, P87-94, and P87-98, yield geologically improbable directions (Fig. 7b). There are no potential vents northwest or southeast of P87-43, but the direction is oriented directly down the slope of Cerro Limitayoc, a pre-existing topographic high, implying gravitational flow off the dome. P87-80 is from a 17-cm-thick pyroclastic flow deposit associated with the contact layer at the base of the upper cooling unit. Small irregularities in the underlying surface may have affected the thin flow. P87-94 has a fairly high eigenvalue, with clustered Kl axes and high L values (Table 2 and Fig. 9d), but its eigenvector is oriented

ERUP1 IVE PROCESSESAND CALDERAFORMATION: CERRO P~-NIZOS, ( t N I-R,\L~\NDES

4-

,

D

3 ~-)

't" a

oQ

axis

+

K2

axis

a

K3

axis

'

Fig. 9. Equal-area, stereonet, lower-hemisphere projections o f four sites from the upper cooling unit See text lbr discussion o f l i n e a l i o n orientations. (a) P87-87, (b) P87-48, (c) P87-83. and (d) P87-94.

north-south, with no reasonable vent location in either direction. It is located several hundred meters south of the b o u n d a r y between upper cooling unit ignimbrite that is confined within a depression to the south and a thick "tongue" to the north that extends eastward to the edge of the plateau (Fig. 7b). The flow may have been diverted southward and captured by the depression. P87-18 and P87-98 are from the same flow unit and have fairly high eigenvalues and L values (Table 2). They are located several hundred meters from the central d o m e complex, and no ignimbrite exposures occur farther inward. The orientation of their K~ axes

can be explained in several ways. If they are perpendicular to flow, then triangulation would indicate a vent location in an area within the central dome complex. In this case, the two sites are near a deflation zone, where flow may be disorganized and particles oriented parallel or perpendicular to the flow direction. ~nother possibility is that the flows responded to the slope of the topographic depression in the southern part of the ignimbrite center, so that the flows turned as they slowed. All upper cooling-unit deposits in this area are confined within this depression, suggesting it had a strong effect on the deposition of ignimbrites.

240

Discussion

Eruption of the Cerro Panizos Ignimbrite cooling units The lower cooling unit shows continuous cooling zonation from non-welded at the base through a densely welded section into a nonwelded top, which indicates no significant eruption hiatuses. Variations in pumice and lithic fragment concentrations are local phenomena, probably due to local flow pulses, and not related to different flow units. The paucity oflithic fragments, the radial distribution of the deposit, and the lack of evidence, such as fallout layers, of eruption column fluctuations, imply steady eruption of hundreds of cubic kilometers of lower cooling unit material from a single vent or a small cluster of vents. The abrupt appearance of lithic fragment concentrations in the uppermost two meters of the lower cooling unit suggests either collapse of the walls of the vent or the opening of a new vent. Welded ignimbrite clasts at this level and throughout the upper cooling unit represent ripped-up pieces of the lower cooling unit and imply that the eruption had continued for a time sufficient to allow welding of deposits. Vent collapse cut off the eruption, leading to a brief hiatus in the eruption and a partial cooling break between the two cooling units. The layered surge and pyroclastic flow deposits at the base of the upper cooling unit were deposited before the lower cooling unit had completely outgassed. The wide areal extent and textural variability of the deposits is probably due to eruption from different vents. No clear evidence of a phreatic component in these deposits was found. The upper cooling unit, comprising many well-defined flow units (Fig. 6c), was probably the result of eruption from many vents. The relatively high lithic fragment content is evidence of vent erosion a n d / o r the opening of new vents. The occurrence of many flow units with fallout layers between them implies un-

MH. ORT

steady eruption columns. Large lateral and vertical variations in welding, and sectorial distribution of flow units, indicate that vent locations migrated during the eruption. The entire Cerro Panizos Ignimbrite is a composite sheet (Smith, 1960) associated with a single eruptive period, rather than the product of two distinct eruptions. Vapor-phase crystallization is continuous from the upper part of the lower cooling unit through the layered pyroclastic flow and surge deposits into the upper cooling unit. The distinction between the two cooling units at the edge of the ignimbrite plateau is indistinct because of vapor-phase crystallization. Welding, which takes place on a time scale of weeks to years (Cas and Wright, 1987), served as a cap to prevent upward gas movement and concentrated gas near the contact. The only erosive contact between the two cooling units is at the base of the surge deposits in Q. Cuevas and is due to the surges themselves. The time between emplacement of the two cooling units was therefore short, and the lower cooling unit was still outgassing when covered by the upper unit.

Post-caldera volcanism The platform lava eruptions, the earliest and largest in volume of the post-caldera eruptions, may represent resurgent magmas that reached the surface and erupted, rather than lifting up intra-caldera ignimbrite to form a resurgent dome (Smith and Bailey, 1968). It is not known why there is no resurgent dome at Cerro Panizos. The length and thickness of the 10 km long flow in the north (Fig. 2 ) suggests either a high eruption rate or fairly low viscosities (no evidence of lava tubes or valley channelling was found). Such conditions may be more conducive to ascent as fast-moving dikes rather than as larger magma bodies associated with resurgence.

ER/;P'[ [VE PROCESSES AND ( ALDERA FORMATION: CERRO P4N IZOS, ('[ NTR ,\I. -\NI)ES

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Stratigraphic evidence for vent location

Pumice clast sizes

Lithic fragments

Sizes of pumice fragments can be used to indicate proximity to the vent if ( 1 ) the pumice does not float in the matrix and (2) pumice sizes at the vent do not vary greatly. The dense pumice of Cerro Panizos was not carried in rafts at the top of the flows, but it is somewhat concentrated upward in the flows, indicating some degree of buoyancy. Pumice sizes in the lower cooling unit do not vary significantly in any single vertical section, so the pumice fragments were probably of fairly constant size at the vent. Pumice fragments in the lower cooling unit are largest near the southern part of the central domes and suggest a vent in that area. Fragment size in the upper cooling unit, however, varies widely between flow units, and pumice data are insufficient to draw conclusions about individual flow units. The largest fragments are found near the central dome area.

Maximum lithic-fragment sizes in ignimbrites, which commonly decrease away from source, do not vary much in the Cerro Panizos Ignimbrite. The dense pyroclastic flows were probably unusually competent, and all available clast sizes may have been within the carrying capacity of the flows. Highly competent flows are postulated to explain the large clasts and poor sorting in the distal deposits of the 1974 nuees ardentes of Volc$,n Fuego (Davies et al., 1978). Concentrations oflithic fragments at the top of the lower cooling unit are interpreted to represent wall-rock avalanches into the conduit during vent collapse or the opening of a new vent. Similar events may explain concentrations and the higher overall content of lithic fragments in the upper cooling unit. Co-ignimbrite lag breccias, commonly associated with vent clearing or opening events (Wright and Walker, 1977: Druitt and Sparks, 1982; Aramaki, 1984: Druitt, 1985; Walker, 1985; Druitt and Bacon, 1986 ), are not present in the Cerro Panizos outflow units, but the lithic-fragmentrich flow units may represent the lateral equivalents of lag breccias (Druitt and Bacon, 1986 ). The lack of near-vent ignimbrite exposures makes it impossible to trace the lithic fragment-rich flows into lag breccias in a deflation zone. The restricted distribution of the orbicular layer at the top of the lower cooling unit indicates they came from a discrete part of the magma chamber tapped only at the end of the eruption of the lower cooling unit (Oft, 1992 ). Other vents may have formed during eruption and emplacement of the upper cooling unit. The presence of boulders of Tertiary sandstone at the base of the upper cooling unit implies vent erosion at that time, as Tertiary sedimentaw rocks in the area were covered by hundreds of meters of ignimbrite.

Thicknesses and welding The lower cooling unit filled most of the topographic irregularities in the area, but its average welding and thickness decrease radially from the central dome complex. The upper cooling unit also decreases in thickness away from the central domes, but the base of the upper cooling unit is more strongly welded than the top in southern outcrops, whereas the reverse occurs in northern outcrops. Welding should be greatest near the vent, as the local topography was inundated by the lower cooling unit, so the welding pattern indicates northward migration of the vents during the eruption. Changing vent locations and unsteady eruption columns likely produced these variations in welding, with each vent a different distance from any given outcrop.

A nisotropy of magnetic susceptibility The AMS flow directions for the lower cooling unit show that the vent area lies in the

242

southern part of the central dome complex (Figs. 2, 7a). The upper cooling unit, on the other hand, is probably not the product of a single vent. Flow directions vary widely and point toward sources that migrated within an area near or north of the lower cooling unit vent (Fig. 7b). Structural evidence for caldera types and location

The stratigraphic and paleomagnetic evidence presented above locate the vent areas for the two cooling units, but do not define the structure of the vent system. The single vent of the lower cooling unit could be associated with an ignimbrite shield or a downsag caldera, but the many vents for the upper cooling unit are probably related to either a collapse or trapdoor caldera.

Evidence for a downsag caldera The center of the depression just south of the central dome complex coincides with the vent area of the lower cooling unit indicated by AMS data (Fig. 7a). It existed during emplacement of the upper cooling unit, because ( 1 ) the base and top of the upper cooling unit within the depression are not parallel, with the base inclined 6 ° and the top about 2 ° toward the depression center (Fig. 6b ), and (2) the upper cooling unit is confined largely to the depression in the southeastern quadrant of the ignimbrite center. Lava-platform flows are also confined inside the depression. The bases of the lava flows dip 1-2 ° toward the depression center, but their tops are fiat. No evidence ofcaldera collapse or ring vents occurs in the depression area, aside from the depression itself. The depression is best explained as the surface expression of a downsag caldera that formed around a central vent during and shortly after eruption of the lower cooling unit (Fig. 10a). The decreasing dips upward from the contact between the lower and upper cool-

M.H. ORT

ing units indicate that most of the subsidence occurred before the emplacement of the upper cooling unit, but that subsidence continued during and possibly after its emplacement. This continued subsidence may be due to: ( 1 ) continued magma withdrawal and subsidence; (2) differential welding and compaction of the thick deposits ponded within the depression; or (3) a combination of these factors. Faults may have propagated upward from the magma chamber/wall rock boundary during subsidence, but they did not reach the surface at Cerro Panizos. Other calderas with a large downsag component have inward-dipping fault blocks that are stepped downward toward the center (Setterfield et al., 1991; A.J. Reedman, pets. commun., 1989). It is unlikely that faults reached the surface early in the Cerro Panizos eruption, but then stopped slipping entirely, leaving no surficial trace. No evidence of fault-bounded caldera collapse occurs until at least the final stages of emplacement of the lower cooling unit.

Evidence for a collapse caldera The central dome complex at Cerro Panizos forms a ring with a southern arc that bisects the downsag caldera (Fig. 2). The AMS-derived vent locations for the upper cooling unit correspond with this ring of domes, which appears to trace caldera collapse ring fractures along which post-caldera magma ascended. Walker (1984), based on a survey of 90 calderas, shows that post-caldera domes that have a ring configuration are generally parallel to, although not necessarily coincident with, the caldera margin. These domes may be equivalent to the moat and ring domes commonly associated with resurgent calderas (e.g., Smith and Bailey, 1968; Du Bray and Pallister, 1991 ). Vent migration, caldera collapse, and ponding within the collapse and downsag calderas can explain the distribution of the upper cooling unit (Fig. 10b). A caldera can trap pyroclastic flows within it, inhibiting outflow ( Val-

ERUPI'IVE PROCESSES AND ('ALDERA FORMATI()N: CERRO P6NIZOS, ('ENTR ~L \ N I ) t i S

entine et al., 1992). Where the ring vents were well inside the downsag caldera south of Cerro La Ramada (Fig. 10b), the flows that escaped the collapse caldera ponded in the downsag depression, with only the most energetic flows (deposited on Cerro Limitayoc) escaping. The northern and western flows erupted from vents close to but inside the edge of the downsag caldera and escaped the depression but did not flow far. They moved radially from their ring vents, forming an arc of emplacement (Fig. l 0b ). The collapse caldera breached or nearly breached the edges of the downsag caldera in the Cerro La Ramada region, so that pyroclastic flows traveled long distances eastward. The northern and southern edges of the "tongue" of upper cooling unit delineate the borders of the zone where the breach occurred (Fig. 10b). Post-caldera effusive volcanism largely covered these structures (Fig. 10c). Platform lava flows filled the downsag caldera and possibly the collapse caldera. The central domes ascended along fractures defined by the collapse caldera and filled any topographic depression remaining in that area. The present morphology is due to these constructional features, rather than caldera subsidence structures.

Regional structural controls on vent location Faults and fold axes in the Cerro Panizos region, which trend north-south, have a strong influence upon the location of late Miocene volcanic vents in the area. The southern domes of the central cluster (Cerros Vicufiahuasi and Anta Cuevas; Fig. 2) and the western margin of the central dome complex form a northsouth volcanic lineament that extends southward through the pre-Cerro Panizos volcanic centers of Cerros Limitayoc and Salle and at least 30 km south to Cerro Pululu. Turner ( 1978 ) shows a north-south fault that stops at the edge of the ignimbrite plateau that, if it continues, marks the eastern edge of the central dome complex. Such faults may have con-

_~43

trolled the location of the Cerro Panizos ignimbrite center. Crustal accommodation of magma withdrawal

Eruptive volumes' Eruptive volumes estimated for the Cerro Panizos Ignimbrite are speculative because of lack of control on caldera-fill volumes. Caldera volumes, calculated using maximum reasonable values for diameter and depth, are: ( 1 ) collapse caldera=226 km ~ (12 km diameter and 2 km depth) and (2) downsag calde r a = 127 km ~ (treated as an inverted cone 18 km diameter and 1 km deep). This gives a maximum total volume of subsidence is 353 km ~. The minimum volume of ignimbrite outside the calderas is 950 km ~, using the average 80-kin diameter and 0.2-kin thickness of the Cerro Panizos Ignimbrite plateau. This is a minimum of 475 km ~ DRE of extracaldera ignimbrite (dense rock=2.4 g/cm ~, average ignimbrite density = 1.2 g/cm ~, an absolute minimum figure, as even the non-welded pumice is denser than I g/cm ~). If the calderas were only half filled by ignimbrite, the collapse caldera contains 113 km 3 of ignimbrite and the downsag caldera would contain 64 km 3. Because of the thicknesses of the intracaldera ignimbrite, its density would be very close to DRE. Thus, the total calculated volume of the Cerro Panizos lgnimbrite is 652 km ~ DRE, almost twice the calculated volume of caldera subsidence. This difference becomes even larger if a significant fraction of erupted material was widely dispersed fallout.

Nature o f subsidence at depth Some compensation for the large amount of erupted material must occur at depth, and this may be related to blocky subsidence in the country rock beneath downsag calderas (Setterfield et al., 1991; A.J. Reedman, pers. corn-

244

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Fig. 10. Maps and cross sections showing the proposed eruptive historx o f C e r r o Panizos. Fhc ~erlical scale for the cross sections is arbitrary, in order to best show the structures. (a) Lower cooling unit, Cerro Panizos Ignimbrile. ('entral xent cruption during emplacement of the lower cooling unit resulted in the f'ormalion e r a downsag caldera, \vith ignimbrite partially ponded within it. (b) Upper cooling unit, Cerro Panizos lgnimbrite. Ring-xent eruption associated with thc eruption of the upper cooling unit resulted in caldera collapse in the northern half of" the dox~nsag caldcra. Ring ~enls propagated t}om the original central vent in an arc to the north, lgnimbrite xs,as pondcd ~ithin the downsag and collapse calderas. (c) Post-caldera volcanism. Lava flows were erupted shortl} after emplacement oi'lhe ('erro Panizos Ign imbritc, filling much of the caldera area. Later, dacile domes and stubby flows were erupted from ~ enls along a ring fracture

mun., 1989). Scandone (1990) describes caldera subsidence in which layers of rock warp into the magma chamber, eventually breaking into blocks that rotate and leave spaces between them to be filled by magma. Models of uneven, blocky collapse are proposed for the Valles caldera (Self et al., 1986) and Grizzly Peak caldera (Fridrich et al., 1991). Such blocky collapse takes time to propagate to the surface and would probably appear as gentle subsidence before fractures breached the surface. A nested downsag-collapse caldera, such as that of Cerro Panizos, could result. A possible contributing factor to a lack of significant collapse is low discharge. Scandone (1990) states that very high eruption rates are needed to produce caldera collapse. At lower discharge, the hot rocks overlying the magma

chamber may deform plastically. Intermediate discharge would delay caldera formation, so more magma would be erupted before collapse. Gentle subsidence predominated during the eruption of the lower cooling unit, but collapse occurred during the emplacement of the upper cooling unit. This implies changes either in the strain rate (directly related to the discharge) or the competence of the roof rocks, or that the rocks had strained to their elastic limit. The competence of the roof rocks is unlikely to have changed over the time of the eruption. No evidence exists of higher discharge in the upper cooling unit, which came from unsteady eruption columns and is generally less welded than the lower cooling unit. The upper cooling unit likely had lower, not higher, discharge. Blocky collapse, with a lengthy period of time to prop-

246

MH, ORT

agate fractures to the surface, is the most likely subsidence mechanism at Cerro Panizos.

Eruption dynamics of Cerro Panizos Ignimbrite

Flow mobility Several lines of evidence indicate that the pyroclastic flows of Cerro Panizos were of relatively low mobility. The terminal ends of flow units at Cerro Panizos are steep and abrupt rather than gradually tapered. At Q. Huasacucho (Fig. 2c), the lower cooling unit thins significantly over a paleo-ridge, filling the nearby valleys while the top of the unit is horizontal. In addition, the lower cooling unit does not occur on the sides of pre-existing mountains, such as Cerro Limitayoc and Sierra de Lipez. Deposits could have drained away as secondary flows (Fisher et al., 1987), but, given the excellent preservation of the volcanic morphology at Cerro Panizos, it seems unlikely that no remnant would be left. Low mobility and confinement of the lower cooling unit to topographic lows are also indicated by anisotropy of magnetic susceptibility (AMS) data. The lower cooling unit pyroclastic flow on the western edge of Cerro Limitayoc moved parallel to the base of the lava dome ("A" in Fig. 11 ). Fluvial deposits at its base suggest that this was a valley in which the pyroclastic flows were channeled. The deposits in this area are intensely altered by vapor phases, which could result from stream-derived steam. The flows of the upper cooling unit, although not highly energetic, had greater mobility than those of the lower cooling unit. Thin deposits of the upper cooling unit occur on the sides of Cerro Limitayoc (Fig. 6a). Flow unit contacts are sub-parallel to the 15 ° slope of the hill and extend as much as 80 m above fiatlying deposits in the valley. The transition from sub-horizontal to inclined contacts is abrupt. These relationships indicate that thin flows

Fig. 11. Map showing orientations of the AMS directions (labelled dark lines) around Cerro Limitayoc. Flow in lower cooling unit followed topography, whereas flows in upper cooling unit had various responses to the topography, depending on their mobility. Strike and dip symbols are from the contact between cooling units.

moved up (or down) the hillside and the deposits are not the "high water mark" of a valley-filling pyroclastic flow. AMS data indicate that the upper cooling unit had diverse interactions with the topography of Cerro Limitayoc. Two sites (B and C) on the northwest side of Cerro Limitayoc had distinct flow directions (Fig. 11 ). The direction at "B" parallels the slope of the dome, indicating the sampled flow climbed and then descended the slope gravitationally after losing momentum. The AMS direction at "C" paralleled the strike of the slope, suggesting that it was advancing more passively along the base of the dome. The many flow units in this section may be a result of interlayering of pyroclastic flows that descended the mountain with others that flowed in the valley. Southeast of the central dome complex, deposition of the upper cooling unit was confined to a large depression. At location "D" (Fig. 11 ), the sampled flow was apparently diverted by the sides of the depression and turned southward. Similar explanations apply to sites "E" and "F" (Fig. 11 ). Upper cooling unit flows reached the edge of the present-day ig-

ERUP1 IVE PROCESSES AND ('ALDERA FORMA.TI()N CERR() PA NIZOS, CENTRAL *\NI)ES

nimbrite plateau in the east-central area (Fig. 2 ), but did not travel as far in the northern and western sectors. Variations in flow behavior within the upper cooling unit reflect distinct flow lobes of different degrees of mobility with sectorial distribution about the source region. Variations in vent location and column height can account for the differences in mobility. The l-m-thick fallout deposit at the base of the Cerro Panizos Ignimbrite indicates that a short-lived Plinian column formed and collapsed within a short time prior to the generation of pyroclastic flows. The weak mobility of the pyroclastic flows of Cerro Panizos implies they did not form from collapse of a high eruption column (Sparks and Wilson, 1976). A pyroclastic fountain, similar to but with greater discharge than those described by Hoblitt ( ! 986) at Mount St. Helens, is a viable model for the generation of the Cerro Panizos pyroclastic flows. With high discharge, material would have flowed sluggishly away without the need for a lofty collapsing column.

Transport mechanisms McTaggart (1960, 1962), Brown (1962), Pai et al. (1972a,b), Sparks (1978), and Wilson (1980, 1984) present models for particle support in pyroclastic flows by fluidization, in which upward-moving gases partially counteract gravitational forces. For complete particle support from fluidization, all particles must be hydraulic equivalents. If not, the lighter particles will be blown out of the flow (elutriated) and the heavier ones will not be fully supported. The m a x i m u m total weight supported by fluidization in pyroclastic flows ranges from 15-70% (Wilson, 1984). Fine ash particles, continuously created by abrasion during flow and then elutriated (Wilson, 1985), are required to trap gases in fluidized pyroclastic flows for long runout distances (Eden et al., 1967). Fluidization lengthens the runout of pyroclastic flows but cannot be the sole particle support mechanism.

247

Some flow units in the upper cooling unit of the Cerro Panizos Ignimbrite have pumice concentrations in their upper parts and lithic concentrations in their lower parts, possible evidence of fluidization. Moderate crystal enrichments in the matrix over the pumice (4075% vs. 30-50%) imply that fine material was winnowed during eruption and flow (Wilson, 1985 ). The lack of segregation structures, such as gas pipes or lithic pods, indicates either that fluidization was weak or that it had ceased some time before deposition, so that such structures were re-incorporated into the flow. The high altitude of eruption and emplacement and low initial gas content inhibited fluidization in the Cerro Panizos pyroclastic flows. Atmospheric pressure at Cerro Panizos is about 0.5 x 105 Mpa, and the area was probably close to its present altitude by late Miocene time (Alpers and Brimhall, 1988 ). Particle support by fluidization is directly related to the product of the gas density and the square of the gas velocity, so a halving of the density of the gas ingested into the eruption column or pyroclastic flow will reduce the particle size supported by that gas by a factor of 2. The arid environment makes significant ingestion of gases by combustion of plants unlikely. The poorly vesicular pumice probably exsolved little juvenile gas into the pyroclastic flows. Thus, the Cerro Panizos pyroclastic flows were poor in fluidizing gases. The high crystal content of the Cerro Panizos Ignimbrite may have further contributed to limited fluidization. Because of the relatively low ash fraction in the crystal-rich matrix, gases were not trapped effectively within the flows (Eden et al., 1967 ). High gas velocities are required to fluidize dense flows (Wilson, 1984) such as the crystal-rich dacite pyroclastic flows ofCerro Panizos. With a low gas content and poor gas retention, they did not retain enough gas to support fluidization over long runout distances. Grain flows, such as sand on the lee slopes of eolian dunes, rely on dispersive pressures

248

between grains to provide upward support, The dispersive pressure is proportional to the shear stress between the moving grains (Bagnold, 1954). Deposition from grain flows is by frictional freezing, rather than particle-by-particle deposition. Pure grain flow is restricted to thin flows on steep slopes. Modified grain flows, in which other particle-support mechanisms (e.g., fluidization) combine with dispersive forces, commonly contain fine-grained material and form flows up to tens of meters thick on gentle slopes (Lowe, 1976). In most natural sediment flows, two or more grain-support mechanisms, such as fluidization and grain collisions, operate (Middleton and Hampton, 1976; Pierson, 1981 ). The most important particle-support mechanism overall is not necessarily dominant at the time of deposition, and evidence of its activity may be erased by other transport processes. For example, a pyroclastic flow may start as a modified grain flow, with particle support mechanisms consisting of fluidization combined with inter-granular dispersive pressures (e.g., particle collisions, flow shear). As the gas pressure within the flow decreases, fluidization supports less of the mass of the flow and the other particle support mechanisms become relatively more important. The fact that the resulting deposits indicate only weak fluidization does not therefore preclude significant support from upward-streaming gases during earlier phases of transport. The relatively fines-poor, crystal-rich nature of the Cerro Panizos Ignimbrite decreased the effectiveness of fluidization, but enhanced modified grain-flow processes, as frequent grain contact maintains the dispersive pressures of modified grain flows. Grain-support mechanisms become significant at grain concentrations above 20% or 30°/0 (Lowe, 1982 ). The presence of fine grains mixed with coarser grains, as in a pyroclastic flow, has the effect of reducing the effective weight of the larger particles and the dispersive pressure required to support them; hence, energy losses are lower

MH. ORT

and velocities higher (Lowe, 1976 ). Positive identification of grain-flow deposits is difficult (Middleton and Hampton, 1976 ), but several characteristics of the Cerro Panizos Ignimbrite suggest that modified grain flow was important. Inverse grading, common in modified grain flows (Bagnold, 1954; Middleton, 1970), occurs in the Cerro Panizos Ignimbrite upper cooling unit flows. Many individual flow units have inversely graded bases, but also have an inverse grading of pumice fragments through much of the flow unit (Fig. 6c). Pumice buoyancy can be explained by fluidization, but the concentration of lithic fragments above the inversely graded base implies that shear in a modified grain-flow controls the lithic-fragment concentrations. Grain contact during movement of the Cerro Panizos pyroclastic flows is indicated by the abundance of broken and abraded crystals in the matrix and their paucity in the pumice. Crystals are commonly in contact with others even in non-welded deposits, and many biotite crystals have a cockscomb-like structure, in which sheets of a crystal fan out at one end, suggesting breakage during flexure over another crystal. In most cases, the crystal over which this breakage occurred is absent, suggesting that breakage occurred before deposition. Pumice clasts in the Cerro Panizos Ignimbrite are generally sub-rounded. No unambiguous signs of fluidization, such as gas pipes, occur in the Cerro Panizos Ignimbrite, Shear and dispersive forces during grain flow destroy or prevent the formation of gas pipe segregations. I postulate that, as the gas contents of the flows declined, fluidization weakened and modified grain-flow processes grew in relative importance, while total flow mobility declined. During the later stages of flow, grain-flow processes destroyed most evidence of fluidization, such as segregation structures. The relatively short runout distances of the Cerro Panizos pyroclastic flows are probably due to the short time that sufficient gas was available for fluidization and the

ERUP'FIVE PRO('ESSES AND ('ALDERA F()RMATION: CERRO F'ANIZOS, CENTRAL a,NDES

relative inefficacy of dispersive pressure as a particle support mechanism in dry flows on low slopes. Modified grain flow, in which fluidization combined with inter-granular dispersive pressures caused by particle collisions and flow shear, is proposed as the dominant flow process in the pyroclastic flows at Cerro Panizos. Acknowledgements Many of the ideas presented here resulted from discussions and field trips with B.L. Coira, R.V. Fisher, M.M. Mazzoni, N.R. Riggs, and J. Meyer. Thoughtful reviews by R.V. Fisher, N.R. Riggs, D. Swanson, and an anonymous reviewer greatly improved the manuscript. J.E. Fryxell made the 39Ar/4°Ar analyses of Cerro Panizos rocks and J. Nakata made the mass spectrometer analyses for the K/Ar dates. M. Fuller and B.B. Ellwood gave access to their paleomagnetism laboratories. Peter Francis kindly provided Landsat TM images. Field assistance by P. Guerstein, A. Sanguinetti, R. Lencina, and A. Benvenutto and burro driving by A. Flores, G. Flores, A. Quispe, and E. Quispe are gratefully acknowledged. Logistical support by B. Coira, A. Perez, C. Tolaba, and R. Quispe was invaluable. This project was supported by Institute for Geology and Planetary Physics grants to R.V. Fisher and Geological Society of America Graduate Research grants to the author. References Alpers, C.N. and BrimhalL G.H., 1988. Middle Miocene climatic change in the Atacama Desert, northern Chile: Evidence from supergene mineralization at La Escondida. Geol. Soc. Am. Bull., 100: 1640-1656. Aquatcr, J., 1979. Estudio del potencial geotermico de la Provincia de Jujuy. Secretaria del Estado de Energia, Buenos Aires. Aramaki, S., 1984. Formation of the Aira Caldera, Southern Kyushu, 22000 years ago. J. Gcophys. Rcs., 89: 8485-8501. Bagnold, R.A., 1954. Experiments on a gravity-free dispersion of large solid spheres in a Newtonian tluid under shear. Proc. R. Soc. London, A225: 49-63.

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