Revista Blasileira de Ceociências 30(4):'745-748, dezembro de 2000

ERUPTIVE STYLE OF MESOPROTEROZOIC A-ryPE CALDERAS IN SOUTHEASTERN MISSOURI, USA

GARY R. LOWELL

RESIJMo ESTIL}S ERUPTIV}S DE CALDEIRAS MESOPROTEROZÓICAS DO TIPO A NO SUDESTE DE MISSOURI, ESTADOS UNIDOS Vulcanismo Mesoproterozóico no sudeste de Missouri foi dominado por erupções de lavas e ignirnbritos tiolíticos de alta ternpe- Ìatuta co¡1 assinatura química-anorogênica. O colapso incremental da caldeira acotnpanhou ciclos Pliniano-Peléano-Efusivo com erup.ções controladas por sítios estruturalmentJcontlolados ao longo das paredes da caldeira. Pelo tnenos quatro caldeiras de fluxos de cinza da classe conr 20-40 km de diâmetro formaram-se a cerra de 1.48 Ga e todas cor¡ feições de modelos clássicos de caldeila em subsidência, Os aspectos coletivos das caldeiras sugercm que ciclos de saîuução-+ erupção expktsiva-+ ,¡ubsidência-+ ret.raçt\o-+ ntudattça de sítio.tiv,etaln papel pl'o- eminente ¡a evo¡ução de-caldeir'äs do tipo A. O desvio das cálãeil'as'de Missouri dos ¡nodelos clássicos é atrib¡ído a particularidades físicas e quírnicas de líquidos do tipo A e que iontrolarn a extensão sinvulcâtrica do padr'ão de falhas regionais pté-existentes.

Palavras-cluves.' caldeiras de fluxo de cinzas, estilo eluptivo, vulcanismo anorogênico, Tetreno St. Frncois

eruption ternperature rhyolite ignimbrites and ABSTRACT Mesop|oterozoic volcanisnr in southeastern Missouli was dominated .by. *of.high iavas with anorogeniichemical signatures. Incremental caldera collapse accompanied Pliirian-Peléan-EffLrsive cycles that wet'e elupted at str.ucturally contrõlled sites along cl"eveloping caldera walls. At least four ash flow caldelas in the 20-40 km diameter class appeared at about 1.48 Ga añd all exhibit features ãt variance ivith classic rnodels fol caldela subsidence. The collective traits of the Missouri calderas suggest

neviítion ofthe Missouri calderas fi.ôm the classic models is attribuiêd to the physicãl and chenlical peculiarities of A-type rnelts and cotltt'ol of synvolcanic extetlsion by the pre-existing regional fault pattern.

Keywords: ash flow caldelas, eruptive style, anorogenic volcanism, St Francois Terrane

INTRODUCTION The classic account of ling-fàult controlled 1.38, and 1.33 Ga. The i.48 Ga event produced thick sequences of subsidence was advanced by Clough et al. (1909) to explain steep rhyolite ignimbrite and lava intruded by coeval lapakivi granite contacts between a thick Cambrian volcanic section and enclosing (Lowell l99l), hybrid ring plutons (Lowell & Young 1999), and metamorphic rocks at Glencoe, Scotland. The salient features of the basaltic dikes (Sylvester 1984). The second cycle formed intr-usions of (Kisvalsanyi Glencoe model (i.e., cylindrical ring fault extending fì'oln surf'ace to "tin granite", magnetite-r'ich syenite, Kiruna-type Fe-ores rnagma chambel and "single stroke" piston-like subsidence of a & Kisvarsanyi 1989), and volcanic domes (Lowell 1998). Olivine cohelent block due to catastrophic magrna withdrawal) were tholeiite dike swarms were emplaced across the region at 1.33 Ga incorporated in virtually all subsequent fèlsic caldera lnodels (\ryilliams (Lowell & Rämö 1999). 1941; Smith & Bailey 1968; and Liprnan 1984). The success ofthese The t.48 Ga episode in the SFT produced significant volurnes of models, particulal'ly in the western USA, has led sorne to view the ignimbrite and lava with A-type chenrical traits and compositions least Glencoe featules as definitive and reject caldera status for volul¡inous ranging from lnetaluminous rhyolite to commendite and at four volcanic accumulations lacking these eletnents. However, Walker calderas along the lnargins of the Missouri Gravity Low (MGL). The trending geopliysical expt'ession ( I 984) demonstrated thât f'lexural subsidence (ring-fracture faults latter is a 90 km by 700 km nolthwest (Guiness et 1982,Darnell et. al. absent) and incremental collapse (rnultiple, non-catastrophic eruptions) of a buried Proterozoic aulacogen aL' recognized include are lnore common in Quaternary calderas than are classic Glencoe 1995). Volcanic subsidence stluctures currently the lèatules. In fact, r'ecent work at Glencoe (Moor:e & Kokelaar 1998) has three overlapping elliptical calderas 20-40 krn in diametel along shown that the classic elements al'e lacking even at the type locality. northeasteln rnargin of the MGL and fourth fault bounded structute on Wood (1984) classifies caldelas as slúekl calcleras (e.g., Mauna the southwestern MGL margin (Fig. 1). The Butler Hil1, Lake Loa), stratocone calderas (e.g., Crater Lake), and ash flow cakleras Killarney, Etninence, and Taum Sauk calderas formed duling the 1.48 (e.g., Valles) and demonstrates that each Inorphological class is Ga episode and are partially bounded and dislocated by northeast and relãtively consistent in tertns of size, effusive products, and tectonic nortliwest trending laults (Fig. 1); they are briefly described below. setting. Àccording to Wood (1984): 1) all known calderas exceeding The Taum Sauk Caldera presel'ves a nearly cornplete recold of 20 km in diameter are ash flow typei2) this caldera class is equally intracaldera processes and is discussed in more detail, is distributed between subduction zone and continental ril't settings; and The most cleeply dissected of the fbur structures the Butler Hill central granite 3) slightly less than 50Vo of such calderas develop central resurgent Calclera (Sides ¿¡ 41.1984) which features a deptÌr-zoned a deeper' -plutons. rnassif with a granophyric roof, a transitional zone, and This paper clescribes non-classical calderas in an intraplate setting rapakivi facies (Lowell 1991 ). The central rnassif is circumscribed by 1999) and intrudes a I km in the Mesoproterozic St. Ft'ancois Terrane of southeastern Missouri. four hybrid ring plutons (Lowell & Young Mountain Caldera subsidence is examined in terms of eruptive and intrusive thick intracaldera ignirnbrite sequence known as the Glassy bounded sequences, parent melt properties, and influence of pre-existing Ignimbrite. Cordell (1979) proposed a subsidence str-ucture gravìty and strùctules. the Missouri calderas are a distinct subtype of Wood's by the Simms Mountain and St. Genevieve faults based on the Hawn Párk Caldera (1984) ash flow caldera class and the style elements described here rnagnetic data (Fig. I ). This structure, named rnay be applicable to calderas in other anorogenic provinces, (Corclett 1979), is interpreted here as a dislocated seglnent of the particularly those of Mesoproterozoic age. Butlel Hill Caldera. It is represented by the most uortheasterly and deepest Precambrian exposures in the SFT (Lowell 1976, Reese & CALDERAS lN THE ST. FRANCOIS TERRANE The St' Lowell 1998). Francois Terrane (SFT) ofsoutheastern Missouri occupies the apex of The southern boundary of Butler Hill Caldera overplints and the Ozark Dome and is the only exposul'e of the Eastern Granite- partially obliterates the older Lake Killalney Caldera (Fig. 1) identified Rhyolite Province of North America. The latter is an extensive buried by Blown (1983, 1989). The southern wall of the Lake Killarney belt of unmetamorphosed Mesoproterozoic igneous rocks extending Caldera is defined by numerous occurrences of wall collapse breccia liom Michigan to southeastern Missouri (Lidiak et al. 1993). The SFT that thin northward. The rnajor intracaldera unit is a 400 rr thick records three magmatic episodes occurt'ing at approximately 1.48'

Depaftment of Geosciences, Southeast Missouri State University, Cape Girardeau, Missouri 63701, USA. E-rnail: glowell@sernovln serno edu 746 Revista Brasileira de Geociências, Volume 30, 2000

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lhyolite ignimbrite known as the Lake Killarney Formation that was the lower sequence suggest deposition fiom multiple eruptive vellts fbrmerly assigned to the Butler Hill Caldera (Sides ¿¡ al. 1984, Lowell along an accommodation zone (Bosworth 1987) of a developing rift 1991). Several small occurrences of F-r'ich granophyre north of the structure (Harrison et al.2000). LKC ling stl'ucture (Fig. l) suggest that this caldera may be intruded by a central glanitic rnassif, At pl'esent there are no published TAUM SAUK CALDERA Volcanic rocks in the westeln SFT (1962) descriptions of volcanic rocks east of the Ironton Fault in the southern were fìrst mapped in detail by R.E. Anderson who concluded SFT but relations in Fig. I suggest they are part of the LKC offset by that they record a simple structural sag. Subsequently, J.E. Anderson the Ironton Fault. However, Brown (1989) proposed that volcanic et al. (1969) reinterpreted the thick volcanic succession as an locks in this area record an independent subsidence structure that he intracaldera sequence and presented evidence for the existence of the by referred to as the Marlow Mountain Caldera. Taum Sauk Caldera (TSC). The caldera concept was challenged (19'72) and The Eminence Caldera is located in a rectangular block bounded by R.E. Anderson (1970) but supported by Berry & Bickford Ketcherside the Shannon County Fault, the southwest rnargin of the MGL, and two Sides (1981) who located a segment of the ring fracture on northeast trending faults (Fig. 1). More than 50 isolated knobs of Mountain (Fig. 2). Sides et al. (1981) reviewed the controversy, noting (1969) ignirnbrite and lava and a single knob of 1.47 Ca granophyre are that one argument presented by J.E. Andersanet aL. was based exposed across an area of about 700 km'. The granophyre has an on erroneous age data, but concluded that the c4ldera concept was agpaitic index > 1.0 and exhibits anorogenic (Ar), WPG, and RRG valid. chemical signatures (Harrison et al. 2000). The volcanic rocks Recent reviews by Lowell (1999, 2000) describe the Taurn Sauk resemble the granophyle in age and chemical traits but have suff'ered Caldera (Fig. 2) as a non-classical subsidence structure that collapsed pronounced K-for-Na exchange and Na loss suggestìng that pristine incrementally duling the 1.48 Ga volcanic episode. The caldela is 30 compositions were probably peralkaline. The upper volcanic sequence km in diameter and encloses about 600 km' of A-type rhyolite contains at least 800 m of quartz-bearing rhyolite lavas and dome ignimbrites, lavas, and air fall units. This intracaldera sequence is rnaterial resting on basal volcaniclastic and air fall deposits. It lies neither domed nor cut by coeval plutons but is modified by unconformably above a thick lower sequence of quartz-poor Phanerozoic reactivation of Precambrian faults and erosion. ignimblites containing minor lava and air fall deposits that exhibit a Intracaldera volcanic units approximate 7 km in total thickness but tlie pronounced northwest vertical foliation. Variable phenocryst maximum stratigraphic thickness at the depocenter is only about L5 rnineralogy, steep foliation, and the tremendous thickness (7-8 km) of Revista Brasileira de Geociências, Volume 30, 2000 747

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gentle synclinal. configuration of tlie km. This thickness dispality is the result of venting along developing eruptions occurrecl. The continued subsidence after cessation of caldera waÌls and centripetal flow toward the depocenter (Fig' 2)' intràcaldera sequence inclicates Berry's (1976) detailed account ofTSC stratigraphy' viewed in the TSR eruptions. with small-scale eruptions of crystal-rich contéxt of "normal ignimbrite sequences" (Sparks et al. 1973), reveals TSC àvents concluded and cleposition o-f minor fluvial and lacustrine 5 Plinian-Peléan-Effusive episodes. These small to moderate volume ignimbrite, air falls, volcaniclastic sediments enclosing tliin cycles were mostly initiated by air fall tuff el'uptions, r'eached sédiments. The latter include in the Proffìt Mountain at'ea and along maxir¡urn intensity during ernplacetnent of crystal poor ignimbrites, stromatolitic carbonate horizons caldera wall at Ketcherside Mountain (Fig 2) whete and concluded wiih crystal rich lava flows. Each cycle produced a the southeastern replaces carbonate strata. depositional fan around its source extending 4-12 km toward the Au-Cu bearing scapolite skarn deþocenter. Eruptive activity shifted along the controlling faults as Mesoproterozoicvolcanism indicated in Fig. 2. SUMMARYANDCONCLUSIONS was dorninated by ring fracture eruptions of The last of the 5 major eruptive events related to the TSC produced in southeastern Missouri high silica rhyolite ignimbrites and lavas with approximately 60 km3bf cryital-rich ignimbrites known as the Taum high temperature, At least 4 ash flow caldelas in the 20-40 Siuk Rhyotite (TSR). The large volume of this unit is anomalous in onãrogeni" chernical traits. appeared at about 1.48 Ga and all exhibit featules at telms ol ðaldera chronology and absence of antecedent ail fàll deposits km diãmeter class the classic model fbr caldera subsidence. The collective and subsequent lava flows. Possibly these rocks recold migration of a variance with calderas indicate that cyclic sattn'atiott-) new batch of magrna into the zone tapped by active vents. On-going traits of the Missouri subsidence-; quenchittg-+,s luft- of's ite played a chemical stuclies of the TSC volcanic sequence may clarify this exp l.os ive ertt¡t tion) their formation. Individual calderas were tilled by uncertainty. The TSR was erupted from at least 3 vent sites along the or'ominent role in volulne Plinian-Peléan-Effusive cycles erupted from southwestêrn caldera wall (Fig. 2) and attained a maxitnum thickness iow to rnediurn eruptive loci linked these sites to produce of about 1200 m in the central caldera. The thickness and location of rnultiple vent sites. Shifting zone circumscribing a zone of piecerneal the depocenter are important because they indicate that morphological a clefacto ring flacture frorn the classic rnodel is attributed to: 1) the clevelópment of the caldera was well advanced when the TSR subsidence. Dèviation 748 Revista Brasileira de Geociências, Volume 30, 2000

physical and chernical peculiarities of A-type rnelts (Eby I 990) and 2) 7 - Shifting eruptive loci are linked by pre-existing/synvolcanic control of synvolcanic extension by the pre-existing orthogonal fault fäults to define a ring fì'actule zone that mirrors the orthogonal pattern. The rnost important style elements of the Missouri calderas fault pattern; are: 8 - Early eluptive products vent frorn structurally controlled sites I - A-type rnelts (= 900'c, low Hro, high F, low viscosity) along developing caldera walls with centlipetal flow toward approximate granite minimum composition at low P; narrow depocenter of developing caldera; cornpositional range eluptive products precludes rnagma of 9 - Late hybrid ring plutons ref'lect renewed influx of basalt after volatiles; chambel zoning except for most silicic rnelt is solid; Magrna ascent was nearly isothermal (eruptive T liquidus T) so 2 - = 10 - Central dorning by melt resurgence is observed in only one eruptives are crystal poor; early Missouri caldera; 3 - Incremental subsidence is due to rnultiple eruptive events of I I - Wall-collapse breccias composed of early volcanic products piston-like small to intermediate volume rather than single-stage, (including basalt) were transported towar-d the depocenter along subsidence related to single large volume catastrophic eruptions; listric boundary faults; fluvial volcaniclastics and lacustrine 4 - Reduced eruptive volumes are related to high magrna ascent that stromatolitic carbonates were deposited in small intracaldera permits pyroclastic eruptions shortly aiter melt saturation; basi ns. 5 - Pre-existing faults were exploited by synvolcanic extension and Acknowledgements The author is grateful fol the opportunity significant control vent location and subsidence exert on to participate in the First Symposium on Volcanism and Associated geometry; Environments. The organizers and scientific cotnmittee are 6 - Degassing forced eruptive activity to shift to new sites above congratulated on a sytnposium of high scientitìc merit convened at a satulated melt; sub-vent "quench granophyres" irnpose barriers site ofoutstanding beauty and hospitality. Special thanks go to Adriane to turtller eruption unless rapid influx of tneteoric water remelts Machado and Márcia Barros Pinho for their many kindnesses during the barrier; my stay in Brazil. To two anonimous referees of Revista Blasileila de Geociências fol the review and suggestions to the tnanuscript.

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