Middle Jurassic volcanism in the Northern and Central Andes
Natalie Romeuf Laboralolre de Pélrologie Magmalique, URA CNRS 1277, CEREGE, BP 80, Universilé d' Aix-Marsellle 111, 13545 Aix-en-Provence Cédex 04, France
Luis Aguirre Laboralolre de Pélrologie Magmalique, URA CNRS 1277, CEREGE, BP 80, UniversHé d' Aix Marseille 111, 13545 Aix-en-Provence Cédex 04, France Presenl address: Departamenlo de Geologla, Universidad de Chile, Casilla 13518, Correo 21, Sanllago Pierre Soler OrsIom, URl H, Départemenl TOA, 213 rue lafayeHe, 75480 Paris Cédex lO, France el Laboralolre de Mlnéralogle, URA CNRS 736, MNHM, 61 rue BuHon, 75005 Paris, France
Gilbert Féraud Inslilul de Géodynamique, URA CNRS 1279,UniversHé de Nice-Sofia Anlipolis,
Pare Valrose, 06034 Nice Cédex, France Etienne Jaillard Misión Orslom, Apartado 17-11-06596, Cuno, Ecuador and Orslom, UR1H, Départemenl TOA, 213 rue LafayeHe, 75480 Paris Cédex lO, France
Gilles Ruffet Inslilul de Géodynamique, URA CNRS 1279,UniversHé de Nice-Sofia Anlipolls, Pare Val rose , 06034 Nice Cédex, France
ABSTRACT
Stratigraphical, petrographical, geochronological and geochemical data available suggest that the various segments 01 the westem margin 01 South America experienced a different geodynamic evolution throughout the Jurassic_ In the early Jurassic, the Northem Andes were characterized by an extensional tectonic regime whereas in the Central Andes subduction led to the emplacement 01 a calc-alkaline magmatism. During the middle Jurassic (Bathonian to Oidordian), an active subduction, perpendicular to the continental margin, generated a magmatic arc (Misahuallí/Colán are) along the Northem Andes, The volcanic products are medium to high K calc-alkaline rocks composad 01 basaltic andesitic t:> myolitic lavas and acid pyroclastic rocks (ignimbrites, unwelded tuffs, volcanogenic sandstones and breccias). A new radiometric 40 Ar_3D Ar data gave an age 01 ca_ 172 Ma lorthe Misahuall r Formation 01 Ecuador, The Central Andes, in the southem coastal region 01 Peru, ,were characterized by an oblique convergence between the Phoenix oceanic plate and the continental margin 01 South America. This subduction produced a medium to high-K, calc-alkaline andesitic volcanic series (Río Grande and Chala lormations), comprising porphyritic basaltic andesites and emplaced as Ilows, intrusive rocks (dykes, 'si Is' and 'stocks') and acid pyroclastic rocks. New geochronologicaI 4°Ar-3DAr data confirm an age 01 ca. 165 Ma lor the andesitic basalt flows 01 the Chala Formation.
Key words : Andes, Pero, Ecuador, Jurassic, Vo/canism, Petrology, Geochemislry, Geochronology, Geodynamics, Active continental margins.
RESUMEN
Volcanismo del Jurásico Medio en los Andes del norte y centrales. Información estratigráfica, petrográfica, geocronológica y geoqurmica disponible sugiere que los dilerentes segmentos del margen occidental de Sudamérica tuvieron una evolución geodinámica dilerente durante el Jurásico. En el Jurásico Inferior, los Andes del Norte
Revista Geo/ÓQica de Chila, Vol. 22, No. 2, p. 245-259, 9 Fi9S., 3 tablas. Decembar 1995. 246 MIDDL:: JURASSIC VOLCANISM IN THE NORTHERN ANO CENTRAL ANDES
se caracterizaron por un régimen tectónico extensional mientras que en los Andes Centrales procesos de subducción generaron un magrratismo calcoalcalino. Durante el Jurásico medio (Batoniano a Oxfordiano) la subducción ortogonal al margen continental generó un arco magmático (Arco Misahuallí-Colán) en los Andes del Norte. Los productos volcánicos de este arco son rocas calcoalcalinas de medio a alto K, que corresponden a andesitas basálticas y riolitas, y rocas piroclásticas ácidas :ignimbritas, tobas, areniscas volcanogénicas y brechas). Se obtuvo una nueva edad radiométrica '"Ar 39Arde ca. 172 Ma para la Formación Misahuallí en Ecuador. Los Andes Centrales, en la región costera meridional del Perú, estuvieron caracterizados por una convergencia oblicua entre la placa oceánica Phoenix y el margen continental de Sudamérica. Este proceso de subducción produjo una serie volcánica andesítica calcoalcalina de medio a alto K (formaciones Río Grande y Chala) que incluye andesitas basálticas porfíricas emplazadas como lavas, diques, 'sills' y 'stocks', y rocas piroclásticas ácidas. Nuevas edades Palabras claves: Andes, Perú, Ecuador, Jurásico, Volcanismo, Petrologfa, Geoqufmica, Geocronologla, Geodinámica, Márgenes conllnen tales activos. INTRODUCTION According to several geodynamic reconstructions, perpendicular to the continental margin was active the Andean margin was the site of a major plate along the Northern Andes, whereas the Central An reorganization duringthe latest Triassic-earliest Juras des were dominated by transform faulting with sic (Aspden et al., 1987; Mourier, 1988; Jaillard et al., restricted subduction. In the latest Jurassic, a major 1990). According ~o these reconstructions, during the reorganization of the plate motion too k place. A new middle Jurassic (Bathonian-Bajocian), a subduction southwest-northeast convergence direction induced the accretion of continental and are terranes in the Northern Andes in northern Peru, Ecuador and Co eo-w 7e-w 76'W lombia and the emplacement of subduction-related '_."', N '-'-"""( COLOMBIA calc-alkaline volcanism in the Central Andes of Peru -¡ (Aspden et al., 1987; Jaillard et al., 1990). The authors t ~:~~. ~"~-.,..J~ present new petrological, mineralogical, geochro 0-5 ) QUITO h / ~._ • (!í..za \. ~ ... - nological and geochemical dataconcerning four Middle .1 1 Jurassic volcanic formations located in these two AbIt.gu.~.Ba_ ·.t' ~ ECUADOR ¡J re,.. f different segments of the Andean margin (Figs. 1 and ¡ ~ )' 2). Two of these formations crop out in the northern Rlobamba e ¡! AMAZONIAN BASIN 2'5 4i J / segment and are located in Ecuador (Misahuallí For mation, Fig. 1) and Northern Peru (Colán Formation, ,.lIt --// Fig. 1), whereas the othertwo (Río Grande and Chala Formations, Fig. 2) are located in the central segment and crop out along the southern coast of Peru. ¡~,:;~~':-, ¡ í ..... ',E_R""U;o--___• Zalmo5l: ,.8 (J These data place new constraints on the geo ~ ...... dynamic interpretation of the Jurassic evolution of the --... ":., "--:.: / Northern and Central Andes . • Plu~. San Felipe. 6'5 ocean FIG. 1. Location 01 Jurassic oulcrops 01 Ihe Northern Andes in till Jurasslc batholiths _ Mlsahuallf aOO Co"n Formatlons S ehaplza Formatlon Ecuador and NOr1hern Peru. N. Romeuf, L. Aguirre, P. Soler, G. Féraud, E. Jail/arrJ and G. Ruffet 247 7Z"'W 70'W N f PERU • C.ravel • Arequlpa I ./ ,.,.1 PACIFIC OCEAN <..... BOLIVIA ) r o'" '-. ~t '· \ Tacna. .1. ' "- ~ . _.~/ CHILE FIG. 2. Locallon 01 Jurassic volcanic outcrops (black areas) in the Central Andes, southern coastal Peru (alter Carlier and Soler, ul1lublished). JURASSIC VOLCANISM IN THE NORTHERN ANDES (ECUADOR AND NORTHERN PERU) MISAHUALU FORMATION Batholith (162 Ma; Aspden and Litherland, 1992). The Misahuallí Formation was previously defined The Misahuallí Formation (Tschopp, 1956) crops as the uppermost part ofthe detritalcontinental Chapiza out in the Subandean zone of Ecuador, to the east of Formation (Tschopp, 1956) and assignedto the Juras the Cordillera Real. Two main outcrop zones are sic/Cretaceous boundary (Hall and Calle, 1982). This observed, one inthe north inthe Napo Dome, between age was based on palynological evidence and a the towns of Baeza and Tena, and the other in the radiometric date of 132 Ma (K/Ar, whole-rock = w.r.), south along the Cóndor Cordillera in the region of obtained from rocks of the Amazon basin (Hall and Zamora (Fig. 1). The predominantlyvolcanic Misahuallí Calle, 1982). However, the authors' field observations Formation consists mainly of basaltic to rhyolitic lava and a new'OAr-39Arplateau age of 172.3±2.1 Ma (Fig. flows and acid pyroclastic deposits (dacitic to rhyolitic 3; Romeuf, 1994) suggest that the Misahuallí Forma ignimbrites and unwelded lapilli tuffs), erupted in a tion, as originally named, consists of two volcanic subaerial environment, but also ineludes subordinate units. This plateau age was obtained on a single grain amounts of intraformational volcanogenic sandstones of amphibole from an andesite (M I-90). Because o, and breccias (Table 1). the low K content (0.6-0.9%) and th3 small size of the The base of the Misahuallí Formation has not been grain, only three significant apparE:nt ages could be observed and its top is covered by the Aptian-Albian obtained. Nevertheless, because 07 the high purity of epicontinental sandstones of the Hollín Formation. the analysed sample, as demonstrated by a constant The volcanic rocks are intruded by large calc-alkaline 37Ar -39 Ar ratio (proportional to th3 Ca/K ratio) ver eo K batholiths (Fig. 1) previously dated from 1 90 to 150 sus temperature, the calculated plateau age of Ma (Herbert and Pichler, 1983; Pichler and AJy, 1983; 172.3±2.1 Ma is probably geologically significant. Aspden etal., 1992; Aspden and Litherland, 1992). The This age corresponds to the Misahuallí Formation Misahuallí Formation seems to be intruded only by the proper, located in the Subandean zone and corres youngest batholiths (ca. 150 Ma) and by the Abitagua ponds to Middle Jurassic age (ca. 190-150 Ma), 248 MIDDLE JURAS SIC VOLCANISM IN THE NORTHERN ANO CENTRAL ANDES TABLE 1. MAIN CHARACTERISTICS OF JURASSIC VOLCANIC FORMATIONS IN THE NORTHERN ANO CENTRAL ANDES. Northern Andes Central Andes Formatlon Mlsahualll Colán Río Grande Chala Locatlon Sub-Andean Zone, Huancabamba Andes Southern coastal Peru Southern coastal Peru Napo Ranga and Northern Peru Cóndor Cordillera Weslern Ecuador Age Middle Jurasslc Middle Jurasslc Middle Jurassic Middle Jurassic Bajoclan Oxfordian Callovian Aalenian to Bajocian Environment Aerial Continental Aerial Continental Lilloral Lilloral Llthology Basahic andesites Basahic andesitas Basahic andesites Basahic andesite flows Andeslles Andesites Acid pyroclaslic rocks and inlrusives Oacites Oacites Acld pyroclaslic rocks RhyolHes Acid pyroclasl ic rocks Acic pyroclasllc rocks Mlneralogy Plaglodase K-i"ich Labradorite Labradorite K·rich Labradorite Labradorite (AnS3.1Ab42.7°r4.2) (AnSS .3Ab39 .30r2.4) (AnssAb40.S0r4.4) (AnS9 .gA~7 .SOr2. 7) (Basahic andesites) (Basahic andesites) (Basahic andesHes) (Basahic andesile flows) to andeslne K-rich Labradorite (AIl3S .2Abso.sOr4) (AnSS.2A~7 .SOr4) (Oacites) (Basaltic andesile inlrusives) Pyroxene Allglle lo dlopslde Augile lo diopslde Augile Ca-rich Augile (WC,44 .3E""SFs10.7) (W°44E"43Fs 1:Y (W037.SE"44.SFs17 .7) (W039.1E""S.9Fs14) whereas the younger one, or uppermost Chapiza from the reworking of the volcanic products of the Member, Jurassic:lCretaceous boundary (ca. 130 Ma) Colán Formation (Mourier, 1988). Callovian bivalves is restrictedtothe Amazonian Basin (Fig. 1), and is not were also reported in the Colán Formation (Mourier, considered further in this study. 1988). The Colán Formation rests unconformably upon the Pliensbachian to Sinemurian Pucará carbo COlAN FORMATION nates (Loughman and Hallam, 1982) and is covered by the Neocomian quartzites of the Goyllarisquizga The Colán Formation (Pardo and Sanz, 1979) Group (Mourier, 1988). The Colán Formation is also represents the southward prolongation of the reworked bythe Tithonian Chicama Formation (Jaillard Misahuallí Formétion in Northern Peru (Fig.1) and and Jacay, 1989). The ColánFormation is restrictedto consists of subaerial basaltic andesitic to dacitic lava the are a west of the Marañon geantieline. To the east, flows and pyroclastic beds (T able 1) that inelude fossil in the East Peruvian Trough, the Colán Formation is wood (Mourier, 1988). Its thickness varies from 1,000 replaced by the contemporaneous continental detrital m to 3,000 m (Mcurier, 1988). Oxfordian ammonites upper Jurassic Sarayaquillo Formation, equivalent of (Perisphinctes sp ., Dichotomosphinctes sp.) occur in the Ecuadorian Chapiza Formation (Mourier, 1988; penecontempora,eous sedimentary beds, derived Jaillard and Jacay, 1989). N. ROI7l6Uf, L. Agulrre, P. Soler, G. Féraud, E. Jalllaro and G. Ruffet 249 JURASSIC VOLCANISM IN THE CENTRAL ANDES (SOUTHERN COAST OF PERU) This volcanism is represented by the Río Grande 1994): a lava flow (CHA-50) and a d~jke (MOR-1 ) cross and the Chala formations situated along the southern cutting the Guaneros Formation in the Tacna region. coast of Peru in the Arequipa region (Fig. 2) . Very detailed age spectra (31 steps) were obtained on plagioclases to give precise information on the composition of the analysed plagioclases (Fig. 3). RIO GRANDE FORMATION Afterthe first four very disturbed apparent ages, MOR- 1 age spectrum shows a low temperature flat region The Río Grande Formation (Rüegg, 1957; 01- with ages around 157 Ma followed by lower ages at chauski, 1980) is composed of two units separated by 154-155 Ma, then by a large region (corresponding to a slight angular unconformity (Aguirre and Offler; 48% ofthe total 39 Ar released) of concordant ages with 1985; Aguirre, 1988). The upper unit has yielded a K a weighted mean of 157.2±0.4 Ma. The 37Arc.f39ArK Ar (w.r.) age of 164±4 Ma obtained on a weakly ratio spectrum (not given) clearl, shows that the altered basaltic andesiteflowofthe upper unit (Aguirre intermediate temperature lowest a;1es correspond to and Offler, 1985; Aguirre, 1988). An Aalenian Breydia alteration phases characterized by lower Ca/K ratios manflasensis assemblage zone was reported from the lower unit of the formation (Rüegg, 1957; Roperch and Carlier, 1992). The upper unit consists mainly of 200 highly porphyritic basaltic andesites and acid l\-fisahualIí 180 ¡--¡ pyroclastic rocks, whereas the lower unit is mainly ~ ~ composed of acid pyroclastic rocks: dacitic to rhyolitic 160 ignimbrites as well as unwelded lapilli tuffs, including -<~ 172.3 ± 2.1 Ma abundant volcanic shards (Table 1) and volcano ;: 140 detritic rocks, sandstones and conglomerates. Some c..~ c.. 120 calcareous beds also occur in the lower unit. -< MI-90 Amphlbole .Ingle groln 200 CHALA FORMATION _ ~':l A-50 Plaglocla ••• I 159 ± 0.5 Ma J 165.8 ± 0.5 Ma =f.'¡ 164 J¡ ':::', The Chala Formation (Fig. 2) comprises a thick ~ J .:- .. 157. 2 ± 04. Ma sequence of basaltic andesite flows intercalated with ~ I -< 140 U r MOR-1 Plaglocla ... ignimbrites, unwelded lapilli tuffs, volcanigenic ;: sandstones and breccias (Olchauski, 1980; Roperch and Carlier, 1992); various generations of cross i 104 -< Chala cutting stocks, sills and dykes are also present. Lava 80 flows and subvolcanic basic intrusives have rather O 20 40 60 80 100 similar petrographic and mineralogical characteristics. %~'Ar Released The Chala Formation is contemporaneous with the sedimentary Guaneros Formation (Jaén et al., 1963) and with the Río Grande Formation (Roperch and FIG.3. '''Arf'''Arage speelra 01 a- an amphibole single grain Irom lhe Carlier, 1992). A 4°Ar_39Ar (w.r.) age of ca. 177 Ma was Misahuallf Formalion (MI-90); b- Iwo plagioclase bulk obtained from a basaltic flow from the base of the samples Irom Ihe Chala Formalion. Error bars are given al la level. Age delerminations by lhe 40Ar_39Ar melhod formation (Roperch and Carlier, 1992). Determinations (single grains and buik samples) Nere performed al lhe by the 4°Ar_39Ar method (w.r.) carried out on small Laboraloire de Géochronologie, Université de Nice (Franca). intrusives cross-cutting the Chala Formation at the 110 The anaiy1ical melhod lor age dele-minations is described and Chala areas gave ages around 157 Ma (Roperch in Aullel (1990) and Zumbo (1993) . The samples were irradiated in the McMaster reaelor (Hamillon, Canada) with and Carlier, 1992). a lo tal lIux 01 8.8xl018 neulronlcm2. The irradialion stan New 4°Ar-39Ar dates (plagioclases) were obtained dard was Ihe amphibole MMhb 1 (Samson and Alexander, on rocks of the Chala Formation (Fig. 3; Romeuf, 1987; 520.4 Ma). 250 MIDDLE JURASSIC VOLCANISM IN THE NORTHERN ANO CENTRAL ANDES (probably affected by younger secondary K-rich released, gives a weighted mean age of 165.8±0.5 phases). This age at 157.2±0.4 Ma probably Ma which may representthe lava flowforrnation. It is, corresponds to the dyke formation. nevertheless, noticeable that the low temperature The CHA-50 age speetrum is more difficult to age of 159±0.5 Ma is almost concordant with the small interpret becausethe lowest lowtemperature apparent 'plateau age' of the previous sample. ages (the first four steps excepted) do not correspond The upper unit of the Río Grande Formation to significantly different 37Arc.?8ArK ratios (spectrum exhibits the same petrological and field characteristics not given). The aut,ors suspect that these lower ages than the Chala Formation. These two broadly contem (weightedmean of 159±O.5 Ma) correspondto albitized poraneous sequences would probably represent the phases observed in thin seetion. The higher tem same volcanic unit. perature tlat region, corresponding to 51 % of the 39Ar PETROGRAPHY AND MINERALOGY PETROGRAPHY euhedral phenocrysts up to 1 cm long, microphe nocrysts and microliths in the groundmass (Table 2). Almost all the lavas are highly porphyritic, In some lavas of the Río Grande and Chala samples sometimes amygdaloidal, with a phenocryst content (e.g., CHA 50 and RG21), the plagioclase constitutes from 17 to 49% (Table 2). The Río Grande lava flows more than 50% of the mineral assemblage (Table 2) and the Chala intrusive rocks are generally richer in as phenocrysts and microliths. In some basaltic phenocrysts than the Misahuallí and Colán lavas. The andesites of the different formations, the plagioclase mineralogy of the basaltic andesites is rather similar, has sieved orpoikilitictextures and sometimes include consistingof phenocrysts of plagioclase, clinopyroxene small pyroxenes and glass. and sometimes pseudomorphosed olivine, with Fe-Ti In the Misahuallí Formation the plagioclase is oxides as phenocrysts and microphenocrysts. The rather K-rich labradorite (An.7.6SAb32-690r2.3-S.S) in the intersertal groundmass is composed of microliths of basaltic andesites and more sodic in the andesites the same minerals and glass. In the Misahuallí and and dacites (An30-4SAbS3.a.0r2.7.9.s).ln the Colán, Chala Colán formations, the dacites contain plagioclase, and Río Grande formations, the plagioclase is hornblende and Ti-Fe oxides as phenocrysts in a labradoritic with compositions AnSl.6SAb29.S00rl.S.3.1 devitrified glassy matrix. Apatite occurs as an (Colán), AnSS.63Ab~-400r2 . 2.6.S (Chala), and An31-61Ab3S. accessory mineral in the dacites and in the pyroclastic 660r2.S.S .• (Río Grande). In all the formations, the rocks. plagioclase has rather restricted compositional varia tion between the core and rim of phenocrysts and the MINERALOGY microliths. In the Misahuallí, Chala and Río Grande forma OUVINE tions, the plagioclase is rather K-rich with a mean Or content of about 4% (Fig. 4). The plagioclase in the Olivine is rather scarce, but appears in basaltic basaltic flows of the Chala and Colán formations are andesites of all formations as phenocrysts.lt is always less K-rich (Or content at about 2%). The Or-contents pseudomorphosed by chlorite, titanite and opaques (Fig. 4) of the plagioclases are rather similar to those although it is identifiable by its morphology. of plagioclases in recent high-K calc-alkaline and shoshonitic lavas ofthe Andean margin (Peru, Lefevre, PLAGIO CLASE 1979; Tata Sabaya volcano, Chile; de Silva et al., 1993) and of Cenozoic high-K calc-alkaline lavas of The plagioclase is ubiquitous and represents the the Sierra Madre Occidental, Mexico (Delpretti, 1987). most abundant mineral in all the formations (plagio The fact is that the high Or content of the plagioclase clase phenocryst content is in the range 2-46%). This seems to be linked to a rather high primary KP mineral, often albitized and sericitized, appears as content of the host rock. ; Colán Misahuallf I Formation Rfo Grande Chala MisahuaUf .-r- Sample RG14 RG21 CHA50 CHA 168 CHA 115 OV13 MI 246 MI 247 MI141 M 1236 ~ c:=;. ji! ~ Lithology Sasaltie Sasaltie Sasaltie Sasallle Sasaltie Sasallle Sasallle Sasallle Daelte Daeite (J) andesite andesite andesite andesite intrusive andeslte andeslte andeslte ~ I -'" p Phenocrysts Phenocrysts ~ Plagiociase 46.0 40.0 2.0 23.5 38.0 21.0 17.0 22.0 Plagioclase 36.0 27.0 iil Altered R Altered pyroxene 0.0 0.0 0.0 2.5 4.0 4.0 3.0 2.0 ferromagnesians 4.5 0.3 ~ (amphibole. blotne) [¡¡ Augne 0.5 6.5 14.0 1.5 4.0 6.5 5.0 2.0 Amphiboie 3.1 ¡¡;"" 2.0 0.0 3.0 0.0 0.0 0.1 Fe-Ti oxides 1.2 0.2 a Olivine 1.0 III Fe-Ti oxides 0.8 1.0 0.3 0.3 0.3 1.0 1.5 2.0 Apatne 0.1 5. p Total phenoerysts 48.3 49.5 16.3 30.8 46.3 32.5 26.6 28.0 Total phenoerysts 44.9 27.5 I Groundmass 69.0 54.0 68.0 Groundmass Glass 25.0 26.0 38.0 30.0 32.0 Devnrilied glass 55.0 72.5 Microlnhs 26.0 7.0 1.5 44.0 41.0 Piagioclases 18.0 44.0 microinhs Total groundmass 51.0 51.0 83.5 69.0 54.0 68.0 74.0 73.0 Total groundmass 55.0 72.5 TOTAL 99.3 100.5 99.7 99.8 100.3 100.5 100.6 101.0 TOTAL 99.9 100.0 Number 01 points 2.260 >2.000 3.790 >2.000 >2.000 2.254 2.313 2.215 2.311 1,082 counted ""~ 252 MIDDLE JURASSIC VOLCANISM IN THE NORTHERN ANO CENTRAL ANDES • Misahuallí 8 r------, CHA 72 • Chala intrusives 7 ... • D Chala flows e 6 Colan .¡, <> "O 5 -¡ • + ••+ • RioGrande •X ex •• X Calc·alkaline rocks trom ~ 4 + Peru SIO, =155.6·63.3) ~XD + :'*: Shoshonites from Peru 3 x- x ir SIO, =(52.9·63.9) • + x +x + Sierra Madre Occidental 2 .¡aD SIO, =(55.2·72.4) x<> -. _ Tata Sabaya <> yolcano + SIO, =152.1·62.3) 2 3 4 5 6 % Or plagioclases FIG. 4. Compartson 01 Or-content In plagloclase versus K.o content 01 whole rock 01 Jurassic volcanic rocks in Norlhern and Central Andes. OIher recent volcanic unlts 01 the Andean margin are included lor relerence Miocene to Recent calc-alkaline rocks wHh SiO. in the range 55.6-63.3% andshoshoniles wHh SiO. in lhe range 52.9-63.9%01 Peru; Lefl!Vre, 1979. Lale Qualemary lo Recenl, Tala Sabaya volcano, Chile wilh SiO.ln the range 52.1-62.3%; de Silva el al., 1993. Terliary high-K volcanic rocks Irom Sierra Madre Occidental 01 Mexico wilh SiO. in the ranga 55.2-72.4%; Delprelli, 1987. The mineral chemislry was delermined by microprobe analyses allhe Unlversité de ~onlpelller (France) using a CAMEBAX probe under operalion conditions 0115 kV, 15 nA and a beam widlh 01 1 J.lm. PYROXENES No orthopyroxene was found in the lavas from the different formations either beca use they are completely Pyroxenes are present in the basaltic andesites pseudomorphosed, or absent. Some petrographical and andesites of all formations as euhedral pheno evidences such as an altered core with a rim composed e rysts , microphenocrysts and as microliths in the of clinopyroxene suggest that orthopyroxene was groundmass. The·~frequently include glass and Fe-Ti originally present in the basaltic andesites and that it oxides and may appear as glomerocrysts. They are is now altered. generally augites with a rather homogeneous composition. In the basaltic andesites of the Northern AMPHIBOLES Andes (Misahualií and Colán formations), they are Ca-richer and plot between the augite and diopside Amph iboles occur in th e acid lavas ofthe M isah uall í fields of the pyroxene composition diagram (Morimoto and Colán Formations. According to Leake's et al., 1988). In some lavas, two generations of classification (1978), they correspond to pargasitic pyroxene seem to be present: the first is always hornblende, ferroan pargasitic hornblende, ferro altered to chlorite, titanite and epidote, whereas the pargasitic hornblende and pargasite. Their rather second is preserved. Their compositions show little high (Na+K)A content, ranging from 0.57 to 0.89, is variations: W03P>-'.6 En4().sls5-13 in the Misahuall í basaltic typical of orogenic lavas belonging to the high-K calc andesites, Wo 41 .. SEn41 .. ls7.18 in the Colán rocks; W03S. alkaline series (Gill, 1981). 43En40 .. 7Fs4.23 in the Río Grande Formation and Wo34. 44En43.SSFs5-22 in lhe basaltic andesitic flows and BIOTITE intrusives of the Chala area. There is no significant compositional var ation between phenocryst rims and Biotite occurs only in rhyolitic unwelded tuffs and cores as well as between the phenocrysts and the ignimbrites from the Misahuallí Formation. It is microliths. The chemistry of these pyroxenes is typical generally completely altered and only a few chemical of orogenic lavas according to their Ca, Na, Cr and Ti analyses are available. Its composition is typical ofthe contents (Leterrier et al., 1982). calc-alkaliné lavas (Abdel-Rahman, 1994). N. Romeuf, L. Aguirre, P. Soler, G. Féraud, E. JaillarrJ and G. Ruffet 253 METAMORPHISM The rocks of the different formations described metamorphism explains the relatively high mobility of here have suffered very low-grade metamorphism elements such as Na, K, Ca, Ba, Rband Sr, expressed (Romeuf, 1994) characterized bythe crystallisation of as plagioclase albitization, olivine pseudomorphism secondary minerals (i) as replacement of the and glass replacement. This phenomenon precludes groundmass, notably ofthe glass, (ii) as pseudomorphs the use of the above elements for discriminating the after igneous minerals and (iii) as void infillings. This geochemical affinity of the lavas. GEOCHEMISTRY The volcanic rocks of the different formations there is no Fe-enrichment during differentiation. The showa medium-high K calc-alkaline affinity typical for REE patterns are characteristic for .:alc-alkaline lavas magmas emplaced at a continental margino AII these with selective enrichment of LREE relative to HREE rocks have suffered strong mobilization of lithophile (LaJYbN=5-10) (Figs. 5 and 8). The multi-element elements (Na, K, Ca, Rb, Sr, Ba .. ). Thus, only immobile patterns of the basaltic andesites exhibit a great elements such as Ti, Zr, Y, Th, Ta, Nb, and RE E, were enrichment in LILE compared to MOR S (Fig. 6) . used to discriminate the magmatic affinity. Selected The basic rocks of this formation plot in the calc whole-rock analyses are presented in table 3. These alkaline fields of several discriminant diagrams (e.g. , rocks were chosen on the basis of their rather low Pearce and Cann, 1973; Fig. 7). alteration pattern. COLAN FORMATION MISAHUALU FORMATION These rocks present similar geochemical char The Misahuallí rocks are calc-alkaline (Figs. 5, 6, acteristics as the Misahuallí volcanic rocks and and 7), suggesting their formation in an active conti correspond to calc-alkaline lavas emplaced in a con nental margino They display typically low Ti0 (0.35- tinental arc setting (Figs. 5-7). They are characterized 2 0.85%), Nb (7-13 ppm) and FeO (6.8-9.3%)contents, by low Ti0 (0.5-0.8%) and Nb (1-5 ppm), high Zr l 2 high Thffa ratios (9-17.5) and relatively high AIP3 (100-400 ppm), AIP3 (16.6-17.2%)and LlLEcontents, (15.5-17.8%), Zr (110-300 ppm), and REE contents; and very high Thffa ratios (45-57). Their REE patterns 100 _____ Chala intrusives 100 -o- Chala flows GI .¡:J!! :!:: -lr-- Río Grande .t; 'tie e o o .c .c O 3Z ~ u uo o a:: a:: 10 10 I I I I I La Ce Nd Sm Eu Gd Dy Yb Lu La Ce Nd Sm Eu Gd lb Dy Yb Lu FIG. 5. REE pallerns lor Jurassic volcanie roeks 01 Norlhem and Central Andes (normalized afler Haskin el al.. 1968) . 254 MIDDLE JURASSIC VOLCANISM IN THE NORTHERN ANO CENTRAL ANDES TABlE 3. SElECTED ANAL YSES OF JURASSIC VOlCANIC ROCKS IN THE NORTHERN ANO CENTRAL ANDES: Misahualli Colán Chala Rlo Grande MI247 MillO MI14l MI145 OY3 OY13 CHA50 CHA124 CHAl68 CHA115 CHAl54 RG14 RGla RG21 57.37 52.03 75.44 55.26 56.73 55.08 Si02 55.42 58.87 65.65 71 .86 61.33 54.85 52.84 63.16 1.58 1.10 0.35 0.96 0.96 0.99 Ti02 0.86 0.71 0.48 0.23 0.52 0.75 1.29 1.23 AI~3 16.86 17.02 16.38 14.81 16.89 16.97 15.14 14.11 15.66 15.09 10.67 17.59 17.33 16.78 Fe~3 3.06 2.39 1.14 2.34 1.53 2.73 7.48 4.86 8.01 7.77 2.06 5.61 4.74 6.46 FeO 4.10 3.20 1.48 0.00 2.06 4.19 1.87 1.59 1.03 1.77 0.08 1.36 1.34 1.66 MnO 0.12 0.15 0.10 0.06 0.08 0.20 0.12 0.16 0.16 0.52 0.02 0.13 0.13 0.12 MgO 3.58 2.38 1.16 0.32 2.82 5.31 4.99 1.48 3.95 5.42 0.41 3.20 2.70 3.12 CaO 6.25 4.24 2.55 0.20 3.68 7.12 8.09 2.53 5.38 5.13 1.04 4.92 5.65 5.46 Na~ 3.06 5.13 4.44 3.28 5.65 2.69 2.81 4.72 0.60 3.05 3.31 3.58 3.23 3.05 K~ 3.56 2.90 3.55 5.15 1.30 1.67 2.11 3.65 3.23 4.61 5.08 3.87 3.71 3.27 P20 5 0.29 0.29 0.19 0.08 0.19 0. 16 0.33 0.37 0.38 0.37 0.09 0.36 0.32 0.33 H~+ 1.98 2.14 1.35 1.37 3.14 2.72 I .SO 1.22 1.99 2.05 1.55 1.72 1.60 1.85 H20 · 0.09 0.12 0.15 0.24 0.13 0.07 0.80 0.12 0.70 0.45 0.15 0.92 0.60 0.57 To'" 99.23 99.54 99.22 99.94 99.32 100.03 99.37 99.20 100.04 99.36 100.25 99.48 99.04 99.34 Li n.d. n.d.. n.d. n.d. n.d. n.d. 67 22 15 24 92 25 34 44 Rb 92 76 100 130 38 44 55 63 139 155 78 153 136 120 Sr 596 667 529 229 621 357 274 239 235 343 27 660 445 530 Ba 1,078 1,230 1,398 1,218 370 605 489 724 749 2,849 545 1,001 2.520 807 Co 22 16 11 5 14 26 26 7 29 25 2 16 16 18 Cu 86 36 21 14 31 90 33 9 58 52 9 21 58 18 Cr 61 26 3 5 29 77 202 5 57 52 7 22 19 22 Ni ' 5 44 7 36 38 48 69 6 18 25 6 13 10 14 V 190 118 57 9 8 213 280 77 234 223 28 198 186 202 Zn 74 66 65 35 39 85 85 77 61 285' 14 174 207 138 Nb ' O 9 8 lO 1 2 8 lO 8 8 8 5 6 5 Zr 136 208 220 156 399 l OS ISO 265 220 195 204 158 162 180 Y 25 24 22 22 11 22 29 42 29 31 28 29 30 30 La 19.00 27.30 30.60 31.60 10.10 14.30 24.20 29.00 27.00 24.00 23.30 24.00 26.00 26.00 Ce 29.00 SO.80 52.70 60.40 21.00 26.10 53.40 66.00 58.00 53.00 39.70 SO.OO 54.00 55.00 Nd 23.00 2410 21.90 22.90 12.20 15.30 26.30 29.80 30.00 29.00 14.90 24.00 25.00 24.00 Sm 4.48 453 3.87 3.95 2.29 3.15 6.32 6.80 7.10 6.30 3.16 5.SO 5.60 5.80 Eu 1.23 127 1.04 0.65 0.78 0.88 1.42 1.51 1.40 1.20 0.72 1.10 1. 10 1.10 Gd 4.41 433 3.45 3.18 2.26 3.53 5.64 5.47 6.10 5.20 3.03 4.SO 4.SO 4.60 Dy 3.91 362 3.07 3.00 1.66 3.52 5.85 5.81 6.20 4.50 4.21 4.20 4.40 4.40 Yb 2.16 196 1.80 2.05 0.59 1.84 3.21 3.73 3.40 2.60 3.53 2.50 2.60 2.60 Lu 0.40 029 0.28 0.32 0.13 0.27 0.51 0.62 n.d. n.d. 0.60 n.d n.d n.d • Rockanalyges were performed al the UniversM d'Aix Marseille 111 (France, Laboratoire de Pétrologie Magmatique) by M.O. Trensz and J.C. Germanique using an ICP-OES (Jobin-Yvon) according to the analytical method 01 Germanique (1994). Several samples were analysed by neutron activation at Cornell University (lthaca, N.Y., U.S.A.) by P. Soler. This study is based upon 68 whole-rock analyses. display selective enrichment in LREE compared to the Fig. 8), low Ti02 «1%) and Nb (5-6 ppm) and high HREE with LaJYbp. in the range 4-10. (Figs. 5 and 8). AIP3 (16.8-17.6) and Zr (150-180 ppm) contents. The They fall within the field represented by the samples enrichment in LlL elements observed in the multi of the Misahuallí Formation. A calc-alkaline, element diagram (Fig. 6) is interpreted as a primary subduction-related affinity was already suggested for feature despite the mobility of some of these elements, the Colán Formation (Mourier, 1988; Soler, 1991). because ofthevery high enrichme nt ofthese elements relative to MORB, the high Zr content of the rocks and RIO GRANDE FORMATION of the high Or-content of the plagioclase. The chemistry of these basaltic andesites suggests CHALA FLOWS AND INTRUSIVE ROCKS a calc-alkaline affinity for the Río Grande lavas. Their REE patterns (Fig. 5) are typical of volcanic are rocks The chemistry of these rocks suggests a calc emplaced at a continental margin with a high LREE alkaline magmatism, emplaced on an active conti enrichment with regard to HREE (LaJYb =5.6-8.3, N nental margin (Figs. 5, 6, and 7)_ A strong element N. Romeuf, L. Aguirre, P. Soler, G. Féraud, E. Jail/ard and G. Ruffet 255 1,000 1,000 ¡:------, 100 100 -o- Chala flows -.-Chala intrusives ID ID -Ir- Río Grande a: a: O O :E :E :ii! 10 10 u :ii! O u a: a:O 0,01 0,01 Sr K,o Rb Ba Th Ta Nb Ce PAZr Hf Sm Tio. y Yb Se Cr Sr K,o Rb Ba Th Ta Nb Ce PAZr H' Sm no. y Yb Se Cr FIG. 6. Spidergrams (Pearce, 1983) lor basic to intermediate Jurassic volcanic rocks 01 Northern and Central Andes. • Misahualll 12 TU100 <> Colán A and B: Low-K Tholei~es <> A Río Grande B: Ocean-Floor Basalts • Misahuallí 10 O Chala flows C and B: Calc-alkaline Basa~s <> Colán • Chala intrusives D: Within-Plate Basa~s '- e Chala 8 .1 A .. Rro Grande :az ••• 6 .J .... ~ e o ~t? o O O O 4 : <> o o 2 o o 10 15 20 25 YbN(ppm) FIG. 8. LaNb versus Yb diagram lor JuraSlSIc volcank: rocks 01 Northern and Central Andes. Zr 25 50 75 Yx3 39 FIG. 7. TV1DO-Zr-3Y diagram (afler Pearce and Cann, 1973) lor steps 01 the 4°Ar_ Ar analyses. Comparison between basic Jurassic volcanlcs 01 Northern and Central Andes. the Or-content in plagioclase and the whole-rock KP Legend as in ligure 4. shows that some of the samples have clearly experienced secondary enrichment (e.g., Fig. 4; sample CHA72). mobility is indicated by KP enrichment in some rocks Nevertheless. the multi-element diagrams show 01 the Chala Formation (KP up to 7% in some an important primary LILE enrichment compared to basalts). This disturbance was detected in the 1irst MORS (Fig. 6). The REE patterns (Fig. 5) 01 the lavas 256 MIDDLE JURAS SIC VOLCANISM IN THE NORTHERN AND CENTRAL ANDES and subvolcanic intrusives are rather similar and 1990). This comparison shows that the compositions of all these lavas are rather similar for several major characteristic of calc-alkaline magmas with a LaJYbN , , in the range 4-7. The Río Grandeflows andthe Chala and trace elements (Si02 Ti02 FeO" MgO, KP, Ba, intrusive rocks el<:hibit a rather similar composition, Sr... ). However, the HREE, Zr, Th, Hf, andTacontents whereas the chemistry of the Chala flows is slightly of the Jurassic lavas are depleted relative to the different, the latters having a lower KP content. This reference series, whereas the LREE, Y, and MnO feature is also reflected in the Or content of the contents are generally higher. Similar variations in the plagioclase present in the basaltic andesites. HREE and LREE contents have been explained by different degrees of partial melting of asthenospheric COMPARISON WITH OTHER ANDEAN VOlCANIC material in Quaternary Andean lavas belonging to the SERIES Chilean Villarrica and Lanín volcanoes (Hickey-Vargas et al., 1989). The Jurassic lavas of Ecuador and Peru The Andean Jurassic volcanic rocks studied here exhibit some chemical characteristics (Nd, Sm, Eu, have been compared with the medium- to high-K, and Zr contents) transitional between the medium-K Quaternary calc-alkaline volcanic series of Chile and calc-alkaline volcanics of the Laguna del Maule (Frey Bolivia. The former were represented by the Laguna et al., 1984), emplaced on a 'normal' continental del Maule series located in the Southern Volcanic crust, and the high-K calc-alkaline rocks of the Neva Zone (SVZ, Frey et al., 1984) and the latter by the dos de Payachata (W6rner et al., 1988; Davidson et Nevados de Payachata series in the Central Volcanic al., 1990), emplaced on a thick continental crust. Zone (CVZ; W6ner et al., 1988; Davidson et al., CONClUSIONS ANO GEOOVNAMIC SETTING The four volcanic units described are broadly Zr content of the basaltic andesites is higher in the contemporaneous and correspond to an important Chala-Río Grande lavas than in the Misahuallí-Colán volcanic activity that took place in two segments of the rocks. These petrological and geochemical differences Andean margin curing the middle Jurassic. observed between the lavas from the Northern Andes In the Northern Andes, the Misahuallí and Colán (Misahuallí-Colán arc in Ecuador and Northern Peru) formations display similar petrological, mineralogical and those of the Central Andes (Chala-Río Grande and geochemical characteristics, which strongly formations of southern coastal Peru) might be suggest the existence of a magmatic arc emplaced on explained by different degrees of partial melting and/ the Andean mar;:¡in in the present-day Subandean or by differences in thickness of the continental crust. zone. This magmatism was associated with an active However, this volcanism indicates that active subduction along the Northern Andes. The age of this subduction took place along the Peruvian segment volcanism was dated, in Ecuador (Misahuallí during the middle Jurassic, but the extensional and Formation), atabout 172Ma(Romeuf, 1994), whereas strike-slip tectonics associated to the formation of the in northern Peru, the Colán volcanic rocks seem to be contemporaneous Yura basin, in the same segment slightly younger IOxfordian). in back-arc position (Vicente, 1981; Vicente, et al., In the Central Andes, the Río Grande and Chala 1982), suggest that this subduction was oblique. Formations exhibit similar characteristics and also According to recent geodynamic reconstructions correspond to a subduction-related medium to high-K of the early Mesozoic evolution of the Andean margin calc-alkaline magmatism. The Misahuallí and Colán (Jaillard et al., 1990), the Northern Andes and the formations constitute a complete differentiation series Central Andes werecharacterized by ditferenttectonic from basalt to dacite and rhyolite, whereas the Chala settings during the middle Jurassic. From the latest Río Grande rocks consist of a broadly bimodal series, Lías to the Kimmeridgian (180-145 to 140 Ma), the with basaltic andesites and acid pyroclastic rocks. Andean margin (Fig. 9) was characterized by a global The Chala-Río Grande lavas are K-richer than the northeast-southwest convergence between the Misahuallí-Colár formations, a fact also reflected in oceanic Phoenix plate and South America (Aspden et the Or-rich comp:>nent of the plagioclase (Fig. 4). The al., 1987; Mourier, 1988; Jaillard et al., 1990). This N. Romeuf, L. Agu¡rre, P. Soler, G. Féraud, E. Jaíllaro and G. Ruffel 257 the Peruvian segment (Jaillard et al., 1990). According to these authors, the southeastward direction 01 the subducting oceanic plate would have induced left lateral translorm motion along the :Jeruvian segment (Aspden et al., 1987; Jaillard et al., 1990) (Fig. 9). The authors' data would support the existence 01 southeastward subduction beneath the Northern An des, resulting in the emplacement 01 the Misahuallí• Colán magmatic arc during the micdle Jurassic (190- 150 Ma) with the intrusion 01 large granodioritic batholiths and the eftusion 01 lavas at the same time FIG. 9. Geodynamic reconslruclions 01 Northern Andean margin lor (172.3±2.1 Ma lor the Misahuallí volcanic rocks and lalesl Lias lo Kimmeridgian periodo modi/ied afler Jaillard el al., 1990. lo show signHlcanl convergence required lor Oxlordian lor the Colán Formation l. The presence 01 produdion 01 Rro Grande and Chala volcanic lormalions. a calc-alkaline volcanism in the southern coastal Peru atthe same period (Bajocian-Bathonian) also indicates the presence 01 a subduction zone in this area. Thus, convergence, broadly orthogonal in the Northern the motion 01 the Phoenix oceanic plate was not just Andes, was responsible lor the southeastward southeastward, but ratherWNW-ESE. Anotherexpla subduction 01 the oceanic plate beneath the western ~ation can be that the Jurassic rlorphology 01 the South American margin and resulted in the lormation Peruvian segment was different Irom the present-day 01 a magmatic arc at the continental margin in Ecua morphology as suggested by the important counter dor, Northern Peru and Colombia (Fig. 9). A turbiditic clockwise rotations recorded by the Jurassic and trough, the existence 01 emergent areas and the Cretaceous volcanic rocks in ooastal Southern Peru scarcity 01 magmatism would suggest that only very and northernmost Chile (Heki et al., 1983; Roperch local subductiontook place contemporeanously along and Carlier, 1992). ACKNOWLEDGEMENTS This study is part 01 a research cooperation project et Géodynamque). The lirst authcr is gratelul to R. on Andean Jurassic volcanism between the UR1 H Marocco (ORSTOM Mission in Ecuador), to A. Egüez (Géodynamique et Concentrations Minérales) 01 the (Escuela Politécnica Nacional de Quito, Ecuador), to Institut Franctais de Recherohe Scientilique et M. Litherland and J.A. Aspden (British Geologioal T echnique pour le Oéveloppement en Coopération Mission 01 Cooperation in Ecuador) and to the mem (ORSTOM) and the Laboratoire de Pétrologie bers 01 the Laboratoire de Pétrologie Magmatique. Magmatique (URA CNRS 1277, Université d'Aix The authors wish to thank G. Carlier (ORSTOM) and Marseille 111, France). This study is part 01 a Ph. O. R.J. Pankhurst (British Antarctic Survey) lor their Thesis carried out at the Laboratoire de Pétrologie helplul comments and revie'NS that greatly improved Magmatique (Université O'Aix-Marseille 111, France). the manuscript. R. Kilian and J.A. Aspden arethanked Field work and laboratory analyses were partly sup lor their carelul and constructive review 01 this ported byORSTOM (UR 1 H, Concentrations Minérales manuscript. REFERENCES Abdal-Rahman A,F. M. 1994. Natura olbiotites Irom alkaline. metamorphism 01 a Jurassic lava Ilow: Río Grande calc-alkaline and peraluminous magmas, Journal of Formation, Peru. Journal of SO:Jth American Earth Petr%gy, Vol. 35, p. 185- 193. Sciences, Vol. 1, p. 343-361 . Aguirre. L. 1988, Chemical mobility during low-grade Aguirre. L.; Offler R. 1985. 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