Journal ofthe Geological Society, London, Vol. 152, 1995, pp. 807-818, 5 figs, 5 tables. Printed in Northern Ireland

Geochronology and stratigraphy of the Roque Nublo Cycle, ,

F. J. PEREZ-TORRADO', J. c. CAR RACE DO^ & J. MANGAS' I Departamento Fisica-Geologia, Facultad Ciencias del Mar, ULPGC, PO Box 550, 35080- de Gran Canaria, Canary Islands () 2Volcanological Station of the Canary Islands, Spanish Research Council (CSIC), PO Box 195, 38080-La Laguna, Tenerife, Canary Islands (Spain)

Abstract: The Roque Nublo Cycle, of Pliocene age, is the second main cycle of subaerial volcanic activity in GranCanaria. During this cycle a stratovolcano at least2500111 high developed in the central part of theisland; its volcanic products covered an area of about540 km2. Thedetailed stratigraphy of the Roque Nublo Cycle, together with a magnetostratigraphic study and six new K-Ar dates, which complement 22 previously published ones, have led to the reconstruction of the volcanic history of this cycle in which six main stages have been distinguished. The revised chronology of the Roque Nublo Cycle implies an overlap in age with the earliest rocks of the third cycle of subaerial activity in Gran Canaria. No volcanic hiatus occurred between the second and third cycles, as was previously thought to be the case.

Keywords: Canary Islands, stratigraphy, geochronology, stratovolcanoes.

Interbedded fossiliferous layers are absent from the volcanic Miocene with an episode of submarine volcanism that sequences of theCanary Islands. Therefore dating the represents 75% of the total volume of the island (Schmincke volcanic formations has been mainly by radiometric (K-Ar) 1982, 1990). The subaerial development of the island can be and, to a lesser extent, palaeomagnetic methods. divided into three main magmatic cycles, called Cycle I or More than a hundred radiometric ages of Gran Canaria Old (Miocene), Cycle I1 orRoque Nublo(Pliocene) and havebeen published since the early 1970s. These have Cycle 111 orRecent (Plio-Quaternary) (Schmincke 1976; mainly been 40K-40Ar (Abdel-Monem et al. 1971; Lietz & McDougall & Schmincke 1976; Araiia & Carracedo 1978; Schmincke 1975; McDougall & Schmincke 1976; Feraud et ITGE 1992). al. 1981; ITGE 1992), andto a lesser extent 4"Ar-'9Ar The Roque Nublo Cycle began at about 5.5 Ma, after a (Bogaard et al. 1988; Clark 1988) and I4C (Nogales & period of volcanic repose of at least 3 Ma following Cycle 1. Schmincke 1969) methods. Twenty two of these, obtained by Atthe beginning of this eruptivereawakening of Gran the 40K-4"Ar method, date lava flows and intrusions of the Canaria,strombolian-type basaltic fissure eruptionswere Roque Nublo Cycle (Table 1). located in thesouthern slopes of the island, producing Despitethe numerous radiometricages published, NW-SE lineations of cindercones and nephelinitic lava several important aspects of the volcanic history of the fields of relatively small surfaceextent. Lateron, the Roque Nublo Cycle remain unclear. For example, the age of volcanic activity was concentrated at the centre of the island the explosive eruptions which generated the Roque Nublo where it remained until the end of the Roque Nublo Cycle breccia deposits andthe age of emplacement of the (Hoernle 1987; Schmincke 1990; P6rez-Torrado 1992). phonolitic plugs stratigraphically located at the end of the During thelatter stage,large amounts of lava flows and cycle are not known. This paper details the stratigraphy and breccia-type pyroclastic deposits(known as Roque Nublo geochronology of theRoque Nublo Cycle, and gives an agglomerates or breccias) were produced. At the end of the evolutionarymodel of this cycle, onthe basis of cycle, endogenous phonolitic plugs were intruded (F6ster et magnetostratigraphy and K-Ar dating. It also demonstrates d. 1968; Anguita 1972, 1973; Schmincke 1976, 1990; Brey & the importance 01 combiningradiometric and mag- Schmincke 1980; Hoernle 1987; Anguita et al. 1989,1991; netostratigraphic methods when dating volcanic formations Pkrez-Torrado 1992). in the Canarian Archipelago. Some of the results discussed The distribution and geometry of theRoque Nublo here have beenpresented previously (Perez-Torrado & deposits, the outwarddips of lava and pyroclastic flow Mangas 1992; PCrez-Torrado et al. 1993). deposits and the radial pattern of a dyke swarm indicates the existence in thecentre of the island of astratovolcano (Roque Nublostratovolcano) (Fig. 1).This stratovolcano Geological framework of the Roque Nublo Cycle may have reached at least 2500 m above sea level (Anguita The Canary Islands, which comprise seven main islands and et al. 1989,1991; PCrez-Torrado 1992; Mehl & Schmincke several islets, are situated in the Atlantic Ocean between N 1992). 27" and N 30" latitudes (Fig. 1). These islands are located on According tothe previously published ages, the time the passive continental margin of the African plate. span of the Roque Nublo Cycle was from 5.5 to 3.4 Ma and Gran Canaria, a nearly circular island with a diameter of the stratovolcano was active from 4.4 to3.4Ma (Abdel- about 40 km and a height of 1949 m, is located in the centre Monem et al. 1971; Lietz & Schmincke 1975; McDougall & of the archipelago.It is the third largest island in surface Schmincke 1976; Feraud et al. 1981; ITGE 1992). These ages area (1532 km'). Its geological history beganduring the imply a relatively long period of volcanic quiescence before 807

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Table 1. Summary of the published K-Ar age determinations (whole rock, 40K/40Ar)for the Roque Nublo Group

Sample No. Formation Site location andpetrologyReferences sample Age' Age' GP (M 4 (M4(M4

GC53 1 Tirajana? Tephrite. Area of Las Fortalezas, McDougall & Schmincke 3.40 f 0.08 3.49 Barranco de Tirajana. (1976) GC52 2 Tirajana? Tephrite. Just below sample No. 1. McDougall & Schmincke 3.49 f 0.08 3.58 ( 1976) GCU-29A 3 Tirajana? Tephrite. Mesa de las Burras, Abdel-Monem et al. (1971) 3.50 f 0.09 3.59 Barranco de Tirajana. 1155 4 Tenteniguada Phonolite. Risco Blanco volcanic dome. Lietz & Schmincke (1975) 3.65 f 0.18 3.75 3.71 f 0.18 3.81 GC- 105 5 Rincon de ? Tephrite with hauyne. Dyke north of Abdel-Monem et al. (1971) 3.75 f 0.12 3.85 Tejeda. P4 6 Riscos de Chapin Ankaramite. Barranco de Tenoya. Lietz & Schmincke (1975) 3.77 f 0.08 3.87 P18 7 Riscos de Chapin Ankaramite. Area of Salto del Negro. Lietz & Schmincke (1975) 3.77 f 0.15 3.87 GC65 8 Riscos de Chapin Phonolite. Road C-811, between Tejeda McDougall & Schmincke 3.81 f 0.09 3.91 and San BartolomC de Tirajana. ( 1976) GC1 9 Tenteniguada Phonolite. Volcanic dome of Montaiia McDougall & Schmincke 3.86 * 0.06 3.96 de 10s Brezos. (1976) GC76 10 Riscos de Chapin Alkali basalt. Barranco de Siberio, near McDougall & Schmincke 3.88 f 0.07 3.98 El Chorrillo. (1976) P15 11 Riscos de Chapin Alkali basalt. Barranco de Tirajana, Lietz & Schmincke (1975) 3.96 f 0.10 4.07 east of Aldea Blanca. 950 12 El Tablero Alkali basalt. Montaiia Molinos. Lietz & Schmincke (1975) 3.99 f 0.10 4.10 4.02 f 0.10 4.13 GC-l612901 13 Riscos de Chapin Basanite. Barranco de Quintanilla. ITGE (1992) 4.15 f 0.10 4.15 1419 14 Riscos de Chapin Alkali basalt. Road from Santa Lucia Feraud et al. (1981) 4.29 f 0.10 4.29 to Temisas. P2 15 Riscos de Chapin Alkali basalt pillow-lava. Lomo de 10s Lietz & Schmincke (1975) 4.25 f 0.09 4.36 Ingleses, northwest of Las Palmas. 4.37 f 0.09 4.49 1244 16 Riscos de Chapin Alkali basalt. Lower sequence of Mesa Lietz & Schmincke (1975) 4.38 f 0.09 4.50 del Junquillo. 4.43 f 0.09 4.55 P12 17 El Tablero Olivine nephelinite. El Tablero de Lietz & Schmincke (1975) 4.86 f 0.15 4.99 . GC-l612908 18 El Tablero Basanitic lava related to the Roque ITGE (1992) 5.01 f 0.09 5.01 Bermejo vent. Road C-810 (-San Nicolb). 799 19 El Tablero Intracanyon nephelinite lava. Las Feraud et al. (1981) 5.07 f 0.10 5.07 Tabladas, north of San Nicol5s de Tolentino. GC25 20 El Tablero Intracanyon olivine nephelinite lava. McDougall & Schmincke 4.95 f 0.13 5.08 Near the end of the Barranco de (1976) Tazartico. GC-l712901 21 El Tablero Basalt. Pino Gordo eruptive vent. ITGE (1992) 5.32 f 0.07 5.32 GC24 22 El Tablero Intracanyon olivine nephelinite lava. McDougall & Schmincke 5.48 f 0.14 5.58 Mouth of the Barranco de Tazartico. ( 1976)

Sample, Site location and sample petrology and Age' are taken from the original references. Age', ages recalculated with the Steiger & Jager (1977) standards. No., refers to Fig. 3. GP, geomagnetic polarity (n.d., polarity not determined).

the onset of Cycle 111, the most recent period of activity of fraction). K and @Ar analyses were made in duplicate in order to GranCanaria which startedat approximately 3Ma increase the accuracy of the dates. K-Ar ages were calculated using (McDougall & Schmincke 1976, sample GC-71). However, international standards (Steiger & Jager 1977). Ages from other the data of the present work show that Cycles and 111 authorsand laboratorieswere recalculated using these standards I1 (see Table 1). overlap. Determinations of geomagneticpolarities were carried out on lava and pyroclastic flow deposits using flux-gate portable K-Ar ages and magnetic stratigraphy magnetometers in more than twenty detailed stratigraphic sections, including most of the previously dated rocks (see Figs 1 & 2 and Tables 1 & 2). Thermaland alternating field standardde- Methodology magnetizationlaboratory procedures were applied in oriented samples to test the stability of the natural remanent magnetization, Six K-Ar dates were obtained fromsamples of lavas that were andto clean secondarycomponents in those volcanic units that considered from thin sectionexamination tobe free of deuteric showed inconsistent measurements. This feature, frequently found alteration and weathering (see Table 2). Laboratory determinations in lavas of originally reversed polarity magnetic remanence, is due werecarried out onthe 98% pure groundmass (74-174pm to a weak secondarycomponent of viscous remanent magnetism

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N

Fig. 1. Geological map of the Roque Nublo volcanic group (modified from ITGE 1992). The location of the lavas dated in this work and of the mag- netostratigraphic sections determined Riscos de Chapin Frn. are indicated. Triangles (Tenteniguada pi~lrrves Magnetostratigraphicsection m- sLB7 I Fmj and stars (El Tablero Fm) indicate El Tablem FM. K-Ar age domes and cinder cones respectively. m ‘It

which is easilyremoved at low demagnetizationfields or suggest that the former age is more reliable than the latter. temperatures (see Table3). However, since sampleBTI-4 corresponds to aphonolitic plug intrudinga lateral facies equivalent tothe flow of Analysis of results sample BTH-2, the respective K-Ar ages are in accordance with their relative stratigraphic position. The new ages are in agreement with therelative According to these new datesthe end of the Roque stratigraphic position of the deposits (see Fig. 2). Nublo Cycle could be extended to 2.7 Ma. Several Roque Sample TEA-2, with an age of 4.6 f 0.2 Ma, corresponds Nublo ignimbrites and lava flows appearinterbedded tothe initialstages of activity of theRoque Nublo between the lava flow dated as 3.4 Ma (No. 1 in Table 1) by stratovolcano.Samples BTP-3, SLB-7 and TRS-6,stratig- McDougall & Schmincke (1976) and samples BTH-2 and raphically equivalent, show consistent ages of 3.8 + 0.1 and BTI-4 (see Tirajana and Tenteniguada formations in Fig. 2). 4.1 rt 0.2 Ma. Samples BTH-2 and BTI-4 of 3.1 f 0.1 and This shows that some eruptive activity related to the Roque 2.7 f 0.1 Ma, respectively, give younger ages than the upper Nublo Cycle occurred in this period of time, implying the limit, 3.4Ma, of theRoque Nublo Cycle proposed in absence of a significant hiatus in volcanic activity between previous work (Abdel-Monem et al. 1971; Lietz & Cycles I1 and 111, at leastin the central area of Gran Schmincke 1975; McDougall & Schmincke 1976; Feraud et Canaria. This important fact was reported by PQez-Torrado al. 1981; ITGE 1992). The petrographic and geochemical & Mangas (1992) andlater corroborated by Hoernle & characteristics of samples BTH-2 and BTI-4 (see Table 2) Schmincke (1993). A possible explanation of the overlap of

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Table 2. K-Ar age determinations from the groundmass of six samples of the Roque Nublo Group.

Sample locationSite (UTM Formation Sample petrology K (%) 40Ar* -40Ar* Age* lc GP altitude and (ppm) and coordinates, altitude 40Ar, (Mal locality)

BTI-4 X = 446.37 Tenteniguada Phonoliticdome. Fresh sample with 6.2960.001183 0.421 2.7 ztO.l + y = 3094.85 abundant phenocrysts of hauyne, z = 1600 m. clinopyroxene,amphibole, titanite, 6.3230.001218 0.291 Roque del Saucillo iron oxides and minorbiotite. The groundmass is fine-grained without visible micro-vesiculation and made of mainly feldspar,clinopyroxene and iron oxides. BTH-2 X = 448.15 Tenteniguada Phonolitic lava flow. Freshsample 4.4250.000926 0.305 3.1 f 0.1 + y = 3093.55 with scantphenocrysts of hauyne, z = 1240 m. clinopyroxene and amphibole.Fine- 4.4720.000983 0.325 Barranco of La Capellanfa grainedgroundmass without visible micro-vesiculation andmade of feld- spar, clinopyroxene and iron oxides. TRS-6 X = 438.75 Riscos de Chapin Trachyandesitic lava flow. Scant phen- 4.8340.001285 0.451 3.8 *O.l - y = 3098.87 ocrysts of clinopyroxene,amphibole z = 1175 m. and haiiyne. Groundmass with hyalo- 4.8150.001292 0.482 Road Tejeda- pilitic and trachytic textures and made of feldspar,clinopyroxene and iron oxides.Scant micro-vesiculation oc- casionally filled with carbonates. SLB-7 X = 448.57 Riscos de Chapin Trachytic lava flow. Fresh sample with 4.5960.001223 0.323 3.8 f 0.1 - y = 3084.30 scant and small phenocrysts of feld- z = 510 m. spar, clinopyroxene, haiiyne, iron 4.5810.001225 0.324 Las Fortalezas. Barranco oxides,titanite and amphibole dis- of Tirajana persed in a fine-grained groundmass made of tabular oriented microlites of feldspar, minor amounts of clinopyro- xene and iron oxides. BTP-3 X = 447.92 Riscos de Chapin Trachytic lava flow. Fresh sample with 4.5690.001312 0.396 4.1 ztO.2 - y = 3088.85 scant phenocrysts of feldspar, haiiyne, z=lOOOrn. clinopyroxene, amphibole, iron oxides 4.664 0.0013410.360 Barranco of La Cagarruta and titanite. Oriented fine-grained groundmass made of tabular micro- lites of feldspar,clinopyroxene and iron oxides. TEA-2 X 453.60 Riscos de Chapin Trachybasaltic lava flow. Abundant 2.4500.000784 0.229 4.6 f 0.2 - y = 3097.50 and large phenocrysts of clinopyro- z = 390 m. xene, feldspar, amphibole, iron oxides 2.4790.000789 0.388 Barranco of San Roque and few very altered olivine. Fine- grainedgroundmass with hyalopilitic texture and signs of palagonitization in the glass, made of feldspar, clinopyro- xene and iron oxides.

40Ar* refers to radiogenic 40Ar. 40At is the total @Ar, equivalent to @Ar* + @Aratm. Theanalyses of K and 40Ar were made in duplicate as a control on the accuracy of the age data. UTM coordinates refer to the Spanish national 1:25000 topographic maps. GP indicates the geomagnetic polarity of the samples and Formation assigns these samples to the formations defined in this work.

volcanic products of theseeruptive cycles may be the compared with an established geomagnetic polarity time coexistence of eruptionsfrom deep and shallow magma scale (Mankinen & Dalrymple 1979), a lack of direct reservoirs. The beginning of Cycle I11 would correspond to correspondence is observed (Fig. 3). All but two of the fissure eruptions in which primitive nephelinitic and samples (No. 5 and 9 in Table 1 and Fig. 3) with radiometric basanitic magma fromdeep reservoirswere erupted.In agescorresponding tothe Gilbertepoch have reverse contrast, the final stages of Cycle I1 would correspond polarity. Whilst sample No. 5 is consistent with the Cochity mainly toeruptions of phonolitic magmas from more event, sample No. 9 corresponds within the error limits to a evolved shallow magma chambers.A similar situationhas reverse part of the geomagnetic polarity time scale. When been documented for the Teide-Pico Viejo complex in the all the K-Ar ages available for the Roque Nublo Cycle are island of Tenerife (Araiia et al. 1989). considered, the Cochity, Nunivak, Sidufjall andThvera When the K-Arages and polarities obtainedare normal polarity events of the Gilbert reverse epoch do not

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Magnetic polarity RISCOS- DE CHAPIN Fm. (RCF) EL TABLERO Frn.

A et I

[ 10 111

PROXIMAL PYRDCUISTK:MEMBER (PTRMI

TENTENIGUADA Frn. AYACATA Fm. CTNF) * 2.7

lted lava flows

l

looI PROXIMAL DlSTAL Fig. 2. Generalized stratigraphic col- umns of the different formations of the Roque Nublo Group. K-Ar ages (asterisks) and magnetic polarities ( + or N is normal polarity; - or R is reverse polarity and / indicates no polarity determination) are also indicated.

appear to be represented in the volcanic materials of the Table 3. Palaeomagnetic results of samples used forK-Ar determination in this work (see Table 2) cycle. These normalevents represent about 30% of the Gilbertepoch. This apparently selective absence of the SampleDecl.' Incl.' Decl.'Incl.' Demag. (AF)GP normal polarity events is difficult to explain if volcanism is (mT) considered to be approximately evenly distributed over this period of time, as suggested by the radiometric ages. BTI-4 297.5 40.6 12.6 49.0 20 + Magnetostratigraphic works on Miocene-Pliocene volca- BTH-2 338.5 48.0 341.3 46.6 20 + nic suitesin Tenerife(Carracedo 1979), Lanzarote TRS-6 350.9 44.7 188.1 -34.9 30 - (Carracedo & Soler 1994) and Fuerteventura (Felix 1989), SLB-7 261.6 - 4.3 208.8 -36.5 20 show how evenseveral hundredmetre thick volcanic BTP-3 222.7 -23.3 215.5 -35.0 20 - successions often comprise one or, at most, a few different TEA-2 285.1 35.8 168.4 -22.2 40 - polarity units, which is not in agreement with the long time Decl.' and Incl.' indicate the declination and inclination before the intervals suggested by thecorresponding radiometric data demagnetization treatment (natural remanent magnetization). sets. An explanation of this apparent discrepancy may lie in Decl.' and Incl.' likewise, after alternating field (AF) demagnetiza- the errors inherent to the radiometric dating method. Loss tion at the field shown (in Mt). of Ar in some volcanic materials by weathering, reheating, GP is the geomagnetic polarity of the samples. orsome other cause, is unrecoverable. Onthe contrary,

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Table 4. Stratigraphic units of the Roque Nublo Group defined by K-Ar AGES different authors PREVIOUS WORK 2.48 FUSTER et al. (1%8) SCHMINCKE (1976, 1990) T ANGUITA (1972, 1973) T BREY & SCHMINCKE (1980) I OrdanchiiicSeries renteniguadaFormation - Kaena Ayacata Fm. 3.01 San Andres Formation -Mammoth 3.15 Nyacata Formation

- 3.40 Presa de Homos Formation

:IV% Roque Nublo Series 4.05 Nunivak 4.20 m 4.32 Sidujfall cg 4.47

-Roque Nubb Serie! Mesa de Junquillo Formation

El Tablero Formation El Tablero Fm.

America Commission on Stratigraphic Nomenclature 1983), are different.

Fig. 3. Comparison of K-Ar ages (circles with error bars) and The El Tablero Formation magnetic polarities (open circles, reverse polarity; full, normal This formation is equivalent to that defined by Schmincke polarity) with the established geomagnetic polarity scale (Mankinen (1976, 1990), although in this work it hasa wider strati- & Dalrymple 1979). The identification of the age determinations graphic extentthan that previously thought. It comprises refers to Tables 1 and 2. cinder cones and related lava flows of nephelinite-basanite post-eruptive spurious magnetic components can be readily composition grouped in NW-SE-trending volcanotectonic eliminated, andthe originalmagnetic polarity reliably alignments. These are located mainly in the central part and defined. on the southern slopes of Gran Canaria (see Fig.l and El The magnetostratigraphic data point to the concentration Tablero Fm in Fig. 2). of the volcanic activity in relatively short single polarity The El Tablero Formation is the oldest and longest lived intervals, usually a small part of a geomagnetic polarity unit formation of theRoque Nublo Groupand consistently (subchrone), separated by long periods of quiescence. This shows reversegeomagnetic polarity. According to the hypothesis is in agreement with theeruptive patterns of radiometricages available, the earliest activity of this central volcanic edifices described by Wadge (1980, 1982). formation took place at approximately 5.5 Ma (sample NO. 22 in Table 1) and the latest at 4.1 Ma (sample No. 12 in Table 1). The majority of the ages obtainedfor this Stratigraphy and geochronology of the Roque Nublo formation (4 out of 7) cluster around 5 Ma (samples No. 17, Cycle 18, 19 and 20 in Table l), an age that can be considered to The different deposits beionging to the Roque Nublo Cycle representthe climax of ElTablero Formation volcanic can be included in a litho-stratigraphic unit with the range of activity. a Group(Roque Nublo Group). Redefinition of the The formation mainly represents the initial stages of previously establishedstratigraphic units (seeTable 4) is activity of the Roque Nublo Group before the formation of necessary because of new data in the recently published theRoque Nublostratocone, from 5.5 to 4.6Ma. The geological maps of GranCanaria (ITGE 1990, 1992), the youngest cindercones and lavas of the formation were detailedstudy of thesedeposits carried out byPCrez- formed in minor flank eruptions that occurred whilst the Torrado (1992), andthe need for acorrect hierarchy of central complex was active. litho-stratigraphicunits in accordance with international norms(Hedberg 1976; NorthAmerica Commission on The Riscos de Chapin Formation Stratigraphic Nomenclature 1983). Thus, the Roque Nublo Thisformation is equivalent tothe 'Pre-Roque Nublo Group canbe divided into six formations,some of them Series' of Faster et al. (1968) and Anguita (1972, 1973) and subdivided in members and/or flows (Table 4 and Figs 1 and the 'Mesa del JunquilloFormation' of Schmincke (1976, 2). Some of these formations correspond wholly or partially 1990) and Brey & Schmincke (1980). It comprises a to previously defined units (Table 4). Where new names conformable succession of lavas, most of them with aa have been assigned this is because the type sections, after morphologies, and minor interbedded strombolian-type which the formations must be named (Hedberg 1976; North pyroclastic and/or epiclastic deposits (more abundant to the

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top of the formation). These deposits fill anetwork of extensive (see Fig. 1 andTirajana Fm in Fig.2). The palaeoravines,excavated in the Mioceneformations, wich Tirajana Fm-Riscos de Chapin Fm boundary coincides with extend radially from the centre of the island in all directions the first deposit of Roque Nublo breccia, the dominant except the S and SW. The thickness andnumber of flow lithology of the TirajanaFormation, in the centraland units within the formation gradually decrease fromthe intermediate sectors of the island and with the beginning of centre of Gran Canaria (up to 350 m and more than 20 lava a succession of epiclastic and Roque Nublo breccia deposits flows) to the coast (less than 25 m and only 1 or 2 lava flows) in the distal parts.A characteristic difference between t.he (see Fig. 1 and Riscos de Chapin Fm in Fig. 2). The two formations is the development of numerous intraforma- chemical composition of the lavas range from alkali tionalunconformities within the TirajanaFormation, basalts-basanites to trachytes-phonolites(PCrez-Torrado whereas the Riscos de ChapinFormation is internally 1992). conformable. The Riscos de ChapinFormation shows reverse The Tirajana Formation comprises two distinct members geomagnetic polarity. The oldest age obtained is4.6 Ma thatappear to undergolaterala facies change. The (sample TEA-2 in Table 2) andthe younger limit is Epiclastic Member is partially equivalent to the ‘Las Palmas approximately 3.9-3.8 Ma (samples BTP-3, TRS-6and Formation’ of Schmincke (1976, 1990) and to the‘Higher SLB-7 in Table 2). Member of the Las Palmas Detritic Formation’ (Gabaldh Two volcanic units, called Flow A and Flow B, which et al. 1989; ITGE 1992). This member is located close to the may be considered to be isochronous marker horizons are coast in the W, N, NEand Esectors of the island. It present in the Riscos de Chapin Formation (see Table 4). includes a succession of alluvial, laharand pyroclastic The former, dated at approximately 4.5-4.4Ma and alkali deposits with some subordinate lavas and has a thickness of basaltic incomposition, represents the transitionfrom less than 70 m. The Pyroclastic Member is equivalent to the subaerial lavas with pahoehoe structures to submarine lavas ‘Roque Nublo Series’ of Flister et al. (1968) and Anguita with pillow and hyaloclastic structures in the coastal areas of (1972, 1973), and the ‘Los Listos Formation’ of Schmincke NE Gran Canaria. Preserved beneath it is a Pliocene marine (1976, 1990) and Brey & Schmincke (1980). This member is bed presentlyfound between 80-150m above sea level. distributedover the centralparts of the island where it Flow B, trachytic in composition and dated at approximately reaches a thickness up to 600m. It includes a succession of 3.9-3.8 Ma (samples BTP-3, SLB-7 and TRS-6 in Table 2 thick Roque Nublo breccia deposits of phonolitic-trachytic and No. 8 in Table l), is located in the central sector of the composition (Mangas et al. 1993) and abundant interbedded island along the Tirajana ravine and the Riscos de Chapin lavas, the compositions of which range from alkali zone. It marks the top of Riscos de ChapinFormation in basalts-basanites to phonolites (PCrez-Torrado 1992). these areas. TheRoque Nublo breccias comprise different units Flow A is a good example of errors in K-Ar dating of a which infill the main palaeovalleys, always with lenticular single unit. Different ages of 4.49-4.36 and 3.87 Ma (samples morphologies, and which have individual thickness ranging No. 15 and 7, respectively, in Table 1) have been obtained from 10-60 m in the central areas of Gran Canaria to 2-5 m by Lietz & Schmincke (1975) from lava flows originally near the coast. They always show non-erosive basal contacts, assumed to be at different stratigraphic position. However, are highly-indurated, lithic-rich (approximately 40% in detailed field observations (PCrez-Torrado 1992) clearly volume) and normally display a massive internal structure. show that thesesamples correspond to a stratigraphically Despite the fact that all the authors who have studied these equivalent flow; both lavas exhibit pillow structures and rest deposits in detail agree on their volcanic origin, the eruption directly on lower shorefacemarine deposits. Lietz & mechanisms that have beenproposed for their formation Schmincke (1975) argued that thesemarine deposits differ widely. Thus, Fuster et al. (1968) proposed that these occurred in two distinct horizons. However, thereare no deposits were originated by Saint Vincent-type plinian sections in which both supposed horizons are present; the eruptions; Anguita (1972, 1973) proposed that the eruptions similarities between the marinedeposits in the different were high intensity vulcanian eruptions;and Brey & sections are so great as to indicate that they belong to the Schmincke (1980) suggested that the Roque Nublo breccias same horizon (PCrez-Torrado 1992). A lava flow stratig- derived from the gravitational downfall of plinian columns. raphically equivalent to Flow A-with hyalopilitic textures PCrez-Torrado (1990, 1992) concludethat they are non- interpreted to have been formed when the lava reached the welded lithic-rich ignimbrites deposited in vulcanian sea-has been dated by ITGE (1992) as 4.15 f 0.1 Ma phreato-magmatic eruptions. (sample No. 13 in Table 1). Analyses of the relationship of A very characteristic Roque Nublo breccia deposit with Flow A with the underlying marine sediments and with the abundant vegetation impressions at its base is present within later Epiclastic Member (Tirajana Formation) deposits, the the Pyroclastic Member, and can be traced for 7 km along above mentioned ITGE (1992) age and the curve of global the Tirajana valley. This deposit has been identified as Flow eustatic changes defined by Haq et al. (1987, 1988) suggest C, athird marker horizon which overlies Flow B of the that the 4.49-4.36 Ma age is the most probable. Riscos de ChapinFormation (Table 4). Schmincke (1990) The Riscos de ChapinFormation represents the first defined the ‘Barranco de Honda de La Cueva Formation’ constructive stage of theRoque Nublostratovolcano, which comprises the above mentioned deposit and other less involving eruptions of low explosivity and the establishment extensive Roque Nublo breccias and epiclastic deposits. of a shallow magmatic chamber (PCrez-Torrado 1992). Nevertheless, this author uses the range ‘formation’ unconventionally by including the ‘Barranco de Honda de La Cueva Formation’ as subunita of the ‘Los Listos The Tirajana Formation Formation’. This formation has a similar geographical distribution to that The TirajanaFormation hasa uniform reverse of the underlying Riscos de Chapin,although itis more geomagnetic polarity throughout both the lava flows and the

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Roque Nublo ignimbrites. Its lower limit can be established However, this interpretation hasbeen superseded by the at approximately 3.9-3.8 Ma, from the age of Flow B of the more recent work (Anguita et al. 1989, 1991; PCrez-Torrado Riscos de Chapin Formation. The upper limit of 3.1 Ma is 1992; Mehl & Schmincke 1992). given by sample BTH-2 of Tenteniguada Formation (see The collapses that producedthe Ayacata Formation Table 2 and Tenteniguada Fm in Fig. 2), a phonolite lava postdate most of the Tirajana Formation (S. J. Day, pers. flow which caps the sequence of lava flows and ignimbrites comm.) and probably postdate alateral equivalent of the that outcrop at the centre of the island. phonolitic lava flow dated at 3.1 f 0.1 Ma (sample BTH-2 in The Tirajana Formation marksa significant change in the Table 2). The intrusive episodesthat resulted in eruptive style during the construction of the Roque Nublo emplacement of the youngest domes of the Tenteniguada stratocone,from the hawaiian-strombolian eruptions Formation (e.g., Roque del Saucillo dome, sample BTI-4 in characteristic of the El Tableroand Riscos de Chapin Table 2) postdate the gravitational collapses that destroyed formationstheto highly explosive vulcanian-phreato- the Roque Nublo stratocone. We tentatively suggest that the magmatic eruptions associated with phonolitic-trachytic age of the Ayacata Formation is less than 3.1 Ma. magmas. The Tenteniguada Formation The Rincbn de Tejeda Formation This is equivalent to the ‘Ordanchitic Series’ of Ftister et al. This formation is partially equivalent to the ‘Presa de Los (1968) and Anguita (1972, 1973), to the‘Tenteniguada Hornos Formation’ of Schmincke (1976, 1990) and Brey & Formation’ of Schmincke (1976,1990) and tothe ‘Risco Schmincke (1980). This mainly intrusive formation is made Blanco Formation’ of Brey & Schmincke (1980). As with the up of volcanic breccias, agglutinates, lavas and numerous Rinc6n de Tejeda Formation, the Tenteniguada Formation plutonic apophyses (alkali gabbros). Also present within it is is largely intrusive. It comprises a group of phonolitic plugs a sequence of epiclastic deposits,probably lacustrine and domes, aligned in a NW-SE orientation (see Fig.1 and sediments. These rocks are intruded by many dykes, which Tenteniguada Fm in Fig. 2). areorientated ina radial pattern.The Rinc6n de Tejeda Three volcanic plugs-Montaiia de Los Brezos, Risco Formationoutcrops a small area in thecentre of Gran Blanco(samples No. 9and 4 in Table 1)and Roque del Canariaand represents the main eruptive vents of the Saucillo (sample BTI-4 in Table 2)-and a lava flow (sample Roque Nublostratovolcano (see Fig. 1 and Rinc6n de BTH-2 in Table 2) have been dated. The volcanic plugs of Tejeda Fm in Fig. 2). Montaiia de 10s Brezosand Risco Blanco, with an age of The stratigraphic boundaries of this formation are not as 3.9-3.8 Ma, appearto represent the lower limit of this clearly defined as in previous formations. We consider the formation,coincident with the Riscos de Chapin-Tirajana lower limit to becoincident with the beginning of the formations boundary. The Roque del Saucillo plug, with an construction of theRoque Nublostratovolcano and, age of2.7 Ma, may correspond to the last activity of the therefore, with the lower limit of the Riscos de Chapin Tenteniguada Formation and of the Roque Nublo Group. Formation (approximately 4.6 Ma). At present theupper The K-Ar age obtained for Risco Blanco (3.78 f limit is undefined. 0.18 Ma)and its reverse magnetic polarity isin good Only one K-Ar age is available forthe Rinc6n de agreement with its stratigraphic position. The Montaiia de Tejeda Formation: this is for adyke which has an age of Los Brezos dome is intruded into Miocene deposits and is 3.85 Ma (sample No. 5 in Table 1) and normal polarity. This isolated from other Roque Nublo Group deposits. The age date is concordant with the Cochity normal polarity event in of 3.96 f 0.6Ma is not in agreement with its normal theGilbert reverse polarity epoch.Dykes from this magnetic polarity according to the established geomagnetic formation show, however, both normal and reverse polarity. time scale (see Fig. 3). Since thereare no reliable We correlate the previously mentioned age with Flow B of stratigraphic criteriawe find the age and stratigraphicposition the Riscos de ChapinFormation and the earliest intrusive of this dome within the TenteniguadaFormation ques- domes of TenteniguadaFormation, with which aclear tionable. The Roque del Saucillo dome (2.7 f 0.1 Ma) as geochemical affinity is observed. well as the rest of the volcanic plugs of the Tenteniguada areaand the associated lavaflows (3.1 f 0.1 Ma) show normal(Gauss Epoch) geomagnetic polarity. This implies The Ayacata Formation thatthe final intrusions of this formationoverlap in time This formation comprises the similary-named formation of with the beginning of the emissions of Cycle 111, at Schmincke (1976, 1990) and Brey & Schmincke (1980) and approximately 3 Ma (McDougall & Schmincke 1976, sample the ‘San AndrCs Formation’ of Schmincke (1990). It GC-71). comprises fan-shaped debris avalanche deposits that extend from the center of the island to the S-SSE coast. These are thought to have originated by gravitational collapses of the Evaluation of eruptive volumes and rates in the southern flank of the stratovolcano(Anguita et al. 1989, Roque Nublo Group 1991; Mehl & Schmincke 1992). Remains of the mechanical Any valid estimate of the surficial extent and volume of the slide contact can beobserved at thecentre of the island Roque Nublo Group must begin by reconstructing the between the Ayacata Formation and the Riscos de Chapin palaeotopography of Gran Canariaprior to deposition of and Tirajana formations (see Fig. 1 and Ayacata Fm in Fig. the group (Fig. 4). In order to be as accurate as possible in 3). Schmincke (1976) and Brey & Schmincke (1980) this reconstruction, a grid of 75 rows X 100 columns at an interpret this contact as the separation of the intra and extra interval of 0.5 km was developed on 12 of the 15 geological crater facies, theirAyacata and Presa de Los HornOS maps (1:25 000) of Gran Canaria (ITGE 1990) (see Fig. 4a). formations being included in theintracrater facies. A palaeotopography value of z (altitude in metres above sea

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Fig. 4. Palaeotopographic reconstruction of the island of Gran Canaria before deposition of the Roque Nublo Group. (a) Part of Gran Canaria in which the palaeotopography has been re- constructed. The numbered geological maps, the UTM coordinates and the grid used for the palaeotopographic re- construction are shown. (b) Surficial extent of the Roque Nublo Group deposits just prior to the gravitational 'A collapses of the stratovolcano. The arrows show the main directions of flow; full triangles, Tenteniguada domes; full m la.s.l.1 stars, El Tablero strombolian cones; full square, main Tejeda alkali-gabbro out- ROOUE NUBLO crop; C indicates the inferred borders of the Roque Nublo stratovolcano vent area. Original, pre-Roque Nublo Group, MIOCENE C) contours at 250 m intervals are marked. I I (c) S-N cross-section of the Roque CROSSSECTION A-A (S-NI Nublo stratovolcano.

level) was assigned to each node of the grid. These values The surficial extent of theRoque Nublo Group was were obtained from two different sources. calculated from the palaeotopographic mapto be about (1) Longitudinal and transverse geological cross-sections 540 km', of which 40 km' comprises new land gained from (35) arranged every 5 columns and rows. the sea (see Fig. 4b). The volumeestimation procedure (2) Lithological profiles completed from wells drilled for outlined above also yields the elevation and morphology of water extraction, this being the only possible way to obtain the Roque Nublo stratovolcano just prior to its gravitational information on this group in places subsequently covered by collapse. This stratovolcano probably reached an elevation deposits of Cycle 111. 75 out of the 250 profiles obtained by above the present sea-level of approximately 2500-3000m two researchprojects (MAC-21 1981; PlanHidrol6gico de and had anasymmetric profile with the steeper slopes to the Gran Canaria 1991) that partially or totally cross the Roque south (see Fig. 4c). The total volume of the Roque Nublo Nublo Group reaching the Miocene substratum were Group deposits is calculated tobe about 200 km3. This analysed. volumeestimate is higherthan the 100 km3 previously Values of z for the top of the Roque Nublo Group were evaluated by Anguita (1972), Schmincke (1976, 1990) and assigned to eachnode, based on geological cross-sections McDougall & Schmincke (1976), andthe 140-160 km3 and field data in those areas where Roque Nublo Group calculated by Hoernle (1987). However,Anguita (1972) rocks are still present. Projections of the dips of the Roque drew attention to the fact that in his conservative calculation Nublo Group from these areas towards the centre of the the effects of the palaeotopography andthe parts of the island were used to estimate the thicknesses of the group in Roque Nublo Group deposits covered by later eruptionshad those areasfrom which it hasbeen removed by erosion. not beenconsidered. The volume of 200 km3 calculated When calculating the increase in the area of the island from above is not corrected for porosity in the ignimbrites, nor these data it was necessary to make a correction to the z for the vesicularity or varying densities of the lavas, and values. This was because the initial deposits of the Roque should not be regarded as a dense-rock-equivalent volume. Nublo stratovolcano coincided with a marine transgression We can nevertheless use the uncorrected volumes as a basis of the island in which the sea level rose to 80-100 m above for calculation of approximateeruptive rates, and in the presentsea level (Navarro et al. 1969; Lietz & particular to compare relativeeruptive rates of the main Schmincke 1975; Gabald6n et al. 1989; ITGE 1990,1992; formations. Perez-Torrado 1992). This correction does not affect the The Riscos de Chapin and Tirajana formations, the main volume estimate. part of the Roque Nublo stratovolcano, form about 90% of

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Table 5. Calculated eruptive rates of the Tirajana and Riscos de about 0.3 Ma. Thisindicates that anabrupt fall in the Chapin Formations, for likely limiting values of volumes of juvenile eruptive rate occurred inthe final stages of theRoque material (not corrected for porosity and density variations) inthe Nublo Group activity. formations and of the age of the boundary between them The eruption rates given in this work can be compared with the values suggested by Wadge (1980, 1982) for active Volumeratio Uncorrected volume of Eruptiverates stratovolcanoes (0.5-5.0 km3 ka-'), or those calculated for juvenilematerials (km' ka-l) the Teide-Pico Viejocentral complex by Ancochea et al. 3.9 Ma 3.8 Ma (1990) of 0.75 km' ka-'. The lower eruption rates obtained boundary boundary for the Roque Nublo stratocone correspond to thelonger 3070 Tirajana Fm = 77 km' 0.0960.1 10 evolutionary period of this volcano (>L5 Ma), compared to Riscosde Chapin Fm = 54 km3 0.077 0.068 the values of Wadge (<0.4 Ma) or those of Ancochea et al. 4050 Tirajana Fm = 66km3 0.083 0.094 (<0.2 Ma) for the volcanoes that were studied by them. RiscosdeChapinFm = 72 km' 0.103 0.090 A model for the evolution of the Roque Nublo the 200 km3 calculated volume. According to the geological Group maps (ITGE 1990, 1992) these formations have at present a The previously published and new geochronological data, volumetric ratio of 40:60. However,erosion has removed together with the new and more detailed stratigraphy, allow more of the Tirajana Formation than the Riscos de Chapin a tentative model forthe evolution of theRoque Nublo Formation,thus an original 30:70 ratio is probably more Group to be proposed. Six successive stages (A to F in Fig. accurate. The uncertainties in this ratio and in the age of the 5) can be distinguished. boundary between the two formations (3.9-3.8Ma) are the (A) Between 5.6 and 4.6 Ma strombolian-type eruptive main sources of uncertainty in the relative eruptive rates of episodes generated cindercones and sparse lava flows. the two formations. The calculations outlined below have These wereconcentrated along volcano-tectonic fissures therefore been repeated for the likely limiting values of oriented in NW-SE directions in the south of the island (A formation volumes and boundary age (Table S) in order to in Fig. 5). The magmas of this initial stage were deep in test the robustness of our conclusions. Weillustrate our origin and of primitive composition (nephelinites to method by describing the calculations for a volumetric ratio basanites); they probably ascended directly from the mantle of 30: 70 and a boundary age of 3.9 Ma. (upper right in Fig. 5). The eruptive rate calculated forthe Riscos de Chapin (B) Volcanic activity at this stage migrated from the Formation, almost entirely composed of lava flows erupted southern slopes of the island to its central sector (B in Fig. between 4.6 and 3.9 Ma, is 0.@77 km' ka-'. For the Tirajana 5). At this stagethe activity was still in the form of Formation, which is madeup mainly of Roque Nublo strombolian eruptionsat vents aligned along NW-SE- ignimbrites, lava flows and epiclastic deposits in an oriented volcanic fissures. estimatedratio of60:25:15, the volumes of thethree (C) At approximately 4.6 Ma a shallow magma chamber components would be 75, 32 and 19 km3 respectively. Thus, was established.Construction of theRoque Nublo the total volume of this formation discounting the epiclastic stratovolcano began by means of low explosivity eruptions deposits and the lithic component of the ignimbrites (40% of (hawaiian to strombolian) (C in Fig. 5). the 75 km?) would have beensome 77km'. Given the (D) Between 4.6 and 3.9Ma theeruptive activity chronostratigraphic limits of the formation, 3.9 and 3.1 Ma, maintained its hawaiian-strombolian character. Lava flows this indicates an eruptive rate of 0.096 km' ka-'. reached thenorthern coast and increased the area of the The ranges of eruptive rates calculated using the island by at least 40 kmz. Magma compositions ranged from different assumptionsoutlined in Table 5 are 0.068- alkali basalts-basanites to trachytes-phonolites. Some deep 0.103 km3 ka-' for the Riscos de ChapinFormation and undifferentiated fissure-fed strombolian cones were formed, 0.083-0.110km' ka-.' for theTirajana Formation. We coexisting with the shallow stratified magma chamber which conclude that the eruptive rates for the two formations were fed the stratovolcano (D in Fig. S). comparable,or that, more likely, that for the Tirajana (E) From 3.9 Ma onwards vulcanian-phreatomagmatic Formation was slightly higher. explosive eventsoccurred in the summit area of the However, according to Schmincke (1976, 1990), stratovolcano,producing interlayered lava flows and McDougall & Schmincke (1976), Hoernle (1987) and ignimbrites (E in Fig. 5). Cindercones with radial Hoernle & Schmincke (1993), the eruption rates decreased distribution patterns, and associated lavas, continue to form progressively after emission of the Riscos de Chapin lavas. on the flanks of the stratovolcano. While magmas related to This discrepancy may be due to three factors: (i) the total the explosive events always show a differentiated chemical volume calculated in this work for the Roque Nublo Group composition (trachytic-phonolitic) whichmay have been deposits is notablyhigher thanthat evaluated by the derived from the uppermost layer of the magmatic chamber, above-mentioned authors; (ii) the age limits assigned to the in the mainly effusive eruptions the source of the magma equivalent formations forthese authors,the Mesa del appears to have been deeper in the chamber or may even Junquilloand Los Listosformations, are slightly different have ascended directly from the mantle (see lower right in fromthose of this work;(iii) thatthe volume of deposits Fig 5). generated duringignimbrite eruptions may have been (F) About 3 Ma ago the Roque Nublo stratovolcano was underestimated in previous work. at its peak of activity and development. The explosive The post-Tirajana Formation phonoliticdomes and chzracter of the volcano changed progressively through to associated lava flows of the Tenteniguada Formation, with a the emission of phonolitic lavas and intrusion of phonolitic volume of less than 1 km3, were emplaced over a period of plugs at its summit area. At this stage theRoque Nublo

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A. Southern sector fissural eruptive activitystage ,,,

(initial ohase of the RNG) 5.6-4.6 Ma - B. Central sector fissural eruptive

lm~t~ve(deep) magmas

incipient C. Initial Roque Nublo stratovolcano stage

D. Predominantly effusive RN Strongstrombolian-type 'Iratified sha"ow Primi'ive(dW magma chamber^..,?. magmas stratovolcano 4> 1 activity

. - .,_, \.., 4.6-3.9 Ma Bn-Ne to Tr-Ph

Explosive Vulcanian-type E. Mature RN 1eruptive actiuity

I' Vulcanian-type explosions alternating with strombolian-type Fig. 5. Schematic representation of the eruptions from adventive vents, -a M- evolution of the Roque Nublo Group. I 1.. " Bn, basanites; Ne, nephelinites; Tr, trachytes; Ph, phonolites.

stratovolcano probably reached an elevation of more than dismantling of the volcanic edifice (Schmincke 1990)-have 2500m above sea level with a pronounced asymmetry. The been postulated. steeper slopes madethe volcano gravitationally unstable towardsthe S-SW flanks (see Fig.4c). Seismic activity The work was supported by the research projects Nos 69-1989 and related to late explosive events or the intrusion of volcanic 22-1993 of theState University Office of theCanary Island plugs may havetriggered gravitational collapses of the Government. We would like to thank V. Soler for his assistance volcano tothe south. These collapses produced massive with the palaeomagnetic sampling and measurements and S.J. Day debris avalanches that fell downslope over distances of up to for sharpening our arguments and correctingour english. Thorough 20 km and eventually reached the south coast. The collapses reviews and comments by J. Marti, J. Gilbert and K. Hoernle are produced a horse-shoe shapedamphitheatre open to the very gratefully acknowledged. south (in the summit).Volcanic activity continued within this amphitheatre with the emplacement of volcanic domes References of phonolitic composition, fumarolic activity and occasional ABDEL-MONEM,A., WAmms, N.D. & GAST,P.W. 1971. Potassium-argon explosions which generated block and ash flow deposits (F ages,volcanic stratigraphy and geomagneticpolarity history of the in Fig. 5). Canary Islands: Lanzarote,Fuerteventura, Gran Canaria and La Thesubsequent evolution of the stratovolcano is Gomera. American Journal of Science, 271, 490-521. ANCOCHEA,E., FUSTER, J.M., IBARROLA,E., CENDRERO,A., COELLO, J., controversial. Two main hypotheses-an explosion caldera HERNAN,F., CANTAGREL,J.M. & JAMOND,C. 1990. Volcanic evolution of with a lake finally filled with sediments and lava products the island of Tenerife (Canary Islands) in the light of new K-Ar data. (Anguita et al. 1989, 1991) andthe progressive erosive Journal of Volcanological Geothermal Research, 44,231-249.

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ANGUITA,F. 1972. La evolucidn magmstica en el Ciclo Roque Nublo (Gran 1109-IV, 1113-1, 1114-1 and 1114-IV). Proyecto MAGNA. Canaria). Estudios Geolbgicos, 28,377-482. - 1992. Memoria y mapa geoldgico a escala 1 : 10 OOOO de la isla de Gran - 1973. Genesis of Roque Nublo formation: a special kind of ignimbrite Canaria. (21-21/21-22). eruption in Gran Canaria. Bull Volcanologique, 37, 111-121. LIETZ, J.& SCHMINCKE, H.U.1975. Miocene-Pliocene sea level changes and -, GARC~ACACHO, L. & ARANA,V. 1989. Field tripto Gran Canaria volcanic episodes on Gran Canaria (Canary Islands) in the light of new (RoqueNublo Caldera). ESFMeeting on Canarian volcanism, K-Ar ages. Palaeogeography Palaeoclimatology Palaeoecology, 18, Lanzarote. Guia de campo. 213-239. --, , COLOMBO,F., GONZALEZCAMACHO, &A. VIEIRA, R. 1991. Roque MAC-21, 1981. Proyecto de planifcacidn de la explotacidn y uso racional de NubloCaldera: a new stratoconecaldera in GranCanaria, Canary 10s recursos de agua en Ius Islas Canarias (inuentario de puntos de agua). Islands. Journal of Volcanological and Geothermal Research, 47,45-63. Comision Intemacional Actuaciones Estado en Materia de Agua en las ARANA,V. & CARRACEW,J.C. 1978. Los volcanes de las Islas Canarias. 111 Islas Canarias. Gran Canaria. Ed. Rueda, Madrid. MANGAS,J., PEREZ-TORRADO,F.J., MASSARE,D. & CLOCCHIATTI,R. 1993. -, BARBERI,F. & FERRARA,G. 1989. El complejo volcinico del Teide-Pico Phonolitic origin of Roque Nublo ignimbrites of Gran Canaria (Canary Viejo. In: ARA~A,V. & COELLO,J. (eds) Los volcanes y la caldera del Islands, Spain) from clinopyroxene melt inclusion studies. European ParqueNacional del Teide (Tenerife, Islas Canarias). Icona,Serie Journal of Mineralogy, 5, 97-106. Ttcnica, 7,101-126. MANKINEN,E.A. & DALRYMPLE,G.B. 1979. Revised geomagnetic polarity ~OGAARD,P,, SCHMINCKE, H.U., FREUNDT, A., HALL,C.M. & YORK,D. 1988. time scale for the interval 0-5 Ma BP. Journal of Geophysical Research, Eruption ages and magma sup~lyrates during the Miocene evolution of 84 (BZ), 615-626. GranCanaria. Single-crystal Ar/39Arlaser ages. Natunvissenschaften, MCDOUCALL,I. & SCHMINCKE,H.U. 1976. Geochronology of Gran Canaria, 25,616-617. Canary Islands: Age of shield building volcanism and other magmatic BREY,G. & SCHMINCKE,H.U. 1980. Originand diagenesis of theRoque phases. Bulletin Volcanologique, 40, 1-21. NubloBreccia, Gran Canaria (Canary Islands). Petrology of Roque MEHL,K. & SCHMINCKE, H.U.1992. Multiple sector collapse of the Pliocene Nublo volcanics, 11. Bulletin Volcanologique, 43, 15-33. RoqueNublo Stratocone on GranCanaria (Canary Islands). I11 CARRACEDO,J.C. 1979. Paleomagnetismo e historia volcdnica de Tenerife. Congreso Geologico EspaAa, VIII Congreso Latinoamericano Geologia, Aula Cultura de Tenerife. 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Received 12 July 1994; revised typescript accepted 21 December 1994.

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