The center of the Vallehermoso Caldera Felsic (OJO: “the center of the caldera” es muy usado si miras en google) Complex: interior of an

ancient volcanic system (La Gomera, Canary Islands)

J.A. Rodriguez-Losada a*, J. Martinez-Frias b

a Department of Soil Science and Geology, University of La Laguna, 38206 La laguna, Tenerife, Canary Islands, Spain

b Laboratorio de Geología Planetaria, Centro de Astrobiologia (CSIC/INTA), 28850 Ctra. De Ajalvir, Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain

*Corresponding author. Fax: +34-922-318311E-mail address: jrlosada@ull.es (J.A. Rodriguez-Losada).

Abstract

This paper tackles the issue of a Mid to Upper Miocene felsic dyke intrusion, which is located around the Vallehermoso and Tamargada district (North of La Gomera Island). This intrusion is built by two main structural patterns: 1) an ENE-WSW dyke intrusion with a dominant subvertical dip and 2) a latter conic dyke intrusion, superimposed to the first one. As a consequence of this latter cone sheet complex, masking and distortion of the ENE-WSW intrusion took place. The cone sheet complex emplacement caused failure of the roof of a shallow magma chamber, forming a caldera collapse with a central sector almost coincident with the centre of the cone sheet. A breccia dominated depressed central area is surrounded by intrusions of nepheline phonolites domes and trachytic-phonolitic dykes. These constitute the so-called “Trachytic and Phonolitic Complex”. Trachytes and nepheline-phonolites are the dominant outcrops. Occasional appearance of haüyne phonolites domes is visible to the NE sector of the cone sheet. Otherwise, small outcrops of intermediate rocks also occur. The whole seems to be affected by dominant meteoric alteration that makes them unavailable for dating studies (Ojo: he quitado la segunda parte de la frase). Other felsic intrusions that also crop out in the Vallehermoso-Tamargada area, are characterized by the occurrence of alkali gabbroids and syenites (Tamargada alkaline intrusions).

On the basis of petrological, geochemical and detailed field studies both trachytes and nepheline phonolites probably evolved from alkali mafic magmas from the Upper Old Basalts formation, through a dominant fractional crystallization process. In addition, haüyne phonolites could proceed from local and latter magmatic processes such as gas transfer, becoming nepheline phonolites into haüyne phonolites. Likewise, our data evidence that a possible co-genetic linkage with the felsic volcanic formation is refused and suggest, on the contrary, the existence of two ancient episodes of felsic magmatism in La Gomera island: the Tamargada alkaline intrusions as the former one, and the more recent represented by the Trachytic and Phonolitic Complex.

Introduction

In the context of the geological features of the volcanism in the Canary Archipelago, the Vallehermoso area and surrounding environment represent one of the less known in the entire Canary Islands and specifically in La Gomera. From the first studies by Buch (1825) and Fristch (1867), La Gomera has been traditionally considered of scarce volcanic interest for the scientific community as reflected by the low number of domestic and international works and publications, in comparison with other Islands of the archipelago. Hausen (1960) characterized the Vallehermoso valley as an erosion caldera. Latter, Rodriguez-Losada (1988) and Cueto et al (1994) found evidences suggesting that the Vallehermoso caldera (VC) was originated by collapse, being the erosion stage a secondary process that modelled the caldera morphology. In an intermediate date, Cendrero (1971) called it “Arco de Vallehermoso” because its arcuate shape, concave to the north, with an average of 5 km. E-W amplitude, and 8 km. South

to North length and a total area of 37 km2 (Fig. 1). The landscape is rugged and strong dips and deep valleys are typical. Several peaks, ranging up to 1.000 m height delimit the south border of the caldera. The felsic subvolcanic formation, which is located at the central part of the VC (28º 08'N 28º 11'N, 17º 14'W 17º 18'W, received different names after its discovery (OJO: Si decimos que jugó un papel importante hay que decir cual y justificarlo). Fernandez-Navarro (1918) suggested that it represents the oldest outcrop of the island. Bravo (1964) considered the felsic formation as a part of the La Gomera Basal Complex. In this sense, it is important to stress that Cendrero (1971) found a clear distinction between the Basal Complex and the Trachytic-Phonolitic Series. Finally, Rodriguez-Losada (1987) carried out a very detailed geological and petrological study of the area, as a part of his Ph.D thesis, identifying the existence of new geological features (e.g. Cone Sheet Dyke Complex), and renaming the whole subvolcanic formation as the Trachytic-Phonolitic Complex (TPC).

The Cone Sheet Dyke Complex (VCSC) is located at the centre of the VC showing a high level of erosion (Rodriguez-Losada, 1987, 1988; Hernan et al, 2000) and two radial dyke swarms (Huertas et al, 2000). Other similar dyke swarms have been previously described in the Canary Archipelago such as that found at the south-western part of the Cañadas Caldera (Tenerife island) (De La Nuez et al., 1989; Ancochea et al., 1999) or the striking Cone-Sheet complex related to the Tejeda Caldera in Gran Canaria island (Schmincke, 1967; Hernan, 1976; Hernan and Velez, 1980; Schirnick et al., 1999).

This paper aims to describe the structural, petrological and geochemical features of the TPC, stressing its geological significance to understand the interaction of the magmatic episodes in the context of the whole evolution of the VC.

Geological setting

The historical record in the study of the geology of La Gomera Island starts with Von Fritch (1867), followed by several descriptions based on the studies of Fernandez-Navarro (1918), Gagel (1925), Muller (1930), Jeremine (1935), Blumenthal (1961) and Bravo (1964), among others. Despite recent works tackle in detail some geological aspects of local areas of La Gomera, the description based on Bravo (1964) continues being the best and most general overview of the geological features of the entire La Gomera Island. Nowadays, it still remains almost unchanged and represents the basis for many other studies in the island. This author recognized two major units: 1) the Basal Complex (the oldest one), which is made up of maphic, ultramaphic plutonic rocks, sediments and pillow lavas, all of them cut by a dense dyke swarm of basaltic, trachytic and phonolitic dykes, and 2) the later volcanic series, characterized by volcano-sedimentary breccias, old basalts, felsic domes and lava flows, recent basalts, all of them unconformably overlying the Basal Complex.

As previously defined, an isolated felsic subvolcanic intrusive formation was differentiated from the Basal Complex by Cendrero (1971). Based of field criteria, this author supposed that the felsic intrusive formation was the oldest subaereal volcanic episode in the island, followed by the Old Basalts that were renamed as Lower Old Basalts and Upper Old Basalts with polygenic breccias between them. Later on, Cubas et al (1994), revised the extension of the Lower Old Basalts and found new exposures of these basalts where the polygenic breccias alternate with them. Further detailed field

studies made necessary to re-arrange the stratigraphic position of the Vallehermoso felsic formation as more modern than the Lower Old Basalts and contemporaneous (or slightly older) than the Upper Old Basalts (Table 1). Fig. 2 shows the geological map of the studied area.

On the basis of K-Ar dating, it has been determined that the age of the Basal Complex ranges between mid to lower-Miocene age (Abdel-Monem et al. 1971; Feraud, 1981; Cantagrel et al., 1984). The volcano-sedimentary breccias, Old Basalts and the felsic intrusive formation (subject of this paper) are of mid to upper-Miocene age and the Horizontal Basalts, felsic domes, felsic lava flows and Recent Basalts are of upper-Miocene to lower-Pliocene age (Table 2).

We consider the use of the term “Trachytic Phonolitic Complex (TPC)” more appropriate than “Trachytic Phonolitic Series” (Cendrero, 1971), because the word “Series”, in addition to its terminological connotation, has a specific local use implying a horizontal sequence of rocks (as for instance in the case of the basaltic series of the Canary Archipelago). The word “Complex” looks like more suitable in order to describe a geological formation made up of intrusive materials and debris breccias with a chaotic distribution. The VC includes: 1) the TPC of La Gomera Island that is constituted by a) VCSC (Vallehermoso Cone Sheet Complex), b) felsic domes, and c) associated polylithic and poligenic breccias, 2) A part of the basal complex that crops out rimming the south, west and northern limits of the caldera, and 3) small outcrops of recent basalts, roughly confined to the E and SE border (Ojo José Antonio he quitado que los 2 y 3 no están estudiados ya que ha quedado claro antes que lo estudiado es lo primero). The most significant feature is a set of outcrops of mixed breccias, occurring in the center of the VC, mainly composed by a chaotic combination of felsic fragments and blocks. The irregular distribution of breccias makes extremely difficult to establish a clear temporal sequence. The breccias are surrounded by felsic intrusive dykes and domes.

OJO JOSE ANTONIO. YO PONDRÍA TODO ESTO DE LOS ANÁLISIS EN LAS TABLAS Y NO EN EL TEXTO, YA QUE ES POCO PARA UN EPÍGRAFE Y ADEMÁS NO ENCAJA BIEN DEL TODO.

Chemical composition of selected samples of silicate and ore minerals appearing in the most representative rocks from the TPC were determined by electron microprobes at the Consejo Superior de Investigaciones Científicas, Madrid, and the Technical Services of the Granada and Oviedo Universities.

Whole rock chemical composition was determined by A.A.S. by using a “PYE UNICAM SP 1900” spectrophotometer (University of La Laguna, Tenerife) after fusion of powder sample and lithium metaborate mixture and latter acid digestion. REE and Trace elements were determined by XRF using a “Phillips PW-1410” spectrometer (University Complutense, Madrid) and by ICP-MS (Activation Laboratories, Canada).

Vallehermoso Cone Sheet Complex

Different patterns of dyke intrusion can be distinguished along various sectors as shown in Fig. 3a. The first analysis of the dyke intrusion in the Vallehermoso area was carried out by Rodriguez-Losada (1987, 1988). He identified, by the first time, the presence of a conic intrusion of felsic dykes, reflected on the surface as a semi-circular pattern which is located a few hundred meters to the south of Vallahermoso village. The author described the structural feature as an incomplete cone-sheet dyke complex, open to the NW, intruded in the Vallahermoso range. The dykes intrude through the ultramaphic rocks of the basal complex as well as through the lower old basalts formation. Trachytes and nepheline phonolites, with a predominant abundance of the first ones, are the dominant petrological types. However, the projection of their composition into the SiO2-Na2O+K2O diagram shows that the transition between both types of rocks is not clear in some cases (Fig. 8). To solve discrepancies between normative and modal data, a normative nepheline-content higher than 10% was applied following Streckeisen (1978).

Textures

Most of the dykes (trachytes and phonolites), vary between aphanitic and porphyritic textures, displaying a fine-grained matrix (Fig. 4 a, b). Trachytic alignment in the matrix is sometimes also present and occasionally the groundmass either does not exhibit clear orientation in their microcrysts or display spherulitic textures made up of radiating needles as response to devitrification processes. Grain sizes can also vary from a most common bimodal size distribution to seriate one. The phenocrysts, when present, show euhedral to subhedral shapes and are composed of anortose, oligoclase-andesine and kaersutite. Most of the matrix is constituted by major alkali feldspar, Na-rich plagioclase, minor nepheline bordered by radiating aegirine, and traces of aegirine, apatite, sphene and ore minerals (Table 3). Iron oxides, chlorites, zeolites and calcite occur as secondary phases. Often, the rocks include xenoliths of mafic rocks with edges clearly discernible from the host rock. Vesicularity is scarce and the vesicles, when visible, are commonly filled by secondary minerals such as calcite. Glassy, chilled margins not greater than 10 cm thick, are also common.

Structural arrangement

Strike and dip of 303 felsic dykes were determined (a magnetic deviation of 10º W was considered for the studied area), following data from the Spanish National Geographic Institute (Fig. 3a). In addition, a total of 148 basaltic dykes related to later volcanic processes were also measured, to evaluate possible structural modifications. Fig. 5 displays the histogram of thickness for all dykes. An almost bimodal distribution of can be deduced from the plot (a - felsic dykes), with two maximum values of 1 m and 3-4 m. In the second diagram (b - basaltic and later dykes), a dominant thickness of 0.5 meters prevails, while an 8% of basaltic dykes intrude as a tabular bodies around 5 meters thick. In general terms, most felsic dykes occur as single intrusions and dyke spacing shows: a) strong variations with very low intrusion frequency near the centre and b) a dramatic increasing of intrusive density and an outward decreasing near the edges resembling coherent dyke-complexes (Walker, 1999). Flexion, brecciation and abrupt thickness changes in single dykes are usually visible affecting them, many times intruded by later injection of more modern dykes of similar strike, dip and petrological features. The basaltic dykes show no significant spatial changes regarding the density of dyke-intrusion in the study area and exhibit an almost dominant NNE-SSW trend.

As previously defined, the systematic measurement of strike and dip of 303 felsic dykes shows a circular pattern with a general dip inward of the intrusive complex. This tendency is noticeably inferred (see Fig. 3a) except for the NW sector, where an unambiguous direction is not evident. Combining the six stereograms in a single one (Fig. 3b), a cone-sheet dyke intrusion can be clearly deduced. The dyke swarm (OJO LA LOCALIZACIÓN ESTÁ YA REPETIDA ARRIBA) exhibits a diametrical extent rising 12 km at the surface. As can be seen in the figures, the dip of dykes show values ranging between 60o-80º towards the centre. Gradual direction changes from the centre to the edge are not observed. Another observed trend, which is partially masked by the cone-sheet, is ENE-WSW. It is difficult to establish a temporal correlation between this one and the former; when both patterns can be simultaneously observed, ENE-WSW trend appears intruded by the cone-sheet. Field work suggests that the ENE-WSW intrusion of felsic dykes took place almost contemporaneously (or even before) the conic intrusion. This is supported by the appearance of a sub-radial pattern probably due to distortion of the ENE-WSW trend induced by the conic complex.

Dome intrusions

Clearly visible intrusions of exhumed felsic domes occur along a semicircular pattern with an estimate diameter of 3 km and the same central point that the conic dyke complex. The domes exhibit various morphologies resembling very thick dykes with “shark wing” shapes disposed in series of several parallel-intruded domes and following the semicircular pattern of the cone-sheet (Fig. 6). They represent roughly a 6% of the total TPC outcrop and are not cut by other dykes. A total of 8 domes have been identified in the area. All of them, except domes 1 and 2, show the morphology described above. These domes display similar features and differ from the others by their shape and their single-body outcrop. and two domes are spatially related with the TPC, but do not appear to be genetically related with it (Fig. 3a). Geochronological data support the previous assumption (at lest one of them, called “Roque del Cano” or dome 1 in the Fig. 3a, were dated as lower Pliocene by Cantagrel et al, 1984); this makes evident that their evolution is linked to the most recent felsic formation of La Gomera Islands. The lack of geochronological data for the rest of the domes is a consequence of their intense alteration. Thus, field criteria are very important to try to deduce their volcanostratigraphic level in the TPC context, in particular their structural concordance with the cone-sheet complex.

Textural and Petrological features.

Textures and petrological characteristics resemble those observed in the dykes. Nepheline phonolites are the most abundant rock types, showing aphanitic and porphyritic textures and a fine-grained matrix. Trachytic alignment in the matrix is also present and occasionally the groundmass does not exhibit any orientation. As previously defined in the Cone Sheet Complex, grain size can vary from a most common bimodal to seriate size. The phenocrysts show both euhedral to subhedral shapes and are composed of anortose, oligoclase-andesine, kaersutite and nepheline. Most of the matrix is constituted by alkali feldspar, Na-rich plagioclase, minor nepheline bordered by radiating aegirine, aegirine and occasional apatite, sphene and ore minerals (Table 4). As secondary, appear iron oxides, chlorites, zeolites and more abundant calcite. Usually,

the rocks include xenoliths of mafic rocks. Small variations in the nepheline modal content from 5 to 15% are detected.

An intriguing issue of the TPC is the appearance of a dyke-like dome composed by Haüyne phonolite, which noticeably differs from the rest of TPC domes. The only outcrop of Haüyne phonolites is located between domes 1 and 3 (not indicated in Fig. 3a) appearing as a small acute crest. These phonolites show porphyritic textures with a groundmass made up of alkali feldspar, radiating aegirine needles and traces of nepheline, showing inhomogeneous trachytic alignment. Euhedral to subhedral phenocrysts of anortose and aegirine augite are common. Anhedral olivine crystals with thick and dark edges are visibles. In some cases, olivine is totally substituted by a groundmass of aegirine, iron oxides and zeolites. Frequently, augite and olivine crystals can be distinguished as xenoliths inclusions of basalts, showing assimilation processes. Haüyne and Kaersutite are also present, occurring as subhedral to anhedral phenocrysts. Inclusions of haüyne inside the feldspars phenocrysts are frequent. In addition, minor apatite, sphene and iron oxides, as well as secondary calcite and zeolites, complete the mineralogical setting of these unusual domes.

Breccias

A series of chaotic breccias occur filling the inner-most area of the VCSC. They conform a circular outcrop of around 3 km in diameter, which is located 1 km SSE from Vallahermoso village (Fig. 2). It is important to note that the central part of the VC is also located at this area. Peripheral normal faults and fractures indicate that this central part of the TPC was originated by the collapse of a volcanic edifice during the first stages of the subaereal evolution of La Gomera Island. This ancient collapse along with later erosion processes triggered that highly eroded deposits of chaotic distributed breccias are now visible. Despite the embroiled outcrop of the breccias, four main types can be differentiated: 1) debris-avalanche breccias; 2) mylonites; 3) tuff-like breccias, and 4) dome-intrusion breccias.

Type 1 is the most abundant in the central area of the TPC. It shows a great variety of lithologic types, though are much more uniform close to the domes, where the dome-intrusion breccias (mainly made of phonolithic fragments), are dominant. Major angular to subrounded fragments and blocks or boulders of trachytic (principal) and nepheline-phonolitic (accessory) composition, and minor intermediate and basaltic clasts comprise the skeleton of the breccias, showing a continuous size series from millimetres to decimetres. Broken crystals of alkali feldspar, minor plagioclase, biotite, ore minerals and apatite traces are the main constituent of the matrix. Finally calcite, appear filling in veins and small cavities and vugs. Type 2 is related to faults, and characterised by irregular rock fragments varying from mm to cm in size, broken crystals and a dark matrix. It exhibits chlorytic alteration and recrystallization processes in the form of microspherulitic aggregates of quartz. Type 3 shows eutaxitic textures, consisting of elongated pumice fragments or bands of them, in which, devitrification processes are present. Thus, the axiolitic texture is characterized by the growing of alkali feldspar and aegirine aggregates from the surface of elongated glass fragments (Fig. 7). The source of such eutaxitic clasts, resembling fragments of ancient welded ignimbrites, still remains unresolved. There are not signs of the existence of layered welded ignimbrites in the area and the analysis of present outcrops also makes improbable a possible correlation between eutaxitic clasts and fragments from eroded deposits of residual welded ignimbrites.

The scarce appearance of dyke-like eutaxitic bodies suggests that eutaxitic clasts could derive from piroclastic felsic dikes in which rheological and eutaxitic textures evolved at the final stages of explosive eruptions (Wolff, 1986).

Alteration features

Intensely altered areas in the felsic rocks are common. The alteration processes have transformed the felsic rocks into a white and powdery rock, locally called “salitre” by its similitude with the salt deposits. The alkali feldspars are still present as relict primary minerals. This processes result in a relative decrease of SiO2, FeO, MnO, MgO, CaO, Na2O, K2O and increase in Al2O3, Fe2O3, TiO2, P2O5 and water content (see Table 9). X-ray diffraction analyses indicate the presence of kaolinite, illite, montmorillonite, chlorite, analcime and calcite, probably as transitional mineral phases of an alteration stage that could evolve to more stable phases such as kaolinite and gibbsite.

The syenite problem

A small WSW to ENE streaking intrusive body of 700 m long and 150 m wide crops out between km. 35 to km. 36 of the main road from San Sebastian de La Gomera to Vallehermoso. It is close to Tamargada, a little village with a sparse population, located about three kilometres East from Vallehermoso. The rocks consist of syenites and alkali gabbroids appearing in the limit between the Basal Complex and the felsic dyke-swarm. The intriguing question is what are the syenites genetically related with?. Two possible responses can be suggested: a) with a final or not, felsic stage of the Basal Complex evolution and b) as non extruded or intruded in dykes felsic magmas and hence genetically linked to the sheet intrusion.

Pretty good correlation between phonolitic cone-sheet swarms and syenites is clearly stated in other similar subvolcanic formation located in the Tejeda Caldera (Gran Canaria) (Schmincke, 1967; Hernan, 1976). The Tejeda syenites lie in the central sector inside the sheet-intrusion, exhibiting an almost perfect spatial connection. However, this is not the case in Vallehermoso, where syenites occur in a peripheral sector, in the limit between the Basal Complex and the sheet intrusion (Fig. 2). Despite the lack of radiometric data, Cendrero (1971) suggested, on the basis of field and petrological data, that syenites and the sheet intrusion are not genetically linked. In addition, Fernandez Santin (1979), Rodriguez-Losada et al (1990) and Rodriguez-Losada and Martinez-Frias (1998) found mineral paragenesis of hydrothermal metamorphism affecting the syenites and two stages (magmatic and hydrothermal) of mineralization mainly composed of oxides (magnetite and ilmenite) and sulphides (pyrite, pyrrhotite) in the metallogenetic history of these rocks. This type of processes were not found in the felsic dykes of the TPC supporting “a” assumption. Thus, concerning to the volcanic evolution of the island, three main felsic volcanic cycles would be assumed in La Gomera Island. The former, related to the Basal Complex, the second, to the TPC studied here and the third and most recent one, related to the Upper Miocene-Lower Pliocene cycle.

Intensive variables

The study of intensive parameters was carried out to try to estimate the equilibrium temperature and oxygen fugacity conditions comparing the felsic volcanic rocks with the Tamargada plutonic felsites. Oxygen fugacity and equilibrium temperature data are given in Table 3c (trachytes on dykes) and 4c (dome phonolites). From the iron and titanium oxides compositions, magnetite-ilmenite pairs indicate an equilibrium temperature of between 1029 and 1149ºC and oxygen fugacities ranging from 10-9.6 to 10-8.3 atm in the trachytic dykes and 873-1004ºC, 10-13.5 to 10-10.5 atm in the dome phonolites.

A revision of the temperature and oxygen fugacity estimations, based on the magnetite and ilmenite pairs in the Tamargada syenites (Rodriguez-Losada and Martinez-Frias, 1998), points to lower values of temperature (around 736 ºC) and oxygen fugacity (10-16 atm) (based on Ghiorso and Sack, 1991). These estimated parameters seem to be close to the temperatures of ilmenite unmixing (Ramdohr, 1980), after magmatic crystallization. From other ore minerals such as pyrite and pyrrhotite crystals, two mineralising stages (magmatic and hydrothermal) were defined as a part of the metallogenetic evolution of the syenites (Rodriguez-Losada and Martinez-Frias, 1998) with no connection to the felsic volcanic complex.

Higher equilibrium temperature and oxygen fugacities correspond to trachytic dykes with peralcaline tendencies while phonolitc domes exhibit lower intensive parameters. This could support the hypothesis, already evidenced by field observations, according to which the phonolitic dome intrusion took place in a late stage of the cone-sheet emplacement, from???? a slightly cooled and decompressed felsic magma.

Whole rock geochemistry of the TPC

Major elements

Chemical composition of rocks is displayed in tables 5 (domes) and 6 (dykes). By plotting the samples in the total-alkali-silica diagram, dykes and domes lie into the trachytes and phonolites fields (Fig. 8). Rocks displaying an intermediate composition are very scarce in the area; nevertheless, three samples, one of them included as xenolith in a trachytic dome, were studied and plotted. According to the previously assumed temporal correlation between upper old basalts and the TPC, chemical data of these basalts were plotted in the same diagram by using data from Ibarrola (1970) and Brandle & Cerqueira (1975). Most of basalts lie within the basanite tephrite field, close to the basanite:alkali basalt and trachybasalt:tephrite field boundaries. Most of them have low normative nepheline contents (around 8.5 %, and mostly less than 5%) and olivine content higher than 10% in four of them (basanites) and less than 10% for the rest of them (tephrites), showing trends parallel to the trachybasalt:tephrite field boundary and its extension (basanite:alkali basalt) as the similar corresponding division of Cox et al. (1979). No clear pattern can be defined between basalts and trachytes:phonolites. Nevertheless, the five intermediate plotted samples within the basaltic trachyandesite field and tephriphonolite:trachyandesite field boundary seem to define a pattern almost parallel to the mentioned limits from the basalts to the felsic samples. Breccias are composed mostly of felsic fragments and hence, their chemical composition approaches to the composition in dykes and domes. Some of them were plotted in the diagram supporting the defined tendency from basic to felsic samples. Nevertheless, their high alteration degree makes necessary to consider this assumption

with caution. The high LOI content in two cases around 7 % suggest mobilization of other elements apart water and CO2 during alteration.

Both felsic dykes and domes concentrate around the trachyte-phonolite boundary from LeBas et al. (1986), except for the haüyne phonolites, which lie exclusively within the phonolite field. According to that, most samples have normative nepheline except two of them with a little normative Q content. In addition, five samples have minor normative C. Here, two groups of different undersaturation can be defined: a weakly undersaturated group with moderate normative nepheline content (Ne < 10 %), including most of the trachytic dykes and almost around 50 % of the sampled domes and a strongly undersaturated one, with higher normative nepheline content (Ne > 10 %) that consist of haüyne phonolites domes, around 10% of total dykes and almost 50 % of domes with the included haüyne phonolites. Both types exhibit different degree in their peralkaline character. Trachytes and phonolites range from weakly to moderate peralkaline tendency, especially distinguishable by the absence or presence of normative aegirine and natrium silicate. Despite the normative calculations, results must be taken with caution at the time to perform geochemical evolutive conclusions and unfortunately, due to the variable alteration degree in almost all sampled rocks, uncertainties in the conclusions are unavoidable. Related to that, normative Q and C must be interpreted not as a result of primary silica oversaturation or initial high alumina content but as latter meteoric and hydrothermal alteration marks occurred in the area.

Trace elements

Table 7 shows abundances of trace elements in the TPC felsic rocks. Trace elements behaviour in differentiated magmas was subject of previous works in the Canary Archipelago (Brandle, 1973; Neumann et al, 1999; Schmincke, 1987; Wolff, 1984; Wolff and Palacz, 1989; Wolff et al, 2000, Zafrilla, 2001 among others). Chondrite-normalized trace elements diagram for Upper Old Basalts and dykes and domes from the TPC were plotted in Fig. 9. Some trace elements abundances in the basalts, was plotted with data from Brandle and Cerqueira (1975). Trachytes and phonolites from TPC show enrichments in Nb, Ta, Th and relative depletion in Rb, K, Pb, Sr and Ti. Zr, which is incompatible by its high solubility in alkaline felsic liquids (Watson and Harrison, 1983), shows strong enrichments in these diferenciated liquids as well. Selected trace elements patterns against Zr content were plotted (Fig. 10). Most of felsic samples exhibit Zr content varying from 1000 to 2000 ppm. There is a clear correlation between the Zr and Ce, La, Rb, Th enrichments. By plotting trace elements abundances in the Upper Old Basalts together with their abundances in the contemporaneous TPC, an initial increase followed by a decrease in Ba and Sr as Zr content rise up can be noted. Respect to the similar behaviour of these two last elements, a diagram of Ba/Sr vs. Zr seems to show an initial ascending trend which is followed by dropping of the Ba/Sr ratio towards the highest Zr abundances. Nb/Ta ratio exhibits no clear correlation with the Zr.

REE elements

REE data from the TPC felsic rocks are displayed in Table 8a. Chondrite-normalized REE plot of felsic dyke and dome samples is presented in Fig. 11. Contents have been normalized with the normalization factors of Taylor and McLennan (1985). Additionally, Syenites and alkali gabbroids from Tamargada were also plotted for

comparison with the TPC trachytes and phonolites. As can be seen in the figure, an initial strong descendant tendency from strong enrichments in LREE to MREE contrast with a moderate to weak declining progression from MREE to HREE. In general, negative Eu anomaly characterizes the nepheline phonolites and trachytes. Even though haüyne phonolites develop similar trend, they show minor REE contents and contrary to the rest of samples do not have Eu anomaly or this one seems to be slightly positive.

Concerning to the syenites problem referred before, additional chondrite-normalized REE plot of the Tamargada syenites and alkali gabbroids was included in the diagram for comparison with respect to the felsic volcanic samples. The Tamargada syenites exhibit weak enrichment in LREE and HREE and depletion towards the MREE with a little positive Eu, Gd and Tb anomalies while alkali gabbroids show a general trend from high contents in LREE to lower in HREE. These variations resemble in part, those exhibit in other syenites, such as those from Benijo (Taganana) in the neighbouring Tenerife Island (Table 8b) and contrast with respect to the ones from Las Cañadas area (data from Wolff et al., 2000). Both Tamargada and Benijo syenites are at lest of mid Miocene age and suffered latter intrusive magmatic, hydrothermal and alteration processes along periods of time that probably has distorted their original REE distribution while quaternary syenites from Las Cañadas show a clearly different pattern, reflecting in part, their more recent magmatic history and the lack of low degree hydrothermal metamorphic processes that affected the others. Despite the broad distribution trends, it is noted an unexpected strong variations and anomalies especially significant towards the HREE in the Tamargada samples which, strongly contrast with a more regular pattern of the TPC trachytes and phonolites. That, suggest distinct evolutionary processes affecting the syenites and support for the previous assumption in which, the magmatic cycle of the Tamargada alkaline intrusions is assumed to be different and earlier to the one of the TPC felsic sheet intrusion.

Geochemical implications of data analysis for the suggested evolution model

Although geochemical data are masked by latter alteration of rocks, taking into account the temporal correlation between the felsic complex (TPC) and the Upper Old Basalts, a genetic link seems to exist from the basalts to the felsic formation. In the assumed geological context of the TPC, this formation rises up as a consequence of a magmatic evolution from the Upper Old Basalts. Examining the Fig. 8, it seems that this evolution departs from alkali basalts towards trachytic and phonolitic terms through a dominant fractional crystallization process. As can be expected in this process, an increase of alkali compounds is followed by an increase in the silica content as can be seen in the Fig. 8. A general decrease in total FeO, MgO, CaO, TiO2 and increase in SiO2, Al2O3, Na2O and K2O is exhibit as the differentiation index (Thornton and Tuttle, 1960) grows up (Fig. 12). The behaviour of some trace elements in the differentiated magmas seems to be in accordance with a differentiation by fractional crystallization. This is visible in the Fig. 10 where, in the basis of that Zr content, as best example of incompatible element, is indicative of fractionation degree by the high solubility of zircon in alkaline felsic liquids (Watson and Harrison, 1983), other trace abundances such as La, Ce, Rb, Th increase as Zr increases. Ba and Sr show initial tendency to increase followed by decreasing towards the most differentiated terms. Both tendencies are correlated in the Ba/Sr vs. Zr diagram where a high content of Ba relative to Sr exhibit a maximum value in the less differentiated felsic rocks. Berlin and Henderson (1969) explain that behaviour by the affinity of these trace elements to the feldspar lattices. Initially, dominant crystallization of ferromagnesian minerals makes Ba and Sr

to progressively accumulate in the residual liquids. But due to latter addition of Ba into the feldspars with respect to Sr, initial Ba/Sr ratio increases while Ba remains in residual liquid. After Ba begins to incorporate into the feldspar lattices, increasing of Ba/Sr ratio stop and afterwards declines with the higher tendency of Sr to remains in the residual liquid with respect to the one exhibit by Ba.

Despite the general mechanism of magmatic evolution by fractional crystallization, other geochemical features such as the presence of haüyne phonolites with their petrological characteristics must be explained in the context of other secondary magmatic processes. As described above, a little outcrop of haüyne phonolites occurs inside the TPC showing petrological and geochemical patterns that contrast with respect to the rest of the felsic rocks. These rocks appear more depleted in REE contents when compared to the nepheline phonolites and trachytes. About their appearance, two alternative hypotheses could be considered here: a) they are linked to another magmatic cycle. As they intrude trough felsic domes of the TPC, this magmatic cycle should be later than the TPC. Following this idea, Hernandez-Pacheco (personal communication) pointed at to a possible correlation between these haüyne phonolites and those outcropping in the neighbouring Teno massif (Tenerife Island), in the form of peripheral intrusions of these in La Gomera Island. This assumption consider that where haüyne felsic rocks appear (Tenerife or La Palma Island), the associated intermediate rocks are haüyne-bearing rocks and the basic terms show peculiar patterns that differs from the ones which, in evolutive terms, starts from alkali basalts and evolve to trachybasalts and finally to trachytes and nepheline phonolites, b) They represent the last differentiated terms of the TPC magmatic cycle where local contamination processes made the haüyne-bearing phonolites to occur. Appearance of these kind of rocks where described previously for alkaline suites by Koster Van Gross and Wyllie (1966), Wellman (1968), Kogarko and Ryabchikov (1969), Kogarko (1974). In this case, high local concentration of volatile compounds such as chlorine, fluorine, sulphur, during the late stages of magmatic evolution by a gas transfer mechanism, became nepheline phonolites into haüyne phonolites. Also, high local concentration of Ti in these haüyne phonolites by this transfer mechanism will favour the development of sphene phenocrysts, as the observed in these rocks. Geochemical differences such as a slightly increase in Mg, Ca, Ba, Rb, Sr and depletion in Zr, REE could be explained by contamination from olivine-augite basalts xenoliths, which are commonly visible in these rocks showing different assimilation degrees.

Nowadays, neither chronological evidences to assess that haüyne phonolites do not belong to the TPC magmatic period nor appropriate intermediate rocks to explain an evolutionary trend to haüyne phonolites exist. General alteration of samples makes geochronological criteria usage unavailable. Despite the lack of data and on the basis of field data, our opinion is that these haüyne differentiates were originated by the b) type mechanism as an anomalous local process superimposed to the differentiation by fractional crystallization.

Discussion and conclusions

Cendrero (1971) defined the TPC as the oldest subaereal volcanic episode occurred in the island and concluded that after an intense erosion process, within which ultramaphic plutonic rocks from the Basal Complex raised to the surface, a series of eruptions emitted blocky lava flows and domes cut by dykes and a new series of domes that originated the visible chaotic breccias.

In the context of this work and mainly in the basis of field studies, the proposed evolution of the TPC is slightly different because it not represent the oldest volcanic episode but previous emissions of basalts in a former shield stage ( Lower Old Basalts) do it. After intense erosion a new volcanic cycle formed a new basaltic shield edifice that hosted the subvolcanic felsic complex (TPC) in the mid-upper Miocene. A general explanation supported in the field criteria revealed that during the development of the second volcanic basaltic cycle, a differentiated magmatic body or bodies system developed and extended from NE-SW. That initiate the following sequence of events: 1) a first intrusive episodes of the TPC intruding through the Basal Complex and the residual Lower Old Basalts of the first shield stage. After a dominant NE-SW striking intrusive felsic volcanic episodes that caused debris avalanches deposits in surface and partial erosion of the second shield volcanic edifice, the magmatic activity focussed in a magmatic body located roughly 1 km SSE from Vallehermoso village; 2) the new magma chamber triggers uplift of a central area by a rapid intrusion of felsic dykes leading to the establishment of the cone sheet swarm. The dykes intruded with a dominant dip more than 65º. Less common almost horizontal sills are present masking the whole conic pattern; 3) a final stage of intrusions drive to the emplacement of a series of very thick nepheline-phonolite dyke-like domes with a distinctive “shark-wing” shape. This last intrusion stage took place in a similar conic pattern that crop out about 500-1500 m from the centre. Late marginal haüyne-phonolite domes coming from different and more localized differentiation processes intruded in a final intrusive phase. Probably, during these last stages of intrusion, a collapse in the central sector of the sheet intrusion started, leading to the formation of a caldera collapse, roughly of 3 to 4 km in diameter. Consequently, a 2 km in diameter central sector of debris-avalanche breccias covered the core of the cone-sheet with outcrops of tectonic breccias distributed along their margins. Intrusion breccias are also visible around the domes (Fig. 13).

Cueto et al (1994), in the basis of previous chronological data, supposed that the Vallehermoso caldera collapse started after the cone-sheet complex intrusion and slightly previous to the upper-Miocene to lower-Pliocene felsic intrusions (faintly older than 4.4 Ma). As they assumed that cone-sheet intruded within the 10.2 Ma to 4.6 Ma temporal range and taking into account that there was not found any other way but field criteria to establish a more precise age for the cone-sheet intrusion, it can be presumed that collapse episodes were triggered after rapid migration of felsic magma from the magma chamber to develop the cone-sheet structure and latter roof instability.

The geometry of the cone-sheet was previously identified by Rodriguez-Losada (1987; 1988) as an incomplete structure undefined or poorly visible along the NW sector centred to the SSE of Vallahermoso village and extended around 12 km in diameter. The conic sheet intrusion was interpreted as superimposed to a major NE-SW pattern. In this occasion, an absence of other associated sheet intrusions was emphasized. In a latter date, Hernan et al (2000) suggest that “the cone-sheets were originally connected to a hypothetical dome-shaped magmatic body whose uppermost part is at present located 1350 m under sea level”. In addition, they disagree with respect to Rodriguez-Losada (1987, 1988) assessing that the cone-sheet exhibit a complete circular pattern and is not open to the NW but closed along the entire perimeter and the less abundance of dykes at the NW sector is only apparent and due to latter deposits that cover it.

Huertas et al. (2000), opposing to the interpretation from Rodriguez-Losada (1987, 1988) found that two radial dyke swarms exist. One of them, centered on the Tamargada area while the other radial dyke swarm center is located around 4 km WSW,

coincidental with the center of the cone-sheet complex. They assessed that an ENE to WSW migration of felsic radial-pattern centers occurred prior to the establishment of the felsic conic dykes and the older one (at the eastern side) is probably related to the Tamargada syenites. This last analysis suggest that a ENE to WSW migration of felsic volcanic systems took place, focussing the felsic activity first in the Tamargada area, roughly where the syenites crops out, and later evolving towards the WSW, at the south from Vallehermoso village.

Related to the first sentence, in which, an entire circular pattern is visible, we disagree with that assessment. After latter field analysis in the area, we confirmed that the NW sector is not only uncovered but also it allows to a deeper access to the root of the volcanic system and no arcuate pattern of dyke intrusion is discernible to complete the in surface circular pattern of the cone-sheet. A real and perceptible fact is the lack of conic pattern in the NW sector. Explanation in order to justify this absence by masking of latter covering deposits is, in our opinion, incorrect. Just at the contrary, no covering deposits exist in the Vallehermoso valley towards the NW from Vallehermoso village, where the deepest erosion levels of the area are accessibles. Probably, another explanation can be argued to explain this uncompleted cone sheet but not by covering sediments.We agree with these authors that the lack of dyke intrusion towards the NW sector is only apparent and probably the cone sheet was complete but we argue that this is due to an erosive explanation instead of masking by sediments. Another point of disagreement is the related to the decreasing deep with the increasing distance to the center of the cone-sheet taking into account that deep of dykes maintain a constant value from the surface to the magmatic focus. In the basis of this model, Hernan et al (2000) calculated the deepness of the uppermost part of the magmatic body as located 1350 m under sea level. In our studies, we have noted an almost constant and strong deep, not dependent on the distance from the dyke swarm centre, according to a model similar to the proposed by Gudmundsson (1998), characterised by curved inclined sheets concave toward the inner sector of the dyke swarm. This is in accordance with the interpretation of Gudmundsson (2002) in which, under a general tensile stress, the general dip of a central sheet intrusion becomes steeper. In that case, prior and probably during the cone-sheet emplacement, a general tensile stress due to the ENE-WSW felsic intrusion was acting in the area. Thus, the proposed geometry by Hernan et al., is in disagreement with the proposed geometry in this work. In addition, the similitude between the cone-sheet from La Gomera and the one existing in the neighbouring Gran Canaria Island ( the Tejeda cone-sheets) concludes when the geometry of the last one is in accordance with the model from Phillips, 1974, where the dip of the curved tilted sheets decreases as the distance from the sheet centre increases, contrary to the TPC, where this phenomenon is not visible.

Another topic for discussion comes from Huertas et al., (2000), where they found two radial dyke swarms focussed first near Tamargada and afterwards towards the WSW (South from Vallehermoso). This migration pattern is in agreement with the occurrence of a ENE-WSW dyke intrusion pattern studied previously by Rodriguez-Losada (1987, 1988), developed prior to the cone-sheet intrusion. In our opinion, both radial dyke swarms are not clear enough to assess about their existence and another alternate interpretation for the, in our opinion, existence of apparent radial patterns is the structural distortion caused by the latter cone-sheet intrusion through the preceding ENE-WSW intrusive system that reciprocally and as was previously advised, conditioned the cone-sheet geometry.

As concluding remarks, several points define the features of the Vallehermoso caldera felsic complex.

1. The existence of a highly eroded caldera collapse ranging 3-4 km in diameter with chaotic breccias dominated central sector that is surrounded by a 2 km diameter semicircular discontinuous outcrop of nepheline phonolite domes resembling thick dykes with “shark wing” shapes. Geochronological data (Cantagrel et al., 1984) and field relations point to a Mid-Upper Miocene age for the TPC.

2. The caldera is related with the second oldest magmatic felsic cycle that constitutes the so-called “Trachytic-Phonolitic Complex (TPC)” (Rodriguez-Losada, 1988) or the “Trachytic-Phonolitc Series” (Cendrero, 1971). Considering the major elements variations diagrams of the felsic cycle together with the ones from the Upper Old Basalts and based on the field relationships and available geochronological data it seems that the trachytic and phonolitic dominated magmas derived from basaltic magmas by fractional crystallization processes in the Upper Old basalts cycle. Minor appearance of other felsic magmas such as haüyne phonolites can be related to later anomalous differentiation processes such as local gas and volatile elements transfer.

3. Appearance of intermediate rocks is scarce. Occasionally, basaltic to trachybasaltic xenoliths of diffuse margins are discernible into the trachytes and phonolites. In some cases, olivine xenoliths are also visible into phonolitic rocks. The low occurrence of intermediate lithologies remains unclear. A possible explanation could point to an incomplete field sampling combined to partial assimilation processes of intermediate magmas to produce maphic trachytes also present in the TPC.

4. In order of account, first trachytes and second nepheline phonolites are the dominant rocks in the area with minor maphic trachytes and occasional haüyne phonolites. Most of then exhibit aphanitic textures and less frequent porphiritic ones. Usually, basaltic and trachybasaltic xenoliths are distinguishable in the felsic rocks and most commonly in maphic trachytes. A special case is remarkable in the haüyne phonolites where basaltic xenoliths are more abundant than in the rest of rocks.

5. Highly altered areas in the felsic rocks are common. The alteration processes became the felsic rocks into a white and dusty rock where alkaly feldspars are still present as relict primary minerals. X-ray diffraction analysis shows appearance of kaolinite, illite, montmorillonite, chlorite, analcime and calcite. Probably, they represent a transitional alteration stage that could evolve to more stable mineral phases such as kaolinite and gibbsite.

6. A potential co-genetic linkage between the Tamargada syenites and the rest of felsic rocks of the TPC is refused in this work. On the basis of the REE diagrams where distribution patterns on syenites and in trachytes and phonolites are quite different and considering the occurrence of low grade metamorphic processes affecting the syenites that are no visible in the trachytes and phonolites, it seems clear that no connection exist between felsic plutonic and volcanic rocks. In that case, a previous magmatic felsic cycle and the former one, probably related to the basal complex evolution, took place in the La Gomera Island. A second (Mid to Upper Miocene) was the studied here and a latter one (Upper Miocene to Lower Pliocene) matching with the recent felsic domes and lavas (“Roques Series” from Cubas, 1978).

7. The geometry of the cone-sheet results in an incomplete circular pattern at the surface. The structure open at the NW sector can be explained by deeper erosion processes towards this sector. No clear dip variation can be noted from the centre to the peripheral sectors. The characteristic strong dip (higher than 65º towards the centre) independent of the horizontal distances to the centre suggests a curved conic pattern concave to the inner part of the cone-sheet. Considering the classical model of Anderson (1936) and the most recent from Phillips (1974) and Gudmundsson

(1998), the TPC cone-sheet matches with the Gudmundsson model. According to Gudmundsson (2002), the existence of a previous field of tensile stress by the ENE-WSW felsic intrusion could condition the resultant geometry of the La Gomera TPC cone-sheet complex.

Acknowledgements

We thank

References

Abdel Monem, A., Watkins, N.D., Gast, W., 1971. Potasium Argon ages, volcanic stratigraphy and geomagnetic polarity history of the Canary Islands: Lanzarote, Fuerteventura, Gran Canaria and La Gomera. Am. J. Sci. 271, 490-521.

Ancochea, E., Huertas, M.J., Cantagrel, J.M., Coello, J., Fuster, J.M., Arnaud, N., Ibarrola, E., 1999. Evolution of the Cañadas edifice and its implications for the origin of the Cañadas Caldera (Tenerife, Canary Islands). J. Volcanol. Geotherm. Res. 88, 177-199.

Anderson, E.M., 1936. The dynamics of formation of cone sheets, ring dykes and cauldron subsidence. Proc. R. Soc. Edingurgh 56, 128-163.

Berlin, R., Henderson, C.B.M., 1969. The distribution of Sr and Ba between the alkali feldspars, plagioclases and groundmass phases of porphyritic-trachytes and phonolites. Geochim. Cosmochim. Acta 33, 247-255.

Blumenthal, M.H., 1961. Rasgos principales de la geología de las Islas Canarias con datos sobre Madeira. Bol. Inst. Geol. Min. of Spain (IGME) 72, 1-130.

Brändle, J.L., 1973. Evolución geoquímica de los materiales sálicos y alcalinos de la isla de Tenerife. Estudios Geológicos 29, 5-51.

Brändle, J.L., Cerqueira, M.I., 1975. Nota sobre la distribución de elementos menores en los magmas basálticos de las series antiguas de las Islas Canarias. Estudios Geologicos 31, 375-383.

Bravo, T., 1964. Estudio geológico y petrológico de la isla de La Gomera. Estudios Geológicos 20, 1-57.

Buch, L.v., 1825. Physikalische besehreibung der Kanarischen Inseln. Berling 201 p.Cantagrel, J.M., Cendrero, A., Fuster, J.M., Ibarrola, E., Jamond, C., 1984. K-Ar

chronology of the volcanic eruptions in the Canarian Archipelago: Island of La Gomera. Bull. Volcanol. 47-3, 597-609.

Cendrero, A., 1971. Estudio geológico y petrológico del complejo basal de la isla de La Gomera (Canarias). Estudios Geológicos 27, 3-73.

Cox, K.G., Bell, J.D., Pankhurst, R.J., 1979. The interpretation of Igneous Rocks. George Allen and Unwin, London, 450 pp.

Cubas, C.R., 1978. Estudio de los domos sálicos de la isla de La Gomera (Islas Canarias). I. Vulcanología. II. Geoquímica. Estudios Geológicos 34, 53-70, 107-129.

Cubas, C.R., Hernan, F., Ancochea, E., Brändle, J.L., Huertas, M.J., 1994. Serie basáltica antigua inferior en el sector de Hermigua. Isla de La Gomera. Geogaceta 16, 15-18.

Cueto, L.A., Barrera, J.L., Gomez, J.A., 1994. La Caldera de hundimiento de Vallehermoso, isla de La Gomera (Canarias). Boletín Geológico y Minero 105-4, 7-12.

De La Nuez, Cubas, C.R., Hernan, F., 1989. Los domos sálicos del Parque nacional del Teide. In Araña, V. And Coello, J. (Editors): Los volcanes y la caldera del Parque Nacional del Teide (Tenerife, Islas Canarias). ICONA (Spain), 177-185.

Feraud, G., 1981. Datations de reseaux de dykes et de roches volcaniques sous-marines par les methodes K-Ar et 40Ar-39Ar. Utilisation des dykes comme marqueurs de paleocontraints. Thesis. Univ. Nice, 146 pp.

Fernandez Navarro, L., 1918. Sur la constitution de l’île de Gomera. C.R. Ac. Sci., Paris, 167, 1038-1040.

Fernandez Santin, S., 1979. Procesos de propilitización en rocas salicas del complejo basal de la isla de La Gomera (Canarias). III Asamblea Nacional de Geodesia y Geofísica III, 1633-1654.

Fritsch, K.V., 1867. Reisebilder von der Canarischen Inseln. Peterman`s Geographischen Mittheilungen, 22.

Gagel, C., 1925. Begleiwort zu der karte von La Gomera mit einen anhang über die calderafrage. Z. Deutsch Geol. Ges. A. Abhandlungen 77, 551-575.

Ghiorso, M.S., Sack, R.O., 1991. Fe-Ti oxide geothermometry: thermodynamic formulation and the estimation of intensive variables in silicic magmas. Contrib. Mineral. Petrol. 108, 485-510.

Gudmundsson, A., 1998. Magma chambers modeled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion statistics. J. Geophys. Res. 103, 7401-7412.

Gudmundsson, A., 2002. Emplacement and arrest of sheets and dykes in central volcanoes. J. Volcanol. Geotherm. Res. 116, 279-298.

Hausen, H., 1960. Las calderas canarias. Ann. Est. Atlanticos 6, 133-194.Hernan, F., 1976. Estudio petrológico y estructural del complejo Traquítico-Sienítico de

Gran Canaria. Estudios Geológicos 32, 279-324.Hernan, F., Velez, R., 1980. El sistema de diques cónicos de Gran Canaria y la

estimación estadística de sus características. Estudios Geológicos 36, 65-73.Hernan, F., Cubas, C.R., Huertas, M.J., Brändle, J.L., Ancochea, E., 2000. Geometría

del enjambre de diques cónicos de Vallehermoso, La Gomera (Islas Canarias). Geogaceta 27, 87-90.

Huerta, M.J., Brändle, J.L., Ancochea, E., Hernan, F., Cubas, C.R., 2000. Distribución de los diques sálicos del Norte de la isla de La Gomera. Geogaceta 27, 91-94.

Ibarrola, E., 1970. Variabilidad de los magmas basálticos en las Canarias orientales y centrales. Estudios Geológicos 26, 337-399.

Jeremine, E., 1935. Contribution a l’etude des iles Hierro et Gomera. Bull. Soc. Franc. Miner. Paris, 58, 350-363.

Kogarko, L.N., Ryabchikov, I.D., 1969. Peculiarities of alkaline magmas rich in volatiles. Geokhimia, 1439-1450.

Kogarko, L.N., 1974. Role of volatiles. In Sorensen, H. (Editor): The alkaline rocks. John Wiley & Sons. 474-487.

Koster van Gross, A.F., Wyllie, P.J., 1966. Liquid inmiscibility in the system Na2O-Al2O3-SiO2-CO2 at pressures to 1 Kbar. Am. J. Sci. 264, 234-255.

LeBas, M.J., Le Maitre, R.W., Streckeisen, A., Zanettin, B., 1986. A chemical classification of igneous rocks based on the total alkali-silica diagram. J. Petrol. 27, 745-750.

Muller, W., 1930. Utersuchungen über die verwitterung und gerteinen von der insel Gomera. Chemie der Erde 4-3, 369-394.

Neumann, E.-R., Wulff-Pedersen, E., Simonsen, S.L., Pearson, M.J., Marti, J., Mitjavila, J., 1999. Evidence for fractional crystallization of periodically refilled magma chambers in Tenerife, Canary Islands. J. Petrol. 40, 1089-1123.

Phillips, W.J., 1974. The dynamic emplacement of cone sheets. Tectonophysics 24, 69-84.

Rodriguez-Losada, J.A., 1987. Un complejo de diques cónicos en la Isla de La Gomera (Islas Canarias). Estudios Geológicos 43(1-2), 41-45.

Rodriguez-Losada, J.A., 1988. El complejo traquítico-fonolítico de la isla de La Gomera ( Islas Canarias). Thesis Univ. Comp. Madrid (Spain) 463/88, 416 p.

Rodriguez-Losada, J.A., Nuez de la, J., Martinez-Frias, J., 1990. Minerales de Fe y Ti en las intrusiones alcalinas de Tamargada. La Gomera ( Islas Canarias). Revista Sociedad Geológica de España 3(1-2), 161-166.

Rodriguez-Losada, J.A., Martinez-Frias, J., 1998. Ancient oxide- and sulphide-mineralization in the islands of Tenerife and La Gomera (Canary Archipelago, Spain). Mineralium Deposita Letter 33, 639-643.

Schirnick, C., Bogaard, P.v.d., Schmincke, H.-U., 1999. Cone sheet formation and intrusive growth of an oceanic island- The Miocene Tejeda complex on Gran Canaria (Canary islands). Geology 27-3, 207-210.

Schmincke, H.-U., 1967. Cone sheet swarm, resurgence of Tejeda Caldera and the early geologic history of Gran Canaria. Bull. Volcanol. 31, 153-162.

Schmincke, H.-U., 1987. Geological field guide of Gran Canaria. Pluto Press, 126 p.Streckeisen, A.L., 1978. Classification and nomenclature of volcanic rocks,

lamprophyres, carbonatites and melilitic rocks. Neues Jahrbuch für Mineralogie Abhandlungen 134, 1-14.

Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312 pp.

Thornton, C.P., Tuttle, O.F., 1960. Chemistry of igneous rocks I: Differentiation Index. Am. J. Sci. 258, 664-684.

Walker, G.P.L., 1999. Volcanic rift zones and their intrusion swarms. J. Volcanol. Geotherm. Res. 94, 21-34.

Watson, E.B., Harrison, T.M., 1983. Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth Planet. Sci. Lett. 64, 295-304.

Wellman, T., 1968. Stability of sodalite in teh system Na2O-AlSiO4-NaCl-H2O. Am. Geoph. Un. Trans. 49, 342-343.

Woff, J.A., 1984. Variation in Nb/Ta during differentiation of phonolitic magma, Tenerife, Canary Islands. Geochim. Cosmochim. Acta 48, 1345-1348.

Wolff, J. A., 1986. Welded-tuff dykes, conduit closure and lava dome growth at the end of explosive eruptions. J. Volcanol. Geotherm. Res. 28, 379-384.

Wolff, J.A., Palacz, Z.A., 1989. Lead isotope and trace element variation in Tenerife pumices: evidence for recycling within an ocean island volcano. Mineral. Mag. 53, 519-525.

Woff, J.A., Grandy, J.S., Larson, P.B., 2000. Interaction of mantle-derived magma with island crust? Trace element and oxygen isotope data from the Diego Hernandez Formation, Las Cañadas, Tenerife. J. Volcanol. Geotherm. Res. 103, 343-366.

Zafrilla, S., 2001. Relationships between magmatic evolution and phonolitic volcanic activity in the Las Cañadas edifice, Tenerife, Canary islands. Thesis. Univ. Barcelona. 306 p.