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: +34-922-318311
E-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 FernandezNavarro (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 upperMiocene 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 ENEWSW 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 MartinezFrias (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 conesheet 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. Chondritenormalized 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 chondritenormalized 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 “sharkwing” 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 conesheet 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
2.
3.
4.
5.
6.
7.
dykes with “shark wing” shapes. Geochronological data (Cantagrel et al., 1984) and
field relations point to a Mid-Upper Miocene age for the TPC.
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.
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.
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.
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.
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).
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
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