GEOCHEMICAL AND PETROGRAPHIC CHARACTERIZATION OF BASALTS SOUTH OF THE AZUERO-SONÁ FAULT, IN THE AZUERO
PENINSULA, PANAMÁ
GEOCHEMICAL AND PETROGRAPHIC CHARACTERIZATION OF BASALTS SOUTH OF THE AZUERO-SONÁ FAULT, IN THE AZUERO
PENINSULA, PANAMÁ
María Margarita Ariza Acero
Graduation Project presented to Los Andes University to obtain the title of Geoscientist
Idael Francisco Blanco Quintero, PhD. Camilo Montes, PhD.
University of the Andes
School of Sciences, Department of Geosciences Bogotá D.C. – Colombia
ABSTRACT
Petrographic and geochemical analysis with geological mapping and some field relationship allow to propose a new tectonic arrangement for the Azuero Accretionary Complex (AAC). Basalts from three locations on the AAC were analyzed: Honda creek, Boca del Quebro beach and Varadero river. All the samples were classified as lava flows of tholeiitic basalts, some with amygdalar texture and slightly hydrothermally alterated. They also exhibit an OIB-like signature, that may correspond also to the enriched group of the Azuero Plateau but further geochronological analyses maybe required to distinguish between these two. In addition, the sample from Varadero river exhibits a little alkaline affinity and an enrichment in REE that maybe associated with the third volcanic stage of la Hoya ocean island. This compositional and textural differences recognized within samples of Honda Creek and previous reports of gabbroic bodies on this locality may lead to the recognition of a third ocean island in the AAC, besides the two previously reported.
RESUMEN
El análisis petrográfico y geoquímico con mapeo de terreno y alguna relación de campo de campo permiten proponer un nuevo arreglo tectónico para el Complejo Acrecionado de Azuero (CAA). Se analizaron los basaltos de tres localidades en el CAA: quebrada Honda, playa Boca del Quebro y río Varadero. Todas las muestras se clasificaron como flujos de lava de basaltos toleíticos, algunas con textura amígdular e hidrotermalmente alteradas. También presentan una firma similar a OIB, que puede corresponder también al grupo enriquecido del plateau de Azuero, pero es posible que se requieran análisis geocronológicos adicionales para distinguir entre estos dos. Adicionalmente, la muestra del río Varadero muestra una pequeña afinidad alcalina y un enriquecimiento en REE que puede estar asociado con la tercera etapa volcánica de la isla oceánica de la Hoya. Esta diferencia de composición, diferencias texturales reconocidas dentro de muestras de Honda Creek y reportes previos de cuerpos gabróicos en esta localidad puede conducir al reconocimiento de una tercera isla oceánica en la CAA, además de las
ACKNOWLEDGEMENTS
First of all I want to thank God for all the valuable things he give me until today: my family and friends, and the opportunity of studying in this University. To my mother who has supported me in all that I have decided to undertake, in every last night, in every happy or difficult moment, with her unconditional love and words of encouragement. To my dad for believing in my abilities and giving me all his love and understanding. To my uncles, my grandparents, my brothers, for giving me their support as they have been able, to give me many happy moments in moments of rest and moments of stress. My friends at school, ‘las viejitas’, with whom I have never lost contact and have given me so many smiles; my physicist and geoscientist friends that let me know different life points of view and different ways of share with others: for the fun moments during the field, in the lab or studying quantum mechanics.
To the University of the Andes for granting me the scholarship Quiero Estudiar, which I hope many others will benefit from and hope to contribute in the future, so that I can study in this great institution. To the professors of Physics and Geosciences with whom I have been able to share, and who every day show different and wonderful paths of sciencel, especially Ph .D. Camilo Montes and Idael Blanco.
To the Field Geology 2015 participants, for all the good moments that made these field trip one of the most enriching experience for my personal and academical life. To the Universidad de los Andes FAPA project of professors Blanco and Montes (P12. 160422.002/001) for financing the analysis and part of the field trips. To the University of Florida, Smithsonian Tropical Research Institute, PCP Pire, Corporación Geológica Ares, Carlos Jaramillo, Aldo Rincon, Jorge W. Moreno, Felipe Lamus, Germán Bayona, and Carlos Armando Rosero for their support during field camps and their contribution with their knowledge. To the laboratory technicians, Ivette and Sofia, for their help while making the thin sections and to the Universidad de Granada where the geochemical analysis were done.
TABLE OF CONTENTS
1. INTRODUCTION ... 1
2. CONCEPTUAL FRAMEWORK ... 2
2.1 Mid-Ocean Ridge Volcanism ... 2
2.2 Oceanic Intraplate Volcanism ... 4
2.3 Oceanic Plateaus ... 5
3. GEOLOGICAL SETTING ... 7
3.1 Caribbean Plate and its margins ... 7
3.2 Panamá Arc and the Azuero Peninsula ... 10
4. METHODOLOGY ... 13
4.1 Field Work ... 13
4.2 Thin section preparation ... 16
4.3 Samples preparation and analytical techniques ... 17
5. RESULTS ... 18
5.1 Mapping and field observations ... 18
5.2 Petrography ... 21
5.3 BSE Images and EDS analysis ... 24
5.4 Geochemistry ... 26
6. DISCUSSION ... 33
7. CONCLUSIONS ... 36
REFERENCES ... 36
1. INTRODUCTION
The Azuero peninsula is divided in two terrains separated by the Azuero-Soná sinistral fault and the Azuero melánge produced as the fracture zone of the fault (both units form ASFZ). The northern terrain is part of the Central American Arc, a natural corridor between North and South America, that is the south west border of the Caribbean Plate (Buchs et al., 2010). The South terrain, on the other hand, has been recognized as part of an accretionary complex of oceanic intraplate volcanoes (seamounts) of a Paleocene to early Middle Eocene origin that came from the Pacific, probably formed at the Galapagos hotspot (Buchs et al., 2009, 2010, 2011).
During the summers of 2014 and 2015 two fieldwork campaigns were done in the Azuero peninsula. With these, it was possible to identify and map new lithological units the area of the accreted seamounts; in particular an important body of olivinic lavas, a sedimentary unit previously associated with the Azuero Plateau (the Azuero part of the Caribbean Plate) and locally different basalt flows (with textural differences) were identified. Geochemical studies of major elements in the zone done by Buchs et al. (2009) have served to associate each unit to different stratigraphic part of the volcanic edifice and stages of the intraplate volcanism. However, these studies do not consider the units described and field observations done during the summer field campaigns. These units can account for a new limit between the Caribbean Plate and the accreted oceanic islands.
From the geochemical studies of major and trace elements, and field observations that lead to a complete map of the zone and several cross-cuts is possible to identify the tectonic setting of genesis of the basaltic flows. For these purpose, new
sampling localities are added to the existing ones, and are compiled with geochemical data of Buchs et al. (2010) to compare them with our results.
2. CONCEPTUAL FRAMEWORK
Basalts are one of the most common rocks in the Earth’s crust, specially in the oceanic floor. These are volcanic rock of mafic composition, i.e. with high content of magnesium and iron silicates and low content of silica. Usually, basaltic rocks have a porphyritic texture, with a very fine matrix and phenocrysts of different phases of pyroxenes, olivine and plagioclase depending on the chemical composition of the magma. These features¡ reflect the formation of basalts as extrusive rocks, because they result from the rapid cooling of basaltic lavas.
Basaltic magmatism occurs in a wide range of tectonic settings such as hotspots, forming flood basalts (that leads possibly to the formation of a plateau) or/and ocean island basalt, and mid-oceanic ridges (MOR). For this reason, basaltic volcanism provides a window into Earth’s mantle (White, 2015). The following sections explain briefly the magmatism in each of these scenarios, as well as some of the most important characteristics to recognize it.
2.1Mid-Ocean Ridge Volcanism
The majority of the oceanic crust is composed of a tholeiitic basalt formed at mid-ocean ridges (MORBs). These are constructive margins where two plates are separating as a result of ‘slab-pull’, the principal force that drives plate motion. Magmatism at MOR rises as a passive consequence of this motion (White, 2015).
Petrographycally, a MORB is a tholeiitic basalt with textures that range from glassy to phyric. Common MORB phenocrysts are Pl, Ol, Mg-Cr spinel. The groundmass mineralogy of MORBs is mainly constituted by Pl and Cpx microlites and Fe-Ti oxide. Chemically, MORB are distinctive basalts because of their low K2O (< 0.2% wt.) and low TiO2 (< 2.0% wt.) contents, and Mg# >65. However, within this range two types of MORBs can be distinguish with the major element composition:
• N-MORB (Normal MORB): with K2O < 0.1% wt. and low TiO2 < 1.0% wt.
• E-MORB (Enriched MORB): with K2O > 0.1% wt. and low TiO2 > 1.0% wt.
With trace element composition this separation can also be done, giving additionally part of the interpretation of the specific source if each type. In one side, N-MORB type exhibits a positive slope in the light rare earth elements (LREE) part (LREE depleted), that corresponds to a depleted mantle source. On the other side, E-MORBs show a LREE enrichment, similar to the enriched mantle xenoliths. Nevertheless, for both types high rare earth elements (HREE) overlaps, and becomes almost flat. Now, when trace element ratios are considered arises new criteria of differentiation between this two type of MORBs and other rocks of the oceanic floor. For example:
• La/Sm ratios in E-MORBs (La/Sm ~ 1.7) is greater than N-MORB ratios (La/Sm < 1.0).
• K/Ba ratios are bigger than a 100 (K/Ba>100) for N-MORB and mid 30s for E-MORB.
From this analysis, and considering isotope geochemistry, authors have concluded that MORBs have more than one source region and the mantle beneath the ocean basins is not homogeneous: N-MORB tap an upper depleted mantle, and E-MORB from the perisphere, a rheological enriched layer proposed by Anderson (1995), or
from a deeper source, probable a plume originated in a lower mantle level, such as the Core Mantle Boundary (Winter, 2015).
2.2Oceanic Intraplate Volcanism
10% of the volume of the oceanic floor is made basalts that come from oceanic intraplate volcanism. These are known as ocean island basalts (OIB), and comprises the seamounts, oceanic chains of islands such as Hawaii, and oceanic plateaus. Despite there are poor geophysical and field data, it is considered that intraplate volcanism is generated by rising mantle plumes, formed in the thermal discontinuity of the CMB (White, 2015). When the head of this plumes, of almost 800 to 1200 km of diameter, reaches the surface it produces a huge basaltic outpouring, the large igneous provinces (LIP) or oceanic plateaus in an oceanic context. The conduit, or tail, marks the hotspots, the surface expression if a plume (Winter, 2010).
Basaltic magmas are characterized by SiO2 composition of less than 45% wt. and is formed at a temperature above 1000 °C. According to composition in major oxides such as SiO2, Na2O, K2O and FeO magmas can be classified in different series depending on the content of alkalis (Na2O+K2O) vs. SiO2 and K2O vs. SiO2. The alkaline series includes those magmas that contain an excess of alkali oxides (Na2O+K2O). On the other side, magmas in the sub-alkaline series have a high content in K2O, but may have different content in Al2O3 and FeO: the tholeiitic magmas have a high content in FeO; the calc-alkaline with high Al2O3; and shoshonitic a higher content of Fe2O3. From these four, two principal magma series
groundmass phases, and Cpx phenocrysts are Mg-rich than the groundmass, if found. On the other hand, in OIA basalts olivine is more common in groundmass as well as Ti-rich augite, amphibole ans alkali-feldspar.
Geochemistry of major elements shows that OITs are similar to MORB, but the first one shows some differences. For example, for the same Mg# OITs have higher K2O, TiO2 and P2O5, and lower Al2O3 than MORB. Comparison between OIB typical series obviously lead to alkali/silica ratio different for each series. Trace element geochemists have reached to the conclusion that LIL and HFS elements are enriched in OIB magmas with respect to MORBs. Now, to distinguish between each magmatic series several ratios of these elements can be used. For example,
• K/Ba: In OIA this ratio is greater than 20 (>20), while OITs range between 25-40.
• Zr/Nb: In N-MORBs is >30, and in OIBs is less than 10.
In chondrite – normalized diagrmas this differences become evident because of the enrichment in LREE of all OIBs, similar to E-MORB, and the steepest ‘negative slope’ for OIAs (Winter, 2010).
2.3Oceanic Plateaus
The oceanic plateaus (OP) are parts of the oceanic crust with an anomalous thickness (7 – >10 km) compared with oceanic crust formed in the mid ocean ridges. For this anomalous thickness and a composition mainly of harzburgite, OP are more buoyant than the oceanic crust of MORB, so they are rarely completely subducted: some fragments remain attached to the continental margin or onto the overriding intraoceanic subduction zone (Kerr et al., 2003). Despite this fact, currently no OP prior to the Cretaceous is found, unlike continental large igneous provinces (LIP). To recognize accreted OP rocks a number of detailed geochemical
data and field observations are required, these are summarized by Kerr et al. (2000) in Table 1.
Tectonical Setting
High MgO
lavas (>14%) Lappm/Nbppm (REE) pattern Pillow lavas Tephra Layers Subaerial Eruption
Oceanic
Plateau yes ≤ 1 predominantly flat
May be common or absent
very few ocasionally
Mid-Ocean
Ridge rare ≤ 1 LREE depleted common very few no
Marginal
Basin rare ≤ 1 predominantly flat common very common no
Oceanic Island rare ≤ 1 predominantly LREE enriched present very few frequently
Volcanic
Rifted Margin yes
contain sequences with ≤ 1 and >>1
flat to LREE enriched
not all lavas
are pillowed occasional common
Arc
(continental
and oceanic) rare >>1 LREE enriched not all lavas are pillowed
very
common frequently
Continental
flood basalt yes
mostly >>1; <10% of flows ≤ 1
flat to LREE
enriched usually absent occasional always
Table 1. Diagnostic geochemical and geological characteristics of volcanic sequences of differente tectonic settings. Taken from Kerr et al. (2000).
Such LIP are formed as a result of an atypical temperature increase in a region of the mantle (>200-300°C over the surrounding material of the upper mantle) that is known as a mantellic plume that ascends from a thermal discontinuity in the deep mantle. Hence, within this plumes a mantle melt is content as a result of an increment in the temperature, with a high MgO content, which is quickly (1-2 m.y.) ejected to the surface covering a large area.
3. GEOLOGICAL SETTING
3.1Caribbean Plate and its margins
The Caribbean Plate (CP) is a fragment of a greater oceanic plateau. The pre-Farallon plate was thickened (~8 to 20 km in some parts) by two pulses of intraplate volcanism one between ~92 – 88 Ma and the other between 76-72 Ma in the Pacific Ocean (Whattam et al., 2015), probably over the Galapagos hotspot (Nerlich et al, 2014), forming the Caribbean Large Igneous Province (CLIP). In the Late Cretaceous it started to move eastward to its current position in the interamerican ocean basin, in response to the opening of the Atlantic Ocean (Buchs et al., 2010) as can be seen in Figure 1. This original spreading center was formed after the rifting of North and South America in the Jurassic (Pindell and Barrett, 1990). Part of the rock formed with this rifting seems to be subducted mainly beneath eastward advancing island arc bordering the plateau. The CLIP has been recognized by various authors such as Kerr et al. (2003) and Buchs et al. (2010 and references therein), by its geochemical signature composed mainly by flat to enriched/depleted chondrite normalized patterns for REE (Kerr et al., 2003). Its plume source is attributed to high MgO lavas.
Figure 1. Evolution of the Caribbean Large Igneous Province (CLIP) considering the subdution initiation theory for NE and SW margins. Taken from Whattam & Stern (2015).
It has been proposed that the plume head-cold lithosphere that generates the flood basalt that lead to the formation of the CLIP triggered the subduction initiation along the Caribbean borders, forming the leeward Antilles and the South Central American Arc, the natural corridor between North and South America (Whattam & Stern, 2015). Buchs et al. (2010) propose that the South Central American Arc initiated in the Late Campanian (~75 – 73 Ma) on top of Conician-Santonian (~89 – 85 Ma) plateau. After that, during Upper Cretaceous to Miocene various oceanic complexes were accreted to the west margin of the arc, such as the Azuero Accretionary Complex (AAC) (see Figure 2).
Figure 2. General setting of the Azuero Marginal Complex (AMC) and the Azuero Accretionary Complex (ACC) at the SW corder of the Caribbean Plate (from Buchs et al., 2010)
3.2Panamá Arc and the Azuero Peninsula
The Panama Arc is one of the main components of the South Central American Arc. It lies on the Panama microplate, located at the junction of this plate with Cocos, Nazcae, South American, Caribbean and Chortis Block (Buchs et al., 2010). It has been recognized by Buchs et al. (2011) that the Panamanian Arc is an example of a margin that has experienced subduction erosion due to the subduction of the Cocos Ridge, that also triggers the uplift of some of this parts. One of the most prominent features of this part of the arc is the Azuero Peninsula, separated from the main part of the arc by the San Joaquín Fault System (see Figure 3).
The Azuero peninsula comprises various tectonostratigraphic units that include various igneous and sedimentary lithologies. The northern part of the geological sequence is defined by Buchs et al. 2010 as the Azuero Marginal Complex (AMC), that comprises the Azuero Arc Group (AAG), the Azuero proto-arc Group, the Azuero Plateau (AP) and the Ocú formation. The AP is made of massive and pillowed lava flows of tholeiitic basalts, with low K2O content and ~89 – 85 Ma old (Kolarsky et al., 1995; Buchs et al., 2009). Textures of these lavas are ophitic to interstitial plagioclase (Plg), clinopiroxene (Cpx), opaque minerals and volcanic glass. Buchs et al. (2010) recognized that the AP group includes two geochemical distinct groups: The first group includes all the flat patterns in chondrite normalized REE-diagrams, that correspond to plateau like affinities. The second group has a more enriched character, a tholeiitic affinity with lower SiO2, Mg# and CaO; and higher TiO2.
The Ocú Fm. is a sedimentary formation that rest upon the AP. It is composed of foraminifera-bearing and Campanian pelagic and hemipelagic limestones with interbeds of basaltic lavas and tuffaceous units that content volcanic clasts from an intermediate-silicic volcanic source. It was deposited under subaerieal conditions, during the location of the protoarc lava flows and early stages of the development of the AAG (Buchs et al., 2011).
In the middle part of the region the Azuero-Soná Fault Zone divide the peninsula in two, separating the AMC as the authoctonus part from a Paleocene to early-middle Eocene accreted complex of ocean islands known as the Azuero Accretionary Complex (AAC) in the SW corner of the peninsula. The fault zone is a major tectonic feature of 1-2 km of wide, formed in the fracture zone of the Azuero-Soná Fault (ASF) by mafic and sedimentary rocks highly faulted, as well as fault rocks. The ASF is a sinistral strike-slip fault with a normal component
(Pérez-Ángel, 2014) characterized by general NW-SE strike and high angle dips, with left-lateral strike-slip motion during Plio-Pleistocene.
The AAC is an example of dismemberment oceanic island volcano. It is principally an igneous marine complex overlapped unconformable by the Tonosí and Covachón Formations. This include five lithologic assemblages defined by Buchs et al. (2011): (1) submarine massive and pillowed lava flows with minor occurrence of pelagic and hemipelagic calcareous sediment and hyaloclastite; (2) submarine sheeted lava flows and scarce pillow lava locally interbedded with shallow marine limestones; (3) clastic deposits composed of basaltic breccias crosscut by basaltic dykes; (4) subaerial massive lava flows locally deposited on top of shallow marine limestones and clastic deposits; and (5) large gabbroic intrusions and dense networks of dykes that crosscut both submarine and subaerial sequences (see Figure 4). Low metamorphism in these units is related to shallow accretion of the complex igneous rocks here are predominantly tholeiitic and alkali basalt and/or gabbro. The author distinguished distinct groups, each one with a distinctive signature. Between these groups two several dismembered, spatially well distributed islands were identified when stratigraphic and field observations were considered: La Hoya island and Punta Blanca accreted in the Middle Eocene. The islands where emplaced along a rift zone during the formation of the island.
Figure 4. Map of lithologic assemblages found in the Azuero Accretionary Complex. Taken from Buchs et al. (2011).
4. METHODOLOGY
4.1Field Work
The south area of the Azuero Peninsula, the part at the south of the Azuero-Soná Fault, was studied in detail during twenty-three days in the 2015 field session of the Azuero project. During this time mapping, sample recollection of in-situ rocks and structural analysis in the zone were made to establish different structural and lithostratigraphic relations between the basaltic lava flows and the other units present in the Azuero Accretionary Complex. For this field work purpose, nine representative fresh rock samples of basalts, including pillow lavas and almost laminar flows, were collected to make thin sections for petrographic analysis and whole-rock chemistry by X-ray fluorescence (XRF), for major elements chemistry,
and inductively coupled plasma mass spectrometry (ICP-MS), for trace elements composition.
Table 2. List of selected samples and its locations to study in this work. Includes the laboratory procedures done for each sample.
During the fieldwork twelve different zones were visited. Of these, seven were selected for the study of basaltic rock because of their structural position and composition previously registered by Buchs et al. (2011), and their easy access described on the field camp session of the Azuero project in 2014.
Sample ID Locality Place Latitude Longitude THIN SECTION
ICP XRF
40620 994212 Boca del Quebro Beach 7.4360 -80.9169 X 40810 993002 Honda Creek 7.4651 -80.8649 X 40719 994608 Quebro River 7.4612 -80.8118 X 40811 993406 Honda Creek 7.4781 -80.8630 X
40409 994202 Honda Creek 7.4602 -80.8657 X X X 40402 994600 Varadero River 7.2783 -80.8636 X X X 40625 993016 Honda Creek 7.4695 -80.8640 X X X 40518 993014 Honda creek 7.4722 -80.8650 X
These localities were Honda and Guayabo Creeks, and Quebro River in in Honda Serranía, Varadero River, Varaderito River, La Playita River and Pavo River. The remaining localities were selected during the field as complementary transects to get a better coverage of the whole area and then an improvement of the geological map of the zone, previously charted by Buchs with less detail in La Honda Serranía. These localities corresponds to Caña Brava Creek, Boca del Quebro Beach, Loma de los Monos and La Barra. The zones selected for sampling for the discussion in this work were Honda Creek, Quebro River, Boca del Quebro Beach, and Varadero River, because the rocks of these places show textural differences in hand sample, and can be used as criteria to distinguish between the different parts of the accretionary part (view Figure). Samples taken from each of the locations mentioned above are listed in Table 2, and some pictures of the visited locations are shown in Figure 4.
Figure 5. Pictures of some of the outcrops visited during the field work. A- La Playita River; B- Guayabo limestones and cherts; C- Picritic lava flows in Guayabo River; and D- Beach outcrops of lava flows interbedded with hemipelagic units. Photos by Gina Roberti and Carlos Rosero.
4.2Thin section preparation
Eight thin sections of the basaltic lava flows in the Azuero Peninsula were done for the petrographical characterization. The thin sections were prepared using the standard procedures at Sample Preparation Lab at University of the Andes as
A
B
C
product, the thinner block adhered to the glass, was reduced to a minimum thickness of 30µm using a diamond polishing disc and silicon carbide of 1000, 600 and 300. Additionally, sample 40518 was used to prepared a polished thin section in Spain during a course of preparation of thin sections of this type.
4.3Samples preparation and analytical techniques
Powder whole-rock samples were obtained by grinding the rock in an Agatha mill. Major elements oxides and Zr compositions were determined on glass beads method, made of 0.6g of powder sample diluted in 6g of Li2B4O7 by a PHILIPS Magix Pro X-ray fluorescence (XRF) equipment at the Centro de Instrumentación Cienctífica (CIC) at the Univesity of Granada. Accuracy was better than ±1.5% for concentrations 10 wt. %. Precisions for Zr and LOI was better than ±4% at 100ppm concentration. The analyses were recalculated to an anhydrous 100 wt. % basis. Trace elements, except Zr, were determined at the CIC at the University of Granada by ICP-Mass Spectrometry (ICP-MS) after HNO3+HF digestion of 0.1000g of sample powder in a Teflon -lined vessel at 180ºC and 200 P.S.I. for 30 minutes evaporation to dryness, and subsequent dissolution in 100 mL of 4 vol. % HNO3.
The scanning electron microscope (SEM) images and energy-dispersive X-ray spectroscopy (EDS) were obtained at Centro de Estudios Geofísicos at Pontificia Universidad Javeriana, from polished-thin section recovered with graphite in the Centro de Microscopia at Universidad of the Andes, with the high-resolution scanning microscope ZEIZZ EVO - HD15, operated at an accelerating voltage of 15-20 kV and 120x to 600x magnification, with a 3.0 nm spatial resolution.
Data are used to apply CIPW norm using the Excel spreadsheet and to construct the different classification and tectonic discrimination diagrams, and also REE and
spider diagrams for trace element compositions. The results are compared with most of the geochemical analysis made on the Azuero peninsula Worner et al. (2009), Corral et al. (2010), Wegner et al. (2011), Buchs et al. (2010, 2011). The geochemical data is divided into four principal regions: the AP, AAG, proto-arc and AAC. This lead to determine the possible tectonic evolution of the southern accretionated part of the Azuero peninsula.
5. RESULTS
5.1Mapping and field observations
The field observations made infield campaign of 2014 and, specially, 2015 are collected and compiled in the geological map of the work area on the Azuero Peninsula done during the field and corrected and digitized by Ortiz-Guerreo (2015) as her thesis project. This one is presented in Figure 7 with the numbers of the localities as a simplified version, and a complete version can be found in the Appendix. The structural cross sections constructed considered the fault in the AAC interpreted by Ayala (2014) using gravity anomalies measurements. Evidence of this fault was observed in 2015 field, where a fault breccia of the fracture zone of this fault and serpentinite associated of it were found in the Caña Brava creek (see Figure 6).
Figure 6. Fault zone in Caña Brava creek. A- Fault breccia (bottom) in contact with country rock (top); B- Serpentinite; C- Fault breccia amplification: matrix of gouge and blocks of basalt, as country rock . Photos by Camilo Montes.
5.2Petrography
The basalt samples studied are mainly amygdalar, with a very fine to fine grain sizes (~ 0.01–1 mm) for matrix crystals and medium grain size for vacuoles. They exhibit interstisial or subophitc texture (Figure 9- A-I). The groundmass of the samples is mainly composed by Plg, Cpx, opaque minerals and devitrified glass, and the amygdales are filled with zeolites, calcite, chlorite or a mixture of these minerals. The basalts also present phenocrysts of olivine (Ol), orthopyroxene (Opx) and amphiboles (Amph), and some perlites in one sample. The plagioclase are euhedral acicular crystals, while clinopyroxenes and opaque present subhedral form. The modal compositions of the non-altered or low altered volcanic samples matrix is 25 - 30% of Plg, 20 - 25% of Cpx and 40 - 45% of volcanic glass. All of these features are characteristic of an ocean island tholeiites as can be seen in volcanic petrographical guide book (MacKenzie et al., 1982) and as it was mentioned in the conceptual framework, thus the petrographic observations over this samples categorize them as tholeiitic basalts.
Using the extinctions angle method, it was determined that the plagioclase compositions correspond to bytownite. It is common to observe ophitic textures between clinopyroxenes and plagioclases, showing intergrowth within the minerals.
Most of the samples present a weak alteration because of the mafic minerals in the matrix have been partly replaced by chlorite (Gifkins et al., 2005). Other alteration minerals are recognized like biotite in the matrix, chlorite filling the vacuoles and as alteration of the matrix, amphibole (as the alteration mineral of some pyroxenes) and calcite; and the devitrification texture in eight of the samples.
According to their geographic location, the samples are divided into three groups: the Honda, the beach and the Hoya groups. Within the Honda group samples
collected on the Honda creek and the Quebro River are included. In this group the glass content of the basalt units gets lower in the south direction following the curse of the creek. Furthermore, the alteration of the three northern samples present chlorite and calcite, while the alteration in the southern samples and the Quebro samples present a zeolite, biotite and a less chlorite phase alteration, and a bigger size in the amygdales. Additionally, the crystal’s size in the bulk of the rock is greater in the southern part and includes a greater content of Cpx.
On the other hand, La Playa group sample, 40620, have fine-size grains in the matrix, the majority of Plg and a portion of glass and Cpx. The Plg crystals are predominantly aligned in one direction (Figure 8-H). This can be recognized as a lava flow (interbedded with sedimentary units). However, the sample was not structurally oriented so a flux direction cannot be interpreted. At last, the Hoya group sample, 40402, have the greatest content of oxides between all the samples (~5%). It also presents an alignment in plagioclase, that could represent the original lava flow; and rare olivine phenocrysts and medium to big amygdales filled with perlite, amorphous volcanic glass formed by the hydration of obsidian.
Figure 8. Pictures of the samples thin sections under polarized light in the optical microscope. A- 40811; B- 40518; C- 40625; D- 40712; E- 40810; F- 40409; G- 40719; H- 40620; I- 40402.
5.3BSE Images and EDS analysis
Back-scattered Electron (BSE) images were obtained using SEM for two different basalts: one of Honda group, sample 40712 that was not recovered with graphite; and the one from La Hoya Group. These are presented in Figure 9.
Figure 9. BSE images of samples 40402 (A-B) and 40712 (40712). Labels with the mineral phases found are included.
Table 3. Chemical formulas of some of the analized points in sample 40402.
# Oxygen
Projection on the number of cations of the mineral
Formula Mineral
Si Ti Al Fe Mn Mg Ca Na K
22 6,79 0,33 2,74 0,82 0,02 0,75 1,84 1,19 0,09 (K0,09)(Na0,16Ca1,84)(Na1,03Mg0,75Mn0,02Fe0,82Al1,53Ti0,33)(Si6,79Al1,21)O22(OH)2 Hornblende
3 0,03 0,90 0,09 1,96 0,00 0,00 0,03 0,00 0,00 FeTi0,9O3 Ilmenita
8 2,35 0,00 1,58 0,02 0,00 0,00 0,64 0,52 0,01 (K0,01Na0,52Ca0,64)(Si2,35Al1,58)O8 Labradorite
8 2,86 0,00 1,10 0,00 0,00 0,00 0,05 1,13 0,00 (Na1,13Ca0,05)(Si2,86Al1,1)O8 Albite
22 7,31 0,10 1,71 0,61 0,00 1,39 2,49 0,77 0,08 (K0,08Ca0,49)Ca2(Na0,77Mg1,39Fe0,61Al1,02Ti0,1)(Si7,31Al0,69) O22(OH)2 Hornblende
22 7,01 0,21 1,86 0,57 0,00 1,33 2,76 0,74 0,05 (K0,05Ca0,76)Ca2(Na0,74Mg1,33Fe0,57Al0,87Ti0,21)(Si7,01Al0,99) O22(OH)2 Hornblende
Table 4. Chemical formulas of some of the analyzed points in sample 40712.
# Oxygen
Projection on the number of cations of the mineral
Formula Mineral
Si Ti Al Fe Mn Mg Ca Na K
22 7,00 0,04 1,21 1,80 0,05 0,53 4,39 0,41 0,06 (K0,06Ca0,94)Ca2(Na0,41Ca1,45Mg0,53Mn0,05Fe1,8Al0,21Ti0,04)(Si7Al1) O22(OH)2 Hornblende
8 2,52 0,01 1,38 0,07 0,00 0,05 0,17 1,13 0,10 (K0,1Na1,13Ca0,17)(Al1,38Si2,52)O8 Oligoclase
6 1,93 0,06 0,31 0,32 0,01 0,46 0,85 0,13 0,02 (K0,06Ca0,94)Ca2(Na0,47Ca0,16Mg1,70Mn0,03Fe1,19Al0,21Ti0,22)(Si7,06Al0,94) O22(OH)2 Hornblende
The energy dispersed X-ray (EDS) analysis show that there is not a single type of plagioclase as shown by the petrography. Instead, there are at least three different minerals, albite, labradorite and oligoclase, in the samples studied with this technique (Table 3). These correspond to a more alkaline phases than originally believed. Also, the EDS analysis showed that the sample content as well an important volume of amphiboles from the Hornblende group. These minerals may had formed as part of the hydrothermal alteration of the pyroxenes in the rock matrix. This alteration, particularly in the sample from La Hoya Group, corresponds to the field observations made in the outcrops of La Playita river, where basalts were found with mineralizations of pyrite as product of this type. In addition to plagioclase and Hornblende, in sample 40712 volcanic glass, ilmenite and gmelinite were also found.
The chemical formulas estimated using the EDS data for all of the minerals mentioned above are shown in Table 3 and Table 4. The number of oxygens considered for the normalization procedure is reported as well. Other compositional spectrums were done during the whole analysis of the samples, but they seem to be a mixture of different minerals.
5.4Geochemistry
Bulk-rock geochemistry of major elements and trace elements was determined for four fresh volcanic samples, and is presented in Table 5 and Table 6. These samples were categorized as amygdalar basalts in the field, based on their mineralogy, textural characteristics and field relations.
Table 5. Major oxides composition of the studied samples.
Sample ID SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 LOI SUM 40402 45,76 14,41 13,77 0,20 6,11 9,65 2,45 0,82 3,99 0,46 1,74 99,36 40409 48,04 11,97 11,85 0,22 10,46 10,52 1,64 0,30 2,36 0,24 1,80 99,40 40625 47,65 13,18 11,50 0,15 6,04 10,64 3,47 0,86 2,57 0,26 3,05 99,37 40712 49,44 11,66 10,60 0,15 9,43 7,75 2,22 3,16 2,23 0,22 2,63 99,49
Table 6. Trace element composition of the studied samples.
Sample
ID Li Rb Cs Be Sr Ba Sc V Cr Co Ni Cu Zn Ga Y Nb Ta Zr Hf Mo
40402 5,74 11,98 0,12 1,43 394,27 198,03 27,92 331,12 66,89 40,36 65,67 90,42 139,99 23,33 36,58 35,24 2,65 277,75 6,43 1,53 40409 1,22 3,70 0,10 0,85 269,63 75,48 25,74 257,86 550,69 56,58 439,19 78,49 110,51 18,54 22,17 17,05 1,30 136,26 3,71 1,18 40625 2,66 28,35 0,15 1,05 162,39 58,93 26,46 275,20 252,43 37,83 108,90 87,64 103,14 22,18 24,98 19,40 1,42 155,78 4,13 1,18
40712 1,84 21,05 0,10 0,76 489,99 211,01 24,09 246,65 273,72 37,79 194,95 95,50 116,16 15,76 21,44 16,25 1,16 128,39 3,56 0,89
Sample
ID Sn Tl Pb U Th La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Zr
40402 2,68 0,01 1,25 0,64 2,53 27,65 62,37 8,24 36,66 8,87 2,93 8,22 1,21 6,96 1,33 3,24 0,45 2,67 0,40 271,80 40409 1,42 0,01 0,28 0,39 1,32 15,15 34,27 4,56 20,46 5,15 1,75 4,84 0,73 4,21 0,82 1,98 0,30 1,67 0,24 139,10 40625 1,60 0,02 1,25 0,48 1,55 17,37 39,02 5,22 23,55 5,75 1,95 5,41 0,82 4,68 0,91 2,18 0,32 1,86 0,26 158,60
According to the TAS classification of volcanic rocks (Le Maitre et al., 1989) samples were classified as can be seen in Figure 10. It can be see that the studied samples do correspond to sub-alkaline basalts, and specifically tholeiitic basalts as the petrography suggests (view Figure 11). Additional classification of the samples was done considering the idealized mineralogical composition, estimated using the CIPW Norm, for the bulk rock geochemistry (see Figure 12). The mineralogical composition obtained is presented in e series for the basalt samples, after Pearce et al. (1977)
Table 7, and the classification diagram proposed by is shown in Figure 12. From
here it can be clearly seen that the mineralogical composition also agrees with the basalt classification. In addition, some of the mineral phases coincides with the results of BSE images and EDS analysis.
Figure 10. Total Alkalis Silica (TAS) classification of volcanic rocks after LeMaitre et al. (1989).
Figure 11. Determination of magmatic sub-alkaline series for the basalt samples, after Pearce et al. (1977)
Table 7. Estimated mineral composition of the samples according to CIPW Norm. Only mineral phases different from zero are reported here.
Sample ID Plg Or Nph Di Hyp Ol Ilm Mag Ap Zrn 40402 55,08 6,02 0 14,8 11,88 4,81 5,08 1,22 1,07 0,04 40409 45,61 2,23 0 20,65 25,71 1,11 3,04 1,07 0,56 0,02 40625 51,87 6,28 2,33 25,89 0 8,74 3,26 1,02 0,59 0,02 40712 36,59 22,77 0 18,67 4,9 12,83 2,79 0,93 0,5 0,02
Figure 12. Classification of the samples based on the ideal mineral composition calculated based on the CIPW Norm (Thompson, 1984).
Li et al. ( 2015) demonstrate that tectonic discrimination diagrams for basalts are not as reliable as they were originally believed. Some of them, however, have shown that they can give an approximation of the origin of the magma of which the rocks are produced when cooling. For example, REE diagrams normalized to the chondrite (McDonough & Sun, 1995) are useful to distinguish if the source is enriched or depleted in LREE. Another example is the spider diagram normalized to N-MORB that is useful to identify mobile trace elements. Diagrams constructed with the trace elements of the samples here studied are presented in Figure 13 A and B, compared with the data for groups I and II of the Azuero Plateau defined by (Buchs et al., 2010).
The patterns shown in REE and Spider diagrams coincide with a OIB-like affinity for the studied rock samples, being highly enriched in the LREE, having a negative slope and a flat pattern in the first one. It also can be seen that the traces in REE diagrams do not coincide exactly with either Azuero Plateau Group I or II (Buchs et al., 2010), however there exist some similarities with the group II specially from the petrographically defined Honda group (samples 40409, 40625 and 40712). Sample 40402 exhibit a more enriched behavior in almost all elements considered for the plot. Two other discrimination diagrams are considered in Figure 13 C and D. Both coincide with the results of REE and multi-elemental diagrams of an OIB source.
Table 8. Other geochemical criteria considered to identify the source magma of the samples.
Sample ID K2O? TiO2? La/Sm La/Nb Zr/Nb
40402 yes yes 3,12 0,78 7,71
40409 yes yes 2,94 0,89 8,16
40525 yes yes 3,02 0,90 8,18
40712 yes yes 2,89 0,86 8,34
Other geochemical, further than discrimination diagrams, can be considered to identify the source of the studied samples as mentioned in the CONCEPTUAL FRAMEWORK. These criteria are calculated evaluated and reported in Table 8, such as the value of different ratios and if the K2O and Ti2O is greater than 0,1% wt.
The evaluation of this numeric criteria and the field observations, when compared with values reported in Table 1 and before it, lead again to an ocean island tectonic setting, that comes from an enriched mantle.
6. DISCUSSION
Buchs et al. (2010; 2011) describe the lithological units of the SW corner of the Azuero Peninsula as a complex of two ocean islands accreted on the middle Eocene. However, the field work in the northern part of the zone of this complex, at the south of the ASFZ, show that within the Honda section exists sedimentary units that in the field are recognized as part of the Ocú formation, so the underlying lavas, that include some pillowed parts as found in the CLIP as well, were classified here as part of the Azuero Plateau. In fact, the geochemical studies of trace elements in the samples of this group show a similar pattern to the plateau enriched group, group II, described by Buchs et al. (2010). Nonetheless, this signature in REE diagrams also coincides with an OIB-like signature, which agrees with the tectonic setting determined using other criteria. They correspond to tholeiitic basalts, with a weak hydrothermal alteration, of probably an ocean island, formed at the Galapagos hotspot with deep mantle material.
Within the samples of Honda group textural and compositional differences are recognized. This lead us to divide the group in two different units of basaltic lava flows that correspond with the field observation reported in the structural cross section of the zone presented by Ortiz-Guerrero (2016). This may suggest the presence of at least two stages of lava ejection for the formation of these ocean island, besides the picritic flows that were also found. Here, the rock samples with smaller amygdales (the southern flow) maybe under the northern flow, that is characterized by a greater vacuole size.
Thus, there exist an important compositional difference, beyond the petrographical differences, between the Honda and la Hoya group. This alkalinization maybe related with the third volcanic stage of the ocean island, as recognized by Buchs et al. (2011 and the references therein).
Urdaneta (2016) reports the finding of gabbroic bodies in the northern part of the Honda creek, and close to the ASFZ, and categorize on of it as part of an ophiolitic sequence. The reevaluation of these data considering the observations and analysis here exposed may suggest that in the Azuero Accretionary complex there is a third ocean island, related to the Honda sequence. However, to constrain this interpretation is necessary to do further geochronological studies in the samples as a diagnostic proof if we're the oldest rocks, within Honda Group are. The samples of age greater than 85 Ma are from the Plateau.
Considering this interpretations and starting from the models presented before by Ortiz-Guerrero (2016) and Buchs et al. (2011) it is possible to give a further tectonic arrangement history for the SW area of the Azuero peninsula since the Campanian. In the late Campanian (>75 Ma) the CLIP was the thickened part of the Farallon plate, with the Ocú formation sedimenting over it. After that, the formation of the proto-arc complex marks the subduction initiation of the Farallon plate under the CLIP between 75 – 73 Ma, leading to the constitution of the current Caribbean Plate, but in a more southerly part of what was originally proposed. Evidence of this ancient convergent margin can be the Bouguer gravitational anomalies as proposed by (Ayala, 2014) and fault rocks and serpentinite from Caña Brava creek. Later, in the Paleocene oceanic islands were forming in an equatorial latitude and during middle Eocene were accreted in the SW of the peninsula, starting with the one that now is found close to the ASFZ, generating the folded sequences observed in the Honda creek (Ortiz-Guerrero, 2016) with a south western vergence. Finally,
the whole accretionary complex, and furthermore and important part of the peninsula, was uplifted when the Cocos Ridge began to be subducted under this sequence.
7. CONCLUSIONS
Various volcanic rock samples were collected south of the Azuero-Soná fault, during the summer campaign of the Azuero Field project. Petrographical and bulk rock geochemical studies were done to these samples. These analyses categorized the samples as amygdale tholeiitic basalts, with a weak hydrothermal alteration and an OIB affinity. When field observations are considered and related to these studies, samples can be further divided in three groups: Honda, La Hoya and Beach group. The Honda group consist in two different textural groups that maybe associated to different parts of an ocean island.
Thus, the characterization of the basaltic samples along the SW corner of the Azuero Peninsula lead to a recognition and detailed characterization of the lava flows of the ocean islands formed in the Paleocene that compound the Azuero Accretionary Complex, attached to the peninsula in the middle Eocene. This characterization also allows the appreciation of the third volcanic stage in the southern ocean island and the establishment of another OI in the northern part of the AAC, close to the Azuero-Soná Fault Zone.
REFERENCES
Ayala, J. (2014). Bouguer Anomaly Model From Western Azuero Peninsula. Universidad de los Andes, Bogotá D.C.
Anderson, D. L. (1995). Lithosphere, asthenosphere, and perisphere. Reviews of Geophysics, 33(1), 125–149. https://doi.org/10.1029/94RG02785
Buchs, D. M., Arculus, R. J., Baumgartner, P. O., Baumgartner-Mora, C., & Ulianov, A. (2010). Late Cretaceous arc development on the SW margin of the Caribbean Plate: Insights from the Golfito, Costa Rica, and Azuero, Panama, complexes.
Geochemistry, Geophysics, Geosystems, 11(7), Q07S24.
Buchs, D. M., Arculus, R. J., Baumgartner, P. O., & Ulianov, A. (2011). Oceanic intraplate volcanoes exposed: Example from seamounts accreted in Panama. Geology, 39(4), 335–338. https://doi.org/10.1130/G31703.1
Buchs, D. M., Baumgartner, P. O., Baumgartner-Mora, C., Bandini, A. N., Jackett, S.-J., Diserens, M.-O., & Stucki, J. (2009). Late Cretaceous to Miocene seamount accretion and mélange formation in the Osa and Burica Peninsulas (Southern Costa Rica): episodic growth of a convergent margin. Geological Society, London, Special Publications, 328(1), 411–456. https://doi.org/10.1144/SP328.17
Buchs, D. M., Baumgartner, P. O., Baumgartner-Mora, C., Flores, K., & Bandini, A. N. (2011). Upper Cretaceous to Miocene tectonostratigraphy of the Azuero area (Panama) and the discontinuous accretion and subduction erosion along the
Middle American margin. Tectonophysics, 512(1–4), 31–46.
https://doi.org/10.1016/j.tecto.2011.09.010
Gifkins, C. C., Herrmann, W., & Large, R. R. (2005). Altered volcanic rocks: a guide to description and interpretation. Centre for Ore Deposit Research, University of Tasmania. Retrieved from http://www.codes.utas.edu.au
Kerr, A. C., White, R. V., & Saunders, A. D. (2000). LIP Reading: Recognizing Oceanic Plateaux in the Geological Record. Journal of Petrology, 41(7), 1041– 1056. https://doi.org/10.1093/petrology/41.7.1041
Kerr, A. C., White, R. V., Thompson, P. M. E., Tarney, J., & Saunders, A. D. (2003). No Oceanic Plateau—No Caribbean Plate? The Seminal Role of an Oceanic Plateau in Caribbean Plate Evolution, 126–168.
Kolarsky, R. A., Mann, P., Monechi, S., Meyerhoff-Hull, D., & Pessagno, E. A. (1995). Stratigraphic development of southwestern Panama as determined from integration of marine seismic data and onshore geology. SPECIAL PAPERS-GEOLOGICAL SOCIETY OF AMERICA, 159-159.
Le Maitre, R. W. B., Dudek, P., Keller, A., Lameyre, J., Le Bas, J., Sabine, M. J., & Zanettin, A. R. (1989). A classification of igneous rocks and glossary of terms: Recommendations of the International Union of Geological Sciences, Subcommission
MacKenzie, W. S., Donaldson, C. H., & Guilford, C. (1982). Atlas of igneous rocks and their textures. Longman.
McDonough, W. F., & Sun, S. -s. (1995). The composition of the Earth. Chemical Geology, 120(3), 223–253. https://doi.org/10.1016/0009-2541(94)00140-4
Meschede, M. (1986). A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb 1bZr 1bY
diagram. Chemical Geology, 56(3), 207–218.
https://doi.org/10.1016/0009-2541(86)90004-5
Nerlich, R., Clark, S. R., & Bunge, H.-P. (2014). Reconstructing the link between the
Galapagos hotspot and the Caribbean Plateau. GeoResJ, 1–2, 1–7.
https://doi.org/10.1016/j.grj.2014.02.001
Ortiz-Guerrero, C. (n.d.). Tectonostratigraphic reconstruction of the Azuero Peninsula; Geologic Mapping and Structural Cross-Sections. Universidad de los Andes, Bogotá D.C.
Pearce, T. H., Gorman, B. E., & Birkett, T. C. (1977). The relationship between major element chemistry and tectonic environment of basic and intermediate
volcanic rocks. Earth and Planetary Science Letters, 36(1), 121–132.
https://doi.org/10.1016/0012-821X(77)90193-5
Pindell, J. L., & Barrett, S. F. (1990). Geological evolution of the Caribbean region: a plate tectonic perspective. The Caribbean region: Boulder, Colorado, Geological Society of America, Geology of North America, v. H, 405-432.
Meschede, M. (1986). A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb 1bZr 1bY
diagram. Chemical Geology, 56(3), 207–218.
https://doi.org/10.1016/0009-2541(86)90004-5
Thompson, R. N. (1984). Dispatches from the Basalt Front. I. Experiments.
Proceedings of the Geologists’ Association, 95(3), 249–262.
https://doi.org/10.1016/S0016-7878(84)80011-5
Urdaneta, M. P. (2016). Geochemical and petrological characterization of gabbroic bodies in the Azuero Peninsula: Regional implications. Universidad de los Andes, Bogotá D.C.
Whattam, S. A., & Stern, R. J. (2015). Late Cretaceous plume-induced subduction initiation along the southern margin of the Caribbean and NW South America: The first documented example with implications for the onset of plate
tectonics. Gondwana Research, 27(1), 38–63. https://doi.org/10.1016/j.gr.2014.07.011
White, W. M. (2015). Section 2. Island Chains, Hot Spots, and Mantle Plumes. Geochemical Perspectives, 4(2), 106–110.