1
Authors: Patino, Vogel, Alvarado, Rose[TAV1]
Introduction
The regional geochemical studies of volcanic rocks in Central America have been focused on the chemical variation of lavas and ophiolites. In contrast, with some exceptions, studies of the ignimbrites have not been common. The[LP2] reasons for this lack of study includes many factors, some of which are: the modern arc was thought to consist predominantly of orogenic andesistes; many early workers thought that the volcanic edifices were younger than the ignimbrites; the silicic rocks were thought to all be derived from mantle derived mafic magmas; the caldera forming events were not considered important; mineral exploration was not focused on the ignimbrites; and the early studies of ignimbrites elsewhere were more focused on understanding ignimbrites as instantaneous samples of magma chambers (and therefore magma chamber processes), rather than including them in a regional tectonic setting. Understanding the regional and spatial geochemical variation of ignimbrites along the recent and old volcanic fronts is important. These studies are critical to better constrain the evolution of the arc and consequent the evolution of the continental crust. This understanding will lead to better predictive[LP3] models for this dangerous type of volcanic eruptions.
There are no studies integrating the variation of silicic volcanic deposits along the whole of Central America. In this paper we focus mainly on silicic ignimbrites of Miocene to Pleistocene age along the Central American volcanic front from Guatemala to Panamá[LP4] – in Belize there are no reports. We review what is known about their occurrence, age, distribution and chemical characteristics and place them in a regional tectonic framework, and explore the origin of these silicic magmas.
Central America has been divided into two main tectonic blocks of different crustal origins: the northern Chortis block and the southern Chorotega block (see other chapters for figure). An important contrast in Central America is that the Chortis block consists of a basement of cr[LP5]ystalline Paleozoic rocks, whereas in the Chorotega block there is no crystalline Paleozoic basement. The Chorotega block is made up of an over-thickened oceanic crust, the Caribbean Large Igneous Province (CLIP), that was emplaced in the Cretaceous (see other chpt for details). There is no consensus on the location of the boundary between the Chortis and the Chorotega blocks (see chpt). However, what is important for this study is that the northern part of the Central American is underlain by old continental crust and the southern part of the arc contains no old continental crust. We explore in this chapter the effect of this changing nature of the crust on the chemical variation of the silicic volcanic rocks. The Central American volcanic front is an excellent locality in which to study the effect of variation of basement rocks on the origin and evolution of silicic magmas.
[LP6]Geographic distribution
Ignimbrites occur from Guatamala to Panamá. Although most Quaternary volcanism has been concentrated near the active arc, Tertiary ignimbrites cover a wide area of the interior, especially in central Honduras and northeastern Nicaragua (Williams and McBirney,UC Pubs in Geol. Sci.). Figure ** shows the geographic and age distribution of the ignimbrites of Central America (Guillermo will make this map).
Age distribution – Strat columns (See table at the of document)
There have been two periods of intensive volcanic activity in Central America in the last 10 Ma (Carr, 1982). The first period occurred between 6 and 3 Ma, and the second period occurred between 1 Ma and the present with the volcanic products ranging in composition from basalt to rhyolite. However, neither of these periods can compare with the volume or extent of volcanic activity that occurred during the middle Miocene, peaking at 14 Ma (Carr et al., 1982). Large ignimbrite deposits cover the early Quaternary volcanic structures in Guatemala and El Salvador, and the younger volcanic centers sit on top of a silicic pyroclastic stratigraphic marker dated at 84 Ka (Los Chocoyos ignimbrite). However, in the southern part of the volcanic front, ignimbrites underlay the Quaternary volcanoes (Carr et al., 1982).
The spacing of the rhyolitic volcanic centers in Guatemala and El Salvador averages 90 km, however this contrasts with the spacing of 25 km for the basaltic-andesitic centers (Carr et al., 1982) – in Costa Rica this spacing is 50 km (this is from Guillermo). These rhyolite volcanic centers (calderas) are offset from the volcanic front, to the north and east (Rose et[LP7]al[TAV8]., ???). Wadge (1984) estimates that for Central America volcanic production rate is of 31-62 km3/Ma/km-arc. Rose et al. (1999) estimated that volume of the silicic pyroclastic deposits, which were less than 200 Ka, were similar in volume of the lavas. The earliest records of large silicic eruptions in Central America in marine basins are based on data from ODP cores. The Ocean Drilling Program (ODP) have recorded two of the largest silicic eruptions in Central America with volumes estimated to be > 100 km3 (Siggurdsson et al., 2000). The first was mid-Eocene (~46 Ma) to early Oligocene (~32), which are correlated with the Matagalpa and Morazán Fms. in Guatemala, El Salvador, Honduras and Nicaragua (Ehrenborg, 1996). The second was Oligocene/Miocene (~23 Ma) to the mid-Miocene, which are correlated with the Chalatenango, Padre Miguel and Coyol Fms. in Guatemala – El Salvador, Honduras and Nicaragua respectively (Ehrenborg, 1996).
Tertiary Ignimbrites
Early Tertiary stratigraphy and geochemistry of ignimbrites in Central America are poorly known. Weisemann (1975) described the Morazán Fm. as the Oligocene to Middle Miocene volcanics in El Salvador. Reynolds (1980, 1987) summarized the stratigraphy of the late Tertiary ignimbrite units in northern Central America. He notes two formations, one from the Middle to Upper Miocene (Chalatenango) and a younger one from the Pliocene (Cuscatlán). The older unit, Chalatenango formation consists of rhyolitic tuffs and lavas. The source of large welded tuffs from the Chalatenango Fm. originated from the Santa Rosa de Lima caldera (Reynolds, 1987). This unit is contemporanous to the lowest formation in the Padre Miguel Group, in Honduras, and the Coyol Group in Nicaragua (Ehrenborg, 1996).
Tertiary ignimbrites in Honduras are widespread but poorly studied and are best known through the work of Williams and McBirney(UC pub) . Quaternary ignimbrites are absent in Honduras, but Miocene and Pliocene are very abundant. These have been roughly divided into two groups, an older, more voluminous group of Miocene age, which occupy a large portion of the central highlands. A younger group, probably of Miocene to Pliocene age, that occurs in the region bordering the Gulf of Fonseca (modify this with more data). Ignimbrites of the older group tend to form extensive plateaus, whereas those of the younger group tend to form the caps of mesas and cuestas adjacent to the Gulf of Fonseca. Ignimbrites from Honduras are dominated by rhyolite, but rhyodacites are nearly as abundant (Williams and McBirney). The mineralogy of the ignimbrites is dominated by quartz, sanidine , plagioclase and biotite. In most rhyolitic units , phenocrysts of quartz and alkali feldspar are more abundant than [LP9]plagioclase[TAV10][LP11]. In some units pyroxene and hornblende are common. Many of the ignimbrites reported by (Williams and McBirney) are crystal rich, containing twenty-five percent or more phenocrysts. Only 14 chemical analyses are available for the silicic ignimbrites (W and M, tables 3 and 5). Rhyolites are the most abundant composition in this group. There are no studies that identify the caldera sources for the ignimbrites.
In Nicaragua there are only limited data about the Tertiary ignimbrites. One of the sources of the ignimbrites from the Coyol Group is El Limon caldera in Nicaragua (Ehrenborg, 1996), and like the Santa Rosa de Lima caldera in Honduras, it is located behind the Neogene volcanic front. The younger unit, Cus[LP12]catlán Fm., has rhyolitic tuffs and volcanic sediments overlain by rhyolitic and basaltic lavas. This formation is contemporaneous with the upper formations of the Padre Miguel Group in Honduras and with the deposits of Las Sierras in Nicaragua (Weyl, 1980). The only unit described in the literature from this time frame is Las Sierras (Weyl, 1980). This unit includes ash-flows of Plio-Pleistocene age located to the west of the modern volcanic front. Viray (2003) has shown that these units have distinct compositions. Detail radiogenic dating and fieldwork is needed to estimate the age and volume of the different units.
The Tertiary ignimbrites of Costa Rica are the best studied in Central America. The earliest Miocene record of silicic volcanism in northern Costa Rica is the voluminous Carbonal dacitic lava flow (7.8 Ma, Gillot et al., 1994). An extensive ignimbrite sequence (ca. 2400 km²) overlies and underlies[LP13] the Carbonal lava and occurs in the Pacific water-sheds of the Guanacaste range, flanking the stratovolcanoes and exposed in fluvial valleys. It is known as the Santa Rosa plateau (Bagaces Formation) and is of Upper Miocene to Late Pliocene age. It consists of a series of pyroclastic flow deposits with minor interbedded lava flows and terrigenous sediments (Gillot et al., 1994, Tournon, 1984; Chiesa et al., 1987, 1992). Some of the oldest pyroclastic deposits in northern Costa Rica are included in the Bagaces Fm., a unit that incorporates numerous deposits of highly welded to unwelded deposits. Dating of these deposits is in pr[LP14]ogress and preliminary dates indicate that they may be less than 8 Ma (Alvarado et al., 1992). The volume of the Bagaces ignimbrite sequence is more than 100±40 km³, DRE of silicic magma [LP15]. A second and third episode of ignimbrite eruption occurred at 2.06 Ma and 4.1 Ma (Vogel et al., in press[LP16]), both with unknown caldera sources and informally named the Sandillal and Montano units respectively[LP17].
Quaternary Ignimbrites
In Central America the Quaternary ignimbrites are almost always related to large calderas. In Guatemala and El Salvador, five major silicic centers occur behind the active arc, and ash-flow deposits cover large areas in these countries (Hahn et al., 1979; Hart, 1993). The total volume of all these silicic deposits (< 200ka) have been conservatively estimated to be between 300 and 500 km3, which is similar to the volume of lavas erupted from the youngest generation of volcanic front volcanoes (Rose et al., 1999).
Rose et al. (1999) presented high-resolution stratigraphy of silicic volcanism during the late Quaternary (<200 ka) from 5 calderas from Guatemala and El Salvador: the Atitlán, Amatitlán, Ayarza, Coatepeque, and Ilopango (Table ?). Atitlán caldera formed by episodic eruptions within the last 12 Ma [TAV18](Newhall, 1980, 1987). The most voluminous unit of recent eruptions, Los Chocoyos (270 km3 DRE), is a stratigraphic marker for almost all of Central America. The youngest silicic eruption (AD 260) reported by Rose et al. (1999) is from Ilopango caldera in El Salvador, which consists of air falls and pyroclastic flows with a volume estimated between 15 and 20 km3 (DRE) of material.
In Nicaragua there are three ignimbrite shields (de Vries). The northern Malpaisillo shield has little topographic expression, whereas the southern mots Las Sierras shield is nearly 1 km above sea level. The Malpaisillo ignimbrite field outcrops around the Monte Galan – Momotombo area. The most recent ignimbrites come from the Monte Galan caldera and older ones probably from the Malpaisillo and San Fenando structures (de Vries). A large ignimbrite shield volcano [LP19]dominates the volcanic front south of Momotomibito with the main caldera enclosing the Masaya volcano. Ignimbrites from this caldera occur 50 km from the center, and have filled valleys near Las Maderas in the Nicaraguan Depression[LP20]. The upper layer of this ignimbrite, near Diramba, has been dated from a carbon sample at 29,500 years BP (Sussman, 1985).
In Nigaragua the most recent silicic eruption reported is from Apoyo, with 11 km3 erupted from Apoyo 23,000 years BP (Sussman, 1985). The Apoyoque ignimbrite has recently been studied by **** and erupted *** years ago with a volume of ****. The Masaya volcano lies within the Las Sierras caldera with the present active volcano is enclosed with the 2,00-4,000 year old Masaya caldera, which was formed during an 8 km3 ignimbrite and surge producing eruption (Williams, 1983 from de Vries)[LP21]
In Costa Rica the youngest silicic deposit in the north is La Ese formation (0.66-0.89 Ma) and is associated with the Guayabo caldera, the precursor to Miravalles volcano. It is a crystal-poor unit (Plag-Qz-Cpx-Hb+/- Ol) Three other episodes of ignimbrite eruptions in northern Costa Rica have been recognized from field relationships and dated. The first is an unnamed silicic flow dated at 1.18 Ma. The next unit is 1.31 to 1.47 Ma tentatively associated with the Guachieplín-Alcantara caldera a precursor to Rincón de La Vieja volcano (Kempter et al., 1997; Zamora et al. in press –see ref Guillermo e-mailed). This is a crystal-rich unit (Plag-Qz-Hb-Cpx-Opx) The oldest unit (1.47 Ma) is a biotite rich flow that is a marker unit and it often directly overlies the Bagaces sequence. This unit is one of the largest known ignimbrite eruptions in the Quaternary time in Guanacaste (Chiesa, 1991). It is a crytal-rich pyroclastic flow deposit (Qz-Biot-Hb-Opx-Plag An30-35) covering an area of ca. 4000 km2, which corresponds to 34 km3 of ignimbrite erupted volume (about 25 km3 of rhyolitic magma).
In the Central Valley region of Costa Rica, the Tiribí Formation represents the latest example of silicic volcanism in the Costa Rica and has been dated at 0.324 Ma and is the best-studied ignimbrite in Costa Rica (Hannah et al., 2002). It is chemically zoned from 55.4 wt. % to 68.4 wt. % SiO2. Crystal content varies based on silica content. The lowest silica samples contain 28% plagioclase (An76), 4% clinopyroxene (Wo44 En47 Fs9) and less than 1% olivine (Fo70-73) and opaques (magnetite and ilmenite). The high-silica samples are crystal poor rhyolites, and are nearly aphyric, with 0 to 2% crystals consisting of plagioclase (An34 to An49), clinopyroxene (Wo47 En44 Fs9), orthopyroxene (Wo3-6 En69-75 Fs 22-29), rare alkali feldspar (Ab48 Or43 An8) and opaques dominated by magnetite. It covers about 500 km2 and has a volume of about 25 km3 DRE.
The Alto Palomo Unit unit consists of a series of poorly welded, dacitic to rhyolitic ignimbrites that have been dated at 0.57 Ma. The Alto Palomo Unit consist of at least two formations. The Alto Palomo Fm. (Plag-Cpx-Hb) and is characterized by mingling of basaltic magma as small streaks and clots in the pumice samples. The Palmito Fm. (Plag-Hb-Bt-Qz) is very poorly known, which the source[LP22]. Other ignimbrites from the Central Valley and vicinity for which we have data have been dated at 6.0, 1.5, 0.92, and 0.44 Ma (Vogel et al., in review).
In Panama there has been only one reference to silicic pyroclastic deposits and this occurs at the El Valle Volcano (Defant et al. 1991). The ages are 0.9-0.2 Ma and are of dacitic composition with higher concentrations of Sr and lower concentrations of Y than any other silicic samples in Central America in our data collection. Defant et al. (1991) interpreted these to be melts of the hot, young subducting slab.
Table x
Age and estimated volume of different formations in Northern Costa Rica
Name Formation / Age (Ma) / Volume (DRE, km³)Guayabo / 0.65-1.18 / 15.5±(increase)
Guachiepilín / 1.31- 1.47 / 25±2 (increase[TAV23])
Bagaces / 8 -16 / 100±40
References: Chiesa (1991), Chiesa (1992), Gillot et al. (1004), Vogel et al. (in review)
Variation along the arc
The most common assemblage of ignimbrites is dominated by plagioclase both as phenocrysts phase and in the groundmass, with lesser amounts of sub-calcic augite, and hypersthene. A less common assemblages contains amphibole as well. In some assemblages only hydrous ferromagnesium minerals are present such as amphibole or biotite , or both . These hydrous minerals are absent all units studied in Nicaragua (Viray 2003), and from the Tiribí Fm, Costa Rica, the youngest unit in Costa Rica (Hannah et al., 2002). In addition, Fe-Ti oxides, apatite, quartz, are common in most ignimbrites. Alkali [LP24]feldspars [TAV25]are rare in all ignimbrites in Central America (Carr et al., 1982; Hannah et al., 2002; Viray, 2003) with the exception that they have been reported to be common in Honduras (Williams and McBirney).
The calderas that produced the large volume of silicic deposts in Central America erupt material that ranges in composition from basalt to rhyolite. Early workers (Pushkar et al., 1968; Carr and Stoiber, 1982?) thought that the ignimbrites become more dacitic as you move southward along the arc. However, recent work has shown that rhyolitic ignimbrites are common in Nicaragua and Costa Rica (Chiesa, 1991; Chiesa et al., 1992; Alvarado and Carr, 1993; Kempter et al., 1996; Viray, 2003; Vogel et al., in press and references therein). In Guatemala and Costa Rica the most primitive materials from the calderas are basaltic andesites and andesites, respectively. In the other regions, basaltic scoria is present. Another regional characteristic is that the Tiribí Fm. from Central Costa Rica and the Coyol Fm. from Nicaragua have a higher alkali content than the rest of the samples from Central America[LP26].
Chemical trends along the arc
Carr et al. (1990) and others have shown that there are geochemical variations along the arc in the modern lavas from Central America (see chpt). These variations have been explained by changes in the crustal thickness and slab input (Carr et al., 2003?). Similar geochemical variations occur along the arc in the silicic ignimbrites (Viray, 2003; AGU abstract). This observation was previously noticed by Pushkar et al. (1968), Weyl (1980), Carr et al. (1982) on a more limited set of geochemical parameters[TAV27][LP28]. Many of the trace element variations in the silicic volcanic products in Central America resembles the modern basaltic-andesitic lavas. The similarity in composition between the modern lavas and the silicic pyroclastic material is a key in understanding the origin of the silicic volcanic rocks in Central America and will be discussed more below.
The data set for the ignimbrites from Costa Rica and Nicaragua cover the widest time range (0.023 Ma to ~12 Ma) that we have complete geochemistry for, and the geochemical data set for Guatemala and El Salvador cover the most recent events (< 0.2 Ma). Thus, we are restricted from making inferences on temporal relationships of composition variations along the arc.
Sources of the magmas and involvement of sediments from the subducting slab have been inferred from trace element ratios of recent lavas (cf. Patino, 2000; Plank et al., 2003). For example in southeastern Nicaragua and northwestern Costa Rica samples from the modern arc (basalt to andesite) have low Ce/Pb, and high Ba/Nb, high Ba/La and high U/Th ratios, indicating a large input from slab fluids. In Central Costa Rica samples from the modern arc (basalt to andesite) have high Ce/Pb and low Ba/Nb and low Ba/La ratios indicating a smaller input from the slab. In the silicic pyroclastic rocks, these ratios mimic those of the recent lavas (see Figures **** Ce/Pb vs distance, Ba/Nb vs distance, Ba/La vs distance, Ce/Pb vs Ba/La – all figures to have lava samples as background). In Nicaragua, Plank et al. (2003) interpreted the change in U/Th ratios form Miocene to recent in the volcanics as reflecting the changes in the nature of organic matter content in the sediment input. The slab contribution for the older magmas had low U content, reflecting low organic matter in the sediments. For the younger magmas, the U in the slab component increases due to the higher organic matter conent in the sediments. Our data for U/Th are also consistent with Plank’s et al., 2003 data with the oldest ignimbrites contain the lowest U/Th ratios and the youngest ignimbrites mimicking the modern arc. These trace element ratios, which have been used to infer sources of the recent arc lavas, show the same trends in the silicic pyroclastic rocks as the recent arc lavas.