Van Wyk De Vries. B., 1993. Tectonics and Magma Evolution of Nicaraguan Volcanic Systems
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Excerpted with Benjamin’s permission from: van Wyk de Vries. B., 1993. Tectonics and magma evolution of Nicaraguan volcanic systems. Unpub. Ph.D. Thesis, Open University, Milton Keynes, UK, 328pp. Abstract Variation of volcano morphology, eruptive style and magma composition are studied with respect to structural environment along the Central American volcanic arc in Nicaragua. Two main types of volcano are encountered: shield, with low-alumina basalts and andesites, and stratocones with high-alumina basalts and andesites. The shields are located near, or within, small grabens and fault zones, while stratocones are constructed on crust unaffected by regional faulting. Two type volcanoes are chosen to study the origin of the observed variations: (1) Concepcion stratocone stands on unfaulted crust, which is locally deforming under the weight of the volcano. Thrusts propagate away from the volcano, while the central region is rifted. Eruptions occur mainly from a central vent and are predominantly pyroclastic. Magmas are influenced by moderate to high-pressure fractionation, creating high-alumina basalts, which upon ascent to a low-pressure environment, fractionate further to andesite, with rapid alumina loss. Mixing is restricted, and has little effect on magma compositions. Magmas probably reside for extended periods within the lower crust, before ascent along a single pathway into an upper crustal chamber. (2) Zapatera, a shield volcano, lies within the Ochomogo fault zone. It is predominantly constructed of thin lava flows erupted from multiple vents. Low-alumina basalts with slightly variable near-primary characteristics are erupted, indicating that separate magma batches rise to high levels. Most rocks are hybrids and magma composition is dependant on mixing of rapidly fractionated magmas at low pressure. In the absence of structural pathways, magmas pond in a high-pressure environment, forming high-alumina basalts. The magmas subsequently rise along a single path to erupt from a central vent, creating a stratocone. Where structural pathways are present rapid magma ascent favours low-pressure differentiation and high eruption rates, creating shield- like constructs. B van Wyk de Vries September 1993 Contents Chapter 1: Introduction 1.1. The Project 1 1.2. Background to Volcanic Studies in Nicaragua 2 1.3. Thesis Layout. 4 Chapter 2. Quaternary Volcanism and Tectonics in Nicaragua 2.0. Introduction 5 2.1. Geology and Structure or Western Nicaragua 5 2.1.1. Regional Setting 5 2.1.2. Morphology 7 2.1.3. General Geology 8 2.1.4. Structure in Western Nicaragua 10 I. The Nicaraguan Depression 10 II. Neotectonic Features: Fault Zones and Grabens 14 IIa. La Pelona Fault Zone. 14 IIb. La Paz Centro Fault Zone 16 llc. Mateare Fault 17 lld. Managua Graben 19 IIe. Ochomogo Fault Zone 21 Ill. Origin of Fault Zones 22 2.1.5. Summary 24 2.2. Quaternary Volcanoes in Nicaragua 25 2.2.1. Shield Volcanoes: Individual Descriptions 27 I. Cosigüina Volcano 27 II. Telica Massif 27 III. Rota Massif 27 IV. El Hoyo 29 V. Zapatera 29 2.2.2. Stratocones: Individual Descriptions 29 I. San Cristobal Massif 29 II. Cerro Montoso and Momotombo 29 III. Mombacho Volcano 31 IV .Concepcion and Maderas 31 2.2.3. Ignimbrite Shield Volcanoes 31 I Malpaisillo Ignimbrite Shield 32 II. Las Sierras and Chiltepe Shields 32 2.2.4. Summary 35 2.3. Magma Composition in Nicaragua 36 2.4. Discussion 46 2.5. Two Type Volcanoes: Choice of Concepcion and Zapatera 47 [Not Scanned] Chapter 3: Geology and Evolution of Concepcion: an Example or a Nicaraguan Stratocone Volcano Chapter 4: Geology and Evolution of Zapatera: an Example or a Nicaraguan Shield Volcano Chapter 5: The Origin of High-Alumina Basalts at Concepcion Chapter 6: A Comparison of Concepcion and Zapatera Volcanoes: the Formation of Stratocones and Shield Volcanoes. Chapter 1 Introduction 1.1. The Project The material erupted at a volcano is the product of magma generation in the source region, its subsequent evolution within the mantle and crust and the mode of eruption. In subduction related arcs the roles of fractional crystallization, mixing and contamination within the crust have been clearly demonstrated (O'Hara 1977, Gil11981, De Paolo 1981). The effect of many crustal parameters, such as its thickness, composition, and structure in controlling the degree to which these processes operate to produce different magmas and eruptive activity has been increasingly appreciated (Carr 1984, Singer and Myers 1991). Most recently it has been suggested that gravitational deformation of volcanic constructs may also have an important role in determining the evolution of the magmatic system immediately underneath (Borgia et al. 1990, van Wyk de Vries et al. in press). This thesis investigates the relationship between structures (faults and localized stress concentrations) and the magmatic and volcanic evolution of selected Nicaraguan volcanoes. Three particular aspects are addressed in detail. Firstly, the evolution of multi-vent shield volcanoes and single vent stratocones: secondly, the empirical observation that low-alumina basalts and andesites are associated with shield volcanoes, while high-alumina basalts and andesites occur at stratocones, and thirdly the response of high-level magma evolution to structures originating in the crust, or by volcano spreading. By studying one distinct area of the Nicaraguan volcanic arc this study provides a general understanding of the processes of magmatic and structural interaction. However, the results can be applied to the regional understanding of volcanic activity in Nicaragua and may be useful in the prediction, and monitoring of volcanic hazards. Nicaragua provides an excellent location in which to study the interaction of crustal structures with magmatic systems. Firstly, there is a clearly marked difference between the two main types of volcanoes: shields and stratocones, and a distinct variation in magma composition and petrography. The shield volcanoes erupt low-alumina basalts and andesites, with small phenocryst sizes (<0.5cm), while the stratocones erupt high-alumina basalts and andesites often containing megacrysts (<3cm). Secondly shield volcanoes are, with only one exception, located in areas of crustal faulting, either grabens or diffuse fault zones, while stratocones are not: the crust on which they stand is unfaulted. Thus there is a clear association of volcano type with structures in the crust. Thirdly, the composition of the crust along the volcanic arc in Nicaragua is considered to be uniform (Carr 1984) and volcano spacing is very close (-25km); thus the major variable in the environment through which magmas pass is likely to be structure. Lastly, near-primary magma compositions are common (Walker et al. 1990, Carr et al. 1993) allowing variations in magma composition to be attributed to either primary (source- derived) or secondary (magma chamber) origins. 1.2. Background to volcanic studies in Nicaragua. First descriptions of the volcanoes in Nicaragua come from Oviedo y Valdez (1855), Von Seebach (1892) and Sapper (1937). In 1538, Oviedo and his compatriots aided in a detailed description of the Masaya volcano in their search for the gold in the active lava lake. Their observations on the melting of iron buckets and disappointment on the cooling of extracted liquid probably represent the first thermal and composition observations of arc magmas. The German explorers of this and the last century were the first to apply scientific thought to the volcanoes. Von Seebach (1892) made elegant sketches and descriptions of the volcanoes and noted that the shape of them varied from low hills to steep cones. Sapper (1937) observed that they are arranged in a linear fashion with distinct breaks, and suggested that magmas rise along fissures, possibly associated with the Nicaraguan 'Graben' (Sapper 1937). The term used by Sapper, who wrote in German, was a specific geomorphological term (Manton 1987), 'quersank', which unfortunately was translated into the more specifically structural term, graben, leading to misinterpretation of the origin of the Nicaraguan Depression in later works. The whole of the Nicaraguan chain was described by McBirney and Williams (1965), who noted that some of the volcanoes were cut by north-trending faults, along which vents had formed. Stoiber and Carr (1973) divided the Central American volcanic arc into segments, broken by zones of transverse faulting, containing volcanoes with a tendency for explosive eruptions. One segment boundary was located at Managua, where the 1972 earthquake had drawn attention to the graben in which the city lies (Brown et al. 1973). Ui (1972) made a petrological study of the volcanoes around Masaya and observed that different magma compositions were erupted at different volcanoes, noting that high-alumina basalts were erupted at the stratocones of Mombacho and Apoyo, while iron and magnesia-rich basalts were erupted at Masaya and the Granada alignment The occurrence of near primary magmas erupted from fault zones was demonstrated by Walker (1984) and these, in conjunction with basic rocks from other Nicaraguan volcanoes have been used to constrain the nature of primary magmas parental to all others along the volcanic front (Carr et al.1993), providing a useful constraint for studies of processes within the crust Detailed petrological investigation of magma evolution at Nicaraguan volcanoes has to date only been made on the Nejapa-Granada alignments (Walker 1984), Cerro Negro (Walker and Carr 1986) and