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Chemistry and petrological evolution of the Lastarria volcanic complex in the north Chilean Andes
Geological Magazine
Abstract and Figures
The Lastarria volcanic complex, along the northern Chile–Argentina border, includes three morphostructural components: Southern Spur, Lastarria (the highest cone, 5697 m) and Negriales (a geographically associated lava field, 5.4 km3). Petrographically, the Lastarria complex consists of pyroxene andesites and pyroxene–amphibole dacites. The whole-rock geochemistry shows a bimodial silica variation between 57 and 68%, with peaks at 59–60% and 61.62% SiO2. Petrographie and chemical data indicate different magmatic sources for Lastarria and Negriales. Whole-rock geochemistry can be explained by crustal contamination and crystal–liquid fractionation, with differences in storage times in magma chambers being a major controlling factor. Strong textural, mineralogical and chemical evidence for magma mixing, shortly before explosive eruptions at Lastarria, suggests that this process may have triggered the violent eruptive volcanic activity which characterizes the latest stages of the main cone. Abundant bombs of banded clear pumice and dark scoria in pyroclastic flow deposits are the texturally heterogeneous products resulting from incomplete mixing homogenization.
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... A shortlived magmatic intrusive event was responsible for major explosive eruptions in 1993 (Calder et al., 2000;Gardeweg et al., 1998Gardeweg et al., , 2011Matthews et al., 1994). Lastarria does not have a historical record of eruptive activity but has exhibited major explosive eruptions during the Holocene (Naranjo, 1992). The time intervals separating the major explosive events could have lasted between 1660 and 2390 ± 40 yr according to uncalibrated accelerator mass spectrometry (AMS) measurements on 14 C radiocarbon dating of organic sediments found along deposits of avalanche and ignimbrites (Naranjo, 2010). ...
... The LVC has produced Upper Miocene/Upper Pleistocene andesiticto-dacitic lava flows and domes, with subordinate basaltic andesite lava flows (Naranjo, 1992(Naranjo, , 2010. Pyroclastic rocks corresponding to Lower Pleistocene dacitic ignimbrites are also present. ...
... Sampling at Lastarria was performed on the northwestern flank and concentrated on Holocene pyroclastic-flow deposits containing scoriaceous lapilli and bombs, banded black and beige pumices, and lithic block components (Table 1; Naranjo, 2010). LRA1 and LRA2 are scoriae fragments with sizes ranging from lapilli to bombs that fall under the "grey IV pyroclastic flow deposit" definition reported by Naranjo (1992), grouped here as "Ignimbrite 3" (Naranjo, 2010) (Table 1 provides a list of equivalent names). This flow was emitted by crater 4, where most of the actual summit fumaroles are located (Aguilera, 2008). ...
Lascar (5592 m a.s.l.) and Lastarria (5697 m a.s.l.) are Chilean active stratovolcanoes located in the Central Volcanic Zone (CVZ; 16°S to 28°S) that have developed on top of a 71 km thick continental crust. Independently of the similarities in their Plinian/Vulcanian eruptive styles, their complex magmatic feeding structures and the origins of their magmatic fluids still necessitate constraints in order to improve the reliability of geochemical monitoring. Here we investigate the petrography, bulk-rock chemistry, and mineral chemistry in products from the 1986–1993 explosive eruptive cycle at Lascar and from several Holocene eruptive sequences at Lastarria. These data are integrated with measurements of the noble gas isotopes in fluid inclusions (FIs) of minerals from the same products as well as in fumarole gases. The geochemistry of minerals and rocks shows that the studied products belong to high-K–calc-alkaline series typical of subduction-related settings, and provide evidence of differentiation, mixing, and crustal assimilation that are higher at Lastarria. The contribution of slab sediments and fluids to magma genesis in the wedge is limited, suggesting a homogeneous mantle beneath CVZ. The deepest crystallization processes occurred at variable levels of the plumbing systems according to the lithostatic equivalent depths estimated with mineral equilibrium geobarometers at Lascar (15–29 km) and Lastarria (~20–40 km). The ⁴⁰Ar/³⁶Ar and ⁴He/²⁰Ne ratios in FIs and fumarole gases indicate the presence of some degree of air contamination in the fluids from both volcanoes. The ³He/⁴He values at Lascar (6.9–7.3 Ra) are relatively homogeneous and comparable to those of fumaroles, suggesting a main zone of magma crystallization and degassing. In contrast, the ³He/⁴He values at Lastarria (5.31–8.01 Ra) vary over a wide range, suggesting various magma storage levels and providing evidence of crustal contamination, as indicated by the rock chemistry. We argue that mantle beneath the two volcanoes has a MORB-like signature of ³He/⁴He, while local crustal contamination explains the lower ratios measured at Lascar.
... Therefore, Sc was selected as the reference element. The rock reference database was constructed from the average concentrations of lava and pyroclastic rocks from the Lastarria volcano (Table 2; Naranjo, 1992, Naranjo, 2010Trumbull et al., 1999;Robidoux et al., 2020). The Log EF values for the 22 trace elements varied betweeñ 0 and 6.4. ...
... N.A.: not applicable/measured.TABLE2 Chemical concentration of trace elements (ppm) in flows and pools of molten sulfur collected in January 2019. Average concentrations of host rocks (lavas and pyroclastic flows) used in enrichment factor computations were extracted fromNaranjo (1992Naranjo ( , 2010,Trumbull et al. (1999), andRobidoux et al. (2020). ...
Molten sulfur is found in various subaerial volcanoes. However, limited records of the pools and flows of molten sulfur have been reported: therefore, questions remain regarding the physicochemical processes behind this phenomenon. A suite of new sulfur flows, some of which active, was identified at the Lastarria volcano (northern Chile) and studied using satellite imagery, in situ probing, and temperature and video recording. This finding provides a unique opportunity to better understand the emplacement mechanisms and mineral and chemical compositions of molten sulfur, in addition to gaining insight into its origin. Molten sulfur presented temperatures of 124–158°C, with the most prolonged sulfur flow reaching 12 m from the source. Photogrammetric tools permitted the identification of levees and channel structures, with an estimated average flow speed of 0.069 m/s. Field measurements yielded a total volume of 1.45 ± 0.29 m³ of sulfur (equivalent to ∼2.07 tons) mobilized during the January 2019 event for at least 408 min. Solidified sulfur was composed of native sulfur with minor galena and arsenic- and iodine-bearing minerals. Trace element analysis indicated substantial enrichment of Bi, Sb, Sn, Cd, as well as a very high concentration of As (>40.000 ppm). The January 2019 molten sulfur manifestations in Lastarria appear to be more enriched in As compared to the worldwide known volcanoes with molten sulfur records, such as the Shiretoko-Iozan and Poás volcanoes. Furthermore, their rheological properties suggest that the “time of activity” in events such as this could be underestimated as flows in Lastarria have moved significantly slower than previously thought. The origin of molten sulfur is ascribed to the favorable S-rich chemistry of fumarolic gases and changes in host rock permeability (fracture opening). Molten sulfur in Lastarria correlates with a peak in activity characterized by high emissions of SO2 and other acid species, such as HF and HCl, in addition to ground deformation. Consequently, molten sulfur was framed within a period of volcanic unrest in Lastarria, triggered by changes in the magmatic-hydrothermal system. The appearance of molten sulfur is related to physicochemical perturbations inside the volcanic system and is perhaps a precursor of eruptive activity, as observed in the Poás and Turrialba volcanoes.
... Ignimbrites of distal origin to the Salares Norte District are located on the periphery of the district, such as the Upper Miocene (ca. 10-12 Ma) Salar Grande Ignimbrite (Naranjo, 1992). These tuffs, with thicknesses of up to 50 m, are subhorizontal to gently folded and overly the Early Miocene to Middle Miocene volcanoclastic sequences. ...
... Lastarria ss is the only active structure of the complex and has evolved through 10 eruptive stages (from 260 ± 20 to 2.46 ± 0.050 ka; Naranjo, 2010). Their volcanic products correspond to andesitic-to-dacitic lava flows and domes, pyroclastic flows and fallout deposits, and debris avalanche deposits (Naranjo, 1992;Naranjo, 2010). Although there are no records of historical eruptive activity, a persistent degassing has been observed since the early 19th century (Casertano, 1963;González-Ferrán, 1995), which is concentrated in four fumarolic fields ( Figure 1) located in the northwestern flank (fumarolic field 1), in the eastern and western rim of crater IV (fumarolic fields 2 and 3) ,and inside of crater V (fumarolic field 4) (Aguilera et al., 2012). ...
One of the major problems in the volcanic surveillance is how data from several techniques can be correlated and used to discriminate between possible precursors of volcanic eruptions and changes related to non-eruptive processes. Gas chemical surveys and measurements of SO2 emission rates performed in the past (2006–2019) at Lastarria volcano in Northern Chile have revealed a persistent increment of magmatic sourced gas emissions since late November 2012, following a 13 years period of intense ground uplift. In this work, we provide new insights into the gas-chemical evolution of Lastarria’s fumarolic discharges obtained from direct sampling (2006–2019) and SO2 emission rates using UV camera and DOAS instruments (2018–2019) and link these to pre-existing information on ground deformation (1998–2016) in order to determine the origin of observed degassing and ground deformation processes. We revise the four mechanisms originally proposed as alternatives by Lopez et al. (Geosphere, 2018, 14 (3), 983–1007) to explain the changes observed in the fluid geochemistry and ground deformation between 2009 and 2012, in order to explain major changes in gas-geochemistry over an extended period between 1998 and 2019. We hypothesize that a continuous sequence of processes explains the evolution in the fluid geochemistry of fumarolic discharges. Two mechanisms are responsible of the changes in the gas composition during the studied period, corresponding to a 1) deep magma chamber (7–15 km depth) pressurized by volatile exsolution (1998–2020), which is responsible of the large-scale deformation; followed by 2) a crystallization-induced degassing (2001–2020) and pressurization of the hydrothermal system (2003-early November 2012), where the former process induced the changes in the gas composition from hydrothermal-dominated to magmatic-dominated, whereas the last produced the small-scale deformation at Lastarria volcano. The changes in the gas composition since late November 2012, which were strongly dominated by magmatic volatiles, produced two consecutive processes: 1) acidification (late November 2012–2020) and 2) depletion (2019–2020) of the hydrothermal system. In this work we have shown that a long-term surveillance of the chemistry of fluid discharges provides valuable insights into underlying magmatic/volcanic processes, and consequently, for forecasting future eruptions.
... Mixing processes are deduced from the presence of plagioclase phenocrysts with reabsorbed edges and sieve textures, the occurrences of biotites, and amphibole phenocrysts with evidence of Fe oxidation, and clinopyroxenes and olivines with alteration rims, as well as the presence of other disequilibrium textures. These features suggest magma-mixing at different stages and levels during the evolutionary stages of Guallatiri volcano, as seen in other CVZ centers (e.g., Lastarria volcano; Naranjo, 1992). It is noteworthy that the occurrence of rounded and reabsorbed plagioclase grains and quenched phenocrysts indicates that magma mixing resulted in a re-homogenization of magmas and therefore was not a short-term pre-eruptive process, as seen in other volcanic centers of the region as Parinacota and Pomerape volcanoes (Wӧrner et al., 1988;Davidson et al., 1990). ...
Guallatiri volcano is an active stratovolcano located in the Arica y Parinacota Region of northern Chile. It belongs to the Central Volcanic Zone (CVZ) of the Andean range and is the youngest and southernmost volcano of the Nevados de Quimsachata volcanic chain. Guallatiri is an ice-capped volcano considered the third-highest risk center in northern Chile with a current activity characterized by two fumarolic fields with strong and persistent gas emissions. This work presents the first detailed geological mapping of the volcano based on stratigraphic, petrographic, morphological, and geochemical data. The volcano has evolved into seven units, whit the initial stages characterized by the widespread and effusive emission of several thick lava flows around the volcanic edifice. The later evolution units involved the extrusion of lava domes at the summit, with a Holocene plinian eruption accompanied by pyroclastic density currents, tephra fallout, and lahars. Furthermore, two parasitic Holocene domes, Tinto and South domes were nested on the volcano's southwestern flank. High eruptive rates (0.36 km³/ka) indicate that Guallatiri volcano has been one of the most active volcanoes during the Late Pleistocene-Holocene. The volcanic products from Guallatiri mainly correspond to trachyandesites, trachydacites, and dacites rich in amphibole and biotite, with minor clinopyroxene and olivine. They belong to the High K2O Calc-Alkaline Series, similar to other active volcanoes of the region. The construction of detailed geological maps of active volcanoes is used as the basis for the generation of hazard and risk maps, which enable the mitigation of associated volcanic risks. In the case of Guallatiri, the occurrence of lahars, pyroclastic flows, and tephra fallout are considered the main volcanic hazards, which could affect the villages of Guallatire and Ancuta, in addition to small hamlets in Bolivia.
... We note that the calculated volatile-source proportions observed at Lastarria are similar to those of other Central Andean volcanoes including Lascar (Tassi et al., 2009) and Tacora (Capaccioni et al., 2011) volcanoes in northern Chile, suggesting that they have a similar crustal or subducted-slab carbonate source. Because the basement rocks within the Lastarria region are mostly composed of Paleozoic intrusive rocks and Tertiary-Quaternary volcanic rocks (Naranjo, 1992;Naranjo and Cornejo, 1992;Mamani et al., 2008), we can assume that the carbonate and organic volatile signatures are derived from the subducted slab, indicating that the subducted slab is the main volatile source at Lastarria. This interpretation is consistent with the findings of Aguilera et al. (2016), who infer a subducted sediment source for condensed gases sampled from Lastarria in 2014. ...
Recent geophysical evidence for large-scale regional crustal inflation and localized crustal magma intrusion has made Lastarria volcano (northern Chile) the target of numerous geological, geophysical, and geochemical studies. The chemical composition of volcanic gases sampled during discrete campaigns from Lastarria volcano indicated a well-developed hydrothermal system from direct fumarole samples in A.D. 2006, 2008, and 2009, and shallow magma degassing using measurements from in situ plume sampling techniques in 2012. It is unclear if the differences in measured gas compositions and resulting interpretations were due to artifacts of the different sampling methods employed, short-term excursions from baseline due to localized changes in stress, or a systematic change in Lastarria's magmatic-hydrothermal system between 2009 and 2012. Integrated results from a two-day volcanic gas sampling and measurement campaign during the 2014 International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) Commission on the Chemistry of Volcanic Gases (CCVG) 12th Gas Workshop are used here to compare and evaluate current gas sampling and measurement techniques, refine the existing subsurface models for Lastarria volcano, and provide new constraints on its magmatic-hydrothermal system and total degassing budget. While compositional differences among sampling methods are present, distinct compositional changes are observed, which if representative of longterm trends, indicate a change in Lastarria's overall magmatic-hydrothermal system. The composition of volcanic gases measured in 2014 contained high proportions of relatively magma- and water-soluble gases consistent with degassing of shallow magma, and in agreement with the 2012 gas composition. When compared with gas compositions measured in 2006-2009, higher relative H2O/CO2 ratios combined with lower relative CO2/St and H2O/St and stable HCl/St ratios (where St is total S [SO2 + H2S]) are observed in 2012 and 2014. These compositional changes suggest variations in the magmatic-hydrothermal system between 2009 and 2012, with possible scenarios to explain these trends including: (1) decompression-induced degassing due to magma ascent within the shallow crust; (2) crystallization-induced degassing of a stalled magma body; (3) depletion of the hydrothermal system due to heating, changes in local stress, and/or minimal precipitation; and/or (4) acidification of the hydrothermal system. These scenarios are evaluated and compared against the geophysical observations of continuous shallow inflation at ~8 km depth between 1997 and 2016, and near-surface ( < 1 km) inflation between 2000 and 2008, to further refine the existing subsurface models. Higher relative H2O/CO2 observed in 2012 and 2014 is not consistent with the depletion or acidification of a hydrothermal system, while all other observations are consistent with the four proposed models. Based on these observations, we find that scenarios 1 or 2 are the most likely to explain the geochemical and geophysical observations, and propose that targeted shallow interferometric synthetic-aperture radar (InSAR) studies could help discriminate between these two scenarios. Lastly, we use an average SO2 flux of 604 ± 296 t/d measured on 22 November 2014, along with the average gas composition and diffuse soil CO2 flux measurements, to estimate a total volatile flux from Lastarria volcano in 2014 of ~12,400 t/d, which is similar to previous estimates from 2012.
... cuenta con numerosas dataciones que indican actividad de 0,33 Ma a Holoceno (Naranjo 2010; Tabla 1). Presenta 5 cráteres alineados N-S y está compuesto por lavas y abundantes depósitos piroclásticos de composición andesítica (Naranjo 1992, Naranjo et al. 2013b. Sobre su flanco oriental se encuentra una pequeña avalancha de detritos producida por un colapso sectorial (Naranjo y Francis 1987). ...
... Un segundo grupo se ubica al oeste del salar de Antofalla, donde se encuentran los volcanes Escorial, Lastarria, Cordón de Azufre y Cerro Bayo Gorbea (e.g. Richards y Villeneuve 2002, Naranjo 1992, 2010, Naranjo et al. 2013 Giordano et al. 2013. ...
In the Central Andes, large (> 500 km²) and long-lived (1–5 Ma) volcanic clusters (LVCs) are less explored and their eruptive history and magmatic regimes less understood than smaller, short-lived (<0.5 Ma), individual stratocones. The Chachani-large volcanic cluster (C-LVC) sizeable volume (c. 290 km³) consists of twelve edifices forming the 1.06–0.64 Ma group of stratovolcanoes and the 0.46–0.05 Ma group of domes coulees and block-lava flow fields. Both groups overlie pre-Chachani lavas and tuffs 1.02–1.27 Ma, and together they have buried large nested craters or a caldera associated with the c. 1.62–1.66 Ma Arequipa Airport ignimbrite. The C-LVC evolved from: (i) homogeneous compositions of the pre-Chachani and Chachani basal eruptive units to (ii) relatively wide compositional variations (53–67 wt% SiO2) between mafic andesite and dacite at moderate eruptive rates (0.27–0.41 km³/ka) for the ‘Old Edifice’ group, and finally to (iii) narrower (57–64 wt% SiO2) andesitic compositions coinciding with extrusive activity at 2.5 times lower eruptive rates (0.12–0.15 km³/ka) for the ‘Young Edifice’ group. The large compositional variations in the Old Edifice group are related to strongly contrasting resident and recharge magma compositions of hybridized lavas. In contrast, the narrow compositional range and lower eruption rate during the second half of the C-LVC eruptive history represent a trend towards more homogeneous, andesitic magma composition with time. Mineral texture and compositional studies provide evidence for disequilibrium and magma mixing in the C-LVC shallow (5–20 km depth range) magma reservoirs. These temporal changes in magma composition document that the transcrustal magma systems of the C-LVC evolved and matured with time by a combination of processes: fractional crystallization, crustal contamination and magma mixing/mingling with variable rates of mafic recharge. This resulted in a shift in time to a steady state, monotonous (andesite) regime as a result of coupling between compositional parameters and thermal conditions, density constraints, and the viscosity/crystallinity of erupted magmas.
Subduction-related volcanism in the Nevados de Payachata region of the Central Andes at 18 °S comprises two temporally and geo-chemically distinct phases. An older period of magmatism is represented by glaciated strato-cones and ignimbrite sheets of late Miocene age. The Pleistocene to Recent phase (_<0.3 Ma) includes the twin stratovolcanoes Volcan Pomerape and Volcan Parinacota (the Nevados de Paya-chata volcanic group) and two small centers to the west (i. e., Caquena and Vilacollo). Both stratovol-canoes consist of an older dome-and-flow series capped by an andesitic cone. The younger cone, i. e., V. Parinacota, suffered a postglacial cone collapse producing a widespread debris-avalanche deposit. Subsequently, the cone reformed during a brief, second volcanic episode. A number of small, relatively mafic, satellitic cinder cones and associated flows were produced during the most recent activity at V. Parinacota. At the older cone, i.e., V. Pomerape, an early dome sequence with an overlying isolated mafic spatter cone and the cone-forming andesitic-dacitic phase (mostly flows) have been recognized. The two Nevados de Payachata stratovolcanoes display continuous major-and trace-element trends from high-K20 basaltic andesites through rhyolites (53%-76% SiO2) that are weil defined and distinct from those of the older volcanic centers. Petrography, chemical composition, and eruptive styles at V. Parina-cota differ between pre-and post-debris-avalanche lavas. Precollapse flows have abundant am-phibole (at SiO2 > 59 wt%) and lower Mg numbers than postcollapse lavas, which are generally less silicic and more restricted in composition. Com-positional variations indicate that the magmas of the Nevados de Payachata volcanic group evolved through a combination of fractional crys-tallization, crustal assimilation, and intratrend magma mixing. Isotope compositions exhibit only minor variations. Pb-isotope ratios are relatively low (2°6pb/2°4pb = 17.95-18.20 and 2°8pb/ 2°4pb = 38.2-38.5); 87Sr/86Sr ratios range 0.70612-0.70707, 143Nd/144Nd ratios range 0.51238-0.51230, and 818OsMow values range from + 6.8%o to + 7.6%o SMOW. A comparison with other Central Volcanic Zone centers shows that the Ne-vados de Payachata magmas are unusually rich in Ba (up to 1800 ppm) and Sr (up to 1700 ppm) and thus represent an unusual chemical signature in the Andean arc. These chemical and isotope variations suggest a complex petrogenetic evolution involving at least three distinct components. Primary mantle-derived melts, which are similar to those generated by subduction processes throughout the Andean arc, are modified by deep crustal interactions to produce magmas that are parental to those erupted at the surface. These magmas subsequently evolve at shallower levels through assimilation-crystallization processes involving upper crust and intratrend magma mixing which in both cases were restricted to end members of low isotopic contrast.
The geochemistry and phenocryst mineralogy of orogenic basalts and andesites are discussed on the basis of an extensive data bank, mainly drawn from the circum-Pacific region and the Mediterranean. The survey reveals major differences between the lavas of continental margins and island arcs, especially in the relative proportions of the different magma compositions which are erupted. All the orogenic lavas are rich in phenocrysts, which are of plagioclase, augite, orthopyroxene, titanomagnetite, olivine, or hornblende in calc-alkaline andesites, but hornblende and biotite in high-K and shoshonitic andesites. The ferromagnesian phenocrysts in all the orogenic lavas are Mg-rich. Correlations between geochemistry and phenocryst mineralogy are believed to reflect the 'blanketing' effect of low-P fractional crystallization in determining the magma characteristics. -A.H.
Major and trace element and strontium isotope data indicate that the Mt. Burney, Reclus, Aguilera, and Hudson volcanoes are the source vents for some of the widespread Holocene tephra layers in southernmost Patagonia. Tentative correlations of these tephra, based on the limited available radiocarbon chronological control, indicate that the eruption of these volcanoes was not synchronous as believed by Vaino Auer, and that the simplistic Holocene tephrochronology proposed by Auer - Tephra I, Tephra II, and Tephra III - is not reliable, neither in southernmost Patagonia nor throughout Patagonia. -from Author
The uniformity of rare earth element patterns in sedimentary rocks is used to provide estimates for the composition of the Post-Archean upper crust. This approximates to granodiorite. The island-arc model for continental growth is used to provide a total crustal composition, from which the upper crust is derived by partial melting, leaving a depleted lower crust. The Archean sedimentary rock patterns reveal a different composition for the upper crust in that era. They closely resemble those of present- day island-arc suites, which the composition of the Archean crust is inferred to be less fractionated, with much less development of high-level granodiorites. Continental evolution appears to be episodic, rather than uniform, with a major period of production of differentiated upper crust at about 2.5 aeons, long recognized as the Archean-Protoerozoic boundary.-from Author
There are well established differences in the chemical and isotopic characteristics of the calc-alkaline basalt—andesite-dacite-rhyolite association of the northern (n.v.z.), central (c.v.z.) and southern volcanic zones (s.v.z.) of the South American Andes. Volcanic rocks of the alkaline basalt-trachyte association occur within and to the east of these active volcanic zones. The chemical and isotopic characteristics of the n.v.z. basaltic andesites and andesites and the s.v.z. basalts, basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere. Conversely, the alkaline lavas are derived from basaltic parent magmas formed from mantle of ‘within-plate’ character. Recent basaltic andesites from the Cerro Galan volcanic centre to the SE of the c.v.z. are derived from mantle containing both subduction zone and within-plate components, and have experienced assimilation and fractional crystallization (a.f.c.) during uprise through the continental crust. The c.v.z. basaltic andesites are derived from mantle containing subduction-zone components, probably accompanied by a.f.c. within the continental crust. Some c.v.z. lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing. The petrogenesis of the c.v.z. lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation, fractional crystallization, and magma mixing during uprise through the continental crust.