Charles R. Stern

Active Andean volcanism: its geologic and tectonic setting

El arco volcanico andino incluye mas de 200 estratovolcanes y, al menos, 12 sistemas de calderas gigantes potencialmente activos, dispuestos en cuatro segmentos separados de la cadena andina conocidos como Zonas Volcanicas Norte, Central, Sur y Austral, y cuya actividad es producto de la subduccion de las placas oceanicas Nazca y Antartica bajo la placa sudamericana. Los cuatro segmentos con volcanismo activo ocurren en zonas donde el angulo de subduccion es relativamente inclinado (25°), y entre ellos existen regiones donde el angulo de subduccion es relativamente plano (<10°) a profundidades 100 km y el volcanismo esta ausente. Las zonas de bajo angulo de subduccion habrian comenzado a formarse durante el Mioceno debido a la subduccion de plateaus y dorsales oceanicas, indicando que la actual segmentacion de la zona de subduccion y el volcanismo andino es un rasgo transitorio relacionado a la actividad tectonica neogena. La relacion genetica entre subduccion y volcanismo ha sido confirmada por estudios geoquimicos que indican que la actividad magmatica se inicia por la deshidratacion y/o fusion de la litosfera oceanica subductada y la interaccion de los fluidos liberados con el manto astenosferico que la sobreyace. Componentes derivados de la corteza continental son tambien incorporados en los magmas andinos a traves de la erosion por subduccion del margen continental y/o asimilacion de material cortical en los magmas derivados del manto. Las variaciones en la tasa de erosion por subduccion y subduccion de corteza continental afectan en forma significativa no solo la quimica de los magmas andinos, sino tambien el acomplamiento mecanico de intraplaca en la zona de subduccion y la dinamica orogenica a lo largo de los Andes. Componentes corticales son mas significativos en los magmas extruidos en la Zona Volcanica Central donde la corteza es extremadamente gruesa (70 km) y las tasas de erosion por subduccion del margen continental alcanzan, posiblemente, a consumir un volumen de rocas equivalente hasta un 4% del volumen de la corteza oceanica subductada, son tambien muy elevadas debido a las condiciones climaticas hiperaridas y el bajo aporte de sedimentos a la fosa. Lavas, flujos piroclasticos, lahares, flujos de detritos producto de colapso sectorial de estratovolcanes, y la caida de tefra son algunos de los peligros y riesgos mas importantes asociados al volcanismo andino. Desde el ano 1532 mas de 25.000 personas han muerto como consecuencia de 600 erupciones con registro historico. La mayor parte de estas muertes ocurrio en 1985 durante la erupcion de los Nevados del Ruiz en Colombia. A pesar de que mas de 20 millones de personas viven a menos de 100 km de distancia de un volcan activo en los Andes, principalmente en los valles interandinos de Colombia y Ecuador y el Valle Central del centro-sur de Chile, en la actualidad, menos de 25 volcanes estan siendo monitoreados para determinar los riesgos potenciales asociados a la actividad volcanica andina.

Rapid magma ascent recorded by water diffusion profiles in mantle olivine

Mechanisms and rates of magma ascent play a critical role in eruption dynamics but remain poorly constrained phenomena. Water, dissolved in mantle minerals as hydrogen and partitioned into the magma during ascent, may provide clues to quantifying magma ascent rates prior to eruption. We determined the dehydration profiles in olivine crystals from peridotite mantle xenoliths within the Pali-Aike alkali basalt from Patagonia, Chile. The results demonstrate that the amount of water stored in the uppermost mantle has likely been underestimated due to water loss during transport. Using experimental diffusion data for hydrogen, we estimate that the xenoliths reached the surface from 60–70 km depth in several hours, a surprisingly rapid rise comparable to ascent rates for kimberlite magmas.

Trace-element and Sr, Nd, Pb, and O isotopic composition of Pliocene and Quaternary alkali basalts of the Patagonian Plateau lavas of southernmost South America

The Pliocene and Quaternary Patagonian alkali basalts of southernmost South America can be divided into two groups. The “cratonic” basalts erupted in areas of Cenozoic plateau volcanism and continental sedimentation and show considerable variation in 87Sr/86Sr (0.70316 to 0.70512), 143Nd/144Nd (ɛNd) and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios (18.26 to 19.38, 15.53 to 15.68, and 38.30 to 39.23, respectively). These isotopic values are within the range of oceanic island basalts, as are the Ba/La, Ba/Nb, La/Nb, K/Rb, and Cs/Rb ratios of the “cratonic” basalts. In contrast, the “transitional” basalts, erupted along the western edge of the outcrop belt of the Pliocene and Quaternary plateau lavas in areas that were the locus of earlier Cenozoic Andean orogenic arc colcanism, have a much more restricted range of isotopic composition which can be approximated by 87Sr/86Sr=0.7039±0.0004, ɛNd, 206Pb/204Pb=18.60±0.08, 207Pb/204Pb=15.60±0.01, and 208Pb/204Pb=38.50±0.10. These isotopic values are similar to those of Andean orogenic are basalts and, compared to the “cratonic” basalts, are displaced to higher 87Sr/86Sr at a given 143Nd/144Nd and to higher 207Pb/204Pb at a given 208Pb/204Pb. The “transitional” basalts also have Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios higher than the “cratonic” and oceanic island basalts, although not as high as Andean orogenic are basalts. In contrast to the radiogenic isotopes, δ18O values for both groups of the Patagonian alkali basalts are indistinguishable and are more restricted than the range reported for Andean orogenic are basalts. Whole rock δ18O values calculated from mineral separates for both groups range from 5.3 to 6.5, while measured whole rock δ18O values range from 5.1 to 7.8. The trace element and isotopic data suggest that decreasing degrees of partial melting in association with lessened significance of subducted slabderived components are fundamental factors in the west to east transition from arc to back-arc volcanism in southern South America. The “cratonic” basalts do not contain the slab-derived components that impart the higher Ba/La, Ba/Nb, La/Nb, Cs/Rb, 87Sr/86Sr at a given 143Nd/144Nd, 207Pb/204Pb at a given 208Pb/204Pb, and δ18O to Andean orogenic arc basalts. Instead, these basalts are formed by relatively low degrees of partial melting of heterogeneous lower continental lithosphere and/or asthenosphere, probably due to thermal and mechanical pertubation of the mantle in response to subduction of oceanic lithosphere below the western margin of the continent. The “transitional” basalts do contain components added to their source region by either (1) active input of slab-derived components in amounts smaller than the contribution to the mantle below the arc and/or with lower Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios than below the arc due to progressive downdip dehydration of the subducted slab; or (2) subarc source region contamination processes which affected the mantle source of the “transitional” basalts earlier in the Cenozoic.

Role of subduction erosion in the generation of Andean magmas

Subduction erosion, or scraping off of the continental crust by subducting oceanic lithosphere, is an important tectonic process along parts of the Andean convergent plate boundary. Estimated rates of subduction erosion increase northward north of the southem Andes, due to decreasing subduction angle and decreasing sediment supply to the trench. West of the central Andes subduction erosion may remove as much as 535 km 3 of crust per 1 km of trench per 1 m.y., or 7 vol% of the subducting oceanic lithosphere. The higher 87 Sr/ 86 Sr ratios of mafic magmas erupted north of the southern Andes may be explained by the higher rates of subduction erosion and increased amounts of contamination of the sub-arc mantle magma source.

Geochemistry of Mesozoic marginal basin floor igneous rocks from southern Chile

Extension behind a Late Jurassic continental margin volcanic arc in southern Chile caused rifting and the development of a narrow marginal basin floored by oceanic crust. Extension ceased and the basin was closed and uplifted in mid-Cretaceous time, so the basin floor is now exposed as the upper part of an autochthonous ophiolite complex composed of gabbros, sheeted dikes, and pillow lavas, with minor plagiogranite and associated siliceous dikes. Many of the rocks are altered. The metamorphic grade increases from zeolite or greenschist facies in the pillow lavas to amphibolite facies in the gabbros, but the maximum intensity of recrystallization occurs in the sheeted dike unit and is associated with loss of Rb and K and increasing K/Rb ratio, contrasting with the effects produced by low-temperature alteration of basalts by sea water. Metamorphic effects seem to be related to hydrothermal convective systems operating at the spreading axis at the time of basin formation. Geochemically, the rocks have affinities with mid-oceanic ridge basalts, but K, Rb, and Ba contents and Ba/Sr and Ce/Yb ratios are higher and K/Rb ratios are lower in the least altered rocks than in mid-oceanic ridge basalts. Similar features are apparent in some other marginal basin basalts. Fractionation trends are tholeiitic, the mafic rocks displaying a wide range of Fe/Mg ratios (0.9 to 5.2) but without any concomitant silica enrichment. Rare-earth elements, TiO 2 , and Zr correlate positively and Cr and Ni negatively with Fe/Mg, while the gabbros have lower contents of some incompatible elements as a result of their cumulate nature. The leucocratic rocks within the mafic complex have been derived from two distinct sources. Some trondhjemites and granophyres have compositions indicating derivation by refusion of continental material bordering the mafic complex. The plagiogranites, however, have a distinctive geochemistry, consistent with an origin by high-level differentiation of the mafic magmas. Such rocks, normally lying in or just below the sheeted dike unit, may be a common if minor component of oceanic crust.

Field and geochemical data bearing on the development of a mesozoic volcano-tectonic rift zone and back-arc basin in southernmost South America

Abstract A broad zone of dominantly subaerial silicic volcanism associated with regional extensional faulting developed in southern South America during the Middle Jurassic, contemporaneously with the initiation of plutonism along the present Pacific continental margin. Stratigraphic variations observed in cross sections through the silicic Jurassic volcanics along the Pacific margin of southernmost South America indicate that this region of the rift zone developed as volcanism continued during faulting, subsidence and marine innundation. A deep, fault-bounded submarine trough formed near the Pacific margin of the southern part of the volcano-tectonic rift zone during the Late Jurassic. Tholeiitic magma intruded within the trough formed the mafic portion of the floor of this down-faulted basin. During the Early Cretaceous this basin separated an active calc-alkaline volcanic arc, founded on a sliver of continental crust, from the then volcanically quiescent South American continent. Geochemical data suggest that the Jurassic silicic volcanics along the Pacific margin of the volcano-tectonic rift zone were derived by crustal anatexis. Mafic lavas and sills which occur within the silicic volcanics have geochemical affinities with both the tholeiitic basalts forming the ophiolitic lenses which are the remnants of the mafic part of the back-arc basin floor, and also the calc-alkaline rocks of the adjacent Patagonian batholith and their flanking lavas which represent the eroded late Mesozoic calc-alkaline volcanic arc. The source of these tholeiitic and calc-alkaline igneous rocks was partially melted upper mantle material. The igneous and tectonic processes responsible for the development of the volcano-tectonic rift zone and the subsequent back-arc basin are attributed to diapirism in the upper mantle beneath southern South America. The tectonic setting and sequence of igneous and tectonic events suggest that diapirism may have been initiated in response to subduction.

Basalt-andesite-rhyolite-H2O: Crystallization intervals with excess H2O and H2O-undersaturated liquidus surfaces to 35 kolbras, with implications for magma genesis

Three rocks representing the calc-alkaline rock series gabbro-tonalite-granite or basalt-andesite-rhyolite were reacted with varying percentages of water in sealed capsules between 600 and 1300°C and pressures to 36 kbars, corresponding to depths of more than 120 km within the earth. For each rock we present complete P-T diagrams with excess water, and the water-undersaturated liquids surface projected from P-T-X_(H_2O) space mapped with contours for constant H_2O contents and with the fields for near-liquidus minerals. All changes in liquidus and solidus slopes can be correlated with changes in mineralogy from less dense to more dense, or with expansion of crystallization fields, without appeal to changes in molar volume of H_2O in liquid and vapor phases. The results indicate that tholeiites and andesites of the calc-alkaline series with compositions similar to the rocks studied are not primary magmas from mantle peridotite at depths greater than about 50 km. Primary andesitic magmas from shallower levels would require very high water contents and we do not believe such magmas could normally reach the surface. The liquids results are consistent with the derivation of andesites with little dissolved water as primary magmas from subducted ocean crust (quartz eclogite), but multi-stage models are preferred. Temperatures required for the generation of andesites by fusion of continental crust are higher than considered reasonable. The evidence precludes the generation of primary rhyolites or granites from the mantle of subducted oceanic crust at mantle depths. Primary rhyolite or granite magmas with moderate water contents (saturated or undersaturated) can be generated in the crust at reasonable temperatures, and could reach near-surface levels before vesiculation. Water-undersaturated granite liquid with residual crustal minerals could constitute plutonic magmas of intermediate composition.

Sr and Nd isotopic and trace element compositions of Quaternary volcanic centers of the Southern Andes

Isotopic compositions of samples from six Quaternary volcanoes located in the northern and southern extremities of the Southern Volcanic Zone (SVZ, 33–46°S) of the Andes and from four centers in the Austral Volcanic Zone (AVZ, 49–54°S) range for 87Sr/86Sr from 0.70280 to 0.70591 and for 143Nd/144Nd from 0.51314 to 0.51255. The ranges are significantly greater than previously reported from the southern Andes but are different from the isotopic compositions of volcanoes in the central and northern Andes. Basalts and basaltic andesites from three centers just north of the Chile Rise-Trench triple junction have 87Sr/86Sr,143Nd/144Nd,La/Yb,Ba/La, and Hf/Lu that lie within the relatively restricted ranges of the basic magmas erupted from the volcanic centers as far north as 35°S in the SVZ of the Andes. The trace element and Sr and Nd isotopic characteristics of these magmas may be explained by source region contamination of subarc asthenosphere, with contaminants derived from subducted pelagic sediments and seawater-altered basalts by dehydration of subducted oceanic lithosphere. In the northern extremity of the SVZ between 33° and 34°S, basaltic andesites and andesites have higher 87Sr/86Sr,Rb/Cs, and Hf/Lu, and lower 143Nd/144Nd than basalts and basaltic andesites erupted farther south in the SVZ, which suggests involvement of components derived from the continental crust. In the AVZ, the most primitive sample, high-Mg andesite from the southernmost volcanic center in the Andes (54°S) has Sr and Nd isotopic compositions and KR/b and Ba/La similar to MORB. The high La/Yb of this sample suggests formation by small degrees of partial melting of subducted MORB with garnet as a residue. Samples from centers farther north in the AVZ show a regionally regular northward increase in SiO2, K2O, Rb, Ba,Ba/La, and 87Sr/86Sr/ and decrease in MgO, Sr,KR/b,Rb/Cs, and 143Nd/144Nd, suggesting increasingly greater degrees of fractional crystallization and associated intra-crustal contamination.

Rocas Verdes ophiolites, southernmost South America: remnants of progressive stages of development of oceanic-type crust in a continental margin back-arc basin

Abstract The Mesozoic Rocas Verdes are a group of mafic igneous complexes in the southernmost Andes. They consist of pillow lavas, dykes and gabbros, interpreted as the upper parts of ophiolites formed along mid-ocean-ridge-type spreading centres that rifted the southwestern margin of the Gondwana continental crust, during the onset of spreading in the South Atlantic, to form the mafic igneous part of the floor of a back-arc basin behind a contemporaneous convergent plate boundary magmatic arc. Mafic dykes and gabbros intrude older continental lithologies along both flanks of the Rocas Verdes, and these same leucocratic country rocks are engulfed in the Rocas Verdes mafic complexes. These relations indicate that the Rocas Verdes ophiolites formed in place and are autochthonous. Zircon U/Pb ages, as well as both chemical and lithostructural characteristics of these ophiolite complexes, suggest that the Rocas Verdes basin formed by ‘unzipping’ from the south to the north, with the southern part beginning to form earlier, and developing more extensively, than the northern part of the basin. The Rocas Verdes ophiolites contain a wealth of information about progressive stages of continental rifting during back-arc basin formation, magmatic and metamorphic processes along mid-ocean-ridge-type spreading centres, and as analogues to Archaean greenstone belts.

Constraints on the interaction between slab melts and the mantle wedge from adakitic glass in peridotite xenoliths

Peridotite xenoliths from the Quaternary Cerro del Fraile basalts, southernmost South America, sample the mantle less than 25 km east of the Andean Austral Volcanic Zone (AVZ), an arc segment characterized by melting of a young, ‘hot’, subducted slab and the eruption of adakites. Many of these peridotite xenoliths are modified by either modal and/or cryptic Na-rich metasomatism, which produced elevated Sr/Y, La/Yb and La/Nb ratios typical of slab melts. Some of the metasomatized xenoliths, derived from a relatively deep and hot portion of the mantle, contain an interconnected network along mineral grain boundaries of high-Mg#, low-Y andesitic glass with major and trace element composition similar to the high-Mg adakites erupted in the AVZ. We interpret this adakitic glass to be a quenched slab melt that has infiltrated the mantle wedge from below. The texture and chemistry of this quenched melt and surrounding mantle minerals suggest that selective assimilation of predominately mantle clinopyroxene, some spinel and minor olivine is an important process in producing high-Mg adakites from primary low-Mg slab melts.