Miguel A. Parada

Multiple sources for the Coastal Batholith of central Chile (31–34°S): geochemical and Sr–Nd isotopic evidence and tectonic implications

The Coastal Batholith of central Chile between 31 and 34°S represents a locus of long-lived (c. 200 Ma) plutonic activity. It is composed of four plutonic complexes: Santo Domingo, Limari, Papudo–Quintero and Illapel. These complexes, which occur as north-trending discontinuous belts that decrease in age eastward, were emplaced during four major episodes of arc magmatism. The Santo Domingo Complex (Carboniferous) comprises hornblende–biotite bearing tonalites and granodiorites with abundant mafic enclaves, and enclave-free granites. The association of enclave-bearing granitoids and enclave-free granites resulted from different degrees of mixing of crustal- and mantle-derived magmas. The Limari Complex (Lower Jurassic) is a bimodal association of crustal leucogranites and mantle-derived gabbros. The lithology of the Papudo–Quintero Complex (Middle Jurassic) varies from hornblende–pyroxene diorite to biotite granite. The main rock types, namely tonalites and granodiorites, commonly contain mafic enclaves. The granitoids and the enclaves have remarkably similar REE patterns and are similar in Sr–Nd isotope composition, which suggest that they represent a cogenetic suite of mantle origin. The Illapel Complex (Cretaceous) consists of hornblende bearing tonalites, trondhjemites and granodiorites that resemble the rocks of TTG suites. The eNd values and initial ratios (Sri) for the mafic rocks of the Carboniferous Complex (eNd: −2.0 and −3.5; Sri: c. 0.7057) differ markedly from those of the Mesozoic mafic rocks (eNd: +1.9 to +5.4; Sri: 0.7033 to 0.7039), the Cretaceous rocks being the most depleted with regard to Sr–Nd. A change from Carboniferous lithosphere-dominated sources to Mesozoic asthenosphere-dominated sources of the magmas that started in the Early Jurassic is recognized. Slab components can be recognized in the source of the Cretaceous rocks. The crustal source of the Lower Jurassic leucogranites was more isotopically depleted and refractory than that of the Carboniferous granites. On the other hand, no crustal participation in the origin of the Middle Jurassic and Cretaceous felsic granitoids is recognized; they are differentiated from mantle-derived magmas. Progressive removal of old lithospheric mantle during the Mesozoic due to a process of lithospheric delamination would explain the profound change in the subcrustal source, as well as the melting and modification towards a more refractory nature of the continental crust.

Granitoids of the Coast Range of central Chile: Geochronology and geologic setting

The Southern Coastal Batholith of central Chile is mainly composed of calc-alkaline granitoids of Late Carboniferous to Permian age. There is no clear evidence of the existence of older plutonic rocks in the batholith. The granitoids are intruded into an accretionary prism which underwent contemporaneous metamorphism. Triassic high-level plutons of limited areal extent occur as post-tectonic bodies in the high P/T metamorphic belt of the subduction complex. Jurassic plutons, which increase in volume toward the northern limit of the area and beyond, are associated with a tectonomagmatic event which, in certain localities, remobilized and re-set the ages of the Paleozoic granitoids. Cretaceous plutons occur to the east of the Southern Coastal Batholith. Initial 87Sr/86Sr ratios of the Paleozoic granitoids (0.706 to 0.707) are higher than those of the Mesozoic plutons (∼0.704) which were derived from a source similar to that of the present-day volcanic rocks of this segment of the Andes. The northern limit of the Paleozoic rocks is presumed to be an east-west fault zone, apparently inactive since Jurassic times. The continuity of the Mesozoic granitoids is not affected by this boundary.

Calbuco Volcano and minor eruptive centers distributed along the Liquiñe-Ofqui Fault Zone, Chile (41°–42° S): contrasting origin of andesitic and basaltic magma in the Southern Volcanic Zone of the Andes

Calbuco volcano is a Late Pleistocene-Holocene composite stratovolcano located at 41°20′ S, in the southern region of the Southern Volcanic Zone of the Andes (SSVZ; 37°–46° S). In contrast to basalt and basaltic andesite, which are the dominant lava types on the volcanic front from 37° to 42° S, Calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths. They have SiO2 contents in the 55–60% range, and have comparatively low K2O, Rb, Ba, Th and LREF abundances relative to other SSVZ centers. Incompatible element abundance ratios are similar to those of most SSVZ volcanics, but 87Sr/86Sr and 143Nd/144Nd are respectively higher and lower than those of adjacent volcanic centers. Basalts from nearby Osorno stratovolcano, 25 km to the northeast, are similar to other basaltic SSVZ volcanoes. However, basalts from several minor eruptive centers (MEC), located east of Calbuco and Osorno volcano along the Liquiñe-Ofqui fault zone (LOFZ), are enriched in Ba, Nb, Th and LREE, and have higher La/Yb and lower Ba/La, K/La and Rb/La. 87Sr/86Sr and 143Nd/144Nd in MEC basalts are respectively lower and higher than those of Osorno and Calbuco lavas. We suggest that MEC basalts were produced by lower extents of mantle melting than basalts from Osorno and other SSVZ stratovolcanoes, probably as a result of lower water content in the source of MEC basalts. Calbuco andesites formed from basaltic parents similar to Osorno basalts, by moderate pressure crystallization of a hornblende-bearing assemblage accompanied by crustal assimilation. Hornblende stability in the Calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks, which are also an appropriate endmember for isotopic trends, together with magma storage at mid-crustal depths. The unique characteristics of Calbuco volcano, i.e. the stability of hornblende at andesitic SiO2 contents, low 143Nd/144Nd and high 87Sr/86Sr, and abundant crustal xenoliths, provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the SSVZ.

Emplacement, petrological and magnetic susceptibility characteristics of diverse magmatic epidote-bearing granitoid rocks in Brazil, Argentina and Chile

Magmatic epidote mEp -bearing granitoids from five Neoproterozoic tectonostratigraphic terranes in Northeastern NE . Brazil, Early Palaeozoic calc-alkalic granitoids in Northwestern NW Argentina and from three batholiths in Chile have been studied. The elongated shape of some of these plutons suggests that magmas filled fractures and that dyking was probably the major mechanism of emplacement. Textures reveal that, in many cases, epidote underwent partial dissolution by host magma and, in these cases, may have survived dissolution by relatively rapid upward transport by the host magma. In plutons where such a mechanism is not evident, unevenly distributed epidote at outcrop scale is armoured by biotite or near-solidus K-feldspar aggregates, which probably grew much faster than epidote dissolution, preventing complete resorption of epidote by the melt. Al-in-hornblende barometry indicates that, in most cases, amphibole crystallized at PG 5 kbar. Kyanite-bearing thermal aureoles surrounding plutons that intruded low-grade metamorphic rocks in NE Brazil support pluton emplacement at intermediate to high pressure. mEp show overall chemical variation from 20 to 30 mol% mole . . percent pistacite Ps and can be grouped into two compositional ranges: Ps and Ps . The highest Ps contents are 20 - 24 27 - 30 in epidotes of plutons in which hornblende solidified under P- 5 kbar. The percentage of corrosion of individual epidote crystals included in plagioclase in high-K calc-alkalic granitoids in NE Brazil, emplaced at 5-7 kbar pressure, yielded estimates of magma transport rate from 70 to 350 m year y1 . Most of these plutons lack Fe-Ti oxide minerals and Fe q3 is . mostly associated with the epidote structure. Consequently, magnetic susceptibility MS in the Neoproterozoic granitoids in NE Brazil, as well as Early Palaeozoic plutons in Argentina and Late Palaeozoic plutons in Chile, is usually low

Crustal xenoliths from Calbuco Volcano, Andean Southern Volcanic Zone: implications for crustal composition and magma-crust interaction

Crustal xenoliths in the 1961 andesite flow of Calbuco Volcano, in the southern Southern Volcanic Zone (SSVZ) of the Andes, consist predominantly of pyroxene granulites and hornblende gabbronorites. The granulites contain plagioclase+pyroxene+magnetite±amphibole, and have pronounced granoblastic textures. Small amounts of relict amphibole surrounded by pyroxene-plagioclase-magnetite-glass symplectites are found in some specimens. These and similar textures in the gabbronorites are interpreted as evidence of dehydration melting. Mineral and bulk rock geochemical data indicate that the granulites are derived from an incompatible trace element depleted basaltic protolith that underwent two stages of metamorphism: a moderate pressure, high temperature stage accompanied by melting and melt extraction from some samples, followed by thermal metamorphism after entrainment in the Calbuco andesite lavas. High ɛNdT values (+4.0 to +8.6), Nd-isotope model ages of 1.7–2.0 Ga, and trace element characteristics like chondrite normalized La/Yb< and La/Nb≤1 indicate that the protoliths were oceanic basalts. Similar oceanic metabasalts of greenschist to amphibolite facies are found in the Paleozoic metamorphic belt that underlies the Chilean coastal ranges. Mineral and bulk rock compositions of the gabbronorite xenoliths indicate that they are cognate, crystallizing from the basaltic andesite magma at Calbuco. Crystallization pressures for the gabbros based on total Al contents in amphibole are 6–8 kbar. These pressures point to middle to lower crustal storage of the Calbuco magma. Neither granulite nor gabbro xenoliths have the appropriate geochemical characteristics to be contaminants of Calbuco andesites, although an ancient sedimentary contaminant is indicated by the lava compositions. The presence of oceanic metabasaltic xenoliths, together with the sedimentary isotopic imprint, suggests that the lower crust beneath the volcano is analogous to the coastal metamorphic belt, which is an accretionary complex of intercalated basalts and sediments that formed along the Paleozoic Gondwanan margin. If this is the case, the geochemical composition of the lower and middle crust beneath the SSVZ is significantly different from that of most recent SSVZ volcanic rocks.

Formation of cristobalite nanofibers during explosive volcanic eruptions

High-resolution transmission electron microscopy (HRTEM) observations of unaltered volcanic air-fall deposits from the ongoing lava dome explosive eruption at Chaiten Volcano, Chilean Patagonia, revealed the presence of highly crystalline silica nanofibers in the respirable fraction of the volcanic ash ( 240 °C), beta form of cristobalite, with average lengths of hundreds of nanometers and widths on the order of tens of nanometers. We propose that the beta-cristobalite nanofibers are formed during explosive eruptions by the reduction of amorphous silica by carbon monoxide to its reactive suboxide SiO, which is later oxidized to form one-dimensional crystalline silica nanostructures. Nucleation and growth of the nanofibers are enhanced by the high surface area of the micrometer- to nanometer-sized fragments of silica glass in the volcanic column. The formation of nanocrystalline cristobalite fibers during explosive lava dome eruptions poses new challenges for the assessment of the short- and long-term health hazards associated with the respirable nanofibrous components of volcanic ash.

Ages and geochemistry of Mesozoic-Eocene back-arc volcanic rocks in the Aysén region of the Patagonian Andes, Chile

Edades y geoquimica de las rocas volcanicas del trasarco del Mesozoico-Eoceno en la region de Aysen de los Andes patagonicos, Chile. Diez y ocho nuevas edades radiometricas (catorce 40Ar-39Ar, cuatro K-Ar) junto con las ya publicadas confirman la existencia de tres eventos volcanicos (previamente definidos por relaciones estratigraficas) en la Patagonia chilena (Region de Aysen) durante el intervalo Mesozoico-Eoceno: Jurasico Medio-Cretacico temprano (160-130 Ma), Cretacico (114-75 Ma) y Eoceno (55-46 Ma). Sobre la base de las caracteristicas geoquimicas e isotopicas de Sr y Nd de las rocas volcanicas estudiadas, se pueden reconocer dos dominios magmaticos Mesozoico-Eoceno: Dominio Magmatico Norte (DMN) y Dominio Magmatico Sur (DMS). Los basaltos y rocas intermedias del DMN tienen afinidades alcalinas y valores isotopicos de Sr-Nd deprimidos a moderadamente deprimidos similares a aquellos derivados de una fuente dominada por material astenosferico. Las rocas volcanicas maficas del DMS tienen un caracter subalcalino y caracteristicas isotopicas de Sr-Nd mas enriquecidas comparables a aquellas derivadas de una fuente litosferica. Las rocas volcanicas felsicas del DMS tienen razones iniciales 87Sr/86Sr mas altas y valores de eNd mas bajos que las rocas volcanicas felsicas de DMN, sugiriendo una mayor contribucion cortical en sus fuentes magmaticas. Las distinciones geoquimicas e isotopicas entre el DMN y el DMS podrian estar influenciadas por la presencia de rocas metamorficas paleozoicas como basamento de las rocas volcanicas del DMS. Por otra parte, la distincion entre los basaltos del DMN y DMS podria corresponder a diferencias en la magnitud de la extension, siendo el DMN donde la extension habria sido mayor y, consecuentemente, la litosfera mas delgada

Mesozoic and cenozoic plutonic development in the Andes of central Chile (30°30′–32°30′S)

Abstract Mesozoic and Cenozoic granitoids of this segment of the Andes occur in three N/S-trending belts: western (WB), central (CB), and eastern (EB). The WB is formed by the Mincha and Illapel superunits; the CB includes the Cogoti superunit and the San Lorenzo unit; and the EB comprises the Rio Grande and Rio Chicharra superunits. KAr and RbSr ages show discrete ranges for each of the belts, with a pronounced eastward migration of magmatism with time: WB, Early Jurassic to Late Cretaceous; CB, early Tertiary; and EB, late Tertiary. The jumps in the sites of the magmatic belts correspond to essentially non-magmatic intervals (86–70 Ma and 39–26 Ma) and may relate to periods of subduction-erosion or changes in the dip angle of the subducted lithosphere. Periods of rapid migration correspond to specific changes in the Pacific Ocean spreading history. The superunits show relatively uniform major element oxide variation. The exceptions are the Limahuida granitoids which have characteristics of a confirm derivation of the parent magmas from the upper mantle with virtually no continental crustal involvment. This distinguishes the Mesozoic-Cenozoic granitoids from those of the Paleozoic belts of Chile, which have values systematically higher than 0.705.

Ages and cooling history of the Early Cretaceous Caleu pluton: testimony of a switch from a rifted to a compressional continental margin in central Chile

The Caleu pluton, in the Coastal Range of central Chile, represents the last magmatic event related to the Early Cretaceous rifting along the western margin of South America. The pluton was emplaced into a c. 10 km thick pile of mainly basalts and basaltic andesites deposited in an Early Cretaceous subsiding basin, and affected by very low-grade metamorphism. The cooling history of the pluton is documented on the basis of U–Pb, 40Ar/39Ar step-heating and fission-track dating. The U–Pb date suggests an age of emplacement in the interval 94.2–97.3 Ma. Rapid subsolidus cooling between 550–500 °C and 250 °C is documented by 40Ar/39Ar plateau ages on amphibole, biotite and plagioclase between 94.9 ± 1.8 and 93.2 ± 1.1 Ma. Slower subsolidus cooling to c. 100 °C is identified at the 94–90 Ma interval by the fission-track thermal model. The geochronological data show that the emplacement of the pluton is coeval with the very low-grade metamorphism of the host rocks. Therefore, this metamorphism is probably not the result simply of burial, but also of a regional thermal gradient related to the plutonism. Exhumation of the pluton started coevally with its emplacement and continued to about 90 Ma, being associated with the closure of the Early Cretaceous rifting. The Caleu plutonism represents an asthenospheric-derived event during maximum extension, and marks a turning point between extensional- and compressional-related magmatism.

Copper geochemistry in salt from evaporite soils, Coastal Range of the Atacama Desert, northern Chile: an exploration tool for blind Cu deposits

The evaporite soil (1.5 to 15 m thick) in the Coastal Range of northern Chile is a serious obstacle to mineral exploration. We conducted a Cu geochemical study in salt beds (mainly gypsum and anhydrite) from this soil. The sampling was distributed over mineralized and barren rocks, in salts of evaporitic soils from hills, slopes, gullies, plains and alluvium-filled valleys. The samples of salt from hills and slopes reflect very well the presence of blind ore deposits, and the magnitude of the Cu anomalies (<200 to >1000 ppm Cu) is independent of the depth at which the mineralization is located. Similar geochemical response has been found in salt from evaporitic soils that cover gullies, plains and alluvium-filled valleys. However, in the last case the magnitude of the Cu anomalies (<20 to >200 ppm Cu) depends on the thickness of the gravels and the depth of leached bedrock that covers the blind deposits. Although the Cu anomalies are spiky, their contrast with the background Cu population is remarkable. Sampling of salt from evaporitic soils appears to be a valuable tool for exploration of buried Cu deposits in the exceedingly arid Coastal Range of the Atacama Desert.