Pre-Pennsylvanian Rocks of Aquidneck and Conanicut Islands, Rhode Island

Guidebook for field trips to the Boston area and vicinity : 68th annual meeting, New England Intercollegiate Geological Conference, October 8-10, 1976: Trip B-1

Geochemical, Isotopic and Single Crystal 40Ar/39Ar Age Constraints on the Evolution of the Cerro Galan Ignimbrites

The giant ignimbrites that erupted from the Cerro Galan caldera complex in the southern Puna of the high Andean plateau are considered to be linked to crustal and mantle melting as a consequence of delamination of gravitationally unstable thickened crust and mantle lithosphere over a steepening subduction zone. Major and trace element analyses of Cerro Galan ignimbrites (68-71% SiO2) that include 65 new analyses can be interpreted by evolution at three crustal levels. AFC modeling and new fractionation corrected d18O values from quartz (+7.63-8.85%o) are consistent with the ignimbrite magmas being near 50:50 mixtures of enriched mantle (87Sr/86Sr ~ 0.7055) and crustal melts (87Sr/86Sr near 0.715-0.735). Processes at lower crustal levels are predicated on steep heavy REE patterns (Sm/Yb = 4-7), high Sr contents (>250 ppm) and very low Nb/Ta (9-5) ratios, which are attributed to amphibolite partial melts mixing with fractionating mantle basalts to produces hybrid melt that rise leaving a gravitationally unstable garnet-bearing residue. Processes at mid crustal levels create large negative Eu anomalies (Eu/Eu* = 0.45-0.70) and variable trace element enrichment in a crystallizing mush zone with a temperature near 800-850ºC The mush zone was repeatedly recharged from depth and partially evacuated into upper crustal magma chambers at times of regional contraction. Crystallinity differences in the ignimbrites are attributed to biotite, zoned plagioclase and other antecrysts entering higher level chambers where variable amounts of near-eutectic crystallization occurs at temperature as low as 680ºC just preceding eruption. 40Ar/39Ar single crystal sanidine weighted mean plateau and isochron ages combined with trace element patterns show that the Galan ignimbrite erupted in more than one batch including a ~ 2.13 Ma intracaldera flow and outflows to the west and north at near 2.09 and 2.06 Ma. Episodic delamination of gravitationally unstable lower crust and mantle lithosphere and injection of basaltic magmas whose changing chemistry reflects their evolution over a steepening subduction zone could trigger the eruptions of the Cerro Galan ignimbrites.Fil: Kay, Suzanne Mahlburg. Cornell University; Estados UnidosFil: Coira, Beatriz Lidia Luisa. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Wörner, Gerhard. Universität Göttingen; AlemaniaFil: Kay, Robert W.. Cornell University; Estados UnidosFil: Singer, Bradley S.. University of Wisconsin; Estados Unido

Pliocene-Pleistocene ash-fall tuff deposits in the intermountain Humahuaca and Casa Grande basins, northwestern Argentina: tracers in chronostratigraphic reconstructions and key to identify their volcanic sources

Ash-fall tuffs of the Pliocene-Pleistocene deposits of Humahuaca and Casa Grande intermountain basins, northwestern Argentina, have been differentiated into two groups based on new geochemical and geochronological data which correspond to the tuffs of the Pliocene-Lower Pleistocene alluvial fan deposits dominated by debris flow, deep sandy gravel braided, and shallow ephemeral lake deposits (Uquia and Mal Paso formations), and those recorded in Pleistocene alluvial fans sheet flood deposits. The two clusters of ages recognized: 4.3 to 2.6 Ma, and 2.2-pre 0.8 Ma, corresponding to these tuff groups, are in agreement with pulses of ignimbrite eruptions in the Altiplano Puna Volcanic Complex (APVC), and Southerm Puna calderas located west of the study region. The ash-fall tuffs of both groups are mainly vitreous to phenocryst-poor of rhyodacite-dacite composition with minor andesites to trachyandesites, characterized by 58-69% SiO2 contents, A/CNK 1-1.4, FeO/MgO (0.8-2.8), which plot in the calc-alkaline range. They can be differentiated based on its immobile trace elements ratios as indicated by a slight enrichment in LREE, higher arc affiliation and somewhat higher Sm/Yb ratios in the case of the younger group, although in both Sm/Yb ratios are indicative of sequestration of HREE in residual hornblende. The new geochemical and geochronological data of those ash fall tuffs point to these as chrono-stratigraphic tracers of the Humahuaca and Casa Grande intermountain basins stratigraphy, during the Pliocene-Pleistocene, also as the key to identify their volcanic sources.Fil: Coira, Beatriz Lidia Luisa. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; ArgentinaFil: Galli, Claudia Inés. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; Argentina. Universidad Nacional de Salta; ArgentinaFil: Mahlburg Kay, Suzanne. Cornell University; Estados UnidosFil: Stockli, Daniel F.. University of Texas; Estados UnidosFil: Flores, Patrocinio Ismael. Universidad Nacional de Jujuy. Instituto de Geología Minera; ArgentinaFil: Eveling, Emilio José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta; Argentina. Universidad Nacional de Salta. Facultad de Ciencias Naturales; Argentin

Cenozoic ash-fall deposits in the Andean foreland basins, Northwest Argentina (23°-26°S) - Key to reconstruct their chrono-stratigraphy and to identify links to the Andean Neogene ignimbrite flare-up

Outstanding Cenozoic ash-fall deposits have been recognized in Andean foreland basins in NW Argentina. The purpose of this contribution is to review their stratigraphic, petrographic, geochemical and geo-chronogical data in the Neogene deposits of the Andean foreland basins at 23°-26°S, between the Puna highland, to the west, and the fold thrust belt to the east. In doing this we evaluate their potential as chrono-stratigraphic markers, analyze the different volcanic events that they represent, evaluate them in relation to the Southern Central Andes volcanism and to determine the pattern of their distribution. Different ash-fall events were recognized at 15-11 Ma; 10–6.8 Ma; 6.4 to 4.8; 4.3 to 2.6; and from 2.1-Ma-Recent across the Neogene foreland basins deposits. Mineralogical and geochemical data of the ash fall deposits were analyzed in each event in order to characterize them during the different pulses. Those data were evaluated with respect to those of the contemporary most voluminous explosive volcanism represented by the Neogene ignimbrite flare up of the Southern Central Andes to test the feasibility to identify their potential sources. The eruptive sources of these ash-fall events were identified as the Neogene giant calderas of the Altiplano Puna Volcanic Complex (APVC), and the Southern Puna (Agua Escondida, Luingo calderas/15-11 Ma; Cerro Aguas Calientes Caldera, Complejo Volcánico Negra Muerta, Ramadas Volcanic Center/10 Ma- 6.8 Ma; Galán and Puripicar calderas/6.4–4.8 Ma; Guacha, Pacana, Puripicar and Galán calderas/4.3–2.6 Ma; Purico Complex and Guacha Caldera 2.1 Ma-Recent. Comprehensive analysis of the ash-fall deposits, across the different basins in a W-E profile at same time has shown that their distribution is consistent with an east-southeast, and to a lesser extent, east northeast dispersion from the Neogene giant calderas as controlled by northwesterly and westerly winds. These results indicate the effectiveness of using stratigraphy, mineralogy, geochemistry and ages in comparing ash-fall tuffs with coeval ignimbrites in order to correlate stratigraphic sequences in foreland basins, identify explosive volcanic events, contemporary emission centers and to put forward their dispersion patterns.Fil: Coira, Beatriz Lidia Luisa. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; ArgentinaFil: Galli, Claudia Inés. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; ArgentinaFil: Mahlburg Kay, Suzanne. Departments Of Earth And Atmospheric Sciences, Cornell; Estados UnidosFil: Alonso, Ricardo Narciso. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Tucumán. Instituto Superior de Correlación Geológica. Grupo Vinculado al INSUGEO- Centro de Estudios Geológicos Andinos; ArgentinaFil: Flores, Patrocinio Ismael. Universidad Nacional de Jujuy. Instituto de Geología Minera; ArgentinaFil: Gonzalez, Edgar David. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; Argentin

Paleogene and Neogene magmatism in the Valle del Cura region: New perspective on the evolution of the Pampean flat slab, San Juan province, Argentina

El Valle del Cura está geológicamente caracterizado por una espesa secuencia volcánica y volcániclástica que registra la evolución del magmatismo terciario de arco y retroarco de la Cordillera Frontal de Argentina sobre la actual zona de subducción horizontal Pampeana entre los 29,5° a 30°S. Durante el Eoceno, se desarrollo una cuenca de retroarco donde se depositó una secuencia volcano-sedimentaria sinorogénica, la Formación Valle del Cura, la que incluye riolitas y tobas dacı´ticas. Estas rocas silíceas poseen débiles signaturas químicas de retroarco, alto contenido de elementos litófilos y un enriquecimiento isotópico en relación con el resto de la secuencia terciaria más joven; la signatura química es consistente con magmas desarrollados en condiciones de una corteza delgada. Le sucede el magmatismo de arco oligoceno-mioceno temprano del Grupo Doña Ana; arco localizado al oeste del límite con Chile. Las volcanitas de Doña Ana corresponden a una suite calcoalcalina producto de fusión en la cuña astenosférica bajo una corteza de espesor normal (30–35 km). Debido a la subhorizontalización de la placa de Nazca, se produce la migración del arco: la Formación Cerro de las Tórtolas, de edad miocena inferior a superior, representa este frente volcánico ubicado en el límite chileno-argentino. Dos etapas se reconocen: la más antigua (16–14 Ma) representa fundidos equilibrados en una corteza de espesor normal (30–35 km), mientras que la más joven (13-10 Ma) posee relaciones de tierras raras indicativas de magmas equilibrados con fases residuales de mayor presión, reflejo de una corteza engrosada (50 km). Las relaciones isotópicas de las lavas más jóvenes son consistentes con un incremento de los componentes corticales, incorporados durante su evolución ya sea en la corteza o introducidos al manto por subducción y erosión de sedimentos en el antearco. El comienzo de la subducción de la dorsal de Juan Fernández en la región, entre los 10 y 12 Ma, es consistente con un pico en los procesos de erosión y subducción en el antearco. Las lavas jóvenes de la Formación Cerro de las Tórtolas y las ignimbritas dacíticas del Mioceno Medio de la Formación Tambo se desarrollaron bajo las condiciones de máxima compresión. El último episodio volcánico significativo en la región corresponde a la Ignimbrita Vacas Heladas, del Mioceno Superior. Son las volcanitas isotópicamente mas enriquecidas, resultado de las altas proporciones de componentes corticales involucrados en su génesis. Luego de este volcanismo y como respuesta a la subhorizontalización de la placa subducida, cesó la actividad volcánica en el Valle del Cura.The Valle del Cura region is characterized by a thick volcanic and volcaniclastic sequence that records the Tertiary arc and backarc magmatic evolution of the Argentine Main Cordillera over the modern Pampean flatslab at 29.5-30°S. During the Eocene, a retroarc basin developed, represented by the Valle del Cura Formation synorogenic volcanosedimentary sequence, which includes rhyolites and dacitic tuffs. These silicic volcanic rocks have weak arc chemical signatures and high lithophile element concentrations and are isotopically enriched relative to the late Oligocene-early Miocene volcanic rocks that followed them. Their chemical characteristics fit with eruption through a thin crust. The Valle de Cura Formation was followed by the Oligocene-early Miocene Doña Ana Group volcanic sequence, which erupted at and near the arc front west of the border with Chile. The Doña Ana Group volcanic rocks have calc-alkaline chemical characteristics consistent with parental magmas forming in a mantle wedge and erupting through a normal thickness crust (35 km). Subsequent shallowing of the downgoing Nazca plate caused the volcanic front to migrate eastward. The volcanic sequences of the middle Miocene Cerro de las Tórtolas Formation erupted at this new arc front, essentially at the Argentine border. Two stages are recognized: an older one (16-14 Ma) in which magmas appear to have erupted through a normal thickness crust (30-35 km) and a younger one (13-10 Ma) in which the steeper REE pattern suggests the magmas last equilibrated with higher pressure residual mineral assemblages in a thicker crust. Isotopic ratios in the younger group are consistent with an increase in original crustal components and crust introduced into the mantle source by forearc subduction erosion. A peak in forearc subduction erosion near 12-10 Ma is consistent with when the main part of the Juan Fernandez Ridge began to subduct beneath the region. In addition to late Miocene Tambo Formation dacitic ignimbrites, the younger Cerro de las Tórtolas Formation volcanic rocks erupted at the height of contractional deformation in the Valle del Cura and to the east. The last important volcanic sequence to erupt in the Valle del Cura is the late Miocene Vacas Heladas Ignimbrite, the most isotopically enriched Tertiary magmas in the Valle del Cura that contain the highest proportion of crustal components. Subsequently volcanism ceased in the region in response to shallowing of the subduction zone.Fil: Litvak, Vanesa Dafne. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Poma, Stella Maris. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Mahlburg Kay, Suzanne. Cornell University; Estados Unido

The Somuncura Large Igneous Province in Patagonia: Interaction of a Transient Mantle Thermal Anomaly With a Subducting Slab

The Oligo-Miocene Somuncura province is the largest (∼55 000 km2) back-arc mafic volcanic field in Patagonia, and one of Earth\u27s largest with no clear link to a hotspot or major extension. Major and trace element and Sr-Nd-Pb isotopic data suggest involvement of a plume-like component in the mantle magma source mixed with hydrous, but not high field strength element (HFSE)-depleted components, from a disintegrating subducting plate. Magmatism is attributed to mantle upwelling related to disturbances during plate reorganization, possibly at a time when the South America plate was nearly stationary over the underlying mantle. Melting was enhanced by hydration of the mantle during Paleogene subduction. Crustal contamination was minimal in a refractory crust that had been extensively melted in the Jurassic. Eruption began with low-volume intraplate alkaline mafic flows with depleted Nd-Sr isotopic signatures. These were followed by voluminous ∼29-25 Ma tholeiitic mafic flows with flat light and steep heavy rare earth element (REE) patterns, intraplate-like La/Ta ratios, arc-like Ba/La ratios and enriched Sr-Nd isotopic signatures. Their source can be explained by mixing EM1-Tristan da Cunha-like and depleted mantle components with subduction-related components. Post-plateau ∼24-17 Ma alkaline flows with steep REE patterns, high incompatible element abundances, and depleted Sr-Nd isotopic signatures mark the ebbing of the mantle upwelling