Geochemistry and Sr, Nd, Pb isotopic compositionnext term of the Central Atlantic Magmatic Province (CAMP) in Guyana and Guinea

The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, extending more than 5000 km north to south, on both sides of the Atlantic Ocean. Its emplacement occurred about 200 Ma ago, at the Triassic–Jurassic boundary, and is linked to the initial breakup of Pangaea. Two areas of the province are studied here: French Guyana/Surinam (South America) and Guinea (West Africa), in order to document the petrogenesis and geodynamical significance of high-Ti and low-Ti basaltic magmas from the CAMP. In Guyana, doleritic and gabbroic dykes are located on the edge of the Guiana Shield, and represent limited volumes of magma. They display low SiO2 (47–50%), high TiO2 (2.5–3.5%) and high FeO tholeiitic trends and show variably enriched trace element patterns ((La/Yb)n=1.5–5.1). Their previous termisotopicnext term signature and ratios of very incompatible elements (εNdi=+5.8 to +4.2, (87Sr/86Sr)i=0.703–0.705, (207Pb/204Pb)i=15.46–15.64) match a depleted PREMA (prevalent mantle)-like source. Their genesis can be modeled by ca. 15% partial melting of a lherzolite source, and a subsequent limited fractional crystallization (5–10%) or a slight upper crustal assimilation–fractional crystallization (AFC, r=0.1, Proterozoic contaminant). In Guinea, in contrast, huge volumes of CAMP magmas were intruded along the Rockelides suture and the West African craton, forming the Fouta Djalon sills and the Kakoulima laccolith. The laccolith is more than 1000 m thick. These features consist of gabbros, dolerites, diorites and mafic (gabbro) and ultramafic (dunite, wherlite) cumulates. Guinean tholeiites show high SiO2 (51–58%), low TiO2 (0.7–1.2%) and FeO trends, with high LILE/HFSE ratios and slight negative Nb–Ta anomalies. previous termIsotopicnext term signatures (εNdi=+0.4 to −5.3, (87Sr/86Sr)i=0.705–0.710, (207Pb/204Pb)i=15.57–15.66) indicate a more enriched source than for Guyana as well as a higher rate of magma–upper crust interaction through an AFC process (r=0.3, Birimian crust contaminant) and, probably, an additional upper crustal contamination for the most differentiated sample. This geochemical study supports the prevalence in Guinea, as for other low-Ti CAMP tholeiites, of a lithospheric mantle source, previously enriched during ancient subduction events, and preferentially reactivated in late Triassic times by edge-driven convection between cratonic and mobile belt domains. A larger contribution from a depleted asthenospheric source is required to generate high-Ti tholeiites in Guyana, which may reflect the development of CAMP rifting towards the initiation of the Central Atlantic oceanic crust

Barren Miocene granitoids in the Central Andean metallogenic belt, Chile: Geochemistry and Nd-Hf and U-Pb isotope systematics Granitoides estériles del Mioceno en la franja metalogénica de los Andes Centrales, Chile: geoquímica e isotopía de Nd-Hf y U-Pb

Four Middle-to-Late Miocene barren plutonic complexes that occur between the giant porphyry copper deposits of the central Chilean Andes were selected for U-Pb LA-ICPMS geochronology and Hf-isotope systematics on single zircon grains. Major and trace elements and Sr-Nd-Hf isotope whole rock geochemical studies were under-taken to compare with slightly younger or coeval barren and fertile intrusive rocks between 32° and 34°S. The studied granitoids yield resolvable crystallization ages of 11.3±0.1 Ma (Cerro Mesón Alto massif), 10.3±0.2 Ma (La Gloria pluton), 14.9±0.2 Ma/14.9±0.1 Ma (Yerba Loca stock) and 11.2±0.1 Ma/14.7±0.1 Ma (San Francisco Batholith). Major and trace elements discard an adakitic signature as suggested for coeval porphyric intrusions at 32°S, slightly younger mineralized porphyries at Río Blanco-Los Bronces deposit and other Cenozoic adakites. Volcanic host rocks are less fractionated than the intrusive rock units. The same observation can be made for the unmineralized northern plutons compared to the southern ones. Initial Sr-Nd isotope data show insignificant variation (0.703761-0.704118 and 0.512758-0.512882), plotting in the mantle array. Trace element enrichment can be explained by addition of subducted-slab fluids and/or terrigenous sediments to the mantle wedge prior to and/or slight crustal input during magma ascent. Zircon grains separated from these barren intrusives share a similar initial εHf i-data variation for the younger age group (10-12 Ma; 7.04-9.54) and show a more scattered range for the older one (14-15 Ma; 8.50-15.34); both sets plot between the DM and CLTUR evolution lines. There is evidence that magma evolution was slightly distinct through time from older to younger barren magmatism, compared to a few fertile porphyritic rocks from Río Blanco-Los Bronces porphyry copper deposit. It is suggested that chronological inconsistencies within these complexes might be related to differential shortening across the NE-SW-trending Yeso Valley transfer fault, assumed as coeval, which also explains the local lack of easterly magmatic are front migration.Cuatro complejos plutónicos del Mioceno Temprano a Medio que se encuentran ubicados entre los depósitos de pórfidos gigantes de cobre de los Andes chileno centrales fueron seleccionados para un estudio de geocronología U-Pb por LA-ICPMS y de isótopos de Hf en granos individuales de circón. Estudios geoquímicos de elementos mayores y trazas y los isótopos de Sr-Nd-Hf en roca permiten compararlos con rocas intrusivas ligeramente más jóvenes o coetáneas estériles y fértiles entre 32° y 34°S. Los granitoides estudiados muestran edades de cristalización de 11,3±0,1 Ma (masivo Cerro Mesón Alto), 10,3±0,2 Ma (plutón La Gloria), 14,9±0,2 Ma/14,9±0,l Ma (Yerba Loca stock) y 11,2±0,1 Ma/14,7±0,l Ma (Batolito San Francisco). Elementos mayores y trazas excluyen una característica adakítica, como ha sido sugerido para los intrusivos porfídicos a los 32°S, los pórfidos mineralizados ligeramente más jóvenes de Río Blanco-Los Bronces y otras adakitas del Cenozoico. Las rocas volcánicas de caja son menos fraccionadas que las unidades de rocas intrusivas. La misma conclusión se obtiene con relación a los plutones de la zona norte comparados a los del sur. Valores isotópico iniciales Sr-Nd muestran una variación insignificante (0,703761-0,704118 y 0,512758-0,512882) en el campo del manto. El enriquecimiento de elementos traza puede ser explicado a través de una adición de fluidos de la placa subducida y/o por sedimentos terrígenos transportados hacia la cuña de manto y/o por un leve aporte cortical durante el ascenso del magma. Granos de circón separados de los intrusivos estériles del grupo más joven (10-12 Ma; 7,04-9,54) comparten valores iniciales de sHf, pero los intrusivos más antiguos (14-15 Ma; 8,50-15,34), en comparación, representan un rango más amplio; ambos rangos de valores se proyectan en la línea de evolución entre el DM y CHUR. Hay evidencia de que la evolución magmática fue ligeramente distinta a lo largo del tiempo desde un magmatismo joven y estéril hacia las rocas porfídicas mineralizadas de Río Blanco-Los Bronces. Se sugiere que la inconsistencia geocronológica entre los distintos complejos podría relacionarse con un acortamiento diferencial a través de una falla de transferencia NE-SO a lo largo del valle del río Yeso, asumida coetánea, lo cual explicaría la falta, a nivel local, de migración del frente del arco magmático hacia el este

Barren Miocene granitoids in the Central Andean metallogenic belt, Chile: Geochemistry and Nd-Hf and U-Pb isotope systematics

Four Middle-to-Late Miocene barren plutonic complexes that occur between the giant porphyry copper deposits of the central Chilean Andes were selected for U-Pb LA-ICPMS geochronology and Hf-isotope systematics on single zircon grains. Major and trace elements and Sr-Nd-Hf isotope whole rock geochemical studies were undertaken to compare with slightly younger or coeval barren and fertile intrusive rocks between 32° and 34°S. The studied granitoids yield resolvable crystallization ages of 11.3±0.1 Ma (Cerro Mesón Alto massif), 10.3±0.2 Ma (La Gloria pluton), 14.9±0.2 Ma/14.9±0.1 Ma (Yerba Loca stock) and 11.2±0.1 Ma/14.7±0.1 Ma (San Francisco Batholith). Major and trace elements discard an adakitic signature as suggested for coeval porphyric intrusions at 32°S, slightly younger mineralized porphyries at Río Blanco-Los Bronces deposit and other Cenozoic adakites. Volcanic host rocks are less fractionated than the intrusive rock units. The same observation can be made for the unmineralized northern plutons compared to the southern ones. Initial Sr-Nd isotope data show insignificant variation (0.703761-0.704118 and 0.512758- 0.512882), plotting in the mantle array. Trace element enrichment can be explained by addition of subducted-slab fluids and/or terrigenous sediments to the mantle wedge prior to and/or slight crustal input during magma ascent. Zircon grains separated from these barren intrusives share a similar initial εHf-data variation for the younger age group (10-12 Ma; 7.04-9.54) and show a more scattered range for the older one (14-15 Ma; 8.50-15.34); both sets plot between the DM and CHUR evolution lines. There is evidence that magma evolution was slightly distinct through time from older to younger barren magmatism, compared to a few fertile porphyritic rocks from Río Blanco-Los Bronces porphyry copper deposit. It is suggested that chronological inconsistencies within these complexes might be related to differential shortening across the NE-SW-trending Yeso Valley transfer fault, assumed as coeval, which also explains the local lack of easterly magmatic arc front migration.31 page(s

Cenozoic tectonostratigraphic evolution and architecture of the central Andes in northern Chile based on the aquine region, western cordillera (19 degrees-19 degrees 30 ' S)

Crustal thickening by horizontal shortening and associated deformation have been broadly considered as prime mechanisms for mountain building in the Central Andes of western South America. However, timing and structural style of Andean orogeny in northernmost Chile remains to be fully understood. By means of this contribution we attempt to unravel the Cenozoic tectonostratigraphy and structural architecture of a narrow segment within the Western Cordillera (western flank of the Altiplano plateau), based on detailed structural mapping, available and new geochronological data gathered from the Aquine region (similar to 19 degrees 15'S). The geology of this area indicates that compressive tectonics dominated for similar to 21 Myr, between 27 Ma and 6 Ma, and that onset of deformation probably occurred in the middle Eocene (after ca. 49 Ma). According to angular unconformities registered throughout the region, three principal compressive episodes have been determined: (1) middle Eocene-late Oligocene (ca. 49-27 Ma); (2) late Oligocene-middle Miocene (ca. 27-14 Ma); and (3) middle-latest Miocene, 14-6 Ma. Paleogene and Neogene structural development within the Aquine region was concomitant to activity on the bordering West and East Vergent Thrust Systems (WTS and ETS), which controlled uplift of the western Altiplano since the late Oligocene-early Miocene, located at the foot of the Precordillera and within the eastern part of the Western Cordillera, respectively. Major structures in the western segment of the region are high-angle, east-vergent, substratum-involving thrusts that affect late Mesozoic and Cenozoic volcanic and sedimentary intra-arc deposits. Unalike, a west-vergent fold-and-thrust belt is developed in the southeastern segment of the region, involving only Miocene deposits. These contrasting structural styles and opposite vergences indicate a combination of thin- and thick-skinned tectonics in a "pop-up" like structural array. Growth strata and sedimentological features in alluvial and fluvial sandstones and conglomerates of the upper Miocene Mauque Formation suggest the occurrence of syntectonic deposition. In addition, gentle angular unconformities within upper Oligocene-upper Miocene arc deposits suggest that sedimentation and volcanism developed synchronic to compressive tectonics throughout the Miocene and coeval to deposition in the active bordering thrust systems (WTS and ETS). The interpreted tectonostratigraphic development of the study area is similar to but slightly diachronic with the Cenozoic evolution in the Belen region (18 degrees-19 degrees S) of northernmost Chile. We interpret that east- and west-vergent thrusts within the Western Cordillera, are deeply rooted in a crustal-scale "pop-up" structure that accommodated Cenozoic deformation on the western flank of the Altiplano. Uplift as a consequence of contraction along the Western Cordillera was coeval to late Paleogene block exhumation and subsequent Neogene development of fold-and-thrust belts on the eastern border of the Altiplano. The main structural characteristics within the Aquine region suggest that inversion of an Upper Cretaceous intra-arc basin was of major relevance for the Cenozoic structural development of the Western Cordillera

Provenance of the Miocene Alto Tunuyán Basin (33°40′S, Argentina) and its implications for the evolution of the Andean Range: Insights from petrography and U–Pb LA–ICPMS zircon ages

The Alto Tunuyán Foreland Basin in western Argentina is located immediately south of the flat-slab segment of the Central Andes and its evolution is directly related to the propagation of structures to the east. Petrographic and geochronologic studies have been performed to determine the provenance of syntectonic sediments in the basin in order to establish their relationship to the Andean orogenic activity. The analysed detrital and igneous zircons in contrast with previous data, allow us to restrict the basin age between ca. 15 and 6 Ma. Sandstones record two main contributions, one from andesitic volcanic rocks and the other from an acidic igneous source, the first probably corresponding to Miocene volcanic rocks from the Principal Cordillera (Farellones Formation) and the second to Permo–Triassic, acidic, igneous rocks from the Frontal Cordillera (Choiyoi Magmatic Province, CMP). Two secondary sources have been recorded, sedimentary and metamorphic; the first one is represented by Mesozoic rocks in the Principal Cordillera and the second by the Proterozoic/early Carboniferous Guarguaráz Complex (GC) in the Frontal Cordillera, respectively. Sandstones from the lower basin deposits (15–11 Ma) register supply pulses from the Farellones Formation reflecting the unroofing of the Principal Cordillera by uplift pulses during the middle Miocene. Sandstones from the upper basin deposits (ca. 11–9 Ma) record an increase in material derived from the CMP, reflecting important uplift of the Frontal Cordillera. A thick, ca. 9 Ma old ignimbrite within the basin indicates an eruption in the Frontal Cordillera. Detrital zircons from the CMP have been detected also in the lower basin deposits, suggesting either recycling of Mesozoic deposits containing CMP zircons or an early paleorelief of the Frontal Cordillera. The good correlation between the age of the detrital zircons of the CMP and the GC in the lower basin deposits supports recycling of Mesozoic sedimentary deposits.Fil: Porras, Hernan. Universidad de Chile; Chile. Universidad Nacional de Costa Rica; Costa RicaFil: Pinto, Luisa. Universidad de Chile; ChileFil: Tunik, Maisa Andrea. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; ArgentinaFil: Giambiagi, Laura Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Deckart, Katja. Universidad de Chile; Chil

Geometry and late Mesozoic-Cenozoic evolution of the Salar de Atacama Basin (22°30′-24°30′S) in the northern Central Andes: New constraints from geophysical, geochronological and field data

The late Mesozoic-Cenozoic evolution of the Central Andes of northern Chile consists of a series of events related to changing conditions along the plate margin, leading to different episodes of compression and extension. A long-lasting basin which records these changes corresponds to the Salar de Atacama Basin. Its tectonic history has been blurred by several interpretations of seismic reflection lines and multiple chronostratigraphic correlations between subsurface-surface units. As such, many aspects of its tectonic evolution are still unclear. In this study, we performed an E-W gravimetric profile across the Barros Arana Syncline and the basin. We analyzed interval velocities obtained from seismic reflection studies and reviewed the profiles closest to the gravity survey. Four U-Pb samples for detrital zircon analysis were obtained from Late Cretaceous-Paleogene units. Geophysical data show a segmentation of physical properties within the basin, with denser units and faster interval velocities around the late Mesozoic-early Cenozoic units, and lighter, slower units in the actual salt pan. The main contrast in physical properties is observed in the evaporitic and fine-grained continental deposits of the San Pedro Formation. Geophysical and geological constraints show that its thickness varies between 800 and 1200 m, and that it was deformed during later events. The U-Pb data show that the rocks within the Barros Arana Syncline reach well into the Paleogene, and that they were mainly deformed during the late Eocene Incaic Event, where an east-verging thrust system also involving Permo-Triassic units was developed, establishing the actual structural configuration. The late Oligocene-early Miocene units were then deposited in a post-orogenic setting, with minor compression. A stronger phase around the late Miocene reactivated previous structures and folded these deposits. This continuous, compressive history of the basin, which is mostly a result of the Incaic Event, is in agreement with the regional evolution of the Central Andes

Geocronología U-Pb e isótopos de Hf-O en circones del batolito de la Costa Pensilvaniana, Chile.

Artículo de publicación SciELOThe Coastal Batholith of south-central Chile between latitudes 33° and 40°S is composed of calc-alkaline granitoids emplaced in a relatively restricted time period. New SHRIMP U-Pb zircon ages on eight quartzdioritic to granitic rocks collected over a distance of 800 km yielded ages between 300 and 320 Ma, Pennsylvanian (late Carboniferous). Lu-Hf isotopic analyses on the same zircon grains have initial εHf(i) values from +1.67 to -5.64. The δ18O ratios for that grains range from 6.4 to 8.6‰. These new isotopic data point to a relative homogeneous source with prominent components of the continental crust. The calculated Mesoproterozoic Depleted Mantel model ages, in addition to the short span of intrusive ages give insights to the position of the proto-Gondwana margin and the changing subduction mechanism at the end of late Paleozoic time.This work was financed through the Chilean Grant FONDECYT 1095099