Southern Cordilleran basaltic andesite suite, southern Chihuahua, Mexico: A link between Tertiary continental arc and flood basalt magmatism in North America

Abstract

Mid‐Cenozoic orogenic andesites and ignimbrites of western Mexico, southwestern New Mexico, and Arizona are commonly capped by basaltic andesites, most from 29–20 Ma. We refer to these mafic lavas as the Southern Cordilleran Basaltic Andesite (SCORBA) suite, and they may constitute the most extensive Cenozoic basaltic suite in North America. The SCORBA suite has trace element and isotopic characteristics of orogenic (arc) rocks (e.g., Ba/Nb>40), and silica content (53–56% SiO2) like the Grande Ronde Basalt, which represents about 80% of the volume of the Columbia River Group. Geochemical and isotopic data are presented on SCORBA lavas and rare mafic lavas (PRE‐SCORBA) interlayered with older ignimbrites from a 700‐km‐long NE‐SW transect of southern Chihuahua, Mexico. SCORBA and PRE‐SCORBA lavas with relatively low K/P (<7) and differing Ba/Nb (50 versus 18) have similar isotopic compositions, arguing against their isotopic signatures being controlled by crustal assimilation. Along the entire length of the transect, the basaltic rocks have εNd and 87Sr/86Sr near bulk Earth and 206Pb/204Pb and 207Pb/204Pb ratios that lie along a 1.7 Ga pseudoisochron. The Pb isotopic variation is geographically controlled, becoming more radiogenic from east to west, reflecting mixing in mantle source regions. The eastern mantle source has low 206Pb/204Pb and is a mixture of an enriched, enriched‐mantle‐like (EMI) component with one or more depleted components, which could include an intraplate component with relatively high Nb/Y (>0.8). The western mantle source contains less of the intraplate component and is more oceanic in character. Overprinting both the eastern and western sources is a Cenozoic subduction component that is responsible for the western radiogenic Pb, and this component fades out inland to the east. This transect crosses the inferred position of the Mojave‐Sonora megashear, previously proposed to be a major lithospheric boundary, separating Proterozoic basement to the east from Phanerozoic basement to the west at the latitude of the transect. Most chemical changes near the inferred position of the megashear are subtle, and they may be gradational rather than abrupt. The uniformity of Sr and Nd isotopic compositions across the inferred position of the megashear indicates that one or more of the following statements is true: (1) Phanerozoic and Proterozoic subcontinental lithospheres are essentially indistinguishable in Sr and Nd compositions in southern Chihuahua, (2) the megashear is not a lithospheric boundary separating Phanerozoic and Proterozoic crust in the vicinity of the transect, or (3) the isotopic signatures were acquired in the asthenosphere rather than in subcontinental lithosphere. The principal difference between the SCORBA suite and the earlier mid‐Cenozoic andesite to rhyolite orogenic suite is average SiO2 content. This difference reflects regional stress regimes at the time of eruption and magmatic plumbing. SCORBA was erupted in a more extensional tectonic environment than the orogenic suite. S...

Magmatismo y tectónica en la Sierra Madre Occidental y su relación con la evolución de la margen occidental de Norteamérica

“…Los conjuntos 1 y 2, y el conjunto 3, han sido defi nidos como el Complejo Volcánico Inferior (CVI) y el Supergrupo Volcánico Superior (SVS), respectivamente (McDowell y Keitzer, 1977). El conjunto 4, en la parte norte de la SMO, ha sido defi nido como una extensión del cinturón de "Basalto-Andesitas del Sur de la Cordillera" (SCORBA por sus siglas en inglés) (Cameron et al, 1989). Finalmente los basaltos del conjunto 5 han sido interpretados como testigos de varios episodios de extensión relacionados con la apertura del Golfo de California (Henry y Aranda-Gómez, 2000).…”

“…Los mayores afl oramientos de estas lavas se distribuyen en una franja de burda dirección NNE entre San Buenaventura, Chihuahua, al norte y Navojoa, Sonora, al sur, pasando por Yécora (Figura 3). Esta rocas han sido incluidas en los denominados Basalto-Andesitas del Sur de la Cordillera (SCORBA por sus siglas en inglés), propuestos por Cameron et al (1989) como una serie regional que se extiende también a Arizona y Nuevo México y marcaría la primera fase de extensión, probablemente en un marco tectónico intra-arco. Las edades reportadas para esta serie en Chihuahua y Sonora varían entre 33 y 17.6 Ma (Cameron et al, 1989 y referencias incluidas;McDowell et al, 1997;Paz-Moreno et al, 2003).…”

“…Basaltos con edades de ~24 Ma se encuentran cubriendo a ignimbritas del Oligoceno también en Nazas (Aguirre-Díaz y McDowell, 1993) y El Rodeo (Aranda-Gómez et al, 2003;Solé et al, en prensa). Cameron et al (1989) incluyen los basaltos de Nazas entre los SCORBA. Sin embargo, estudios posteriores (e. g., Luhr et al, 2001;Aranda-Gómez et al, 2003) han demostrado que estas rocas alcalinas (hawaiitas) tienen fi rmas geoquí-micas más afi nes a los basaltos intraplacas típicos de la provincia Basin and Range mexicana.…”

“…Estas últimas se refi eren particularmente al volcanismo máfi co postignimbrítico. Por otro lado, es interesante observar que las andesitas basálticas reportadas en estas muestras, coinciden grandemente con el campo de las andesitas defi nidas como SCORBA, lo cual confi rma la posible extensión de esta provincia hacia la parte norte de la SMO sugerida por Cameron et al (1989). La composición de las rocas del CVI, indicada también en la Figura 10, es en general similar al resto de las rocas de la SMO; sin embargo, a diferencia de estas últimas, las rocas del CVI están más restringidas a la región subalcalina, y no tienen un carácter bimodal (e. g., Valencia-Moreno et al, 2001y 2003; además, el rango en los valores de sílice es más restringido hacia ambos lados.…”

Pulling apart the Mid to Late Cenozoic magmatic record of the Gulf of California: is there a Comondú Arc?

“…Dated rocks have been grouped into four main compositional groupings: basalt (includes basaltic andesites and tholeiitic, calc-alkaline and alkaline varieties); andesite (includes calc-alkaline and adakitic-type compositions); dacite and rhyolite (includes high-silica rhyolites and peralkaline compositions). Important features of the diagram are: (1) the silicic dominant character of the Oligocene ignimbrite pulse; (2) the appearance of basalts during the Oligocene silicic ignimbrite pulse and increase in the frequency of basaltic eruptions up to the start of the Early Miocene pulse; these basaltic eruptions correspond to SCORBA of Cameron et al (1989); (3) the bimodal character of the Early Miocene pulse; (4) the increase in andesitic compositions beginning c. 20 Ma until c. 13 Ma; (5) the abrupt decline in rhyolite magma generation and eruption beginning c. 19 Ma when andesite-dacite eruptions were more predominant; (6) an abrupt return to bimodal volcanism at c. 13 Ma with a concomitant decline in andesitic eruptions; and (7) after 10 Ma, volcanism becomes fundamentally basaltic to coincide with the onset of seafloor spreading in the GoC beginning as early as c. 6 Ma (Lizarralde et al 2007). Diagram based on 1496 radiometric ages, and age data plotted using Isoplot (Ludwig 2003).…”