Contrasting styles of Pre‐Cenozoic and Mid‐Tertiary crustal evolution in northern Mexico: Evidence from deep crustal xenoliths from La olivina

Abstract

The principal deep crustal rock types found at the La Olivina xenolith locality in southeastern Chihuahua, Mexico, are mafic granulites, paragneisses, and intermediate‐ to silicic‐composition orthogneisses. These granulite facies xenoliths are interpreted in terms of two age groups, pre‐Cenozoic and mid‐Tertiary, based on previous ion probe dating of zircons from the xenoliths and on isotopic comparisons of the xenoliths to rocks of known age. The mafic granulites have Pb, Nd, and Sr isotopic compositions identical to those of Oligocene volcanic rocks from the La Olivina region. Compositionally, they are olivine‐normative gabbroic cumulates, and they precipitated from two or more mid‐Tertiary basalt to dacite or rhyolite assimilation/fractional crystallization series. Mineral assemblages in the xenoliths record pressures of ≤7.2 kbar or depths of <25 km. If these are the maximum pressures the rocks experienced and if the crust was >35 km thick in Oligocene time as inferred from regional tectonic considerations, then the mafic granulites cannot be samples of basaltic magmas underplated near the crust‐mantle boundary. The cumulate protoliths for the mafic granulites probably formed in magma chambers well above the Moho. The mafic granulites are plausibly representative of kilometers of new crust that formed in mid‐Tertiary time beneath the extensive ignimbrite fields of Mexico. Most orthogneiss xenoliths are pre‐Cenozoic, and they are rocks associated with the late Paleozoic Ouachita Orogeny and older events (e.g., Proterozoic basement and Paleozoic arc rocks). The Ouachita Orogeny was a collision event involving North America and a continental plate to the south, and the results of this study indicate that La Olivina is located above the southern plate. The paragneiss xenoliths overlap in isotopic composition with Carboniferous flysch units exposed in the Marathon uplift of west Texas. These sedimentary rocks and the sedimentary protoliths of the paragneiss xenoliths were derived from the southern plate and deposited before the orogeny in the ocean basin that separated North America and the southern plate. The paragneisses were not metamorphosed to granulite facies until mid‐Tertiary time. Pre‐Cenozoic and mid‐Tertiary crustal evolution followed very different paths in northern Mexico. For example, Nd isotopic evidence for crustal recycling is much more evident in rocks associated with the Paleozoic convergence than in rocks produced during mid‐Tertiary magmatism. Furthermore, mafic rocks are very rare in the pre‐Cenozoic xenolith suite, but they dominate the mid‐Tertiary one. These contrasting characteristics of the pre‐Cenozoic and mid‐Tertiary xenolith suites are interpreted to reflect differences in tectonic environment and crustal thickness. Preceding the collision event, the southern plate had an Andean‐type margin, and the abundant evidence for crustal recycling during this time implies that the crust was thick. The rarity of pre‐Cenozoic mafic xenoliths suggests that Proterozoic and Paleozoic lower crust m...

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

“…Sin embargo, rocas graníticas de edad grenvilliana (~1.0 Ga) cortadas por diques anfi bolíticos se reconocen en la parte centro-oriental de Chihuahua, en la Sierra de Los Filtros y la Sierra El Carrizalillo (Ruiz et al, 1988b;McDowell y Mauger, 1994) (Figura 3). También se han reconocido de manera indirecta en xenolitos de rocas volcánicas recientes en El Potrerillo y La Olivina, en el noreste y sureste de Chihuahua, respectivamente (Ruiz et al, 1988b;Cameron et al, 1992). Estos afl oramientos sugieren que el basamento antiguo de Norteamérica cruza por debajo de la SMO, y podría suponerse que ha modifi cado de alguna manera la composición de las rocas volcánicas cenozoicas.…”

“…Este parece ser un mecanismo razonable para explicar las tendencias lineares observadas en suites de basalto-andesitariolita observadas, por ejemplo, en la región de Batopilas (Cameron et al, 1980). Por otro lado, el estudio del mecanismo involucrado en la generación del enorme volumen de rocas ignimbríticas de la SMO (~300 000 km 3 ), se convirtió a partir de la década de1980 en un tema muy polémico (e. g., Cameron et al, 1980aCameron et al, , 1980bCameron y Hanson., 1982;Verma, 1984;Ruiz et al, 1988a;Cameron et al, 1992;McDowell et al, 1999;Albrecht y Goldstein, 2000). Por lo general, los estudios realizados en busca de una solución a este confl icto han recurrido al uso de fi rmas isotópicas grabadas en las rocas volcánicas.…”

“…Esto sugiere que el 80% del volumen inicial quedó como material residual, agregando algunos kilómetros de nueva corteza gabroica debajo de la SMO (Ruiz et al, 1988b;Cameron et al, 1992). De acuerdo con Cameron et al (1986), los modelos de AFC para la SMO sugieren un volumen menor al 25% de corteza asimilada.…”

“…De acuerdo con dichos autores, el evento magmático SMO no constituyó un periodo importante de formación de corteza. Por el contrario, Cameron et al (1992) consideran que muchas de las rocas granulíticas son del Terciario medio, y representan kilómetros de corteza nueva formada en ese tiempo debajo de México.…”

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

“…Henry & Aranda-Gomez 2000;Pallares et al 2007). The anomalously high Ba/Nb ratios (.200) approach values for Paleozoic-Proterozoic silicic orthogneiss and Tertiary plutonic-derived, glass-rich silicic xenoliths analysed from the La Olivina basaltic centre in northern Chihuahua by Cameron et al (1992), which have Ba/Nb ratios between c. 90 and 371 and 100-726, respectively. Compare with Figure 12 of Ewart et al (1992Ewart et al ( ).…”