No 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...

No 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...

No abstract