CONTRASTING STYLES OF PRE-CENOZOIC AND MIDTERTIARY CRUSTAL EVOLUTION IN NORTHERN MEXICO - EVIDENCE FROM DEEP CRUSTAL XENOLITHS FROM LA-OLIVINA

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 less-than-or-equal-to 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 may have delaminated in response to crustal overthickening associated with the Ouachita collision event.