Joaquin Ruiz

Enhanced precision, accuracy, efficiency, and spatial resolution of U‐Pb ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry

Abstract The transition from Laramide syntectonic sedimentation of the lower Eocene Willwood Formation to the post-Laramide volcanogenic sedimentation of the middle Eocene Wapiti Formation was studied in the upper South Fork Shoshone River Valley, Wyoming. To better understand the regional age, paleogeography, and provenance of volcaniclastic sandstones in the lower stratified member of the Wapiti Formation, we sampled three units for detrital zircon U/Pb geochronology (n=241). The maximum depositional age for the sandstone units within the lower-most, middle, and upper-most units are 49.03 Ma, 49.44 Ma, and 48.99 Ma, respectively, which is consistent with previous geochronologic and paleontologic studies. These ages also are consistent with rocks deposited immediately prior to the emplacement of the Heart Mountain slide. Detrital-zircon age spectra show a transition from a mixed (recycled?) provenance, consistent with drainage from the west, composed of minor primary Eocene volcanic contributions to one dominated by primary Eocene and Archean contributions from the northern Absaroka volcanoes and the Laramide Beartooth Uplift. Thus, uplift and unroofing of the Beartooth Plateau was occurring during the deposition of the oldest members of the Wapiti Formation.

Nd isotopic ages of crust formation and metamorphism in the Precambrian of eastern and southern Mexico

Sm-Nd ages for garnets in the three Precambrian exposures of eastern and southern Mexico demonstrate that they belong to the Grenville tectonothermal event. The Sm-Nd garnet ages of 0.95 Ga for the Oaxacan Complex and 0.90 Ga for the Huiznopala Gneiss, Molango and the Novillo Gneiss, Ciudad Victoria, are postdated 75 Ma by Rb-Sr ages on biotites. Both sets of data document a cooling history following Grenville metamorphism at or before 1.0 Ga ago. Our garnet data are consistent with a blocking temperature for Sm-Nd in that mineral around 600° C suggested by Humphries and Cliff (1982).The three Precambrian occurrences have Nd chemical ages of separation from depleted mantle (TDM) grouped in the range 1.40–1.60 Ga. This may result from derivation of the rocks from actual crustal protoliths which had been separated from the mantle 0.5 Ga before the Grenville Orogeny. It is much more likely, however, that crustal materials of 1.7 Ga or older age were mixed with mantle-derived products during Grenville events to produce intermediate TDM ages andɛNd values around zero 1.0 Ga ago.

Oaxaquia, a Proterozoic microcontinent accreted to North America during the late Paleozoic

Grenville-age granulite facies rocks in southern, central, and northeastern Mexico have distinctive geologic features that suggest a common tectonic evolution. The similarities include northwest-trending structural grain from Oaxaca to Tamaulipas, massif-type anorthosite-charnockite complexes, protoliths rich in sedimentary rocks of shallow-marine platform or continental rift-related facies devoid of calc-alkaline volcanic rocks, common metamorphism under granulite facies conditions, U-Pb zircon ages of about 1.0 Ga., and an apparently common history of uplift and cooling. Altogether, this evidence strongly suggests a coherent geologic history for this block. Paleontologic data from the overlying sedimentary sequences indicate that Oaxaquia was not part of Laurentia during most of the Paleozoic. This precludes emplacement of Oaxaquia in its present position by simple lateral displacement from the southern United States as well as a Taconic time of emplacement. Oaxaquia was probably emplaced to its present position during late Paleozoic time. The concept of a Mesoproterozoic “Oaxaquia” microcontinent extending for about 1000 km in Mexico needs to be considered in the reconstruction of the Grenville orogen as a whole and for the Paleozoic tectonic interactions between eastern Laurentia and western Gondwana.

Proterozoic and Phanerozoic basement terranes of Mexico from Nd isotopic studies

Nd isotopic data were collected on Precambrian crystalline rocks exposed in northern, eastern, and southern Mexico, as well as from lower crustal xenoliths from central Mexico, in order to constrain the age and character of the Mexican basement. The data indicate that basement belonging to the Grenville (1.0 Ga) tectonothermal event extends from Los Filtros, in Chihuahua, northern Mexico, to Oaxaca, in southern Mexico. These rocks all have average Nd crustal residence times (TDM ages) in the range 1.60 to 1.35 Ga. We infer that this results from mixing average 1.9 Ga or older recycled continental crust with 70% to 90% newly derived mantle-crustal material during the Grenville orogeny. To the west of the Precambrian, the basement contains large amounts of Phanerozoic (probably Paleozoic) crust, identified from lower crustal xenoliths with TDM ages less than 1.0 Ga. The crust represented by these xenoliths may have been emplaced as suspect terranes in Mesozoic Cordilleran events. Alternatively, the apparent Paleozoic crust that underlies parts of central Mexico may connect to the Paleozoic metamorphic Acatlan complex in southern Mexico, and together they would constitute a continuation of the Appalachian-Caledonian orogenic belt through Mexico. Our data do not preclude either of these two models.

Geochronology and Nd isotopic data of Grenville-age rocks in the Colombian Andes: new constraints for Late Proterozoic-Early Paleozoic paleocontinental reconstructions of the Americas

Abstract New U Pb zircon crystallization ages and 40Ar/39Ar cooling ages from the Colombian Andes confirm the existence of rocks metamorphosed during the Orinoquian Orogenic Event (ca. 1.0 Ga) of northern South America. eNd (t = 1.1 Ga) for these rocks range from −3.9 to +0.91, which is interpreted as a mixture of Late Archean-Early Proterozoic crust with juvenile material produced during the 1.1 Ga orogenic event. The Colombian Grenville age rocks are part of a much longer metamorphic pericratonal belt, sporadically exposed along the Andes, in western-central Peru, southern Bolivia and northern Argentina. In addition, Nd model (TDM) ages for the Colombian rocks range from 1.9 to 1.45 Ga, similar to those obtained in the Grenville Province of the eastern U.S. and in the Mexican basement, placing constraints on Late Proterozoic-Early Paleozoic paleocontinental reconstructions.

Isotopic studies of the Acatlan complex, southern Mexico: Implications for Paleozoic North American tectonics

The Paleozoic Acatlan complex and Grenville-age Oaxaca terrane of southern Mexico have been suggested to be the southern continuation of North American orogenic systems. The Oaxaca terrane yields interpreted ages of ∼1.0 Ga and has many of the characteristics typical of Grenville belt rocks. The Paleozoic Acatlan complex consists of multiply deformed metasedimentary rocks, schists, granitoids, and eclogites that have been compared to rocks of the Appalachian belt. The northward extension of both the Oaxaca terrane and Acatlan complex, how-ever, are obscured by younger rock cover as they enter central Mexico. Furthermore, the configuration of these orogenic systems in southern Mexico is the reverse of that of the rest of North America, with the Grenville Oaxaca terrane to the east of the Paleozoic Acatlan complex. Isotopic studies show that the Acatlan complex records three tectonothermal events. The Sm-Nd whole-rock/mineral isochrons from schists as well as eclogites yield metamorphic ages of 410-380 Ma. This age of metamorphism is supported by U-Pb zircon data from a granitoid which yields an age of 370 ± 34 Ma. A later intrusion of a large stock in the Late Pennsylvanian (287 ± 2 Ma) was probably closely followed by a less significant deformational event. Small granitic intrusions and migmatites were later emplaced at 205-170 Ma (Rb-Sr and Sm-Nd mineral/ whole rock). The metasedimentary rocks and Paleozoic granitoids of the Acatlan complex have present-day e Nd of -9 to -11, with crustal residence ages (T DM ) of 1.3-1.6 Ga. These rocks must have been derived at least in part from a Proterozoic source area, and it is significant that crustal residence ages are identical to those of the Oaxaca terrane. The Oaxaca terrane, along with some South American Precambrian complexes, of which the Oaxaca terrane was probably once a part, may be considered the most likely source areas for the Acatlan complex. The less extensive eclogite, trondjhemite, and amphibolite bodies in the Acatlan complex yield model ages that show them to be juvenile additions to the crust. Neodymium model ages of the Acatlan complex are unlike those of some accreted crustal blocks of the Pacific margin, such as the Alexander, Stikine, and Wrangellia terranes, which have little signs of crustal recycling. Similarities between the Acatlan complex and the Acadian belt, as well as current Paleozoic paleogeographic and paleomagnetic reconstructions, suggest that the Devonian metamorphic event that affected these two areas was the result of a Laurentia-Gondwana collision. A later collision in late Carboniferous time caused deformation in the Acatlan complex, plutonic activity in southern Mexico, and deformation in the Ouachita, Marathon, and Appalachian belts. Both the Acatlan and Oaxaca terranes would have been continuous with South America until the break-up of Pangea in the Mesozoic era.

Nd-Sr isotope composition of lower crustal xenoliths — Evidence for the origin of mid-tertiary felsic volcanics in Mexico

Isotopic data were collected on lower crustal xenoliths to constrain the Mexican lower crust as source material for the mid-Tertiary Sierra Madre Occidental, which is one of the largest silicic volcanic piles known. The xenoliths are predominantly pelitic gneisses and mafic orthogneisses that were brought to the surface on the eastern edge of the Sierra Madre Occidental by recent alkalic basalts. The pelitic gneisses are uniform in mineral assemblage and contain garnet+quartz+plagioclase+sanidine+rutile +sillimanite/kyanite+graphite. The orthogneisses are plagioclase, garnet and/or spinel bearing two pyroxene granulites. Available geothermometric and geobarometric data show that the xenoliths equilibrated at temperatures and pressures consistent with those of the mantle/crust boundary in those areas.The xenoliths range from 46.2 to 67.2 SiO2. Paragneisses are in general more silicic than the orthogneisses. The xenoliths have Rb concentrations between 0.4 and 97 ppm but most samples are very low, with less than 3 ppm Rb. The Sr isotopic ratios of orthogneisses from the lowermost crust throughout most of northern Mexico are very similar and range from ca. 0.705 to 0.706. Previous studies indicate that these rocks have measured ɛ(inNd) values between+2 and −5. Paragneiss xenoliths are generally more radiogenic in Sr isotopic ratio, up to 0.730, and have lower ɛNd values of−11.The Nd and Sr isotopic characteristics of the orthogneisses are similar to those of the voluminous mid-Tertiary ignimbrites of the Sierra Madre Occidental. The xenoliths cannot represent cumulate material produced during the mid-Tertiary volcanism because they are Paleozoic or older. Consequently, based on Sr and Nd isotopic data, the silicic ignimbrites could comprise up to 100% lower crustal material.

Rhenium behavior in molybdenite in hypogene and near-surface environments: Implications for Re-Os geochronometry

Abstract Rhenium is concentrated mainly in molybdenite (MoS2) and occurs as a major cation in only a few rare minerals. This affinity makes molybdenite an ideal mineral for Re-Os geochronometry, but research on the behavior of Re in molybdenite is limited. Infrared microscope, XRD, back-scattered electron (BSE), and microprobe techniques have been used in this study to document Re behavior in molybdenite affected by hypogene and near-surface processes. In the hypogene environment, both 3R and 2H molybdenite can experience Re loss during hydrothermal alteration, which also causes increased infrared transparency (IR). Alteration at temperatures as low as ~ 150°C can cause Re loss in the presence of advecting fluids, and will affect Re-Os dating if it occurs long after primary mineralization. Re loss in 3R and 2H molybdenite under supergene conditions does not increase IR transparency. Rhenium is not incorporated into supergene ferrimolybdite, but is enriched in K-Al-silicate intergrowths which may be illite(?). These minute intergrowths are present in several samples and would not be detected in a simple optical examination. In the supergene environment elemental Os is stable, whereas Re is not. Rhenium in molybdenite may be removed by supergene fluids after some has decayed to 187Os, causing erroneously old ages, or it may be adsorbed into the illite intergrowths, creating ages which are too young. In the weathering environment, Proterozoic molybdenites have altered to Re-enriched powellite, which can be detected using back-scattered electron imagery. Combined microprobe, XRD, BSE, and infrared microscopy can be successfully used to detect alteration in molybdenite prior to dating. These techniques are non-destructive and should be performed before any molybdenite is dated by the Re-Os system.

Crust–mantle interaction in large igneous provinces: Implications from the Re–Os isotope systematics of the Columbia River flood basalts

The source and evolution of magmas that form large igneous provinces is a controversial topic. At the center of the debate is whether the different reservoirs — continental crust, asthenosphere or sub-continental lithosphere — contribute to the formation and evolution of these provinces. The isotopic systematics of Re and Os offer significant new information to constrain this debate. Because Os is compatible and Re incompatible during mantle melting, the different possible melt reservoirs will develop distinct Os isotopic signatures over time. Therefore, the source of magmas and the different possible contaminants should be readily distinguishable using the Os isotopic system alone. We have focused our study on the Re and Os composition of the Columbia River basalt Group (CRBG), which is one of the youngest and best studied continental flood basalts provinces in the world. Samples from throughout the well documented stratigraphic column, that represent primitive and contaminated magmas, as well as samples emplaced in stable North American Craton and Mesozoic accreted terrane were analyzed. The initial 187Os/188Os ranges from the lowest value of ∼0.13 in the most primitive Imnaha Formation and increases to values as high as 0.4 in flows from formations emplaced in the accreted terrane. Samples located on the craton have initial 187Os/188Os isotopic ratios that vary from ∼1 to ∼3. The data conclusively demonstrate that continental crust, most likely mafic lower crust, played an important role in the evolution of this flood basalt province. These data also show that the sub-continental lithospheric mantle (SCLM) was not significantly involved in the formation or modification of these continental flood basalts.

Source contamination versus assimilation: an example from the Trans-Mexican Volcanic Arc

Abstract Volcanic samples representing a wide range of lithologies and compositions were collected from the Miocene to Quaternary age Michoacan-Guanajuato volcanic field (MGVF) in the Trans-Mexican volcanic belt (TMVB). The samples were analyzed for major and trace elements, and 87Sr/86Sr and 187Os/188Os in an effort differentiate the importance of source contamination and assimilation in continental arc magmatism. Re concentrations in the MGVF samples range from 0.03 to 0.13 ppb and Os concentrations range from 0.05 to 0.001 ppb. The 87Sr/86Sr of the samples vary little, ranging from 0.7037 to 0.7047, despite a wide range in major element composition. However, the 187Os/188Os vary greatly, from 0.135 to 0.410. Decreasing Os concentration and increasing 187Os/188Os show a clear relation with indicators of fractionation such as MgO or Ni. A plot of 187Os/188Os versus Ba/Nb for all samples from the MGVF show two distinct trends: (1) a wide variation in Ba/Nb (50–200) associated with minor variations in 187Os/188Os (∼0.135–0.145), and (2) increasing 187Os/188Os (0.145–0.40) associated with restricted Ba/Nb (35–70). These trends are best explained through a dynamic multi-component process. Fluids are released from the subducting slab, resulting in melting of the overlying asthenospheric wedge. The pristine fluids have high Ba and low Re and Os concentrations. The resulting melts have variable Ba/Nb, but unradiogenic 187Os/188Os. Superimposed upon these melts are both assimilation and fractional crystallization processes, which affect both the Ba/Nb and 187Os/188Os systems as they ascend into the lower crust.