Reference Number: 2003
GeoREF Number: 2006-010959
Publication Year: 2005
Abstract: "We have identified five distinct isotope provinces in northwestern Mexico in a study of the Sr, Nd, and Pb isotope geochemistry of Laramide and mid-Tertiary igneous rocks in Sonora and Chihuahua as well as adjacent portions of southwestern United States. Province A, in northern Sonora, is characterized by very unradiogenic Nd and Pb isotopic compositions and radiogenic Sr isotopic compositions. Unradiogenic Nd and radiogenic Sr and Pb isotopic compositions characterize Province B, in central Sonora. Provinces A and B are flanked to the east and south by Provinces C, D, and E, which are characterized by more radiogenic Nd and less radiogenic Sr isotopic compositions than observed in Provinces A and B. Rocks from Provinces C, D, and E have similar Sr and Nd isotopic compositions, although the variation observed in rocks from Province E is more restricted than in Provinces C and D. These provinces are unique, however, in their Pb isotopic compositions; rocks from Province C are the least radiogenic whereas those from Province E are the most radiogenic. The identified isotope provinces correlate well with the constraints imposed by the geology of the southwestern United States and northwestern Mexico. The primary factor controlling the geographic distribution of isotope provinces in northwestern Mexico appears to be the age of the crust that the magmas passed through and with which they interacted. Provinces A and B are interpreted to reflect the area underlain by old, early Proterozoic basement in northwestern Sonora; the significance of the difference in Pb isotopic compositions between rocks in Provinces A and B is not clear. We interpret Province C, located in northern Chihuahua and adjacent regions of Texas, to reflect the distribution of mid-Proterozoic crust of the North American craton that has been overprinted by Grenvillian deformation. Province D is located in a belt running from central Chihuahua into southwestern Texas and is interpreted to reflect the distribution of exotic crust that was accreted to the southern margin of the North American craton during Ouachita convergence; this crustal block consists of a Paleozoic arc sequence developed on Proterozoic basement. Lastly, we interpret Province E, located in southern Sonora and Chihuahua, as exotic Jurassic-Cretaceous arc crust accreted during the late Mesozoic. The observed distribution of isotope provinces in northwestern Mexico is not consistent with the Mojave-Sonora megashear hypothesis. In particular, the NNE-trending eastern boundary of the two western isotope provinces (Provinces A and B) does not show the 700-800 km of left-lateral offset that would be expected if this boundary was intersected by the Mojave-Sonora megashear."
Descriptors: absolute age; alkaline earth metals; Arizona; basement; Cenozoic; Chihuahua Mexico; Cretaceous; dates; deformation; faults; geochemistry; Grenvillian Orogeny; igneous rocks; intrusions; isotope fractionation; isotope ratios; isotopes; Jurassic; K/Ar; Laramide Orogeny; lead; magma contamination; magmatism; Mesozoic; metals; Mexico; Mojave-Sonora Megashear; Nd-144/Nd-143; neodymium; Oligocene; orogeny; Paleogene; Pb-206/Pb-204; Precambrian; Proterozoic; radioactive isotopes; rare earths; Rb/Sr; shear zones; Sierra Madre Occidental; Sonora Mexico; Sr-87/Sr-86; stable isotopes; strontium; systems; tectonics; Tertiary; Texas; U/Pb; United States; Upper Cretaceous; upper Precambrian
Classification: 16 Structural geology; 02D Isotope geochemistry
Source: "Special Paper - Geological Society of America, vol.393, pp.671-692, 2005"
map_coordinates: "Latitude:N263000,N323000 Longitude:W1083000,W1150500; Latitude:N254500,N320000 Longitude:W1030000,W1080000"
Contributors:Cole, Jennifer M; Rasbury, E Troy; Hanson, Gilbert N; Montanez, IsabelP; Pedone, Vicki A
Reference Number: 1973
GeoREF Number: 2005-031003
Publication Year: 2005
Abstract: "Sedimentary rocks and associated fossils are the direct record of pastlife and its environmental context. Few methods allow direct andprecise dating of this important archive, making it difficult tocorrelate climate, biologic, and tectonic events preserved interrestrial records. We sampled lacustrine tufa calcite from themiddle member of the Miocene Barstow Formation to further test thefeasibility of uraniumlead dating of terrestrial carbonates. Fivesamples yield U-Pb ages (2sigma ) of 14.81+ or -0.39 Ma, 15.30+ or-0.25 Ma, 15.39+ or -0.15 Ma, 16.14+ or -0.40 Ma, and 16.24+ or -0.23Ma. These precise ages are consistent with existing ages from K-Ar and(super 40) Ar/ (super 39) Ar dating of intercalated volcanic ashes.Our most precise U-Pb age on tufa has an uncertainty of <1% (150ka), which is equivalent to uncertainties for K-Ar ages from the sametime period. Our results indicate that U-Pb dating of carefullysampled and screened sedimentary carbonates provides much needed ageconstraints in purely terrestrial sequences and can be a powerful toolfor correlation even in deposits with structural complexity and rapidfacies changes."
Descriptors: absolute age; Barstow Formation; calcite; California; carbonates;Cenozoic; chemically precipitated rocks; correlation; dates;depositional environment; lacustrine environment; Miocene; Neogene;precision; San Bernardino County California; sedimentary rocks;terrestrial environment; Tertiary; tufa; U/Pb; United States
Classification: "03, Geochronology"
Source: "Geological Society of America Bulletin, vol.117, no.3-4, pp.276-287,Apr 2005"
map_coordinates: "Latitude:N350000,N351000 Longitude:W1170000,W1171500"
Reference Number: 8804
GeoREF Number: 2005-047432
Publication Year: 2005
Abstract: "The Miocene-Quaternary Jemez Mountains volcanic field (JMVF), the site of the Valles caldera, lies at the intersection of the Jemez lineament, a Proterozoic suture, and the Cenozoic Rio Grande rift. Parental magmas are of two types: K-depleted silica-undersaturated, derived from the partial melting of lithospheric mantle with residual amphibole, and tholeiitic, derived from either asthenospheric or lithospheric mantle. Variability in silica-undersaturated basalts reflects contributions of melts derived from lherzolitic and pyroxenitic mantle, representing heterogeneous lithosphere associated with the suture. The K depletion is inherited by fractionated, crustally contaminated derivatives (hawaiites and mugearites), leading to distinctive incompatible trace element signatures, with Th/(Nb,Ta) and La/(Nb,Ta) greater than, but K/(Nb,Ta) similar to, Bulk Silicate Earth. These compositions dominate the mafic and intermediate lavas, and the JMVF is therefore derived largely, and perhaps entirely, from melting of fertile continental Jemez lineament lithosphere during rift-related extension. Significant variations in Pb and Nd isotope ratios ( (super 206) Pb/ (super 204) Pb = 17.20-18.93; (super 143) Nd/ (super 144) Nd = 0.51244-0.51272) result from crustal contamination, whereas 87Sr/86Sr is low and relatively uniform (0.7040-0.7048). We compare the effects of contamination by low- (super 87) Sr/ (super 86) Sr crust with assimilation of high- (super 87) Sr/ (super 86) Sr granitoid by partial melting, with Sr retained in a feldspathic residue. Both models satisfactorily reproduce the isotopic features of the rocks, but the lack of a measurable Eu anomaly in most JMVF mafic lavas is difficult to reconcile with a major role for residual plagioclase during petrogenesis."
Descriptors: "assimilation, Cenozoic, depletion, Jemez Lineament, Jemez Mountains, Jemez Mountains volcanic field, lava, mafic composition, magma contamination, magmas, Miocene, Neogene, New Mexico, partial melting, Quaternary, Rio Grande, Tertiary, United States, Valles Caldera"
Classification: "05A,: Igneous and metamorphic petrology"
Source: "Journal of Petrology, vol.46, no.2, pp.407-439, Sept 2005"
Contributors:Conly, A G; Brenan, J M; Bellon, H; Scott, S D
Contributors:Harlan, Stephen S; Snee, Lawrence W; Reynolds, Mitchell W; Mehnert,Harald H; Schmidt, R G; Sheriff, Steve D; Irving, Anthony J
Reference Number: 1986
GeoREF Number: 2005-062998
Publication Year: 2005
Abstract: "We report new super 40 Ar/ super 39 Ar and K-Ar dates from theUpper Cretaceous Adel Mountain Volcanics of northwestern Montana andspatially related Tertiary igneous rocks. The Adel Mountain volcanicfield consists of about 900 square kilometers of lavas, associatedvolcaniclastic strata, and intrusions that lie astride the easternmostfolds of the Montana disturbed belt of the Cordilleran fold and thrustbelt. The Adel Mountain volcanic rocks have been intensely deformed byfolds and thrust faults along their southwestern margin but areessentially undeformed to the east. Prior to isotopic dating, the ageof the Adel Mountain Volcanics was the subject of debate, with ageassignments ranging from Late Cretaceous to early Tertiary. Isotopicdates reported here demonstrate that the Adel Mountain Volcanics areclearly Late Cretaceous and that the volcanic rocks were probablyemplaced during an approximately 2- to 3-million-year interval betweenabout 76 to 73 mega-annum Ma. The new dates from the Adel MountainVolcanics are significant in that they provide a more refined andreliable age for the Late Cretaceous cratonic paleomagnetic referencepole for North America. The dates from the Adel Mountain Volcanics, aswell as those from spatially related younger intrusions, also provideimportant constraints on the age of fold and thrust-belt deformationalong the eastern margin of the Montana disturbed belt. Syntectonicdeformation of the Adel Mountain Volcanics, as well as apparentfolding and faulting of Tertiary quartz monzonite sills, indicatesthat contractional deformation clearly spanned the Late Cretaceous andmay have extended to as young as the Paleocene/Eocene boundary atabout 55.5 Ma. Elsewhere, posttectonic field relationships indicatethat deformation may have ended prior to 60 Ma. Complexities in fieldrelationships with respect to folds and faults shown by the earlyTertiary intrusions as well as complications in the argon systematicsindicate that these interpretations must be considered preliminary.Further field work and structural studies and additionalhigh-precision geochronology are needed in order to place limits onthe cessation of contractional deformation in this part of Montana.Unambiguously posttectonic dikes 47.5 Ma that cut all deformed rocksand structures in the area indicate that disturbed belt deformationhad clearly ceased by the early middle Eocene prior to the onset ofwidespread crustal extension in this part of the northern Cordillera."
Descriptors: absolute age; Adel Mountain Volcanics; anticlinoria; Ar/Ar; Cenozoic;Cretaceous; dates; deformation; dikes; extension; feldspar group; foldand thrust belts; folds; framework silicates; igneous rocks;intrusions; K/Ar; lithostratigraphy; Mesozoic; Montana; monzonites;northwestern Montana; paleomagnetism; plagioclase; plutonic rocks;silicates; sills; tectonics; Tertiary; United States; UpperCretaceous; USGS
Classification: 05A Igneous and metamorphic petrology; 16 Structural geology
Source: "U. S. Geological Survey Professional Paper, Report: P 1696, 29 pp.,2005"
map_coordinates: "Latitude:N470000,N471500 Longitude:W1114500,W1121500"
Contributors: Thompson, R N; Ottley, C J; Smith, P M; Pearson, D G; Dickin, A P;
Reference Number: 2029
GeoREF Number: 2006-083239
Publication Year: 2005
Abstract: " The <80 ka basalts-basanites of the Potrillo Volcanic Field (PVF) form scattered scoria cones, lava flows and maars adjacent to the New Mexico-Mexico border. MgO ranges up to 12.5%; lavas with MgO<10.7% have fractionated both olivine and clinopyroxene. Cumulate fragments are common in the lavas, as are subhedral megacrysts of aluminous clinopyroxene (with pleonaste inclusions) and kaersutitic amphibole. REE modelling indicates that these megacrysts could be in equilibrium with the PVF melts at approximately 1.6-1.7 GPa pressure. The lavas fall into two geochemical groups: the Main Series (85% of lavas) have major- and trace-element abundances and ratios closely resembling those of worldwide ocean-island alkali basalts and basanites (OIB); the Low-K Series (15%) differ principally by having relatively low K (sub 2) O and Rb contents. Otherwise, they are chemically indistinguishable from the Main Series lavas. Sr- and Nd-isotopic ratios in the two series are identical and vary by scarcely more than analytical error, averaging (super 87) Sr/ (super 86) Sr = 0.70308 (SD = 0.00004) and (super 143) Nd/ (super 144) Nd = 0.512952 (SD = 0.000025). Such compositions would be expected if both series originated from the same mantle source, with Low-K melts generated when amphibole remained in the residuum. Three PVF lavas have very low Os contents (<14 ppt) and appear to have become contaminated by crustal Os. One Main Series picrite has 209 ppt Os and has a gamma (sub Os) value of +13.6, typical for OIB. This contrasts with published (super 187) Os/ (super 188) Os ratios for Kilbourne Hole peridotite mantle xenoliths, which give mostly negative gamma (sub Os) values and show that Proterozoic lithospheric mantle forms a thick Mechanical Boundary Layer (MBL) that extends to approximately 70 km depth beneath the PVF area. The calculated mean primary magma, in equilibrium with Fo (sub 89) , has Na (sub 2) O and FeO contents that give a lherzolite decompression melting trajectory from 2.8 GPa (95 km depth) to 2.2 GPa (70 km depth). Inverse modelling of REE abundances in Main Series Mg-rich lavas is successful for a model invoking decompression melting of convecting sub-lithospheric lherzolite mantle (epsilon Nd = 6.4; T (sub p) approximately 1400 degrees C) between 90 and 70 km. Nevertheless, such a one-stage model cannot account for the genesis of the Low-K Series because amphibole would not be stable within convecting mantle at T (sub f) approximately 1400 degrees C. These magmas can only be accommodated by a three-stage model that envisages a Thermal Boundary Layer (TBL) freezing conductively onto the approximately 70 km base of the Proterozoic MBL during the approximately 20 Myr tectonomagmatic quiescence before PVF eruptions. As it grew, this was veined by hydrous small-fraction melts from below."
Descriptors: alkali basalts; basalts; basanite; Cenozoic; chemical properties; chemical ratios; convection; Dona Ana County New Mexico; geochemistry; igneous rocks; Las Cruces New Mexico; lithogeochemistry; lithosphere; mantle; New Mexico; picrite; Potrillo volcanic field; Quaternary; United States; volcanic fields; volcanic rocks
Classification: " 02C Geochemistry of rocks, soils, and sediments; 05A Igneous and metamorphic petrology"
Source: " Journal of Petrology, vol.46, no.8, pp.1603-1643, Aug 2005"
map_coordinates: "Latitude N321842,N321842 Longitude W1064642,W1064642"