Todd B. Housh

San Quintín Volcanic Field, Baja California Norte, México: Geology, petrology, and geochemistry

The San Quintin Volcanic Field (SQVF) is unique for the Baja California peninsula as the only known location of intraplate-type mafic alkalic volcanism and the only known source of peridotitic and granulitic xenoliths. It consists of 10 distinct Quaternary volcanic complexes. The oldest cones mainly erupted primitive magmas (Mg # > 64)(Mg # = 100 × Mg/(Mg + (0.85 × FeTotal))), which carried occasional small xenoliths. As the SQVF evolved with time, differentiated magmas (Mg # < 64) became increasingly common, but primitive magmas, virtually devoid of xenoliths and unusually rich in olivine phenocrysts, dominanted at the youngest cones. Abundances of incompatible elements declined during evolution of the SQVF, implying a temporal increase in the extent of partial melting in the mantle, or progressive exhaustion of these elements in the source. Samples from two cones, Mazo and Ceniza, show relatively low Ce/Pb, eNd, and 206Pb/204Pb and high 87Sr/86Sr, which we interpret as evidence for crustal contamination of these magmas. Small isotopic variations for the other cones are collectively interpreted to reflect involvement of at least three mantle components beneath the SQVF. Ranges in isotopic composition overlap for primitive and differentiated rocks, supporting fractional crystallization as the mechanism for deriving the latter from the former. Most differentiated rocks can be successfully modeled by fractional crystallization of olivine, plagioclase, clinopyroxene, and spinel from primitive parents. The largest and most abundant xenoliths were carried by differentiated magmas, indicating that fractional crystallization took place within the mantle, below the level of peridotite entrainment, and reflecting the importance of fractionation-elevated volatile contents for driving these differentiated magmas rapidly to the surface. Primitive rocks of the SQVF are unusual compared to other reported intraplate-type mafic alkalic suites from around the world in having relatively high Al2O3 and Yb, as well as low La/Yb and CaO/Al2O3. These characteristics and trends of rising Al2O3 and falling CaO with decreasing incompatible element abundances are all consistent with origins for the SQVF primitive magmas by progressive partial melting of spinel lherzolite at unusually shallow levels in the mantle.

Tracking exhumation of Andean ranges bounding the Middle Magdalena Valley Basin, Colombia

The shortening history of the Andes is important for understanding retroarc deformation along convergent margins and forcing mechanisms of Cenozoic climate. However, the timing of uplift in the northern Andes is poorly constrained, with estimates ranging from Cretaceous to Pliocene. Detrital zircon U-Pb ages from the Middle Magdalena Valley Basin in Colombia reveal two provenance shifts during Cenozoic time. The fi rst shift occurs between early and late Paleocene strata, where U-Pb results show a switch from Proterozoic-dominated to Phanerozoic-dominated age spectra. We attribute this change to uplift-related exhumation of the Central Cordillera. The second shift occurs between middle-late Eocene and late Oligocene strata, where increased Grenville ages and diminished Mesozoic ages can be linked to uplift of the Eastern Cordillera. Our results show that signifi cant pre-Neogene deformation affected the northern Andes, underscoring the potential importance of Andean uplift on the dynamics of Paleogene climate.

Lead Isotopic Study of Early Proterozoic Wopmay Orogen, Nw Canada: Role of Continental Crust in Arc Magmatism

Lead isotopes are sensitive indicators of crustal interaction in arc magmatism because of the large contrast in lead concentration between mantle-derived magmas and continental crust and the three independent decay paths in the U-Th-Pb system. Preserved within Wopmay Orogen (1.94 to 1.84 Ga) are the remnants of three magmatic arcs. The isotopic composition of lead in alkali feldspars separated from a diverse suite of samples from all three arcs fall in a relatively tight cluster with

Ancient isotopic characteristics of Neogene potassic magmatism in Western New Guinea (Irian Jaya, Indonesia)

^{206}Pb/^{204}Pb = 15.59-15.88

Use of electrothermal vaporization for volatility-based separation of Rb–Sr isobars for determination of isotopic ratios by ICP-MS

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Nature of the crust beneath west-central Chihuahua, Mexico, based upon Sr, Nd, and Pb isotopic compositions at the Tomóchic volcanic center

^{207}Pb/^{204}Pb = 15.27-15.36