Drew S. Coleman

The Sierra Crest Magmatic Event: Rapid Formation of Juvenile Crust during the Late Cretaceous in California

The Late Cretaceous was a period of extremely voluminous magmatism and rapid crustal growth in the western United States. From approximately 98 to 86 Ma, greater than 4000 km2 of exposed granodioritic to granitic crust, including the largest composite intrusive suites in the Sierra Nevada batholith, were emplaced in eastern California. Plutons intruded during this period include the highest peaks in the Sierra; we informally refer to this as the Sierra Crest magmatic event. Field, petrologic, geochemical, and geochronologic data indicate that, although they comprise an insignificant volume of exposed rocks (less than 100 km2), mafic magmas were intruded contemporaneously with each episode of intermediate and high-silica magmatism in the event. This observation attests to the fundamental importance of high-alumina basaltic magmas during crustal-growth episodes in continental arcs. Geochemical data for suites of coeval plutonic rocks of the Sierra Crest magmatic event, ranging in composition from basalt to ...

Geochronology of the Proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province

The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf-slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc-continent collision along the western margin of Laurentia.

SHRIMP‐RG U‐Pb Zircon Geochronology of Mesoproterozoic Metamorphism and Plutonism in the Southwesternmost United States

Mesoproterozoic intrusive and granulite‐grade metamorphic rocks in southern California have been inferred to be exotic to North America on the basis of perceived chronologic incompatibility with autochthonous cratonal rocks. Ion microprobe geochronology indicates that zircons in granulite‐grade gneisses, dated at 1.4 Ga using conventional methods, are composed of 1.68–1.80‐Ga cores and 1.19‐Ga rims. These Early Proterozoic gneisses were metamorphosed at extremely high temperatures and moderate pressures during emplacement of the 1.19‐Ga San Gabriel anorthosite complex. The lack of a 1.4‐Ga metamorphic event suggests that Proterozoic rocks in this region, rather than being exotic to North America, may in fact be a midcrustal window into Mesoproterozoic crustal evolutionary processes in southwestern North America.

Uptake and mobility of uranium in black oaks: implications for biomonitoring depleted uranium-contaminated groundwater

In a preliminary study, the uptake and the mobility of uranium (U) by black oak trees (Quercus velutina) were assessed by measuring the isotopic composition of tree rings in two mature oak trees in a heavy metal contaminated bog in Concord, MA. The bog is adjacent to a nuclear industrial facility that has been processing depleted uranium (DU) since 1959. Over the past 40 years, DU has been leaking from an onsite holding basin and cooling pond down gradient to the bog where the oaks are located. Because DU has no source outside the nuclear industry, contamination from the industrial facility is readily discernable from uptake of natural U by measuring isotopic compositions. Isotope ratio analysis confirms the occurrence of DU in bark, sapwood and heartwood tree rings dating back to 1937, pre-dating the introduction of DU at the site by at least 20 years. Isotope dilution analysis indicates high concentrations of U (>3 ppb) in sapwood that drop rapidly to relatively constant concentrations (0.3-0.4 ppb) in heartwood. These data indicate that once incorporated into tree cells, U is mobile, possibly by diffusion through the tree wood. Concentrations of U in sapwood are approximately equal to average U concentrations in groundwater onsite over the past 10 years, suggesting that oak trees can be used as present-day bioindicators of U-contaminated groundwater. We suggest that regional sampling of oak bark and sapwood is a reasonable, inexpensive alternative to drilling wells to monitor shallow groundwater U contamination.

Cretaceous dikes within the Jurassic Independence dike swarm in eastern California

Field relationships and new U-Pb geochronology reveal that a significant number of dikes associated with the Independence dike swarm, both within the type locality and elsewhere, are of Cretaceous age. A new U-Pb zircon age for a diorite dike in the type locality of the Independence dike swarm in California confirms that some mafic dikes in the swarm are coeval with previously dated Late Jurassic felsic dikes. Cretaceous dikes in the eastern Sierra Nevada batholith are located near—and may be genetically related to—coeval mafic and felsic plutons. However, the presence of Cretaceous dikes throughout the swarm demonstrates that this is not a local phenomenon, and caution therefore should be exercised when using Independence-like dikes as temporal markers. We have not identified any consistent petrographic or chemical criteria that distinguish Jurassic Independence dikes from Cretaceous dikes. However, Jurassic dikes in the Sierra Nevada commonly contain a sinistral shear fabric that is absent in both dated Cretaceous dikes and undated dikes that intrude dated Cretaceous plutons. Despite intruding at least 60 m.y. apart, the orientations and petrology of the Jurassic and Cretaceous dikes are remarkably similar. Because dikes were intruded over a time period characterized by large changes in the orientation of plate convergence west of the Sierran arc, it is unlikely that the uniform structural orientation of Jurassic and Cretaceous dikes resulted from regional stresses imposed by the subducting plate. Instead, the orientation of Jurassic and Cretaceous dikes (and of many Sierran Cretaceous plutons) may have been controlled by preexisting faults or by the orientation of the continental margin.

Early to Middle Proterozoic Construction of the Mojave Province, Southwestern United States

Abstract Zircon and monazite U-Pb geochronology of rocks in the western Mojave province of the southwest US reveals that the Proterozoic arc exposed there shares an intrusive and deformational history with rocks exposed further east in the Yavapai and Mazatzal belts after approximately 1780 Ma. Consequently, it seems likely that the Mojave province was contiguous with these other Laurentian provinces by that time. Isotopic and geochronologic data also suggest that Mojave province plutonic rocks inherit their distinctive isotopic compositions, at least in part, from an enriched lithospheric mantle source and interaction with sedimentary rocks containing Archean detritus.

Determining the source of felsitic lithic material in southeastern New England using neodymium isotope ratios

We introduce the use of Nd isotopes as a high-precision technique that can be used in conjunction with visual, petrographic, and trace element analyses to source indistinct felsitic debitage. This approach allows sourcing of debitage from southeastern New England, including samples that previously could not be constrained to one source. Combined with trace element data, isotopic data also provide the possibility of sourcing debitage to a particular quarry site within a volcanic complex, and even to a particular ash flow within a quarry site. Determining the origin of debitage so precisely is important for understanding acquisition, trade, and exchange networks in southeastern New England, where distances separating quarry sites within an individual volcanic complex (Lynn–Mattapan) are greater than distances between different volcanic complexes (Lynn–Mattapan, Blue Hill, and Wamsutta). The sourcing of debitage to Mattapan quarries suggests that Middle Archaic populations in the northwest part of the Boston Basin obtained nonlocal lithic material primarily from sources south of the basin.