Jonathan S. Miller

Contrasting stratified plutons exposed in tilt blocks, Eldorado Mountains, Colorado River Rift, NV, USA

Abstract Roof-to-floor exposures of mid-Miocene plutons in tilt blocks south of Las Vegas, NV, reveal distinct but strongly contrasting magma chamber statigraphy. The Searchlight and Aztec Wash plutons are well-exposed, stratified intrusions that show a similar broad range in composition from ∼45–75 wt.% SiO 2 . Homogeneous granites that comprise about one-third of each intrusion are virtually identical in texture and elemental and isotopic chemistry. Mafic rocks that are present in both plutons document basaltic input into felsic magma chambers. Isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with minor crustal contamination, whereas more felsic rocks are hybrids that are either juvenile basaltic magma+crustal melt mixtures or products of anatexis of ancient crust+young (Mesozoic or Miocene?) mafic intraplate. Despite general similarities, the two plutons differ markedly in dimensions and lithologic stratigraphy. The Searchlight pluton is much thicker (∼10 vs. 3 km) and has thick quartz monzonite zones at its roof and floor that are absent in the Aztec Wash pluton. Isotopic and elemental data from Searchlight pluton suggest that the upper and lower zones are cogenetic with the granite; we interpret the finer grained, slightly more felsic upper zone to represent a downward migrating solidification front and the lower zone to be cumulate. In contrast, the upper part of the Aztec Wash pluton is granite, and a heterogeneous, mafic-rich injection zone with distinct isotopic chemistry forms the lower two-thirds of the intrusion. Similar mafic rocks are relatively sparse in Searchlight pluton and do not appear to have played a central role in construction of the pluton. Large felsic and composite dikes that attest to repeated recharging and intrachamber magma transfer are common in the Aztec Wash pluton but absent in the Searchlight pluton. Thus, although both intrusions were filled by similar magmas and both developed internal stratification, the two intrusions evolved very differently. The distinctions may be attributable to scale and resulting longevity and/or to subtle differences in tectonic setting.

Construction of a pluton: Evidence from an exposed cross section of the Searchlight pluton, Eldorado Mountains, Nevada

A top-to-bottom cross section of the Searchlight pluton is exposed in a large tilt block in the Colorado River extensional corridor of southern Nevada. Hornblende barometry and geologic relations indicate that the pluton was about 10 km thick, extending from approximately 3 to 13 km depth. The pluton is stratified with about 2 km of fine-grained quartz monzonite below the roof, 2 km of granite in the center, and 6 km of coarser, more mafic quartz monzonite at the bottom. The lower unit has a pronounced magmatic foliation that was subhorizontal prior to tilting. Contacts between the units are gradational over a few centimeters to about 20 m. Geometry, field relations, and elemental and isotopic data suggest that the three units mark the terminal stages of evolution of magma that filled a very thick magma chamber. The upper unit formed as a solidification front that migrated downward from the roof, and the middle unit granite and the lower unit represent complementary segregated melt and crystal-rich cumulate. Thus, the dominant part of the pluton appears to have solidified in monotonic fashion from more or less uniform magma that was compositionally similar to the upper unit. The basal quartz monzonite and the lower part of the granite enclose widely scattered, synplutonic hornblende gabbro and diorite pods that range from centimeter to kilometer scale. These rocks are commonly fine grained and formed as quenched mafic melts. They are isotopically distinct from the main sequence and represent discrete injections into the magma chamber. The mafic magmas were contaminated by, but did not strongly affect, the main-sequence magma. The Nd and Sr isotopic compositions of the main-sequence magma are intermediate between those of regional crust and enriched mantle-derived basalts like those represented by the mafic pods. This suggests that the principal magma was a hybrid with about 60% mantle component, and thus the pluton represents both addition to and internal reorganization of the crust

The Coso geothermal field: A nascent metamorphic core complex

Investigation of the Coso Range using seismicity, gravity, and geochemistry of rocks and fluids, supports the interpretation that the structure hosting the geothermal resource is a nascent metamorphic core complex. The structural setting is a releasing bend in a dextral strike-slip system that extends from the Indian Wells Valley northward into the Owens Valley. This tectonic setting results in NW-directed transtension, which is accommodated by normal and strike-slip faulting of the brittle upper 4–6 km of the crust, and shearing and ductile stretching below this depth, accompanied by shallow igneous intrusions. Focal mechanisms of some small earthquakes that have occurred from 1996 to the present beneath the Coso Range exhibit depth-dependent rotation of seismic P and T axes, indicating that the local orientations of the principal stresses likely favor resolved shear stress on low-angle faults. These small earthquakes occur near the base of seismicity, which we interpret as coincident with the brittle-ductile transition. Geochemical results show a significant asthenospheric influence in the isotopic composition of rocks and fluids, indicating that the crust is thinned within the Coso structure. Thinned upper crust is underlain by a more dense mafic body at depths of 10 km or less. This is consistent with observed gravity anomalies and models. The mafic body may represent cumulates left over from the fractional crystallization of rhyolite, which occurs as endogenous domes at Coso, or it could be a sheeted-dike complex in the upper mid-crustal area. Transtension began at 2–3 Ma, and continues today. Using a long-term crustal deformation rate of 2 mm/yr, we infer that the basal detachment fault commonly observed in fully exhumed metamorphic core complexes will reach the surface in two to four million years.

Tertiary extension‐related volcanism, Old Woman Mountains area eastern Mojave Desert, California

Tertiary volcanism in the Old Woman Mountains area, eastern California overlapped temporally and spatially with lithospheric extension during the early Miocene. Field relations and restoration of movement along major faults indicate that the primary locus of magmatism probably lay along or flanked the axis of the Piute Mountains. The maximum age of volcanism is unknown but is probably about 20–21 Ma (after regional extension began). Most of the volcanism terminated after emplacement of the 18.5 Ma Peach Springs Tuff. Contrary to conventional models which suggest that extension-related volcanism is basaltic or bimodal, the volcanic rocks of the Old Woman Mountains area represent a calc-alkaline continuum dominated by mafic andesite; the most mafic rocks are mildly alkalic. Compositional variability within the suite is the result of mixing a mantle component with silicic crust, accompanied initially by minor olivine fractionation. Petrographic and field observations indicate that both magma mixing and assimilation of granitic crust have operated to produce the observed chemical variation. Trace elements and isotopes preclude binary mixing and suggest that mixing involved several crustal end members. This is consistent with the heterogeneous nature of both the exposed basement and a lower crustal xenolith suite sampled by a Tertiary dike. The mildly alkalic nature of the mafic lavas, the enrichment in large ion lithophiles and light rare earth elements, and the relatively radiogenic character of the most primitive uncontaminated basalt suggest a source similar to enriched mantle lithosphere. Alternatively, these geochemical and isotopic signatures could be attributed to contamination of asthenospheric basalt with mafic lower crust or low-degree partial melts of this crust.

Ireteba Pluton, Eldorado Mountains, Nevada: Late, Deep‐Source, Peraluminous Magmatism in the Cordilleran Interior

The Ireteba pluton is a ∼66 Ma \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Hafnium, oxygen, neodymium, strontium, and lead isotopic constraints on magmatic evolution of the supereruptive southern Black Mountains volcanic center, Arizona, U.S.A.: A combined LASS zircon-whole-rock study

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Zircon growth and recycling during the assembly of large, composite arc plutons

\end{document} granite emplaced at the eastern edge of the Cordilleran plutonic belt in southeastern Nevada. In common with other Cordilleran peraluminous granites, its mineralogy, major element chemistry, isotopic composition, and abundant Proterozoic zircon inheritance document crustal origin. Distinctive trace element chemistry, field relations, and inherited components further constrain its genesis. High Sr concentrations, low heavy rare earth elements, and absence of negative Eu anomalies indicate that the Ireteba magma was extracted from a residue relatively rich in garnet and poor in feldspar; rounded quartz is probably a resorbed, high‐P liquidus phase or restite. The granite shares with adakites (slab‐derived arc magmas) and Archean granitoids the Sr–rare earth element signature of deep‐source origin. Nd‐Sr isotopic compositions indicate a dominantly crustal origin for the granite, but it is less mature than the underlying ancient Mojave crust. The granite is apparently a hybrid derived primarily from the ancient crust but with a less mature component as well: either Jurassic igneous rock, as suggested by sparse 150–170 Ma zircon cores, or juvenile mafic magma, as implied by abundant synplutonic mafic rocks, or both. Influx of basaltic magma during the waning stages of Cordilleran convergence may have triggered melting in the deep, thickened crust, with basaltic magmas being trapped beneath the less dense crustal melts. Like other relatively young peraluminous granites of the Cordilleran Interior, including the Idaho‐Bitterroot batholith, the Ireteba pluton may reflect changing conditions during the waning stages of plutonism.

Geology and geochronology of the Spirit Mountain batholith, southern Nevada: Implications for timescales and physical processes of batholith construction

Studies of plutons indicate that they are the result of a complex interplay of magmatic processes occurring during magma generation, ascent, and emplacement. A critical tool for deciphering these processes is high-precision geochronology, which can help determine the timing and rates of magmatism in the crust. We conducted a field and U-Pb geochronological study of the Cretaceous Black Peak intrusive complex in the North Cascades of Washington State to investigate magmatism at a detailed scale and to refine estimates of plutonic construction rates. High-precision chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb geochronology was carried out on 31 samples from five mapped intrusive phases. Field relations in the Black Peak intrusive complex show intrusive contacts that vary from sharp to gradational. Whole-rock Sm/Nd, zircon oxygen isotopes, and zircon trace elements were obtained on subsets of representative samples. The U-Pb geochronology from the Black Peak intrusive complex documents batholith intrusion over 4.5 m.y. and suggests that magmatism was semicontinuous for a minimum of 3.5 m.y. Individual samples display age dispersion in single-zircon dates that ranges from ∼105 yr to several 106 yr, with a general increase in the age range for younger samples. Whole-rock eNd and zircon δ18O for all Black Peak intrusive complex samples indicate that magmas were derived from mantle and crustal sources and that all magmas were isotopically homogenized prior to zircon saturation. Ti-in-zircon temperatures from zircon cores are generally above calculated zircon saturation temperatures, which suggests that most Black Peak intrusive complex magmas were zircon undersaturated in the melt source region. A range of thicknesses was considered, and a thickness of ∼10 km for the Black Peak intrusive complex gives an average intrusion rate of ∼1.1 ×10–3 km3/yr, which is high enough to sustain a magma reservoir in the shallow crust. The field evidence and long overall duration of intrusion are incompatible with the entire Black Peak intrusive complex being molten at any one time, but the larger, more compositionally homogeneous domains in the Black Peak intrusive complex are likely the solidified remnants of mushy magma bodies with ∼105 yr durations. These data suggest that the Black Peak intrusive complex may have remained “mushy” for long periods of time (105 yr) and may indicate that the spread in dates within individual samples is best interpreted as either antecrystic recycling and/or protracted autocrystic growth.