Search for: All records

This study examined the origin of graphite mineralization in the Mesoproterozoic Adirondack Highlands. Flake graphite was mined from the historic mining district around Ticonderoga, New York, USA, from the 1850s to the 1920s. Most of the production and reserves from the district are disseminated graphite from feldspathic quartz schists (e.g., Dixon-American Graphite Co. mine, Faxon property, Flake Graphite Company). Protolith sediments of the schists were deposited in the Trans-Adirondack backarc basin at ca. 1.25 Ga and were metamorphosed to high grade at 1.18−1.15 Ga during the accretionary Shawinigan orogeny and at 1.09−1.02 Ga during the Ottawan phase of the Grenvillian orogeny. Major elements and carbon isotopes of graphite from schists (δ13C = −28.1‰ to −5.4‰ relative to Vienna Peedee belemnite) are consistent with derivation from organic carbon in protolith sandstones that devolatilized during metamorphism. Some small mines and prospects in the district mined graphitic calcite marbles and clinopyroxene ± scapolite skarns associated with a pegmatite suite dated by U-Pb in zircon to 1.04−1.01 Ga (e.g., Lead Hill and Crown Point deposits), and rare graphite veins are observed at several deposits. Hydrothermal flake graphite from pegmatites, skarns, and veins are distinct from graphite in schist-hosted deposits and have δ13C values that cluster around −7‰ to −5‰, consistent with deposition from igneous CO2. Covariation of carbon and oxygen stable isotopes and U-Pb zircon geochronology are not compatible with precipitation from fluids derived from country rock metapelites or marbles. Some graphite districts elsewhere show compelling evidence for remobilization and redeposition of sedimentary-derived carbon as hydrothermal graphite, sometimes as thick veins producing world-class, high-grade deposits. In the Adirondacks, there is evidence for two generations of graphite mineralization with distinct carbon sources: syngenetic graphite with an organic origin in Shawinigan quartz schists and epigenetic graphite with an igneous origin in Ottawan skarns and pegmatites. 

Abstract: This dataset contains bulk-rock geochemical and isotopic data for igneous and host-rock samples from the Cretaceous Sierra Nevada arc, California. Reported data include F, Cl, Br, and I concentrations measured in solid fractions and water-soluble fractions, along with major element concentrations, selected trace element concentrations, water contents, and hydrogen isotope compositions where available. Samples include plutonic rocks, volcanic and shallow intrusive rocks, roof-pendant lithologies, and xenoliths. 

Abstract The northern Sierra Nevada batholith was emplaced into and across a series of accreted crustal belts that vary considerably in their ages and lithologies. Unlike batholithic segments to the south, the northern Sierra comprises smaller, spatially distinct plutons where geologic relations with the host basement can be observed. Intermediate to felsic plutons were sampled as arc‐perpendicular transects at the latitude of Lake Tahoe and zircon Lu‐Hf and trace element analysis was performed in order to assess the relative impacts of temporal and spatial variability of arc magmatism on zircon geochemistry. Trends through time in the Hf data are complex, whereas there is an abrupt step from juvenile values in plutons intruding western belts (+12.3 to +14.4) to more evolved values in those intruding the Northern Sierra terrane to the east (−0.6 to +5.2). A similar pattern is observed in several zircon trace element signatures, including pronounced steps toward higher U/Yb, Dy/Yb and Ce/Y from the western belts into the Northern Sierra terrane to the east. The step is approximately coincident with the Feather River terrane, which is interpreted to mark the suture between the oceanic lithosphere to the west and the North American continental lithosphere to the east. The observed links between variation in zircon Lu‐Hf and trace element concentration and basement domain indicate that northern Sierran zircons incorporate, and are sensitive to, the crustal tracts into which they are emplaced. Preliminary application of our results to provenance analysis of Great Valley strata indicates changing provenance through time in the adjacent forearc. 

Abstract The Mineral King pendant is an ~15-km-long, northwest-striking assemblage of Permian to mid-Cretaceous metavolcanic and metasedimentary rocks that form a steeply dipping wall-rock screen between large mid-Cretaceous plutons of the Sierra Nevada batholith (California, USA). Pendant rocks are generally well layered and characterized by northwest-striking, steeply dipping, layer-parallel cleavage and flattening foliation and steeply northwest-plunging stretching lineation. Northwest-elongate lithologic units with well-developed parallel layering and an absence of prominent faults or shear zones suggests a degree of stratigraphic continuity. However, U-Pb zircon dating of felsic metavolcanic and volcanosedimentary rocks across the pendant indicates a complex pattern of structurally interleaved units with ages ranging from 277 Ma to 101 Ma. We utilize a compilation of 39 existing and new U-Pb zircon ages and four reported fossil localities to construct a revised geologic map of the Mineral King pendant that emphasizes age relationships rather than lithologic or stratigraphic correlations as in previous studies. We find that apparently coherent lithologic units are lensoidal and discontinuous and are cryptically interleaved at meter to kilometer scales. Along-strike facies changes and depositional unconformities combine with kilometer-scale tight folding and structural imbrication to create a complex map pattern with numerous discordant units. Discrete faults or major shear zones are not readily apparent in the pendant, although such structures are necessary to produce the structural complications revealed by our new mapping and U-Pb dating. We interpret the Mineral King pendant to be structurally imbricated by a combination of kilometer-scale tight to isoclinal folding and cryptic faulting, accentuated by, and eventually obscured by, pervasive flattening and vertical stretching that preceded and accompanied emplacement of the bounding mid-Cretaceous plutons. Deformation in the Mineral King pendant represents a significant episode of pure-shear-dominated transpression between ca. 115 Ma and 98 Ma that adds to growing evidence for a major mid-Cretaceous transpressional orogenic event affecting the western U.S. Cordillera. 

Ongoing investigations of halogen element (F, Cl, Br, I) concentrations in rocks and minerals in the Cretaceous Sierra Nevada Batholith, CA, aim to elucidate the spatio-temporal distribution and budget of these important elements in a “typical” continental convergent margin arc. Using a 3.0 kW Axios (Panalytical) wavelength-dispersive X-ray fluorescence spectrometer (XRF) equipped with a Ge 111 crystal to eliminate second order interferences on Cl-Kα lines, the Pomona College XRF lab has undertaken a campaign-style study of Cl in pressed powder samples of metamorphic and metavolcanic rocks in the Sierra. This work complements pyrohydrolysis + ion chromatography (IC) and ICP-MS analyses the research team is undertaking at the University of Texas – Austin. Both labs quadruply wash powders to eliminate Cl contributions from decrepitated fluid inclusions or grain boundary deposits. Intercomparison between the two labs show correlation (r2 = 0.98) between analyses of the same unknown samples, but decreased accuracy of XRF (>30% relative) below 30 µg/g. Despite lower precision, XRF characterization is a relatively rapid and less labor intensive means to identify key Cl variations among rock types and to select samples for full analysis of all four halogen elements by pyrohydrolysis + IC (Cl,F) and ICP-MS (Br,I). Results thus far indicate that Mg- to Al-rich pelites ranging in metamorphic grade from phyllite to migmatite vary widely in Cl: 50–500 µg/g; cordierite-biotite hornfels are typically elevated in Cl (200–400 µg/g) and other lithologies such as skarns and amphibolite are highly varied (50–600 µg/g Cl); a localized study of a high temperature (650–750°C) migmatites surrounding a gabbro-diorite complex shows low and relatively uniform Cl (100 ± 50 µg/g) in the migmatites. This fundings suggests that Cl may have been mobilized into melts during biotite dehydration melting in the migmatites. Metavolcanic rocks vary from 20 to over 2000 µg/g Cl, suggesting post-eruptive exchange with exogenous fluids during hydrothermal alteration and metamorphism. Metavolcanic packages in different pendants, screens and septa show some localized patterns in Cl concentration that are being explored further. 

Abstract We explore the growth of lower-continental crust by examining the root of the Southern California Batholith, an ~500-km-long, paleo-arc segment of the Mesozoic California arc that lies between the southern Sierra Nevada Batholith and northern Peninsular Ranges Batholith. We focus on the Cucamonga and San Antonio terranes located in the eastern San Gabriel Mountains where the deep root of the Mesozoic arc is exhumed by the Quaternary Cucamonga thrust fault. This lower- to mid-crustal cross section of the arc allows us to investigate (1) the timing and rates of Mesozoic arc construction, (2) mechanisms of sediment incorporation into the lower crust, and (3) the interplay between mantle input and crustal recycling during arc magmatic surges. We use U-Pb detrital zircon geochronology of four quartzites and one metatexite migmatite to investigate the origin of the lower-crustal Cucamonga metasedimentary sequence, and U-Pb zircon petrochronology of 26 orthogneisses to establish the timing of arc magmatism and granulite-facies metamorphism. We find that the Cucamonga metasedimentary sequence shares broad similarities to Sur Series metasedimentary rocks in the Salinia terrane, suggesting that both were deposited in a late Paleozoic to early Mesozoic forearc or intra-arc basin marginal to the Southern California Batholith. This basin was progressively underthrust beneath the arc during the Middle Jurassic to Late Cretaceous and was metamorphosed during two high-grade (>750 °C), metamorphic events at ca. 124 Ma and 89–75 Ma. These metamorphic events were associated with 100 m.y. of arc magmatism that lasted from 175 Ma to 75 Ma and culminated in a magmatic surge from ca. 90 Ma to 75 Ma. Field observations and petrochronology analyses indicate that partial melting of the underthrust Cucamonga metasedimentary rocks was triggered by the emplacement of voluminous, mid-crustal tonalites and granodiorites. Partial melting of the metasedimentary rocks played a subsidiary role relative to mantle input in driving the Late Cretaceous magmatic flare-up event. 

Garnet U‐Pb dating by laser ablation‐inductively coupled plasma‐mass spectrometry requires the development of matrix‐matched reference materials of variable chemistry and U mass fraction for accurate analysis. Additional calibration of existing primary reference materials is also justified based on the relatively poor calibration of some of the widely available primary reference materials that are currently utilised by the geoscience community. We present a micro sampling workflow combined with a refined ID‐TIMS methodology for the generation of high precision (~ 0.1%) U‐Pb dates from domains within garnet single crystals. Using this workflow, we calibrated two new natural andradite reference materials, the Jumbo andradite (And₉₉; 110.34 ± 0.03 (0.04) [0.13] Ma,n= 7, MSWD = 1.21) and the Tiptop andradite (And₈₇; 209.57 ± 0.11 (0.13) [0.26] Ma,n= 6, MSWD = 1.39). We also present additional calibration of the widely utilised Willsboro‐Lewis andradite primary reference material (And₉₀; 1024.7 ± 9.5 (9.6) [9.6] Ma (2s; overdispersed),n= 6). Wafers of the Jumbo and Tiptop andradite reference materials are available from the authors upon request.