Search for: All records

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. 

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.

 

Halogens (F, Cl, Br, I) are primary components of volcanic gas emissions and play an essential role in continental arc magmatic environments due to their solubility in fluids that generate metallic ore deposits. Despite their ubiquity, the behavior and budget of halogens in continental arc environments are poorly constrained. We investigated the plutonic and volcanic halogen budgets in intermediate-to-felsic igneous rocks (56–77 wt% SiO2) from the Sierra Nevada (California) - a Mesozoic continental arc where plutonic and volcanic outcrops can be correlated via their geographic, compositional, and geochronologic framework. We measured the halogen concentrations of bulk rock powders and their leachates via ion chromatography (F, Cl) and ICP-MS (Br, I). Halogen concentrations in our rock powders range between 107–727 μg/g F, 13–316 μg/g Cl, 2–323 ng/g Br, and 1–69 ng/g I. In contrast, leachates yielded 3–4 orders of magnitude less Cl and F, one order of magnitude less I, and similar amounts of Br compared to their corresponding bulk rocks. Preliminary data show no significant differences between volcanic and plutonic samples, suggesting that halogen concentrations in these rocks are insensitive to shallow fractionation. Although F and I exhibit no correlation with major element compositions, Cl and Br display negative trends with increasing SiO2 and K2O, and positive trends with increasing Fe2O3T, MnO, MgO, CaO, and TiO2, suggesting mafic minerals as important hosts of structurally bound halogens. Overall, Sierran plutonic rocks display low halogen contents (max. F, Cl = 727, 315 μg/g), consistent with biotite- and apatite-bearing granitoids reported in [1]. This work suggests that halogens do not preferentially enrich in shallow plutonic or volcanic portions of a continental arc system and that mafic mineral phases likely serve as primary reservoirs of these elements in intermediate-to-felsic igneous rocks. These hypotheses will be further investigated in future work through in-situ analysis of halogen concentrations in crystals. [1] Teiber, Marks, Wenzel, Siebel, Altherr & Markl (2014), Chemical Geology, vol. 374–375, pp. 92–109, doi: 10.1016/j.chemgeo.2014.03.006 

The chemical and isotopic characteristics of a solidified pluton represent the integration of magmatic and sub-solidus processes operating across a range of spatial and temporal scales during pluton construction, crystallization, and cooling. Disentangling these processes and understanding where chemical and isotopic signatures were acquired requires the combination of multiple tools tracing processes at different time and length scales. We combine whole-rock oxygen and Sr-Nd isotopes, zircon oxygen isotopes and trace elements, and mineral compositions with published high-precision U-Pb zircon geochronology to evaluate differentiation within the bimodal Guadalupe Igneous Complex, Sierra Nevada, California (USA). The complex was constructed in ~300 k.y. between 149 and 150 Ma. Felsic magmas crystallized as centimeter- to meter-sized segregations in gabbros in the lower part of the complex and as granites and granophyres structurally above the gabbros. A central mingling zone separates the mafic and felsic units. Pluton-wide δ18O(whole-rock), δ18O(zircon), and Sr-Nd isotopic ranges are too large to be explained by in situ, closed-system differentiation, instead requiring open-system behavior at all scales. Low δ18O(whole-rock) and δ18O(zircon) values indicate assimilation of hydrothermally altered marine host rocks during ascent and/or emplacement. In situ differentiation processes operated on a smaller scale (meters to tens of meters) for at least ~200 k.y. via (1) percolation and segregation of chemically and isotopically diverse silicic interstitial melt from a heterogeneous gabbro mush; (2) crystal accumulation; and (3) sub-solidus, high-temperature, hydrothermal alteration at the shallow roof of the complex to modify the chemical and isotopic characteristics. Whole-rock and mineral chemistry in combination with geochronology allows deciphering open-system differentiation processes at the outcrop to pluton scale from magmatic to sub-solidus temperatures over time scales of hundreds of thousands to millions of years.

 

The Laramide orogeny is a pivotal time in the geological development of western North America, but its driving mechanism is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with the Southern California Batholith (SCB) which caused the angle of subduction beneath the continent to shallow and led to shut-down of the arc. Here, we use over 280 zircon and titanite Pb/U ages from the SCB to establish the timing and duration of magmatism, metamorphism and deformation. We show that magmatism was surging in the SCB from 90 to 70 Ma, the lower crust was hot, and cooling occurred after 75 Ma. These data contradict plateau underthrusting and flat-slab subduction as the driving mechanism for early Laramide deformation. We propose that the Laramide orogeny is a two-stage event consisting of: 1) an arc ‘flare-up’ phase in the SCB from 90-75 Ma; and 2) a widespread mountain building phase in the Laramide foreland belt from 75-50 Ma that is linked to subduction of an oceanic plateau.

 

The halogens (F, Cl, Br, I) are cycled into the crust via subduction. The presence of F and Cl in arc settings impacts melt viscosity, igneous phase relations, and thermodynamic properties of magma in the pluton-to-volcano system, whereas the systematics of Br and I in melt systems are poorly understood. Mass balance constraints show that more halogens are subducted with the slab than are released during volcanism and passive degassing, suggesting that a halogen sink may exist in the lithosphere. Despite this, the halogen content of the upper continental crust of arc systems and distribution of halogens between plutonic and volcanic arc rocks are poorly quantified. This study presents whole rock halogen (F, Cl, Br, I) concentrations for 22 unaltered, geospatially- and temporally-related Cretaceous granitoid, hypabyssal plutonic, and volcanic rocks from the Sierra Nevada, California. This sampling approach allows direct comparison of plutonic and volcanic counterparts to make inferences about the pluton-volcano relationship. Because F behaves more incompatibly than Cl, Br, and I, late-stage fluid exsolution from melts may concentrate F in plutonic rocks and Cl, Br, and I in volcanic rocks. These whole rock halogen data provide a first-order approximation of the proportion of subducted halogens that are stored in the upper continental crust, and where along the magmatic plumbing path they are stored with important implications for their role in primary igneous processes such as pluton crystallization and volcanism. Ultimately, the results from this work will serve as the preliminary data for a larger study, provide insight into the magnitude of the roles the halogens play during primary igneous processes, and add to the limited halogen data on arc rocks. 

Cretaceous plutons in the Sierra Nevada provide a ~35 million year record of magma production in the Cordillera. Some periods of Sierran magmatism were exceptionally productive: the 95-85 Ma Sierra Crest event and the 101-98 Ma period in the axial Sierra Nevada batholith yielded some of the largest plutons; however, the transitions into and out of these intervals of building of big plutons and high net magma flux periods are not well-studied. To better understand the the transitions between high levels of magma flux in the Sierra arc, we have focused on a suite of small, granite to granodiorite plutons in the Kings River and Monarch Divide regions of Kings Canyon National Park. Notable among them are the Tehipite Dome, White Divide, Kennedy Lakes, Dougherty Peak, Cartridge Pass, Arrow, and Pyramid plutons. These plutons lie between the large middle Cretaceous plutonic suites of the axial Sierra Nevada (e.g. Mitchell Suite) and the Mount Whitney and John Muir suites of the Sierra Crest. New U-Pb zircon of ~97-92 million years old among the plutons confirms their transitional placement and era. Geochemically, the granite suites show distinct geochemical arrays from the coeval granodiorite plutons, suggesting that the two are not related by fractionation or degree of magma mixing. Radiogenic and stable isotopes also point to these plutons as distinct from each other. It follows that the mixed bulk chemistry and isotopic character of the Kings River and Monarch intrusions likely reflects a switchover in source during diminished magma flux. Thus, they appear to be recording heterogeneous source and plumbing systems in the transition that may be otherwise erased during more efficient, high-flux magmatism. 

The Sierra Nevada Batholith is a record of copious magmatism caused by subduction of the Farallon oceanic plate under the western margin of North America during much of the Mesozoic Era, between 256 and 80 Ma. The diversity of rocks produced during these sub-surface interactions depends on several variables, including fluid availability, melt source, and mantle partial melt emplacement geometry (Ducea et al., 2015). The analysis of zircon is particularly appealing because zircon is a robust mineral that endures periods weathering and erosion and commonly lingers as detrital crystals in the rock record. It thus has the potential to add value as a lens into global magmatism and planetary evolution given its use as a thermometer (Watson and Ferry, 2007), and measure of magma source composition (Davies et al. 2021). Several researchers suggest that zircon can be a useful tool for constraining depth of crystallization (Tang et al. 2020). Building on thesis work on the utility of europium anomalies in zircon to model depths and, by proxy, crustal thickness for batholithic granitoids, this project provides additional data and insight to understand spatially and temporally varied trends of the arc’s plutonic record. Magma emplacement occurs in pulses and typically exhibits an eastward younging trend during the Mesozoic (Chen and Moore, 1982). Chinen (2022) found that the arc’s Western Margin exhibits both younging and thickening trends towards the east. Recent research exposed the issues associated with traditional cerium anomaly calculation because of a reliance on lanthanum, a poorly analyzed element (Loader et al., 2022). We incorporate these new methods to calculate zircon metrics for our data; this project further constrains the precision of interpretations about geochemical trends using laboratory analysis and zircon because it draws on a large and prolific database of plutonic trace element geochemistry. Because multiple magmatic and environmental processes affect zircon crystallization compositions, we use broad suites of zircon (e.g. rare earth elements, oxygen isotopes) and whole rock (XRF, trace elements, isotopes, additional minerals) geochemical analyses to elucidate aspects of previous research (Brady and Lackey, 2022; Chinen, 2022) and to build upon noted trends of the plutonic Cordilleran record. 

The Sierra Nevada Batholith (SNB) records copious Mesozoic magmatism and is an important touchstone for understanding crustal growth at continental convergent margins. Recent research in the SNB has focused on defining magmatic cyclicity and arc “flare ups” based on the ages, magma production rates, and radiogenic isotope heterogeneities of the plutonic and volcanic rocks found throughout the batholith. Two main intervals at ca. 170–148 Ma and ca. 125–85 Ma delivered >95% of the magmas in the exposed plutonic bulk in the SNB and suggest elevated emplacement rates and hotter-than-usual magmas, though the Cretaceous is by far the most productive era and the most promising for understanding the factors modulating magmatic flux. The mid-Cretaceous of the Sierra (ca. 105–98 Ma) saw the appearance of conspicuous, high-silica (>65 wt.% SiO2; average ~71%) granitic plutons of similar chemical nature that span a large geographic area, breaking the well-established west-to-east “younging” trend found in the more common rocks of intermediate compositions. This study focuses on thirteen of these high-silica granites: the Bullfrog, Independence, McGann, Rawson Creek, and Spook Plutons of the eastern Sierra; and the Shaver Intrusive Suite, Grant Grove, Case Mountain, Coyote Pass, Dennison Peak, and Frys Point Plutons of the western/central Sierra. Whole rock geochemistry, zircon trace elements, and radiogenic isotope ratios (Sr and Nd) in these high-silica granites show some transitional patterns with other contemporaneous and geographically related plutons of intermediate compositions, suggesting fractionation trajectories; however, some distinct dissimilarities are observed, including: 1) elevated, but highly varied initial 87Sr/86Sr ratios, 2) elevated fluorine in granites, and 3) hotter apparent zircon saturation conditions. These geochemical data, hotter conditions, and higher flux suggest that mantle conditions favored more crustal melting and crustal source input than at any other time in the Cretaceous. We conclude that the granitic outburst of the mid-Cretaceous was a flare up like no other.