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Abstract Plutons offer an opportunity to study the extended history of magmas at depth. Fully exploiting this record requires the ability to track changes in magmatic plumbing systems as magma intrudes, crystallizes, and/or mixes through time. This task has been difficult in granitoid plutons because of low sampling density, poorly preserved or cryptic intrusive relationships, and the difficulty of identifying plutonic volumes that record the contemporaneous presence of melt. In particular, the difficulty in delineating fossil magma reservoirs has limited our ability to directly test whether or not high-SiO2 rhyolite is the result of crystal-melt segregation. We present new high-precision U-Pb zircon geochronologic and geochemical data that characterize the Miocene Searchlight pluton in southern Nevada, USA. The data indicate that the pluton was built incrementally over ~1.5 m.y. with some volumes of magma completely crystallizing before subsequent volumes arrived. The largest increment is an ~2.7-km-thick granitic sill that records contemporaneous zircon crystallization, which we interpret to represent a fossil silicic magma reservoir within the greater Searchlight pluton. Whole-rock geochemical data demonstrate that this unit is stratified relative to paleo-vertical, consistent with gravitationally driven separation of high-SiO2 melt from early-formed crystals at moderate crystallinity. Zircon trace-element compositions suggest that our geochronologic data from this unit record most of the relevant crystallization interval for differentiation and that this process occurred in <150 k.y.
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Trace elements in zircon and Mesozoic history of Sierra Nevada plutonism: fractionation, thickening, and oxidation
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.
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- Award ID(s):
- 2105370
- PAR ID:
- 10514003
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Abstracts with Programs
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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The Wasatch Mountains expose an oblique profile through the Alta and Little Cottonwood stocks (LCS) owing to 20° eastward tilt in the footwall of the Wasatch Fault. The cross section spans the upper 11 km of the crust beneath the Eocene paleosurface exposed in Park City, UT. Previous titanite and zircon U-Pb petrochronology established 10 Myr of simultaneous magmatism and hydrothermal metamorphism both in the deeper LCS and in the shallower Alta stock which likely was the conduit between the LCS and cogenetic Keetley volcanic deposits. Hydrothermal metamorphism within and surrounding the Alta stock was synchronous with and most likely driven by emplacement of LCS and migrated from within the Alta stock and contact aureole to margins of the stock suggesting an evolving permeability structure during and after the crystallization of the LCS. New titanite U-Pb petrochronology from the LCS and stock-bounding Wasatch Fault Zone indicate that 1) the LCS was constructed in two phases, an earlier ~36–34 Ma and a younger ~32–25 Ma phase, 2) the presence of both magmatic and hydrothermal titanite as recorded by trace element chemistry, and 3) a pre-Wasatch Fault ductile shear zone likely accommodated magma emplacement at crustal strain rates beginning around 32 Ma. Principal component analysis of LCS trace element data distinguishes two end-member titanite populations along the first component axis: a magmatic population with high REE and a metamorphic population with low REE and high Sr, Sc, V, Cr, Fe, Al, Pb, and particularly W. The second principal component is defined by variance in the REE interpreted to record fractionation by titanite crystallization from melt. The initial ~36–34 Ma phase of LCS construction overlaps with magmatism within the Alta stock conduit and Keetley volcanic rocks and is only found on the western, deepest portion of the LCS. Trace element chemistry of ~36–34 Ma titanites lacks the low REE, high W population suggesting that hydrothermal water released by crystallizing magma did not percolate through these rocks. Low REE, high W titanites are restricted to the structurally higher second phase of the LCS. Despite this relationship, not all samples in the second LCS phase contain the hydrothermal population, which suggests spatially complex magma emplacement and/or later hydrothermal permeability structure.more » « less
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null (Ed.)Abstract The spatial and temporal distribution of arc magmatism and associated isotopic variations provide insights into the Phanerozoic history of the western margin of South America during major shifts in Andean and pre-Andean plate interactions. We integrated detrital zircon U-Th-Pb and Hf isotopic results across continental magmatic arc systems of Chile and western Argentina (28°S–33°S) with igneous bedrock geochronologic and zircon Hf isotope results to define isotopic signatures linked to changes in continental margin processes. Key tectonic phases included: Paleozoic terrane accretion and Carboniferous subduction initiation during Gondwanide orogenesis, Permian–Triassic extensional collapse, Jurassic–Paleogene continental arc magmatism, and Neogene flat slab subduction during Andean shortening. The ~550 m.y. record of magmatic activity records spatial trends in magma composition associated with terrane boundaries. East of 69°W, radiogenic isotopic signatures indicate reworked continental lithosphere with enriched (evolved) εHf values and low (<0.65) zircon Th/U ratios during phases of early Paleozoic and Miocene shortening and lithospheric thickening. In contrast, the magmatic record west of 69°W displays depleted (juvenile) εHf values and high (>0.7) zircon Th/U values consistent with increased asthenospheric contributions during lithospheric thinning. Spatial constraints on Mesozoic to Cenozoic arc width provide a rough approximation of relative subduction angle, such that an increase in arc width reflects shallower slab dip. Comparisons among slab dip calculations with time-averaged εHf and Th/U zircon results exhibit a clear trend of decreasing (enriched) magma compositions with increasing arc width and decreasing slab dip. Collectively, these data sets demonstrate the influence of subduction angle on the position of upper-plate magmatism (including inboard arc advance and outboard arc retreat), changes in isotopic signatures, and overall composition of crustal and mantle material along the western edge of South America.more » « less
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Nearly two decades since the first oxygen isotope (δ18O) studies of zircon in the Sierra Nevada Batholith, California, USA, a far more extensive picture of spatial and temporal patterns of magmatic δ18O has emerged in parallel with a tenfold increase in geochronologic coverage, and many new radiogenic isotope (Sr, Nd, Hf) analyses. Over this time, models of Cordilleran-type arc systems have sought to elucidate flare-ups of magmatism as cyclic, with radiogenic isotope “excursions” tracing variable input of crust and mantle into arc magmas [e.g., 1]. Such models haven't incorporated oxygen isotopes to full advantage because of apparent complexity in the signals they record [2]. New, single zircon δ18O analyses—of plutonic, volcanic, and detrital zircon—from the Sierra amplifiing the findings of previous studies [e.g., 3], that δ18O records are well-suited for detecting relatively fast (<10 million year) recycling of subducted supracrustal rock and accreted terranes in forearc settings. Such recycling is not resolved by radiogenic isotope systems. A wealth of new volcanic δ18O zircon data from the Sierra, along with δ18O of hydrothermal minerals like skarn garnet, also records periods of significant δ18O “pull-downs” as lower-δ18O hydrothermal waters alter surface rocks whose assimilation subsequently embeds these surface signals in silicic volcanic systems. Such re-melting and volcanic episodes are often brief (< 5 million years) and small volume, so have often been overlooked, however such, δ18O values may be key to detecting plutonic from volcanic zircon in detrital records when used in conjunction with trace elements. Low-δ18O domains are becoming recognized in other arcs and to be useful to detect episodic resampling of crustal domains [4]. Morover, discovery of fossil low-δ18O systems in screens of wallrock in mid-crustal levels [e.g., 5] documents wholesale rapid burial of these domains in arcs, during transitions to episodes of shortening or transpression. All together, zircon δ18O uniquely traces surface- to-source transport and recycling in Cordilleran arcs as it relates to changing arc stress regime, at periods that may fail to be recorded in excursions of radiogenic isotopes, such as relaxation of stress regimes in upper plate domains. [1] DeCelles, P. G. et al. Nature Geoscience 2, 251-257 (2009) ; [2] Chapman, J. B. et al. Lithos 398- 299, (2021); [3] Lackey, J. S., et. al. J. Petrology 49, 1397–1426 (2008); [4] Turnbull, R. E. et al. Gondwana Res. 121, 436-471; [5] Ryan-Davis, J. et al. Contributions to Min. and Pet 174, 19 (2019)more » « less
