An official website of the United States government
Abstract The southern Coast Mountain batholith was episodically active from Jurassic to Eocene time and experienced four distinct high magmatic flux events during that period. Similar episodicity has been recognized in arcs worldwide, yet the mechanism(s) driving such punctuated magmatic behavior are debated. This study uses zircon Hf and O isotopes, with whole-rock and mineral geochemistry, to track spatiotemporal changes in southern Coast Mountains batholith melt sources and to evaluate models of flare-up behavior and crust formation in Cordilleran arc systems. Zircon Hf isotope analysis yielded consistently primitive values, with all zircon grains recording initial εHf between +6 and +16. The majority (97%) of zircons analyzed yielded δ18O values between 4.2‰ and 6.5‰, and only five grains recorded values of up to 8.3‰. These isotopic results are interpreted to reflect magmatism dominated by mantle melting during all time periods and across all areas of the southern batholith, which argues against the periodic input of more melt-fertile crustal materials as the driver of episodic arc magmatism. They also indicate that limited crustal recycling is needed to produce the large volumes of continental crust generated in the batholith. Although the isotopic character of intrusions is relatively invariant through time, magmas emplaced during flare-ups record higher Sr/Y and La/Yb(N) and lower zircon Ti and Yb concentrations, which is consistent with melting in thickened crust with garnet present as a fractionating phase. Flare-ups are also temporally associated with periods when the southern Coast Mountains batholith both widens and advances inboard. We suggest that the landward shift of the arc into more fertile lithospheric mantle domains triggers voluminous magmatism and is accompanied by magmatic and/or tectonic thickening. Overall, these results demonstrate that the magmatic growth of Cordilleran arcs can be spatially and temporally complex without requiring variability in the contributions of crust and/or mantle to the batholith.
more »
« less
Understanding Oxygen Isotopes in Cordilleran Batholiths: A 190 Million Year, Top-to-Bottom Perspective from the Sierra Nevada
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
- PAR ID:
- 10514015
- Publisher / Repository:
- 10th Hutton Symposium on Granites and Related Rocks
- Date Published:
- Journal Name:
- 10th Hutton Symposium on Granites and Related Rocks
- Format(s):
- Medium: X
- Location:
- Baveno, Italy
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
We present geochemical data from gas samples from ~1200 km of arc in the Central Volcanic Zone of the Andes (CVZA), the volcanic arc with the thickest (~70 km) continental crust globally. The primary goals of this study are to characterize and understand how magmatic gases interact with hydrothermal systems, assess the origins of the major gas species, and constrain gas emission rates. To this end, we use gas chemistry, isotope compositions of H, O, He, C, and S, and SO2 fluxes from the CVZA. Gas and isotope ratios (CO2/ST, CO2/CH4, H2O/ST, δ13C, δ34S, 3He/4He) vary dramatically as magmatic gases are progressively affected by hydrothermal processes, reflecting removal and crustal sequestration of reactive species (e.g., S) and addition of less reactive meteoric and crustal components (e.g., He). The observed variations are similar in magnitude to those expected during the magmatic reactivation of volcanoes with hydrothermal systems. Carbon and sulfur isotope compositions of the highest temperature emissions (97–408 ◦C) are typical of arc magmatic gases. Helium isotope compositions reach values similar to upper mantle in some volcanic gases indicating that transcustal magma systems are effective conduits for volatiles, even through very thick continental crust. However, He isotopes are highly sensitive to even low degrees of hydrothermal interaction and radiogenic overprinting. Previous work has significantly underestimated volatile fluxes from the CVZA; however, emission rates from this study also appear to be lower than typical arcs, which may be related to crustal thickness.more » « less
-
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
-
The Mineral King pendant in the Sierra Nevada batholith (California, USA) contains at least four rhyolite units that record high-silica volcanism during magmatic lulls in the Sierran magmatic arc. U-Th-Pb, trace element (single crystal spot analyses via sensitive high-resolution ion microprobe–reverse geometry, SHRIMP-RG), and bulk oxygen isotope analyses of zircon from these units provide a record of the age and compositional properties of the magmas that is not available from whole-rock analysis because of intense hydrothermal alteration of the pendant. U-Pb spot ages reveal that the Mineral King rhyolites are from two periods, the Early Jurassic (197 Ma) and the Early Cretaceous (134–136 Ma). These two rhyolite packages have zircons with distinct compositional trends for trace elements and δ18O; the Early Jurassic rhyolite shows less evidence of crustal influences on the rhyolites and the Early Cretaceous rhyolite shows evidence of increasing crustal influences and crystal recycling. These rhyolites capture evidence of magmatism during two periods of low magmatic flux in the Sierran Arc; however, they still show that magmas were derived from interactions of maturing continental crust, increasing from the Early to Late Jurassic. This finding likely reflects the transition of the North America margin from one of docking island arcs in the Early Jurassic to one of a more mature continental arc in the Early Cretaceous. This also shows the utility in examining zircon spot ages combined with trace element and bulk isotopic composition to unlock the petrogenetic history of altered volcanic rocks.more » « less
-
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.more » « less
