The Breccia of Frog Lakes: Record of Mafic Arc Magmatism in the Mesozoic Sierra Nevada, California

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The evolution of the Mesozoic western margin of North America in California is characterized by a change in tectonic regimes. After the emplacement of the Golconda thrust during the Sonoma orogeny in early Triassic time, the passive western margin changed to a convergent margin with subducting oceanic crust. Onset of arc magmatism is recorded by the volcanic section of Saddlebag Lake pendant in the east-central Sierra Nevada and includes welded tuffs, mafic flows, and volcanic breccias. The welded tuffs and mafic breccias provide insight into the diversity of volcanic processes during early evolution of the Sierran arc. The Mesozoic volcanic section of the Saddlebag Lake pendant (SLP) overlies foreland basin sediments derived from the eroding Golconda allochthon. The initial volcanic unit, the tuff of Black Mountain, is overlain by the conglomerate of Cooney Lake, which contains continental-derived sediment similar to the Candelaria Fm, and no volcanic clasts. Stratigraphically above the conglomerate is the 224 Ma tuff of Saddlebag Lake, which underlies the breccia of Frog Lakes. The breccia of Frog Lakes thus represents the earliest stratigraphic record of mafic volcanism in the Mesozoic Sierran arc. Basaltic to andesitic clasts found within the breccia of Frog Lakes are geochemically similar to modern arc-derived andesites, enriched in fluid-mobile LILEs, indicating that water had been introduced into the mantle wedge by the subducting plate and consequently depleted in less-mobile HFSEs, especially niobium. A subaqueous setting is indicated by the presence of a fine-grained, laminated sedimentary succession between the tuff of Saddlebag Lake and the breccia of Frog Lakes, together with jigsaw fragmentation of Frog Lakes breccia clasts, fluidal margins of some of these clasts, and localized fine-grained laminated sedimentary zones within clast-rich horizons. Although the arc setting remained subaqueous throughout deposition of at least the basal SLP Mesozoic section, the landscape became dominated by volcanism that continued at least into Early Jurassic time.

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... The notable exception is in the east-central Sierra Nevada, where Lahren (1993, 1999) documented a caldera complex at Tioga Pass, Cali-fornia, and where dates of 232-218 Ma were obtained on volcanic rocks interpreted to be outfl ow from that caldera complex. Depositional features of these vol canic rocks, including angular fragmentation and fi ne ash-rich laminations, suggest that they were erupted subaqueously (Douglas et al., 2011). In the Mineral King pendant, a tuff dated as 220 Ma (N. ...
... By Late Triassic time, a continental magmatic arc, which may previously have been offshore and thus unavailable to contribute detritus to the continental interior, constituted a subaerial series of volcanoes along the western coast from the central Mojave Desert in California to Sonora, Mexico. In Figure 11, the shoreline is inferred to cross the arc in the southern Sierra Nevada based on marine facies in volcanic and sedimentary rocks in the central and northern Sierra Nevada (Busby-Spera, 1984Douglas et al., 2011). ...
Upper Triassic sandstones in diverse locations in eastern California, southern Arizona, and on the Colorado Plateau (USA) yield detrital zircons that are remarkably similar in age and geochemistry, leading to the hypothesis that they are temporally related and were derived from similar sources. Volcaniclastic sandstone from the lowest Vampire Formation in eastern California, the Sonsela Member of the Chinle Formation at Petrified Forest National Park, northeastern Arizona, and the herein-named Waterman formation in southern Arizona yield zircons that range in age from ca. 205 to ca. 235 Ma. Together with the similar range of ages, these zircons uniformly have Th/U ratios between similar to 0.2 and 2. In addition, the Waterman formation contains zircon grains with an age range from ca. 225 to 250 Ma, but with markedly lower Th/U ratios of 0.1-0.2, and a distinctively older group with ages to ca. 280 Ma. In a general sense, variations in Hf concentrations and Yb/Gd ratios support the discrimination of grains based on age and Th/U. We use age and geochemical data from the zircons to infer that these units capture a slice of time during development of the early Mesozoic Cordilleran magmatic arc along western North America. Plutonic rocks that record magmatism in the arc are Permian-Triassic in age, and match zircon ages in the detrital grains, thus providing a view of which parts of the arc were actively eroding into the stream systems that deposited the three units. Streams diverged from a common source that maintained a relatively uniform magma composition over time, as indicated by a narrow range of Th/U values, as well as tapping a somewhat different source evidenced by a grouping in which Th/U ratios are lower. Once the streams left the highlands of the arc and the depocenter of the lowest Vampire Formation, they diverged, such that one flowed to the area of the Colorado Plateau while the second flowed toward southern Arizona. At the same time, a stream system originating in the older, Sonoran part of the arc flowed from the south into southern Arizona.
Marine volcaniclastic rocks in the Sierra Nevada preserve a critical record of silicic magmatism in the early Sierra Nevada volcanic arc, and this magmatic record provides precise minimum age constraints on subduction inception and tectonic evolution of the early Mesozoic Cordilleran convergent margin at this latitude. New zircon Pb/U ages from the Ritter Range pendant and regional correlations indicate arc inception no later than mid-Triassic time between 37°N and 38°N. The regional first-order felsic magma eruption rate as recorded by marine volcanic arc rocks was episodic, with distinct pulses of ignimbrite emplacement at ca. 221–216 and 174–167 Ma. Ignimbrites range from dacite to rhyolite in bulk composition, and are petrographically similar to modern arc-type, monotonous intermediate dacite or phenocryst-poor, low-silica rhyolite. Zircon trace element geochemistry indicates that Jurassic silicic melts were consistently Ti-enriched and light rare earth-enriched and U-depleted in comparison to Triassic melts of the juvenile arc, suggesting Jurassic silicic melts were hotter, drier, and derived from distinct lithospheric sources not tapped in the juvenile stage of arc construction. Pulses of ignimbrite deposition were coeval with granodioritic to granitic components of the underlying early Mesozoic Sierra Nevada batholith, suggesting explosive silicic volcanism and batholith construction were closely coupled at 1–2 million-year time scales.
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