Axel K. Schmitt

The Geysers - Cobb Mountain magma system, California (Part 1): U-Pb zircon ages of volcanic rocks, conditions of zircon crystallization and magma residence times

Abstract Combined U-Pb zircon and 40 Ar/ 39 Ar sanidine data from volcanic rocks within or adjacent to the Geysers geothermal reservoir constrain the timing of episodic eruption events and the pre-eruptive magma history. Zircon U-Pb concordia intercept model ages (corrected for initial 230 Th disequilibrium) decrease as predicted from stratigraphic and regional geological relationships (1σ analytical error): 2.47 ± 0.04 Ma (rhyolite of Pine Mountain), 1.38 ± 0.01 Ma (rhyolite of Alder Creek), 1.33 ± 0.04 Ma (rhyodacite of Cobb Mountain), 1.27 ± 0.03 Ma (dacite of Cobb Valley), and 0.94 ± 0.01 Ma (dacite of Tyler Valley). A significant (∼0.2–0.3 Ma) difference between these ages and sanidine 40 Ar/ 39 Ar ages measured for the same samples demonstrates that zircon crystallized well before eruption. Zircons U-Pb ages from the underlying main-phase Geysers Plutonic Complex (GPC) are indistinguishable from those of the Cobb Mountain volcanics. While this is in line with compositional evidence that the GPC fed the Cobb Mountain eruptions, the volcanic units conspicuously lack older (∼1.8 Ma) zircons from the shallowest part of the GPC. Discontinuous zircon age populations and compositional relationships in the volcanic and plutonic samples are incompatible with zircon residing in a single long-lived upper crustal magma chamber. Instead we favor a model in which zircons were recycled by remelting of just-solidified rocks during episodic injection of more mafic magmas. This is consistent with thermochronologic evidence that the GPC cooled below 350° C at the time the Cobb Mountain volcanics were erupted.

Jurassic accretionary complex and ophiolite from northeast Turkey: No evidence for the Cimmerian continental ribbon

Permian-Triassic and Late Cretaceous accretionary complexes, ascribed to the consumption of two distinct oceans, the Paleo- and Neo-Tethys, are exposed over extensive areas in the Eastern Mediterranean region. However, a separating continental ribbon, the so-called Cimmeride continent, between the Paleo- and Neo-Tethys during early Mesozoic time cannot be defined. Here we report a previously unknown Early Jurassic metamorphic oceanic accretionary complex and ophiolite from northeast Turkey, bounded by oceanic accretionary complexes of Permian-Triassic and Late Cretaceous age to the north and the south, respectively, without a continental domain in between. This special tectonic position and widespread coexistence of Permian-Triassic and Late Cretaceous accretionary complexes alongside the Izmir-Ankara-Erzincan suture imply that (1) the southern margin of Laurasia in the eastern Mediterranean region grew by episodic accretionary processes from late Paleozoic to end-Mesozoic time without involvement of a Cimmerian continental ribbon, and (2) the Paleo-Tethys and northern branch of the Neo-Tethys were not distinct oceans in the Eastern Mediterranean region.

Increased sediment accumulation rates and climatic forcing in the central Andes during the late Miocene

New zircon U-Pb ages from six volcanic ashes in the Subandean foothills provide for the first time well-constrained depositional ages of the late Cenozoic strata in this region. The radiometric ages indicate 12.4 ± 0.5 Ma, 7.93 ± 0.26 Ma, and 5.94 ± 0.20 Ma depositional ages for the bases of Yecua, Tariquia, and Guandacay Formations, respectively. We used these zircon ages to determine the accumulation rates of the late Cenozoic foreland sediments in the Subandes. Our results show a fourfold increase (from 130 to 628 m/m.y.) in sediment accumulation rates between ca. 7.9 and 6 Ma during the deposition of Andean-derived coarsening- and thickening-upward sandstone-dominated Tariquia strata. This increased accumulation rate correlates well with monsoon intensification and climate variability in South America, which was accompanied by the development of fluvial megafan paleodrainage networks in the central Andes. This correlation suggests that climate may have been an important factor between the late Miocene and Pliocene.

Voluminous low δ18O magmas in the late Miocene Heise volcanic field, Idaho: Implications for the fate of Yellowstone hotspot calderas

We report oxygen isotope compositions of phenocrysts and U-Pb ages of zircons in four large caldera-forming ignimbrites and post-caldera lavas of the Heise volcanic field, a nested caldera complex in the Snake River Plain, that preceded volcanism in Yellowstone. Early eruption of three normal δ 18 O voluminous ignimbrites with δ 18 O quartz = 6.4‰ and δ 18 O zircon = 4.8‰ started at Heise at 6.6 Ma, and was followed by a 2‰–3‰ δ 18 O depletion in the subsequent 4.45 Ma Kilgore caldera cycle that includes the 1800 km 3 Kilgore ignimbrite, and post-Kilgore intracaldera lavas with δ 18 O quartz = 4.3‰ and δ 18 O zircon = 1.5‰. The Kilgore ignimbrite represents the largest known low-δ 18 O magma in the Snake River Plain and worldwide. The post-Kilgore low δ 18 O volcanism likely represents the waning stages of silicic magmatism at Heise, prior to the reinitiation of normal δ 18 O silicic volcanism 100 km to the northeast at Yellowstone. The occurrence of low δ 18 O magmas at Heise and Yellowstone hallmarks a mature stage of individual volcanic cycles in each caldera complex. Sudden shifts in δ 18 O of silicic magmas erupted from the same nested caldera complexes argue against any inheritance of the low δ 18 O signature from mantle or crustal sources. Instead, δ 18 O age trends indicate progressive remelting of low δ 18 O hydrothermally altered intracaldera rocks of previous eruptions. This trend may be generally applicable to older caldera complexes in the Snake River Plain that are poorly exposed.

Extrusion vs. duplexing models of Himalayan mountain building 3: duplexing dominates from the Oligocene to Present

The Himalaya is a natural laboratory for studying mountain-building processes. Concepts of extrusion and duplexing have been proposed to dominate most phases of Himalayan evolution. Here, we examine the importance of these mechanisms for the evolution of the Himalayan crystalline core via an integrated investigation across the northern Kathmandu Nappe. Results reveal that a primarily top-to-the-north shear zone, the Galchi shear zone, occurs structurally above and intersects at depth with the Main Central thrust (MCT) along the northern flank of the synformal Kathmandu Nappe. Quartz c-axis fabrics confirm top-to-the-north shearing in the Galchi shear zone and yield a right-way-up deformation temperature field gradient. U-Pb zircon dating of pre-to-syn- and post-kinematic leucogranites demonstrates that the Galchi shear zone was active between 23.1 and 18.8 Ma and ceased activity before 18.8–13.8 Ma. The Galchi shear zone is correlated to the South Tibet detachment (STD) via consistent structural fabrics, lithologies, metamorphism, and timing for four transects across the northern margin of the Kathmandu Nappe. These findings are synthesized with literature results to demonstrate (1) the broad horizontality of the STD during motion and (2) the presence of the MCT-STD branch line along the Himalayan arc. The branch line indicates that the crystalline core was emplaced at depth via tectonic wedging and/or channel tunnelling-type deformation. We proceed to consider implications for the internal development of the crystalline core, particularly in the light of discovered tectonic discontinuities therein. We demonstrate the possibility that the entire crystalline core may have been developed via duplexing without significant channel tunnelling, thereby providing a new end-member model. This concept is represented in a reconstruction showing Himalayan mountain-building via duplexing from the Oligocene to Present.

U-Pb zircon chronostratigraphy of early-Pliocene ignimbrites from La Pacana, north Chile: implications for the formation of stratified magma chambers

Abstract High spatial resolution U–Pb dates of zircons from two consanguineous ignimbrites of contrasting composition, the high-silica rhyolitic Toconao and the overlying dacitic Atana ignimbrites, erupted from La Pacana caldera, north Chile, are presented in this study. Zircons from Atana and Toconao pumice clasts yield apparent 238 U/ 206 Pb ages of 4.11±0.20 Ma and 4.65±0.13 Ma (2σ), respectively. These data combined with previously published geochemical and stratigraphic data, reveal that the two ignimbrites were erupted from a stratified magma chamber. The Atana zircon U–Pb ages closely agree with the eruption age of Atana previously determined by K–Ar dating (∼4.0±0.1 Ma) and do not support long (>1 Ma) residence times. Xenocrystic zircons were found only in the Toconao bulk ignimbrite, which were probably entrained during eruption and transport. Apparent 238 U/ 206 Pb zircon ages of ∼13 Ma in these xenocrysts provide the first evidence that the onset of felsic magmatism within the Altiplano–Puna ignimbrite province occurred approximately 3 Myr earlier than previously documented.

Linking rapid magma reservoir assembly and eruption trigger mechanisms at evolved Yellowstone-type supervolcanoes

The geological record contains evidence of volcanic eruptions that were as much as two orders of magnitude larger than the most voluminous eruption experienced by modern civilizations, the A.D. 1815 Tambora (Indonesia) eruption. Perhaps nowhere on Earth are deposits of such supereruptions more prominent than in the Snake River Plain–Yellowstone Plateau (SRP-YP) volcanic province (northwest United States). While magmatic activity at Yellowstone is still ongoing, the Heise volcanic field in eastern Idaho represents the youngest complete caldera cycle in the SRP-YP, and thus is particularly instructive for current and future volcanic activity at Yellowstone. The Heise caldera cycle culminated 4.5 Ma ago in the eruption of the ∼1800 km3 Kilgore Tuff. Accessory zircons in the Kilgore Tuff display significant intercrystalline and intracrystalline oxygen isotopic heterogeneity, and the vast majority are 18O depleted. This suggests that zircons crystallized from isotopically distinct magma batches that were generated by remelting of subcaldera silicic rocks previously altered by low-δ18O meteoric-hydrothermal fluids. Prior to eruption these magma batches were assembled and homogenized into a single voluminous reservoir. U-Pb geochronology of isotopically diverse zircons using chemical abrasion–isotope dilution–thermal ionization mass spectrometry yielded indistinguishable crystallization ages with a weighted mean 206Pb/238U date of 4.4876 ± 0.0023 Ma (MSWD = 1.5; n = 24). These zircon crystallization ages are also indistinguishable from the sanidine 40Ar/39Ar dates, and thus zircons crystallized close to eruption. This requires that shallow crustal melting, assembly of isolated batches into a supervolcanic magma reservoir, homogenization, and eruption occurred extremely rapidly, within the resolution of our geochronology (103–104 yr). The crystal-scale image of the reservoir configuration, with several isolated magma batches, is very similar to the reservoir configurations inferred from seismic data at active supervolcanoes. The connection of magma batches vertically distributed over several kilometers in the upper crust would cause a substantial increase of buoyancy overpressure, providing an eruption trigger mechanism that is the direct consequence of the reservoir assembly process.

Solving the Martian meteorite age conundrum using micro-baddeleyite and launch-generated zircon

Invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites. However, the interpreted ages of the ejected igneous crust of Mars differ by up to four billion years, a conundrum due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards Earth. Here we solve the conundrum by combining in situ electron-beam nanostructural analyses and U–Pb (uranium–lead) isotopic measurements of the resistant micromineral baddeleyite (ZrO2) and host igneous minerals in the highly shock-metamorphosed shergottite Northwest Africa 5298 (ref. 8), which is a basaltic Martian meteorite. We establish that the micro-baddeleyite grains pre-date the launch event because they are shocked, cogenetic with host igneous minerals, and preserve primary igneous growth zoning. The grains least affected by shock disturbance, and which are rich in radiogenic Pb, date the basalt crystallization near the Martian surface to 187 ± 33 million years before present. Primitive, non-radiogenic Pb isotope compositions of the host minerals, common to most shergottites, do not help us to date the meteorite, instead indicating a magma source region that was fractionated more than four billion years ago to form a persistent reservoir so far unique to Mars. Local impact melting during ejection from Mars less than 22 ± 2 million years ago caused the growth of unshocked, launch-generated zircon and the partial disturbance of baddeleyite dates. We can thus confirm the presence of ancient, non-convecting mantle beneath young volcanic Mars, place an upper bound on the interplanetary travel time of the ejected Martian crust, and validate a new approach to the geochronology of the inner Solar System.

Recovering tectonic events from the sedimentary record: Detrital monazite plays in high fidelity

Measurement of detrital zircon U-Pb ages has become the method of choice for single crystal–based investigations of provenance for both modern and ancient sediments. Recent studies, however, demonstrated the failure of zircon to record major tectonic events in source terranes, revealing the need for a more robust provenance methodology. A direct comparison between the utility of crystallization ages of detrital zircon and monazite as provenance indicators has been made using modern river alluvium derived from known sources. While detrital zircon does not fully record the multiple collisional phases that are the hallmark of the Appalachian orogen, detrital monazite accurately records all the major tectonic events. The physical and petrogenetic differences between zircon and monazite are the primary factors for differing detrital age spectra. Zircon, owing to its extreme refractory nature, skews detrital age spectra toward older ages and limits its ability to record low-grade thermotectonic events in orogens. Monazite recrystallizes over a broader range of metamorphic conditions than does zircon. Consequently, monazite has the potential to record metamorphic events that might otherwise be absent from the detrital zircon record, thus providing a more accurate record of source terranes in regions characterized by moderate thermal events.

Dynamics of deformation and sedimentation in the northern Sierras Pampeanas: An integrated study of the Neogene Fiambalá basin, NW Argentina

The thick-skinned Sierras Pampeanas morphotectonic domain of western and northwestern Argentina (27°S–33°S) is characterized by reverse-fault–bounded basement blocks that delimit internally deformed, Neogene sedimentary basins. Foreland-basin evolution in this part of the Andes is still not very well understood. For example, challenging questions exist as to how thick-skinned deformation develops, if there are distinct spatiotemporal trends in deformation and exhumation, how such deformation styles influence sedimentation patterns, and whether or not broken foreland basins are related to regional plate-tectonic processes, such as flat-slab subduction. The Fiambala basin of the northwestern Sierras Pampeanas is the largest of several intermontane basins in the transition to the southern margin of the Puna Plateau. This basin preserves a thick continental Neogene sequence that provides information on the dynamics of thick-skinned deformation and resulting sedimentation. The Fiambala basin contains ~4 km of fluvial-alluvial sedimentary rocks that comprise the Tamberia, Guanchin, and Punaschotter Formations. U-Pb geochronology of ashes intercalated within the Fiambala stratigraphic sequence demonstrates that these sedimentary rocks are late Miocene to Pliocene (8.2 ± 0.3 Ma to 3.05 ± 0.4 Ma) in age. Sedimentology and provenance data indicate that the source of the Tamberia Formation was located to the west of the modern western basin-bounding range. The Guanchin and Punaschotter Formations record input from local sources, including the modern basin-bounding range to the west and the southern Puna Plateau to the north, suggesting reorganization of the catchment area at ca. 5.5 Ma. The coarsening-upward trends recorded by the fluvial Tamberia and Guanchin Formations indicate enhanced tectonics and relief during sedimentation. The Punaschot-ter conglomerates record alluvial-fan sedimentation and local sources. Fault kinematic data document a contractional regime, characterized by E-W and NE-SW shortening, active throughout the middle-late Miocene and Pliocene. Furthermore, a comparison between the Fiambala basin and similar sedimentary basins in the Sierras Pampeanas (e.g., Bermejo foreland basin) and the Eastern Cordillera leads us to propose that the study area originally constituted an integral part of a continuous and more extensive foreland-basin system (thin-skinned) for much of its early history. Our data suggest coeval intrabasin deformation along strike from the Bermejo region northward to the Eastern Cordillera. The coeval change at ca. 6 Ma from a regional to more compartmentalized (thick-skinned) tectono-sedimentary environment in the regions adjacent to the Eastern Cordillera, the southern Puna margin, and other sectors within the Sierras Pampeanas domain may thus reflect a regional tectonic process related to flat subduction. Our data, combined with existing sedimentological and petrological evidence, imply that the passage from steep to flat subduction occurred synchronously from ~30°S to ~26°S.