The Sierra de Pie de Palo of northwest Argentina preserves middle to lower crustal metamorphic rocks that were penetratively deformed during Ordovician accretion of the Precordillera terrane to the Gondwana margin. New structural, petrologic, and geochronologic data from a 40 km structural transect reveals that the Sierra de Pie de Palo preserves a middle to lower crustal ductile thrust complex consisting of individual structural units and not an intact ophiolite and cover sequence. Top‐to‐the‐west thrusting occurred intermittently on discrete ductile shear zones from ∼515 to ∼417 Ma and generally propagated toward the foreland with progressive deformation. Ordovician crustal shortening and peak metamorphic temperatures in the central portion of the Sierra de Pie de Palo were synchronous with retro‐arc shortening and magmatic flare‐up within the Famatina arc. Accretion of the Precordillera terrane resulted in the end of arc flare‐up and the onset of synconvergent extension by ∼439 Ma. Continued synextensional to postextensional convergence was accommodated along progressively lower grade shear zones following terrane accretion and the establishment of a new plate margin west of the Precordillera terrane. The results support models of Cordilleran orogens that link voluminous arc magmatism to periods of regional shortening. The deformation, metamorphic, and magmatic history within the Sierra de Pie de Palo is consistent with models placing the region adjacent to the Famatina margin in the middle Cambrian and not as basement to the Precordillera terrane.

Pozzolanic reaction of volcanic ash with hydrated lime is thought to dominate the cementing fabric\ud and durability of 2000-year-old Roman harbor concrete. Pliny the Elder, however, in first century CE\ud emphasized rock-like cementitious processes involving volcanic ash (pulvis) “that as soon as it comes\ud into contact with the waves of the sea and is submerged becomes a single stone mass (fierem unum\ud lapidem), impregnable to the waves and every day stronger” (Naturalis Historia 35.166). Pozzolanic\ud crystallization of Al-tobermorite, a rare, hydrothermal, calcium-silicate-hydrate mineral with cation\ud exchange capabilities, has been previously recognized in relict lime clasts of the concrete. Synchrotron-based\ud X-ray microdiffraction maps of cementitious microstructures in Baianus Sinus and Portus\ud Neronis submarine breakwaters and a Portus Cosanus subaerial pier now reveal that Al-tobermorite\ud also occurs in the leached perimeters of feldspar fragments, zeolitized pumice vesicles, and in situ\ud phillipsite fabrics in relict pores. Production of alkaline pore fluids through dissolution-precipitation,\ud cation-exchange and/or carbonation reactions with Campi Flegrei ash components, similar to processes\ud in altered trachytic and basaltic tuffs, created multiple pathways to post-pozzolanic phillipsite and\ud Al-tobermorite crystallization at ambient seawater and surface temperatures. Long-term chemical\ud resilience of the concrete evidently relied on water-rock interactions, as Pliny the Elder inferred. Raman\ud spectroscopic analyses of Baianus Sinus Al-tobermorite in diverse microstructural environments\ud indicate a cross-linked structure with Al3+ substitution for Si4+ in Q3\ud tetrahedral sites, and suggest\ud coupled [Al3++Na+\ud ] substitution and potential for cation exchange. The mineral fabrics provide a geoarchaeological\ud prototype for developing cementitious processes through low-temperature rock-fluid\ud interactions, subsequent to an initial phase of reaction with lime that defines the activity of natural\ud pozzolans. These processes have relevance to carbonation reactions in storage reservoirs for CO2 in\ud pyroclastic rocks, production of alkali-activated mineral cements in maritime concretes, and regenerative\ud cementitious resilience in waste encapsulations using natural volcanic pozzolans

Ancient Roman syntheses of Al-tobermorite in a 2000-year-old concrete block submerged in the Bay of Pozzuoli (Baianus Sinus), near Naples, have unique aluminum-rich and silica-poor compositions relative to hydrothermal geological occurrences. In relict lime clasts, the crystals have calcium contents that are similar to ideal tobermorite, 33 to 35 wt%, but the low-silica contents, 39 to 40 wt%, reflect Al 3+ substitution for Si 4+ in Q 2 (1Al), Q 3 (1Al), and Q 3 (2 Al) tetrahedral chain and branching sites. The Al-tobermorite has a double silicate chain structure with long chain lengths in the b [020] crystallographic direction, and wide interlayer spacing, 11.49 Å. Na + and K + partially balance Al 3+ substitution for Si 4+ . Poorly crystalline calcium-aluminum-silicate-hydrate (C-A-S-H) cementitious binder in the dissolved perimeter of relict lime clasts has Ca/(Si+Al) = 0.79, nearly identical to the Al-tobermorite, but nanoscale heterogeneities with aluminum in both tetrahedral and octahedral coordination. The concrete is about 45 vol% glassy zeolitic tuff and 55 vol% hydrated lime-volcanic ash mortar; lime formed <10 wt% of the mix. Trace element studies confirm that the pyroclastic rock comes from Flegrean Fields volcanic district, as described in ancient Roman texts. An adiabatic thermal model of the 10 m 2 by 5.7 m thick Baianus Sinus breakwater from heat evolved through hydration of lime and formation of C-A-S-H suggests maximum temperatures of 85 to 97 °C. Cooling to seawater temperatures occurred in two years. These elevated temperatures and the mineralizing effects of seawater and alkali-and alumina-rich volcanic ash appear to be critical to Al-tobermorite crystallization. The long-term stability of the Al-tobermorite provides a valuable context to improve future syntheses in innovative concretes with advanced properties using volcanic pozzolans.

The Franciscan subduction complex of California is considered a type example of a subduction-accretion system, yet the age of subduction initiation and relationship to the tectonic history of western North America remain controversial. Estimates for the timing of Franciscan subduction initiation are largely based either indirectly on regional tectonic arguments or from the ages of high-grade blocks within mélange. Many of the high-grade blocks record counterclockwise pressure-temperature paths with early amphibolite overprinted by later eclogite and blueschist; however, their origin and significance with respect to subduction initiation have been debated. In contrast, some high-grade blocks show evidence for clockwise pressure-temperature paths and an early eclogite assemblage overprinted by later amphibolite. Zircon U-Pb ages from inclusions in garnet and Lu-Hf estimates of initial garnet growth ages from these samples record early eclogite metamorphism at~176 Ma. Matrix zircon U-Pb ages and Lu-Hf estimates of final garnet growth ages record a barroisite-amphibolite assemblage overprint of eclogite at~160 Ma. Combined with petrologic data and existing geochronology, the data suggest that (1) Franciscan subduction was underway by no later than 180 Ma, (2) continuous subduction metamorphism occurred for at least 100 Ma, and (3) Franciscan subduction initiation predated the formation of the overlying Coast Range Ophiolite, supporting models that form the ophiolite above an east dipping Franciscan subduction zone.

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Studies of ancient and active subduction zones are critically important to understanding processes of interplate coupling, crust-mantle recycling and arc magmatism. Dating subduction metamorphism along prograde and retrograde paths in order to constrain such processes, however, has proved to be extremely difficult. The complex thermal history of subduction systems poses significant challenges to low-T geochronometers and subduction-zone assemblages commonly lack suitable minerals for higher temperature geochronology. Garnet and lawsonite, however, are critical index minerals of high-and low-T subduction-zone metamorphism and are well suited to Lu-Hf geochronology. In addition, the closure temperature for Lu-Hf in garnet and lawsonite should be sufficiently high that an age will date mineral crystallization and therefore the timing of subduction zone metamorphism. The relative stability and timing of garnet and lawsonite formation will be controlled by the bulk composition, peak metamorphic conditions and shape of the P-T path experienced by a particular sample. To test the influence of metamorphic conditions and P-T path on Lu-Hf geochronology, garnet and lawsonite-bearing samples were dated from rocks where lawsonite formed after garnet along a retrograde path, stabilized prior to garnet along a prograde path and formed contemporaneously with garnet. In the Franciscan Complex, the ages of garnet-epidote amphibolite, garnet-epidote blueschist, garnetlawsonite blueschist and lawsonite blueschist range from c. 166-130 Ma and generally decrease with decreasing metamorphic grade, consistent with previous studies. Garnet-lawsonite blueschist/eclogite formed along an apparent prograde path at Ward Creek records an apparent age of c. 152 Ma. Lower temperature lawsonite blueschist at Ward Creek, however, failed to provide a geologically significant date and likely reflects isotopic disequilibrium at low temperatures. The apparent temperature-time history from Franciscan Complex Lu-Hf ages most likely reflects samples derived from various portions of the subduction zone or that were subducted and metamorphosed at different times in the thermal evolution of the subduction zone. In the Sivrihisar Massif, lawsonite eclogite and garnet-lawsonite blueschist record distinctly different ages of 91.1 AE 1.3 Ma and 83.3 AE 1.8 Ma. The different ages date the timing of high-P metamorphism within each protolith and suggest that garnet-lawsonite eclogite metamorphism pre-dated garnet-lawsonite blueschist metamorphism in these samples by c. 8 Ma. The age of lawsonite eclogite metamorphism extends the timing of high-P metamorphism and requires subduction initiation prior to 91 Ma. The results indicate that the Lu-Hf system provides a reliable tool for dating the wide range in P-T conditions of subductionzone metamorphism. Lawsonite dating, in particular, provides a reliable method by which to date low-T retrograde and prograde metamorphism in the absence of garnet. Lawsonite may not be ideal for geochronology if sufficient garnet coexi...

The Famatina margin records an orogenic cycle of convergence, metamorphism, magmatism, and extension related to the accretion of the allochthonous Precordillera terrane. New structural, petrologic, and geochronologic data from the Loma de Las Chacras region demonstrate two distinct episodes of lower crustal migmatization. The first event preserves a counterclockwise pressure-temperature path in kyanite-K-feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at ∼12 kbar and 868 • C at 461 ± 1.7 Ma. The shape of the pressure temperature path and timing of metamorphism are similar to those of regional midcrustal granulites and suggest pervasive Ordovician migmatization throughout the Famatina margin. One-dimensional thermal modeling coupled with regional isotopic data suggests Ordovician melts remained at temperatures above their solidus for 20-30 Ma following peak granulite facies metamorphism, throughout a time period marked by regional oblique convergence. The onset of synconvergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of Precordillera terrane accretion at ∼436 Ma. The second migmatite event was regionally localized and occurred at ∼700 • C and 12 kbar between 411 and 407 Ma via vapor saturated melting of muscovite. Migmatization was synchronous with extension, exhumation, and strike-slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the Chilenia terrane.

Dating zircon by U-Pb in the Pleistocene Bishop tuff is challenging because of the low concentrations of radiogenic Pb and the relatively large correction required for disequilibrium in the intermediate daughter products. The dates can be difficult to interpret, because the absolute precision on the dates is similar in magnitude to timescales of crystallization. Interpretations of the duration of zircon crystallization span two orders of magnitude and appear to depend on the analytical approach (bulk analysis or microbeam analysis). To reconcile the differing interpretations, we present new SIMS and ID-TIMS zircon Pb/U dates, including some grains that are dated by both techniques. Both the SIMS and ID-TIMS dates have similar distributions as previous results. Normalized to a Th/U melt = 2.81, SIMS spot dates from interior regions of sectioned grains range from 769±31 ka to 845±28 ka (2σ) and ID-TIMS dates have a range from 760±7 ka to 793±6 ka. The double-dated grains demonstrate an average of 31 ka difference between the bulk grain age and the interior spot age, demonstrating 10 ka-scale age variation within individual grains. This level of variability precludes the assignment of a geological meaning to a mean (or weighted-mean) zircon date. A previous ID-TIMS investigation of single BT zircon that showed apparent statistically significant clustering of zircon ages was compromised by an incorrect treatment of the covariance structure of 230 Th-corrected Pb/U data. We show approximate and exact methods for the correct treatment, which demonstrate substantially more scatter in that data than previously recognized. A close investigation of available partition coefficient data show that they are not adequately precise for <±10 ka corrections, and moreover, evidence for disequilibrium uptake of U and Th means that equilibrium partition coefficients may not be useful, even if determined precisely. Mismatches between the U-Th concentration systematics of zircon and of glasses suggest that there are gaps in our understanding of the relationship between the two phases, lending uncertainty to the accuracy of disequilibrium corrections that use measured glass compositions. Given these uncertainties in the disequilibrium correction, caution is urged when correcting dates with nominal precisions that are better than ±10 ka. Nevertheless, the youngest zircon ages very likely constrain the eruption age of the Bishop Tuff to < 775 ka, which conflicts with some estimates of the 40 Ar-39 Ar sanidine age of eruption. In detail, we show that the use of currently available partition coefficients for the correction of the 230 Th deficit limits the absolute precision of Pb/U dates to >±10 ka (2σ) when this method is used, but also show that the amount of trace element uptake (including U, Th, and Hf) varies significantly between crystal faces, and therefore bulk equilibrium may not be a realistic assumption. The liquid line of descent defined by U-Th systematics of glasses are different from the U-Th systematics of zircon, requiring ...