Emily M. Peterman

Phase transformations of continental crust during subduction and exhumation: Western Gneiss Region, Norway

Whether quartzofeldspathic rocks transform to (U)HP minerals during subduction - and back to low-pressure minerals upon exhumation - remains one of the more profound questions pertaining to collisional orogenesis. Garnet-bearing quartzofelds- pathic gneisses from the Western Gneiss Region, Norway provide an opportunity to answer this question. High-precision Sm-Nd garnet geochronology of these gneisses documents garnet growth from 418 to 398 Ma. Garnet zoning in two samples implies growth during subduction-related increase in pressure from 0.5 GPa and 550 � C to 1.7 GPa and 700 � C. Zoning in all other garnets suggests growth over rather narrow P-T ranges from 1.0-1.6 GPa and 725-800 � C during decompression, possibly accompanying melting. If these samples constitute a representative suite, (i) the dearth of (U)HP garnets suggests that most of the quartzofeldspathic gneisses in the Western Gneiss Region did not transform to eclogite-facies parageneses during subduction; (ii) the abundance of garnets grown during decompression indicates that the major period of densification was during exhumation. The widespread metastability of quartzofeldspathic rocks during subduction is substantially different from the findings of previous work and suggests commensu- rately less subduction of continental crust before the slab is positively buoyant.

Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops

Atom probe yields geologically meaningful ages from nanoscale Pb-enriched dislocation loops in discordant zircon. Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 μm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The 207Pb/206Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages.

Growth conditions of symplectic muscovite + quartz: Implications for quantifying retrograde metamorphism in exhumed magmatic arcs

Symplectic intergrowths are important markers of incomplete retrograde metamorphism. Because they represent arrested reactions that were limited by diffusion, these textures provide constraints on how pressure and temperature varied following peak-grade metamorphism. For example, symplectic intergrowths of muscovite + quartz that formed in upper amphibolite facies sillimanite + K-feldspar wall rocks permit assessment of the thermal history of a magmatic arc post-intrusion. Wall rocks surrounding the last plutons to be emplaced within the Peninsular Ranges batholith, (Southern California, USA, to Baja California, Mexico), present an ideal setting to investigate retrograde textures that elucidate arc evolution. During intrusion, wall rocks reached 625 ± 25 °C and exceeded muscovite + quartz stability. Post-intrusion, muscovite + quartz symplectites grew at the expense of K-feldspar in response to falling temperature. Ti-in-quartz measurements reveal that symplectic quartz grew at ≥375 °C, which reflects temperatures as much as 250 °C lower than peak-grade metamorphism. This result has significant implications for the mechanical and thermal evolution of Peninsular Ranges batholith arc crust.

Dissolution-reprecipitation metasomatism and growth of zircon within phosphatic garnet in metapelites from western Massachusetts

Abstract Highly restitic garnet-kyanite-phlogopite metapelitic schists from the Goshen Dome of western Massachusetts contain: a population of prograde monocrystalline, megacrystic garnet, some with significant P in substitution for Si; precipitates of hydroxylapatite and rutile; and <1 μm zircon crystals of undetermined origin and abundance on the order of 105/mm3. The unusual P content and the abundant internal precipitate suite are similar to features reported in garnet from ultrahigh-pressure (UHP) and mantle settings, suggesting a potential (U)HP origin for the garnet megacrysts. Zircon included in megacrysts is surrounded by radial fractures, indicating in situ volumetric expansion or new growth. Cores display rare earth element (REE) profiles and cathodoluminescence (CL) zoning consistent with magmatic growth, and yield only Paleozoic dates (447–404 Ma). The embayed core–rim boundary is marked by a several micrometers wide band of CL-dark zircon enriched in Y, P, U, and Th that is interpreted as the accumulation of redistributed xenotime component from the original zircon rim during metamorphism. Outside of this band, the rim has elevated Hf, Th/U << 1, and steep heavy REE profiles. The metamorphic rims yield concordant dates from 400 to 381 Ma. Matrix zircon grains have magmatic cores (1726–415 Ma) with similar core-rim boundaries enriched in Y, P, U, and Th. Metamorphic rims on matrix zircon yield slightly younger dates (393–365 Ma) and are compositionally heterogeneous. The difference between the youngest core and oldest rim indicates a short interval (ca. 4 Ma) between deposition of detrital zircon and the onset of metamorphism in the earliest Acadian. The oldest zircon rim dates are found within phosphatic garnet megacrysts of possible very high-pressure origin. The compositional uniformity of these rims indicates equilibrium with a single source; the anomalous composition suggests a combination of dissolution-reprecipitation and new growth of zircon that is derived from garnet. The range in both composition and dates indicates that matrix zircon rims formed in response to local changes in mineralogy and fluid/melt composition and/or availability. New growth of zircon on these grains cannot be confirmed, suggesting that dissolution-reprecipitation reactions during continued metamorphism may be the dominant mechanism that formed these rims. The data collectively suggest that dissolution-reprecipitation may be a common mechanism for producing metamorphic rims on zircon that does not require additional Zr and Hf, which are limited within most metamorphic settings.