Accessory Mineral Eu Anomalies in Suprasolidus Rocks: Beyond Feldspar

Accessory mineral Eu anomalies (Eu/Eu*) are routinely measured to infer changes in the amount of feldspar over time, allowing accessory mineral U‐Pb dates to be linked to the progressive crystallization of igneous and metamorphic rocks and, by extension, geodynamic processes. However, changes in Eu/Eu* can reflect any process that changes the relative availability of Eu2+ and Eu3+. We constructed partitioning budgets for Sm, Eu2+, Eu3+, and Gd in suprasolidus metasedimentary rocks to investigate processes that can influence accessory mineral Eu anomalies. We modeled three scenarios: (1) closed‐system, equilibrium crystallization; (2) fractionation of Eu by feldspar growth during melt crystallization; and (3) removal of Eu by melt extraction. In the closed‐system equilibrium model, accessory mineral Eu/Eu* changes as a function of fO2 and monazite stability; Eu/Eu* changes up to 0.3 over a pressure‐temperature range of 4–12 kbar and 700–950°C. Fractionation of Eu by feldspar growth is modeled to decrease accessory mineral Eu/Eu* by ~0.05–0.15 per 10 wt% feldspar crystallized. Melt extraction has a smaller effect; removal of 10% melt decreases accessory mineral Eu/Eu* in the residue by ≤0.05. Although these models demonstrate that fractionation of Eu by feldspar growth can be a dominant control on a rocks u budget, they also show that the common interpretation that Eu/Eu* only records feldspar growth and breakdown is an oversimplification that could lead to incorrect interpretation about the duration and rates of tectonic processes. Consideration of other processes that influence Eu anomalies will allow for a broader range of geological processes to be investigated by petrochronology.Plain Language SummaryMetamorphic rocks—rocks in which new minerals grew in response to increase in pressure and temperature related to deep burial or subduction—and igneous rocks—rocks that formed as magmas cool and crystallize—provide a direct record of how Earth’s continents have moved and changed through time. To read this record, geologists need to be able to measure the ages of metamorphism and magmatism: When did it happen? How long did it last? How does it relate to other rocks around the world? A common approach to addressing these questions is using U‐Pb dating of the minerals zircon, monazite, and apatite. The elements these minerals incorporate are indicative of how hot and how deep in the Earth they were when they grew. In this study we explore how geologists can use the concentrations of the element Europium (Eu) in these minerals to provide new insights into the geological meaning of U‐Pb dates, leading to more robust interpretations of Earth’s plate tectonic history.Key PointsEu anomalies in suprasolidus rocks record any process that changes the relative availability of Eu2+ and Eu3+, not just feldspar growthDisequilibrium is required for feldspar growth to strongly influence accessory mineral Eu anomaliesComparing accessory mineral Eu anomalies and Sr concentrations leads to more robust interpretation than evaluating Eu anomalies alonePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/156481/1/ggge22268_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156481/3/ggge22268.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156481/2/ggge22268-sup-0001-2020GC009052-Text_SI-S01.pd

Subduction Initiation Recorded in the Dadeville Complex of Alabama and Georgia, Southeastern United States

The Dadeville Complex of Alabama and Georgia (southeastern United States) represents the largest suite of exposed mafic-ultramafic rocks in the southern Appalachians. Due to poor preservation, chemical alteration, and tectonic reworking, a specific tectonic origin for the Dadeville Complex has been difficult to deduce. We obtained new whole-rock and mineral geochemistry coupled with zircon U-Pb geochronology to investigate the magmatic and metamorphic processes recorded by the Dadeville Complex, as well as the timing of these processes. Our data reveal an up-stratigraphic evolution in the geochemistry of the volcanic rocks, from forearc basalts to boninites. Our new U-Pb zircon crystallization data—obtained from three amphibolite samples—place the timing of forearc/protoarc volcanism no later than ca. 467 Ma. New thermobarometry suggests that the Dadeville Complex rocks subsequently experienced deep, high-grade metamorphism, at pressure-temperature conditions of \u3e7 kbar and \u3e760 °C. The data presented here support a model for formation of the Dadeville Complex in the forearc region of a subduction zone during subduction initiation and protoarc development, followed by deep burial/underthrusting of the complex during orogenesis

Tectonic Mode Switches Recorded at the Northern Edge of the Australian Plate During the Pliocene and Pleistocene

We report new data from medium‐high grade metamorphic rocks found at the northern margin of the Lengguru Fold Belt in West Papua. The study involved a systematic analysis of cross‐cutting structures to establish the relative timing of deformation, together with isotopic dating to define when these tectono‐thermal events occurred. These data show that the region underwent multiple episodes of deformation within the last six million years. Metamorphic mineral growth was associated with the development of ductile shear zones. This episode occurred during a phase of crustal stretching associated with the formation of a metamorphic core complex. Metamorphic zircon growth at 4.9 to 5.3 Ma was documented in two of the dated samples. These data are interpreted to post‐date the peak pressure and temperature conditions of the phase of regional crustal stretching. The shear fabrics associated with the metamorphic core complex were later overprinted by at least two generations of folds. The change in mode from crustal extension to shortening reflects a tectonic mode switch. A subsequent mode switch is documented by numerous brittle extensional faults that cross‐cut the earlier formed ductile fabrics. We interpret ca. 0.75-1.51 Ma (U-Th)/He age data to reflect cooling associated with the later stages of crustal shortening (marked by folds) or the later extensional unroofing of the peninsula. This work demonstrates that an orogen can record multiple tectonic mode switches within several million years. These outcomes should be considered in studies of ancient orogens where analytical uncertainties associated with isotopic dating may mask short‐lived mode switches

Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians

White mica 40Ar/39Ar analyses may provide useful constraints on the timing of tectonic processes, but complex geological and thermal histories can perturb Ar systematics in a variety of ways. Ductile shear zones represent excellent case studies for exploring the link(s) between dynamic re-/neo-crystallization of white mica and coeval enhanced fluid flow, and their effect on 40Ar/39Ar dates. White mica 40Ar/39Ar dates were collected from compositionally similar granites that record different episodes of deformation with proximity to the Dover Fault, a terrane-bounding strike-slip shear zone in the Appalachian orogen, Newfoundland, Canada. 40Ar/39Ar data were collected in situ by laser ablation and by step heating single crystals. Results were compared to each other and against complementary U-Pb zircon and monazite, and K-Ar fault gouge analysis. Although step-heat 40Ar/39Ar is a widely applied method in orogenic settings, this dataset shows that relatively flat step-heat 40Ar/39Ar spectra are in contradiction with wide spreads in in-situ 40Ar/39Ar dates from the same samples, and that plateau dates in some cases yielded mixed dates of equivocal geological significance. This result indicates that the step-wise release of Ar from white mica likely homogenizes and obscures spatially-controlled Ar isotope reservoirs in white mica from sheared rocks. In contrast, in situ laser ablation 40Ar/39Ar analysis preserves the spatial resolution of 40Ar reservoirs that have been variably reset by deformation and fluid interaction. This study therefore suggests that laser ablation is the best method for dating the timing of deformation recorded by white mica. Final interpretation of results should be guided by microstructural analysis, estimation of deformation temperature, chemical characterization of white mica, and complementary chronometers. Overall the dataset shows that granitic protoliths were emplaced between 430-422 Ma (U-Pb zircon). High strain deformation along the Wing Pond Shear Zone occurred between ca. 422-405 Ma (U-Pb monazite and 40Ar/39Ar). Subsequent patchy Ar loss in white mica occurred locally during low T shear (40Ar/39Ar). K-Ar dating of authigenic illite in fault gouge from the broadly co-linear brittle Hermitage Bay Fault indicates that slip along the terrane boundary persisted until at least the Mississippian

Discussion of 'Metamorphic P - T and retrograde path of high-pressure Barrovian metamorphic zones near Cairn Leuchan, Caledonian orogen, Scotland'

K. Aoki, B. F. Windley, S. Maruyama & S. Omori reply: First, we thank Viete, Oliver & Wilde for their interesting and thought-provoking comments on the timing of the high-pressure granulite facies (HGR) metamorphism recorded in metamorphic rocks at Cairn Leuchan, Scotland, published by Aoki et al. (2013). Based on new metamorphic data of garnetites and garnet-amphibolites at Cairn Leuchan and new zircon U-Pb ages of amphibolitized eclogite at Tomatin, we suggested in our publication that the HGR metamorphism was retrograde after eclogite facies before the c. 470 Ma 'Barrovian metamorphism'. Viete, Oliver & Wilde however speculate that the HGR metamorphism at Cairn Leuchan may have occurred at c. 1000 Ma, as a result of their new U-Pb zircon age of the Cowhythe Gneiss at Portsoy and from previous studies of the geological structure and geochronology. We are grateful for this opportunity to describe, albeit in a preliminary manner, our new understanding and tectonic model of the Caledonian orogen in Scotland and western Ireland of which the Barrovian metamorphism is a key component. A reply to a comment is not the correct place to propose an entirely new paradigm for such a classic orogen, but we will present our model more fully in a future publication.. Copyright © Cambridge University Press 2014

Clumped‐Isotope Geothermometry and Carbonate U–Pb Geochronology of the Alta Stock Metamorphic Aureole, Utah, USA: Insights on the Kinetics of Metamorphism in Carbonates

To assess thermal and kinetic influences on atomic mobility and mineral (neo)crystallization, clumped‐isotope abundances of calcite and dolomite were measured alongside dolomite cation ordering and U–Pb dates, across metamorphic grade within the c. 35–30 Ma Alta stock contact metamorphic aureole, Utah, USA. Average Δ47 values of dolomite inside the metamorphic aureole reflect the blocking temperature of dolomite (300°C–350°C) during cooling from peak temperatures. Dolomite Δ47 values outside the metamorphic aureole record a temperature of ∼160°C. At the talc isograd, dolomite Δ47 values abruptly change, corresponding to a decrease of ∼180°C over <50 m in the down‐temperature direction. This observed step in dolomite Δ47 values does not correlate with cation ordering in dolomite or U–Pb dates, neither of which correlate well with metamorphic grade. The short distance over which dolomite Δ47 values change indicates strong temperature sensitivity in the kinetics of dolomite clumped‐isotope reordering, and is consistent with a wide range of clumped‐isotope reequilibration modeling results. We hypothesize that clumped‐isotope reordering in dolomite precedes more extensive recrystallization or metamorphic reaction, such as the formation of talc. Dolomite U–Pb analyses from inside and outside the metamorphic aureole populate a single discordia ∼60 Myr younger than depositional age (Mississippian), recording resetting in response to some older postdepositional, but premetamorphic process.Key Points:Dolomite clumped isotopes record an abrupt ∼180°C decrease, over <50 m, at the talc isograd of the Alta contact metamorphic aureoleThis step feature suggests strong thermal sensitivity in clumped‐isotope reordering that precedes talc neocrystallizationClumped‐isotope geothermometry may provide new insights into processes controlling metamorphism and reaction kinetics in carbonatesPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/167127/1/ggge22474.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167127/2/2020GC009238-sup-0001-Supporting_Information_SI-S01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167127/3/ggge22474_am.pd