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The southern Appalachians preserve evidence for three Paleozoic orogenies that contributed to construction of the composite southern Appalachian orogen, including the Taconic (480-440 Ma), Neoacadian (380-340 Ma), and Alleghanian (330-280 Ma) events. However, the complexity of thermal-metamorphic overprinting and polydeformation has impeded efforts to examine questions related to tectonic processes such as the crustal escape flow hypothesis in the southern Appalachians. To address this, new monazite and xenotime laser ablation split-stream U-Pb and hornblende 40Ar/39Ar dates have been produced for the Blue Ridge (BR) and Inner Piedmont (IP), and these data are being compiled with all previously available geo-thermochronological and quantitative P-T data to construct P-T-t histories for different parts of the orogen. Monazite U-Pb dates from prograde monazites in the North Carolina BR yield a clear Taconic (464-441 Ma) metamorphic signal for conditions up to granulite facies, which is interpreted to result from development of a Taconic accretion-subduction complex. Following the Taconic arcs collision, this part of the BR was cooled during Neoacadian and Alleghanian uplift and exhumation pulses, as indicated by thermochronologic dates spanning a wide range of closure temperatures (~550-220 °C). In the IP and Sauratown Mountains window, U-Pb dates of mostly prograde monazite growth yield a dominant Neoacadian signal (369-358 Ma), which corroborates previous age estimates for IP Barrovian metamorphism up to sillimanite II grade. In the IP, hornblende 40Ar/39Ar ages of 380-345 Ma generally indicate syn-Neoacadian cooling below ~500 °C. In the IP between the Brevard and Brindle Creek fault zones, Y-rich monazites yield younger dates (~330 Ma) that overlap with hornblende 40Ar/39Ar yield ages (335-324 Ma). Combined, these ages are interpreted to reflect post-Neoacadian reactivation and retrogression of the Brevard fault zone and potential folding(?) of the Brindle Creek fault zones during early Alleghanian deformation. This retrograde deformation persists until at least 297 Ma, as reflected by xenotime dates in the Brevard zone (311-297 Ma). Future work will address how the timing and extent of metamorphism, deformation, and exhumation may vary south of the present study area. 

In the last two decades, crustal channel and escape flow, wherein long-wavelength ductile flow of lower crustal material transports mass and heat out of the collision zone, have remained among the most impactful ideas proposed to explain shortening accommodation in continental collisions. In the Inner Piedmont (IP), southern Appalachians, channel and escape flow have been previously proposed for the Devonian-Mississippian Neoacadian orogeny, and the deep exhumational level of the IP relative to other orogens in which channel flow has been proposed makes it ideal for testing the channel and escape flow models. In the IP channel flow model, the Brevard fault zone (BFZ) footwall is interpreted to buttress orogen-normal crustal flow of the hot IP in northwestern North Carolina and drive escape flow to the southwest. However, the polymetamorphic and deformational history of the southern Appalachians has made it difficult to isolate the spatial and temporal extent of thermal and deformational events driving flow of the interpreted channel. To address this, we use in situ laser ablation split stream monazite (Mz) U-Pb geochronology and geochemistry coupled with quantitative P-T data to define the extent and conditions of Paleozoic metamorphic events in the southern Appalachians of North Carolina. In this area, northwest of the BFZ, Mz dates indicate mostly Taconic (~462 Ma) and minor Neoacadian metamorphism (~368 Ma) whereas IP data show Neoacadian metamorphism (~363–330 Ma) with no Taconic ages. IP Mz also records a transition over time from HREE-poor to HREE-rich compositions, indicating Mz growth associated with both garnet growth and breakdown, respectively. This, along with diffuse chemical profiles and resorption textures in garnet, suggests that IP Mz records prograde to retrograde metamorphism. Furthermore, P-T estimates from the eastern Blue Ridge of northwestern NC are 5–9 kbar and 565–730 °C, whereas peak Neoacadian metamorphism in the IP core reached 5–8 kbar and 750–850 °C. We interpret this to indicate that the BFZ footwall acted as both a thermal and rheological boundary in northwestern NC during Neoacadian metamorphism, supporting earlier interpretations. Future work will assess the timing and conditions of metamorphism further south into the Blue Ridge and IP of South Carolina, Georgia, and Alabama. 

The Appalachian Mountains expose one of the most complete deeply exhumed orogenic belts in the world. These rocks provide the opportunity to understand tectonic processes in the mid- to lower- crust that can be linked to upper crustal processes interpreted from less exhumed orogenic belts. However, 3 Paleozoic orogenies (Taconic, Neoacadian, Alleghanian) in the southern Appalachians produced a complicated thermal-metamorphic history that is poorly understood. Recently obtained monazite U-Pb ages in the western, central, and eastern Blue Ridge of Tennessee and the Carolinas range from 459 to 441 Ma, indicating that this part of the Blue Ridge preserves Taconic (Ordovician) metamorphic mineral assemblages and were not significantly reheated during Neoacadian (Devonian) or Alleghanian (Mississippian) orogenesis. Five published garnet Sm-Nd ages from the eastern Blue Ridge in Alabama and Georgia of 331 to 320 Ma indicate widespread Alleghanian metamorphism. The northwestern extent of these Alleghanian metamorphic rocks is constrained by a garnet Sm-Nd age of 357±3 Ma from NW of the transtensional Goodwater-Enitachopco fault. However, published metamorphic age constraints are lacking SE of and along strike to the NE of the Alleghanian rocks. We report new garnet Sm-Nd ages for northern Georgia that constrain the extent of the Alleghanian metamorphic rocks. Garnet-staurolite-hornblende gneiss in the Pumpkinvine Creek Formation yields an Alleghanian age of 323±3 Ma (MSWD=6.6, N=7). To the NE, garnet-muscovite-biotite gneiss from within the structural window at Brasstown Bald and migmatiticsillimanite- and spinel-bearing garnet-biotite neiss from outside the window at Blood Mountain have ages of 446±6 (MSWD=0.7, N=4) and 448±8 (MSWD=6, N=7) Ma, respectively. These 2 indistinguishable ages confirm the premetamorphic stacking of thrust sheets exposed in the structural window. Comparison of these new ages indicates post metamorphic displacement on the Allatoona fault between the Dahlonega terrane and the western Blue Ridge. Additional garnet ages spatially distributed across the Piedmont of east central Alabama and the Murphy belt of NE Georgia extent are currently in-progress. The full data set will be used to test tectonic models including possible out-of-sequence thrusting and crustal channel flow. 

The southern Appalachians record three Paleozoic collisional events, including the Taconic (Ordovician), Neoacadian (Devonian-Mississippian), and Alleghanian (Carboniferous-Permian) orogenies. The complex nature of thermal and structural overprinting related to these events, coupled with a lack of widespread modern geo-, thermo-, and petrochronologic studies here has limited our ability to unravel the precise timing, spatial extent, and conditions of Paleozoic deformation and metamorphism. In the Blue Ridge (BR) and Inner Piedmont (IP) of Tennessee, North Carolina, and Georgia, which represents the orogenic core of the composite southern Appalachians, new monazite laser ablation split stream (LASS) analyses, amphibole 40Ar/39Ar dates, and metamorphic phase equilibria models are integrated with pre-existing geo- and thermochronology data to test holistic models of Paleozoic orogenesis. In the BR west of the Brevard fault zone (BFZ), monazite U-Pb dates are 459-441 Ma and are related to a pronounced Taconic metamorphic peak (to upper amphibolite facies) during development of an eastern Laurentian subduction-accretionary complex, followed by exhumation and cooling during later Neoacadian and Alleghanian thrust stacking, indicated by thermochronologic data. In the BFZ and the IP to the east, monazite U-Pb dates range from 373-356 Ma and delimit the timing of peak Neoacadian kyanite-sillimanite II metamorphism in the IP driven by accretion and partial subduction of Laurentian and mixed-affinity IP rocks beneath the overriding Carolina superterrane. The relatively clear separation of Taconic and Neoacadian monazite dates across the BFZ indicate that this shear zone acted as a Neoacadian thermal-rheologic transition zone that partitioned SW-directed crustal “escape” channel flow of melt-weakened material, as proposed by earlier studies. Late Paleozoic monazite U-Pb dates derived from within the BFZ (~335 Ma) and in the southeasternmost parts of the IP (~324 Ma) reflect Alleghanian reactivation of the BFZ and the northwesternmost extent of Alleghanian Barrovian metamorphism, respectively, but the majority of the BR and the IP in the study area reveal no evidence of post-Neoacadian metamorphic overprinting. 

In the southern Appalachians, questions persist regarding the spatial extent and conditions of metamorphism for the Taconic, Neoacadian, and Alleghanian orogenic events. Ongoing research seeks to investigate the viability of the channel and/or escape flow models as potential mechanisms for lower crustal flow during orogenesis. Thus, metamorphism due to these events must be constrained in the central and eastern Blue Ridge (CBR, EBR) and Inner Piedmont (IP) provinces, which are key components of these models. In this contribution, we present one part of this ongoing research—recent results of monazite-xenotime U-Pb geochronology and rare earth element (REE) geochemistry from western North Carolina. Monazite and xenotime from six metasedimentary and metavolcanic samples collected from the CBR and EBR northwest of the Brevard Fault Zone (BFZ) yield Taconic U-Pb dates (> 400 Ma) and show no evidence of Neoacadian or Alleghanian mineral growth or resetting. Chondrite-normalized REE abundances for the EBR samples show minimal depletion in heavy REE (HREE) relative to light REE (LREE). Two mylonitic samples located adjacent to or within the BFZ yield both Taconic and Neoacadian dates; REE concentrations and petrography suggest that the youngest date, c.339 Ma, records retrograde xenotime and monazite growth during garnet breakdown following peak Neoacadian metamorphism and is not indicative of early Alleghanian prograde influence. In the Brevard and Brindle Creek thrust sheets of the IP, monazite and xenotime U-Pb dates from four metasedimentary samples yield Neoacadian (c. 340-360 Ma) to very early Alleghanian (c. 322-335 Ma) dates; however, the Alleghanian dates are limited to the easternmost portion of the Brindle Creek thrust sheet near the Central Piedmont Suture. Monazites from samples in the IP record varying, but pronounced, depletion in HREE relative to LREE. Combined with petrographic evidence of garnet resorption and monazite-xenotime rim growth and corresponding U-Pb dates, IP rocks likely record prograde Neoacadian metamorphism followed by retrograde monazite-xenotime growth prior to the main Alleghanian pulse. The abovementioned models are supported by these data, but additional geochemical and piezometric analyses are needed to better elucidate their impact during Neoacadian orogenesis. 

Abstract We present microbeam major- and trace-element data from 14 monzodiorites collected from the Malaspina Pluton (Fiordland, New Zealand) with the goal of evaluating processes involved in the production of andesites in lower arc crust. We focus on relict igneous assemblages consisting of plagioclase and amphibole with lesser amounts of clinopyroxene, orthopyroxene, biotite and quartz. These relict igneous assemblages are heterogeneously preserved in the lower crust within sheeted intrusions that display hypersolidus fabrics defined by alignment of unstrained plagioclase and amphibole. Trace-element data from relict igneous amphiboles in these rocks reveal two distinct groups: one relatively enriched in high field strength element concentrations and one relatively depleted. The enriched amphibole group has Zr values in the range of ∼25–110 ppm, Nb values of ∼5–32 ppm, and Th values up to 2·4 ppm. The depleted group, in contrast, shows Zr values <35 ppm and Nb values <0·25 ppm, and Th is generally below the level of detection. Amphibole crystallization temperatures calculated from major elements range from ∼960 to 830 °C for all samples in the pluton; however, we do not observe significant differences in the range of crystallization temperatures between enriched (∼960–840 °C) and depleted groups (∼940–830 °C). Bulk-rock Sr and Nd isotopes are also remarkably homogeneous and show no apparent difference between enriched (εNdi = 0·1 to –0·1; 87Sr/86Sri = 0·70420–0·70413) and depleted groups (εNdi = 0·3 to –0·4; 87Sr/86Sri = 0·70424–0·70411). Calculated amphibole-equilibrium melt compositions using chemometric equations indicate that melts were highly fractionated (molar Mg# <50), andesitic to dacitic in composition, and were much more evolved than bulk lower continental crust or primitive basalts and andesites predicted to have formed from hydrous melting of mantle-wedge peridotite beneath an arc. We suggest that melts originated from a common, isotopically homogeneous source beneath the Malaspina Pluton, and differences between enriched and depleted trace-element groups reflect varying contributions from subducted sediment-derived melt and sediment-derived fluid, respectively. Our data demonstrate that andesites and dacites were the dominant melts that intruded the lower crust, and their compositions mirror middle and upper bulk-continental crust estimates. Continental crust-like geochemical signatures were acquired in the source region from interaction between hydrous mantle-wedge melts and recycled subducted sediment rather than assimilation and/or remelting of pre-existing lower continental crust. 

Abstract The southern Coast Mountain batholith was episodically active from Jurassic to Eocene time and experienced four distinct high magmatic flux events during that period. Similar episodicity has been recognized in arcs worldwide, yet the mechanism(s) driving such punctuated magmatic behavior are debated. This study uses zircon Hf and O isotopes, with whole-rock and mineral geochemistry, to track spatiotemporal changes in southern Coast Mountains batholith melt sources and to evaluate models of flare-up behavior and crust formation in Cordilleran arc systems. Zircon Hf isotope analysis yielded consistently primitive values, with all zircon grains recording initial εHf between +6 and +16. The majority (97%) of zircons analyzed yielded δ18O values between 4.2‰ and 6.5‰, and only five grains recorded values of up to 8.3‰. These isotopic results are interpreted to reflect magmatism dominated by mantle melting during all time periods and across all areas of the southern batholith, which argues against the periodic input of more melt-fertile crustal materials as the driver of episodic arc magmatism. They also indicate that limited crustal recycling is needed to produce the large volumes of continental crust generated in the batholith. Although the isotopic character of intrusions is relatively invariant through time, magmas emplaced during flare-ups record higher Sr/Y and La/Yb(N) and lower zircon Ti and Yb concentrations, which is consistent with melting in thickened crust with garnet present as a fractionating phase. Flare-ups are also temporally associated with periods when the southern Coast Mountains batholith both widens and advances inboard. We suggest that the landward shift of the arc into more fertile lithospheric mantle domains triggers voluminous magmatism and is accompanied by magmatic and/or tectonic thickening. Overall, these results demonstrate that the magmatic growth of Cordilleran arcs can be spatially and temporally complex without requiring variability in the contributions of crust and/or mantle to the batholith. 

The concept of long-wavelength ductile flow of lower crustal material, or channel flow, has emerged to explain the evolution of large hot orogens. In this model, growth of heat producing crust during collision leads to melt-weakening and flow of lower crust in response to tectonic forcing or long-wavelength gradients in gravitational potential energy. In the Himalayan-Tibetan (HT) orogen where the model was originally proposed, it has been hypothesized that a Miocene orogen-normal channel was active and that there was a more recent switch to orogen-parallel “escape” flow as the front of the orogen began to deform as a thrust wedge. However, because this hypothesized HT orogenic channel is largely subsurface it cannot be directly examined, making it difficult to test these hypotheses. The Inner Piedmont (IP), southern Appalachians has been proposed to be an exhumed orogenic channel based on inverted metamorphic isograds, extensive migmatization, and a large-scale curved mineral lineation pattern that is consistent with a shift from orogen-normal to orogen-parallel flow. To test the viability of the channel flow model in the IP, we construct pressure-temperature-time (P-T-t) paths and compare these to existing models which indicate that peak temperatures and residence times will differ between thrust wedge and channel flow models. The P-T-t paths are constructed using isochemical phase diagram sections (pseudosections), garnet compositions, monazite geochronology, and 40Ar/39Ar thermochronology to define prograde to retrograde conditions and residence times. The channel flow models require temperatures above 700-750°C to initiate and maintain flow. Preliminary pseudosections from the northern IP Brindle Creek fault zone indicate prograde to peak conditions of 815–820 °C and 7.9–9.3 kbar, and retrograde conditions of 720–730 °C and 5.3–5.4 kbar based on observed garnet compositions and sample mineralogy (Qtz + Pl + Bt + Sil + Grt ± Ms ± Ep ± Ilm ± Rt). Pseudosections are still being revised, however if confirmed, the P-T conditions are compatible with channel flow in the IP. Future model revisions and age data from samples forming a transect across the IP and into the adjacent Carolina superterrane and eastern Blue Ridge will be used to compare the P-T-t histories between the prdoposed channel and surrounding units.