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.
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DEFINING METAMORPHIC TIMING, EXTENT, AND CONDITIONS IN THE SOUTHERN APPALACHIAN BLUE RIDGE AND INNER PIEDMONT—INSIGHTS FROM MONAZITE XENOTIME GEOCHEMISTRY AND GEOCHRONOLOGY
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.
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- Award ID(s):
- 1802730
- NSF-PAR ID:
- 10471372
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Abstracts Geological Society of America
- ISSN:
- 0435-3986
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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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.more » « less
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The southern Appalachians preserve a composite thermal and deformational history that involves at least four orogenic pulses and spans >800 Myr from the middle Mesoproterozoic to the late Paleozoic. This complex history has made the discrete separation of metamorphic and deformational events persistently challenging, particularly for Paleozoic events. To address this, we report results from a new suite of amphibole 40Ar/39Ar data from the eastern Blue Ridge and Inner Piedmont (Tugaloo and Cat Square terranes) of western North Carolina to constrain the timing of cooling from thermal conditions greater than ~550 °C. Samples were collected along an orogen-normal transect from the Newton window, near the Central Piedmont suture, to the eastern Blue Ridge, west of the Grandfather Mountain window. Amphibole gneiss from the Spruce Pine area and Wilson Creek gneiss from the Grandfather Mountain window record dates of 380 and 496 Ma, respectively. Across the Brevard Fault zone, one sample from the western Inner Piedmont Tugaloo terrane yields a date of 377 Ma. Southeast of the Brindle Creek fault in the Cat Square terrane, three samples yield dates of 344-320 Ma while two samples from Tugaloo rocks of the Newton window yield dates of 335-330 Ma. Together, these dates suggest regional cooling of the Inner Piedmont progressed from northwest to southeast following the Neoacadian orogeny. The data also indicate either that high-grade (>550 °C) Alleghanian metamorphism may have only reset the amphibole40Ar/39Ar systematics in the most southeastern parts of the Inner Piedmont, near the Central Piedmont suture, or that these high-grade Neoacadian rocks were not exhumed from the mid-crust until the Alleghanian. Additionally, our new amphibole 40Ar/39Ar dates northwest of the Brevard Fault zone are not reset during Neoacadian orogenesis. Collectively, we interpret that high-grade metamorphism in this part of the southern Appalachians must have occurred prior to the Alleghanian, placing limits on the northwesternmost extent of mid- to high-grade Alleghanian metamorphism in this part of southern Appalachians. Additional data from the Sauratown Mountains window to the northeast and South Carolina and Georgia to the southwest may further constrain the spatial extent of the Neoacadian thermal event in the southern Appalachians.more » « less
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Abstract The tectonometamorphic evolution of the southern Appalachians, which results from multiple Paleozoic orogenies (Taconic, Neoacadian, and Alleghanian), has lacked a consensus interpretation regarding its thermal‐metamorphic history. The Blue Ridge terranes have remained the focus of the debate, with the interpreted timing of regional Barrovian metamorphism and associated deformation ranging from early (Taconic) to late Paleozoic (Alleghanian). New monazite U‐Pb geochronology and thermobarometric data are integrated with previously reported geo‐ and thermochronology to delimit the Paleozoic thermal‐metamorphic evolution of these terranes. Monazite compositional, textural, and U‐Pb age systematics are remarkably consistent for all samples, yielding a single dominant age mode for each sample. The western, central, and eastern Blue Ridge terranes yield weighted mean monazite U‐Pb ages of 450–441, 459–457, and 458–453 Ma, respectively. Thermodynamic modeling using mineral assemblages yields peak conditions of 600°C–650°C and 5.8–8.9 kbar for staurolite and kyanite grade western Blue Ridge units, including the stratigraphically youngest unit in the Murphy syncline, which also yields a weighted mean monazite U‐Pb age of 441 Ma. The Taconic metamorphic core of the central Blue Ridge yields peak conditions of 775°C and ∼11.5 kbar. Combined, these ages indicate that the relatively intact Barrovian metamorphic progression mapped across the Blue Ridge of Tennessee, North Carolina, and northern Georgia is solely of Ordovician (Taconic) age. Synthesis of this new data with existing geo‐ and thermochronology support a model of Barrovian metamorphism resulting from construction of a Taconic accretionary wedge and subduction complex, followed by post‐Taconic unroofing during Neoacadian and Alleghanian thrusting.
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1130/abs/2021AM-370757
