scholarly journals Linking metamorphism, magma generation, and synorogenic sedimentation to crustal thickening during Southern Appalachian mountain building, USA

Lithosphere ◽  
2019 ◽  
Vol 11 (5) ◽  
pp. 722-749
Author(s):  
H.H. Stowell ◽  
J.J. Schwartz ◽  
S.B. Ingram ◽  
J. Madden ◽  
C. Jernigan ◽  
...  
Keyword(s):  
Regional Metamorphism ◽  
Foreland Basin ◽  
Progressive Increase ◽  
Crustal Thickening ◽  
Garnet Growth ◽  
Metamorphic Rocks ◽  
Blue Ridge ◽  
Magma Generation ◽  
The North ◽  
North American Craton

Abstract The nature of metamorphism, magma compositions, the spatial distribution of plutons, and foreland sediments reflect, in part, the character and thickness of continental crust. We utilized metamorphic pressure-temperature-time (P-T-t) paths, garnet Sm-Nd ages, zircon U-Pb ages, and pluton compositions to estimate paleocrustal thickness and temporal changes in crustal magma sources in the Blue Ridge of the southernmost Appalachians. Garnet Sm-Nd ages for amphibolite-facies metamorphic rocks range from 331 ± 4 to 320 ± 3 Ma. Low- and high-Sr/Y plutons that intruded these metamorphic rocks have zircon U-Pb ages of 390 ± 1 to 365 ± 1 Ma and 349 ± 2 to 335 ± 1 Ma, respectively. Therefore, garnet growth began during regional metamorphism synchronous with or shortly after intrusion of the youngest high-Sr/Y trondhjemite plutons. Phase diagram sections and thermobarometry indicate that garnet growth initiated at ∼5.8 kbar and 540 °C and grew during temperature increases of 60–100 °C and pressure increases of 2–3 kbar. The older, low-Sr/Y magmas are inferred to have been sourced in the crust at depths <∼30 km, insufficient for garnet to be stable. However, the younger, high-Sr/Y magmas are inferred to have been sourced at >30 km depths where garnet was stable. Hafnium isotopic compositions for all the plutons, but one, exhibit a range from negative initial εHf(i) to weakly positive initial εHf(i), indicating incomplete mixing of dominantly crustal sources. Our data require minimum crustal thicknesses of ∼33 km at 331 Ma; however, Alleghanian crustal thicknesses must have locally reached 39 km, based on crustal reconstruction adding the Alleghanian thrust sheet beneath the eastern Blue Ridge. We infer the presence of hot, tectonically thickened crust during intrusion of the early Alleghanian high-Sr/Y plutons and conclude that garnet growth and plutonism reflect a progressive increase in crustal thickness and depth of magma generation. The crustal thickening was synchronous with deposition of Mississippian to early Pennsylvanian sediments in the foreland basin of the Appalachian orogen between 350 and 320 Ma. This crustal thickening may have preceded emplacement of the Alleghanian thrust sheets onto the North American craton.

Evidence for early fluid channelization, Pikwitonei granulite domain, Manitoba, Canada

1992 ◽  
Vol 29 (8) ◽  
pp. 1701-1716 ◽  
Author(s):  
Julie K. Vry ◽  
Philip E. Brown
Keyword(s):  
Carbon Isotope ◽  
Fluid Inclusions ◽  
Regional Metamorphism ◽  
Fluid Phase ◽  
Garnet Growth ◽  
High Grade ◽  
Low Oxygen ◽  
The North ◽  
Rock Types ◽  
Lake Region

The results of field mapping and carbon isotope and phase equilibria studies suggest that two different, locally controlled fluid regimes existed during at least the early phases of high-grade metamorphism in the north Cauchon Lake region, Pikwitonei granulite domain, Manitoba, Canada. During the prograde stages of high-grade "anticlockwise" regional metamorphism, rocks already metamorphosed to at least sillimanite grade were thermally metamorphosed at temperatures near 900 °C by the intrusion of a charnockitic magma. It is likely that this magma released an oxidizing, CO2-bearing, probably CO2-rich fluid phase while the region was still at relatively shallow depths. Fluid migration was channelized along the intrusive contact, and local fluid buffering characterized many of the country rocks. The light carbon isotope values of graphites (gr) and CO2 in cordierites (crd) in pelitic lithologies (δ13Cgr = −41.8 to −30.4; δ13Ccrd = −31.8 to −34.9), and the low oxygen fugacities in many samples rule out infiltration of these units by large amounts of an externally derived CO2-rich fluid phase. Texturally early CO2-rich fluid inclusions occur in the cores of garnets in a variety of rock types along the intrusive contact. These fluid inclusions were probably trapped during early garnet growth at high temperatures and relatively low pressures, and appear to have undergone limited or no subsequent reequilibration. They do not appear to provide direct information about the highest regional metamorphic temperature and pressure conditions to have affected the region (750 °C and 7 kbar (1 kbar = 100 MPa)) but may instead retain evidence of the prograde metamorphic path. These studies demonstrate the importance of local controls on the sources, compositions, timing, and transport of metamorphic fluids in the north Cauchon Lake region.

scholarly journals Large-wavelength late Miocene thrusting in the North Alpine foreland: Implications for late orogenic processes

2019 ◽  
Author(s):  
Samuel Mock ◽  
Christoph von Hagke ◽  
Fritz Schlunegger ◽  
István Dunkl ◽  
Marco Herwegh
Keyword(s):  
Late Miocene ◽  
Foreland Basin ◽  
Crustal Thickening ◽  
Tectonic Activity ◽  
Local Geometry ◽  
European Alps ◽  
The North ◽  
Nappe Tectonics ◽  
Alpine Foreland ◽  
Large Wavelength

Abstract. Additional to classical nappe tectonics, the Oligocene to mid-Miocene post-collisional evolution of the Central European Alps was characterized by vertically directed tectonics, with backthrusting along the Insubric Line and the subsequent uplift of the External Crystalline Massifs (ECMs). Thereafter, the orogen experienced axis-perpendicular growth when deformation propagated into its external parts. For the North Alpine foreland between Lake Geneva and Lake Constance, in the past, this has been kinematically and spatially linked to the uplift and exhumation of the ECMs. Based on apatite (U-Th-Sm)/He thermochronometry, we constrain thrusting in the Subalpine Molasse between 12–4 Ma, thus occurring coeval to main deformation in the Jura fold-and-thrust belt (FTB) and late stage exhumation of the ECMs. However, this pattern of tectonic activity is not restricted to areas which are bordered by ECMs, but is consistent along the northern front of the Alps between Geneva and Salzburg. Therefore, late Miocene foreland deformation is not necessarily a consequence of uplift and exhumation of the ECMs. While the local geometry of the Subalpine Molasse results from lateral variations of the mechanical stratigraphy of the foreland basin sediments, we suggest that the large-wavelength tectonic signal is the response to a shift in tectonic forces possibly caused by deep-seated geodynamic processes. This resulted in a change from dominantly vertical to horizontal tectonics and orogen-perpendicular growth of crustal thickening. We constrain the onset of this major tectonic change to ca. 12 Ma in the North Alpine foreland, resulting in thrusting and folding in the Subalpine Molasse west of Salzburg and in the Jura FTB until at least 4 Ma.

Geochemical characterization of mid-Cretaceous granitoids of the Kootenay Arc in the southern Canadian Cordillera

1993 ◽  
Vol 30 (5) ◽  
pp. 1076-1090 ◽  
Author(s):  
Alan D. Brandon ◽  
Richard StJ. Lambert
Keyword(s):  
British Columbia ◽  
Regional Metamorphism ◽  
Crustal Thickening ◽  
Precambrian Basement ◽  
Crustal Anatexis ◽  
Volcanic Arcs ◽  
North American Continent ◽  
Isotopic Compositions ◽  
The North ◽  
Host Rocks

Within southeast British Columbia, mid-Cretaceous granitoid batholiths are exposed in the Omineca Belt, a north–south-trending metamorphic and plutonic orogenic belt. The Bugaboo, Horsethief Creek, and Fry Creek batholiths are post-kinematic with respect to regional metamorphism that affected their host rocks, and are composed of hornblende and biotite granodiorites and granites in all three batholiths, and two-mica granites in Fry Creek. The biotite granites are weakly peraluminous, have initial εSr ranging from + 36 to + 56 and initial εNd ranging from −4.8 to −7.5, and overlap the range of Nd–Sr isotopic compositions for Precambrian basement gneisses and Proterozoic metasediments found in southeast British Columbia. The initial 206Pb/204Pb versus 208Pb/204Pb and 207Pb/204Pb for these granitoids lie in bands between model lower and upper crustal Pb isotopic compositions above the Northern Hemisphere Reference Line for oceanic basalts. These granitoids have high Nb and Rb and low Ba abundances relative to granites found in volcanic arcs. The two-mica granites are strongly peraluminous and have εSr ranging from + 170 to + 470, εNd ranging from −10 to−21, and more radiogenic initial Pb-isotope ratios than the biotite granites. The two-mica granites have trace element compositions similar to those of granites found in within-plate and collisional tectonic settings.We favor a model for crustal anatexis of Precambrian basement gneisses and Proterozoic metapelites to produce the mid-Cretaceous biotite granites and two-mica granites, respectively. Crustal anatexis was likely in response to crustal thickening that occurred during the Mesozoic in southeast British Columbia as exotic terranes collided with and were accreted to the western edge of the North American continent.

Contact metamorphism of the Tethyan Sedimentary Sequence, Upper Mustang region, west-central Nepal

2020 ◽  
Vol 157 (11) ◽  
pp. 1917-1932 ◽  
Author(s):  
Iva Lihter ◽  
Kyle P. Larson ◽  
Sudip Shrestha ◽  
John M. Cottle ◽  
Alex D. Brubacher
Keyword(s):  
Regional Metamorphism ◽  
Contact Metamorphism ◽  
Metamorphic Rocks ◽  
Upper Crust ◽  
Sedimentary Sequence ◽  
The North ◽  
Central Nepal ◽  
West Central ◽  
Metamorphic Temperature ◽  
Local Movement

AbstractThe Upper Mustang region of west-central Nepal contains exposures of metamorphosed Tethyan Sedimentary Sequence rocks that have been interpreted to reflect either contact metamorphism related to the nearby Mugu pluton or regional metamorphism associated with the North Himalayan domes. New monazite geochronology results show that the Mugu leucogranite crystallized at c. 21.3 Ma, while the dominant monazite age peaks from the surrounding garnet ± staurolite ± sillimanite schists range between c. 21.7 and 19.4 Ma, generally decreasing in age away from the pluton. Metamorphic temperature estimates based on Ti-in-biotite and garnet–biotite thermometry are highest in the specimens closest to the pluton (648 ± 24°C and 615 ± 25°C, respectively) and lowest in those furthest away (578 ± 24°C and 563 ± 25°C, respectively), while pressure estimates are all within uncertainty of one another, averaging 5.0 ± 0.5 kbar. These results are interpreted to be consistent with contact metamorphism of the rocks in proximity to the Mugu pluton, which was emplaced at c. 18 ± 2 km depth after local movement across the South Tibetan detachment system had ceased. While this new dataset helps to characterize the metamorphic rocks of the Tethyan Sedimentary Sequence and provides new constraints on the thickness of the upper crust, it also emphasizes the importance of careful integration of metamorphic conditions and inferred processes that may affect interpretation of currently proposed Himalayan models.

Metamorphic rocks of the 1985 Tibet Geotraverse, Lhasa to Golmud

1988 ◽  
Vol 327 (1594) ◽  
pp. 203-213 ◽  
Keyword(s):  
Tibetan Plateau ◽  
Regional Metamorphism ◽  
Metamorphic Grade ◽  
Metamorphic Rocks ◽  
The Tibetan Plateau ◽  
Crustal Anatexis ◽  
Prograde Metamorphism ◽  
Lhasa Terrane ◽  
The North ◽  
Peak Metamorphism

Two examples of uplifted basement have been studied in the Lhasa Terrane of the Tibetan Plateau. The Nyainqentanglha orthogneisses are bounded by staurolite-garnet schists to the north which record prograde metamorphism at 5 .0 ± 1.3 kbar, 610 ± 70 °C. Garnet sillimanite xenoliths within the orthogneiss suggest that peak temperatures reached at least 700 ± 70 °C at 5.1 ± 2 .5 kbar. These P / T fields reflect high T /low P metamorphism during Eocene subduction, and indicate that the syntectonic Nyainqentanglha orthogneiss was emplaced at depths greater than 10 km. Sillimanite-bearing assemblages from the Amdo gneisses in the northern Lhasa Terrane provide evidence of crustal anatexis at temperatures > 680 °C. This event is poorly constrained in time but is probably Cambrian or earlier. Within the Kunlun Terrane, biotite and garnet isograds north of the Xidatan Fault indicate an increase in metamorphic grade from north to south, reaching peak metamorphism at 470 ± 30 °C, 4 .3 ± 1.5 kbar synchronous with the emplacement of the Triassic batholith. Regional metamorphism was followed by uplift of at least 2 km before emplacement of post-tectonic, early Jurassic granites.

scholarly journals Late Miocene thrusting in the North Alpine foreland: Driven by a deep-seated process and shaped by the local mechanical stratigraphy

2019 ◽  
Author(s):  
Samuel Mock ◽  
Christoph von Hagke ◽  
Fritz Schlunegger ◽  
István Dunkl ◽  
Marco Herwegh
Keyword(s):  
Late Miocene ◽  
Large Scale ◽  
Foreland Basin ◽  
Crustal Thickening ◽  
Tectonic Activity ◽  
European Alps ◽  
Strain Partitioning ◽  
Central Alps ◽  
The North ◽  
Alpine Foreland

Abstract. Besides classical emplacement and accretion related nappe tectonics, the Oligocene to middle Miocene post-collisional evolution of the central European Alps was also characterized by pronounced vertically directed tectonics. These are expressed by backthrusting along the Insubric Line and the subsequent uplift of the External Crystalline Massifs (ECMs). During late Miocene times, the Central Alps experienced lateral growth when deformation propagated into the external parts of both the pro- and retro-side of the orogen. For the North Alpine foreland, pro-wedge propagation of deformation has been kinematically and spatially linked to the uplift and exhumation of the ECMs. In this paper, we investigate the young exhumation history of the North Alpine foreland. Based on low-temperature apatite (U-Th-Sm)/He thermochronometry, we constrain thrusting in the Subalpine Molasse between 12 Ma and 5 Ma, thus occurring coeval to the main deformation phase in the adjacent Jura fold-and-thrust belt (FTB) and late stage exhumation of the ECMs. However, this pattern of tectonic activity is not restricted to areas which are bordered by the ECMs, but is consistent along the entire front of the Central Alps, regardless of its hinterland architecture. The local-scale pattern of strain partitioning and style of deformation is governed by lateral variations in the sedimentary foreland basin architecture. We suggest that the large-scale constant tectonic signal is the response to a shift in tectonic forces caused by a deep-seated geodynamic process. This resulted in a change from dominantly vertical to orogen-scale horizontal tectonics and an associated orogen-perpendicular growth of crustal thickening. We constrain the onset of this major tectonic change to ca. 15 Ma in the Southern Alps and ca. 12 Ma in the North Alpine foreland, where it results in (re-)activation of thrusts in the Subalpine Molasse and folding in the Jura FTB.

Advances in the Geochronology of the Rice Lake – Beresford Lake Area, Southeastern Manitoba

1971 ◽  
Vol 8 (5) ◽  
pp. 572-579 ◽  
Author(s):  
Andrew Turek ◽  
Zell E. Peterman
Keyword(s):  
Greenstone Belt ◽  
Regional Metamorphism ◽  
Granitic Rocks ◽  
Metamorphic Rocks ◽  
Lake Area ◽  
The South ◽  
Quartz Veins ◽  
The North ◽  
Granite Emplacement ◽  
Second Period

The Rice Lake – Beresford Lake area consists of an easterly trending Precambrian greenstone belt which is flanked on the north by granitic rocks and on the south by granitic and metamorphic rocks. Analyses of an intrusive, late tectonic quartz monzonite at Black Lake, yield an isochron age of 2735 ± 55 m.y. with an initial 87Sr/80Sr composition 0.7019 ± 0.0008. This age is a minimum for the granitic rocks to the south of the greenstone belt and is the oldest age obtained in this area. On the other hand, analyses of a quartz diorite pluton intrusive into the greenstones give a metamorphic age of 2555 ± 70 m.y. with an initial 87Sr/86Sr of 0.7016 ± 0.0012. Mylonite zones are developed along the northern and southern boundaries of the greenstone belt. Analyses of whole-rock samples from these zones yield an age of 2345 ± 100 m.y. with initial 87Sr/88Sr 0.7044 ± 0.0024, and is the youngest age obtained in the area.These new data, combined with our previous work, indicate three major events. The first and oldest event is 2730 ± 50 m.y. and is interpreted as a period of regional metamorphism, granite emplacement, and the emplacement of gold–quartz veins which postdate the greenstones. A second period of metamorphism and granite emplacement occurred at 2530 ± 40 m.y. and it affected the area as a whole, as evidenced by updated mineral ages. The third and youngest event, 2345 ± 100 m.y., is recorded by the mylonites and may represent epeirogenic movement in the area.

Tectonism and metamorphism along a southern Appalachian transect across the Blue Ridge and Piedmont, USA

2021 ◽  
pp. 143-180
Author(s):  
Harold Stowell* ◽  
Elizabeth Bollen* ◽  
Matthew P. McKay* ◽  
J. Ryan Thigpen* ◽  
Hannah F. Dickson* ◽  
...  
Keyword(s):  
Crustal Thickening ◽  
Garnet Growth ◽  
Blue Ridge ◽  
Tectonic History ◽  
Southern Appalachian ◽  
The World ◽  
Higher Temperature ◽  
Orogenic Belts ◽  
Temperature Time ◽  
Sillimanite Schist

ABSTRACT The Appalachian Mountains expose one of the most-studied orogenic belts in the world. However, metamorphic pressure-temperature-time (P-T-t) paths for reconstructing the tectonic history are largely lacking for the southernmost end of the orogen. In this contribution, we describe select field locations in a rough transect across the orogen from Ducktown, Tennessee, to Goldville, Alabama. Metamorphic rocks from nine locations are described and analyzed in order to construct quantitative P-T-t paths, utilizing isochemical phase diagram sections and garnet Sm-Nd ages. P-T-t paths and garnet Sm-Nd ages for migmatitic garnet sillimanite schist document high-grade 460–411 Ma metamorphism extending south from Winding Stair Gap to Standing Indian in the Blue Ridge of North Carolina. In the Alabama Blue Ridge, Wedowee Group rocks were metamorphosed at biotite to staurolite zone, with only local areas of higher-temperature metamorphism. The Wedowee Group is flanked by higher-grade rocks of the Ashland Supergroup and Emuckfaw Group to the northwest and southeast, respectively. Garnet ages between ca. 357 and 319 Ma indicate that garnet growth was Neoacadian to early Alleghanian in the Blue Ridge of Alabama. The P-T-t paths for these rocks are compatible with crustal thickening during garnet growth.

scholarly journals Metamorphic Age Comparison and Its Implications between the Zuoquan and Zanhuang Complexes in the Central North China Craton, Based on LA-ICP-MS Zircon U–Pb Dating

Minerals ◽  
2019 ◽  
Vol 9 (12) ◽  
pp. 780 ◽  
Author(s):  
Ling-Ling Xiao ◽  
Min-Hua Chen
Keyword(s):  
Phase I ◽  
Phase Ii ◽  
North China Craton ◽  
North China ◽  
Regional Metamorphism ◽  
Metamorphic Rocks ◽  
The North ◽  
South Central ◽  
Icp Ms ◽  
Two Phases

The Trans-North China Orogen (TNCO) is well-known as an early Precambrian orogenic belt in the North China Craton (NCC). Three meaningful groups of metamorphic ages have been reported in the metamorphic complexes throughout the TNCO, including c. 1.85 Ga, c. 1.95 Ga, and c. 2.5 Ga. The spatial distributions and relationships of these ages provide notable insight into the formation timing and tectonic evolution of the NCC basement. The Zuoquan and Zanhuang complexes are exposed in the south–central TNCO and are adjacent to the Eastern Block. In order to place new constraints on the timing of two phases of metamorphism that occurred in the complexes, combined U–Pb and rare earth element analyses were performed on zircons from different types of metamorphic rocks. Uranium–Pb zircon dating in this study shows that two groups of metamorphic ages of 1.88–1.85 Ga and 2.48–2.46 Ga were commonly recorded by metamorphic rocks in the Zanhuang and Zuoquan complexes, respectively. Our previous geochronological studies showed that metamorphic ages of c. 2.51 Ga and c. 1.90 Ga were locally recorded in the Zanhuang and Zuoquan complexes, respectively. These data indicate that metamorphic rocks in the two complexes underwent at least two phases of metamorphism, i.e., 2.51–2.46 Ga (Phase I) and 1.90–1.85 Ga (Phase II). In combination with previous studies regarding reaction microstructures, metamorphic pressure–temperature paths, and geochronology, the Phase II metamorphic ages are interpreted to be linked to the collision between the Western and Eastern Blocks along the TNCO between 1.97 Ga and 1.80 Ga, whereas the Phase I metamorphic ages, as a result of an earlier and extensive tectono-thermal event that occurred in the Eastern and Western Blocks of the NCC, were related to underplating of mantle-derived magma. It is inferred that the rocks with c. 2.51–2.46 Ga metamorphic ages in the two complexes formed in the Eastern Block and underwent regional metamorphism during that period, and then were tectonically involved in the TNCO and experienced c. 1.90–1.85 Ga metamorphism. Metamorphic peaks occurred at different crustal levels in the orogen, resulting in distinct metamorphic ages and peak conditions preserved by metamorphic rocks in the two complexes.

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