scholarly journals An anticlockwise metamorphic P-T path and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

2018 ◽  
Author(s):  
Carly Faber ◽  
Holger Stünitz ◽  
Deta Gasser ◽  
Petr Jeřábek ◽  
Katrin Kraus ◽  
...  
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Large Scale ◽  
Structural Data ◽  
Geothermal Gradient ◽  
Continental Collision ◽  
Crustal Thickening ◽  
Northern Norway ◽  
Nappe Complex ◽  
T Path

Abstract. This study investigates the Caledonian metamorphic and tectonic evolution in northern Norway, examining the structure and tectonostratigraphy of the Reisa Nappe Complex (RNC; from bottom to top, Vaddas, Kåfjord and Nordmannvik nappes). Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and P-T conditions of deformation and metamorphism that formed the nappes and facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating followed by thrusting. An early Caledonian S1 foliation in the Nordmannvik Nappe records kyanite-grade partial melting at ~ 760–790 °C and ~ 9.4–11 kbar. Leucosomes formed at 439 ± 2 Ma (U-Pb zircon) in fold axial planes in the Nordmannvik Nappe indicate that compressional deformation initiated while the rocks were still partially molten. This stage was followed by pervasive solid-state shearing as the rocks cooled and solidified, forming the S2 foliation at 680–730 °C and 9.5–10.9 kbar. Multistage titanite growth in the Nordmannvik Nappe records this extended metamorphism between 444 and 427 Ma. In the underlying Kåfjord Nappe, garnet cores record lower P-T (590–610 °C and 5.5–6.8 kbar) but a similar geothermal gradient as the S1 migmatitic event in the Nordmannvik Nappe, indicating formation at a higher relative position in the crust. S2 shearing in the Kåfjord Nappe occurred at 580–605 °C and 9.2–10.1 kbar, indicating a considerable pressure increase during nappe stacking. Gabbro intruded in the Vaddas Nappe at 439 ± 1 Ma, synchronously with migmatization in the Nordmannvik Nappe. In the Vaddas Nappe S2 shearing occurred at 630–640 ºC and 11.7–13 kbar. Titanite growth along the lower RNC boundary records S2-shearing at 432 ± 6 Ma. It emerges that early Silurian heating (~ 440 Ma), probably resulting from large-scale magma underplating, initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual nappe units. This tectonic style contrasts subduction of mechanically strong continental crust to great depths.

scholarly journals Anticlockwise metamorphic pressure–temperature paths and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

Solid Earth ◽  
2019 ◽  
Vol 10 (1) ◽  
pp. 117-148 ◽  
Author(s):  
Carly Faber ◽  
Holger Stünitz ◽  
Deta Gasser ◽  
Petr Jeřábek ◽  
Katrin Kraus ◽  
...  
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Large Scale ◽  
Structural Data ◽  
Continental Collision ◽  
Crustal Thickening ◽  
Western Gneiss Region ◽  
Nappe Complex ◽  
T Path ◽  
Tectonic Style

Abstract. This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kåfjord, and Nordmannvik nappes) in northern Troms, Norway. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure–temperature (P–T) conditions of deformation and metamorphism during nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P–T path attributed to the effects of early Silurian heating (D1) followed by thrusting (D2). At ca. 439 Ma during D1 the Nordmannvik Nappe reached the highest metamorphic conditions at ca. 780 ∘C and ∼9–11 kbar inducing kyanite-grade partial melting. At the same time the Kåfjord Nappe was at higher, colder, levels of the crust ca. 600 ∘C, 6–7 kbar and the Vaddas Nappe was intruded by gabbro at > 650 ∘C and ca. 6–9 kbar. The subsequent D2 shearing occurred at increasing pressure and decreasing temperatures ca. 700 ∘C and 9–11 kbar in the partially molten Nordmannvik Nappe, ca. 600 ∘C and 9–10 kbar in the Kåfjord Nappe, and ca. 640 ∘C and 12–13 kbar in the Vaddas Nappe. Multistage titanite growth in the Nordmannvik Nappe records this evolution through D1 and D2 between ca. 440 and 427 Ma, while titanite growth along the lower RNC boundary records D2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.

Origin of syn-collisional granitoids in the Gangdese orogen: Reworking of the juvenile arc crust and the ancient continental crust

2021 ◽  
Author(s):  
Yu-Wei Tang ◽  
Long Chen ◽  
Zi-Fu Zhao ◽  
Yong-Fei Zheng
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Continental Collision ◽  
Late Eocene ◽  
Igneous Rocks ◽  
Early Eocene ◽  
Southern Tibet ◽  
Crustal Rocks ◽  
Mafic Magmas ◽  
T Values

Granitoids at convergent plate boundaries can be produced either by partial melting of crustal rocks (either continental or oceanic) or by fractional crystallization of mantle-derived mafic magmas. Whereas granitoid formation through partial melting of the continental crust results in reworking of the pre-existing continental crust, granitoid formation through either partial melting of the oceanic crust or fractional crystallization of the mafic magmas leads to growth of the continental crust. This category is primarily based on the radiogenic Nd isotope compositions of crustal rocks; positive εNd(t) values indicate juvenile crust whereas negative εNd(t) values indicate ancient crust. Positive εNd(t) values are common for syn-collisional granitoids in southern Tibet, which leads to the hypothesis that continental collision zones are important sites for the net growth of continental crust. This hypothesis is examined through an integrated study of in situ zircon U-Pb ages and Hf isotopes, whole-rock major trace elements, and Sr-Nd-Hf isotopes as well as mineral O isotopes for felsic igneous rocks of Eocene ages from the Gangdese orogen in southern Tibet. The results show that these rocks can be divided into two groups according to their emplacement ages and geochemical features. The first group is less granitic with lower SiO2 contents of 59.82−64.41 wt%, and it was emplaced at 50−48 Ma in the early Eocene. The second group is more granitic with higher SiO2 contents of 63.93−68.81 wt%, and it was emplaced at 42 Ma in the late Eocene. The early Eocene granitoids exhibit relatively depleted whole-rock Sr-Nd-Hf isotope compositions with low (87Sr/86Sr)i ratios of 0.7044−0.7048, positive εNd(t) values of 0.6−3.9, εHf(t) values of 6.5−10.5, zircon εHf(t) values of 1.6−12.1, and zircon δ18O values of 5.28−6.26‰. These isotopic characteristics are quite similar to those of Late Cretaceous mafic arc igneous rocks in the Gangdese orogen, which indicates their derivation from partial melting of the juvenile mafic arc crust. In comparison, the late Eocene granitoids have relatively lower MgO, Fe2O3, Al2O3, and heavy rare earth element (HREE) contents but higher K2O, Rb, Sr, Th, U, Pb contents, Sr/Y, and (La/Yb)N ratios. They also exhibit more enriched whole-rock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7070−0.7085, negative εNd(t) values of −5.2 to −3.9 and neutral εHf(t) values of 0.9−2.3, and relatively lower zircon εHf(t) values of −2.8−8.0 and slightly higher zircon δ18O values of 6.25−6.68‰. An integrated interpretation of these geochemical features is that both the juvenile arc crust and the ancient continental crust partially melted to produce the late Eocene granitoids. In this regard, the compositional evolution of syn-collisional granitoids from the early to late Eocene indicates a temporal change of their magma sources from the complete juvenile arc crust to a mixture of the juvenile and ancient crust. In either case, the syn-collisional granitoids in the Gangdese orogen are the reworking products of the pre-existing continental crust. Therefore, they do not contribute to crustal growth in the continental collision zone.

scholarly journals Th–U–total Pb monazite geochronology records Ordovician (444 Ma) metamorphism/partial melting and Silurian (419 Ma) thrusting in the Kåfjord Nappe, Norwegian Arctic Caledonides

2019 ◽  
Vol 70 (6) ◽  
pp. 494-511 ◽  
Author(s):  
Grzegorz Ziemniak ◽  
Karolina Kośmińska ◽  
Igor Petrík ◽  
Marian Janák ◽  
Katarzyna Walczak ◽  
...  
Keyword(s):  
Partial Melting ◽  
White Mica ◽  
Stability Field ◽  
Monazite Dating ◽  
Nappe Complex ◽  
Migmatitic Gneiss ◽  
T Path ◽  
The Stability ◽  
Definition Of ◽  
Nappe Emplacement

Abstract The northern extent of the Scandinavian Caledonides includes the Skibotn Nappe Complex of still debated structural position. This paper is focused on part of this complex and presents new U–Th–total Pb monazite dating results for the migmatitic gneiss of the Kåfjord Nappe. The rocks show mineral assemblage of garnet + plagioclase + biotite + white mica + kyanite + rutile ± K-feldspar ± sillimanite. Thermodynamic modelling suggests that garnet was stable at P–T conditions of ca. 680–720 °C and 8–10 kbars in the stability field of kyanite and the rocks underwent partial melting during exhumation following a clockwise P–T path. This episode is dated to 444 ± 12 Ma using chemical Th–U–total Pb dating of the Y-depleted monazite core. Second episode highlighted by growth of secondary white mica resulted from subsequent overprint in amphibolite and greenschist facies. Fluid assisted growth of the Y-enriched monazite rim at 419 ± 8 Ma marks the timing of the nappe emplacement. Age of migmatization and thrusting in the Kåfjord Nappe is similar to the Kalak Nappe Complex, and other units of the Middle Allochthon to the south. Nevertheless, the obtained results do not allow for unambiguous definition of the tectonostratigraphic position of the Skibotn Nappe Complex.

Variscan crustal thickening in the Maures-Tanneron massif (South Variscan belt, France): new in situ monazite U-Th-Pb chemical dating of high-grade rocks

2015 ◽  
Vol 186 (2-3) ◽  
pp. 145-169 ◽  
Author(s):  
Emilien Oliot ◽  
Jérémie Melleton ◽  
Julie Schneider ◽  
Michel Corsini ◽  
Véronique Gardien ◽  
...  
Keyword(s):  
Partial Melting ◽  
Large Scale ◽  
Crustal Thickening ◽  
Variscan Orogeny ◽  
High Grade ◽  
Variscan Belt ◽  
Massif Central ◽  
Internal Zone ◽  
Chemical Dating

AbstractAge constraints on the protoliths, deformation, metamorphism and melting events are key parameters when correlating different continental lithospheric remnants among each other and disentangling their evolution within large-scale orogens. In situ U-Th-Pb chemical dating on monazites using Electron Probe Micro-Analyser (EPMA) has been performed on eight samples throughout the Variscan Maures-Tanneron massif (SE France) in order to date the medium to high-tectonothermal events related to the Variscan orogeny.Results indicate a polyphased crustal evolution : (i) U-Th-Pb ages obtained in polygenetic monazite grain cores gave inherited Upper Ordovician (456 ± 11 Ma) age, highlighting the large scale occurrence of the Ordovician magmatic activity in the North Gondwanian margin. An Early Devonian (404 ± 10 Ma) age may date a protolith emplacement related to calc-alkaline supra-subduction magmatism or could be associated to an early medium-grade metamorphism, prior to collisional stage. (ii) The crustal thickening stage has been further recorded in prograde metamorphic monazites formed during the underthrusting and subsequent nappe stacking events, under amphibolite facies conditions. This stage is dated between 382 ± 11 (Middle Devonian) and 331 ± 5 Ma (Late Visean). (iii) An orogenic partial melting event took place during Middle Carboniferous and is accompanied by the crystallization of crustal peraluminous magmas (Plan-de-la-Tour granite, 329 ± 3 Ma).This contribution demonstrates the capacity of monazite to record the prograde path of rocks during increasing metamorphic conditions related to stages of crustal thickening, and the robustness of the U-Th-Pb chronometer in monazite despite the overprinting of high-grade thermal events, including partial melting. The age ranges of the different orogenic stages reported in this study are in good agreement with those reported in adjacent Variscan Corsica and Sardinia; while correlations with other nearest Variscan massifs like the Argentera massif in the southwestern Alps or the French Massif Central remain more hypothetic. The Internal Zone of the Maures-Tanneron massif, and more widely the Internal Zone of the Maures-Tanneron-Corsica-Sardinia segment, is part of the southern orogenic root system of the Variscan belt.

Alpine high-pressure metamorphism at the Almyropotamos window (southern Evia, Greece)

2000 ◽  
Vol 137 (4) ◽  
pp. 367-380 ◽  
Author(s):  
YONATHAN SHAKED ◽  
DOV AVIGAD ◽  
ZVI GARFUNKEL
Keyword(s):  
High Pressure ◽  
Large Scale ◽  
Structural Data ◽  
Late Eocene ◽  
Crustal Thickening ◽  
Lower Grade ◽  
The South ◽  
Northern Greece ◽  
High Pressure Metamorphism ◽  
Early Oligocene

The Alpine orogenic belt of the Hellenides has been strongly reworked by ductile and brittle extensional tectonics. Extensional structures have affected the central Aegean region and obliterated much of the original orogenic architecture since at least early Miocene times. In the area of Almyropotamos (on the island of Evia, flanking the western part of the Aegean) a unique remnant compressional nappe stack involving Tertiary metamorphic rocks has been preserved. This nappe sequence comprises a high-pressure rock unit on top of a lower grade unit. The upper unit (South Evia Blueschist Belt) is thought to be the westward continuation of the Cycladic blueschist belt metamorphosed at high-pressure conditions during Late Cretaceous–Eocene times. The underlying unit (the Almyropotamos Unit) is a continental margin sequence covered by a flysch and containing Lutetian nummulites, indicating that this unit accumulated sediments until at least late Eocene times.In the present study we analyse the petrology of the Almyropotamos nappe stack and define the P–T conditions of each of the different rock units exposed there. The presence of glaucophane, lawsonite rimmed by epidote, and jadeite (70 mol.%) suggest that peak P–T conditions in the South Evia Blueschist Belt reached approximately 10–12 kbar and 350–450 °C. Unlike previous studies, which estimated that the underlying Almyropotamos Unit reached only greenschist-facies conditions, glaucophane relics and Si-rich phengites were found by us in this unit. These indicate that high-pressure metamorphism and crustal thickening in this part of the Aegean lasted until at least the late Eocene or early Oligocene. We note that in this respect the architecture of southern Evia resembles that of northern Greece (Olympos, Ossa). Our structural data indicate that rock units in the Almyropotamos area record different folding phases, with the South Evia Blueschist Belt having a more complex fold history than the underlying Almyropotamos Unit. The entire nappe stack shares large-scale folds which are E–W trending, and locally overturned-to-the-south, and which may represent (at present coordinates) N–S contraction and nappe transport.

Active continental deformation and regional metamorphism

1987 ◽  
Vol 321 (1557) ◽  
pp. 47-66 ◽  
Keyword(s):  
Continental Crust ◽  
Large Scale ◽  
Regional Metamorphism ◽  
Focal Depth ◽  
Rheological Behaviour ◽  
Crustal Thickening ◽  
Upper Crust ◽  
Length Scales ◽  
Vertical Movements ◽  
Continental Deformation

The vertical and horizontal distribution of present-day continental deformation is examined to see how tectonic movements may be related to large wavelength perturbations to the temperature and pressure experienced by rocks in the crust. Earthquakes are generally restricted to the upper part of the continental crust. The lower crust is usually aseismic and assumed to be weaker. The uppermost mantle beneath continental regions has minor seismic activity that does not account for much deformation, but probably indicates an important strength contrast between the lower continental crust and the upper mantle. The maximum focal depth of earthquakes in any region appears to be limited by temperature, with most restricted to material colder than 350± 100 °C in the crust and colder than 700± 100 °C in the mantle. At length scales long compared with the thickness of the brittle upper crust, the deformation in regions of continental extension or shortening appears to be continuous, even though, in reality, discontinuous movement on faults occurs. This probably indicates that the deformation is dominated by distributed flow in the ductile portion of the lithosphere and not by the behaviour of the thin brittle upper crust. The distribution of seismicity, elevation contrasts and vertical movements at the surface suggests that there is little spatial separation between the brittle deformation in the upper crust and the ductile deformation below on length scales larger than the lithosphere thickness. For this reason, and because of the short thermal time constant of the crust, long-wavelength perturbations to the thermal regime are more influenced by the behaviour of the lithosphere as a whole than by the precise geometry of deformation in the crust. Large-scale regional metamorphism in zones of shortening may result from the re-establishment of the initial geotherm in thickened crust when the lower part of the lithosphere detaches and falls into the asthenosphere. In regions of extension, an increased geothermal gradient is unlikely to result in regional metamorphism unless magmatic augmentation to the heat supply is important. However, if the stretched region is covered by thick sediments, the basement may experience a small increase in temperature and remain significantly hotter than it would be if there were no sediment cover. While unlikely to account for significant metamorphism, this effect may strongly influence the rheological behaviour of the lithosphere in extending regions. The rapid vertical movements associated with syn- or post-orogenic normal faulting in regions of large-scale crustal thickening are probably at least as important in exhuming mid-crustal metamorphosed rocks, and in disrupting patterns of isograds, as those associated with erosion.

scholarly journals Eclogitic metamorphism in the Alpine far-west: petrological constraints on the Banchetta-Rognosa tectonic unit (Val Troncea, Western Alps)

2021 ◽  
Vol 114 (1) ◽  
Author(s):  
Alberto Corno ◽  
Chiara Groppo ◽  
Pietro Mosca ◽  
Alessandro Borghi ◽  
Marco Gattiglio
Keyword(s):  
Geothermal Gradient ◽  
Continental Collision ◽  
Western Alps ◽  
Tectonic Unit ◽  
Equilibrium Modeling ◽  
Sedimentary Sequences ◽  
Significant Temperature ◽  
The Alps ◽  
T Path ◽  
Oceanic Domain

AbstractThe Banchetta-Rognosa tectonic unit (BRU), covering an area of 10 km2 in the upper Chisone valley, consists of two successions referred to a continental margin (Monte Banchetta succession) and a proximal oceanic domain (Punta Rognosa succession) respectively. In both successions, Mesozoic meta-sedimentary covers discordantly lie on their basement. This paper presents new data on the lithostratigraphy and the metamorphic evolution of the continental basement of the Monte Banchetta succession. It comprises two meta-sedimentary sequences with minor meta-intrusive bodies preserving their original lithostratigraphic configuration, despite the intense Alpine deformation and metamorphic re-equilibration. Phase equilibrium modeling points to a metamorphic eclogitic peak (D1 event) of 20–23 kbar and 440–500 °C, consistent among three different samples, analyzed from suitable lithologies. The exhumation P–T path is characterized by a first decompression of at least 10 kbar, leading to the development of the main regional foliation (i.e. tectono-metamorphic event D2). The subsequent exhumation stage (D3 event) is marked by a further decompression of almost 7–8 kbar associated with a significant temperature decrease (cooling down to 350–400 °C), implying a geothermal gradient compatible with a continental collision regime. These data infer for this unit higher peak P–T conditions than previously estimated with conventional thermobarometry. The comparison of our results with the peak P–T conditions registered by other neighboring tectonic units allows to interpret the BRU as one of the westernmost eclogite-facies unit in the Alps.

INFLUENCE OF PULSED MAGMATIC ACTIVITY, LATENT HEAT, AND PARTIAL MELTING ON THE STRENGTH OF THE CONTINENTAL CRUST

2020 ◽  
Author(s):  
Aleksi Rantanen ◽  
◽  
David Whipp ◽  
Jussi S. Heinonen ◽  
Lars Kaislaniemi ◽  
...  
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Latent Heat ◽  
Magmatic Activity
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