scholarly journals Use of thermal modeling to assess the tectono-metamorphic history of the Lugo and Sanabria gneiss domes, Northwest Iberia

2009 ◽  
Vol 180 (3) ◽  
pp. 179-197 ◽  
Author(s):  
James E. Alcock ◽  
José R. Martínez Catalán ◽  
Ricardo Arenas ◽  
Alejandro Díez Montes
Keyword(s):  
Thermal Relaxation ◽  
Thermal Response ◽  
Crustal Thickening ◽  
Gneiss Domes ◽  
Metamorphic History ◽  
Thrust Sheets ◽  
History Of ◽  
Granite Magmatism ◽  
Geochronological Evidence ◽  
Northwest Iberia

Abstract The Lugo and Sanabria domes in Northwest Iberia have well constrained metamorphic and structural histories. Both occur in the Iberian autochthon and resulted from late-Variscan extensional collapse following crustal thickening related to the Variscan collision. The two domes developed beneath large thrust sheets, are cored by sillimanite-orthoclase anatectic gneiss, preserve evidence of a steep thermal gradient (≈ 1 °C MPa−1), and exhibit a distinct decrease in metamorphic grade to the east in the direction of nappe movement. Geochronological evidence indicates that the lower crust melted within ≈ 30 Ma of initial crustal thickening and that dome formation occurred within 50 Ma. The histories of the two domes are considered as the basis for one-dimensional finite-difference models of thermal response to changes in crustal thickness. Results from thermal models suggest that thickening was limited to the crust, provide a numeric explanation for timing and nature of granite magmatism, and indicate that high-temperature metamorphism and crustal anatexis may result directly from thermal relaxation, eliminating the need for significant mantle thermal contribution. Also, the models show that small differences in thickness of large, wedge-shaped thrust sheets can explain distinct P-T paths experienced by different limbs of the domes.

scholarly journals Monazite U–Th–Pb geochronology of the Central Metasedimentary Belt Boundary Zone (CMBbz), Grenville Province, Ontario Canada

2018 ◽  
Vol 55 (9) ◽  
pp. 1063-1078 ◽  
Author(s):  
Michelle J. Markley ◽  
Steven R. Dunn ◽  
Michael J. Jercinovic ◽  
William H. Peck ◽  
Michael L. Williams
Keyword(s):  
Shear Zone ◽  
Crustal Thickening ◽  
Boundary Zone ◽  
Grenville Province ◽  
Metamorphic History ◽  
Crustal Thinning ◽  
Tectonic Significance ◽  
History Of ◽  
Central Gneiss Belt ◽  
Scale Shear

The Central Metasedimentary Belt boundary zone (CMBbz) is a crustal-scale shear zone that juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt of the Grenville Province. Geochronological work on the timing of deformation and metamorphism in the CMBbz is ambiguous, and the questions that motivate our study are: how many episodes of shear zone activity did the CMBbz experience, and what is the tectonic significance of each episode? We present electron microprobe data from monazite (the U–Th–Pb chemical method) to directly date deformation and metamorphism recorded in five garnet–biotite gneiss samples collected from three localities of the CMBbz of Ontario (West Guilford, Fishtail Lake, and Killaloe). All three localities yield youngest monazite dates ca. 1045 Ma; most of the monazite domains that yield these dates are high-Y rims. In comparison with this common late Ottawan history, the earlier history of the three CMBbz localities is less clearly shared. The West Guilford samples have monazite grain cores that show older high-Y domains and younger low-Y domains; these cores yield a prograde early Ottawan (1100–1075 Ma) history. The Killaloe samples yield a well-defined prograde, pre- to early Shawinigan history (i.e., 1220–1160 Ma) in addition to some evidence for a second early Ottawan event. In other words, the answers to our research questions are: three events; a Shawinigan event possibly associated with crustal thickening, an Ottawan event possibly associated with another round of crustal thickening, and a late Ottawan event that resists simple interpretation in terms of metamorphic history but that coincides chronologically with crustal thinning at the base of an orogenic lid.

Formation and evolution of a subduction-related mélange: The example of the Rocca Canavese Thrust Sheets (Western Alps)

2019 ◽  
Vol 132 (3-4) ◽  
pp. 884-896 ◽  
Author(s):  
Manuel Roda ◽  
Michele Zucali ◽  
Alessandro Regorda ◽  
Maria Iole Spalla
Keyword(s):  
Numerical Model ◽  
Mantle Wedge ◽  
Western Alps ◽  
Metamorphic History ◽  
Orogenic Wedge ◽  
Thrust Sheets ◽  
History Of ◽  
Natural Data ◽  
Formation And Evolution ◽  
Subduction Wedge

Abstract In the Sesia-Lanzo Zone, Western Alps, the Rocca Canavese Thrust Sheets (RCT) subunit is characterized by a mixture of mantle- and crust-derived lithologies, such as metapelites, metagranitoids, metabasics, and serpentinized mantle slices with sizes ranging from meters to hundreds of meters. Structural and metamorphic history suggests that the RCT rocks experienced a complex evolution. In particular, two different peak conditions were obtained for the metabasics, representing different tectono-metamorphic units (TMUs), namely, D1a under eclogite facies conditions and D1b under lawsonite-blueschist-facies conditions. The two TMUs were coupled during the syn-D2 exhumation stage under epidote-blueschist-facies conditions. The different rocks and metamorphic evolutions and the abundance of serpentinites in the tectonic mixture suggest a possible subduction-related mélange origin for the RCT. To verify whether a subduction-related mélange can record tectono-metamorphic histories similar to that inferred for the RCT, we compare the pressure-temperature evolutions with the results of a 2-D numerical model of ocean-continent subduction with mantle wedge serpentinization. The predictions of the numerical model fully reproduce the two peak conditions (D1a and D1b) and the successive exhumation history of the two TMUs within the subduction wedge. The degree of mixing estimated from field data is consistent with that predicted by the numerical simulation. Finally, the present-day location of the RCT, which marks the boundary between the orogenic wedge (Penninic and Austroalpine domains) and the southern hinterland (Southalpine domain) of the Alpine chain, is reproduced by the model at the end of the exhumation in the subduction wedge. Therefore, the comparison between natural data and the model results confirms the interpretation of the RCT as a subduction-related mélange that occurred during exhumation within a serpentinized mantle wedge.

Pressure—temperature—time ( P — T — t ) histories of erogenic belts

1987 ◽  
Vol 321 (1557) ◽  
pp. 27-45 ◽  
Keyword(s):  
Thermal Relaxation ◽  
Granite Gneiss ◽  
Crustal Thickening ◽  
Crustal Extension ◽  
Gneiss Domes ◽  
Short Time Span ◽  
Differential Uplift ◽  
Metamorphic Terranes ◽  
Short Time ◽  
Temperature Time

Thermal modelling shows that a cycle of crustal thickening and erosion reproduces many of the characteristics of medium-pressure metamorphic terranes. In contrast, the structural and metamorphic features of high-pressure terranes suggest rapid exhumation, possibly tectonically as fault-bounded blocks. Low-pressure metamorphism requires an augmented heat supply. Such terranes are characterized by granite—gneiss domes, and evidence of crustal extension, and hence may be the result of the mechanically likely orogenic sequence of early thickening followed by extension. Whether earlier isograd sequences are extended, condensed, or reset depends upon the relative rates of deformation and thermal relaxation, and when the deformation occurs relative to the thermal peak of metamorphism. Detailed determinations of relations between deformation events and metamorphism is made difficult by the contrast between continuous metamorphic evolution and short time-span deformation events. Combined microstructural and geochronological studies, together with a consideration of the distribution of isograds will give most information on complex, polymetamorphic histories, and allow distinction between regional and local features, especially those due to differential uplift.

Evolution of the granulites and sub-division of the granulite facies in Ceylon

1972 ◽  
Vol 109 (5) ◽  
pp. 435-443 ◽  
Author(s):  
D. J. A. C. Hapuarachchi
Keyword(s):  
Granulite Facies ◽  
Metamorphic History ◽  
History Of ◽  
Division Ii ◽  
Geochronological Evidence

SummaryThree subfacies of the granulite facies are recognized: garnet–diopside–quartz subfacies; pyroxene–granulite subfacies; and hornblende–granulite subfacies The erection of a garnet–diopside–quartz subfacies follows De Waard's suggestion (1965) of a garnet–clinopyroxene subfacies. Two major divisions of the hornblende–granulite subfacies are now recognized, (i) garnet–biotite division, (ii) cordierite division. Three sub-divisions of the former are suggested on the basis of critical basic assemblages. Since wollastonite has been found to have a limited areal occurrence within the latter, a two-fold sub-division of the cordierite division is suggested on the basis of the assemblages, (a) calcite–quartz, (b) wollastonite. A seven-fold sub-division of the granulite facies is thus proposed for Ceylon rocks. An attempt is made to trace briefly the major events in the metamorphic history of the rocks supported by available geochronological evidence.

The role of excess oxygen for modeling high-Mn, low-Ca garnets in metapelites from the northern Central Metasedimentary Belt of the Grenville Province, Ontario, Canada

2021 ◽  
Vol 58 (1) ◽  
pp. 21-37
Author(s):  
Erik Duesterhoeft ◽  
Peter Raase ◽  
Manuel Duguet ◽  
Robert Michael Easton
Keyword(s):  
Field Gradient ◽  
Crustal Thickening ◽  
Excess Oxygen ◽  
Rock Composition ◽  
Grenville Province ◽  
Metamorphic History ◽  
Direct Pressure ◽  
Thrust Zone ◽  
History Of

The Bancroft terrane and the associated Central Metasedimentary Belt boundary thrust zone represent the northern part of the Central Metasedimentary Belt (CMB) of the Canadian Grenville Province. Only a few direct pressure and temperature calculations based on phase equilibrium petrology methods exist in the central Bancroft terrane, and this study applies thermodynamic approaches such as garnet isopleth geothermobarometry to fill this gap and investigate the metamorphic history of the northern CMB. Four metapelitic rock samples were collected in the vicinity of the enigmatic Bancroft shear zone, which approximates the border between the Bancroft terrane and the Elzevir terrane to the south. Garnet isopleths for these samples only intersect if a certain amount of excess oxygen is added to the bulk rock composition corresponding to a Fe3+/Fetot ratio of 0.33–0.38. The northernmost sample records metamorphic peak conditions of approximately 1 GPa and 780 °C, whereas the southernmost sample, which is located in the Elzevir terrane, records a peak metamorphic pressure of approximately 0.9 GPa at a temperature of 520 °C. The latter result contradicts previous pressure estimates of the region and the proposed metamorphic field gradient but is based on a poorly constrained sample in terms of thermodynamic modeling. Hence, we conclude that the metamorphic field gradient in the northern CMB conceals two different P–T trajectories. Such a scenario is commonly observed in crustal thickening models and suggests that the cold upper plate (Elzevir terrane) was thrust over the warm lower plate (Bancroft terrane) in a northwesterly direction.

scholarly journals Blueschist mylonitic zones accommodating syn-subduction exhumation of deeply buried continental crust: the example of the Rocca Canavese Thrust Sheets Unit (Sesia–Lanzo Zone, Italian Western Alps)

2021 ◽  
Vol 114 (1) ◽  
Author(s):  
Manuel Roda ◽  
Michele Zucali ◽  
Luca Corti ◽  
Roberto Visalli ◽  
Gaetano Ortolano ◽  
...  
Keyword(s):  
Microstructural Analysis ◽  
Oceanic Lithosphere ◽  
Western Alps ◽  
White Mica ◽  
X Ray ◽  
Metamorphic History ◽  
Thrust Sheets ◽  
History Of ◽  
Tectonic Contact ◽  
Elemental Maps

AbstractThe Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of the Sesia–Lanzo Zone (SLZ), bounded by the Periadriatic (Canavese) Lineament that separates the Alpine subduction complex from the Southalpine domain. The RCTU is limited to the south by the Lanzo Massif (LM) and to the east by the Eclogitic Micaschists Complex (EMC). Particularly the tectonic contact area of the RCTU, adjacent to the neighbouring SLZ and the LM is characterised by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ) that was active under blueschist-to greenschist-facies conditions. Despite the dominant mylonitic structure, some rocks (garnet-bearing gneiss, garnet-free gneiss and orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. The scarcity of superposed structures and the small size of relicts impose a detailed microstructural analysis supported by chemical investigation to reconstruct the tectono-metamorphic history of the MZ. Therefore, we integrated the classical meso- and microstructural analysis approach with a novel quantitative technique based on the Quantitative X-Ray Map Analyzer (Q-XRMA), used to classify rock-forming minerals starting from an array of X-ray elemental maps, both at whole thin section and micro-domain scale, as well as to calibrate the maps for pixel-based chemical analysis and end-member component maps, relevant for a more robust conventional geothermobarometer application as well for calculating reliable PT pseudosections. Pre-Alpine relicts are garnet and white mica porphyroclasts in the garnet-bearing gneiss and biotite and K-feldspar porphyroclasts in garnet-free gneiss and orthogneiss, respectively, providing no PT constraints. The Alpine evolution of the MZ rocks, has been subdivided in three deformation and metamorphic stages. The first Alpine structural and metamorphic equilibration stage (D1 event) occurred at a pressure of ca. 1.25–1.4 GPa and at a temperature of ca. 420–510 °C, i.e. under blueschist-facies conditions. The D2 event, characterised by a mylonitic foliation that is pervasive in the MZ, occurred at ca. 0.95–1.1 GPa and ca. 380–500 °C, i.e. under epidote-blueschist-facies conditions. The D2 PT conditions in the MZ rocks are similar to those predicted for the blocks that constitute the RCTU mélange, and they overlap with the exhumation paths of the EMC and LM units. Therefore, the RCTU, EMC and LM rocks became coupled together during the D2 event. This coupling occurred during the exhumation of the different tectono-metamorphic units belonging to both continental and oceanic lithosphere and under a relatively cold thermal regime, typical for an active oceanic subduction zone, pre-dating Alpine continental collision.

Timing of fluorite mineralization and exhumation events in the east Central Alborz Mountains, northern Iran: constraints from fluorite (U–Th)/He thermochronometry

2021 ◽  
pp. 1-17
Author(s):  
Behnam Shafiei Bafti ◽  
István Dunkl ◽  
Saeed Madanipour
Keyword(s):  
Thermal Relaxation ◽  
Geothermal Gradient ◽  
Source Rocks ◽  
Lower Amount ◽  
Mining District ◽  
Northern Iran ◽  
East Central ◽  
Central Alborz ◽  
Ore Mineralization ◽  
History Of

Abstract The recently developed fluorite (U–Th)/He thermochronology (FHe) technique was applied to date fluorite mineralization and elucidate the exhumation history of the Mazandaran Fluorspar Mining District (MFMD) located in the east Central Alborz Mountains, Iran. A total of 32 fluorite single-crystal samples from four Middle Triassic carbonate-hosted fluorite deposits were dated. The presented FHe ages range between c. 85 Ma (age of fluorite mineralization) and c. 20 Ma (erosional cooling during the exhumation of the Alborz Mountains). The Late Cretaceous FHe ages (i.e. 84.5 ± 3.6, 78.8 ± 4.4 and 72.3 ± 3.5 Ma) are interpreted as the age of mineralization and confirm an epigenetic origin for ore mineralization in the MFMD, likely a result of prolonged hydrothermal circulation of basinal brines through potential source rocks. Most FHe ages scatter around the Eocene Epoch (55.4 ± 3.9 to 33.1 ± 1.7 Ma), recording an important cooling event after heating by regional magmatism in an extensional tectonic regime. Cooling of the heated fluorites, as a result of thermal relaxation in response to geothermal gradient re-equilibration after the end of magmatism, or exhumation cooling during extensional tectonics characterized by lower amount of erosion are most probably the causes of the recorded Eocene FHe cooling ages. Oligocene–Miocene FHe ages (i.e. 27.6 ± 1.4 to 19.5 ± 1.1 Ma) are related to the accelerated uplift of the whole Alborz Mountains, possibly as a result of the initial collision between the Afro-Arabian and Eurasian plates further to the south.

Tectono-metamorphic history of the ophiolitic Lento unit (northern Corsica): evidences for the complexity of accretion-exhumation processes in a fossil subduction system

2007 ◽  
Vol 20 (1-2) ◽  
pp. 99-118 ◽  
Author(s):  
Nicola Levi ◽  
Alessandro Malasoma ◽  
Michele Marroni ◽  
Luca Pandolfi ◽  
Matteo Paperini
Keyword(s):  
Metamorphic History ◽  
History Of ◽  
Subduction System
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