The sources of metamorphic heat during collisional orogeny: the Barrovian enigma

2019 ◽  
Vol 56 (12) ◽  
pp. 1309-1317 ◽  
Author(s):  
Paul D. Ryan ◽  
John F. Dewey
Keyword(s):  
Rapid Heating ◽  
Regional Metamorphism ◽  
Thermal Relaxation ◽  
Geothermal Gradient ◽  
Medium Temperature ◽  
Mineral Growth ◽  
Metamorphic Mineral ◽  
Lithospheric Extension ◽  
Western Ireland ◽  
Collisional Orogeny

The problem of the observed very rapid advection of heat into metamorphic thrust stacks is reviewed. Conductive models relying on the thermal relaxation of a thickened crust will not produce the observed Barrovian (medium temperature, medium pressure) assemblages within some short-lived orogens (e.g., western Ireland and Timor). Studies of the rate and timing of metamorphic mineral growth suggest that this is commonly faster than predicted by thermal relaxation. Barrovian assemblages are localised in some orogens (e.g., the Alps) but extensive in others (e.g., the Himalayas). Metamorphic mineral growth brackets deformation; consequently, slow growth is inconsistent with the rapid uplift of many orogens. Thus, no single mechanism can account for the development of Barrovian assemblages during collisional orogeny. The only mechanisms that can supply large amounts of heat for regional metamorphism quickly (<10 Myr) are: rapidly thinning the lithosphere without stretching it (e.g., by plume thermal erosion, slab drop-off, or delamination); by emplacing magma into the crust (modest deep mafic underplate and (or) very large amounts of mafic and silicic magma emplaced into the middle and upper crust); or obducting hot nappes of arc with a thin ophiolite forearc (“hot iron” mechanism). Frictional and viscous heating produces local rapid heating but not fast regional heating. Back-arc or any kind of lithospheric extension increases the geothermal gradient and heat flow but does not heat rocks up. We suggest that magmatic advection of heat-associated lithospheric thinning or “hot iron” overthrusting of an arc/ophiolite are the primary sources of heat in short-lived orogens.

Evolution of regional metamorphism during back-arc stretching and subsequent crustal shortening in the 1.9 Ga Wopmay Orogen, Canada

1987 ◽  
Vol 321 (1557) ◽  
pp. 199-218 ◽  
Keyword(s):  
Regional Metamorphism ◽  
Thermal Relaxation ◽  
Time Interval ◽  
Magmatic Arc ◽  
Mineral Growth ◽  
Metamorphic Mineral ◽  
Mineral Assemblages ◽  
Continental Magmatic Arc ◽  
Uplift And Erosion ◽  
Structural Levels

The stratigraphic units, structural elements and metamorphic mineral assemblages of a regional metamorphic culmination in the 1.9 Ga Wopmay Orogen are exposed over greater than 30 km of composite structural depth, in a series of oblique sections produced by cross folding. Regional metamorphism developed continuously in three sequential, rapidly changing thermo-tectonic régimes within an evolving continental magmatic arc. At ca . 1900 Ma, stretching of intra-arc crust resulted in the accumulation of clastic sediment and bimodal volcanic rift-fill deposits. The onset (first stage) of regional metamorphism is marked by high- T low P mineral assemblages, condensed metamorphic zonal sequences and extensive areas of high-grade gneisses devoid of associated plutons. These features are interpreted in terms of a high thermal gradient related to stretching and thinning of the continental lithosphere. Five to ten million years after stretching, following deposition of a west-facing sedimentary prism, a suite of 1896—1878 Ma plutons was emplaced into the rift and margin deposits as they underwent subhorizontal shortening and deformation during the Calderian Orogeny. Thrusted and folded syn-orogenic foredeep deposits are also intruded by the syn-tectonic plutons. At high and intermediate structural levels, syn-tectonic metamorphic mineral growth and metamorphic zonal sequences which are spatially related to the plutons, document heat advection into the deforming marginal prism and mark a second stage of regional metamorphism related to the emplacement of the plutonic bodies. Inverted mineral isograds in autochthonous Proterozoic units beneath a basal décollement record downward thermal relaxation of isotherms following east-directed Calderian transport of the deformed, thickened, and still hot marginal prism over a relatively cold basement. Derivation of multi-point P - T trajectories from post-tectonic, poikiloblastic garnets charts metamorphic mineral growth during uplift and erosion of the internal zone, documenting the third (final) stage of regional metamorphism in Wopmay Orogen. The short erosional time interval (less than 11 Ma) between tectonic thickening and the end of uplift constrains the heat required for this last metamorphic stage to be inherited from the two preceding thermo-tectonic régimes: epicontinental stretching and the emplacement of the syn-tectonic plutonic suite.

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.

Origin of corundum within anorthite megacrysts from anorthositic amphibolites, Granulite Terrane, Southern India

2020 ◽  
Vol 105 (8) ◽  
pp. 1161-1174
Author(s):  
Shreya Karmakar ◽  
Subham Mukherjee ◽  
Upama Dutta
Keyword(s):  
High Pressure ◽  
Grain Boundaries ◽  
Chemical Potential ◽  
Regional Metamorphism ◽  
Phase Relations ◽  
Southern India ◽  
Triple Junctions ◽  
Medium Temperature ◽  
Hp Metamorphism ◽  
Basic Rocks

Abstract Growth of corundum in metamorphosed anorthosites and related basic-ultra-basic rocks is an exceptional feature, and its origin remains elusive. We describe the occurrence of and offer an explanation for the genesis of corundum in anorthositic amphibolites from ~2.5 Ga old basement of the Granulite Terrane of Southern India (GTSI). The studied amphibolites from two localities, Manavadi (MvAm) and Ayyarmalai (AyAm), contain anorthite lenses (An90–99) with euhedral to elliptical outline set in a finer-grained matrix of calcic plagioclase (An85–90) and aluminous amphibole (pargasite-magnesiohastingsite). The lenses, interpreted as primary magmatic megacrysts, and the matrix are both recrystallized under static condition presumably during the regional high pressure (HP) metamorphism (~800 °C, 8–11 kbar) at ~2.45 Ga. Corundum occurs in the core of some of the recrystallized anorthite lenses (An95–99) in two modes: (1) Dominantly, it forms aggregates with magnetite (with rare inclusion of hercynite; in MvAm) or spinel (and occasionally hematite-ilmenite; in AyAm). The aggregates cut across the polygonal grain boundaries of the anorthite and contain inclusions of anorthite. (2) Corundum also occurs along the grain boundaries or at the triple junctions of the polygonal anorthite grains, where it forms euhedral tabular grains, sieved with inclusions of anorthite or forms skeletal rims around the recrystallized anorthite, such that it seems to be intergrown with anorthite. Combined petrological data and computed phase relations are consistent with growth of corundum in an open system during regional metamorphism in the presence of intergranular fluids. Two mechanisms are proposed to explain the formation of the corundum in the amphibolites: (1) corundum + magnetite/spinel aggregates formed dominantly by oxy-exsolution of pre-existing Al-Fe-Mg-(Ti)-spinel. This pre-existing spinel may be primary magmatic inclusions within the anorthite phenocrysts or could have formed due to reaction of primary magmatic inclusions of olivine with the host anorthite. Pseudosections of fO2-nH2O-T-P in the CaO–FeO–MgO–Al2O3–SiO2–H2O (CFMASH) system indicate that fO2 and H2O strongly influence the formation of corundum + amphibole from the initial magmatic assemblage of anorthite (phenocrysts) + spinel ± olivine (inclusions). (2) The corundum with anorthite presumably formed through desilification and decalcification of anorthite, as is indicated by computed phase relations in isobaric-isothermal chemical potential diagrams (µSiO2-µCaO) in parts of the CASH system. Growth of corundum in this mode is augmented by high activity of anorthite in plagioclase, high pressure, and low-to-medium temperature of metamorphism. This study thus presents a new viable mechanism for the origin of corundum in anorthositic amphibolites, and basic-ultra-basic rocks in general, which should provide new insight into lower crustal processes like high-pressure metamorphism.

Conditions and timing of metamorphism in the southern Abitibi greenstone belt, Quebec

1995 ◽  
Vol 32 (6) ◽  
pp. 787-805 ◽  
Author(s):  
W. G. Powell ◽  
D. M. Carmichael ◽  
C. J. Hodgson
Keyword(s):  
Greenstone Belt ◽  
Regional Metamorphism ◽  
Geothermal Gradient ◽  
Fault Zones ◽  
Low Grade ◽  
Metasedimentary Rocks ◽  
Abitibi Greenstone Belt ◽  
The North ◽  
Major Fault ◽  
Facies Transition

Regional metamorphism, ranging in grade from the subgreenschist-facies to the greenschist–amphibolite-facies transition, affects all Archean supracrustal rocks (>2677 Ma) in the Rouyn–Noranda area. Contact metamorphic minerals associated with the posttectonic Preissac–Lacorne batholith (2643 Ma) show no evidence of a regional retrograde event. Accordingly, the age of regional metamorphism can be bracketed between 2677 and 2643 Ma. Three reaction isograds were mapped in subgreenschist-facies metabasites, dividing the low-grade rocks into three metamorphic zones: the pumpellyite–actinolite zone, the prehnite–pumpellyite zone, and the prehnite–epidote zone. In addition, the pumpellyite–actinolite–epidote–quartz bathograd, corresponding to a pressure of approximately 200 MPa, occurs on both sides of the Porcupine–Destor fault. Low-pressure regional metamorphism is also indicated both by the occurrence of an actinolite–oligoclase zone, and the persistence of pre-regional-metamorphic andalusite. The coincidence of andalusite and the actinolite-oligoclase zone indicates that pressure was <330 MPa at the greenschist-amphibolite transition. The geothermal gradient during metamorphism was approximately 30 °C/km. Regionally, isograds dip shallowly to the north and trend subparallel to lithological and structural trends. Metamorphic minerals in metabasites define tectonic fabrics only near major fault zones and in zones of CO2 metasomatism. In biotite zone metasedimentary rocks the schistosity is defined by mica and amphibole. These textures indicate that metamorphism and fabric development were coeval. However, the actinolite–epidote isograd cuts the Porcupine–Destor fault, indicating that regional metamorphism postdates movement along this fault. The strong fabrics associated with the Porcupine–Destor and Larder Lake–Cadillac faults must have developed through a process dominated by flattening strain.

The metamorphism of the Dalradian rocks of western Ireland and its relation to tectonic setting

1987 ◽  
Vol 321 (1557) ◽  
pp. 243-270 ◽  
Keyword(s):  
Thermal Evolution ◽  
Tectonic Setting ◽  
Regional Scale ◽  
Thermal Relaxation ◽  
The South ◽  
Volcanic Arc ◽  
High Pressure Metamorphism ◽  
Almost Everywhere ◽  
Western Ireland ◽  
Single Locality

The metamorphic evolution of Dalradian rocks exposed in the NW Mayo, Ox Mountains and Connemara inliers of western Ireland is reviewed, and new data and revised calculations are presented. There is evidence at a single locality for an early episode of moderately high-pressure metamorphism with the production of crossite—epidote schists. Subsequent Barrovian-style metamorphism overprinted this almost everywhere in NW Mayo and resulted in further heating to produce chlorite—biotite and garnet zones with the garnet isograd near 400—450 °C, and a staurolite—kyanite zone for which conditions were P = 8 ± 2 kbar (1 bar = 10 5 Pa) and T = 620 ± 30 °C. In Connemara, rare staurolite—kyanite schists are now inferred to have formed at lower pressures than those in the Barrovian zones because ilmenite is stable rather than rutile. The metamorphic zones now mapped result from regional scale heating by synorogenic intrusions exposed to the south, and took place at pressures of 4-6 kbar. There is, however, strong evidence to suggest progressive uplift during this heating phase, especially in the south. It is concluded that the thermal evolution of complex areas such as these can usefully be broken down into a series of modules, each corresponding to metamorphism in a distinct tectonic setting. The sequence identified in the Irish Dalradian is: (i) recrystallization during burial in a low heat flow setting; (ii) thermal relaxation and possibly uplift; and, in Connemara only, (iii) further heating and uplift, probably in the roots of a volcanic arc.

The rapakivi granites of S Greenland—crustal melting in response to extensional tectonics and magmatic underplating

1992 ◽  
Vol 83 (1-2) ◽  
pp. 173-178 ◽  
Author(s):  
P. E. Brown ◽  
T. J. Dempster ◽  
T. N. Harrison ◽  
D. H. W. Hutton
Keyword(s):  
Regional Metamorphism ◽  
Three Dimensional ◽  
Shear Zones ◽  
Granulite Facies ◽  
Crustal Melting ◽  
Lithospheric Extension ◽  
Magmatic Underplating ◽  
Mantle Component ◽  
Juvenile Crust ◽  
Structural Levels

ABSTRACTEarly Proterozoic rapakivi intrusions in S Greenland occur as thick sheets which have ramp–flat geometry and were intruded along the median planes of active ductile extensional shear zones. These shear zones and their intrusions were linked via transfer zones in a major three-dimensional framework. At high structural levels (c. 6 km) the rapakivi intrusions developed thermal aureoles which overprint the regional assemblages, whereas at deeper levels in the regional structure they are contemporaneous with regional metamorphism. Thermobarometry on the regional and contact assemblages indicates low pressure granulite facies conditions (200–400 MPa, 650°-800°C) suggesting very high thermal gradients. The rapakivi suite and associated norites have low initial 87Sr/86Sr together with positive εNd values, indicating the involvement of predominantly young crust and/or mantle component in the generation of the igneous suite. It is considered that the voluminous norites are closely related to the mafic melts which underplated the juvenile crust to trigger the generation of the monzonitic rapakivi suite. Taken together, the data are consistent with a model of Proterozoic lithospheric extension, thinning of relatively juvenile continental crust and compression of mantle isotherms, resulting in high crustal heat flow, mafic underplating, and crustal melting with emplacement of magmas along a linked network of extensional shear zones.

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