scholarly journals Ultra-High Pressure Metamorphism and Geochronology of Garnet Clinopyroxenite in the Paleozoic Dunhuang Orogenic Belt, Northwestern China

Minerals ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 117
Author(s):  
Zhen Li ◽  
Hao Wang ◽  
Qian Zhang ◽  
Meng-Yan Shi ◽  
Jun-Sheng Lu ◽  
...  
Keyword(s):  
High Pressure ◽  
Secondary Ion Mass Spectrometry ◽  
Volcanic Rocks ◽  
Orogenic Belt ◽  
Northwestern China ◽  
Stability Field ◽  
Uhp Metamorphism ◽  
Ultra High Pressure ◽  
Metamorphic Facies ◽  
Tectonic Exhumation

Ultra-high pressure (UHP) metamorphism is recorded by garnet clinopyroxenite enclaves enclosed in an undeformed, unmetamorphosed granitic pluton, northeastern Paleozoic Dunhuang orogenic belt, northwestern China. The protoliths of the garnet clinopyroxenite might be basic or ultrabasic volcanic rocks. Three to four stages of metamorphic mineral assemblages have been found in the garnet clinopyroxenite, and clockwise metamorphic pressure–temperature (P-T) paths were retrieved, indicative of metamorphism in a subduction environment. Peak metamorphic P-T conditions (790–920 °C/28–41 kbar) of garnet clinopyroxenite suggest they experienced UHP metamorphism in the coesite- or diamond-stability field. The UHP metamorphic event is also confirmed by the occurrence of high-Al titanite enclosed in the garnet, along with at least three groups of aligned rutile lamellae exsolved from the garnet. Secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic titanite indicates that the post-peak, subsequent tectonic exhumation of the UHP rocks occurred in the Devonian period (~389–370 Ma). These data suggest that part of the Paleozoic Dunhuang orogenic belt experienced UHP metamorphism, and diverse metamorphic facies series prevailed in this Paleozoic orogen. It can be further inferred that most of the UHP rocks in this orogen remain buried.

scholarly journals First report of ultra-high pressure metamorphism in the Paleozoic Dunhuang orogenic belt (NW China): Constrains from <i>P</i>-<i>T</i> paths of garnet clinopyroxenite and SIMS U-Pb dating of titanite

2020 ◽  
Author(s):  
Zhen M. G. Li ◽  
Hao Y. C. Wang ◽  
Qian W. L. Zhang ◽  
Meng-Yan Shi ◽  
Jun-Sheng Lu ◽  
...  
Keyword(s):  
High Pressure ◽  
Northwest China ◽  
Orogenic Belt ◽  
Stability Field ◽  
Uhp Metamorphism ◽  
High Pressure Metamorphism ◽  
Ultra High Pressure ◽  
Mineral Assemblages ◽  
Metamorphic Facies ◽  
Tectonic Exhumation

Abstract. Ultra-high pressure (UHP) metamorphism is recorded by garnet clinopyroxenite enclaves enclosed in an undeformed, unmetamorphosed granitic pluton, northeastern Paleozoic Dunhuang orogenic belt, northwest China. Three to four stages of metamorphic mineral assemblages have been found in the garnet clinopyroxenite, and clockwise metamorphic pressure-temperature (P-T) paths were retrieved, indicative of metamorphism of a possible subduction environment. Peak metamorphic P-T conditions (790~920 °C/28~41 kbar) of garnet clinopyroxenite suggest that they experienced high pressure to UHP metamorphism, and the UHP metamorphism occurred in the coesite- or diamond-stability field. The UHP metamorphic event is further confirmed by the occurrence of high-Al titanite enclosed in the garnet, along with at least three groups of aligned rutile lamellae exsolved from within the garnet. SIMS U-Pb dating of metamorphic titanite indicates that the post peak, subsequent tectonic exhumation of the UHP rocks occurred in the Devonian (~ 389~370 Ma). These data suggest that part of the Paleozoic Dunhuang orogenic belt experienced UHP metamorphism, and diverse metamorphic facies series prevailed in this orogen in the Paleozoic. It can be further inferred that most of the UHP rocks of this orogen are now buried in the depth.

Ultra-high pressure aluminous titanites in carbonate-bearing eclogites at Shuanghe in Dabieshan, central China

1996 ◽  
Vol 60 (400) ◽  
pp. 461-471 ◽  
Author(s):  
D. A. Carswell ◽  
R. N. Wilson ◽  
M. Zhai
Keyword(s):  
High Pressure ◽  
Phase Equilibria ◽  
Amphibolite Facies ◽  
Central China ◽  
Stability Field ◽  
Ultra High Pressure ◽  
Metamorphic Fluids ◽  
Metamorphic Terrane ◽  
The Stability ◽  
Peak Assemblage

AbstractPetrographic features and compositions of titanites in eclogites within the ultra-high pressure metamorphic terrane in central Dabieshan are documented and phase equilibria and thermobarometric implications discussed. Carbonate-bearing eclogite pods in marble at Shuanghe contain primary metamorphic aluminous titanites, with up to 39 mol.% Ca(Al,Fe3+)FSiO4 component. These titanites formed as part of a coesite-bearing eclogite assemblage and thus provide the first direct petrographic evidence that AlFTi−1O−1 substitution extends the stability of titanite, relative to rutile plus carbonate, to pressures within the coesite stability field. However, it is emphasised that A1 and F contents of such titanites do not provide a simple thermobarometric index of P—T conditions but are constrained by the activity of fluorine, relative to CO2, in metamorphic fluids — as signalled by observations of zoning features in these titanites.These ultra-high pressure titanites show unusual breakdown features developed under more H2O-rich amphibolite-facies conditions during exhumation of these rocks. In some samples aluminous titanites have been replaced by ilmenite plus amphibole symplectites, in others by symplectitic intergrowths of secondary, lower Al and F, titanite plus plagioclase. Most other coesite-bearing eclogite samples in the central Dabieshan terrane contain peak assemblage rutile often partly replaced by grain clusters of secondary titanites with customary low Al and F contents.

Pressure–temperature–deformation paths of closely associated ultra-high-pressure (diamond-bearing) crustal and mantle rocks of the Kimi complex: implications for the tectonic history of the Rhodope Mountains, northern Greece

2006 ◽  
Vol 43 (12) ◽  
pp. 1755-1776 ◽  
Author(s):  
E Mposkos ◽  
A Krohe
Keyword(s):  
High Pressure ◽  
White Mica ◽  
Temperature Deformation ◽  
Low Grade ◽  
Uhp Metamorphism ◽  
Northern Greece ◽  
Crustal Rocks ◽  
Ultra High Pressure ◽  
T Path ◽  
Static Annealing

The ultra-high-pressure (UHP) Kimi complex (uppermost eastern Rhodope Mountains) is a tectonic mixture of crustal and mantle derived associations. Pressure–temperature (P–T) paths and microtextural and geochronological data reveal that crustal and mantle parts juxtaposed against each other at a depth corresponding to ~15 kbar (1 kbar = 100 MPa) had separate ascend histories. The crustal rocks comprise amphibolitised eclogites, orthogneisses, marbles, and migmatitic pelitic gneisses. The latter document UHP metamorphism within the dehydration-melting range of pelitic gneisses, with maximum P–T conditions of >45 kbar at ~1000 °C, as determined by diamond inclusions in garnet and rutile needle exsolutions in Na-bearing garnet. Decompression was combined with only little cooling before 15 kbar, followed by more significant cooling between 15 and 10 kbar. This P–T path probably reflects ascent of UHP rocks within a subduction channel, followed by accretion in the lower crust of a thickened wedge. Although the first ascend phase was probably rapid, the overall time span for UHP metamorphism and final exhumation may have extended over more than 70 Ma. A U–Pb sensitive high-resolution ion microprobe (SHRIMP) age on zircons of ±149 Ma was suggested to date the UHP metamorphism, whereas Rb–Sr white mica and U–Pb zircon ages from syn-shearing pegmatites of ±65 Ma constrain medium- to low-grade shearing and final exhumation of UHP rocks. Mantle parts consisting of spinel–garnet metaperidotites and garnet pyroxenites reached maximum P–T conditions in the garnet-peridotite field at T > 1200 °C and P > 25 kbar. This was associated with plastic flow and followed by severe near isothermal cooling to T < 800 °C at 15 kbar and static annealing. A garnet–clinopyroxene whole-rock Sm–Nd age from a garnet pyroxenite of ±119 Ma probably reflects the age of metamorphic mantle processes (static annealing following the high P/high T strain episode), rather than constraining the age of UHP metamorphism.

scholarly journals Dating of ultramafic rocks from the Western Alps ophiolites discloses Late Cretaceous subduction ages in the Zermatt-Saas Zone

2017 ◽  
Vol 155 (2) ◽  
pp. 298-315 ◽  
Author(s):  
GISELLA REBAY ◽  
DAVIDE ZANONI ◽  
ANTONIO LANGONE ◽  
PIETRO LUONI ◽  
MASSIMO TIEPOLO ◽  
...  
Keyword(s):  
High Pressure ◽  
Late Cretaceous ◽  
Middle Jurassic ◽  
Late Jurassic ◽  
Western Alps ◽  
Ultramafic Rocks ◽  
Uhp Metamorphism ◽  
Ultra High Pressure ◽  
Cretaceous Subduction

AbstractThe Zermatt-Saas Zone was part of the Middle to Late Jurassic Tethyan lithosphere that underwent oceanic metamorphism during Mesozoic time and subduction during Eocene time (HP to UHP metamorphism). In upper Valtournanche, serpentinite, metarodingite and eclogite record a dominant S2 foliation that developed under 2.5±0.3 GPa and 600±20°C during Alpine subduction. Serpentinites contain clinopyroxene and rare zircon porphyroclasts. Clinopyroxene porphyroclasts show fringes within S2 with similar compositions to that of grains defining S2. Zircon cores show zoning typical of magmatic growth and thin fringes parallel to the S2 foliation. These features indicate crystallization of clinopyroxene and zircon fringes during HP syn-D2 metamorphism, related to the Alpine subduction. The U–Pb zircon dates for cores and fringes reveal crystallization at 165±3.2 Ma and 65.5±5.6 Ma, respectively. The Middle Jurassic dates are in agreement with the known ages for the oceanic accretion of the Tethyan lithosphere. The Late Cretaceaous - Paleocene dates suggest that the Zermatt-Saas Zone experienced high-pressure to ultra-high-pressure (HP–UHP) metamorphism at c. 16 Ma earlier than previously reported. This result is in agreement with the evidence that in the Western Alps the continental Sesia-Lanzo Zone reached the subduction climax at least from 70 Ma and was exhumed during ongoing oceanic subduction. Our results are further evidence that the Zermatt-Saas ophiolites diachronically recorded heterogeneous HP–UHP metamorphism.

scholarly journals Decompressional equilibration of the Midsund granulite from Otrøy, Western Gneiss Region, Norway

2019 ◽  
Vol 70 (6) ◽  
pp. 471-482
Author(s):  
Johanna Holmberg ◽  
Michał Bukała ◽  
Pauline Jeanneret ◽  
Iwona Klonowska ◽  
Jarosław Majka
Keyword(s):  
High Pressure ◽  
Thermodynamic Model ◽  
Thermodynamic Modelling ◽  
Ultrahigh Pressure ◽  
Stability Field ◽  
Uhp Metamorphism ◽  
Limited Evidence ◽  
Western Gneiss Region ◽  
Widespread Phenomenon ◽  
The Stability

Abstract The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no, or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otrøy, a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsund granulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmenite and titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT system resulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths of XMg (Mg/Mg+Fe) in garnet, albite in plagioclase and XNa (Na/Na+Ca) in clinopyroxene in the stability field of melt + plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of 1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effect of equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HT at lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previously thought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR and other HP–UHP terranes worldwide.

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