Zircon U-Pb dating of Yushugou terrain of high-pressure granulite facies in southern Tianshan Mountain and its geological significance

2000 ◽  
Vol 19 (1) ◽  
pp. 23-28
Author(s):  
Runsan Wang ◽  
Yan Wang ◽  
Huimin Li ◽  
Dingwu Zhou ◽  
Juli Wang
Keyword(s):  
High Pressure ◽  
Granulite Facies ◽  
Tianshan Mountain ◽  
Geological Significance ◽  
Southern Tianshan ◽  
High Pressure Granulite

Discussion of ‘Metamorphic P–T and retrograde path of high-pressure Barrovian metamorphic zones near Cairn Leuchan, Caledonian orogen, Scotland’

2014 ◽  
Vol 151 (4) ◽  
pp. 758-763 ◽  
Author(s):  
K. Aoki ◽  
B. F. Windley ◽  
S. Maruyama ◽  
S. Omori
Keyword(s):  
High Pressure ◽  
Granulite Facies ◽  
Geological Structure ◽  
Metamorphic Rocks ◽  
New Paradigm ◽  
Zircon Age ◽  
Tectonic Model ◽  
High Pressure Granulite ◽  
Future Publication ◽  
Western Ireland

K. Aoki, B. F. Windley, S. Maruyama & S. Omori reply: First, we thank Viete, Oliver & Wilde for their interesting and thought-provoking comments on the timing of the high-pressure granulite facies (HGR) metamorphism recorded in metamorphic rocks at Cairn Leuchan, Scotland, published by Aoki et al. (2013). Based on new metamorphic data of garnetites and garnet-amphibolites at Cairn Leuchan and new zircon U–Pb ages of amphibolitized eclogite at Tomatin, we suggested in our publication that the HGR metamorphism was retrograde after eclogite facies before the c. 470 Ma ‘Barrovian metamorphism’. Viete, Oliver & Wilde however speculate that the HGR metamorphism at Cairn Leuchan may have occurred at c. 1000 Ma, as a result of their new U–Pb zircon age of the Cowhythe Gneiss at Portsoy and from previous studies of the geological structure and geochronology. We are grateful for this opportunity to describe, albeit in a preliminary manner, our new understanding and tectonic model of the Caledonian orogen in Scotland and western Ireland of which the Barrovian metamorphism is a key component. A reply to a comment is not the correct place to propose an entirely new paradigm for such a classic orogen, but we will present our model more fully in a future publication.

Asymmetric zoning profiles in garnet from HP-HT granulite and implications for volume and grain-boundary diffusion

1999 ◽  
Vol 63 (2) ◽  
pp. 227-238 ◽  
Author(s):  
P. J. O’Brien
Keyword(s):  
High Pressure ◽  
High Temperature ◽  
Grain Boundary ◽  
Boundary Diffusion ◽  
Sharp Decrease ◽  
Fluid Phase ◽  
Granulite Facies ◽  
Grain Boundary Diffusion ◽  
Diffusion Models ◽  
High Pressure Granulite

AbstractDetailed electron-microprobe line profiles and small-area compositional maps of zoned garnets in a sample of high-pressure-high-temperature granulite show features inconsistent with commonly applied diffusion models. Larger grains of an early garnet generation have their highest Ca contents in domains away from the rim or inclusions but show a sharp fall in Ca balanced by increased Mg and Fe (and slightly higher XMg) towards inclusions and the rim. In domains with secondary biotite, the sharp decrease in Ca is accompanied by variations in XMg dependent upon proximity to biotite, thus producing one-sided, asymmetric profiles with XMg lower against biotite. As a consequence, rim compositions of the same grain are different on the sides adjacent and away from biotite and there is no relationship between grain size and rim XMg. Such a zoning pattern requires that grain-boundary diffusion is as slow as volume diffusion and implies the absence of a diffusion-enhancing grain-boundary fluid phase during the majority of the rock's high-temperature exhumation history. Diffusion models ignoring this probability could yield either cooling rates that were too fast, or extrapolated ages based on closure temperature models that were too old.A second garnet generation in the same rock, grown in a Ca-rich domain resulting from kyanite breakdown, has irregularly distributed patches, identified by compositional mapping, containing higher Ca than the first-formed garnet but at lower XMg. Use of such garnet compositions for geothermobarometrical determination of the high-pressure granulite stage would clearly lead to erroneous results. The presence of such contrasting garnet compositions in a granulite-facies rock is clearly evidence of disequilibrium, and further supports the proposition that there was a lack of an effective transport medium even at the mm scale.

A Discussion on the evolution of the Precambrian crust - Geochemistry of granulite facies rocks and problems of their origin

1973 ◽  
Vol 273 (1235) ◽  
pp. 429-442 ◽  
Keyword(s):  
High Pressure ◽  
Continental Crust ◽  
Major Element ◽  
Granulite Facies ◽  
Sr Isotope ◽  
Average Surface ◽  
Magmatic Rocks ◽  
Mineral Associations ◽  
High Pressure Granulite ◽  
Critical Mineral

Occurrences of granulite facies rocks are widespread in continental regions where they mostly are parts of stable shield areas. Granulite facies terrains are classified as low-, medium- or high-pressure terrains on the basis of critical mineral associations. Special interest is attached to the medium- and highpressure terrains, as they are representative of the deepest crustal levels available for study in any areal extent on the surface, and may give information about the composition of the lower continental crust. Granulite facies terrains are mainly composed of metamorphic and metasomatic rocks, but magmatic rocks with primary igneous textures interpreted as formed by crystallization of magmas under granulite facies conditions are frequent in some areas. Examples of such rocks are anorthosites, gabbros and mangerites. The low-pressure—high-temperature granulite facies rocks are chemically indistinguishable from the amphibolite facies gneisses with which they characteristically occur. It is therefore important to make a distinction between these and the higher pressure types. The medium- to high-pressure granulite facies terrains are characterized by a less ‘acidic’ average major element compositions, and significant depletions in Rb, Cs, Th and U compared with average surface shield compositions. Available data also indicate low initial Sr isotope ratios, even in the gneissic types. In the author’s opinion the important problem associated with granulite facies rocks is not that of their origin, but rather of their importance as constituents of the continental crust, and how they attained their present chemistry.

Local rapid exhumation and fast cooling in a long-lived Paleoproterozoic orogeny

2020 ◽  
Author(s):  
Y Zou ◽  
x Chu ◽  
Q L Li ◽  
R N Mitchell ◽  
M G Zhai ◽  
...  
Keyword(s):  
High Pressure ◽  
Temporal Resolution ◽  
Plate Tectonics ◽  
Granulite Facies ◽  
High Grade ◽  
Promising Alternative ◽  
High Pressure Granulite ◽  
Fast Cooling ◽  
Thermal Excursion ◽  
Uht Metamorphism

Abstract The exhumation and cooling rates of high-grade metamorphic rocks are crucial for inferring orogenic processes and understanding the regimes of heat transport in Earth's crust. Quantification of these rates remains challenging for Precambrian terranes, because the temporal resolution of geochronology becomes coarser in deeper geologic time. This limitation is partly reflected by a striking lack of Proterozoic or older short-duration events (<10 Myr), most documented cases of fast metamorphism are confined to the Phanerozoic. In this study, we use garnet geospeedometry to explore the metamorphic rates of Paleoproterozoic high-grade rocks from two representative areas within the long-lived (1.95–1.80 Ga) Jiao-Liao-Ji orogenic belt, North China Craton. The pelitic granulites in the Taipingzhuang area record high-pressure granulite-facies (HPG) metamorphism of ∼12 kbar and ∼800 °C, followed by a fast decompression-cooling to ∼5 kbar and ∼600 °C within ∼5 Myr, at ca. 1.87 Ga. The pelitic granulites in the Rizhuang area document a brief (<1 Myr) thermal excursion to ultra-high-temperature (UHT) metamorphism of ∼8 kbar and ∼940 °C at ca. 1.85 Ga, followed by a fast cooling to ∼600 °C within 1–5 Myr. In light of available geological data, the fast decompression-cooling of HPG granulites is interpreted as the syn-collisional exhumation of thickened lower crustal segments at ca. 1.87 Ga, most likely through tectonic extrusion. The thermal excursion transiently reaching UHT conditions is inferred to be triggered by localized syn-metamorphic mafic intrusions in association with magmatic underplating during post-collisional extension at ca. 1.85 Ga. These metamorphic pulses were interspersed within the protracted Paleoproterozoic orogenesis and require geodynamic processes resembling modern plate tectonics. Notably, these ancient rapid events are beyond the temporal resolution of commonly-used in-situ geochronology that tends to yield apparent longer durations given errors and uncertainties. We therefore note that most ancient metamorphic rates might be underestimated using geochronological data, and recommend garnet geospeedometry as a promising alternative approach. The largely similar rates recorded by Paleoproterozoic and Phanerozoic orogens, as well as high-pressure metamorphism at 1.9–1.8 Ga, support the operation of modern plate tectonics in Paleoproterozoic time.

Extensional exhumation of a high-pressure granulite terrane in Payer Land, Greenland Caledonides: structural, petrologic, and geochronologic evidence from metapelites

2002 ◽  
Vol 39 (8) ◽  
pp. 1169-1187 ◽  
Author(s):  
Jane A Gilotti ◽  
Synnøve Elvevold
Keyword(s):  
High Pressure ◽  
Granulite Facies ◽  
Crustal Thickening ◽  
Low Grade ◽  
Granulite Facies Metamorphism ◽  
High Pressure High Temperature ◽  
High Pressure Granulite ◽  
Two Samples ◽  
Facies Metamorphism ◽  
Southern Half

The Payer Land gneiss complex is unique among the mostly amphibolite-facies, mid-crustal gneiss complexes in the East Greenland Caledonides due to its well-preserved, regional high-pressure (HP) granulite-facies metamorphism. High-pressure – high-temperature (HP–HT) assemblages are recognized in mafic, ultramafic, granitic, and metasedimentary lithologies. Anatectic metapelites contain the assemblage garnet + kyanite + K-feldspar + antiperthite (exsolved ternary feldspar) + quartz ± biotite ± rutile and record approximately the same peak metamorphic conditions (pressure (P) = 1.4–1.5 GPa, temperature (T) = 800–850°C) as those of the neighboring mafic HP granulites. The HP granulite-facies metamorphism is Caledonian based on in situ U–Th–Pb electron microprobe dating of monazite from two samples of the aluminous paragneiss. The monazites are found along garnet–kyanite phase boundaries, as inclusions in garnet and kyanite, and within small leucocratic melt pods (K-feldspar + plagioclase + kyanite ± garnet) within the HP–HT paragneisses. Mylonitic equivalents of the metapelites contain a detrital monazite age signature that suggests the Payer Land paragneisses correlate with other Mesoproterozoic metasedimentary sequences in the area. The gneisses form a metamorphic core complex that is separated from the overlying low-grade sedimentary rocks of the Neoproterozoic Eleonore Bay Supergroup by an extensional detachment. This newly recognized Payer Land detachment is part of a system of prominent extensional faults located in the southern half of the Greenland Caledonides (i.e., south of 76°N). The HP granulites preserve the deepest level of crust exposed in this southern segment of the orogen and attest to significant crustal thickening.

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