Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen, southern Central Asian Orogenic Belt: Constraints from P-T path and zircon U-Pb dating

2021 ◽  
Author(s):  
Wenbin Kang ◽  
Wei Li
Keyword(s):  
Continental Crust ◽  
Garnet Amphibolite ◽  
Central Asian ◽  
Mineral Assemblages ◽  
Southern Central ◽  
Isothermal Decompression ◽  
T Path ◽  
Thinning Process ◽  
Temperature Time ◽  
Beishan Area

<p>Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen, which connects the Tianshan Orogen to the west and the Mongolia-Xing’anling Orogen to the east. According to the microstructures, mineral relationships, and geothermobarometry, four stages of mineral assemblages have been identified as follows: (1) a pre-peak stage, which is recorded by the cores of garnet together with core-inclusions of plagioclase (Pl<sub>1</sub>); (2) a peak stage, which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase (Pl<sub>2</sub>) + amphibole (Amp<sub>1</sub>) + Ilmenite (Ilm<sub>1</sub>) + biotite (Bt<sub>1</sub>), developed at temperature-pressure (P-T) conditions of 818.9–836.5 °C and 7.3–9.2 kbar; (3) a retrograde stage, which is recorded by garnet rims + plagioclase (Pl<sub>3</sub>) + amphibole (Amp<sub>2</sub>) + orthopyroxene (Opx<sub>1</sub>) + biotite (Bt<sub>2</sub>) + Ilmenite (Ilm<sub>2</sub>), developed at P-T conditions of 796.1–836.9 °C and 5.6–7.5 kbar; (4) a symplectitic stage, which is recorded by plagioclase (Pl<sub>4</sub>) + orthopyroxene (Opx<sub>2</sub>) + amphibole (Amp<sub>3</sub>) + biotite (Bt<sub>3</sub>) symplectites, developed at P-T conditions of 732 ± 59.6 °C and 6.1 ± 0.6 kbar. Moreover, the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 ± 4.7 Ma for the protolith and 281.4 ± 8.5 Ma for the peak metamorphic age. Therefore, the mineral assemblage, P-T conditions, and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t (Pressure-Temperature-time) path, indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian, then the southern Beishan area underwent a continental crust tectonic thinning process.</p>

Quantifying the growth of continental crust through crustal thickness and zircon Hf-O isotopic signatures: A case study from the southern Central Asian Orogenic Belt

2021 ◽  
Author(s):  
Yujian Wang ◽  
Dicheng Zhu ◽  
Chengfa Lin ◽  
Fangyang Hu ◽  
Jingao Liu
Keyword(s):  
Continental Crust ◽  
Crustal Thickness ◽  
Orogenic Belt ◽  
Central Asian Orogenic Belt ◽  
Crustal Thickening ◽  
Crustal Growth ◽  
Isotopic Signatures ◽  
Central Asian ◽  
Southern Central ◽  
Juvenile Crust

Accretionary orogens function as major sites for the generation of continental crust, but the growth model of continental crust remains poorly constrained. The Central Asian Orogenic Belt, as one of the most important Phanerozoic accretionary orogens on Earth, has been the focus of debates regarding the proportion of juvenile crust present. Using published geochemical and zircon Hf-O isotopic data sets for three belts in the Eastern Tianshan terrane of the southern Central Asian Orogenic Belt, we first explore the variations in crustal thickness and isotopic composition in response to tectono-magmatic activity over time. Steady progression to radiogenic zircon Hf isotopic signatures associated with syn-collisional crustal thickening indicates enhanced input of mantle-derived material, which greatly contributes to the growth of the continental crust. Using the surface areas and relative increases in crustal thickness as the proxies for magma volumes, in conjunction with the calculated mantle fraction of the mixing flux, we then are able to determine that a volume of ∼14−22% of juvenile crust formed in the southern Central Asian Orogenic Belt during the Phanerozoic. This study highlights the validity of using crustal thickness and zircon isotopic signatures of magmatic rocks to quantify the volume of juvenile crust in complex accretionary orogens. With reference to the crustal growth pattern in other accretionary orogens and the Nd-Hf isotopic record at the global scale, our work reconciles the rapid crustal growth in the accretionary orogens with its episodic generation pattern in the formation of global continental crust.

The origins of granulites: a metamorphic perspective

1989 ◽  
Vol 126 (3) ◽  
pp. 215-247 ◽  
Author(s):  
S. L. Harley
Keyword(s):  
Large Range ◽  
Crustal Extension ◽  
Mineral Assemblages ◽  
Moderate Rate ◽  
Complex Models ◽  
Isothermal Decompression ◽  
Normal Thickness ◽  
Thinning Process ◽  
Tectonic Exhumation ◽  
Very High

AbstractAlthough many recent reviews emphasize a uniformity in granulite pressure–temperature (P–T) conditions and paths, granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation. A thorough survey of over 90 granulite terranes or occurrences reveals that over 50% of them recordP–Tconditions outside the 7.5 ± 1 kbar and 800 ± 50 °C average granulite regime preferred by many authors. In particular, an increasing number of very high temperature (900−1000 °C) terranes are being recognized, both on the basis of distinctive mineral assemblages and geothermobarometry. Petrogenetic grid and geothermobarometric approaches to the determination and interpretation ofP–Thistories are both evaluated within the context of reaction textures to demonstrate that the large range inP–Tconditions is indeed real, and that both near-isothermal decompression (ITD) and near-isobaric cooling (IBC)P–Tpaths are important. Amphibolite–granulite transitions promoted by the passage of CO2-rich fluids, as observed in southern India and Sri Lanka, are exceptional and not representative of fluid-related processes in the majority of terranes. It is considered, on the contrary, that fluid-absent conditions are typical of most granulites at or near the time of their recorded thermal maxima.ITD granulites are interpreted to have formed in crust thickened by collision, with magmatic additions being an important extra heat source. Erosion alone is not, however, considered to be the dominant post-collisional thinning process. Instead, the ITD paths are generated during more rapid thinning (1−2 mm/yr exposure) related to tectonic exhumation during moderate-rate or waning extension. IBC granulites may have formed in a variety of settings. Those which show anticlockwiseP–Thistories are interpreted to have formed in and beneath areas of voluminous magmatic accretion, with or without additional crustal extension. IBC granulites at shallow levels (< 5 kbar) may also be formed during extension of normal thickness crust, but deeper-level IBC requires more complex models. Many granulites exhibiting IBC at deep crustal levels may have formed in thickened crust which underwentveryrapid (5 mm/yr) extensional thinning subsequent to collision. It is suggested that the preservation of IBC paths rather than ITD paths in many granulites is primarily related to therateandtimescaleof extensional thinning of thickened crust, and that hybrid ITD to IBC paths should also be observed.Most IBC granulites, and probably many ITD granulites, have not been exposed at the Earth's surface as a result of the tectonic episodes which produced them, but have resided in the middle and lower crust for long periods of time (100−2000 Ma) following these events. The eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation.

scholarly journals Geochronological and Geochemical Constraints on the Petrogenesis of Permian Dolerite Dyke Swarms in the Beishan Orogenic Belt, NW China

2021 ◽  
Vol 9 ◽  
Author(s):  
Gang Xu ◽  
Jun Duan ◽  
Wenbin Gao ◽  
Rongmin Wang ◽  
Zhen Shi ◽  
...  
Keyword(s):  
Orogenic Belt ◽  
Geochemical Data ◽  
Early Permian ◽  
Nw China ◽  
Dolerite Dyke ◽  
Central Asian ◽  
Southern Central ◽  
Dyke Swarms ◽  
Geochemical Constraints ◽  
Beishan Area

Extensive Early Permian mafic-ultramafic intrusions, doleritic dykes, and basalts crop out within the Beishan area, southern Central Asian Orogenic Belt (CAOB). We present new geochronological and geochemical data for Gubaoquan dolerite dyke swarms in the Beishan orogenic belt. Zircon U-Pb Dating of the Gubaoquan dykes indicates that they were emplaced during the Early Permian (280.7 ± 4 Ma), that was coeval with Yinaoxia and Podong mafic dykes in Beishan area. The dykes are characterized by low Mg# (47–84) in the clinopyroxene crystals, and the content of whole-rock Fe2O3 (t), MgO, and alkali (Na2O + K2O) range from 12.5–17.4, 4.06–5.51, and 2.8–4.4 wt.%, respectively. The samples from the Gubaoquan dykes have high and variable Ba/La (5.93–14.2) and Ba/Nb (15.0–37.3) ratios but low Th/Yb (0.17–0.24) ratios. The rocks show slightly enrichments in LREE, HFSE, Th, and Hf, and depletion in Nb and Ta. The εNd (t = 280 Ma) values and initial 87Sr/86Sr ratios of the Gubaoquan dykes show variations ranging from 6.4 to 6.8 and 0.706240 to 0.707546, respectively. These data suggest that the parental magmas for the Gubaoquan dykes were probably derived from partially decompressed melting of upwelling depleted asthenosphere mantle that was metasomatized previously by subducted fluids.

Supplemental Material: Tectonic origin of the Bainaimiao arc terrane in the southern Central Asian orogenic belt: Evidence from sedimentary and magmatic rocks in the Damao region

2020 ◽  
Author(s):  
Hai Zhou ◽  
Guochun Zhao ◽  
et al.
Keyword(s):  
Volcanic Rocks ◽  
Orogenic Belt ◽  
Central Asian Orogenic Belt ◽  
Isotopic Data ◽  
Magmatic Rocks ◽  
Central Asian ◽  
Southern Central ◽  
Samples And Sampling ◽  
Tectonic Origin

Table S1: Summary of the samples and sampling positions in this study (sampling sites are marked in Fig. 3); Table S2: U-Pb age data for zircons of (meta-)sedimentary and volcanic rocks in this study; Table S3: Lu-Hf isotopic data for zircons of (meta-)sedimentary and volcanic rocks in this study.

Isotopic structure and evolution of the continental crust in the East Transbaikalian segment of the Central Asian Foldbelt

Geotectonics ◽  
2011 ◽  
Vol 45 (5) ◽  
pp. 349-377 ◽  
Author(s):  
E. Yu. Rytsk ◽  
V. P. Kovach ◽  
V. V. Yarmolyuk ◽  
V. I. Kovalenko ◽  
E. S. Bogomolov ◽  
...  
Keyword(s):  
Continental Crust ◽  
Central Asian
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