P–T–t–D records of Early Palaeozoic Andean‐type shortening of a hot active margin: The Dunhuang block in NW China

High‐pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra‐subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–...

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Bibliographic Details
Published in:Journal of metamorphic geology 2023-01, Vol.41 (1), p.59-96
Main Authors: Soldner, Jérémie, Štípská, Pavla, Schulmann, Karel, Yuan, Chao, Anczkiewicz, Robert, Jiang, Yingde, Koziarska, Marta, Zhang, Le, Zhang, Yunying, Wang, Xinyu
Format: Article
Language:English
Subjects:
Age
Andean‐type orogeny
Balancing
Decoupling
Deformation
Devonian
Domains
Dunhuang block
Earth Sciences
Evolution
Fabrics
Garnet
garnet Lu–Hf and Sm–Nd dating
Garnets
Geological processes
HP granulite
Isotopes
Lenses
Lithosphere
Melting
Metamorphism
Mineral assemblages
Orogeny
Palaeozoic
Paleozoic
Peak pressure
Plates (tectonics)
Pressure
P–T–t–D path
Rock
Rocks
Sciences of the Universe
Sedimentary facies
Silurian
Subduction
Subduction (geology)
Tectonics
Temperature
Thickening
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Summary:High‐pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra‐subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–time‐deformation (P–T–t–D) paths. In the current study, P–T–t–D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N‐S trending sub‐vertical fabric (S2) preserved in low‐strain domains and (3) upright folds (F3) associated with a ubiquitous steep E‐W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8–11 kbar and ~850–900°C during crustal melting (M2) followed by partial re‐equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re‐equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P–T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure–temperature (P–T) evolution M3 suggests formation of a mélange‐like association during the late stages of orogeny. The high‐pressure event D1‐M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and
ISSN:0263-4929
1525-1314
DOI:10.1111/jmg.12688