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The widespread migmatites in the northwestern part of the Sulu Orogen, China, indicate regional anatexis that is of great significance when discussing the tectonic evolution of this continental orogenic belt. Cathodoluminescence (CL) images, U–Pb ages, and in situ trace element compositions of zircons from four pegmatite veins within these migmatites provide clear evidence for the nature of the post-collisional evolution of the Sulu Orogen. The inherited zircon cores reveal that the protoliths of the migmatites were middle Neoproterozoic magmatic rocks (810–620 Ma) of the South China Block. The protoliths underwent two partial melting events. The mantle domains of the inherited zircons record a Late Triassic (222.0–204.0 Ma) partial melting event that occurred during the exhumation and retrograde metamorphism, after ultrahigh-pressure (UHP) metamorphism. Subsequent newly grown zircons record a Middle–Late Jurassic to Early Cretaceous (164.1–125.5 Ma) anatexis event, indicating that the late Mesozoic anatexis started before ca. 164.1 Ma, reached a peak at ca. 152.1 Ma, and ceased at ca. 125.5 Ma. Combined with previous results of studies on the Sulu orogen, the late Mesozoic anatexis suggested that the thickened crust of the Sulu Orogen had started to become unstable before 164.1 Ma. The duration of ~164.1–137 Ma corresponds to a period of transition in the tectonic regime of the Sulu Orogen, enabling the early high-temperature ductile deformation. After ca. 137 Ma, the tectonic regime was fully transformed into extension and the Sulu Orogen underwent rapid thinning and collapse, thus leading to the late medium–low temperature ductile deformation (137–121 Ma) and laying the foundations for the large-scale magmatic emplacement during the late Early Cretaceous (127–115 Ma). These two partial melting events together promoted the rapid exhumation of the Sulu UHP rocks. [ABSTRACT FROM AUTHOR]

The study on deformation characters of the Liaohe Group, exposed in eastern Liaoning Province, NE China, provides a key constraint on the tectonic evolution of the Palaeoproterozoic Liao–Ji active belt in the eastern part of the North China Craton (NCC). In this paper, the deformation pattern and mechanism of the Dashiqiao Fm. from Liaohe Group in Hupiyu area have been systematically studied. The present results show that the structure of the investigated Dashiqiao Fm. exhibits a SW bedding‐parallel thrust shearing which is dominated by simple shearing. Simultaneously, S‐SL tectonites were formed under directional flattening. The deformed quartz grains show obvious dislocation slip which is mainly controlled by the low temperature system of the basal slip system, while calcites mainly develop in low temperature with twin slip. Based on microstructural analysis coupled with fabric features, we estimated that the strain rate is 10−12.78–10−8.32 s−1 under the deformation temperature of 350–400°C with a differential stress of 27.3–44.9 MPa. Our macrostructural–microstructural investigation together with previous researches indicates that the Liaohe Group suffered a bedding‐parallel sliding with top‐to‐outside away from the granite body in the core of Hupiyu anticline, corresponding to the uplift of the Hupiyu granite in the early extensional stage. Subsequently, the Hupiyu area suffered SW–NE compression which resulted in the overturn of the southern limb of the Hupiyu anticline and the top‐to‐SW thrusting. [ABSTRACT FROM AUTHOR]

The Mianhuadi mafic complex in the Ailao Shan-Red River (ASRR) shear zone, SW China, consists of granulite and mylonite, formed by granulite-facies peak metamorphism and retrograde amphibolite-facies metamorphism, with later local mylonitic deformation. Textures and mineral relics indicate that the protolith, represented by the mineral assemblage (M 0 ) of olivine, clinopyroxene, plagioclase and Fe-Ti oxides, was emplaced as a mafic intrusion in the Late Permian as part of the Emeishan large igneous province. Two kinds of metamorphic mineral assemblages were identified: the peak assemblage (M 1 ) of garnet + orthopyroxene + clinopyroxene + plagioclase + magnetite + ilmenite, and the retrograde assemblage (M 2 ) composed of hornblende surrounding orthopyroxene and garnet, and hornblende + spinel symplectite around garnet. Pressure-temperature estimates yield 657–722 °C and 7–9 kbar, and 649–676 °C and 5–6 kbar for the M 1 and M 2 stages, respectively, roughly defining a clockwise P-T path with near-isothermal decompression following the peak metamorphism. These metamorphic conditions are quite unlike those (pressure up to 14 kbar) of the Late Permian-Middle Triassic metamorphic event in the region. In addition, foliated biotite from mylonites has an 40 Ar- 39 Ar plateau age of 34.3 ± 0.4 Ma, indicating that the ASRR shear zone was active at or before the Late Eocene. Combined with previous data, our new results indicate that the granulite-facies metamorphism of the Mianhuadi mafic complex occurred most likely during the Eocene, followed by rapid exhumation and ductile sinistral shearing in the Late Eocene. The formation of the granulite was likely related to the upwelling of the asthenosphere due to the delamination of overthickened lithospheric mantle after the India-Asia collision. [ABSTRACT FROM AUTHOR]

Lithological and geochemical differentiation in ductile shear zone is essential to understanding element mobility during ductile deformation in the mid-lower crust. The Shagou ductile shear zone characterized with intensive compositional layering is investigated to understand element behaviors during high temperature ductile deformation. Geochemistry and zircon U–Pb chronology suggest the protoliths were magmatic rocks crystallized at ca. 735-723 Ma, overprinted by ductile deformation at ca. 198–184 Ma. Microstructure, quartz fabric, and thermometer coupled with barometer constrain the deformation to occur at 4.50–6.12 kbar and 633–713 °C. Field observations, geochemical and zircon U–Pb geochronological data suggest that the amphibolitic and gneissic mylonites were generated from geochemical differentiation during high-temperature ductile shearing. The geochemical contents of progressively deformed mylonites show systematic increasing or decreasing with enhancing mylonitization. Silica displays remarkably decreasing with enhancing mylonitization, whereas Mg, Fe, Mn, Ti and P tend to migrate to higher strain, more acid environment. REEs, LILEs, HFSEs, Zn and V display negative correlations with increasing mylonitization, suggesting that nearly all the trace elements migrate to lower strain in the amphibolitic mylonites. However, HREEs, HFSEs, Cr, Ni, Zn and V exhibit positive correlations with increasing mylonitization in the gneissic mylonites. It implies that these elements tend to migrate to higher strain, more acid environment during ductile deformation. • High temperature ductile deformation induced lithological and geochemical differentiation. • Silica decreases with progressive mylonitization. • Mg, Fe, Mn, Ti and P tend to migrate to higher strain and more acid environment. • REEs, LILEs, HFSEs, Zn and V migrate to lower strain area in amphibolitic mylonites. • HREEs, HFSEs, Cr, Ni, Zn and V increase with mylonitization in gneissic mylonites. [ABSTRACT FROM AUTHOR]

The early Paleozoic geological evolution of the South China Craton composed of die Yangtze and Cathaysia Blocks has been die focus of long debate. The Cathaysia block has been central to die controversy regarding convergent margin versus intraplate environment in the early Paleozoic. In order to address the early Paleozoic evolution of Cathaysia, we undertook a systematic study of the stratigraphic sequences, deformational features and geochronology of magmatic event. Our results show that (1) during the early Paleozoic, the Jiangnan domain of the SE Yangtze block was characterized by a carbonate platform and the Cathaysia block by a graptolite-facies clastic rock assemblage, (2) in the Cathaysia block, a littoral-neritic depositional environment prevailed in Cambrian whereas a neritic-bathyal setting dominated during the early-middle Ordovician, and (3) die Late Ordovician depositional sequence in Cathaysia witnessed a period of transition from neritic-bathyal to littoral-land environment, marking the initial uplift process. Paleo-current measurements on the crossbeds revealed northwestward and westward transport directions, suggesting a source area to the east-southeast. All samples collected from the Cambrian-Ordovician strata show similar chemical characteristics; they have negative εNd(t) values (-9.7 to -13.7) and two-stage eNd(t) model ages at ca.2.04 to 2.36 Ga. This suggests that the early Paleozoic rocks were derived from die eroded Paleoproterozoic basement, and little or no mande component was identified. During the Silurian, the Cathaysia block underwent strong folding, dirusting, weak metamorphism and large-scale anatexis accompanied by granitoid emplacement, building the South China Fold Belt. The maximum shortening is estimated at 67 percent. A kinematic analysis of die ductile sheared rocks revealed a fan-shape dirust pattern, with top-to-the southeast in die southeastern and top-to-the northwest in the northwestern Cathaysia block. Zircon U-Pb dating of four granitic plutons yielded 206Pb/238U ages of 435 ± 4 Ma, 424 ± 5 Ma, 428 ± 3 Ma and 427 ± 2 Ma. All die zircon εHf(t) values are negative (-6 to -9) and show a peak of two-stage Hf model ages around 1.9 Ga, indicating that die Silurian granitic magma was derived from the recycling of Paleoproterozoic basement. Major features of die early Paleozoic South China Fold Belt include die lack of early Paleozoic ophiohtes and volcanic rocks, the absence of coeval HP-type blueschists, and die absence of mantle-derived juvenile magmatic rocks. Consequently, a subduction-collision-type orogeny is excluded. The magmatism most probably took place in an intraplate tectonic setting with little or no input of mantle components. We therefore conclude diat die South China Fold Belt was an intraplate orogen, and is possibly related to die global early Paleozoic continental assembly. [ABSTRACT FROM AUTHOR]

In the Dabieshan, the available models for exhumation of ultrahigh-pressure (UHP) rocks are poorly constrained by structural data. A comprehensive structural and kinematic map and a general cross-section of the Dabieshan including its foreland fold belt and the Northern Dabieshan Domain (Foziling and Luzenguang groups) are presented here. South Dabieshan consists from bottom to top of stacked allochtons: (1) an amphibolite facies gneissic unit, devoid of UHP rocks, interpreted here as the relative autochton; (2) an UHP allochton; (3) a HP rock unit (Susong group) mostly retrogressed into greenschist facies micaschists; (4) a weakly metamorphosed Proterozoic slate and sandstone unit; and (5) an unmetamorphosed Cambrian to Early Triassic sedimentary sequence unconformably covered by Jurassic sandstone. All these units exhibit a polyphase ductile deformation characterized by (i) a NW–SE lineation with a top-to-the-NW shearing, and (ii) a southward refolding of early ductile fabrics.The Central Dabieshan is a 100-km scale migmatitic dome. Newly discovered eclogite xenoliths in a Cretaceous granitoid dated at 102 Ma by the U–Pb method on titanite demonstrate that migmatization post-dates HP–UHP metamorphism. Ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming. Along the dome margins, migmatite is gneissified under post-solidus conditions and mylonitic–ultramylonitic fabrics commonly develop. The north and west boundaries of the Central Dabieshan metamorphics, i.e. the Xiaotian–Mozitan and Macheng faults, are ductile normal faults formed before Late Jurassic–Early Cretaceous. A Cretaceous reworking is recorded by synkinematic plutons.North of the Xiaotian–Mozitan fault, the North Dabieshan Domain consists of metasediments and orthogneiss (Foziling and Luzenguang groups) metamorphosed under greenschist to amphibolite facies which never experienced UHP metamorphism. A rare N–S-trending lineation with top-to-the-south shearing is dated at 260 Ma by the 40Ar/39Ar method on muscovite. This early structure related to compressional tectonics is reworked by top-to-the-north extensional shear bands.The main deformation of the Dabieshan consists of a NW–SE-stretching lineation which wraps around the migmatitic dome but exhibits a consistently top-to-the-NW sense of shear. The Central Dabieshan is interpreted as an extensional migmatitic dome bounded by an arched, top-to-the-NW, detachment fault. This structure may account for a part of the UHP rock exhumation. However, the abundance of amphibolite restites in the Central Dabieshan migmatites and the scarcity of eclogites (found only in a few places) argue for an early stage of exhumation and retrogression of UHP rocks before migmatization. This event is coeval to the N–S extensional structures described in the North Dabieshan Domain. Recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in Triassic–Liassic times. The main foliation is deformed by north-verging recumbent folds coeval to the south-verging folds of the South Dabieshan Domain. An intense Cretaceous magmatism accounts for thermal resetting of most of the 40Ar/39Ar dates.A lithosphere-scale exhumation model, involving continental subduction, synconvergence extension with inversion of southward thrusts into NW-ward normal faults and crustal melting is presented. [Copyright &y& Elsevier]