Abstract High-pressure mafic granulites (retrograded eclogites?) were discovered as minor lenses enclosed in garnet-kyanite-cordierite gneiss from the Badu Complex of the East Cathaysia terrane in South China. These rocks consist mainly of garnet, clinopyroxene, hornblende, quartz, and rutile/ilmenite with or without omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. Mineral textures and reaction relationships suggest three metamorphic stages: (1) an eclogite-facies stage (M1) characterized by the mineral assemblage of garnet + clinopyroxene (omphacite) + hornblende + rutile + quartz; (2) a high-pressure granulite-facies (M2) stage mainly represented by garnet + clinopyroxene + plagioclase + hornblende + rutile + quartz in the matrix; and (3) an amphibolite retrograde stage (M3) defined by hornblende + plagioclase + ilmenite + quartz symplectites surrounding garnet porphyroblasts. Conventional geothermometers and geobarometers in combination with phase equilibria modeling constrain metamorphic P–T conditions of 15.8–18.2 kbar/625–690 °C (M1), 11.8–14.5 kbar/788–806 °C (M2), and 5.4–6.4 kbar/613–668 °C (M3), respectively. Two-staged decompression processes are defined after the peak pressure, which suggests a two-staged exhumation of these deeply buried rocks. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating and trace element analysis show that the high-pressure metamorphism occurred at 240–244 Ma. Complete early Mesozoic orogenic processes characterized by initial subduction and/or crustal thickening and subsequent exhumation followed by rapid uplift are reconstructed for this part of the East Cathaysia terrane, South China.
The Cover Feature shows a warrior directing a horse, originally from an alto-relievo of the Tang Dynasty in China. The picture is a metaphor of a visible-light-induced δ C(sp3)−H amination of aliphatic alcohols catalyzed with iron. The hydroxy substitute acts as the directing group to realize the selectivity of the reaction through an alkoxyl radical-involved 1,5-hydrogen atom transfer. More information can be found in the Research Article by Z. Jue et al.
EPMA data, monazite U-Pb data and P-T estimates using conventional geothermometers and geobarometers for felsic granulite, Chencai Complex, West Cathaysia terrane, South China
An iron-catalyzed remote C(sp3 )-H amination of alcohols through 1,5-hydrogen atom transfer is developed. This protocol provides a method to generate δ-C(sp3 )-N bonds from primary, secondary, and tertiary alcohols under mild conditions. A wide substrate scope and a good functional group tolerance are presented. Mechanistic studies show that a LMCT course of an Fe-OR species and a chlorine radical-induced hydrogen abstraction of an alcohol are possible to generate the alkoxy radical intermediate.
When modern-style plate tectonics started and operated on a specific old craton has been a hot topic involving the early Earth's evolution. In order to address this issue on the evolution of the Neoarchean North China Craton (NCC), we investigated a newly identified successive magmatic rock suite of tonalite-trondhjemite-granodiorite (TTG)-sanukitoid in the Datong-Huai’an Complex. Geochemically, TTG gneisses in the Datong-Huai’an Complex can be divided into high-pressure (HP) and low-pressure (LP) TTG rocks. The HP TTGs are characterized by steep rare earth element (REE) patterns, obvious negative Nb, Ta and Ti anomalies and positive δEu anomalies, high Sr/Y, Nb/Ta and (La/Yb)N ratios, and positive Hf (t) (+2.1 to +8.7) and δ18O values (Ave 5.5‰ to 6.0‰). Their protolith is interpreted as the consequence of partial melting of a subducted oceanic slab with garnet and minor rutile as residual phases. The LP TTGs feature flat REE patterns, slightly negative Nb, Ta and Ti anomalies and negative δEu anomalies, low Sr/Y, Nb/Ta and (La/Yb)N ratios, positive Hf (t) (+3.4 to +5.9) and high δ18O values (Ave 5.9‰ to 6.1‰). They may have been generated by partial melting of mafic lower crust with residual plagioclase and amphibolite in the source. The sanukitoid rocks show high MgO, Cr and Ni concentrations but relatively low (La/Yb)N values, positive ɛHf (t) values (+1.5 to +5.4), and higher δ18O values (Ave 1.9‰ to 8.7‰), suggesting that they were originated from partial melting of mantle peridotite previously modified by slab-derived or sediment-derived melts. Zircon U–Pb dating results reveal that the HP TTGs formed at ~2538 Ma and ~2479–2441 Ma, whereas the LP TTGs and sanukitoids formed at ~2518 Ma and ~2517–2485 Ma, respectively. Combining rock assemblages, geochemical features and geological data, we propose that this long-lived magmatism can be divided into three stages of ~2.55-2.52 Ga, ~2.52-2.48 Ga and ~2.48-2.44 Ga. The Datong-Huai’an Complex may have developed along an active continental margin and depicts modern-style plate tectonics with continuous steep subduction, slab rollback and back-arc extension in the late Neoarchean.
Abstract In worldwide orogenic belts, crustal-scale ductile shear zones are important tectonic channels along which the orogenic root (i.e., high-grade metamorphic lower-crustal rocks) commonly experienced a relatively quick exhumation or uplift process. However, their tectonic nature and geodynamic processes are poorly constrained. In the Trans–North China orogen, the crustal-scale Zhujiafang ductile shear zone represents a major tectonic boundary separating the upper and lower crusts of the orogen. Its tectonic nature, structural features, and timing provide vital information into understanding this issue. Detailed field observations showed that the Zhujiafang ductile shear zone experienced polyphase deformation. Variable macro- and microscopic kinematic indicators are extensively preserved in the highly sheared tonalite-trondhjemite-granodiorite (TTG) and supracrustal rock assemblages and indicate an obvious dextral strike-slip and dip-slip sense of shear. Electron backscattered diffraction (EBSD) was utilized to further determine the crystallographic preferred orientation (CPO) of typical rock-forming minerals, including hornblende, quartz, and feldspar. EBSD results indicate that the hornblendes are characterized by (100) <001> and (110) <001> slip systems, whereas quartz grains are dominated by prism <a> and prism <c> slip systems, suggesting an approximate shear condition of 650–700 °C. This result is consistent with traditional thermobarometry pressure-temperature calculations implemented on the same mineral assemblages. Combined with previously reported metamorphic data in the Trans–North China orogen, we suggest that the Zhujiafang supracrustal rocks were initially buried down to ∼30 km depth, where high differential stress triggered the large-scale ductile shear between the upper and lower crusts. The high-grade lower-crustal rocks were consequently exhumed upwards along the shear zone, synchronous with extensive isothermal decompression metamorphism. The timing of peak collision-related crustal thickening was further constrained by the ca. 1930 Ma metamorphic zircon ages, whereas a subsequent exhumation event was manifested by ca. 1860 Ma syntectonic granitic veins and the available Ar-Ar ages of the region. The Zhujiafang ductile shear zone thus essentially record an integrated geodynamic process of initial collision, crustal thickening, and exhumation involved in formation of the Trans–North China orogen at 1.9–1.8 Ga.
The megacryst orthoamphibole rocks from Hengshan Mts,Shanxi Province are comprised of orthoamphibole,kyanite,staurolite,cordierite,quartz,rutile,ilmenite and a small amount of plagioclase and calcic amphibole.They have the similar SiO2 contents to intermediate-basic rocks,but much higher MgO,total Fe2O3 and lower CaO,Na2O and K2O contents than the latter.The rocks can be grouped into two types.Type I orthoamphibole rocks contain mega-grained garnet porphyroblasts,and relatively lower MgO,TiO2,Na2O and K2O but higher Al2O3 and total Fe2O3 contents.This type are similar to IAB in HFSE and REE and contain 143Nd/144Nd=0.511361~0.511894 with tDM(Nd)=2.39~2.60Ga and similar eNd(t) values to that of the metabasic rocks in the Wutai Complex.TypeⅡorthoamphibole rocks contain a large amount of orthoamphibole without garnet,and higher MgO,TiO2,Na2O and K2O but lower Al2O3 and total Fe2O3,which are similar to MORB in HFSE and REE.According to the petrography and geochemistry,we infer that protolith of the orthoamphibole rocks may have been originated from hydrothermal alteration of basic volcanics equivalent to the mafic rocks in Wutai Complex.This hydrothermal alteration caused the depletion of Ca,Na and K and relative enrichment of Mg(Fe),and probably resulted in formation of large amount of hydrous minerals such as chlorite.The present mineral assemblages in the orthoamphibole rocks were formed due to a high-pressure amphibolite facies metamorphism with a subsequent isothermal decompression.The megacrysts may have been resulted from prograde dehydrations of the extremely fluid-rich rocks during the pre-peak stages,and metamorphic differentiation related to these prograde dehydrations may have led to a further modification of the bulk rock compositions.
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