Jinjiang Zhang

Paleo‐Pacific subduction‐accretion: Evidence from Geochemical and U‐Pb zircon dating of the Nadanhada accretionary complex, NE China

The Nadanhada Terrane, located along the eastern margin of Eurasia, contains a typical accretionary complex related to paleo-Pacific plate subduction-accretion. The Yuejinshan Complex is the first stage accretion complex that consists of meta-clastic rocks and metamafic-ultramafic rocks, whereas the Raohe Complex forms the main parts of the terrane and consists of limestone, bedded chert, and mafic-ultramafic rocks embedded as olistolith blocks in a weakly sheared matrix of clastic meta-sedimentary rocks. Geochemical data indicate that the Yuejinshan metabasalts have normal mid-ocean ridge basalt (N-MORB) affinity, whereas the Raohe basaltic pillow lavas have an affinity to ocean island basalts (OIB). Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses of gabbro in the Raohe Complex yield a weighted mean 206Pb/238U zircon age of 216 ± 5 Ma, whereas two samples of granite intruded into the complex yield weighted mean 206Pb/238U zircon ages of 128 ± 2 and 129 ± 2 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb zircon analyses of basaltic pillow lava in the Raohe Complex define a weighted mean age of 167 ± 1 Ma. Two sandstone samples in the Raohe Complex record younger concordant zircon weighted mean ages of 167 ± 17 and 137 ± 3 Ma. These new data support the view that accretion of the Raohe Complex was between 170 and 137 Ma, and that final emplacement of the Raohe Complex took place at 137–130 Ma. The accretion of the Yuejinshan Complex probably occurred between the 210 and 180 Ma, suggesting that paleo-Pacific plate subduction was initiated in the Late Triassic to Early Jurassic.

Evolution and time of formation of the Hohhot metamorphic core complex, North China: new structural and geochronologic evidence

The Hohhot metamorphic core complex (MCC) is one of the typical MCCs in the North China craton. Its fault systems consist of the master Hohhot detachment zone (HHDZ) on the southern flank of the Daqing Shan, and the lowermost and uppermost northern detachment zones on the northern flank. Ductile deformation temperatures of three zones were estimated as 500 ± 50°C, 650 ± 50°C, and 400 ± 50°C, respectively, by analysis of microstructures of minerals and quartz [c] crystallographic axis fabrics using electron backscattered diffraction. These measurements suggest that previous 40Ar/39Ar ages could not represent the time of the high-temperature (>500°C) ductile deformations. Therefore, we used U-Pb zircon ages of mylonitized and non-mylonitized granitic intrusions to constrain the timing of the early high-temperature shearing. Strongly mylonitized granites and weakly mylonitized granites in the lowermost northern detachment zone yielded zircon U-Pb ages of 148 ± 1 and 140 ± 1 million years respectively. A syn-kinematic pluton in the lower plate of the MCC gave a U-Pb age of 142 ± 1 million years. These allow us to speculate on the possibility that SE-directed, early tangential, high-temperature ductile shearing probably was initiated during ca. 148–140 Ma (or ca. 142 Ma) at depth, with the thrust events occurring at shallow levels. A strongly mylonitized granitic dike and a non-mylonitized pluton in the master HHDZ yielded ages of 142 ± 1 and 132 ± 2 million years respectively. A non-mylonitized pluton intrusive into the uppermost northern detachment zone was dated at 131 ± 1 Ma. All these suggest that major extensional ductile shearing along the detachment zones took place during ca. 140–132 Ma. Using these new U-Pb ages, combined with previously published 40Ar/39Ar cooling ages that range from 127 Ma to 119 million years for the master HHDZ and supradetachment basins, we discuss and derive the time of formation process of the MCC. This is one of only a few cases of detailed study of timing for the development of an MCC from earlier deep-level shearing to later thermal uplifting (doming).

Polar Mohr constructions for strain analysis in general shear zones

Abstract The polar Mohr diagram is a useful tool for strain analysis in general shear zones. Polar Mohr diagrams for general shear zones can be constructed with the following measured data: (a) the stretches of two line markers which were originally perpendicular to each other; (b) the ratio of their stretches and the principal directions; (c) the stretches in the shear direction and in the principal direction; and (d) the stretches in any two directions. With these diagrams, the angle between eigenvectors, v , can be obtained, and the kinematic vorticity number W k in the shear zone and the ratio of pure shear rate e to simple shear rate γ during deformation can be computed with the formulae W k = cos v and W k = cos[cot(2e/γ)].

Petrogenesis and evolution of late Mesozoic granitic magmatism in the Hohhot metamorphic core complex, Daqing Shan, North China

Late Mesozoic granitoid plutons of four distinct ages intrude the lower plate of the Hohhot metamorphic core complex along the northern margin of the North China craton. The plutons belong to two main groups: (1) Group I, deformed granitoids (148 and 140 Ma subgroups) with high Sr, LREE, and Na2O, low Y and Yb contents, high Sr/Y and La/Yb ratios, weak or no Eu anomalies, low Rb/Ba ratios, similar initial 87Sr/86Sr values (0.7064–0.7071) and low Mg# (<37 mostly, 100 × molar MgO/MgO + FeO t ); (2) Group II, non-deformed granitoids (132 and 114 Ma subgroups) with low Sr, relatively low Na2O, high Y and Yb contents, pronounced negative Eu anomalies, high Rb/Ba ratios, and initial 87Sr/86Sr values (0.7098–0.7161). The two groups share geochemical similarities in ϵNd(t) (–11.3 to –15.4) and T DM2 ages (1.85–2.18 thousand million years) as well as Hf isotopic ratios in zircons. Geochemical modelling (using the MELTS code) suggests that similar sources but different depths of magma generation produced the early, high-pressure low-Mg adakitic granitoids and late, low-pressure granitoids with A-type characteristics. The early granitoids likely represent a partially melted, deep-seated, thickened lower continental crust that involved a minor contribution from young materials, whereas the later group partially melted at shallower depths. This granitic magmatic evolution coincided with the tectonic transition from crustal contraction to extension.

Syn-deformation P-T paths of Xiaoqinling metamorphic core complex

TheP-T paths of the Xiaoqinling metamorphic core complex (XMCC) have been investigated with the Gibbs method by researching the compositional changing of zoned epidotes which formed during syn-deformation metamorphism. TheseP-T paths indicate that the XMCC had experienced the following thermodynamics processes: firstly, near isobaric falling slightly in temperature in lower crust; secondly, fast decompression and rising in temperature during extensional uplifting to middle crust level; and finally, isobaric falling in temperature in middle crust. The upwelling and emplacement of the deep magma may be a major factor during the uplifting processes of the metamorphic core complex.

U-Pb Dating and Lu-Hf Isotopes of Detrital Zircons From the Southern Sikhote-Alin Orogenic Belt, Russian Far East: Tectonic Implications for the Early Cretaceous Evolution of the Northwest Pacific Margin

The Sikhote-Alin orogenic belt in Russian Far East is comprised of several N-S trending belts, including the Late Jurassic to Early Cretaceous accretionary prisms and turbidite basin which are now separated by thrusts and strike-slip faults. The origin and collage of the belts have been studied for decades. However, the provenance of the belts remains unclear. Six sandstone samples were collected along a 200-km long east-west traverse across the major belts in the southern Sikhote-Alin for U-Pb dating and Lu-Hf isotope analysis to constrain the provenance and evaluate the evolution of the northwest Pacific margin at this time. The result reveals that the sediments from the main Samarka belt was mainly from the adjacent Bureya-Jiamusi-Khanka Block (BJKB); the eastern Samarka belt and the Zhuravlevka turbidite basin were supplied by detritus from both the North China Craton (NCC) and the BJKB; the Taukha belt was mainly fed by sediments from the NCC; whereas the data from the Sergeevka nappes are insufficient to resolve their provenance. In the Late Jurassic to Early Cretaceous, collision and subduction was important in the initial collage of most belts in Sikhote-Alin. However, merely E-W trending collage cannot explain the increasing importance of the NCC provenance from west to east. It is proposed that the main Samarka belt was located adjacent to the BJKB when deposited, whereas the other belts were father south to accept the materials from the NCC. Sinistral strike-slip faulting transported the eastern belts northwards after their initial collage by thrusting.

Midcrustal shearing and doming in a Cenozoic compressive setting along the Ailao Shan-Red River shear zone

The Cenozoic Xuelong Shan antiformal dome is located along the northern segment of the Ailao Shan-Red River shear zone in Yunnan, China. Subhorizontal foliation in the gneiss core is recognized, representing a broad top-to-NE shear initiated under amphibolite facies conditions and propagating into greenschist facies in the mantling schist and strike-slip shear zone. Microfabrics of crystallographic-preferred orientations (CPOs) in quartz suggest that the deformation temperatures increased with increasing structural depth from the upper crust (300–500°C) in the mantling schist to the midcrust (15 km or more, ≥650°C) in the gneissic core. This trend is mirrored by variations in the metamorphic grade of the syn-kinematic mineral assemblages and microstructures, which range from garnet + amphibole + biotite + sillimanite + rutile + feldspar in the core to garnet + staurolite + biotite + epidote + muscovite within the limb units. The dome experienced the following deformation history: (1) a broad top-to-NE shear in the subhorizontal foliation of the gneiss core during the first stage of deformation (D1); (2) opposing reverse-sense shear along the two schist limbs of the dome during contraction-related doming (D2–D3); (3) sinistral strike-slip shearing within the eastern limb (D4); and (4) extensional deformation (D5). The structural-thermal patterns suggest the antiformal dome formation was roughly coeval with top-to-NE ductile shearing in the midcrust of Tibet at 32 Ma or earlier. A major implication is that there was a phase of contractional ductile deformation in the region prior to the initiation of strike-slip deformation.

Interpretation of the experimental data provided by Gómez-Rivas and Griera (2012) in terms of the MEM-criterion

The systematical experiments carried out by Gómez-Rivas and Griera (2012) demonstrate that the ductile shear zones initiate at ∼55° to σ1 just as predicted by the MEM-criterion. However, the data are explained in terms of dilatancy, which requires many prerequisites and implies that the ±55° angle is only valid for the used material. In contrast, the ∼55° predicted by the MEM-criterion is material independent, which makes it widely applicable to explaining the development of ductile shear zones in nature.

SEM/EBSD analysis of quartz cementation and compaction microstructures during diagenesis of sandstone

Compaction and silicon cementation are the dominant processes reducing porosity and permeability in quartzose sandstones during diagenesis. Despite the wealth of information about quartz cementation, there are still unanswered questions related to mechanisms of growth of quartz cement and the diagenesis processes. In this study we present an electron backscatter diffraction (EBSD) analysis, combined with optics and cathodoluminescence (CL) information, for the quartz sandstones from the Upper Triassic Xujiahe Formation of Sichuan Basin, in order to reveal the microstructural and crystallographic features of the silica cementation and detrital grain during the compaction. The EBSD is a crucial technique to provide essential crystallographic data on the quartz grain and its cement. Quartz cement is shown to be syntaxial to its host quartz grain. EBSD data-based orientation maps show dauphiné twinning and low angle boundary to be common in the host grains and quartz cement of the samples. The dauphiné twins occurred in grain-grain contacts and in cement-crystal boundaries, and commonly crossed grain cement boundaries. These features indicate that there may be two types of dauphiné twins, one inherited twins from the source area and the other developed by compaction-induced grain boundary deformation. These investigations suggest that strong mechanical compaction may occur after and/or during quartz cement growth in the later diagenesis of the Xujiahe sandstones. EBSD has a capability of revealing microstructural information and regarding mechanisms of diagenesis crystal growth in quartzose sandstones.

New indicators from bedding-parallel beef veins for the fault valve mechanism

Beef structures (bedding-parallel veins of fibrous calcite) are widespread within the Lower Triassic carbonate rocks in the Sichuan Basin of China, especially within clay-rich strata of low permeability. In the veins, fibrous calcite occurs in the outer zones, and coarse equant calcite in the inner zones. At least two generations of calcite crystallization took place during aqueous alteration, at the same time as deformation recorded by the calcite. The first-generation calcite fibers are at steep angles to the hydrocarbon-bearing host beds, and they grew vertically against the force of gravity at a time when the source rocks were maturing. Second-generation calcite occurs as coarse equant grains that sealed pores via localized fluid flow during horizontal tectonic compression, so that shear stresses acted at the fracture margins. Shearing of the host rock was accommodated in part by dissolution-precipitation creep (DPC), grain rotation, and grain slippage, recorded in crystallographic preferred orientations (CPOs) of the host calcite grains beside the crack walls. The beef veins formed during high pore-fluid overpressures along hydrofractures. We propose that the bedding-parallel veins with beef structures are evidence of a “crack-seal slip” fault valve process during hydrocarbon generation. The hydrocarbon-bearing calcite beef structure may be a good indicator of oil or gas migration, and of the flow direction of aqueous solutions.