Abstract The generally accepted foreland interpretation of the Nanpanjiang basin has been regarded as providing support for the hypothesized Indosinian collision between the South China and Indochina blocks, and it has led to the long‐standing view that crustal shortening dominated the Triassic intracontinental development of South China. Here, we examine the stratigraphic architecture, magmatism and gross structural features of the Nanpanjiang basin. Outcrop studies indicate that a north‐stepping axial turbidite system was confined by intrabasinal faults, the active development of which is indicated by mass‐transport complexes. Field mapping combined with U–Pb dating of baddeleyite and zircon reveals that magmatism in the southern part of the basin coincides with a distinct episode of bimodal volcanism from ca. 244 to 242 Ma, and is unrelated to the middle Permian Emeishan plume. Differences in tectonic subsidence between hanging‐wall and footwall blocks, as that has been quantified by the backstripping of composite sections, show that synsedimentary dip slip along intrabasinal faults was of normal sense. Meanwhile, a role for strike‐slip deformation is registered by the left‐lateral offset of middle Permian and Middle Triassic piercing points and the exceptional preservation in outcrop of syndepositional negative flower structures. For these reasons, therefore, the pre‐Norian Triassic Nanpanjiang basin is thought to represent a transtensional back‐arc setting, a result that leads to an unresolved paradox: in adjacent parts of South China to the east, coeval development involves crustal thickening, nonmarine sedimentation and granitic plutonism. A new lateral decoupling model consistent with palaeomagnetic evidence for large‐scale block rotation is tentatively proposed here as a working hypothesis to explain the observed intracontinental evolution of South China during the Triassic assembly of eastern Asia. We propose that such decoupling may relate in part to the configuration of inherited crustal weaknesses.
Orogens with multiple (ultra)high‐pressure ((U)HP) and (ultra)high‐temperature ((U)HT) metamorphic events provide a complex but telling record of oceanic and continental interaction. The Early Paleozoic history of the “Heart of China,” the Qinling orogenic collage, offers snapshots of at least three (U)HP and two (U)HT metamorphic events. The preservation of remnants of both oceanic and continental domains together with a ≥110 Myr record of magmatism allows the reconstruction of the processes that resulted in this disparate metamorphism. Herein, we first illuminate the pressure‐temperature‐time (P‐T‐t) evolution of the Early Paleozoic (U)HP and (U)HT events by refining the petrographic descriptions and P‐T estimates, assess published, and employ new U/Th‐Pb zircon, monazite, and titanite, and 40 Ar‐ 39 Ar phengite geochronology to date the magmatic and metamorphic events. Then we explore how the metamorphic and magmatic events are related tectonically and how they elucidate the affinities among the various complexes in the Qinling orogenic collage. We argue that a Meso‐Neoproterozoic crustal fragment—the Qinling complex—localized subduction‐accretion events that involved subduction, oceanic‐arc formation, and back‐arc spreading along its northern margin, and mtantle‐wedge exhumation and spreading‐ridge subduction along its southern margin.
Abstract The East Kunlun Orogenic Belt (E-KOB) stands out as one of the most prominent basin-mountain geomorphic features in the northern interior of the Tibetan Plateau. It records a series of accretion-collision events from the Mesozoic to the Cenozoic. In particular, with the uplifting of the Tibetan Plateau, the E-KOB experienced intracontinental deformation and exhumation in the Cenozoic. Clarifying the exhumation history of the E-KOB is crucial to define the growth time and mechanism of the Tibetan Plateau. In this study, we apply detrital zircon fission-track (ZFT) and apatite fission-track (AFT) analyses on modern river sands in order to constrain the regional exhumation history of the eastern E-KOB. Four peak ages have been identified and interpreted as results of rapid exhumation correlated with intracontinental deformation. Two older peak ages at 144.7–141.0 and 114.6–82.1 Ma are in good accordance with the collision time of the north-south Lhasa-Qiangtang Block along the Bangong-Nujiang suture zone and the subsequent progressive deformation stage toward the north. Peak age at 60.9–45.3 Ma is coeval with the initial timing of the India-Asia collision. The youngest peak age at 25.1–18.3 Ma matches well with the extensive outward and upward growth of the Tibetan Plateau during the Oligocene to Miocene time. The Cretaceous and early Cenozoic rapid exhumations suggest that the E-KOB has been involved in the intracontinental deformation induced by collisions of the Lhasa-Qiangtang and India-Asia from the south. It implies that the northern Tibetan Plateau likely has been elevated or was a structural high before the Eocene. In addition, some of our detrital samples show a younger ZFT peak age than the AFT peak age. We attributed this data bias to the contribution of hydrodynamic sorting and/or lithological difference. The combination of ZFT and AFT dating has advantages in eliminating interfering age signals in detrital thermochronology.
The Upper Weihe River (UP‐WHR) basin is located along the northeastern Tibetan Plateau. With the northeastward expansion of the Tibetan Plateau since the late Cenozoic, it has involved foreland propagation and undergone obvious surface uplift. In order to determine the latest differential rock uplift and river incision, longitudinal profiles for 12 major tributaries of the UP‐WHR were extracted. Among them, 11 tributaries display uneven profiles with “slope‐break” knickpoints, suggesting that they are in a transient state and that the change in base level caused by tectonic forces may respond to the channel evolution. In addition, channel steepness index ( k sn ) was calculated to detect the spatial variations of the rock uplift rate. The results show that the north margin of West Qinling (WQL) and the south Liupan Shan (LPS) areas have a high uplift rate. Reconstruction of the paleochannel indicates that the Weihe River has an average incision of 354 ± 130 m since it formed, and the south tributaries have a higher incision of 144 ± 25 m than the north. An average river erosion rate of 0.25–0.3 m/ka was estimated, the WQL has a higher erosion rate of 0.1–0.12 m/ka than Longzhong Basin and the south LPS. This uplift and river incision can be correlated to the northeastward growth of the Tibetan Plateau since the late Cenozoic.
Abstract The Archean-Paleoproterozoic high-grade basement of Yudongzi complex is a key to understanding the early Precambrian crustal evolution of the Yangtze craton. It comprises mainly orthogneiss, paragneiss and amphibolite, whose protoliths are tonalitic-trondhjemitic-granitic (TTG), sedimentary and basic-intermediate volcanic rocks, respectively. The TTG gneiss, amphibole plagiogneiss and biotite plagiogneiss yield magmatic zircon LA-ICP-MS U–Pb ages of 2815 ± 18 Ma (MSWD = 0.92), 2692 ± 26 Ma (MSWD = 0.59) and 2449 ± 4 Ma (MSWD = 0.94), respectively. Metamorphic overgrowths on zircon from amphibolite have an age of 1848 ± 5 Ma (MSWD = 0.71). TTG gneisses show medium Sr/Y and variable high (La/Yb) N ratios with low Y and Yb contents. They are characterized by positive Eu anomaly and distinct depletion of HREE together with negative Nb, Ta and Ti, implying amphibole, garnet and minor rutile as residual phases. Their positive e Hf (t) values of +2.1 to +8.1 with T DM2 of ca. 2.80–3.10 Ga suggest significant reworking of juvenile crust. Amphibole plagiogneisses display a strong enrichment of LREEs and depletion of Nb, Ta and Ti. Additionally, a relative enrichment of Ba, Rb, Pb and Zr, as well as high Cr and Ni contents and Mg # values, imply a mantle source with the addition of continental crust material. Zircon e Hf (t) values vary between −0.9 and +3.9, suggesting a proportionally significant input of juvenile material and therefore interaction between the mantle and pre-existing continental crust. Biotite plagiogneisses show negative e Hf (t) values between −3.4 and −0.1 with a few positive e Hf (t) values ranging from +0.1 to +1.5. Together with T DM2 ages of ca. 2.80–3.00 Ga, these e Hf (t) values suggest that these rocks were mainly generated by reworking of ancient crust. Thus, the Yudongzi complex exposed in the northern part of the Yangtze craton has experienced significant reworking of juvenile crust at ca. 2.80 Ga and subsequent crustal growth at ca. 2.70 Ga, followed by a second stage of reworking of ancient crust at ca. 2.45 Ga. During the Late Paleoproterozoic, the Yudongzi complex was probably involved in the amalgamation of the Paleoproterozoic supercontinent Columbia, and affected by a post-collisional metamorphic event at ca.1.85 Ga.
Abstract The East Kunlun Orogenic Belt (EKOB) in the northern margin of the Tibet Plateau is characterized by widespread granitic plutons, which are keys to understanding the tectonic evolution of the EKOB. The Zhiyu pluton, newly recognized in the central part of the EKOB, mainly consists of monzogranites, biotite granites and quartz diorites. Their LA-ICPMS zircon U-Pb results show formation ages of 447 ± 1.6 Ma, 448 ± 2.5 Ma and 408 ± 1.8 Ma. The monzogranites and biotite granites are characterized by relatively high Sr (208–631 ppm), low Y (4.28–15.82 ppm) and Yb (0.44–1.59 ppm) contents, thus resulting in elevated Sr/Y (30–105) and (La/Yb)N (4–79) ratios, indicating geochemical features of adakitic rocks. These adakitic granites are medium- to high-K, calcic or calc-alkaline in composition, and display a weak peraluminous character. They have low MgO (0.57–1.84 wt.%, average 1.01 wt.%), Mg# (40–53, average 45), as well as low Cr (3.67–17.98 ppm, average 7.19 ppm) and Ni (2.59–9.30 ppm, average 4.71 ppm) contents. These rocks are enriched in LREE, and show negligible or variable positive Eu anomalies (Eu/Eu∗ = 0.61–3.80, average 1.45) and significant negative Nb and Ta anomalies. Majority of the zircon grains from these adakitic granitic rocks have positive eHf(t) values of 0.09–5.21 with two-stage model ages ranging from 1.1 Ga to 1.6 Ga. These features are compatible with those of adakitic rocks derived from a thickened lower crust in the garnet stability field. Their formation is mainly controlled by the process of crust thickening following the closure of the Qimantag Ocean. The younger quartz diorites belong to medium- to high-K, calc-alkalic or alkali-calcic and metaluminous series, and exhibit a relatively high MgO (2.23–5.18 wt.%) and Mg# (40–56, average 50.11), with significant LREE enrichment and negative Eu anomalies, as well as depletion of Nb, Ta. In addition, the quartz diorites have an enriched eHf(t) values ranging from −5.25 to −3.19. Combining the regional tectonic data, we infer that the younger quartz diorites were derived from enriched lithospheric mantle sources and contaminated by the crustal materials in a post-collision extensional setting.
The East Kunlun Orogenic Belt of the northern Tibetan Plateau can be divided by the Qimantagh–Xiangride, Aqikekulehu–Kunzhong and Muztagh–Buqingshan sutures into, from north to south, the Northern Qimantagh, Central Kunlun and Southern Kunlun belts. The Yazidaban ophiolitic mélange, located in the westernmost region of the Qimantagh–Xiangride suture between the Northern Qimantagh and Central Kunlun belts, consists predominantly of serpentinite, basalt, diabase and andesite. The serpentinites are characterized by low total rare earth element (ΣREE) concentrations and depletion of mid-REEs, showing an ophiolitic ultramafic affinity. The basalts have low ΣREE concentrations, a slight enrichment in light REEs, depletion of large ion lithophile elements and insignificant fractionation of high field strength elements, which are attributed to an incompatible element-enriched mid-ocean ridge basalt. Both diabases and andesites are characterized by high ΣREE, strong enrichment in light REEs, depletion of Nb, Ta, P and Ti, and significant fractionation of high field strength elements. These geochemical characteristics indicate that the diabase and andesite were mostly generated in a subduction-related setting. Igneous zircons from the diabase yielded a 206 Pb/ 238 U age of 421.5 ± 2.2 Ma (MSWD = 0.44), representing the formation time of the subduction-related rocks. Together with the regional geology, these constraints on the ultramafic rock, basalt, andesite and diabase suggest that the Yazidaban ophiolitic mélange represents the remnants of oceanic crust from a back-arc basin and the associated subduction-related magmatic rocks. Supplementary material: Details of U–Th–Pb laser ablation inductively coupled plasma mass spectrometry data of zircons from the diabase, and major and trace element compositions for the samples from the Qimantagh mélange, East Kunlun Orogen are available at https://doi.org/10.6084/m9.figshare.c.4283219
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