Zuyi Zhou

Origin of atoll garnets in eclogites and implications for the redistribution of trace elements during slab exhumation in a continental subduction zone

Abstract Detailed electron- and ion-microprobe analyses were carried out on atoll-shaped and normal garnets in ultrahigh-pressure (UHP) metamorphic eclogite from Dabie, east-central China. Compositional profiles of both normal garnets and rings of atoll garnets show well-preserved growth zoning with a decrease in Mn, Ca, and heavy rare earth elements (HREEs), and an increase in Mg toward rims. Manganese and middle rare earth element (MREE) enrichments are observed near garnet rims. Island- and peninsula-shaped garnet inside atolls are homogeneous in major elements and show the same composition as garnet rims, whereas the HREE concentrations are similar to those of the normal garnet cores. Electron back-scatter diffraction (EBSD) analyses show that the island- and peninsula-shaped garnet fractions inside atolls have crystallographic orientations identical to that of the atoll rings. These observations suggest that the atoll garnets were formed by the consumption of earlier-formed cores by fluid released from both hydroxyl exsolution from the nominally anhydrous minerals (NAMs) and lawsonite decomposition at the onset of exhumation, (i.e., garnet breakdown was from the inside and re-growth from outside to inside). Though somewhat restricted, this study reveals that because garnet and zircon act as sinks for HREEs and Zr, respectively, the majority of released HREEs and Zr were likely re-incorporated into newly grown garnet and zircon during the Dabie UHP metamorphic slab exhumation.

Protracted oceanic subduction prior to continental subduction: New Lu-Hf and Sm-Nd geochronology of oceanic-type high-pressure eclogite in the western Dabie orogen

Abstract Oceanic-type high-pressure eclogite from the Huwan mélange in the western part of the Dabie orogen, north of the classic ultrahigh-pressure terrane, has been investigated with combined Lu-Hf and Sm-Nd geochronology. The eclogites are thought to have formed during the subduction of oceanic crust prior to the subduction of the Yangtze craton beneath the North China block. Garnet Lu-Hf and Sm-Nd garnet-whole rock dates yield similar ages of 257.4 ± 1.4 and 252.5 ± 2.7 Ma, the youngest ages reported thus far from this mélange. Despite the partially preserved prograde major- and traceelement zoning in garnets, the Lu-Hf and Sm-Nd ages mostly reflect the high-pressure eclogite-facies metamorphism instead of the early phase of garnet growth. The data define a new high-pressure age population for the Dabie orogen, which provides a more detailed insight into the subduction history of the Yangtze craton before continental collision. The new Lu-Hf and Sm-Nd dates for these oceanictype eclogites, combined with existing geochronological information, range from 315 to 253 Ma. This broad interval suggests that the overall subduction of oceanic crust occurred over a period of ~60 Ma and that different slices with distinct pressure-temperature histories underwent HP metamorphism at distinct times. A model of a continuous evolution from oceanic to continental subduction is favored, and the onset of the deeply continental subduction occurred probably not before ca. 257 Ma.

Late Cretaceous-Cenozoic exhumation history of Tiantangzhai region of Dabieshan Orogen: Constraints from (U-Th)/He and fission track analysis

Integrated fission track and (U-Th)/He analysis is carried out on 6 apatite and 6 zircon samples from a near vertical section in The Tiantangzhai region at the core of the present Dabieshan orogen. The result shows that the region experienced cooling/exhumation during the Late Cretaceous and Early Tertiary period. Age-elevation relationships for different dating systems and different minerals suggest a pulse of rapid exhumation at ∼110 Ma before present, preserved in the structurally highest samples. At lower elevations, ages begin to decrease with decreasing elevation, suggesting lower exhumation rates since 90 Ma. Two periods of different exhumation rates are identified since 90 Ma. The average apparent exhumation rate for the period of 43.4–22.5 is 0.062 km/Ma, whereas that for the period of 76.4–47.4 Ma is 0.039 km/Ma.

Tracing a late Mesozoic magmatic arc along the Southeast Asian margin from the granitoids drilled from the northern South China Sea

Abstract The granitoid suites encountered by drilling in the northern South China Sea (SCS) remain important for understanding the evolution of the late Mesozoic Southeast Asian continental margin. They comprise a range of rock types including diorite, tonalite, granodiorite, monzogranite and syenogranite with SiO2 spanning 56.4–76.8%. Newly acquired secondary ion mass spectrometry (SIMS) U–Pb ages of samples from 14 boreholes indicate two key magmatic episodes: Late Jurassic (161.6–148.2 Ma) and Early Cretaceous (136.5–101.7 Ma). Jurassic magmatism probably began in late Middle Jurassic time, documented by the dates of inherited zircons. The granitoids are dominated by metaluminous to weakly peraluminous I-type granites, are transitional between magnesian and ferroan, and encompass calc-alkaline, high-K calc-alkaline, and shoshonitic series. The geochemical signatures suggest that these granitoids were mostly generated in a normal continental arc environment. Notable features of the I-type samples are well-defined negative Nb–Ta–Ti anomalies typical of arc-related magmas. Taken together, the late Mesozoic arc granites of the SCS, the accretionary wedge of the Palawan terrane to the southeast, and the zone of lithospheric extension north of the SCS throughout Southeast China, define a southeast-to-northwest trench-arc-backarc architecture for the late Mesozoic Southeast Asian continental margin whose geodynamic setting is related to subduction of the Palaeo-Pacific slab beneath the Asian continent. Two key subduction episodes are recognized, one in Late Jurassic and the other in Early Cretaceous time.

Rapid exhumation of the Tianshan Mountains since the early Miocene: Evidence from combined apatite fission track and (U-Th)/He thermochronology

Combined apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometries can be of great value for investigating the history of exhumation of orogenic belts. We evaluate the results of such a combined approach through the study on rock samples collected from the Baluntai section in the Tianshan Mountains, northwestern China. Our results show that AFT ages range from ∼60 to 40 Ma and AHe ages span ∼40–10 Ma. Based on the strict thermochronological constraints imposed by AHe ages, forward modeling of data derived from AFT analyses provides a well-constrained Cenozoic thermal history. The modeled results reveal a history of relatively slow exhumation during the early Cenozoic times followed by a significantly accelerated exhumation process since the early Miocene with the rate increasing from <30 m/Myr to >100 m/Myr, which is consistent with the inference from the exhumation rates calculated based on both AFT and AHe age data by age-closure temperature and mineral pair methods. Further accelerated exhumation since the late Miocene is recorded by an AHe age (∼11 Ma) from the bottom of the Baluntai section. Together with the previous low-temperature thermochronological data from the other parts of the Tianshan Mountains, the rapid exhumation since the early Miocene is regarded as an important exhumation process likely prevailing within the whole range.

Crystal-size distribution and composition of garnets in eclogites from the Dabie orogen, central China

Abstract Crystal-size and spatial distributions of minerals in metamorphic rocks provide insight into their nucleation, growth environment, and the metamorphic evolution of their host rocks. Episodic nucleation and growth histories for garnets in eclogites from the Dabie orogen are revealed by variations in crystal size, geochemical zoning, and mineralogy of inclusions in garnets. The studied garnets show pseudo-lognormal crystal-size distributions (CSDs), prograde chemical zoning patterns indexed by mineral inclusions, and are mainly distributed at random. Constant growth rate for each episode is proposed based on the mineralogy of inclusions in garnets and the chemical zoning patterns. The CSD shapes were evaluated in terms of a size-dependent proportionate growth model and a thermally accelerated, diffusion-controlled model but neither is consistent with the geochemical data and the concordant occurrence of mineral inclusions in garnet. Initial increasing nucleation rate followed by a subsequent, medial stage of nearly constant growth and finally declining nucleation rate is inferred from the CSD data. This study suggests that linking chemical analysis with textural analysis is crucial to avoid misleading interpretation solely through CSD shapes of minerals in metamorphic rocks.

Tracing an Early Jurassic magmatic arc from South to East China Seas

Drilling has revealed suites of magnesian granite and diorite emplaced in Early Jurassic time (198-195 Ma) and an arc-related low-temperature (678 to 696 °C) magmatism in NE South China Sea. These rocks have 87Sr/86Sri (0.705494 to 0.706623) and eNdt (−0.9 to +2.2) as evidence of evolved mantle-derived magmas, coupled with enriched fluid-mobile elements Cs to K and Pb implying involvement of subduction-zone fluids. Another Early Jurassic granodiorite (zircon U-Pb 187 Ma) drilled from the SW East China Sea, is a magnesian high-K calc-alkaline, is comparably confined to a range of low-temperature (~675 °C) arc-related granite, characterized by enrichment of fluid-mobile elements and Nb-Ta depletion. Its Sr-Nd isotopes (87Sr/86Sri = 0.705200, eNdt = 1.1) suggest a product of evolved mantle-derived melts. Together with detrital igneous zircons from Paleocene sequences, these observations reveal an Early Jurassic arc-related low-temperature (600 to 740 °C) magmatism in the SW East China Sea. These arc-related granitoids, along with those from SE Taiwan, could define an Early Jurassic NE-SW-trending Dongsha-Talun-Yandang magmatic arc zone along the East Asian continental margin paired with Jurassic accretionary complexes from SW Japan, E Taiwan to the W Philippines. This arc-subduction complex assembly was associated with oblique subduction of the paleo-Pacific slab beneath Eurasia, presumably responsible for early Jurassic lithospheric extension in South China block.

Early Mesozoic orogeny in Fujian, southeast China

Abstract The Mesozoic sedimentary successions of west Fujian evolved from Lower Triassic deep marine fine turbidites to Upper Triassic and Jurassic coal-bearing molasse deposits, and represent a typical foreland basin sedimentary sequence. This foreland basin was generated by an early Mesozoic continental collision to the south, between the South China block and the South China Sea block. This collision led to the formation of Permo-Triassic S-type granites, and a first stage of thrusting in a NNW-SSE direction. Post-collisional convergence resulted in the accumulation of thick molasse deposits. Two main unconformities, within the molasse and separating the molasse from the underlying flysch, necessitate a re-evaluation of the traditional concepts of ‘Indosinian’ and ‘Yanshanian’ movements in SE China. The tectonic and stratigraphic evolution of the west Fujian foreland basin, together with the structural features in the basin, show that the Early Mesozoic orogeny in the region was a mild and continuous process.

Late- and post-Variscan evolution of the Ardennes in France and Belgium: constraints from apatite fission-track data

Abstract Apatite fission-track (AFT) analyses were performed on 13 Late Palaeozoic samples in order to unravel the late- to post-Variscan evolution of the Ardennes. The dated AFT ages cover a range from 290±33 Ma to 168±12 Ma, and the mean confined track lengths correspond to a unimodal distribution, with means varying between 13.1±0.1 µm and 11.7±0.3 µm. These ages for the sedimentary rocks are clearly younger than the respective stratigraphic ages, indicative of a cooling through the apatite partial annealing zone after post-depositional complete annealing. All available AFT data (290–146 Ma) from this region might be classified as three groups, that is 290–229 Ma, 218–198 Ma and 190–146 Ma, at least in correlation with three exhumation events. Using an inverse model, four major cooling episodes are identified from the modelled temperature–time (T–t) paths. The first rapid cooling (4.2–5.4 °C Ma−1, 320–300 Ma) corresponds to the late-Variscan rapid thrusting that ceased at about 300 Ma. The second cooling episode (0.2–4.0 °C Ma−1, up to 230 Ma) activated differentially, and was probably controlled by the post-Variscan transtension. The third cooling regime (0.1–0.3 °C Ma−1, 230–45 Ma) in the Ardennes Allochthon is slow, and represents a long-term and slow exhumation. In the Brabant Parautochthon, however, it is subdivided into 0.7 °C Ma−1 (225–110 Ma) and 0.2 °C Ma−1 (110–45 Ma). The last accelerated cooling (0.7–1.1 °C Ma−1, since 45 Ma) that affected the whole Ardennes is associated with a south–north compression during the Pyrenean phase.

The Evolution of the South China Sea Basin in the Mesozoic–Cenozoic and Its Significance for Oil and Gas Exploration: A Review and Overview

The greater South China Sea (SCS) Basin is composed of basins of different generations and styles. These polyhistory basins formed in complicated geologic settings and evolved through different tectonic regimes. Based on a classical basin classification scheme and data from previous studies, we summarize the evolution of tectonic environments of the SCS in the Mesozoic–Cenozoic into a Late Triassic–middle Eocene divergent-convergent cycle and a late Eocene–present divergent-convergent cycle. The two cycles are in turn composed of four evolutionary phases, which are (1) Late Triassic–Middle Jurassic divergent continental margin setting, (2) Late Jurassic–middle Eocene convergent intracontinental setting, (3) late Eocene–Miocene divergent continental margin setting, and (4) Pliocene–present convergent continental margin setting. We identify temporal sequence and spatial distribution of major polyhistory basins in the SCS associated with the four basin evolutionary phases in the two tectonic cycles. Each basin corresponds to a specific pressure, space, and temperature, and overprinting of the basin caused changes in pressure, space, and temperature with time. Unraveling this complex and dynamic nature of the polyhistory basins can be instrumental in assessing the hydrocarbon potential and exploration risk in the SCS.