Wenbin Zhu

Neoproterozoic ages of the Kuluketage diabase dyke swarm in Tarim, NW China, and its relationship to the breakup of Rodinia

The widely exposed Kuluketage diabase dyke swarm, Tarim Block, NW China, has been considered to have been emplaced in Permian times. New precise zircon U–Pb SHRIMP ages for two samples from the dyke swarm yield Neoproterozoic ages of 823.8 ± 8.7 Ma and 776.8 ± 8.9 Ma. Correlated with peaks of magmatism in South China and Australia at c. 825 Ma and c. 780 Ma, these two new ages provide significant information for palaeocontinental reconstructions. The prolonged duration of the magmatic events, combined with regional stratigraphic relationships, imply that the Tarim Block may have been affected by a mantle plume during the breakup of Rodinia.

Neoproterozoic to Paleozoic long-lived accretionary orogeny in the northern Tarim Craton

The Tarim Craton, located in the center of Asia, was involved in the assembly and breakup of the Rodinia supercontinent during the Neoproterozoic and the subduction-accretion of the Central Asian Orogenic Belt (CAOB) during the Paleozoic. However, its tectonic evolution during these events is controversial, and a link between the Neoproterozoic and Paleozoic tectonic processes is missing. Here we present zircon U-Pb ages, Hf isotopes, and whole-rock geochemical data for the extensive granitoids in the western Kuruktag area, northeastern Tarim Craton. Three distinct periods of granitoid magmatism are evident: circa 830–820 Ma, 660–630 Ma, and 420–400 Ma. The magma sources, melting conditions (pressure, temperature, and water availability), and tectonic settings of various granitoids from each period are determined. Based on our results and the geological, geochronological, geochemical, and isotopic data from adjacent areas, a long-lived accretionary orogenic model is proposed. This model involves an early phase (circa 950–780 Ma) of southward advancing accretion from the Tianshan to northern Tarim and a late phase (circa 780–600 Ma) of northward retreating accretion, followed by back-arc opening and subsequent bidirectional subduction (circa 460–400 Ma) of a composite back-arc basin (i.e., the South Tianshan Ocean). Our model highlights a long-lived accretionary history of the southwestern CAOB, which may have initiated as part of the circum-Rodinia subduction zone and was comparable with events occurring at the southern margin of the Siberian Craton, thus challenging the traditional southward migrating accretionary models for the CAOB.

Subducted Precambrian oceanic crust: geochemical and Sr–Nd isotopic evidence from metabasalts of the Aksu blueschist, NW China

Abstract: The Aksu Proterozoic blueschist terrane in northwestern China is regarded as one of the oldest well-substantiated Precambrian blueschist terranes in the world. Previous work has focused on the high-pressure metamorphism: both the age and P–T conditions of the metamorphic event have been well discussed. However, little attention has been paid to protolith identification of the blueschist terrane. In this paper, we present geochemical and Sr–Nd analyses of metabasalt samples from the blueschist terrane. The results show that the protoliths of these mafic schists were enriched mid-ocean ridge basalt (E-MORB). Crystal fractionation has led to differentiation of trace element concentrations. This provides the opportunity to obtain a protolith Sm–Nd isochron age of 890 ± 23 Ma (MSWD = 0.68). We conclude that the Aksu blueschist terrane represents part of an ocean crust at the NW edge of Rodinia around Tarim at c. 890 Ma. It experienced subduction and exhumation followed by accretion to the Tarim craton. If the north margin of present Tarim faced outboard of Rodinia during subduction, it might form one section of the arcuate subduction zone around the supercontinent. Alternatively, the subduction zone may have been located between Tarim and Australia and would later be closed.

Fault growth and linkage: Implications for tectonosedimentary evolution in the Chezhen Basin of Bohai Bay, eastern China

The Chezhen Basin has highly representative structural features of the Jiyang depression, Bohai Bay area. The structural geometry and the boundary fault linkage exert a strong influence on basin development and depositional environment. Based on structural analysis, at least six early fault segments are identified in the northern boundary of the Chezhen Basin. These fault segments are important in controlling stratal architecture and distribution. The antecedent structures controlled subbasin initiation and development. The Cenozoic rift initiated in the early Eocene with the development of six isolated fault segments associated with deposition of the Es4 member. During the deposition of the lower Es3 member, these six fault segments quickly linked and formed the present architecture frame. Fault linkage has not resulted in a redistribution of displacement. With the expansion of the Chezhen Basin, the depocenters of the upper strata were kept in nearly the same sites until the early Miocene, then the activity of the Chengnan fault ceased. Fault linkage is a significant event in basin evolution, and its process may be very rapid. However, the fault linkage exerted considerable control on sedimentation and evolution of the basin. This study demonstrates that it is necessary to integrate structural and stratigraphic data to reconstruct the temporal and spatial evolution of normal fault zones.

Thermotectonic evolution of Precambrian basement rocks of the Kuruktag uplift, NE Tarim craton, China: evidence from apatite fission-track data

The Kuruktag uplift is located directly northeast of the Tarim craton in northwestern China. Neoarchaean-to-Neoproterozoic metamorphic rocks and intrusive rocks crop out widely in the uplift; thus, it is especially suited for a more complete understanding of the thermal evolution of the Tarim craton. Apatite fission-track (AFT) methods were used to study the exhumation history and cooling of these Precambrian crystalline rocks. Nine apatite-bearing samples were collected from both sides of the Xingdi fault transecting the Kuruktag uplift. Pooled ages range from 146.0 ± 13.4 to 67.6 ± 6.7 Ma, with mean track lengths between 11.79 ± 0.14 and 12.48 ± 0.10 μm. These samples can be divided into three groups based on age and structural position. Group A consists of five samples with AFT apparent ages of about 100–110 Ma and is generally associated with undeformed areas. Group B comprises three specimens with AFT apparent ages lower than 80 Ma and is mostly associated with hanging wall environments close to faults. Group C is a single apatite sample with the oldest relative apparent age, 146.0 ± 13.4 Ma. The modelled thermal history indicates four periods of exhumation in the Kuruktag uplift: late-Early Jurassic (180 Ma); Late Jurassic–Early Cretaceous (144–118 Ma); early-Late Cretaceous (94–82 Ma); and late Cenozoic (about 10 Ma). These cooling events, identified by AFT data, are assumed to reflect far-field effects from multi-stage collisions and accretions of terranes along the south Asian continental margin.

Synchronous crustal growth and reworking recorded in late Paleoproterozoic granitoids in the northern Tarim craton: In situ zircon U-Pb-Hf-O isotopic and geochemical constraints and tectonic implications

Identifying the relative contribution of various crustal and mantle materials in the source of granitoids is crucial for the study of granite petrogenesis and crustal growth. Extensive and diverse late Paleoproterozoic metamorphosed granitoids are exposed in the western Kuruktag block, northern Tarim craton, marking an important tectonothermal event. Here, we report sensitive high-resolution ion microprobe (SHRIMP) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U-Pb ages, in situ zircon Hf-O isotopic data, and whole-rock geochemical data for a variety of granitoids, including monzogranite, quartz diorite/quartz monzonite, garnet-bearing granodiorite, tonalite, and trondhjemite. Geochronological data show that all these granitoids were emplaced in a relatively short period at ca. 1.93–1.94 Ga and were immediately metamorphosed at ca. 1.91–1.92 Ga. In situ zircon Hf-O isotopic data suggest that both mantle-derived juvenile materials and supracrustal rocks (i.e., the Xingditag Group) were involved in magma genesis. The sodic trondhjemite and tonalite were generated by partial melting of newly underplated mafic lower crust originating from both depleted and relatively enriched mantle, with some assimilation of sedimentary materials. In contrast, the other (mostly high-K) granitoids (monzogranite, quartz diorite/quartz monzonite, and garnet-bearing granodiorite) were produced by partial melting of metasedimentary rocks due to intrusion of, and mixing with, variable amounts of mantle-derived mafic magmas, suggesting that these granitoids may also have recorded substantial crustal growth. These petrogenetic interpretations imply that most granitoids in the study area were a result of synchronous crustal growth and reworking, which must be carefully considered in models of continental growth. The adakitic trondhjemite and tonalite have high Sr/Y, La/Yb, Gd/Yb, Nb/Ta, and Zr/Sm ratios, suggesting partial melting with a rutile eclogite residue and implying significant crustal thickening (>50 km). This tectonothermal event most likely occurred in an Andean-type continental arc and was followed by continental collision at ca. 1.85 Ga along the newly recognized late Paleoproterozoic North Tarim orogen. Such an accretion-to-collisional orogen implies an interior position of this area within the Columbia (or Nuna) supercontinent. Based on geological correlations, a Tarim–North China connection is suggested for Columbia reconstruction.

Cenozoic inversion of the East China Sea Shelf Basin: implications for reconstructing Cenozoic tectonics of eastern China

The structural styles of the East China Sea Shelf Basin are described based on the interpretation of seismic profiles. The basin is divided into West and East Rift regions. It has undergone four phases of compressive structural inversions during the late Palaeocene Oujiang Event, the late Eocene Yuquan Event, the early Miocene Huagang Event, and the latest Miocene Longjing Event. Structural inversions have superimposed on the Cenozoic extensional rift basin during these compression events, forming regional unconformity reflectors (T4°, T3°, T2°, and T1°). The Changjiang Depression located in the north of the West Rift Region is characterized by a number of NW-, EW-, to NE-trending faults. However, the other depressions in the East China Sea Shelf Basin are dominated by NNE- to NEE-trending faults. During the Palaeocene, the Taibei Depression was in a marine environment, whereas the Changjiang Depression was terrestrial, although both depressions are located in the West Rift Region. The Hongzhouwan–Okinawa Fault separates the East China Sea Shelf Basin into south and north. The structural styles and the sedimentary environment change longitudinally and transversely, which are controlled by basement structures. The depocentre of the East China Sea Shelf Basin has gradually migrated and became younger from west to east. The structural inversion also becomes younger from west to east, but the intensity of inversion increases from south to north. These structural inversion events reveal a periodic compression and imply that the compressive uplift of eastern China resulted from subduction of the Pacific Plate as well as collision of the Luzon arc and the Asian continent rather than the remote effect of India–Asian collision.

Mid-Neoproterozoic (ca. 830-800 Ma) metamorphic P-T paths link Tarim to the circum-Rodinia subduction-accretion system

Long-lived exterior accretionary orogeny shapes tectonothermal evolution of the peripheral building blocks of supercontinents and leads to considerable crustal growth. However, such accretionary orogeny has only been locally recognized for the Rodinia supercontinent. Here a suite of newly discovered mid-Neoproterozoic high-grade metamorphic rocks in the northern Tarim Craton, NW China, are used to test the exterior accretion hypothesis for Rodinia. These rocks occur as dark-colored mafic and calc-silicate boudins in impure marbles and mica schists. Geochemical data suggest a protolith of arc-related basalts metasomatized by Ca-rich fluids. Mineral assemblages, phase diagram modeling, and mineral compositions for a garnet pyroxenite and a garnet clinopyroxene gneiss reveal upper amphibolite to high-pressure granulite facies peak metamorphism (660–700°C, 11–12 kbar) following a counterclockwise P-T path, which is characterized by prograde burial and heating, followed by near-isothermal burial and retrograde exhumation and cooling. This P-T path is interpreted to have recorded crustal thickening of an earlier magmatic arc transformed to a fore arc by subduction erosion and subsequent burial along bent isotherms near the subduction channel. All studied samples record ca. 830–800 Ma metamorphic zircon U-Pb ages, which probably date the early exhumation and cooling according to Ti-in-zircon temperatures, zircon rare earth element patterns, and Hf isotopes. This is the first mid-Neoproterozoic P-T-t path in Tarim, and it provides metamorphic evidence for a mid-Neoproterozoic advancing-type accretionary orogeny, which is coeval with the initial breakup events of Rodinia and thus links Tarim to the circum-Rodinia accretion system, supporting the peripheral subduction model.

Provenance and Crustal Evolution of the Northern Yangtze Block Revealed by Detrital Zircons from Neoproterozoic–Early Paleozoic Sedimentary Rocks in the Yangtze Gorges Area, South China

Detrital zircon U-Pb ages of six sedimentary rocks in the northern Yangtze Block reveal three major age groups of 740–890 Ma, 1.9–2.1 Ga, and 2.4–2.55 Ga and two minor groups of 2.6–2.7 and 2.9–3.1 Ga. Although Lu-Hf isotope analysis suggest both juvenile crustal growth and reworking of old crust for all the age populations, juvenile input occurred mainly in the Archean and Neoproterozoic time, whereas Paleoproterozoic is a period dominated by recycling of preexisting crust. The predominant Neoproterozoic detritus is mainly derived from coeval igneous rocks distributed along the western and northwestern margin of the Yangtze Block. However, most of the pre-Neoproterozoic sediments cannot be matched with exposed basement rocks within the Yangtze Block. The similar age patterns of the sedimentary rocks in this study imply only a slight variation in the source regions from the Neoproterozoic to Silurian.

Approximately 1.78 Ga mafic dykes in the Lüliang Complex, North China Craton: Zircon ages and Lu‐Hf isotopes, geochemistry, and implications

Mafic dyke swarms are excellent time markers and paleostress indicators. Numerous late Paleoproterozoic mafic dykes are exposed throughout the Trans-North China Orogen (TNCO). Most of these dykes trend NW-SE or NNW-SSE, nearly parallel to the orogen, while a series of E-W trending mafic dykes are restricted in the Luliang and southern Taihang areas in the central segment of the TNCO. These dykes were mostly considered to be linked with breakup of the supercontinent Columbia previously. In this study, 16 mafic dykes were investigated in the Luliang Complex. Zircon LA-ICP-MS dating of four samples yields magmatic crystallization ages of 1.78–1.79 Ga. These dykes belong to the tholeiite series and consist of basalt, basaltic andesite, and andesite. They are enriched in LREE and LILE and depleted in HFSE, and have negative zircon eHf(t) values of −1.7 to −12.2. The E-W trending mafic dykes show similar geochemical and isotopic features compared to the NW-SE trending dykes in other complexes. They were most likely originated from a lithospheric mantle metasomatized by subduction-related fluids and later emplaced along extensional fractures in a postcollisional setting. NW-SE trending fractures were formed due to gravitational collapse and thinning of the lithosphere. E-W trending fractures in the central segment of the orogen constitute a transverse accommodation belt to equilibrate the different amounts of extension between the northern and southern TNCO. The impact of the postorogenic extension might have continued to approximately 1680 Ma as evidenced by the presence of abundant approximately 1750–1680 Ma anorthosite-gabbro-mangerite-rapakivi granite suites (AMCG-like) occurring in the northern NCC.