Rongfeng Ge

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.

Late Mesozoic rift evolution and crustal extension in the central Songliao Basin, northeastern China: constraints from cross-section restoration and implications for lithospheric thinning

The Songliao Basin, the largest oil-producing basin in China, was the centre of late Mesozoic rifting and lithospheric thinning in northeastern China. However, the rifts are still poorly revealed due to a thick cover of subsidence successions. By structural interpretation and sequential restoration of cross-sections based on new 2D seismic data and well data, this study presents the structural style, basin evolution, and horizontal crustal extension of the central Songliao Basin. We have developed a novel method to retrieve the regional extension principal strains. The results enable an assignment of rifting into two episodes. The earlier episode (ca. 157–130 Ma) was dominated by distributed faulting of numerous planar normal faults trending NNE–SSW, NNW–SSE, or near NS, probably reflecting pre-existing basement fabrics; in contrast, the later episode (ca. 130–102 Ma) was controlled by localized extension along several major listric faults. Horizontal crustal extension during rifting is estimated to have been 11–28 km (10.6%–25.5%), with the long-term average rate varying from 0.20 to 0.51 mm yr–1. Regional horizontal strains show a gradual evolution from biaxial extension at the beginning of rifting to WNW–ESE uniaxial stretching during the later rifting episode. Brittle crustal extension is interpreted to have been associated with vertical strain due to tectonic stretching, which is estimated to have contributed more in thinning the lower crust than the mantle lithosphere. Accordingly, a two-episode dynamic model is proposed to explain rifting in the Songliao Basin. We suggest that the earlier event was dominated by delamination of the thickened continental lithosphere, whereas the later event was probably controlled by regional crustal detachment due to slab subduction and stagnancy of the Izanagi lithospheric plate.

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.

Remnants of Eoarchean continental crust derived from a subducted proto-arc

Newly discovered remnants of Earth’s oldest continental crust are linked to plate tectonics 3.7 billion years ago. Eoarchean [3.6 to 4.0 billion years ago (Ga)] tonalite-trondhjemite-granodiorite (TTG) is the major component of Earth’s oldest remnant continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics started in the early Earth. TTGs are mostly generated by partial melting of hydrated mafic rocks at different depths, but whether this requires subduction remains enigmatic. Recent studies show that most Archean TTGs formed at relatively low pressures (≤1.5 GPa) and do not require subduction. We report a suite of newly discovered Eoarchean tonalitic gneisses dated at ~3.7 Ga from the Tarim Craton, northwestern China. These rocks are probably the oldest high-pressure TTGs so far documented worldwide. Thermodynamic and trace element modeling demonstrates that the parent magma may have been generated by water-fluxed partial melting of moderately enriched arc-like basalts at 1.8 to 1.9 GPa and 800° to 830°C, indicating an apparent geothermal gradient (400° to 450°C GPa−1) typical for hot subduction zones. They also locally record geochemical evidence for magma interaction with a mantle wedge. Accordingly, we propose that these high-pressure TTGs were generated by partial melting of a subducted proto-arc during arc accretion. Our model implies that modern-style plate tectonics was operative, at least locally, at ~3.7 Ga and was responsible for generating some of the oldest continental nuclei.

A 4463 Ma apparent zircon age from the Jack Hills (Western Australia) resulting from ancient Pb mobilization

Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half-billion years of Earth’s history. Determining accurate and precise crystallization ages of these ancient zircons is a ...