Bikou Terrane Research Articles

Age, provenance and tectonic setting of the Tonian–Cryogenian clastic successions in the northwest Bikou terrane, NW Yangtze Block, Central China

Studies on the widely-exposed Neoproterozoic geological records in the Bikou terrane (northwestern Yangtze Block, Central China) have produced different tectonic models (e.g., arc, rift and ridge settings), confusing the relationships between the Bikou and Yangtze blocks in the Neoproterozoic. In addition to the Neoproterozoic Bikou Group igneous rocks and Hengdan Group sedimentary strata which have been well investigated, clastic sedimentary records in the westernmost regions are also crucial for understanding the Neoproterozoic tectonic evolution of the terrane. Here, we target a 3000 m-thick siliciclastic outcrop section along the Bailongjiang River and conduct sedimentological, petrographic, elemental geochemical and zircon U-Pb geochronological analyses to constrain depositional time, sediment provenance and tectonic settings. The outcrop indicates an overall coarsening-upward sedimentary succession, from deep-water sequences to ice-water sequences with dropstone textures. Two volcanic tuffaceous rocks from the basal and middle parts of the succession were dated as 773.2 ± 5.9 Ma and 758.0 ± 8.7 Ma, respectively. The upper parts are characterized by thick, continuous, poorly-sorted glacial diamictites and are conformably overlain by Ediacaran strata. We contend that these diamictites are most likely equivalents to the widespread Cryogenian strata in the Yangtze Block. While provenance analysis results demonstrate felsic bedrock-dominated source terranes with minor mafic contributions, the dropstone gravel components, sandstone petrography and heavy mineral compositions and fine-grained sedimentary rock element concentrations are evidently variable vertically, revealing temporal variations in sediment source-to-sink systems in this region during the late Tonian–Cryogenian. We suggest that the late Tonian deposits were most likely fed by proximal mixed sources with continental arc-related and recycled orogenic terranes, whereas the Cryogenian deposits were dominated by detritus from recycled orogenic terranes. Climate change during the Neoproterozoic glacial-interglacial cycles also played a crucial role in the variable deposit compositions by regulating sediment transport agents and abilities. We favor that the Bikou terrane connected with the Yangtze Block under convergent settings during the early-middle Neoproterozoic and was subsequently under extensional settings. Such findings are a significant step in our understanding of the role of the Yangtze Block in global Neoproterozoic tectonic and climatic frameworks.

Formation and evolution of an Early Cambrian foreland basin in the NW Yangtze Block, South China

The Ediacaran to Cambrian succession in the NW Yangtze Block has long been considered to have formed in a passive margin. The Ediacaran is dominated by thick carbonate rocks, whereas the Lower Cambrian comprises a thick clastic succession. The transition from carbonate to clastic rocks and the provenance and tectonic setting of the clastic succession are poorly understood. Stratigraphic correlation shows a distinct stratigraphic absence from the Lower Cambrian to Devonian in the Bikou terrane. A regional seismic profile shows a wedge geometry of the Lower Cambrian from NW to SE. Outcrops reveal an overall coarsening-upward Lower Cambrian succession. The petrographic analysis of clastic rocks indicates immaturity, implying a proximal source. Palaeocurrent measurements of clastic rocks point to dominant SE-vergent orientations. The age spectra of detrital zircons imply that they were derived from Early Cambrian continental arcs and older continental crust. Geological, geophysical and geochemical evidence indicates that an Early Cambrian foreland basin was formed in the NW Yangtze Block. This foreland basin appears to have been strongly influenced by orogenic loading northwestward. We propose that this orogenic event should be named the Motianling orogeny, the origin of which may be related to subduction of the Proto-Tethys ocean beneath the NW Yangtze Block. Supplementary material: Supplementary tables A–D and a petrological description of detrital zircons are available at https://doi.org/10.6084/m9.figshare.c.6332980

Geochemistry, zircon U-Pb chronology and Hf isotope composition of the Heishan’gou iron deposit in the Bikou Terrane, central China: Implication for the genesis of the Yudongzi banded iron formations

The Yudongzi banded iron formations (BIFs) are widely spread in the Neoarchean Yudongzi Group of the Bikou Terrane, Central China. Their sedimentary and metamorphic ages, however, remain controversial, either their forming or reworking processes. In this contribution, we conducted an integrated study of zircon U-Pb-Hf isotope geochemistry, as well as mineral and bulk-rock geochemistry on the Heishan’gou iron deposit, the largest iron deposit of the Yudongzi BIFs, to constrain their formation and metamorphism ages, source of ore-forming material and tectonic setting. Mineral chemistry analysis indicates that the amphibolite, host-rock of the Heishan’gou deposit suffered amphibolite facies metamorphism. LA-ICP-MS U-Pb dating of zircon grains from orthometamorphic host-rocks suggests that the Heishan’gou deposit was formed at the early Neoarchean (2781 ± 43 Ma for amphibolite and 2776 ± 43 Ma for plagioclase-hornblende schist), and metamorphosed at ca. 2480 ± 26 Ma and 1818 ± 44 Ma, respectively. The low contents of aluminum, titanium and high field strength elements in BIFs suggest that they are detritus-free marine chemical precipitates. The rare earth elements (REE) and Y patterns of BIFs display seawater-like signatures with light REE depletion, and positive La and Y anomalies. Meanwhile, their positive Eu anomalies suggest that submarine hydrothermal fluid might also be involved in the precipitation of BIFs. Combined with zircon Hf isotope data and geochemical compositions of amphibolites, it is proposed that the Yudongzi BIFs should be formed at back-arc basin setting. Compared with the representative Archean continent nucleus of the Yangtze Block (Kongling Complex), the Yudongzi Complex experienced distinct crustal evolutionary history. It indicates that the Yudongzi Complex might represent the accretionary block combined to the Yangtze continental nucleus later than the Paleoproterozoic.

Geochronology, geochemistry, and isotopic composition of the early Neoproterozoic granitoids in the Bikou Terrane along the northwestern margin of the Yangtze Block, South China: Petrogenesis and tectonic implications

The early Neoproterozoic granitoids exposed in the Bikou Terrane preserve vital information for understanding the Tonian tectonic evolution along the northwestern margin of the Yangtze Block, South China. However, their accurate forming age, petrogenesis, and tectonic attributes remain ambiguous. This study reports new results on zircon U–Pb geochronology, zircon Hf isotopes, whole-rock geochemistry, and Sr–Nd isotopes of the Baiquesi and Bahaihe quartz monzonite, Shilingou monzogranite, and Maliupu granodiorite from the Bikou Terrane. Detailed zircon U–Pb dating results reveal that the quartz monzonite and monzogranite were formed at ca. 860 Ma, whereas the granodiorite was emplaced at ca. 825 Ma. The quartz monzonite and monzogranite samples exhibit certain linear correlations in the Harker diagram, congruent distribution patterns of rare earth elements, and consistent isotopic compositions, indicating that they shared the same magma source, whereas the monzogranite was formed by differentiation crystallization of primitive quartz monzonite melt. The quartz monzonite samples are calc-alkaline in composition and have high ratios of Sr/Y and (La/Yb)N, coinciding well with the adakitic rocks. The relatively high contents of MgO, Ni, Cr, and Sr, low contents of Yb and Y, low ratios of La/Ce, Rb/Sr, and Th/La, and positive values of εHf(t) (+4.8–+6.7) and εNd(t) (+1.7–+2.1) indicate that they were formed by the interaction between slab-derived melt and mantle wedge. This mechanism may be triggered by a slab rollback process in an arc-related setting. The Maliupu granodiorite samples are classified as calc-alkaline I-type granitoids, and have low concentrations of MgO, Cr, Ni, and Y, and low ratios of Sr/Y and (La/Yb)N. They are featured by highly evolved εHf(t) values from −15.0 to −10.9 and εNd(t) values from −11.8 to −11.9, with Hf and Nd model ages ranging between 2.63 and 2.20 Ga, implying they were derived from partial melting of the ancient lower mafic crust and were formed in a subduction-related setting. Therefore, the new data, combined with those from previous studies in the Bikou Terrane, provide an essential link in the evidence chain for the hypothesis that the northwestern periphery of the Yangtze Block was in a persistent subduction-related tectonic regime in the early Neoproterozoic.

Late Triassic successive amalgamation between the South China and North China blocks: Insights from structural analysis and magnetic fabrics study of the Bikou Terrane and its adjacent area, northwestern Yangtze block, central China

ABSTRACT The Bikou Terrane, located at the conjunction of the Longmenshan fold-thrust belt and the west Qinling orogenic belt in centeral China, was involved in the Late Triassic collision between the South China and North China blocks. The Bikou Terrane has preserved crucial information on structural geometry and kinematics of Triassic tectonics, and is therefore of great importance for reconstructing the Paleo-Tethyan evolutionary history. However, multi-phase tectonic events of the Bikou Terrane are unsettled. This work presents detailed structural analysis based on both the field and laboratory works, which reveals three phases of deformation events in Bikou and its adjacent areas, including top-to-the-SW shearing related to SW-ward thrusting (DI) mainly to the north of the Bikou Terrane, top-to-the-NNW shearing related to NNW-ward thrusting (DII) in the Bikou Terrane, and strike-slip faulting (DIII) locally developed in the northern Bikou Terrane. Anisotropy of magnetic susceptibility (AMS) study and related structural analysis not only support the multiphase deformation but also reveal a gradual transition from the DII-related magnetic fabrics to the DIII-related magnetic fabrics in the Bikou Terrane. Integrating published geochronological data, it is constrained that DI occurred at ca. 237–225 Ma, DII occurred at ca. 224–219 Ma, and DIII possibly occurred during the Early Cretaceous. Based on regional tectonics, the DI event corresponds to the collision between the South Qinling block and the Bikou Terrane, and the DII event reflects the intracontinental amalgamation between the Bikou Terrane and the Yangtze block, which indicates a Late Triassic successive amalgamation from the North China block to the South China block. Intracontinental adjustment represented by the strike-slip (DIII event) occurred after the final amalgamation between the North China and South China blocks. By applying AMS on deciphering structural geometry and multi-phase deformation, our study suggests that AMS is a useful tool for structural analysis.

Inhomogeneous crust-mantle interaction and Triassic tectonic escape of a Proterozoic microplate: A tale of the Bikou Terrane

Compressional stress in collisional orogenic belts open a free boundary for the lateral extrusion of rigid blocks. Here we trace the history of tectonic escape of a Proterozoic microplate and the associated inhomogeneous crust-mantle processes from magmatic records in the Bikou Terrane, which is the largest microblock located among the North China Block, South China Block and Songpan-Ganzi Orogenic Belt. Zircon U-Pb geochronology indicates that the Triassic granitoid intrusions in the Bikou Terrane emplaced at 220–210 Ma. Petrographical, whole-rock geochemical and isotopical features suggest that the granitoids derived from the partial melting of lower crust, with inhomogeneous contribution of mafic magma derived from partial melting of subcontinental lithospheric mantle. Compared with the Yangba pluton in northeastern Bikou Terrane, the five intrusions in the southwestern Bikou Terrane display relatively lower REE concentrations, more negative Eu anomalies (δEu = 0.24–0.93), and lower Nb/Ta and Zr/Hf ratios with the values of 14–20 and 37–45. Furthermore, they display higher (87Sr/86Sr) i from 0.7050 to 0.7075 and lower εNd(t) from −9.4 to −4.7. These features imply that the granitic intrusions in southwestern Bikou Terrane were primary derived from the partial melting of the lower crust with lesser input of mantle materials than the Yangba pluton, indicating inhomogeneous crust-mantle interaction in the Bikou Terrane. We calculated the Moho depth of northeast and southwest Bikou Terrane as ~60 km and ~ 40–50 km, respectively. We propose that the Bikou Terrane escaped southwestward in Late Triassic following a transition in the stress regime of compression from NE-SW to NW-SE, in addition to the slab break-off from east to west. The lateral extrusion resulted in the inhomogenous crust-mantle interaction.

Petrogenesis and tectonic implications of the Neoproterozoic mafic intrusions in the Bikou Terrane along the northwestern margin of the Yangtze Block, South China

The Neoproterozoic mafic intrusions in the Bikou Terrane along the northwestern margin of the Yangtze Block record crucial information on evaluating the tectonic evolution of the Yangtze Block, South China. However, their petrogenesis and tectonic significance are still poorly constrained. In this paper, we present the results of a comprehensive study of zircon U–Pb geochronology and whole-rock geochemistry from the gabbro-diorites and gabbros of the Bikou Terrane. Zircon U–Pb dating results indicate that these gabbro-diorite and gabbro samples were formed coevally at ca. 880 Ma. The gabbro-diorite samples are calc-alkaline in composition and have intermediate contents of SiO2, Mg#, Ni, and Cr. They are enriched in LREEs, and depleted in HFSEs (i.e., Nb, Ta, Zr, and Hf). They have relatively high ratios of Th/Yb, and low ratios of Nb/Yb, Nb/La, Th/Nd, Th/Zr, and U/Th, indicating that they were derived from partial melting of a mantle wedge source metasomatized by slab-derived melts. The gabbro samples are commonly classified as sub-alkaline tholeiitic rocks, and have constantly low to medium values of MgO, Mg#, Cr, and Ni, with distinct spikes at Th, U, Nb and Ta concentrations. They show an E-MORB-like enriched LREEs, low ratios of Nb/La, Ta/Yb, Th/La, and Nb/Zr, and medium ratios of Nb/Yb and Th/Yb, implying that they were most probably originated from the interaction between the upwelling enriched asthenospheric mantle and the mantle wedge peridotite. This mechanism may be triggered by the slab rollback process in a subduction-related environment. Our new investigations, along with available observations from other scholars, support the persistent subduction-related system along the northwestern margin of the Yangtze Block during the early Neoproterozoic.

Further constraints on a Neoproterozoic active continental margin from sandstones of the Hengdan Group in the Bikou Terrane, northwestern margin of the Yangtze Block, South China

Abstract The Neoproterozoic Hengdan Group in the northwestern margin of the Yangtze Block preserves archives for the debated early to middle Neoproterozoic (ca. 835–720 Ma) tectonic evolution of the margin of the Yangtze Block, and could potentially shed lights on the breakup mechanism of the Rodinia supercontinent. However, its depositional age, provenance and tectonic setting remain controversial. We address these issues through studies of petrography, geochemistry and detrital zircon U-Pb ages on the Hengdan Group sandstones. The Hengdan Group sandstones have low compositional and textural maturity and experienced low degree of chemical weathering and sediment recycling. They were mainly sourced from proximal acidic to intermediate arc-related igneous rocks, and their modal compositions, trace elemental contents (i.e., La, Ce, Th, and ΣREE) and ratios (i.e., LaN/YbN and Eu/Eu*) are comparable to those deposited in an active continental margin. Detrital zircons from the Hengdan Group sandstones yield similar unimodal age distributions at 950–740 Ma, with the youngest age clusters at ca. 730 Ma. Combining with previous work, the Hengdan Group was most likely deposited during ca. 733–717 Ma in a forearc basin and its primary provenance was probably from the nearby arc-related acidic to intermediate igneous rocks. In addition, the northwestern margin of the Yangtze Block was most likely in an active continental margin setting between ca. 835 and 720 Ma, and hence the slab retreating during a long-term oceanic subduction is favored as the dominant breakup driving force in the supercontinental margin(s).

Neoproterozoic active margin in the northwestern Yangtze Block, South China: new clues from detrital zircon U–Pb geochronology and geochemistry of sedimentary rocks from the Hengdan Group

AbstractThe Yangtze Block in South China constitutes an important Precambrian landmass in the present East Asian continent. The Neoproterozoic sedimentary successions of the Hengdan Group in the NW Yangtze Block record essential information for deciphering the Neoproterozoic tectonics along the NW margin. However, its depositional age, provenance and tectonic properties remain uncertain. Here, a combined analysis of detrital zircon U–Pb dating and geochemistry is performed on representative samples from the Hengdan Group. Concordant dating results of samples from the bottom and upper parts constrain the maximum depositional age atc.720 Ma. Detrital zircon age patterns of samples reveal a uniformly pronounced age peak atc.915–720 Ma, which is consistent with the magmatic pulses in domains at the NW end of the Yangtze Block. In addition, these samples display left-sloping post-Archaean Australian shale (PAAS)-normalized rare-earth element patterns and variable trace element patterns, resembling sediments accumulated in a basin related to an active continental margin geodynamic setting. Provenance analysis reveals that the main sources featured intermediate to felsic components, which experienced rapid erosion and sedimentation. These integrated new investigations, along with previous compilations, indicate that the Hengdan Group might have been deposited in a fore-arc basin controlled by subduction beneath the Bikou Terrane. Thus, such interpretation further supports proposals for subduction-related tectonics along the western margin of the Yangtze Block during the early Neoproterozoic.

Phase equilibria and geochronology of Triassic blueschists in the Bikou terrane and Mesozoic tectonic evolution of the northwestern margin of the Yangtze Block (SW China)

The northwestern margin of the Yangtze Block is an important part of the northeastern-most margin of the Tibetan Plateau and is composed of several terranes. However, how and when many terranes or microplates accreted to the Yangtze Block remain to be resolved. Among these, the Bikou terrane consists of Archean to Paleoproterozoic basement and Neoproterozoic volcanic rocks. Blueschists occur as lenses along a ductile shear zone in the southern margin of the Bikou terrane. The age of blueschist facies metamorphism remains an unsolved issue but is important to understand the tectonic evolution. The mineral assemblage of the blueschists is sodic amphibole, epidote, chlorite, albite and quartz. The calculated pseudosection constrains the peak P-T conditions of the blueschist-facies metamorphism to be 5.2–6.5 kbar at 350–385 °C. The blueschists contain zircon grains with an age of 765.2 ± 11 Ma, representing the age of the protolith similar to mafic volcanic rocks in the Bikou terrane. Deformed granitic veins in the blueschists have been subjected to blueschist facies metamorphism and contain phengites with Si contents of 3.5–3.6 per formula unit (pfu). The deformed veins contain zircon grains with ages of 232.3 ± 6.8 Ma and 230.3 ± 4.3 Ma, indicating that blueschist metamorphism is younger than 230 Ma. The new data, combined with previous studies, suggest that the Bikou terrane was accreted to the Yangtze Block in the Late Triassic during the collision between the North China and South China blocks. The blueschist belt, which marks the Late Triassic accretionary boundary between the Bikou terrane and the Yangtze Block, merged with the Qinling-Dabie Orogen to the northeast.

Provenance and depositional mechanism analyses of the Yangtianba Formation, northwestern margin of the Yangtze Block, southwestern China

The Yangtianba Formation in the northwestern Bikou Terrane preserves sedimentary archives for the Early to Middle Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block (YB). However, its provenance and depositional mechanism are still matter of debates. This paper presents geochronological and petrographic studies of two granite gravel samples and one coarse-grained sandstone sample as well as facies analysis of the conspicuous Yangtianba Formation conglomerates (YFC). The weighted average 206Pb/238U ages of the two granite gravels are 743 ± 6 Ma and 762 ± 4 Ma, respectively. The detrital zircon U-Pb ages of the sandstone sample peak at ca. 880 to 740 Ma, close to the depositional age of the Yangtianba Formation. Together with the dominantly angular to subangular morphology and clear oscillatory or broad zoning of these detrital zircons, the Yangtianba Formation is likely sourced from the Early to Middle Neoproterozoic igneous rocks. Facies analysis of the YFC shows they are stacked bundles of fining- and thinning-upward units of conglomerates-sandstone, suggesting a decreasing supply of coarse clastic materials from the episodic mass flow (debris flow) in the submarine channels rather than tillites. Combining with the previous work, the Yangtianba Formation probably mainly sourced from the Neoproterozoic igneous rocks in the Micangshan-Hannan area in the northwestern margin of the YB and is subaqueous sediment gravity flow deposits rather than glacial-marine deposits. These findings favor the model that the northwestern margin of the YB may be an active continental margin setting during the Early- to Middle- Neoproterozoic (ca. 835 to 720 Ma).

Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block (southwestern China): Evidence from sandstone geochemistry and detrital zircon U-Pb ages of the Hengdan Group

The Hengdan Group in the northwestern part of the Bikou Terrane preserves sedimentary archives for the debated early to middle Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block (YB), while its depositional age, provenance and tectonic setting remain controversial. This research has reported the geochemical and geochronological studies on the sandstones in the Hengdan Group. Our work indicates that the sandstones in the Hengdan Group experienced low degree of chemical weathering and sedimentary recycling, and are mainly sourced from acidic to intermediate igneous rocks. In addition, their elemental contents (i.e., La, Ce, Th and ΣREE) and ratios (i.e., LaN/YbN and Eu/Eu*) are comparable to those deposited in an active continental margin setting. All samples yield similar age distribution, having a dominated age range at ca. 850 to 720 Ma. Together with the dominantly angular to subangular morphology and clear oscillatory or broad zoning internal structure of these aged detrital zircons, indicating that the Hengdan Group should be proximally sourced from the early to middle Neoproterozoic igneous rocks. In addition, the youngest group of detrital zircon U-Pb ages of the sandstones in the Hengdan Group suggest that the onset and ending of the deposition of this group is later than ca. 741 Ma and ca. 722 Ma, respectively. Combining with previous work, the Hengdan Group was most likely deposited during ca. 741 to 717 Ma in a forearc basin setting at the northwestern margin of the YB, and with its primary provenance probably from nearby acidic to intermediate igneous rocks. In addition, all these above further supports that the northwestern margin of the YB should be a long-lived active continental margin setting during early to middle Neoproterozoic (ca. 835 to 717 Ma) and, the South China Block should be located at the relative peripheral location of the Rodinia supercontinent.

Study on the magnetite genesis and metallogenic age in Chedu gold deposit, Bikou block

The Chedu gold deposite is located in the south rim of bikou terrane. It is the first BIF-type gold deposit in shaanxi province. Magnetite and quartz bands can be seen in the magnetite quartzite type gold ore. According to the electronic probe analysis results, we find that the ratio ofTFeO to oxide is from 98.90% to 99.87% and the levels of TiO2, MgO, A12O3 and MnO are low in single particle magnetite. It is suggested that the magnetite in gold ore is the cause of sedimentary metamorphism. In addition, a total of 22 of the 35 electronic probe analysis data implying that the magnetite was not visible. The maximum is 0.105%. Through the analysis of geochemical samples, the gold grade is proportional to the iron content.The isotope age of the Bikou group is between 764 and 1611 Ma. The indirect method proves that the Chedu magnetite quartzite gold deposit was formed in the Middle Proterozoic-Neoproterozoic.

Study on The Genesis of The Chedu Gold Deposit in Bikou Block

The Chedu gold deposite is located in the south rim of bikou terrane. It is the first BIF-type gold deposit in shaanxi province. Experiment with electronic probes and scanning electron microscopy, we first discovered silver gold mine and calaverite wrapped in single-grain magnetite.According to the results of hydrogen and oxygen isotope analysis of quartz in gold ore, the value of δ18O is from 16.4 ‰ to 16.6 ‰ and δD is from 16.4 ‰ to 16.6 ‰. This value is similar to the hydrogen and oxygen isotope values of quartz in BIF type iron ore. The discovery of wrapped gold and non-visible gold in magnetite suggests that gold is formed earlier or at the same time as the magnetite crystallizes. It is of great significance to study the genesis mechanism of BIF type gold deposit.

Geology and geochemistry of the Jianchaling hydrothermal nickel deposit: T–pH–fO2–fS2 conditions and nickel precipitation mechanism

The Jianchaling nickel deposit in the Bikou Terrane (Shaanxi Province, China) occurs along the boundaries between granite porphyry and carbonated ultramafic rocks (carbonated serpentinite, talc–carbonate rocks, and listwaenite). Serpentine– magnetite, serpentine– magnesite– magnetite, and magnesite– talc– quartz– pyrite– violarite– millerite– chalcopyrite assemblage formed in carbonated ultramafic rocks during hydrothermal activities. Ni-bearing sulphides, coexisting with magnesite, postdated magnetite in carbonated ultramafic rocks. Compared with serpentinite, Ni, Co, Cu, Mn, and Pb concentrate in talc–carbonate rocks. The fact that the NiO contents of magnetite decrease with progressive carbonation of serpentinite suggests that Ni from magnetite concentrated in fluid and contributed to the formation of the Jianchaling nickel deposit. Sulphides precipitated from fluid with log fO2 value varying from −34.5 to −31.8 and log fS2 value varying from −10.3 to −9.2. High pH and HS− activities triggered by transformation of serpentine into magnesite–talc–quartz assemblage promoted precipitation of Ni-bearing sulphides, and finally formed the Jianchaling hydrothermal nickel deposit.

Detrital zircon U–Pb geochronology and provenance of Lower Cretaceous sediments: Constraints for the northwestern Sichuan pro-foreland basin

Detrital zircons were recovered from several units within the Lower Cretaceous sedimentary cover of the northwestern Sichuan Basin, and dated using U–Pb geochronology to assess their provenance and depositional setting. Five major age populations were identified in the U–Pb age spectra: a Neoarchean to late Palaeoproterozoic group (composed of two distinct age populations at 2754–2151 and 2102–1627Ma), a Neoproterozoic group (979–621Ma), an Early Ordovician to Early Carboniferous group (478–326Ma), an Early Permian to Late Triassic group (297–205Ma), and an Early Jurassic to earliest Cretaceous group (188–138Ma). Each of these age populations corresponds with one or several potential sources around the NWSB; the Neoarchean to late Palaeoproterozoic zircons are primarily derived from the South Qinling Belt, the northern Yangtze Craton, the Bikou terrane, and the Songpan–Ganzi terrane, with an additional contribution from the southwestern Yangtze craton in the Neoproterozoic. Recycling of sediments in the Songpan-Ganzi terrane and the South Qinling Belt may be the source of the Early Ordovician to Early Carboniferous zircon population. Early Permian to Late Triassic zircons are predominantly sourced from Triassic granitoids in the South Qinling Belt, as well as the Late Triassic Songpan–Ganzi turbidites during their eastward transportation. Recycling of Upper Jurassic to Lower Cretaceous sediments in the Qinling orogen most likely supplied the observed population of Jurassic to Early Cretaceous age zircons. Detrital zircon age distributions in Lower Cretaceous sediments record a pro-foreland basin setting characterized by intracontinental subduction. Oblique or tangential compression arising from clockwise rotation of the South China Block (SCB) served as an additional driver for the formation of the Cretaceous pro-foreland basin, and reactivation and inheritance may produce differentiation.

Detrital zircon geochronology of Devonian quartzite from tectonic mélange in the Mianlue Suture Zone, Central China: provenance and tectonic implications

ABSTRACTDevonian quartzite occurs as blocks within a phyllite matrix in Puziba area of the Mianlue Suture Zone (MLSZ) in central China. The depositional time of the quartzite is younger than 425 Ma (mainly Early Devonian), constrained by the zircon U–Pb geochronology data from the quartzite, cross-cutting relationships with granite, and palaeontology evidence. The detrital zircons in the quartzite show typical magmatic features with four main age peaks at: 2676–2420 Ma (11.6% of the population), 1791–1606 Ma (4.8%), 997–817 Ma (26.5%), and 597–425 Ma (17.5%). In combination with the zircon εHf(t) values, we propose that the quartzite in the MLSZ was sourced from Neoproterozoic and Palaeozoic magmatic and sedimentary rocks in the South Qinling Block and the South China Block (particularly from the Bikou Terrane), with minor contributions from Archaean and Palaeoproterozoic magmatic units from both of the South and North China blocks. The blocks of quartzite, slate, marble, metasandstone, and chert blocks in the phyllite matrix in the Puziba area show a typical block-in-matrix texture in a tectonic mélange, and provide significant evidence from sedimentary rock blocks rather than ophiolite or volcanic rock for the existence of the MLSZ.

Magma mixing and crust–mantle interaction in the Triassic monzogranites of Bikou Terrane, central China: Constraints from petrology, geochemistry, and zircon U–Pb–Hf isotopic systematics

Mafic microgranular enclaves (MMEs) are common in the monzogranites from the Yangba pluton in the Bikou Terrane of central China. Zircon LA-ICP-MS U–Pb dating yields ages of 208.7±0.7Ma and 209.3±0.9Ma for the host monzogranites, and 211.9±0.8Ma for the MMEs, indicating formation from coeval magmas. The field occurrence and textures including spheroidal shapes, transitional contacts, igneous mineral assemblages, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclase, indicate that the MMEs crystallized from mafic magma that was injected into and mingled with the host felsic magma. The host monzogranites are intermediate-felsic, metaluminous, and lack typical peraluminous minerals or alkaline mafic minerals, suggesting their I-type affinity. The monzogranites display depletion in high field strength elements, with Nb and Ta anomalies, and enrichment in large ion lithophile elements and light rare earth elements (LREE), with slightly negative Eu anomalies (Eu/Eu∗=0.72–0.93). The MMEs from the Yangba pluton are intermediate, metaluminous, and have higher K2O, Al2O3, Fe2O3T, MgO, Ni, and Cr contents than the pluton. The MMEs also display much higher total rare earth element (REE) concentrations and REE patterns that are sub-parallel to those of the host rocks, enrichments in Rb, Th, U, K, and LREE, and depletions in Ba, Nb, Ta, and Ti. These features are similar to those of the host rocks, indicating crust–mantle interaction during their petrogenesis.Zircons from the MMEs have highly variable εHf(t) values (−5.5 to +8.7) with corresponding two-stage Hf model ages (TDM2) of 1.14 to 1.42Ga, indicating they were derived from depleted mantle with crustal contamination. In contrast, zircons from host monzogranite show εHf(t) values ranging from −1.7 to +2.7, with TDM2 of 1.13 to 1.21Ga, corresponding to that of the basement rocks in the Bikou Terrane, which indicates that these granitic magmas were probably derived from the Neoproterozoic juvenile lower crust with minor contribution of Mesoproterozoic ancient lower crust.The integrated petrology and elemental and isotopic geochemistry suggest that the MMEs and host monzogranites were generated by the interaction of a granitic magma and a mafic magma in the lower crust. We envisage that Neoproterozoic subcontinental lithosphere mantle-derived mafic magma intruded the lower continental crust during Late Triassic, following the Late Permian to Early Triassic collision between the South China and North China Blocks.

Sedimentary fill history of the Huicheng Basin in the West Qinling Mountains and associated constraints on Mesozoic intracontinental tectonic evolution

The Qinling Orogenic Belt is divided commonly by the Fengxian-Taibai strike-slip shear zone and the Huicheng Basin into the East and West Qinling mountains, which show significant geological differences after the Indosinian orogeny. The Fengxian-Taibai fault zone and the Meso-Cenozoic Huicheng Basin, situated at the boundary of the East and West Qinling, provide a natural laboratory for tectonic analysis and sedimentological study of intracontinental tectonic evolution of the Qinling Orogenic Belt. In order to explain the dynamic development of the Huicheng Basin and elucidate its post-orogenic tectonic evolution at the junction of the East and West Qinling, we studied the geometry and kinematics of fault zones between the blocks of West Qinling, as well as the sedimentary fill history of the Huicheng Basin. First, we found that after the collisional orogeny in the Late Triassic, post-orogenic extensional collapse occurred in the Early and Middle Jurassic within the Qinling Orogenic Belt, resulting in a series of rift basins. Second, in the Late Jurassic and Early Cretaceous, a NE-SW compressive stress field caused large-scale sinistral strike-slip faults in the Qinling Orogenic Belt, causing intracontinental escape tectonics at the junction of the East and West Qinling, including eastward finite escape of the East Qinling micro-plate and southwest lateral escape of the Bikou Terrane. Meanwhile, the strike-slip-related Early Cretaceous sedimentary basin was formed with a right-order echelon arrangement in sinistral shear zones along the southern margin of the Huicheng fault. Overall during the Mesozoic, the Huicheng Basin and surrounding areas experienced four tectonic evolutionary stages, including extensional rift basin development in the Early and Middle Jurassic, intense compressive uplift in the Late Jurassic, formation of a strike-slip extensional basin in the Early Cretaceous, and compressive uplift in the Late Cretaceous.

Post-collisional plutonism with adakitic signatures: The Triassic Yangba granodiorite (Bikou terrane, northern Yangtze Block)

The post-collisional Yangba granodiorite intruded into the Bikou metasedimentary-volcanic group, southern Mianlue Suture, central China. The host granodiorites contain many mafic microgranular enclaves which have acicular apatite, phenocrysts of host granodiorites, implying that the enclaves have been incorporated as magma globules into host granodioritic magma and undergone rapid cooling. The variation trends of major and trace elements between enclaves and host rocks suggest a mixing and mingling process with respect to their petrogenesis. The mafic microgranular enclaves are characterized by shoshonite with SiO2≤63%, σ (4.54–6.18)〉3.3, high K2O content (4.22%–6.04%), K2O/Na2O〉1; in the K2O-SiO2 diagram, all the samples plot in the shoshonite field, which are enriched in LILE and LREE, with obvious Nb, Ta negative anomalies, indicating a subducting fluid-metasomatised mantle source. Zircon LA-ICP-MS dating of the granodiorites yielded an age of 215.4±8.3 Ma, indicating they were formed during the late-orogenic or post-collisional stage (≤242±21 Ma) of the South Qinling Mountain Belt. The host granodiorites have many close compositional similarities to high-silica adakites from supra-subduction zone setting, but tend to have a higher concentration of K2O (3.22%–3.84%) and Mg#. Chondrite-normalized rare-earth element patterns are characterized by high ratios of (La/Yb)N, the extreme HREE deple-tion and a lack of significant Eu anomalies. In conjunction with the high abundances of Ba and Sr as well as the low abundances of Y and HREE, these patterns suggest a feldspar-poor, garnet ± amphibole-rich fractionation mineral assemblage. High Mg# values demonstrate that the host granodiorites were contaminated by enclave magma. On a whole, integrated geological and geochemical studies suggested the Yangba granodiorites and their mafic micro-granular enclaves resulted from mixing of enriched mantle-derived shoshonitic magma and thickened lower crust-derived felsic magma. In combination with previous studies we consider that the Yangba granodiorites were likely to represent underplating activities and delamination of the lower crust during the late orogenic stage in western Qinling Orogenic Belt.