Keda Cai

Two primary mechanisms responsible for continental crustal growth at convergent margins have been acknowledged, including continental arc magmatism and allochthonous terrane accretion. As such, intra‐oceanic arcs (IOAs), which represent potential accretion terranes, have been regarded as one of the fundamental contributors. It is noted that current IOAs, mainly outcropping in the western Pacific, show remarkable diversity in their crustal composition and rheological properties. However, it remains unclear how the diversity affects the accretion processes of IOAs and further determines their accretion efficiencies. Here, we conducted 2‐D geodynamic modeling to explore these issues. Our models reveal the accretion process and efficiency relying on compositional differentiation and rheological stratification. Specifically, nascent IOAs characterized by a partially molten zone at the level of Moho, where buoyant crust can be separated from dense mantle, result in accretion efficiencies as high as ∼76.4% when slab break‐off took place concurrently. In contrast, mature arcs with cold thermal gradients show a coherent crust‐mantle transition and complete subduction, giving rise to the lowest accretion efficiency. Some of these modeling results align with geophysical observations from modern IOAs, such as the Izu‐Bonin arc and Kyushu‐Palau ridge, which display different accretion processes and efficiencies. The others are likely plausible mirrors of ancient orogenic belts, including the Cenozoic Kohistan arc and the Paleozoic Central Asian Orogenic Belt.

The Paleo‐Asian Ocean (PAO), was located between the Siberian Craton and the Tarim‐North China Craton, and its subduction‐closure history are important for studying the evolution of the Central Asian Orogenic Belt that may have uncovered impacts on paleoclimate. However, when and how the PAO closed remains controversial. Additionally, the potential link between the Permian aridification in the northern North China Craton and the subduction‐closure processes of the PAO also remains elusive. The closure locations of the PAO are at the South Tianshan Belt, Beishan Belt, Solonker Belt, and Changchun‐Yanji Belt, respectively. Here, we apply a LightGBM model to estimate the crustal thickness of the four segments, combined with big geodata analysis methods and geological evidence, to address these scientific issues. The ∼300, ∼280, ∼260, and ∼240 Ma crustal thickening and corresponding zircon ε_Hf(t) isotopic pull‐downs of these belts, coupled with flora distribution, sedimentary records, and paleomagnetic data, support that the PAO closed diachronously in a scissor style from west to east during the Late Carboniferous to the Middle Triassic. The new estimate regarding crustal thickness for northern North China by the LightGBM model was 47 ± 8 km during the Early Permian, which corresponded to an elevation of 2.2 ± 0.8 km. Furthermore, the profile of the new paleo‐elevation of the northern North China suggests that the topography and elevation are remarkably different from those of the Andean Altiplano plateau, which argues against the interpretation of an Andean‐type orogenic plateau resulting in the Permian aridification of the northern North China Craton.

The mineral assemblages, chemistry, and textures of igneous rocks can record crucial information on magmatic processes in transcrustal magmatic systems. To effectively identify such processes, we present systematic petrological, mineralogical, and geochronological data for a suite of lamprophyre dikes that intruded early flood basalts in the Tuoyun basin of Western Tianshan. The lamprophyre dikes show ocean island, basalt-like trace element patterns and depleted Sr-Nd isotope compositions, suggesting that they were derived from a depleted mantle source. Apatite U-Pb dating reveals that the lamprophyre dikes were emplaced at 66 Ma. These lamprophyre dikes consist of three groups of mineral assemblages: (1) Type-I Clinopyroxene (Cpx); (2) Amphibole (Amp) core and Apatite (Ap); and (3) Amp rim, Type-II Cpx, K-feldspar (Kfs), and Plagioclase (Pl). These mineral assemblages are in chemical disequilibrium and correspond to three magma reservoirs within the transcrustal magmatic system. Textural and geochemical features demonstrate that Type-I Cpx represents antecrysts captured from lower crustal crystal mushes. The Amp cores have the same rare earth element patterns as their enclosed Type-I Cpx inclusions, demonstrating that the cores were produced through peritectic reactions consuming Cpx. The third assemblages occur as microlites formed by shallow crystallization of evolved melts. Thermobarometric calculations suggest a lower crust magma reservoir at 20–30 km depth, a middle crust magma reservoir at ∼15 km depth, and a shallow upper crust magma reservoir at <5 km depth, making up a magma plumbing system of the lamprophyre dikes. The transcrustal magmatic system involves multiple stages of open-system processes, including the recycling of early-formed crystals, multiple magma replenishment, peritectic reactions, and crystal fractionation, resulting in the formation of lamprophyre dikes.

The late‐stage union of Pangea was associated with the convergence of Siberia with Laurussia, but the exact timing remains unclear. The orogenic duration of the Kazakhstan block can provide geochronological constraints as it connects Siberia, Baltica, and Tarim. Zircon petrochronology offers a reliable approach for ascertaining the lifespan of an ancient orogen. In this study, we explore three phases of magmatism recorded in detrital zircons from late Paleozoic and Mesozoic sandstone‐siltstones in Western Tianshan, that is, 400–470 Ma, 320–380 Ma, and 280–320 Ma. Based on their age‐propagated Hf isotopes, melt SiO₂ contents, and crustal thicknesses, our findings suggest that the southern limb of Kazakhstan underwent the early Paleozoic amalgamation of microcontinents with arcs, the late Paleozoic maturation of an Andean‐like continental arc, and the late Carboniferous collision of Kazakhstan with the Junggar oceanic basin and the Tarim craton. Such characteristics manifest the long‐term orogenic progression of Kazakhstan. Combining published timelines and paleolatitudes of major orogens and blocks, we propose that the Kazakhstan block welded northeastern Pangea along with the cessation of these orogenic activities around it. Consequently, by docking Kazakhstan with surrounding cratons, the fundamental configuration of Pangea could have been established in the late Carboniferous.

Apatite, Ca₅(PO₄)₃(F,Cl,OH), is an accessory mineral that is widely present in rocks, usually enriched in trace elements and halogens, such as REEs, S, Mn, Sr, F, Cl, etc. The concentration of the above elements in apatite relies largely on the chemical composition of the corresponding melt from which they crystallized, which can well reflect the magma–fluid evolution as well as the related metallogenic processes. Therefore, apatite research is widely used in the fields of magma genesis, isotope dating, isotope tracing, and mineral exploration. The micro-analysis of apatite mainly uses instruments such as an Electron Probe Micro-Analyzer (EPMA), Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), and Large Geometry-Secondary Ion Mass Spectrometry (LG-SIMS). In contrast, rapid and convenient Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) has not yet been applied to quantitative analysis of minerals. In this study, Durango apatite was at first used as the test sample to explore the analytical accuracy (precision and correctness of analysis or measurement) as well as the influencing factors for quantitative analysis of trace elements and halogens in apatite by TOF-SIMS. We further explore the feasibility of using NIST 610 glass as a standard reference material for quantitative analysis of trace elements in other matrices (e.g., apatite). The results show that TOF-SIMS quantification of apatite is feasible. When Durango apatite was used as a standard reference material for quantification, TOF-SIMS quantified trace elements (Sr, Mn, and ⁵⁴Fe) and halogens (F and Cl) in apatite with a precision of up to 4.41% (RSD) and a correctness of up to 0.07%. NIST 610 glass can be used as a standard reference material for quantitative analysis of apatite by TOF-SIMS, and the error between measured and true values is within a factor of 3.

High-Sr/Y granitoids in continental settings are sometimes erroneously regarded as the products derived from partial melting of thickened/delaminated mafic lower curst under relatively higher pressures (>1.5 GPa) in a collisional orogenic setting. In fact, multiple magmatic processes in the trans-crustal magma system, such as recycling of antecrysts, crustal assimilation, and fractional crystallization, can create or modify the primary “adakitic” signature. As a result, the generation of adakitic magmas in continental settings remains controversial from a bulk-rock perspective. Here, we address the origin of adakitic plutonic rocks through geochemical and textural characterization of rock-forming minerals in the pyroxene-bearing Zhuyuan granodiorite, West Qinling, China. The Zhuyuan granodiorite formed in a post-collisional setting and primarily consists of resorbed orthopyroxene, three types of clinopyroxene, amphibole, two types of plagioclases, K-feldspar, biotite, and quartz. Type-1 Cpx has high XMg (70.0–81.7). Type-2 Cpx displays normal zoning and decreasing XMg (80.9 to 71.5) from the core to rim. Type-3 Cpx is reversely zoned, where the rims have higher XMg (75.5–86.9), Ni, Cr, suggesting a recharge event. Orthopyroxene has high-Ni and -Cr contents, as well as high XMg (80.9–82.8), indicative of antecrysts that grew in mafic magma reservoirs. The injection of magmas from different sources is supported by sieve-textured plagioclase and crystal size distributions of non-poikilitic amphibole. Finally, non-sieve textured plagioclase, biotite, K-feldspar, and quartz are late-crystallized phases, indicative of an orthocrystic origin. The melts in equilibrium with these orthocrysts display significantly higher Sr/Y values than the magma batches that crystallized other mafic phases (i.e., amphibole, clinopyroxene, and orthopyroxene). Thus, we propose that the system involved an initial high-Sr/Y melts in equilibrium with the orthocryst assemblage was generated by water-fluxed melting of intermediate to felsic sources. The addition of low Sr/Y non-orthocrysts (e.g., amphibole and pyroxene) and associated melt diluted the original “adakitic signal” in the magma reservoir and drove the bulk composition to more mafic values. Consequently, the Zhuyuan pyroxene-bearing granodiorite represents a mixture of crystals with diverse origins and distinct magma batches of various compositions (from felsic to mafic compositions). Our study emphasizes that the origin of adakitic granitoids cannot be clearly deciphered without geochemical analysis of the constituent minerals. We also suggest that Sr/Y values in plutons should be cautiously used in paleo-crustal thickness estimates in collisional settings because of possible open system scenarios as described here.

Arc‐arc collision represents one of crucial geological processes to shape mountain belts on the Earth. The deeply‐eroded Central Asian Orogenic Belt (CAOB) is characterized by the collision of multiple arc systems and provides a natural laboratory to understand arc‐arc collision in 4D given the exposure of crustal rocks at different levels. Here we focus on the ∼NW‐SE Irtysh fault zone, which represents a major suture zone within the CAOB to bound arc systems of the Chinese Altai and the West/East Junggar in NW China. Our structural work, for the first time, delineates the spatial position of the western segment of the Irtysh Fault between the Chinese Altai and the West Junggar, and demonstrates a phase of brittle strike‐slip deformation with sinistral kinematics in the Permian. An earlier episode of ∼NE‐SW shortening event affected both southern Chinese Altai and northern West Junggar, forming ∼NW‐SE fold structures. Both ∼NE‐SW shortening and sinistral strike‐slip faulting overlap in time with the collision of the Chinese Altai and the West Junggar during the latest Carboniferous to Permian as constrained by new detrital zircon data, thus indicating a transition from orthogonal collision to oblique convergence between the Chinese Altai and the West Junggar. A comparison with ductile deformation at deep structural levels along the eastern segment of the Irtysh Fault between the Chinese Altai and the East Junggar allows for investigation of arc‐arc collision in 4D, which highlights deformation decoupling across different crustal levels, with an intermediate episode of orogen‐parallel flow recorded within deep crustal rocks.

The Tianshan orogenic belt, part of the Central Asian Orogenic Belt, offers an opportunity to examine the complexities of an orogenic system that records long‐term intracontinental deformation. The Tianshan have been reactivated multiple times since the Mesozoic, but the mechanisms and driving forces of these various orogenic events are not well constrained. Moreover, the spatial exhumation pattern of the entire Tianshan remains poorly studied. We present new zircon and apatite (U‐Th)/He and apatite fission track thermochronological data for samples from the northwestern part of the Chinese Western Tianshan. They indicate three distinctive phases of rapid cooling in the late Carboniferous‐early Permian, Late Triassic‐Early Jurassic, and Cretaceous. The first phase can be linked to uplift and exhumation related to the subduction/closure of the Paleo‐Asian Ocean, while the episodic cooling during the Late Triassic‐Early Jurassic (250–190 Ma) and Cretaceous (115–80 Ma) are interpreted as related to uplift and exhumation associated with strike‐slip deformation and Mesozoic clockwise or anticlockwise rotation of the Junggar basin. Our new data, in concert with a compilation of previously published data from elsewhere in the region, reveal that the Tianshan underwent a greater amount of exhumation in the southern section, and less exhumation took place to the north. All available data also support the notion that the exhumation process has been essentially the same in tectonic blocks along strike since late Paleozoic. During the Cenozoic, the Tianshan experienced large‐scale, rapid exhumation starting in the late Miocene (12–10 Ma) and not the early Miocene as has been previously proposed.

Supplemental data (analytical methods, results, etc.) for the granitoids from the Northern Yili Block.