Convergent Continental Margin Research Articles

Timeframe of eastern Paleo-Tethys closure: Constraint on the Songpan-Ganzi Complex by big data-based multiproxy provenance analysis

The Songpan-Ganzi Complex (SGC) is one of the largest turbidite complexes on Earth. The source-to-sink processes of the SGC are closely linked to the closure of the Paleo-Jinsha Ocean in the eastern Paleo-Tethys realm, so the spatiotemporal transitions in SGC provenance provide the potential to constrain the closure time. A comprehensive multiproxy provenance investigation, integrating detrital zircon UPb geochronology, whole-rock SmNd isotope data, and paleocurrent analysis, shed light on the origin of the Triassic SGC turbidites. In this study, it was determined that a significant proportion (exceeding 70%) of the detrital material comprising the SGC turbidites was sourced from the northwestern margin of the Yangtze Craton and the Qinling Orogen. A comprehensive and quantitative provenance study revealed that the Songpan-Ganzi turbidites primarily originated from collisional orogenic belts at convergent continental plate margins and secondarily from magmatic arcs and other tectonic units surrounding the basin. In this study, it was demonstrated that the SGC contains important information on the convergence of Eurasia and the tectonic evolution of surrounding terranes. (1) The South Qinling Orogen experienced significant rapid exhumation during the Ladinian to early Carnian. (2) The closure of the Paleo-Jinsha Ocean most likely occurred during the transition period between the upper and lower members of the Zhuwo Formation (ca. 231 Ma, middle Carnian). (3) During the Late Triassic, the crust of the Yidun Terrane underwent significant vertical accretion, resulting in a topography exceeding that of the Jomda-Weixi arc by the late Norian.

Zirconium stable isotope fractionation during intra-crustal magmatic differentiation in an active continental arc

Zirconium (Zr) stable isotope variations occur among co-existing Zr-rich accessory phases as well as at the bulk-rock scale, but the petrologic mechanism(s) responsible for Zr isotope fractionation during magmatic differentiation remain unclear. Juvenile magma generation and intra-crustal differentiation in convergent continental margins may play a crucial role in developing Zr isotope variations, and the Northern Volcanic Zone of the Andes is an ideal setting to test this hypothesis. To investigate the influence of these processes on Zr stable isotope compositions, we report δ94/90ZrNIST of whole rock samples from: 1) juvenile arc basalts from the Quaternary Granatifera Tuff, Colombia; 2) lower crust-derived garnet pyroxenites (i.e., arclogites), hornblendites, and gabbroic cumulates found in the same unit; and 3) felsic volcanic products from the Doña Juana Volcanic Complex, a dacitic composite volcano in close proximity to and partially covering the Granatifera Tuff. The basalts have δ94/90ZrNIST values ranging from −0.025 ± 0.018 ‰ to +0.003 ± 0.015 ‰ (n = 8), within the range of mid-ocean ridge basalts. The dacites have δ94/90ZrNIST values ranging from +0.008 ± 0.013 ‰ to +0.043 ± 0.015 ‰ (n = 14), slightly positive relative to the Granatifera and mid-ocean ridge basalts. In contrast, the (ultra)mafic cumulates have highly variable, predominantly positive δ94/90ZrNIST values, ranging from −0.134 ± 0.012 ‰ to +0.428 ± 0.012 ‰ (n = 15). Individual grains and mineral fractions of major rock-forming phases, including garnet (n = 21), amphibole (n = 9), and clinopyroxene (n = 18), were analyzed from 8 (ultra)mafic cumulates. The mineral fractions record highly variable Zr isotopic compositions, with inter-mineral fractionation (Δ94/90Zrgarnet-amphibole) up to 2.067 ‰. Recent ab initio calculations of Zr–O bond force constants in rock-forming phases predict limited inter-mineral Zr isotope fractionation in high-temperature environments, suggesting that the large fractionations we observe are not the product of vibrational equilibrium processes. Instead, we propose a scenario in which large Zr isotopic fractionations develop kinetically, induced by sub-solidus Zr diffusion between coexisting phases via changes in Zr distribution coefficients that arise from changes in temperature. Altogether, Zr isotope variability in this calc-alkaline continental arc setting exhibits no correlation with indices of magmatic differentiation (e.g., Mg#, SiO2), and is not a simple function of fractional crystallization. Furthermore, the garnet clinopyroxenite cumulates studied here represent density-unstable lower arc crust material; consequently, material with isotopically variable δ94/90Zr can be recycled into the mantle as a consequence of lower crustal foundering.

From foredeep to orogenic wedge-top: The Cretaceous Songliao retroforeland basin, China

Songliao Basin, the largest Mesozoic intracontinental nonmarine basin in eastern China, initiated during the latest Jurassic as a backarc extensional basin; rifting failed and thermal cooling controlled subsidence through the early Late Cretaceous. Integrating 2-D and 3D reflection seismic and borehole data with regional geological studies, we interpret sedimentary sequence and structural patterns of the Coniacian-Maastrichtian fill of Songliao Basin as defining a retroforeland basin system developed after 88 Ma (marked by the T11 unconformity in the basin), including (1) significant increase in the thickness of the Nenjiang Formation eastward towards orogenic highlands of the Zhangguangcai Range and the convergent continental margin; (2) a shift of detrital provenance in the basin from north to southeast; and (3) propagation of E-W shortened structures, increasing eastward in amplitude, frequency, and degree of inversion toward the orogen. During latest Cretaceous, foreland basin fill progressively deformed, as the foredeep evolved to a wedge-top tectonic setting, marked by the basin-wide T04 unconformity within the upper Nenjiang Formation at 81.6 Ma. Much of the basin was brought into the orogenic wedge and eroded by the end of the Cretaceous. Late Jurassic/Early Cretaceous backarc rifting of uncratonized basement comprised of accreted terranes likely facilitated and localized the foreland. Synrift normal faults reactivated and extensively inverted as thrust faults are prominent in the eastern 1/3 of the basin, whereas folds developed above detachments in shaley early post-rift strata dominate the western 2/3 of the basin. Songliao foreland development likely was driven by changing plate dynamics and collision along the Pacific margin after 88 Ma.

Triassic granites in the West Qinling Orogen, China: implications for the Early Mesozoic tectonic evolution of the Paleo-Tethys ocean

ABSTRACT The Western Qinling Orogen is marked by secular Triassic magmatism, a thorough understanding of the mechanism of which could provide critical insights into the reconstruction of the Palaeo-Tethys tectono-magmatic activity. Here, we present new geochemical, geochronological and Lu-Hf isotopic data for three batholiths and review regional data. These batholiths have similar rock assemblages (mainly granodiorite and monzogranite), which are characterized by dominantly plagioclase, quartz, and K-feldspar, subsequently amphibole and biotite. Zircon U-Pb data reveal that they represent the long-duration magmatism during the Triassic (ca. 251–223 Ma). Mineralogical characteristics and geochemical affinities manifest that these rocks in the Zeku district can be explicitly delineated as high-K, calc-alkaline, weakly fractionated I-type granites. They yield significantly negative εHf(t) values ranging from −12.81 to −1.26 with old two-stage mantle depleted model ages between 2054 and 1319 Ma. In tandem with Th/Nb (0.98), Th/La (0.39) and La/Nb (2.40) ratios and moderate pressure conditions (ca. 7–10 kbar), the studied granites were derived from partial melting of the Middle Paleoproterozoic to Middle Mesoproterozoic lower crust source region with minor mafic hydrous magma addition. Our new understandings, in conjunction with the temporal-spatial distribution characteristics of magmatism in the West Qinling Orogen, as well as the regional tectonic evolution, suggest that the superimposed orogeny evolved from the northward subduction of the Palaeo-Tethys Ocean (264–225 Ma) through syn-collision (225–215 Ma) to post-collision (beginning at ca.215 Ma) between the North China Craton and South China Block. In this scenario, at the convergent continental margin, the oceanic plate may have undergone melting producing the melts parental to the Zeku granites.

The nature of Paleoproterozoic basement in the northern Yangtze and its geological implication

Archean-Paleoproterozoic crystalline basements are sporadically distributed in the Yangtze Craton, but the fundamental problems about the formation and evolution of these Precambrian basements have not been completely solved. In this study, Precambrian basement rocks in the Feidong Complex provide an excellent opportunity for inspecting the evolution of the Yangtze Craton. It is composed mainly of Paleoproterozoic granitic gneiss (2470–2434 Ma), amphibolite (2482–2461 Ma) and potassic granite (2002–1963 Ma). Granitic gneisses show typical I-type affinity, their relatively enriched Nd-Hf isotopic components (εNd(t) = –1.3 to –1.2; εHf(t) = –5.2 to 1.1) indicate the Archean crustal growth (3.3–3.1 Ga) and subsequent reworking during Early Paleoproterozoic. Meanwhile, the arc-like geochemical signature as well as εNd(t) (3.1–3.8) and εHf(t) (–1.9–3.0) values in amphibolite reveal the interaction between lithospheric mantle wedge peridotite and slab-derived fluids. Early Paleoproterozoic magmatic activities in the Feidong Complex most likely occurred at convergent continental margin related to a subduction event. Furthermore, a comparison of crustal evolution in various Precambrian continental blocks scattered in the Yangtze Craton proves their independence prior to Paleoproterozoic. The potassic granites show typical S-type granites affinity with high Sr/Y, Rb/Sr ratios, their negative εNd(t) (–15.5 to –11.5) and εHf(t) (–12.8 to –7.7) values indicate the reconstruction of Archean continental crust related to continental collision. In conjunction with broadly coeval magmatism and metamorphism events distributed in Yangtze Craton, it is speculated that the orogeny is linked to the internal assembly and convergence event of Yangtze and Columbia supercontinent, respectively.

Provenance of the early Paleozoic sedimentary succession in the Lancang Block, SW China: Implications for the tectonic evolution of the northern margin of Gondwana

• Sediments of the late Ordovician Lancang Group in the Lancang Block, SW China, were sourced mainly from the northern margin of Gondwana . • Provenance change of the sediments might reflect initial subduction of the Proto-Tethys in the Ordovician . • The Lancang Block had affinity to Gondwana during the early Paleozoic. In the present study, we report zircon U-Pb ages, Hf isotopic compositions, and geochemistry for sedimentary rocks of the early Paleozoic Lancang Group in the Lancang Block of the western Yunnan Tethyan orogenic belt, SW China, to reveal the nature of sedimentary provenance with further implication on the orogenic evolution. Detrital zircon grains yield two major U-Pb age populations at 1300–900 Ma and 600–500 Ma, while their ε Hf (t) values range from –28.7 to + 13.0 with Hf model ages mostly cluster at ∼ 2200–2100 Ma. These features on zircon U-Pb age patterns and Hf isotopic compositions are extremely similar to those of exposed rocks in North India. It is noteworthy that the sedimentary provenance had changed dramatically from the lower to the upper sequences of the Lancang Group, as highlighted by the increase of Pan-African and younger Grenvillian (1100–900 Ma) materials accompanied by the decrease of older Grenvillian (>1100 Ma) sediments. Besides, whole rock Nd model ages also increase from the basal sequences upwards. Combining these findings with those of previous studies, we propose that the Lancang Block was adjacent to the northern margin of Gondwana during the early Paleozoic with sediments of the Lancang Group dominantly sourcing from North India. The provenance change recorded in the Lancang Group might have resulted from a transition from passive to convergent continental margin initialed by the subduction of Proto-Tethys in the late Ordovician along the northern margin of Gondwana.

An early Paleozoic active continental margin basin along the southern Chinese Altai: Evidence from high-grade paragneisses in the Fuyun region

High-grade gneisses are widespread in the Chinese segment of the Altai orogen, but their nature is not yet well-constrained with interpretations varying from Precambrian basement to Paleozoic meta-sediments. Such a controversy has hindered our understanding of the early Paleozoic geologic history of the Altai orogen. This study presents whole-rock geochemistry, zircon U-Pb and Hf isotopic data for paragneisses from the Qiongkuer Domain of the southern Chinese Altai, aiming to reveal their sedimentary and metamorphic history and shed light on the geodynamics of the Altai orogen. The paragneisses contain detrital zircons with mostly euhedral shapes, which, together with their weak HREE fractionation and Zr-Hf depletion relative to continental crust, may imply short-distance transport and low maturity of the sediments. In addition, these rocks have felsic compositions, high Rb contents and negative Eu anomalies, as well as low La/Sc and high Co/Th ratios, possibly indicating an acidic-intermediate igneous provenance. Our results show that the detrital zircons from the paragneisses are dominantly <i>ca.</i> 535 to 435 Ma old, some have Neoproterozoic ages, and only a few have Mesoproterozoic or Archean ages. Since the youngest detrital zircon population record an early Silurian weighted mean age of 441 Ma, and an early Devonian (411 Ma) granite intruded these paragneisses, we infer that their protoliths were deposited in the Silurian. The cumulative distribution curves of zircon age spectra of the paragneisses are comparable to those of sediments at convergent continental margins. The large spread of εHf(t) values (−9.5 to +12.9) of the predominant Paleozoic zircons also reflects possible active margin settings which contain both juvenile and reworked materials in the source. Therefore, the protoliths of the studied paragneisses were immature sediments mostly deposited at an active continental margin during the early Paleozoic and sourced mainly from proximal igneous rocks. Similar detrital zircon age spectra of early Paleozoic sequences from the Chinese Altai, Mongolia Altai, and Khovd Zone support the existence of a large accretionary wedge developed along the western margin of the Ikh-Mongol Arc system, resulting from continuous northeast-dipping oceanic subduction.

Ca. 835–823 Ma doming extensional tectonics in the west Jiangnan accretionary orogenic belt, South China: Implication for a slab roll-back event

The Neoproterozoic Jiangnan accretionary orogenic belt recorded the accretion and collision of the Yangtze and Cathaysia blocks to form a stablized South China Block, but related geometry and kinematics is poorly constrained, leading to largely varied tectonic models. Here, we present detailed field investigation and kinematic analysis of the plutonic-metamorphic complexes in the Yuanbaoshan and Sanfang areas of the west Jiangnan orogenic belt, which enables identification of extensional granite-cored domes. In the dome margins, down-dipping lineations display a radial pattern and dome dominated foliations are extensively developed. The shearing structures within the plutonic-metamorphic complexes display extensional shearing surrounding the Yuanbaoshan and Sanfang granitic dome cores. Gneissic granites and massive ones from both the Yuanbaoshan and Sanfang plutons yield comparable crystallization ages of ca. 835–823 Ma that are within age errors of each other, as are the sheared recrystallized asymmetric quartz veins and mylonites dated at 831 Ma. Overall ages of the deformed Sibao Group and the undeformed overlying Danzhou Group, along with those of the granite plutons and mylonites, suggest formation of the granite-cored domes at ca. 835–823 Ma, coeval to the timing of emplacement of the granitic plutons. Locally, top-to-the-E thrusting structures are also observed in the west Yuanbaoshan and Sanfang areas and are inferred as at ca. 860–835 Ma, coinciding well with E- or SE- directed structures developed elsewhere in the Jiangnan orogenic belt, but in contrast with doming extensional shearing structures. Therefore, overall geometry and kinematics in the west Jiangnan belt indicate development of granitic dome related extensional ductile shearing deformation dated at ca. 835–823 Ma and a possible top-to-the-E compressional ductile thrusting deformation within 860–835 Ma. Given the previously inferred regional geology observations, along with age and chemical data across the Jiangnan orogenic belt, the dominant extensional shearing deformation in the region argue for a slab roll-back event within an accretionary belt, typical of domes-and-basins structures formed in accretionary convergent continental margin. The top-to-the-E thrusting is here interpreted as corresponding to compressional regime generated by the west directed subduction of oceanic crust beneath the northern Guangxi continental margin arc in the west Jiangnan orogenic belt.

Building the Proterozoic basement of the western Xing'an-Airgin Sum Block in the eastern Central Asian Orogenic Belt and its implications for the Nuna breakup and Rodinia assembly

The Xing’an-Airgin Sum Block (XAB) is located in central and northern Inner Mongolia, North China, and belongs to one of the oldest blocks in the Central Asian Orogenic Belt (CAOB). Previous studies have shown that there are Mesoproterozoic granites in the XAB; however, the formation process of the Precambrian basement, and also the tectonic evolution and origin of the block are unclear. In this work, the Mesoproterozoic and early Neoproterozoic stratigraphic sequences, ages and the related sedimentary basin types of the western XAB are reported for the first time. Our study reveals that the Mesoproterozoic to early Neoproterozoic stratigraphic sequence contains the Baoyintu Group and Airgin Sum Group with respective depositional ages of 1516–1392 Ma, and 1237–921 Ma. Sedimentation analysis suggests that the Baoyintu Group and the Airgin Sum Group formed in the coastal marine environment with clastic and carbonate provenances and the shallow marine clastic sedimentary environment, respectively. The age distribution patterns of detrital zircons indicate that there were two different types of sedimentary basins, including two convergent basins formed in ∼ 1400 and ∼ 930 Ma, and a collisional basin developed in ∼ 1237 Ma. However, the 1392–1379 Ma syenogranites and alkali-feldspar granites, which are geochemically similar to A-type granites, were generated by the partial melting of mixed Paleo- and Mesoproterozoic crustal materials in an extensional setting. The newly identified early Neoproterozoic I-type granite (921 Ma) was derived from reworking of the Paleo- to Mesoproterozoic crustal source in a hydrous, garnet-free and plagioclase residual condition. These geochemical signatures, coupled with the three sedimentary stages included on the block, suggest that the 1.4–1.3 Ga period recorded an extensional regime at a convergent continental margin (e.g., back-arc basin), whereas the early Neoproterozoic period was likely controlled by a continental arc setting. In summary, the tectonic evolution process of the XAB documented by three sedimentary and two magmatic events could be its response to the supercontinent cycle from the Nuna breakup to the Rodinia assembly.

Late Mesozoic diorites of the middle Gangdese magmatic belt of southern Tibet: New insights from SHRIMP U-Pb dating and Sr-Nd-Hf-O isotopes

The Gangdese magmatic belt, which formed by Mesozoic northward subduction of Neo-Tethyan oceanic lithosphere and Early Mesozoic India-Asia collision, is an excellent natural laboratory for study of subducting slabs, crustal growth, the reworking of convergent continental margins and interaction of crust-mantle in general. Although many studies have examined this belt, the petrogenesis of arc magmas and the geodynamic architecture of the subducting slab are still contentious. Discoveries of igneous rock exposures are important in reconstructing the tectonic regime and evolution of southern Tibet, and provide new constraints on unresolved issues. Here we report two newly identified intermediate rocks, shedding new light on both the formation of arc-type magmas, and the tectonic evolution of the Neo-Tethys during the Late Mesozoic. Zircon SHRIMP II U-Pb dating indicates that a Late Cretaceous gabbro diorite pluton formed at 90.3 ± 1.4 Ma, and a Late Jurassic diorite porphyry pluton crystallized at 157.9 ± 2.3 Ma. The former has positive zircon εHf(t) values of 8.03–10.86 and whole-rock εNd(t) values of 4.11–4.37, with δ18Ozircon values of 5.84‰–6.70‰, slightly above the values of mantle zircon (5.3‰ ± 0.6‰). The latter also has positive zircon εHf(t) (10.29–12.77) and whole-rock εNd(t) (5.54–5.78) values, with normal mantle-like zircon δ18O values of 5.19‰ to 6.00‰. All samples of both plutons display arc-type signatures characterized by enrichment in light rare earth elements (LREEs) and depletion in high field strength elements (HFSEs) (e.g., Nb, Ta, Ti and P). Based on multiple isotopic (Sr–Nd–Hf–O) and whole-rock geochemical characteristics, we conclude that the Late Cretaceous gabbro diorite was derived from partial melting of depleted mantle that had been metasomatized by subducted-sediment-derived melts (~5 vol%). The Late Jurassic diorite porphyry is inferred to be a product of fractional crystallization of primary andesitic magmas with limited contents of Fe-Ti oxide and amphibole, and minor plagioclase accumulation. Pelagic sediments and enriched materials were a negligible component of the precursor magmas of the diorite porphyry, relative to the gabbro diorite pluton. Considering the temporospatial distribution of igneous rocks over the whole Gangdese magmatic belt, we suggest that Late Jurassic magmatism was related to normal angle subduction of the Yarlung Tsangpo Neo-Tethyan oceanic lithosphere, and early Late Cretaceous magmatism was triggered by rollback of the Neo-Tethyan oceanic slab.

An Early Garnet Redox‐Filter as an Additive Oxidizer in Lower Continental Arc Crust Traced Through Fe Isotopes

AbstractThe magmatic differentiation of cooling arc lavas on their way to the surface is dominated by fractional crystallization, which predominantly occurs at lower crustal arc levels (25–40 km) at continental margins. The magmatic storage in the deep crust is complex and remote, and rarely studied compared to shallow magma fractionation, but carries key information for the formation and evolution of the lower continental crust. At convergent continental margins, mantle‐derived melts must pass through a complex system of rocks in the lower arc crust. In particular, melts inevitably transition through deep “hot” (melt‐rich) or “cold” (i.e., water‐rich) zones, collectively termed here lower crustal arc zones (LoCAZ) to account for both scenarios. Their role in driving melt modification, including formation and oxidation of calc‐alkaline suites, remains unclear. Here we report stable Fe isotope compositions of 40 well‐characterized Neoproterozoic mafic dikes from the northwestern margin of the Yangtze Block, representing three stages of continental arc evolution. Rocks from the Tongde dikes display strong correlations between δ57Fe values and trace element indices of garnet fractionation (e.g., high La/Yb and depleted heavy REE patterns). An increase in heavy Fe isotopes with garnet fractionation indicates a redox‐filtering of melts through LoCAZ. We thus propose that retention of Fe2+ in deep crustal garnet is one driver for elevated fO2 and the calc‐alkaline trend in continental margin rocks. With continental arc rocks being on average more oxidized than intra‐oceanic arc lavas, garnet redox‐filtering may be an additive oxidizer in thicker arc crust. The Tongde dike swarms may represent a rare, volumetrically small, direct example of melts ejected out of the lower arc crust. The complex mixing and mingling of melts reported for most arcs and their plutons may obscure this redox process contributing to continental crust growth.

Tectonic and Evolution Characteristics of Basins in Andaman Sea

The study on deposition and structural development history of the basins in Andaman Sea involved tectonic patterns, tectonic stress field features, deposition sequence and source provenance analysis. Through building up the important cross‐section and faults system, combining with the regional tectonic evolution and using the geological, geophysical and geochemistry data, this paper analyzed the tectonic evolution characteristics of the basins in Andaman Sea. The basins in Andaman Sea have experienced pre‐rifting, syn‐rifting and inherit compression‐shear transformation since Late Miocene. The early‐period normal faults which were formed during the early stage of the pre ‐rifting were developed in the basin. The regional fractures were greatly influenced by Sagaing fault. The developed faults and folds in the late period were related to the direction of Sagaing fault and Indian Plate subduction from NNE. The basins in Andaman Sea have many diapiric structures, including accretionary wedge mixing medium diaper, magmatic arc diapir and mud diapir caused by back‐arc striking-slip fault. Among them, the accretionary wedge mixing medium diapir is a special diapiric structure in the convergent continental margin of Burma sea. Special geographic position decides its individual tectonic stress field and the geotectonic evolution of basin. Intensive study on the basins in Andaman Sea will be in favor of exploring the oil and gas of convergent continental margin and play an instructive role in the development of plate tectonics.

Unravelling the P-T-t history of three high-grade metamorphic events in the Epupa Complex, NW Namibia: Implications for the Paleoproterozoic to Mesoproterozoic evolution of the Congo Craton

The Epupa Complex of northern Namibia constitutes the south-western margin of the Archean Congo Craton of central Africa. We present new petrological and geochronological data for metasedimentary migmatites that decode the poorly-known Paleoproterozoic to Mesoproterozoic evolution of this remote part of the craton. Detrital magmatic zircons with concordant 207Pb/206Pb ages between 1898 and 1774 Ma are interpreted to demonstrate the formation of the metasedimentary protoliths through reworking of Paleoproterozoic granitoids of a magmatic arc in a back-arc basin. SHRIMP U-Pb zircon analyses of metamorphic rims around the detrital grains linked with constraints from pseudosection modelling revealed three distinct high-grade metamorphic events in the migmatites. Late Paleoproterozoic regional HT-LP metamorphism between 1740 and 1720 Ma constitutes the oldest event and only affected the rocks of the northernmost part of the Epupa Complex (Eyao Unit). Heating to mid-amphibolite facies peak P-T conditions of 720 °C and 4 kbar caused the partial replacement of early biotite-sillimanite by garnet-cordierite assemblages during melt-producing reactions in metapelites. The near-isobaric heating-cooling P-T paths and the high geothermal gradient (60 °C/km) are consistent with HP-LP metamorphism in a back-arc basin setting with up-rising basic melt as heat source. Early Mesoproterozoic (1530 Ma) HT-LP metamorphism is recorded by rare Mg-rich Opx-Crd rocks that are restricted to the westernmost part of the Eyao Unit. Near-isobaric P-T paths culminate at granulite facies peak-conditions of 830 °C and 2 kbar. This local HP-LP event is interpreted to record contact metamorphism related to the coeval emplacement of early-Mesoproterozoic granitoids or the emplacement of intimately associated gabbros. Metapelitic Grt-Bt-Sil migmatites record the third high-grade metamorphic event that was active at ca. 1330 Ma and is restricted to the southern part of the Epupa Complex (Orue Unit). Sillimanite pseudomorphs after kyanite and late cordierite coronas around garnet indicate a clockwise P-T path during regional upper-amphibolite facies metamorphism that peaks at lower crustal conditions of 770 °C and 7.5 kbar. The clockwise P-T path is interpreted to reflect crustal thickening through magmatic accretion of basic melts that produced the coeval Kunene Intrusive Complex, the largest massif-type anorthosite in the world. The metamorphic events detected in the migmatites record a change of the geotectonic position of the Epupa Complex from a convergent continental margin during the late Paleoproterozoic to an intracratonic position in the Mesoproterozoic. The migmatites of the Eyao Unit were affected by an intense hydrothermal alteration during the Pan-African orogeny (ca. 520 Ma) that also caused the partial re-setting of the U-Pb zircon isotope system.

Tracking decratonization process along a cratonic edge through late Permian to late Triassic magmatic flare-up in northwestern Liaoning, North China Craton

Resolving magmatic tempos of multiple scales along convergent plate margins presents a pivotal task in any regional tectonic and supercontinental reconstructions, as is of paramount importance for the northern North China Craton (NCC) that converged with the Central Asian Orogenic belt (CAOB) during the Paleozoic to early Mesozoic. This study applies the integrated zircon U–Pb dating and geochemical tracing to late Permian to late Triassic intrusive flare-up in northwestern Liaoning, with three episodes of granitic suites deciphered. Their contrasting elemental and isotopic features called for three distinct crustal end-members involved in their formation: ancient amphibolitic protolith, Paleozoic-accreted mafic meta-igneous ingredients and relaminated juvenile intermediate arc plutons. Sequential crustal anatexis of these multiply juxtaposed protoliths in a felsic hot zone tends to yield successive parental magmas for forming three granite suites. While the late Permian (260–250 Ma) granites, with their felsic adakitic affinity and variably evolved isotopic compositions (87Sr/86Sri = 0.70637 to 0.70659, εNd(t) = −16.7 to −10.2, zircon εHf(t) = −18.1 to +1.1), are consistent with partial melts of major ancient crustal materials and minor newly-underplated mafic ingredients, the middle Triassic (242–240 Ma) granites show a more juvenile isotope signature (εNd(t) = −11.8 to −4.7, zircon εHf(t) = −14.7 to +4.1) and attest to distinctively higher input from newly-underplated mafic lower crust in their formation. By contrast, the late Triassic (227–223 Ma) granites exhibit an A-type magmatic affinity. Their radiogenic whole-rock Nd (εNd(t) = −3.7 to +1.1) and zircon Hf (εHf(t) = +0.7 to +12.0) isotopic values suggest magma derivation from high-temperature fusion of dominantly relaminated juvenile charnockitic protolith. What follows is one phase of shoshonitic mafic dykes that fingerprint a slab-melt metasomatized asthenosphere source with depleted isotopic compositions (87Sr/86Sri = 0.70374 to 0.70388, εNd(t) = +3.5 to +4.1). Synthesizing these events with the previously recognized ones leads to a first-order duration of ca. 260–218 Ma for the flare-up and its further differentiation into three secondary pulses including ca. 260–250 Ma appinite and granite, ca. 242–238 Ma ferroan granites and ca. 227–218 Ma A-type granites and mafic dykes. While such primary duration is typical of post- collisional/orogenic magmatic tempos of convergent continental margins, three secondary pulses encapsulate successive geodynamic processes from crustal thickening through orogenic collapse to lithospheric dripping in the aftermath of the Paleo-Asian oceanic closure. Featuring a coupled scenario of lithospheric thinning and crustal growth, the northwestern Liaoning case of cratonic-edge destruction serves as a prelude but also a contrast to the late Mesozoic large-scale decratonization across the eastern NCC possibly due to oceanic subduction of the paleo-Pacific plate.

Seismic constraints on a remnant Mesozoic forearc basin in the northeastern South China Sea

The northeastern of the South China Sea (SCS) has undergone tectonic inversion from a Mesozoic convergent continental margin to a Cenozoic divergent continental margin. The remnant subduction-related structures, such as volcanic arc and forearc basin, likely preserve key clues to reveal the dynamic mechanism of this tectonic inversion. We applied ocean bottom seismometers (OBS) data, combining with multi-channel seismic data, gravity data, drilling data, and petrologic observations, to reveal geophysical and structural features of the Dongsha Basin, a Mesozoic basin in the northeastern SCS. The Mesozoic strata are characterized by large thickness (~1–8 km), relatively high P-wave velocities (3.5–5.5 km/s), high densities (2.55–2.65 g/cm3), alternating folds, and thrust faults, which are significantly different from the features of Cenozoic strata. The Dongsha Basin is flanked by a Mesozoic granitic arc to the northwest and an inferred Palaeo-Pacific subduction zone to the southeast, indicating that it is geographically located in the Mesozoic forearc region. Considering the similar Mesozoic strata detected in the southern margin of the SCS and the contemporaneous accretionary complex outcrops observed in the Palawan Continental Terrane, we propose that the Mesozoic strata in these two regions originated from a common Mesozoic forearc basin, which was broken by the forearc rifting and subsequently separated by the spreading of Cenozoic SCS. Accordingly, a continuous Palaeo-Pacific subduction system is reconstructed, including the “arc-forearc-accretionary wedge” tectonic sequences. Reviewing the mechanisms of forearc rifting globally and the retreat of volcanic arcs onshore South China, we speculate that rollback of the Palaeo-Pacific subduction slab might be one of the causes for the forearc rifting and the initial extension of the SCS.

Depositional Characteristics and Formation Mechanisms of Deep-Water Canyon Systems along the Northern South China Sea Margin

Submarine canyon systems are sites for coarser clastic sediment accumulations in the deep-water domains, having the most potential for hydrocarbon reservoirs. Based on the interpretation of high resolution 2D/3D seismic and drilling data, depositional characteristics of three large deep-water canyon systems on the South China Sea northern margin have been analyzed. The Central Canyon System has a deep incision geomorphology extending from east to west, featured by distinct canyon segmentations, multi-provenance sediment supplies and multi-stage canyon fillings. The Pearl River Canyon System’s formation is closely related to the development of Pearl River Delta. Its vertical stacking and migrating canyon patterns have changed over time. The depositional architectures and evolution of the recent Penghu-Gaoping Canyon System respond to tectonic movements along the Taiwan-Luzon convergent continental margin. The main controlling factors of the formation and evolution of these three canyon systems include the tectonic setting, sediment supply, sea level change and paleo-geomorphology, among which the former two are dominant. The Penghu-Gaoping Canyon System formed along the subduction structural zone, directly indicating a typical tectonic origin. Numerous seismic data show that the Central Canyon and Pearl River Canyon systems are obviously affected by tectonics, associated local topography and sediment supply.

Iron isotope fractionation in hydrous basaltic magmas in deep crustal hot zones

Mafic magmatism within deep crustal hot zones plays an important role in the crustal formation and evolution. High-pressure garnet crystallization drives magma differentiation and Fe isotope fractionation at the base of the lower crust. The ∼840 Ma intrusion and the associated ∼830 Ma dikes from the Tongde region in South China consist mainly of gabbro and diorite, which record complicated magmatic processes of the deep crustal zones and provide new insights into the Fe isotope fractionation. Rocks from the Tongde intrusion have high Sr/Y ratios (61–163) and variable δ57Fe values (+0.07 to +0.21‰ relative to IRMM-014) that initially increase then decrease with progressive magmatic differentiation. All samples do not display obvious imprint of deuteric fluid exsolution, and residual garnet in the mantle source did not fractionate the Fe isotope compositions of the primary magmas significantly. The variable Fe isotope signatures in this study can be reproduced by sequential fractional crystallization of olivine/pyroxene and garnet in the deep crust followed by amphibole saturation in the upper section. The host rocks of the intrusion underwent diffusive exchange with the dikes. Removal of both isotopically light- and heavy-Fe minerals well explains the small Fe isotope fractionation in mafic and intermediate rocks. The high-Sr/Y suites of the intrusion and dikes have an average δ57Fe of +0.11 ± 0.02‰ (2SD), similar to the value of arc crust at convergent continental margins. Therefore, garnet fractionation is an important process in arc roots, which drives the melts evolving to calc-alkaline series. Fractional crystallization of hydrous basaltic magmas under high pressures in deep crustal hot zones makes a significant contribution to the formation and evolution of continental crust.

The genesis of felsic magmatism during the closure of the Northeastern Paleo-Tethys Ocean: Evidence from the Heri batholith in West Qinling, China

To better understand the origin of voluminous silicic rocks in a convergent continental margin, we conducted an integrated study in which we have obtained geochronological, mineralogical, and isotopic (including whole-rock Sr–Nd–Pb, in-situ zircon Hf) data of the Heri batholith in West Qinling on the NE Tibetan Plateau. The batholith is composed of metaluminous to weakly peraluminous granodiorites (235–233 Ma) and porphyritic granodiorites (230–223 Ma) with an I-type affinity. Both lithologies share similar major, trace elemental and SrNd isotopic compositions. Detailed elemental data demonstrate that these granodioritic rocks underwent fractional crystallization of hornblende and apatite, with plagioclase (i.e. sieve-textured plagioclase cores) accumulation to some extent. Except for porphyritic granodiorites, the Pb isotopes for other analyzed samples are characterized by high radiogenicity and uniformity ((206Pb/204Pb)t: 17.263–18.472, (206Pb/204Pb)t: 15.571–15.591, and (206Pb/204Pb)t: 38.032–38.304), together with limited variations in initial Sr ((87Sr/86Sr)t: 0.707251–0.708103) and Nd (εNd(t) = −7.1 to −6.3) isotopes with two-stage model ages (TDM2) of 1.58–1.52 Ga. These factors collectively point to a derivation from the Mesoproterozoic basement rocks at the lower crustal level, or a comprehensive mixing of different-age components that generated an average crustal residence age. The SrNd isotopic compositions of the porphyritic granodiorites are strikingly similar to those of granodiorites. Compared with the experimental melt compositions of amphibolites, the Heri granitoids are probably derived from an amphibolitic source under fluid-absent conditions due to the incongruent breakdown of amphibole and biotite. Based on the temporal–spatial distribution of granitic intrusions in West Qinling and the regional tectonic evolution, our interpretation is that the Heri batholith was formed during the initial collision between the North China Craton (NCC) and the South China Craton (SCC), which was accompanied by the closure of the Paleotethyan Ocean. Considering both previously published data and our new data, we propose that the Heri granitoids were mainly generated by the partial melting of lower crustal amphibolites, with minor mantle-derived melts.

Post-collisional denudation of an orogenic belt traced from geochronological and bulk-rock geochemical records of the western Korean Peninsula

ABSTRACT The western Korean Peninsula preserves evidence for multiple Proterozoic to Phanerozoic orogenic processes along the convergent continental margins. After the Late Palaeozoic to Early Mesozoic collisional stage within the long-lived tectonic Wilson cycles, the area underwent post-collisional extensional tectonics, creating the Late Triassic to Early Jurassic basins on top of the collisional orogenic belt. Here, we present the LA-MC-ICP-MS U-Pb ages of detrital zircons and bulk-rock geochemical data from post-collisional basin fills (viz., the Nampo and Gimpo groups and the Seokmun Formation) and the overlying volcano-sedimentary sequence (Oseosan Volcanic Complex). Our results suggest that (1) the Nampo Group was supplied with detritus mainly from nearby felsic igneous rocks or geochemically equivalent migmatitic basement gneisses, while the Gimpo Group and Seokmun Formation were predominantly sourced from (meta-)sedimentary rocks, likely reflecting their structural positions in the orogenic belt in terms of the exhumed basement-dominant hinterland side vs. the cover rock-dominant foreland side of the mountain belt; (2) the ages of the youngest zircon group in the Nampo Group and the overlying Oseosan Volcanic Complex (ca. 1854 age peak in the lowermost unit and ca. 164 Ma age peak in the uppermost unit) tend to approach the sedimentation ages with ascending stratigraphic order, indicating a transition from a collisional to a convergent tectonic setting during sedimentation. The results from this study provides important insights into the sedimentary response to the evolution of the collisional orogenic belt along the western Korean Peninsula and elucidates the Mesozoic tectonic transition from collisional (closure of the Paleo-Tethys Ocean) to convergent (subduction of the Paleo-Pacific Ocean) processes in the Far East Asian continental margin.

Nature and origin of the volcanic ash beds near the Permian–Triassic boundary in South China: new data and their geological implications

AbstractPermian–Triassic boundary (PTB) volcanic ash beds are widely distributed in South China and were proposed to have a connection with the PTB mass extinction and the assemblage of Pangea. However, their source and tectonic affinity have been highly debated. We present zircon U–Pb ages, trace-element and Hf isotopic data on three new-found PTB volcanic ash beds in the western Hubei area, South China. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircons yields ages of 252.2 ± 3.6 Ma, 251.6 ± 4.9 Ma and 250.4 ± 2.4 Ma for these three volcanic ash beds. Zircons of age c. 240–270 Ma zircons have negative εHf(t) values (–18.17 to –3.91) and Mesoproterozoic–Palaeoproterozoic two-stage Hf model ages (THf2) (1.33–2.23 Ga). Integrated with other PTB ash beds in South China, zircon trace-element signatures and Hf isotopes indicate that they were likely sourced from intermediate to felsic volcanic centres along the Simao–Indochina convergent continental margin. The Qinling convergent continental margin might be another possible source but needs further investigation. Our data support the model that strong convergent margin volcanism took place around South China during late Permian – Early Triassic time, especially in the Simao–Indochina active continental margin and possibly the Qinling active continental margin. These volcanisms overlap temporally with the PTB biocrisis triggered by the Siberian Large Igneous Province. In addition, our data argue that the South China Craton and the Simao–Indochina block had not been amalgamated with the main body of Pangea by late Permian – Early Triassic time.