Tectonic Transition Research Articles

Organic matter accumulation under the Middle-Upper Ordovician tectonic transition on the western margin of the North China platform.

The tectonic of the Middle and Late Ordovician in the western margin of the north China Platform is complex, and the accumulation models of organic matter of the Wulalike Formation formed during this period are still unclear. Total organic carbon (TOC) content, mineral composition, organic carbon isotope composition, as well as the major and trace elements in the shale samples were all measured in this study. The Wulalike Formation was formed during a tectonic transition from a passive continental margin to an active continental margin. Hydrocarbon-forming organisms are mainly algae. Primary productivity, terrigenous input, and sedimentation rates influence input, dilution, and degradation of organic matter. The degree of degradation, redox, and basin restriction control the preservation of organic matter. The TOC of the Wulalike Formation is determined by the input and preservation degree of organic matter at different sedimentary periods. The organic matter accumulation model in the Wulalike Formation is controlled by regional structures, dominated by sedimentation rate and preservation conditions.

Top-to-south shear at the base of the eastern Tethyan Himalayan Sequence during the Eocene-Oligocene Himalayan orogeny

The early mountain building processes in the Himalayan orogen are still not clear because of extensive deformation and metamorphism since the Miocene. A large gently dipping ductile shear zone, referred as the Tethyan Himalayan Décollement (THD), is defined here as the sole décollement of the south-verging Tethyan fold-and-thrust belt in the Lhozag-Cuona area of the eastern Himalayan orogen. The ~4 km-thick THD is characterized by a top-to-south shear sense, moderate T/P Barrovian to high T/P Buchan type metamorphism and Eocene-Miocene partial melting. Zircon UPb dating of metasedimentary rocks and granitic gneisses from the THD yields protolith ages of the Late Cambrian to Early Ordovician. Based on structural analysis, zircon UPb ages, monazite UPb ages and mica 40Ar/39Ar thermochronology, the THD was the boundary shear zone at the top of the Greater Himalayan Crystalline Complex (GHC) and accommodated the persistent north-south shortening in the Tethyan Himalayan Sequence (THS) from ~50 Ma to 20–17 Ma. From ~20–17 Ma, the top-to-north South Tibetan Detachment System (STDS) was predominantly activated to juxtapose the unmetamorphosed or low-grade THS over the GHC. This tectonic transition can be attributed to the roof collapse in the eastern Himalaya (younger than that of the central-western Himalaya), which triggered rapid exhumation of the GHC and the northern Tethyan Himalayan gneiss domes. Hence, the THD was the predecessor of the STDS and a prolonged pathway for leucogranitic melts from the Eocene to early Miocene. The transition from the THD-controlled crustal thickening in the Eocene and Oligocene to the STDS-controlled extrusion in the Miocene shed insights on a new synthesis of the tectonic wedging model for the Himalayan evolution.

Late Paleoproterozoic within-plate mafic magmatism in the western Yangtze Block, South China Craton: Implications for the initial break-up of the Columbia supercontinent

The Paleoproterozoic was an important period for the tectonic-magmatism evolution of the Yangtze Block, which coincided with the assembly and break-up of the global Columbia (also known as Nuna) supercontinent. However, an essential issue, when the Yangtze Block initiated rifting from the supercontinent, remains controversial. This paper presents new zircon chronology, Lu-Hf isotopic data, and whole-rock geochemical analyses of the Bajiaojing and Xiaoqingshan meta-diabases in the western Yangtze Block to address this issue. The magmatic zircons from the Bajiaojing and Xiaoqingshan meta-diabases yielded U-Pb ages of 1675 ± 38 Ma and 1719 ± 10 Ma, representing an episode of mafic magmatism during the Late Paleoproterozoic. Moreover, the abundances and elemental ratios of high-field-strength elements (HFSEs, e.g., Y, Ta, Th, and Hf) in both meta-diabases are very similar to those of within-plate and mantle plume magmas. This implies that these Late Paleoproterozoic mafic rocks were likely derived from a mantle plume source in a continental within-plate extensional setting. Their magmatic source was relatively heterogeneous in isotopic and geochemical compositions, as inferred from significant differences in εHf(t) values and certain HFSE ratios (e.g., Nb/Yb, Ta/Yb, Th/Yb, Nb/Y, and Zr/Y) among samples. Such source heterogeneity might be attributed to interactions between the mantle plume and the overlying lithosphere. Combined with published εHf(t) values of detrital zircons and whole-rock εNd(t) values of (meta-)magmatic rocks, we speculate that the Yangtze Block experienced a tectonic transition from plate convergence to continental extension at ∼ 1.7 Ga. This means that the block began to rift from the Columbia supercontinent as early as 1.7 Ga. The Yangtze Block then evolved progressively as an isolated fragment until it was amalgamated into the Rodinia supercontinent.

Development of the Arabian-Nubian Shield along the Marsa Alam-Idfu transect, Central-Eastern Desert, Egypt: geochemical implementation of zircon U-Pb geochronology

The magmatic complex along the Marsa Alam-Idfu transect, Central-Eastern Desert of Egypt, represents the northern segment of the Arabian–Nubian Shield (ANS), which developed within the framework of the East African Orogen. The basement rocks of the Arabian-Nubian Shield have been developed through three distinct phases of magmatic activity: the island-arc, the syn-orogenic, and the post-orogenic phases. Transitioning of the magmatic phases from the syn-orogenic to the post-orogenic, identifies changing the tectonic regime from a compressional to an extensional setting. The scarcity of comprehensive regional geochronological data that rely on precise isochron methods, such as the zircon U-Pb technique, could limit the comprehensive understanding of this region’s geological and tectonic history. That would raise a number of uncertainties ranging from the timing of the different magmatic activities and timing of changes in the tectonic regime to the existence of the pre-Pan-African crust in the CED. Our study provides new insights into the aforementioned uncertainties through zircon U-Pb dating of different rock units along the Marsa Alam-Idfu transect, CED, Egypt. The resulting ages ranged from 729 ± 3 Ma to 570 ± 2 Ma, constraining the temporal evolution of the ANS in the studied region into (1) the island-arc phase, represented by a metamorphic sample with an age of 729 ± 3 Ma. (2) the syn-orogenic phase, represented by calc-alkaline and alkaline granitic samples with ages ranging from 699 ± 4 Ma to 646 ± 2 Ma. These two phases indicate initiation of the compressional subduction regime in the CED since 729 ± 3 Ma and being dominated till 646 ± 2 Ma. (3) the post-orogenic phase, represented by metavolcanics, volcanic rocks, and alkaline plutonic samples with ages ranging from 623 ± 3 Ma to 570 ± 2 Ma. This phase suggests dominance of the compressional-to-extensional tectonic transition setting from 623 ± 3 Ma to 600 ± 1 Ma along with the Dokhan volcanism and activation of post-collision tensional regime activated at 582 ± 3 Ma. Our findings discourage the proposed dominance of the island-arc and syn-orogenic phases in the CED and the classical restriction of older magmatic activity to calc-alkaline granitic rocks and younger magmatic activity to alkaline granitic rocks. Additionally, we identified evidence of local magmatic sources by dating five grains with Mesoproterozoic (pre-Arabian–Nubian Shield) xenocrysts with ages ranging from 1549 ± 4 to 1095 ± 25 Ma.

The “greywacke problem” explored in the Neoproterozoic of Saxo-Thuringia: new insights into sediment composition and metamorphic overprint

AbstractGreywackes make up a substantial part of the Cadomian basement of Saxo-Thuringia. Here, their classification as greywackes and the timing of metamorphic overprint are re-evaluated using a multi-method approach. Immature monotonous greywacke sequences from the Lausitz (Lausitz Block) and Leipzig groups (North Saxon Anticline), as well as from the eastern Thuringian Basin and parts of the Weesenstein Group (Elbe Zone) probably belong to a coherent unit, based on microscopic investigations supported by SEM Automated Mineralogy analyses and point counting data. However, due to the low matrix content (< 15%), the sedimentary rocks are more likely classified as lithic sandstones. The heterogeneous composition and in particular the highly mature character of the Clanzschwitz Group (North Saxon Anticline) and parts of the Weesenstein Group (Seidewitz Formation) suggest a younger, Late Cambrian to Early Ordovician sedimentation age. Typically, the metamorphic overprint of the “greywacke units” is very weak. Previous assumptions of Cadomian contact metamorphism triggered by Early Cambrian intrusions (ca. 540 Ma) could not be confirmed due to the local differences in the determined metamorphic ages. Late Cambrian to Early Ordovician (521–461 Ma) Th–U–Pb monazite ages are likely related to the tectonic transition from the collisional regime of the Cadomian orogeny to extensional processes in the course of the opening of the Rheic Ocean. Sporadic Late Ordovician (458–445 Ma) Th–U–Pb monazite and K–Ar fine-fraction ages were also obtained but the specific thermal trigger is still subject of debate. The Permo-Carboniferous metamorphic ages (314–286 Ma) indicate high-temperature metamorphism related to the post-Variscan extensional processes of Central Europe during this period. The youngest dated monazites are Jurassic in age and may have grown in association with the hydrothermal activity known from Central Europe at that time. Graphical abstract

The Indosinian Granitoids of the Songpan–Garze–West Kunlun Orogenic Belt, China: Distribution, Petrochemistry, and Tectonic Insights

During the Indosinian orogeny, the Songpan–Garze–West Kunlun orogenic belt experienced significant tectonic and magmatic activity, leading to the widespread emplacement of granitoid bodies. This study provides a detailed petrochemical and geochemical analysis of these granitoids, offering new insights into their tectonic settings and magmatic evolution. The granitoids of this belt are systematically categorized into arc calc-alkaline and arc tholeiitic granitoids (ACG and ATG), cordierite peraluminous and muscovite peraluminous granitoids (CPG and MPG), potassium calc-alkaline granitoids (KCG), and peralkaline granitoids (PAG) suites. ACG and ATG types dominate early magmatism (230–190 Ma), reflecting a convergent tectonic setting, while KCG and MPG types magmatism, respectively, emerged 10–20 Myr and 15–25 Myr later, during post-collisional extensional phases. Geochemical analyses show that ACG and ATG granitoids follow calcic and calc-alkalic trends, while KCG and MPG display alkalic characteristics. These findings align with the region’s tectonic transition from the closure of the Paleo–Tethys Ocean to Late Triassic transpressional deformation. This study enhances the understanding of granitoid petrogenesis and provides valuable implications for regional tectonic evolution and mineral exploration.

Geochronological assessment of the Arabian-Nubian Shield plutonic intrusions in the arc assemblages along the Qift-Quseir transect, Central Eastern Desert of Egypt

The ophiolitic mélange and granitic intrusions along the Qift‒Quseir transect in the Central Eastern Desert (CED) of Egypt are parts of the Egyptian Nubian Shield (ENS), which is the northern segment of the East African Orogeny (EAO). The Arabian-Nubian Shield (ANS) is the longest Neoproterozoic belt on Earth, and it was formed during the East and West Gondwanaland collisions within the framework of the EAO. The ANS basement rocks were developed during three distinct phases of magmatic activity: the island arc and syn-collisional phases, identifying a compressional tectonic regime, and a post-collisional phase that identifies changing the tectonic regime into an extensional type. The geochronological assessment of these magmatic activities is essential for understanding the regional geology and tectonic development of the ANS. In our study, we dated different rock units along the Qift‒Quseir transect and revealed ages ranging from the Late Tonian (820 ± 8 Ma) to the Late Ediacaran (563 ± 4 Ma). These ages were associated with three different magmatic pulses: (1) a seafloor spreading and island arc phase (ophiolite and related rocks), represented by sample QQ05, which was dated from 820 ± 8 Ma; (2) a syn-collisional phase, represented by samples QQ08 and QQ10, dated from 733 ± 10 Ma and 729 ± 10 Ma, respectively; and (3) a post-collisional phase, represented by all the other samples, dated from the Ediacaran at 603 ± 9 Ma to 563 ± 5 Ma. These results showed that the post-collisional phase was dominant, especially in terms of the alkali-feldspar granites, relative to ophiolitic rocks, and the syn-collisional granites in the CED. Initiation of the Dokhan Volcanic eruptions at 639 ± 2 Ma gave us the date of the compressional-to-extensional tectonic transition setting, and the post-collisional tensional regime was activated at 603 ± 9 Ma. Additionally, we identified evidence of local magmatic sources by dating 11 grains of Paleo-to Meso-Proterozoic xenocrysts with ages ranging from 1876 ± 18 to 1070 ± 13 Ma (i.e., components of the pre-Arabian-Nubian Shield).

Sedimentary provenance supports a mid-paleozoic tectonic connection between the Junggar and Altai terranes in central Asia

The provenance of Precambrian detritus in the Junggar and Altai terranes provides crucial constraints on the peri-Siberian accretionary tectonic evolution in the middle Paleozoic. The Precambrian detrital zircons have no coeval magmatic equivalents in the Junggar terrane but show U–Pb age spectra and εHf(t) values comparable to those in the Altai terrane. The correlations suggest that the old detrital materials in the Junggar and Altai terranes were most likely derived from the Siberia craton and adjacent Tuva-Mongolian microcontinent. Paleozoic zircons in the Junggar terrane display a εHf(t) pattern from large spread to dominantly positive values at ca. 420–410 Ma. Such an abrupt change points to an accretionary tectonic transition from an advancing to retreating mode during mid-Paleozoic time, synchronous with similar tectonic switch occurring in the Altai terrane. Taking into account the temporal and spatial relations in sedimentation, tectonism and arc magmatism, we propose that the Junggar terrane had once collided onto the peri-Siberian Altai terrane to receive abundant old detritus from the Siberian continent in the Silurian–early Devonian. They were subsequently separated at ca. 420–410 Ma, possibly due to the slab rollback of the subducting Paleo-Asian Ocean (PAO) plate. These results constrain an Early Paleozoic tectono-paleogeographic boundary of the CAOB along the North Tianshan–Solonker suture zone, and also imply a long-lived PAO subduction was responsible for the Neoproterozoic to Paleozoic accretionary orogenesis at the margins of southern Siberia, eastern Kazakhstan, and northern Gondwana.

Two-stage Mesozoic oceanic subduction and related mantle metasomatism beneath the South Qiangtang terrane with implications for post-collisional magmatism

Subduction-related metasomatism at convergent margins is a crucial process for the lithosphere evolution of the overriding plate. In the central Tibetan Plateau, tectonic transitions and related mantle metasomatism history of the oceanic subduction beneath the South Qiangtang terrane (SQT) remain ambiguous, which impeded understanding the geological evolution of the SQT during the Mesozoic and interpreting the geodynamic process responsible for the Cenozoic post-collisional magmatism. This paper integrated the new and published geochronological and geochemical data from the Jurassic-Cretaceous magmatic rocks in the western part of the SQT to explore the variations of magmatism and processes of mantle modifications. The Jurassic arc magmatism is characterized by the southward younging migration with increasing zircon εHf (t) values and the east–west linear distribution of ca. 155–150 Ma slab-derived adakites. After a magmatic gap at ca. 145–130 Ma, the Early Cretaceous magmatism (130–100 Ma) with distinct juvenile isotopic compositions reinitiated firstly in the south of the Jurassic magmatic belt. In comparison, a younger phase of felsic rocks (120–100 Ma) spatially and geochemically overlaps the northern Jurassic granitoids with ancient crustal sources. These spatial–temporal-geochemical covariations of magmatism could be best explained by the southward transference of oceanic subduction at the earliest Cretaceous that was induced by the Jurassic accretion of an oceanic plateau onto the SQT. Under such subduction regimes, several high-Nb mafic rocks in the northern and southern magmatic belts which were formed by partial melting of the slab-melts-metasomatized lithospheric mantle respectively recorded the Jurassic and Cretaceous mantle metasomatism beneath the SQT, based on their spatial and geochemical relationships with two phases of the adakites. Combined with other geological evidence, these results contribute to elucidating the Mesozoic geodynamic evolution of the oceanic subduction beneath the SQT, which further has implications on the deep dynamic processes responsible for post-collisional magmatism.

The Early Neoproterozoic Andean-Type Orogenic and Within-Plate Magmatic Events in the Northern Margin of the Yangtze Craton during the Convergence of the Rodinia Supercontinent

Although considered a crucial component of the Rodinia supercontinent, it remains uncertain how the Yangtze craton relates to the accretion and breakup of Rodinia. Here, the Huanglingmiao granitic complex (HGC), an intermediate-acid rock series that intruded on the southern Kongling terrane of the northern Yangtze craton margin, is investigated to help resolve this conundrum. Our analysis indicates that these rocks consist of tonalite, trondhjemite, granodiorite, oligoporphyritic granodiorite, porphyric biotite granodiorite, and fine- to medium-grained granodiorite dyke compositions. Collectively, this assemblage is further subdivided into two categories by their temporal, spatial, and geochemical features into early TTG-like and later granitic–dioritic units, which are composed of tonalite, trondhjemite, granodiorite, porphyritic granodiorite, and the fine- to medium-grained granodiorite dykes, respectively. Zircon U-Pb dating yields ages of 865~850 Ma for the TTG-like rocks, 844~825 Ma for the porphyritic granodiorites, and ~800 Ma for the granodiorite dykes. Combined with geochemical evidence, the data suggest that the early- and late-series rocks were formed by a partial melting of Mesoproterozoic and Paleoproterozoic crustal materials, respectively, suggesting that the vertical layering of the crust controlled the composition of the independent units. In addition, isotopic evidence points to different sources for the various rocks in the Kongling terrane and that mantle-derived materials influenced the early-series lithologies. Combined with previous studies on the northern margin of the Yangtze craton, it is inferred that the early-series rocks formed in an active continental margin environment, while the late-series rocks display within-plate boundary formation characteristics. The multiple magmatic activities revealed by this study record sequential partial melting with tectonic transition characteristics from an Andean-type to within-plate magmatism in the northern margin of the Yangtze craton. Taken together, these observations point to a strong association between these rocks, convergence, and incorporation of the northern Yangtze craton margin into the Rodinia supercontinent during the Tonian Period.

Characterization and origin of high-Al chromitites: A case study of chromite deposit in the Kudi ophiolite in the NW Tibetan Plateau

Chromitite occurrences in ophiolites are commonly classified into high-Cr (Cr# > 60) and high-Al (Cr# < 60) varieties. High-Cr chromitites have been extensively studied, whereas the origin of high-Al chromitites remains enigmatic mainly due to strong alteration of the relevant rocks. This study undertook a comprehensive examination of alteration-free high-Al chromitites and their hosting peridotites from the Kudi ophiolite in the NW Tibetan Plateau, complemented by a synthesis of global data of high-Al and high-Cr chromite deposits. Our results reveal that harzburgites, hosting both types of chromite deposits, exhibit similar major oxide compositions in their constituent minerals. In contrast, the constituent minerals in the dunite envelopes and chromitites display distinct major and trace element compositions, which are probably controlled by the compositions of infiltrating melts. For both chromite deposits, the forsterite (Fo) contents of olivine increase from harzburgite (90.0–91.6) to dunite (89.9–94.6) and chromitite (90.0–97.2). Mineral compositions of olivine show significant variabilities at the contact boundaries between densely disseminated and sparsely disseminated chromitites. These features suggest that fluid immiscibility plays an important role in the formation of high-Al chromitite. A continuous spectrum of major oxide compositions of chromite from high-Al to high-Cr chromitites suggests a progressive shift in the parental magma compositions from mid-ocean ridge basalt-like to boninitic melts, implying a rapid tectonic transition during subduction initiation.

A Two‐Stage Geodynamic Model for Post‐Collisional Potassic‐Ultrapotassic Magmatism in Southeast Tibet

AbstractPost‐collisional potassic‐ultrapotassic rocks can provide key clues to the change of the recycled material type and/or tectonic transition in subduction‐related zones. Despite continental materials widely recognized in their sources, it remains unclear whether such continental materials were contributed by former oceanic subduction or recent continental subduction. Here we address this issue by systematically investigating previously reported and our new chemical and Sr–Nd–Pb isotopic compositions of the post‐collisional K‐rich rocks in Southeast Tibet. Kink‐like compositional variations provide solid evidence for a primary control of fractional crystallization on the evolution of these K‐rich magmas. Their primary melts are demonstrated to have been produced by partial melting of phlogopite‐bearing peridotites in subcontinental lithospheric mantle (SCLM). The trace element and Sr–Nd–Pb isotopic signatures argue against involvement of the deeply subducted Indian continent but suggest a great contribution from sediments in oceanic slabs. A thinned (∼70–100 km) and hot (∼55–70 mW/m2) lithosphere is also unraveled beneath Southeast Tibet during the potassic‐ultrapotassic magmatism. Together with geophysical data, here we suggest a two‐stage geodynamic model for post‐collisional potassic‐ultrapotassic magmatism in Southeast Tibet: (a) Before the Indian‐Asia continental collision, phlogopite/K‐richterite‐bearing SCLM sources were formed through oceanic subduction‐related metasomatism; (b) After the Indian‐Asia continental collision, asthenosphere upwelling induced by post‐collisional tectonic extension or deep subduction of the Indian continental slab caused lithospheric thinning, partial melting of pre‐existing phlogopite/K‐richterite‐rich SCLM and thus K‐rich magmatism. This study provides new insights into the role of oceanic subduction and continental collision in post‐collisional potassic‐ultrapotassic magmatism.

Slab break-off of the Kalamaili oceanic slab revealed by the latest Carboniferous mafic–ultramafic rocks in eastern North Tianshan (NW China)

Identifying the tectonic transition from oceanic subduction to collision is crucial for tracking the final stage evolution of ancient orogenic belts. In this study, we present new geochronological and geochemical data for the Mozbaysay mafic–ultramafic complex in the Balikun area, eastern North Tianshan of the southern Central Asian Orogenic Belt. This complex had intruded into the late Carboniferous volcano-sedimentary rocks and is comprised mainly of hornblende gabbro and lherzolite. Zircon U-Pb ages of the hornblende-gabbros reveal that this complex was emplaced at ca. 305 Ma. Geochemical analyses suggest these mafic–ultramafic rocks are characterized by slight enrichment of light rare earth elements (LREEs) and relatively depleted heavy rare earth elements (HREEs), resembling enriched mid-ocean ridge basalt (E-MORB). They also exhibit restricted (87Sr/86Sr)i ratios (0.702396–0.704295) and εNd(t) values (+7.0 to +9.1), indicative of a depleted mantle source with minimal crustal contamination. Incompatible element ratios (i.e., Nb/Ta, Zr/Hf, Rb/Nb, and Ba/Nb) suggest the involvement of subducted slab-derived aqueous fluids in their mantle source. These collectively indicate that the parental magmas of the Mozbaysay mafic–ultramafic rocks may have been generated by a mixed mantle source consisting of the E-MORB-like asthenospheric mantle, subcontinental lithospheric mantle (SCLM), and hydrous fluids from subducted slab. Furthermore, a slab break-off model is proposed to explain the generation of these latest Carboniferous mafic–ultramafic rocks. Integrating these findings with regional geological data, we propose that the tectonic transition from subduction (slab roll-back) to collision (slab break-off) along the Kalamaili suture zone occurred at ca. 305–300 Ma.

New constraints from in-situ U-Pb ages and fluid inclusions of calcite cement and structural analysis on multiple stages of strike-slip fault activities in the northern Tarim Basin, NW China

Dating the activity of complex faults and fracture systems is crucial for creating reliable geological models for various tectonics and subsurface engineering applications. This study presents a comprehensive study integrating U–Pb fracture cement dating, trace elements, and fluid inclusion temperature analysis with seismic analysis of faults, fault diagenesis, and burial history studies to better constrain faulting and fracture activities in an intra-cratonic strike-slip fault system in the northern Tarim Basin. Seismic profiles indicate at least three distinct phases of fault activity corresponding to the Middle Ordovician, Permian, and Paleogene periods. Fracture cementation and crosscutting relationships corroborate the identification of three fracturing stages. U–Pb dating of fractured cement has widely detected Middle Ordovician and Early Permian age intervals. Fluid inclusion homogenization temperatures from the fractured cements, ranging across < 50 °C, 70–130 °C, and 150–180 °C, correspond to three episodes of rapid subsidence during the Ordovician, Permian, and Neogene, respectively. These results suggest three phases of fault/fracture reactivation in the Middle Ordovician (prior to 470 Ma), Early Permian (prior to 295 Ma), and Cenozoic. The fault/fracture reactivation in the Ordovician is closely related to the regional tectonic transition from extension to compression, while fault and fracture reactivation in the Early Permian may be related to hydrothermal activity associated with large-scale igneous province and oil emplacement. Fault/fracture activity in the Cenozoic may be related to a reduction in subsidence, gradual reduction of geothermal gradients, and massive oil emplacement. This research underscores the significance of integrating geochemical and subsurface datasets for accurately determining the timing of faulting and fracturing in sedimentary basins.

Jurassic tectonic regime transition of the southwestern Yangtze Block, SW China: evidence from sedimentary provenance analysis

The Lufeng Basin (LFB) in central Yunnan is located on the southwestern margin of the Yangtze Block in the eastern segment of the Tethys tectonic domain, where the Paleo-Tethys and the Paleo-Pacific tectonic domains converge. Jurassic red beds in this basin record the closure of the Paleo-Tethys Ocean and key information of the tectonic transition driven by the spreading of the Paleo-Pacific plate. We present sedimentary provenance analysis on these Jurassic red beds based on heavy mineral assemblages and detrital zircon geochronology. The results indicate that the strata in this basin were deposited after 214 Ma, and the Early Jurassic Lufeng Formation (Lufeng Fm.) mainly sourced from the west of the LFB with minor contributions from the western Yunnan. In contrast, the sedimentary provenance of the Middle Jurassic Chuanjie Formation (Chuanjie Fm.) and Laoluocun Formation (Laoluocun Fm.) shifted to the proximal ancient basement surrounding the basin and the east of the LFB. The regional unconformity between the Early and the Middle Jurassic may have resulted from the subduction of the Paleo-Pacific plate, jointly influenced by two tectonic domains. The sedimentary provenance shift in the Middle Jurassic may be related to the local uplift of the southwestern Yangtze Block caused by the subduction of the Paleo-Pacific plate. The tectonic regime transition associated with this highest peak of local uplift occurred at ∼170 Ma.

Early Jurassic A-type granite and monzodiorite from the Baoji batholith: Implication for tectonic transition from post-collision to post-orogenic extension in the Qinling Orogenic Belt, China

Introduction: The early Jurassic granitoids in the Qinling Orogenic Belt (QOB) play a crucial role in understanding the tectonic implications for the geological evolution of China. To elucidate the early Jurassic tectonic setting of QOB, we performed a comprehensive analysis of zircon U-Pb ages, whole-rock geochemistry, and in situ zircon Lu-Hf isotopes from early Jurassic monzodiorite and Kfeldspar granite within the Baoji batholith in western QOB.Geochronology Method and Results: The intrusions yielded zircon U-Pb ages of 186 ± 2 Ma and 188 ± 2 Ma, respectively.Geochemistry Results: The monzodiorites are characterized by relatively high MgO, Rb, Th, U, and LREE contents, as well as low P, Ti, and HREE contents. They also exhibit high Nb/Ta ratios (20.6–23.4). The zircon εHf(t) values for the monzodiorite sample range from −4.36 to 6.47, indicating significant contributions from a fertile continental lithospheric mantle with the involvement of crustal components. The K-feldspar granites are enriched in K2O+ Na2O, Rb, Zr, Hf, and Nb, and lower Ba, Sr, Ti, and P. They exhibit high Nb/Ta and Ga/Al ratios but low Y/Nb and Yb/Ta ratios. Their geochemical characteristics reveal an A-type granite affinity with elevated zircon saturation temperatures (848°C–900 °C). Additionally, the K-feldspar granite exhibits REE and trace element patterns similar to those observed in the monzodiorite. However, a wide range of zircon εHf(t) values (−4.72 to 3.98), differing from those of the monzodiorite, indicate that the parental magma of the K-feldspar granite experienced magma mixing between a monzodioritic magma and a crustal-derived felsic magma.Discussion: These findings suggest that both A-type K-feldspar granite and monzodiorite likely formed during post-orogenic processes. Additionally, the QOB commenced its postorogenic evolution as an extensional tectonic environment during the early Jurassic period.

Extreme geochemical fractionation during mantle melting: Insights from Hf-Nd isotopically ultra-depleted eclogite

Extreme fractionation of elements and isotopes in mafic igneous rocks is abnormal in deciphering the source nature and melting conditions of mafic magmatism. Especially, identification of geochemically ultra-depleted mafic melts and their mantle sources has great bearing on the property of crust-mantle differentiation at plate margins. This is illustrated by extreme Hf-Nd isotope fractionation in ultrahigh-pressure eclogites from the Sulu orogen in east-central China. In addition to the previous finding of ultrahigh εNd(0) values, we report here new data of whole-rock trace elements and Lu-Hf isotopes in eclogites and related rocks from the same region. The present results show extremely high Lu/Hf ratios and abnormally high εHf(0) values of up to 576 for the eclogites, significantly different from the garnet amphibolites and other eclogite-facies metamorphic rocks in the same orogen. This feature is coupled with the ultrahigh εNd(0) values as well as the severe depletion of light rare earth elements (LREE) and high field strength elements (HFSE). Because HFSE and LREE are immobile in aqueous solutions and the effect of melt extraction is insignificant during the continental deep subduction, the extreme fractionation of Lu/Hf and Sm/Nd indicate their origination from a geochemically ultra-depleted mantle source. These eclogites have the depleted mantle Hf model ages of 1.27 Ga to 1.61 Ga, similar to the depleted mantle Nd model ages of 1.39 Ga to 1.67 Ga as previously reported. This suggests that the protolith of the extremely high εHf-εNd eclogites was a kind of mafic igneous rocks derived from fractional crystallization of geochemically ultra-depleted mafic melts, which were produced by partial melting of the highly refractory lithospheric mantle during a series of seafloor spreading initiation-failure cycles at a divergent plate margin after the breakup of supercontinent Columbia in the Early Mesoproterozoic. The mafic igneous rocks were located in a passive continental margin in the Late Paleozoic and experienced deep subduction and exhumation in the Triassic, giving rise to the presently studied eclogites. The ancient geochemical signatures were retained without considerable influence by mantle convection, providing insights into the nature of crust-mantle differentiation during the tectonic transition from supercontinental breakup to seafloor spreading beneath the sub-ridge lithospheric mantle.

New P-T path of Huwan high-pressure unit from Hong'an orogen, Central China Orogenic Belt: Implications for oceanic to continental subduction and exhumation in the subduction zones

Oceanic to continental subduction is a pivotal process in plate tectonics, yet characterizing this tectonic transition and subsequent exhumation remains a challenging task. This study integrates our novel pressure-temperature (P-T) estimates on a metabasite lens and its hosting metasedimentary rock in the Xiongdian area of the Huwan high-pressure unit of the Hong'an orogen, Central China Orogenic Belt, with existing geochronological data from the same and neighboring outcrops. Our preliminary but comprehensive findings reveal the evolution of this subduction transition process. After the opening at ca. 430–400 Ma the Paleotethyan ocean subducted northward and formed a warm oceanic subduction zone at ca. 400–385 Ma, marked by relatively low P and high T conditions (∼610–740 °C and ∼1.5–1.8 GPa; M1 stage). Subsequently, the Paleotethyan oceanic plate continued its subduction and cooled the overlying mantle until its final closure during ca. 385–300 Ma. This phase led to the subduction of South China Block (SCB) and the formation of a cold continental subduction zone at ca. 300–270 Ma, featured by relatively high P and low T conditions (∼570–610 °C and ∼2.1–2.7 GPa; M2 stage). Following the delamination from the subducted SCB continental lithospheric mantle, the detached high-pressure rocks experienced a two-stage accelerated exhumation process, i.e., early slow exhumation (∼0.7 ± 0.4 mm/a) to shallower mantle depth (∼610–730 °C and ∼1.5–2.1 GPa; M3 stage) at ca. 270–243 Ma and later rapid exhumation (>2.5 ± 0.9 mm/a) to approximately the Moho depth (∼550–600 °C and ∼0.8–1.3 GPa; M4 stage) at ca. 243–233? Ma. To facilitate the buoyancy-driven exhumation of eclogite blocks from their maximum depth (M2 stage), a minimum of ∼16–25 vol% of low-density metasedimentary rock is required. The increasing buoyancy caused by decreasing densities of the high-pressure rocks with exhumation (from M2 to M4 stage), slab break-off induced rebound forces, and other tectonic factors may jointly explain the accelerated exhumation pattern. Our density results also emphasize that, partly owing to a stronger contribution from a buoyancy-driven mechanism, the high-pressure rocks would be preferentially exhumed in warm to extremely warm oceanic subduction zones compared to their intermediate to cold counterparts.

Initial sedimentation after the closure of the Sumdo Paleo-Tethys Ocean: Implications for early Mesozoic tectonics in the East Tethys domain

The closure of the Paleo-Tethys Ocean and subsequent collisional orogenesis in the early Mesozoic played an important role in the formation of the Qinghai–Tibetan Plateau. However, the final closure of the southernmost Paleo-Tethys branching ocean basin in the Qinghai–Tibetan Plateau—known as Sumdo Paleo-Tethys Ocean—and collision between Central and Southern Lhasa terranes remain controversial. In particular, the tectonic significance of early Mesozoic volcanism and sedimentation in the Lhasa terrane is unknown, but crucial for understanding the final stages of ocean closure. Here, we investigate the depositional ages and provenance of detrital zircon in clastic sedimentary rocks, as well as the petrogenesis of the interbedded andesite from the volcano-sedimentary sequence of the Late Triassic to Early Jurassic Xionglai Formation in the Tangjia-Sumdo area. Detrital zircon dates from the Xionglai Formation define five age peaks at 180–200 Ma, 230–340 Ma, 500–620 Ma, 1000–1240 Ma, and 1600–1800 Ma. Detrital zircon trace elements and Hf isotopic characteristics are interpreted to represent derivation from an accretionary wedge and later magmatism related to oceanic slab break-off as well as a minor component from coeval magmatic rocks in the Nyainrong-Nang area in northern Lhasa terrane. Interbedded andesite has a magnesian andesite signature and yields ages of 192–184 Ma with zircon εHf(t) values that range from −3 to +1. We interpret these data as the andesite being sourced from sediment-derived melts that interacted with the overlying mantle wedge in a post-collision extensional setting. Our study suggests that the closure of the Sumdo Paleo-Tethys Ocean and subsequent collision between Central Lhasa and Southern Lhasa terrane may have happened in Late Triassic-Early Jurassic. The final closure of the multi-Paleo-Tethys Ocean and subsequent collision in the Qinghai–Tibetan Plateau may have induced initial subduction of the Meso-/Neo-Tethys Ocean, representing the tectonic transition from collision/post-collision to subduction/accretion in the Qinghai–Tibetan Plateau during the Late Triassic to Early Jurassic.

Detrital zircon REE and tectonic settings

Earth's long-term history is preserved in the rock archive, yet this record is incomplete, hindering our understanding of how tectonic processes have evolved and shaped our planet's evolution. The physio-chemical resilience of zircon and its ability to be rapidly analyzed for age, isotopic, and elemental data has made it a key phase in unraveling Earth's long-term evolution. The europium anomaly (Eu/Eu*) and the differentiation degree between light and heavy rare earth elements (LREEN¯/HREEN¯) of zircons positively correlate with the pressure of magma crystallization because high pressure inhibits plagioclase crystallization and promotes garnet growth. However, this correlation can be influenced by fluctuations in redox conditions, protolith composition, and water content of magma, whose stability varies in different tectonic settings. Therefore, the correlation coefficient between Eu/Eu* and LREEN¯/HREEN¯ of detrital zircons (rDz) provides a new proxy to help distinguish tectonic settings. We evaluate this new proxy using available global detrital zircon REE data from different tectonic settings in modern and deep times. The data from well-constrained simple tectonic settings suggest that rDz is higher in convergent settings (0.53–0.85) than in collisional settings (0.12–0.51). Convergent settings can also yield relatively low rDz (0.28–0.55) when there is a contribution of a mantle plume lying below the overriding plate of a subduction zone.When studying a detrital zircon archive derived from a large distributive province that includes sources from a variety of settings, known provenance information should be used to disentangle mixed source signals. Also, only considering detrital zircon grains that formed a short time (e.g., 50 or 100 Myr) prior to deposition can reduce the likelihood of multi-cycle distal mixing of contemporaneous but tectonically distinct zircons in a sample. Additionally, greater confidence in interpretations of changes in tectonic settings may be derived from evolution patterns of rDz instead of isolated values. A decreasing trend can likely be interpreted as a transition from convergent to collisional or mantle plume settings, whereas an increasing trend implies an opposite tectonic transition.