Black shales serve as primary archives of organic carbon, preserving critical records of elemental cycling dynamics within carbon sequestration systems. Ancient lakes are critical nodes in the carbon-element coupling system, yet full understanding of this remains challenging. This investigation employs an integrated stratigraphic-geochemical methodology to elucidate volcanic material–induced perturbations in Fe-S-P coupling mechanisms and their consequent impacts on organic carbon preservation within Upper Triassic lacustrine shales of the Ordos Basin, North China Platform. Petrographic analysis reveals that organic carbon enrichment demonstrates genetic associations with volcanic inputs, though peak carbon accumulation intervals stratigraphically lag behind tuff deposition events. The volcanic inputs introduced substantial Fe, P, and S into the lacustrine basin. Volcanogenic atmospheric fertilization stimulated proliferation of coccolithophore, lamellibranch, and actinopterygian assemblages within the lacustrine ecosystem, yet sustained nutrient replenishment over million-year time scales proved geochemically untenable. Crucially, volcanic-derived phosphorus became effectively sequestered via iron oxide adsorption and biomineralization processes in sedimentary matrices during this stage. Sulfate influx modulated the dominant bacterially mediated redox pathway from dissimilatory iron reduction to bacterial sulfate reduction, thereby transitioning the redox regimes to possibly euxinic conditions. During this process, Fe3+ in phosphorus-bearing iron (oxides) was reduced to Fe2+ and subsequently removed via pyrite formation. Subsequent phosphate remobilization from phosphorus-rich sediments under anoxic bottom-water conditions established a positive feedback mechanism that sustained lacustrine productivity over millions of years, ultimately facilitating exceptional organic matter preservation. This research highlights lacustrine nutrient cycling dynamics as critical amplifiers of carbon burial following volcanic material perturbations, providing mechanistic insights into the functioning of lacustrine carbon sinks and long-term carbon sequestration.
The tectonic evolution of the Central Asian orogenic belt is associated with the subduction and closure of the Paleo-Asian Ocean. However, whether the Paleo-Asian Ocean abutting the northern North China craton underwent southward subduction and when the initial subduction occurred remain debated. The Baoerhantu accretionary complex, situated in the northern part of the Bainaimiao arc in the southeastern Central Asian orogenic belt, plays a pivotal role in elucidating the early Paleozoic tectonic evolution of the Paleo-Asian Ocean. According to field mapping, the Baoerhantu accretionary complex exhibits a typical “block-in-matrix” structure of mélanges and a classic ocean plate stratigraphic sequence, characterized by exotic blocks of diverse origins within a metamorphosed siliceous matrix. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon U-Pb dating of these blocks and matrix reveals ages of ca. 511−468 Ma for the metagabbro-metabasalt blocks and a maximum depositional age of 458 Ma for the protolith of the metasandstone (matrix), recording subduction-accretion events of the Paleo-Asian Ocean during the early Paleozoic. Geochemical characteristics suggest that serpentinized dunite blocks originated from a depleted, refractory oceanic plate basement that had been influenced by subducted slab−derived fluids/melts. Metagabbros and metabasalts can be divided into mid-ocean-ridge basalt−type and arc-type units. Mid-ocean-ridge basalt−type rocks exhibit affinities to suprasubduction zone−type ophiolites and formed through partial melting of a depleted asthenospheric mantle, whereas arc-type rocks are products of partial melting of a hydrous, depleted mantle wedge. This research identified the first suprasubduction zone−type ophiolite within the Baoerhantu accretionary complex in Inner Mongolia, which formed in a forearc region, recording initial subduction occurring during the late Cambrian. Combined with the coeval Tulinkai and Central Jilin suprasubduction zone−type ophiolites and their associated island arcs, these units constitute an initial “trench-arc” subduction system, recording southward subduction of the Paleo-Asian Ocean abutting the northern North China craton during the earliest early Paleozoic.
Beryllium (Be) is a critical metal whose enrichment mechanisms in anatectic pegmatites remain poorly constrained. A major challenge in understanding their genesis lies in the reliable identification of source rocks. This study examines Be-pegmatites in the Kelumute-Jideke field of the Chinese Altai in northwestern China to assess the utility of isotopic tracers and to constrain the origin of Be-rich melts. Systematic monazite U-Pb geochronology, whole-rock geochemistry, and Nd-Hf isotopic analyses of pegmatites, granites, and metasedimentary rocks are integrated together with partial melting simulations to determine petrogenetic conditions. Studies reveal a clear superiority of Nd isotopes over Hf isotopes in source discrimination. The Sm-Nd system is less susceptible than the Lu-Hf system to isotopic disequilibrium during melting, and magmatic monazite Nd isotopes in pegmatites preserve a robust record of initial melt composition. Nd isotopic data conclusively indicate that the Triassic Be-pegmatites and coeval granites were produced by partial melting of Early Devonian S-type biotite granites. This interpretation is supported by consistent Nd crustal evolution trends and consistent model ages (TDM2 = 1.3−1.2 Ga), ruling out significant contributions from direct metasedimentary anatexis or fractional crystallization. In contrast, the significant Hf isotope disequilibrium indicates the low-temperature anatexis of Devonian granite dominated the generation of the Be melt. A characteristic of anatectic pegmatites, in which Hf isotopic values decrease with increasing temperature, can be effectively distinguished from the uniform Hf isotopic values of highly fractionated pegmatites. Partial melting modeling demonstrates that muscovite-dehydration melting of the Devonian granite protolith under low-temperature, medium-pressure conditions (∼4 kbar, 640−700 °C) was essential for Be enrichment, efficiently concentrating Be released during mica breakdown.
The features of arc magmas are inherited from the modified lithospheric mantle sources. Within oceanic subduction zones, the subducted sediment-derived fluids play a critical role in governing the geochemical variations of arc magmas. Nevertheless, it remains unclear whether the chemical signatures of arc magmas preserved from these sediments are indicative of extreme paleoenvironmental conditions on Earth’s surface. Here, we present whole-rock Mo-Zn isotopes for sequential arc magmas in Northeast Asia that chronologically document the subductions of the Paleo-Asian Ocean and Paleo-Pacific plates during the Early Permian to Early Jurassic. The Early Permian (293 Ma) and the Late Triassic (213−202 Ma) to Early Jurassic (185−183 Ma) mafic igneous rocks have Zn isotopic compositions (δ66Zn = 0.20‰−0.30‰) similar to those of mid-ocean-ridge basalts (MORBs; δ66Zn = 0.28‰ ± 0.06‰). Notably, the Early Permian basaltic rocks exhibit significantly lighter Mo isotopic signatures (δ98Mo = −0.99‰ to −0.07‰) compared to the depleted MORB mantle (DMM; δ98Mo = −0.204‰ ± 0.008‰). In contrast, the Late Triassic−Early Jurassic gabbros display heavier Mo isotopic compositions (δ98Mo = −0.18‰ to 0.54‰). These results, along with average oxygen fugacity values of 3.14 (n = 6, the Early Permian basalts) and 2.52 (n = 7, the Late Triassic and Early Jurassic gabbros), suggest that the Early Permian mafic arc magmas were sourced from a lithospheric mantle modified by oxidized sediment-derived fluids, whereas the Late Triassic−Early Jurassic gabbros were sourced from a lithospheric mantle modified by reduced sediment-derived fluids. Our investigation demonstrates that arc magmas exhibit limited geochemical variability in their Zn isotope signatures despite extreme environmental perturbations, but redox-sensitive isotopes (such as Mo) could serve as sensitive tracers of recording climatic fluctuations, especially in paleo-surface redox events.
Geodynamic models and geological evidence increasingly support that the South China Sea (SCS) opened through rifting and breakup of the southeastern South China continental margin during the Cenozoic. However, direct magmatic records that capture the transition from initial extension to seafloor spreading remain scarce, hindering a refined understanding of the mechanisms driving continental breakup and SCS opening. In this study, we report a newly identified 34.82 Ma enriched mid-ocean-ridge basalt−like syn-rift basalt from Shantou, South China, accompanied by whole-rock geochemistry, Sm-Nd-Pb isotopic data, and thermochronological constraints from the host granite. The geochemical and isotopic signatures indicate derivation from a metasomatized lithospheric mantle that was initially modified by subducted paleo−Pacific components. Zircon U-Pb and apatite fission-track data reveal Cenozoic lithospheric thinning and thermal perturbation coeval with basalt emplacement, which we link to slab-pull−driven extension associated with proto-SCS subduction. Together with slightly older syn-rift basalts in the area, the Shantou basalt defines a time-resolved syn-rift magmatic sequence recording progressive lithospheric thinning. These results provide direct geochemical and thermal constraints on the rift-to-breakup transition along the South China margin and identify proto-SCS−controlled extension as the primary driver of the early magmatic response preceding SCS opening.
The emergence of continental crust fundamentally shaped Earth's surface environment by modulating global climate and supplying nutrients to the oceans. However, the timing and extent of this emergence remain highly debated, between a postulated widespread event at ca. 2.5 Ga and earlier, localized occurrences evidenced by Paleoarchean shallow-marine sediments. We analyzed the Caozhuang supracrustal series, which represents the oldest shallow-marine sedimentary rocks preserved in the Eastern Ancient Domain (EAD) of the North China craton, and examined the timing and mechanism of the first large-scale continental emergence using isostatic calculations. Analyses of heavy minerals, whole-rock geochemistry, zircon morphology, and inclusions indicate that the dominant protolith of the Caozhuang quartzite was quartz sandstone with high maturity. Detrital zircon U-Pb-Hf and trace element data constrain its depositional age to the middle Paleoarchean (ca. 3.46 Ga). The crustal thickness inferred from the Eu/ Eu* ratios of detrital zircons, combined with isostatic modeling, indicates that the EAD tent with an increase in the depth of partial melting of the Anshan tonalite-trondhjemitegranodiorite source rocks. A comparable sedimentary-magmatic evolution (involving shallow-marine sediments and potassic granites) also accompanied the middle to late Paleoarchean emergence of other cratons (Pilbara, Kaapvaal, and Singhbhum cratons). Therefore, we propose that the emergence of Earth's earliest continents began during the middle to late Paleoarchean, initiating a profound transformation of Earth's surface environment that set the stage for later climatic and biogeochemical evolution.
For more than a century, researchers have debated the route of the Snake River across the northern Cordillera and U.S. Pacific Northwest, including the associated incision history of Hells Canyon. Here, we use detrital zircon U-Pb provenance analysis of Miocene strata upstream and downstream of Hells Canyon to constrain the evolution in fluvial pathways over time. Downstream of Hells Canyon, we show that the ca. 10-6 Ma Clarkston Heights gravel was dominantly sourced from the nearby Salmon and Clearwater rivers but also with substantial input from drainages that now flow into the western Snake River Plain. These same western Snake River Plain drainages were contributing to time-equivalent Lake Idaho strata, upstream of Hells Canyon, indicating a Hells Canyon fluvial connection between the western Snake River Plain and Columbia basin before 6 Ma. Results from Lake Idaho strata suggest a two-phased lacustrine history in the western Snake River Plain: Before ca. 6.0 Ma, lake strata were derived from local tributaries, suggesting that the western Snake River Plain was isolated from the modern upper Snake River system. Between ca. 4.3 Ma and 2.2 Ma, the source area expanded to include eastern tributaries in conjunction with migration of high-standing topography of the Yellowstone hotspot. Our results challenge the long-held hypothesis of "capture" of the Snake River through Hells Canyon. Instead, we infer that Hells Canyon was a long-established route for outflow of lakes occupying the western Snake River Plain, possibly intermittently, during a ca. 10-2 Ma phase of slow regional incision, followed by rapid incision in Hells Canyon starting ca. 2 Ma in conjunction with erosion of the topographic barrier impounding Lake Idaho.
Porphyritic intrusions in the southeast Beartooth Mountains and porphyritic cobbles from the Torchlight conglomerate of the Cenomanian Frontier Formation in Bighorn basin (Montana-Wyoming, USA) bear striking resemblance. We utilize geologic mapping, petrography, geochemistry, and geochronology to characterize rocks from both locations and test whether the Beartooth Mountains area was the provenance for Bighorn basin cobbles. Mapping on Line Creek Plateau in the Beartooth Mountains shows three units with porphyritic phenocrysts consisting dominantly of plagioclase, plagioclase + potassium feldspar, and plagioclase + quartz. Petrography shows embayed quartz, opacitic rims, and zoned plagioclase. Geochemistry characterizes the intrusions as trachydacite to trachyte and trace elements exhibit enrichment in light rare earth elements and slight depletion in heavy rare earth elements relative to primitive mantle. Zircon U-Pb dating shows substantial Archean inheritance and ca. 104-92 Ma dates; some samples display multiple mid-Cretaceous date populations. Bighorn basin porphyritic cobbles exhibit similar modal mineralogy, near-identical geochemistry, and Archean inheritance and mid-Cretaceous geochronology. We suggest the porphyritic cobbles were sourced from basement-hosted porphyritic intrusions now exposed in the Beartooth Mountains, given that geochemistry does not agree with coeval volcanic sources along the Frontier Formation depositional fairway (Idaho batholith) or sedimentary-hosted Beartooth Mountains porphyritic sills intruded into Cambrian shale. Stratigraphic evidence does not support kilometer-scale mid-Cretaceous Beartooth Mountains exhumation to expose basement-hosted porphyritic intrusions, and we therefore present a volcanic hypothesis to bring rocks to the surface. This mid-Cretaceous multimillion-year magmatism is unexplained by tectonic models for the north-central Laramide foreland, and thus we further explore the emplacement processes, spatiotemporal significance, and tectonic implications of these rocks.
New 40Ar/39Ar and U/Pb geochronology constrains the depositional age of the Middle Miocene fossil locality known as Napudet on the west side of Lake Turkana in Kenya. Napudet is the type locality for Nyanzapithecus alesi, represented by a remarkably complete fossil skull of an infant ape. Abundant fossil trees in the stratigraphic section (“Emunyan beds”) from which the N. alesi specimen was exhumed indicate that Napudet was a Middle Miocene forest. 40Ar/39Ar geochronology from basalt flows brackets the depositional age of the Emunyan beds between 14.05 ± 0.34 Ma and 13.52 ± 0.13 Ma (2σ). A laser ablation U/Pb dating approach based on geochemical image mapping of carbonate petrified wood found within the Emunyan beds yields ages of 13.19 ± 0.61 Ma using a Tera-Wasserburg diagram to correct for common Pb and 14.11 ± 0.95 Ma using a total−Pb/U−Th isochron (2σ). Both ages are consistent with the bracketing basalts and provide an opportunity to examine different approaches for dating carbonates. Carbonate U/Pb dating represents an exciting avenue for directly dating terrestrial sequences.
Proterozoic metamorphism and deformation of the southern margin of the Superior craton in the Lake Superior region is attributed to the Penokean orogeny (1890−1830 Ma). This model includes a period of crustal inversion in which Archean basement blocks were exhumed through overlying Paleoproterozoic strata, producing the corridor of gneiss domes that parallels the trend of the Penokean orogen across the northern Midcontinent, USA. However, recent geologic mapping and 40Ar/39Ar geochronology challenge this interpretation, suggesting instead that the gneiss dome structures reflect younger episodes of tectonic activity along the southern margin of Laurentia. In absence of integrated pressure-temperature-time-deformation constraints for these rocks, interpretations are largely limited to their final cooling history, making it difficult to both identify the tectonic forces that shaped the architecture of the Penokean orogenic belt and assess the extent to which later Proterozoic tectonism modified the southern Superior craton. We address this problem with an approach joining thermodynamic modeling, garnet and accessory mineral geochronology, and microstructural analysis for several metamorphic rocks across the gneiss dome corridor. The U-Pb ages of titanite reveal that the Proterozoic geometries of exhumed basement gneiss domes are governed by preexisting Archean structures. Garnet Lu-Hf geochronology constrains the timing of prograde-to-peak metamorphism in the Penokean orogenic belt. Granulite facies metamorphism is related to the final stages of the Penokean orogeny at 1837 Ma and localized in a belt of high-grade rocks near a major Penokean suture. Garnet Lu-Hf ages of samples adjacent to gneiss domes reflect regional metamorphism following the accretionary phase of the Penokean orogeny, between 1825 Ma and 1782 Ma, which we suggest reflects continued crustal thickening related to convergence farther south during this time interval. Combination of garnet microstructures and Sm-Nd ages reflects later exhumation of gneiss domes and buried metasedimentary rocks by ca. 1750 Ma, consistent with previously published 40Ar/39Ar cooling ages across the region. Reset Lu-Hf and Sm-Nd garnet ages and U-Pb ages of syn-kinematic titanite reflect reactivation of primary Penokean structures during this period of basement uplift. These data document significant modification of the Penokean orogen and the Archean crust of the southern Superior province between 1800 Ma and 1700 Ma. Tectonic activity during this interval coincides with collisional events recognized in western Laurentia, suggesting that the period immediately following the Penokean orogeny may be a broadly important time for crustal growth and modification in proto-North America.
The formation of porphyry Cu deposits (PCD) is related to deep magmatic systems, which serve as sources for porphyry magmas and metal- and sulfur-bearing mineralizing fluids. However, the interactions among deep magmas, mid- to upper-crustal magma chambers, and shallow porphyritic intrusions that trigger PCD formation remain poorly understood. This study focuses on the Xiongcun porphyry Cu-Au district in the Gangdese belt of the southern Tibetan Plateau, which experienced two distinct mineralization events: an earlier event related to a hornblende quartz diorite porphyry (HQDP), followed by a later event with a quartz diorite porphyry (QDP). to the Xiongcun PCD formation is investigated based on zircon textures, in situ zircon trace element and U-Pb isotope data, and the geochemical compositions of zircon-hosted ogy and trace element compositions indicate that the mid- to upper-crustal magma chamexperienced mafic magma recharge at ca. 174.9 +/- 0.4 Ma, followed by the emplacement of the HQDP (173.2 +/- 0.4 Ma) and the trace element concentrations from both porphyries indicate that magmatic evolution was fractional crystallization, reflecting a hytite inclusions reveal a pronounced increase in volatile concentrations, particularly sulfur, likely provided by mafic magma recharge rejuvenating the mid-to upper-crustal magma chamber. Compared with the earlier and low-grade HQDP, the younger and more fertile QDP has lower zircon Gd/Yb ratios, and higher Eu/Eu*, (Ce/Nd)/Yb ratios, sulfur concentrations, and apatite XF/XCl ratios. Therefore, the formation of the high-grade Au-rich PCD centered on QDP is attributed to a protracted magmatic evolution in the mid-to upper-crustal magma chamber. During this prolonged period, the QDP likely underwent multiple mafic magma recharges and extensive fractional crystallization, leading to increasing volatile concentration, particularly sulfur, ultimately reaching volatile saturation. This study highlights the critical role of mafic magma recharge in the formation of the Xiongcun PCD, as it supplied essential ore-forming volatiles, especially sulfur, to the mid-to upper-crustal magmatic system.
Archean cratons commonly experienced a protracted evolutionary history with multiphase tonalite-trondhjemite-granodiorite (TTG) and mafic-ultramafic magmatism and corresponding polymetamorphism. Reliably reconstructing the pressure-temperature-time (P-T-t) history of Archean polymetamorphic cratons is crucial for understanding the tectonic regimes that operated on early Earth. In this paper, we conduct a comprehensive study of petrography, mineral chemistry, phase equilibrium modeling, and zircon and titanite dating for three representative samples from the Mengjiatun area in the West Shandong massif, North China Craton. Three phases of metamorphism are identified. The first phase is upper amphibolite to granulite facies with an anticlockwise P-T path, and the peak condition is 790-830 degrees C and similar to 8 kbar, registering a geothermal gradient of 27-28 degrees C/km of the low-P/T type. The second phase is lower amphibolite facies marked by clockwise P-T paths and peak conditions of 610-650 degrees C and 7-9 kbar, corresponding to a geothermal gradient of 20-21 degrees C/km of the medium-P/T type. The third phase is lower amphibolite facies with a possible clockwise P-T path and a peak P-T condition at similar to 500 degrees C and similar to 4 kbar, revealing a geothermal gradient of similar to 34 degrees C/km of the low-P/T type. Zircon and titanite dating results and age compilation indicate the metamorphic ages of the three-phase metamorphism are 2.70-2.66 Ga, 2.64-2.58 Ga, and 2.54-2.48 Ga, which are coeval with the TTG and mafic-ultramafic magmatic activity at 2.74-2.66 Ga, 2.64-2.58 Ga, and 2.56-2.50 Ga, respectively. The polyphase metamorphism, coupled with coeval magmatism, could be attributed to the multiphase sagduction of supracrustal rocks into underlying granitoid intrusions, which was in turn dynamically driven by the episodic upwelling of asthenospheric mantle with accretion of mantle-derived magmas. This multiphase sagduction model is of significance for other Archean polymetamorphic cratons.
The isotope characteristics of the continental crust are fundamental to understanding its formation and evolution. We present zircon O and Hf isotope data for 115 Archean magmatic rocks (>2.58 Ga) from the Rae Province (northern Canada), providing constraints that have been missing from the global Archean archive thus far. Zircon from >2.65 Ga, dominantly tonalite−trondhjemite−granodiorite samples, show mantle-like δ18O, while εHfinitial and two-stage depleted mantle model ages (TDM) are variable and define four broad isotope domains. Two domains comprise relatively juvenile crust (TDM < 3.0 Ga) that define a central spine of the Rae Province. These are flanked to the west and east by two separate domains of more evolved crust with Meso- to Eoarchean histories (TDM > 3.0 Ga). A decrease in εHfinitial with age within each isotope domain is most consistent with infracrustal reworking of local crust. Relative to pre−2.65 Ga crust, zircon from 2.65−2.58 Ga potassic granitoids show a greater range in δ18O, whereas their εHfinitial are more homogenized. Zircon yielding δ18O >6.5‰ are dominant in the northern Rae Province, where sedimentary sequences are most abundant and likely reflect widespread erosion and sediment/crustal recycling following the emergence of >2.65 Ga crust. The relatively homogeneous εHfinitial is most consistent with an infracrustal origin and mixing with local sediment. The zircon isotope data are used to evaluate and refine boundaries of crustal blocks, and to interrogate models for the construction of the Rae Province. The isotope signatures of the Rae Province are consistent with the global Archean archive for the emergence of the depleted mantle and cratonization.
Detrital garnet grains sampled along the margin of East Antarctica (70°E−175°E) yield laser ablation Lu−Hf dates that record the timing of regional metamorphism related to all major orogenies that built this large segment of the continent. Detrital garnet dates from the Ross Sea margin form a large population at ca. 600 Ma, marking the onset of convergence along the Pacific margin of East Antarctica during the Ross Orogeny. Marine sediments offshore northern Victoria Land and Terre Adélie Land contain minor ca. 600 Ma detrital garnet populations derived from the Ross orogen and large Paleoproterozoic populations at ca. 2000−1700 Ma from the Mawson Craton. The Wilkes Land offshore sediments include a prominent detrital garnet population at ca. 1200 Ma (Albany-Fraser-Wilkes Orogeny) and a minor distribution of ca. 600 Ma grains, possibly derived distally from the Ross orogen. Detrital garnet in marine sediments from Prydz Bay are dominated by Rayner Orogeny (ca. 1000−900 Ma) and Kuunga Orogeny (ca. 500−600 Ma) ages. Detrital garnet Lu-Hf populations tend to be systematically ≥10 m.y. older than the major populations of detrital zircon, titanite, and phosphate dates from the same areas. These age differences reflect the tendency of garnet to record the prograde and peak metamorphic history of orogenesis, rather than anatexis and the retrograde and cooling histories that are typically recorded by other detrital mineral geochronometers. Our findings demonstrate the potential of detrital garnet Lu-Hf geochronology for mapping orogenic provinces at the continental scale and reconstructing more complete metamorphic histories from the sedimentary archive.
The southeastern margin of the Tibetan Plateau exhibits a complex tectonic evolution and distinctive geomorphic features, yet previous studies have been limited in scope, lacking comprehensive comparisons across key regions. In this study, we present a substantial new dataset of low-temperature thermochronology, providing critical constraints on the region’s exhumation history. By integrating these results with existing geomorphological, structural, and geophysical evidence, we offer a more comprehensive perspective on the differential uplift and fault activity that have shaped the southeastern Tibetan Plateau during the Cenozoic. More specifically, exhumation along the Jinhe-Qinghe thrust belt exhibits significant along-strike differences, with the northern segment experiencing more substantial late Cenozoic thrusting and exhumation compared to the southern segment. It is suggested that present-day thermal history contrasts across the fault may, in part, reflect inherited topographic features rather than mere recent tectonic processes. In addition, low-temperature thermochronological data indicate that the Miocene was a key period of crustal shortening and deformation across the southeastern Tibetan Plateau, with widespread evidence of rapid cooling and exhumation along major fault zones during this time. Our findings further demonstrate that late Cenozoic crustal deformation, uplift, and shortening in this region have been predominantly controlled by large-scale fault systems. In contrast, the regions between these major faults exhibit significantly older cooling ages and a more stable thermo-tectonic history, suggesting limited late-stage exhumation. Finally, contour maps of thermochronological ages of the southeastern Tibetan Plateau illustrate systematic spatial variations in exhumation, reinforcing the critical role of fault-controlled deformation.
Crustal thickness exerts a key control on the formation of porphyry Cu deposits by modulating magmatic differentiation and Cu enrichment processes. While geochemical proxies like Sr/Y ratios are established tracers of crustal thickness, their relationship with magmatic Cu content remains ambiguous. This study integrates a global geochemical compilation with a detailed case study of the Yidun Arc, eastern Tibet, to resolve this relationship. We establish a robust inverse correlation between crustal thickness and magmatic Cu content on a global scale. In the Yidun Arc, we document a pronounced north-south gradient in Cretaceous crustal thickness. This gradient directly controlled the spatial distribution of fertile, high-Sr/Y magmas, explaining the exclusive localization of Triassic and Cretaceous porphyry deposits in the thickened southern segment. Our results confirm that crustal thickness acts as a critical geodynamic filter for porphyry Cu deposit formation, providing a powerful framework for mineral exploration.
Jurassic orogenesis has had profound effects on the crustal evolution of the North American Cordillera from Alaska through Baja California, Mexico. While often contractional in nature, the orogenic evolution included components of strike-slip, convergent, and extensional tectonic regimes, some of which occurred over dramatically short and punctuated intervals. Within the central part of the cordillera, two Jurassic orogenic events, the Siskiyou and Nevadan orogenies, have been identified in the Klamath Mountains province of Oregon and California. These two orogenic events are thought to have resulted from amalgamation (contractional +/- transcurrent motion) of distinct but spatially related terranes along major shear zones. Regional high-grade metamorphism has been variously assigned to one or the other event, and magmatism has been considered to have occurred before, between, and after these events. Precise timing of the tectonic and thermal history of the province has been uncertain due to a paucity of precise age data. New U-Pb (zircon) ages from pinning plutons indicate that the Middle Jurassic terrane amalgamation (Siskiyou orogeny) ended by ca. 172 Ma. However, regional metamorphism originally thought to be related to Siskiyou orogenesis is, instead, Late Jurassic in age, based on new single-crystal U-Pb (zircon) dates from leucosomes and Sm-Nd dating of prograde garnet, plus U-Pb (rutile) and published 40Ar/39Ar (hornblende) cooling ages. Near-peak metamorphic conditions were reached by ca. 157 Ma. A second terrane amalgamation event (Nevadan orogeny) was coeval with this peak metamorphism, during which low-temperature metavolcanic rocks were thrust beneath high-temperature m & eacute;lange of the Rattlesnake Creek terrane, effectively quenching the high-grade metamorphism. Moreover, rather than being punctuated by tectonic activity, plutonism continued with little or no break from pre-Siskiyou time to post-Nevadan time, reaching a volumetric peak in the Late Jurassic, ca. 158 Ma. The close timing of peak plutonism with peak metamorphic conditions suggests that heat for high-grade metamorphism was related to transcrustal magma transport and differentiation, driven and enhanced by mantle up-welling associated with Middle Jurassic slab breakoff and a westward shift in subduction. Our new geochronologic data, combined with our interpretation of post-Siskiyou slab breakoff, indicate that the Siskiyou and Nevadan orogenies, although closely related in time, were distinct in their thermal, and perhaps, tectonic evolution.
Unraveling the tectono-thermal history of medium-to low-grade belts would place important constraints on the regional tectonic evolution. There exists a Barrovian metamorphic belt in the L & uuml;liang Group, Trans-North China orogen of the North China craton. Representative rock samples from chlorite zone, biotite zone, garnet zone, and staurolite-kyanite zone have been collected to delineate the pressure-temperature-time (P-T-t) evolution. A two-stage prograde P-T path characterized by heating first and then pressurizing is recovered from the garnet zone by phase equilibria modeling. The peak P-T conditions are constrained to be similar to 7.0 kbar and 560 degrees C. Decompression-dominated P-T paths involving peak pressure (6.8-9.2 kbar, 515-565 degrees C) and peak temperature (4.6-6.5 kbar, 560-615 degrees C) stages are obtained from the staurolite-kyanite zone. Metamorphic zircon from the staurolite-kyanite zone and garnet from the garnet zone yield U-Pb ages of 1850 +/- 31 Ma and 1882 +/- 67 Ma, respectively. Biotite from the biotite zone gives an 40Ar/39Ar age of 1762 +/- 3 Ma. The geochronological results indicate that metamorphism of the L & uuml;liang Group is younger than the formation age of the Trans-North China orogen (ca. 1.95 Ga) but is coeval with the subduction-collision orogeny (1.90-1.82 Ga) along the northern margin of the North China craton. The distribution of the Barrovian metamorphic belt is also parallel to the latter orogen (E-W trending). Consequently, combining these results with field observa tions and regional geological evolution, it is inferred that the genesis of Barrovian metamorphism in the L & uuml;liang Group may be related to the stress propagated from the 1.90-1.82 Ga orogeny. The heat sources and formation mechanisms of Barrovian metamorphism globally may vary case by case.
The initial enrichment and mobilization of rare earth elements (REEs) in granites are of crucial importance in the generation of widely distributed ion-adsorption REE deposits in South China. In this paper, we compile geochronological, geochemical, and mineralogical data on granites from ion-adsorption REE deposits, as well as geochemical data from global granites, to investigate the mechanisms of REE enrichment and mobilization in granites and their association with specific tectonic settings in South China. The granites associated with ion-adsorption REE deposits are all high-K calc-alkaline to shoshonitic and were generated by crustal reworking in high-temperature extensional settings. In conjunction with the observation that REE concentrations in granites demonstrate significant positive correlations with crystallization temperatures as well as Th concentrations and Th/U ratios, it is deduced that the high temperature conditions in South China’s extensional settings promoted initial REE enrichment in the granites through the melting of REE- and Th-enriched accessory minerals. In addition, the F-rich conditions, as evidenced by the occurrence of F-rich minerals in the granites, may have further enhanced REE concentrations by increasing the solubility of REEs in the melts. The parent granites of heavy REE (HREE) deposits are all high-silica muscovite granites that belong to the fractionated granites and exhibit flat REE patterns. The occurrence of metamictized zircons indicates that these granites evolved to the magmatic-hydrothermal transition stage. Magmatic evolution led to decreasing light REE (LREE) contents, whereas HREE contents remained consistent and even sometimes increased, resulting in the relative enrichment of HREEs in the evolved granites and thereby contributing to the generation of HREE deposits. The high geothermal gradient and F-rich conditions in the extensional settings in South China also favored magmatic evolution. There are two types of parent granites for LREE deposits. The first type contains abundant primary REE-rich accessory minerals of allanite, titanite, and apatite, which account for a substantial proportion of the REE budget in these granites. The common phenomenon in these granites is the alteration of these minerals and the formation of secondary REE-fluorocarbonates, which favors LREE mineralization during weathering because the altered minerals and secondary REE-fluorocarbonates are more easily weathered and release LREEs for mineralization. The second type contains few of these accessory minerals but still exhibits high REE contents with an obvious negative Ce anomaly. It is concluded that the high-oxygen-fugacity meteoric water carrying the REE ions derived from the weathering profiles resulted in REE enrichment within the rocks.
The Ediacaran Shuram carbon isotope excursion (CIE; 573−568 Ma) is widely recorded on many paleocontinents. The Doushantuo-Pertatataka acritarchs have long been considered to occur exclusively or predominantly in pre-Shuram Ediacaran strata. The absence of precise age constraints and uncertainty in the stratigraphic relationship of the fossil occurrences with the Shuram CIE greatly hindered the global correlation and our understanding of the coevolution of Ediacaran biotas and environment. We report, for the first time, the occurrence of the Shuram excursion on the Tuva-Mongolia terrane and correlate it with those of South China and Siberia cratons, thus indicating an Ediacaran age for the phosphorites of the Kheseen Formation. A new acritarch assemblage with a high diversity is recovered from the Kheseen Formation of the Murun section that is well exposed in the Khuvsgul basin, northwest Mongolia. Diverse acritarchs occur in the horizon recording the Shuram excursion, suggesting that their last appearances are younger than previously thought. New paleontological and chemostratigraphic data at the Murun section extend the spatial and temporal distributions of Ediacaran acritarchs, facilitating stratigraphic correlation with South China and Siberia cratons, and other paleocontinents, and lessening the role of environmental drivers in the evolution of Ediacaran acritarchs.
