Boulder, Colo., USA: The Geological Society of America regularly publishes articles online ahead of print. GSA Bulletin topics studied this month include the nature and dynamics of China and Tibet. Other locations include the Colorado Plateau, New Mexico, Arizona, the Arctic, and the Pampean Plains of Argentina, and the Lake Superior region. You can find these articles at https://bulletin.geoscienceworld.org/content/early/recent .
Sedimentology of the latest Permian to Early Triassic in the terrestrial settings of the North China Basin: Low-latitude climate change during a warming-driven crisis
Kaixuan Ji; Paul B. Wignall; Jinnan Tong; Yingyue Yu; Wenwei Guo ...
Abstract: In terrestrial settings, the Permo-Triassic mass extinction is commonly linked to major changes in sedimentological and climatic conditions that include a switch from meandering to braided fluvial systems and increased aridity. We examined the predominantly terrestrial strata of North China to reveal that, on the contrary, there was little substantial sedimentological change during the Permo-Triassic mass extinction. The crisis level occurs in the upper Sunjiagou Formation, where a range of environments, including low-sinuosity rivers, distally terminating alluvial fans, and mud-dominated coastal plains are recorded. A major sedimentological change occurred slightly higher, within the lowest Triassic, and involved a switch to braided fluvial and shallow lacustrine/fluviodeltaic conditions of the Liujiagou Formation. Later in the Early Triassic, fine-grained, playa lake, and alluvial plain facies became widespread (Heshanggou Formation). Paleosols are present throughout the basin fill and record a transition from Aridisols and Vertisols in the Late Permian to Inceptisols in the Early Triassic followed by gleyed Aridisols, Vertisols, and Inceptisols later in the Early Triassic. Previously reported aeolian facies from Lower Triassic strata were not encountered in this study and were unlikely to have occurred given the prevalence of lacustrine and fluvial conditions at this time. Overall, the North China Basin experienced an increase in seasonal rainfall in the latest Permian and a base-level rise that introduced marine conditions into the southwest of the basin. During the Early Triassic, seasonal or interannual rainfall was frequent, with strong evaporation in a hot climate. Overall, substantial sedimentological changes occurred in the terrestrial Permo-Triassic environments of North China, but despite earlier claims from studies in North China and elsewhere, there was no abrupt transition in fluvial styles around the Permo-Triassic mass extinction.
Detached structural model of the Keweenaw fault system, Lake Superior region, North America: Implications for its origin and relationship to the Midcontinent Rift System
James M. DeGraff; Brad T. Carter
Abstract: The Keweenaw fault system along Lake Superior's south shore in Michigan, USA is one of the most significant fault systems associated with North America's Midcontinent Rift System. Reverse slip has thrust Portage Lake Volcanics (ca. 1.1 Ga) southeastward over Jacobsville Sandstone (ca. 1.0 Ga). Growing consensus in the 1970s about a major rift beneath Lake Superior led to the idea that reverse movement on the Keweenaw fault involved inversion of a rift-bounding normal fault. To test this idea, we integrated bedrock geology maps, subsurface mining data, and geophysical data to build a well-constrained cross-section from the Minnesota, USA-Ontario, Canada border to central Upper Michigan. The cross-section shows the Keweenaw and Hancock faults to be parts of a reverse, listric system detached deep within the layered volcanic section. Correlation of offshore seismic units with onshore stratigraphic units defines a gradual change in stratal dip from 55° NW near the faults to horizontal near the center of Lake Superior. This stratal geometry implies that onshore listric fault geometry continues offshore and that the fault system soles into a basal detachment at 12−15 km depth. Absence of second-order folds northwest of the peninsula indicates that the fault system does not ramp downward from the basal detachment through the lower crust. The observations and model preclude ideas that the Keweenaw fault was a rift-bounding normal fault inverted by post-rift crustal shortening, or that it formed by upward propagation of a deeper normal fault undergoing inversion. Instead, our results imply that the fault initiated as a detached thrust in relatively brittle upper crust and that shortening of relatively ductile lower crust occurred on other structures.
Terrestrial paleoclimate transition associated with continental weathering and drift during the Aptian−Albian of East Asia
Jianliang Jia; Changsheng Miao; Wenquan Xie
Abstract: Reconstruction of Aptian−Albian paleoclimate obtained from marine records remains a challenging topic, but studies on coeval terrestrial paleoclimate and trigger mechanisms have lagged substantially. In this study, new multiproxy data from mudrocks in the Fuxin Basin of NE China provide a high-resolution terrestrial climate record from East Asia. Here, we demonstrate the occurrence of terrestrial climate cooling during the late Aptian (118−113 Ma), which interrupted the mid-Cretaceous warming shown in global records. Nearly uniform long-term global climate trends attributable to tectonism, volcanism, and weathering occur in Early Cretaceous terrestrial and marine records. In the Fuxin Basin, the long-term terrestrial climate was characterized by increasing temperatures during the late early Aptian, gradual cooling during the late Aptian, and subsequent enhanced warming during the early Albian. Moreover, chemical weathering and humidity during these intervals were low, moderate to high, and then moderate, respectively. A markedly reduced high-elevation paleogeomorphology under strong continental weathering during the late Aptian increased the variability in chemical weathering fluxes as the Eurasian plate in NE China drifted SE during the Early Cretaceous and then NE during the Late Cretaceous. We suggest that a combination of enhanced continental weathering and weakened plate drift induced changes in atmospheric CO2, while the geographic setting ultimately led to cooling in the Fuxin Basin during the late Aptian. Our results illustrate the importance of exploring long-term tectonic-climatic-biotic feedbacks to improve our understanding of tectonic processes and ecological transitions across various spatiotemporal scales.
Environmental conditions and mechanisms restricting microbial methanogenesis in the Miquan region of the southern Junggar Basin, NW China
Haijiao Fu; Yueguo Li; Xianbo Su; Detian Yan; Shuguang Yang ...
Abstract: Early microbial gas has been sealed in reservoirs in the Miquan region of NW China, with little or no supply of current microbial gas. To date, the environmental conditions and mechanisms restricting microbial methanogenesis are still unclear in the Miquan region. Thus, in this study, a series of gas and water samples from coalbed methane (CBM) exploitation wells and in situ coal samples were collected and analyzed to determine the potential for current microbial gas generation, the methanogenic pathways, the source of nutrients, the influence of the environmental conditions on in situ microbial communities and their methanogenesis, and the mechanisms restricting microbial methanogenesis. The gas-production simulation experiments revealed that the existing microbes in the coalbed water were less efficient at converting coal into methane under the approximate in situ conditions, which further verified that there was little or no supply of current microbial gas. The stable isotope compositions of the gas samples suggested that carbon dioxide (CO2) reduction was the dominant metabolic pathway for generating CBM, whereas the methanogenic communities contained a mixture of acetoclastic and methylotrophic methanogens in local areas. The nutrients available for the microbes mainly included the total dissolved nitrogen (TDN) and total dissolved carbon (TDOC), and the in situ dissolution of the coals was a significant source of the TDN, whereas the TDOC was mainly supplied by surface water. The microbes in the different tectonic settings were significantly controlled by different combinations of environmental factors, and there was no single environmental factor that completely dominated the spatial variability of the microbial communities. The gradual stagnation of the water environment led to an increase in salinity and a decrease in nutrients, which were likely the main factors restricting microbial methanogenesis under in situ conditions. Combined with the results of the rate-limiting stages of the anaerobic fermentation, the mechanisms restricting microbial methanogenesis can be finally determined in the Miquan region. These discoveries presented in this case study provide a significant supplement to the geological theory of CBM accumulation, and have a guiding significance for CBM development in the Miquan region.
Diatom evidence for a groundwater divide that limited the extent of Lake Estancia, New Mexico, USA, highstands during the Last Glacial Maximum
Kirsten M. Menking; Rebecca J. Bixby; Savannah M. Cutler
Abstract: A high-resolution time series of diatoms from the Estancia Basin of central New Mexico, USA, reveals decadal to millennial changes in water chemistry during the Last Glacial Maximum (LGM) and suggests that groundwater leakage limited the size of Lake Estancia highstands. Previous work showed that the lake expanded at least five times during the LGM, with each highstand reaching the same elevation; numerical modeling experiments showed that highstands required increases in precipitation of 1.5−2 times that of modern times but also demonstrated that these conditions could have lasted for only a few decades or the lake would have overflowed the basin's sill. The fact that the lake returned to the same elevation suggests that highstands were climatologically similar. Here we present an alternative scenario, that once the lake reached a particular threshold volume, groundwater leakage prevented further expansion, which means that precipitation increases may have been larger than previously thought. The LGM diatom sequence is dominated by freshwater planktonic species in the Lindavia ocellata complex, the freshwater to brackish littoral species Pseudostaurosira brevistriata, and the brackish to saline benthic species Diploneis cf. smithii, all of which show decadal to centennial oscillations that were likely driven by solar cycles. Canonical correspondence analyses using mineralogical time series as proxies for wetter and drier conditions reveal that diatom assemblages varied with salinity and pH/alkalinity and suggest that groundwater outflow led to variable water chemistry for different highstands of the same magnitude. Nonmetric multidimensional scaling confirms the uniqueness of each highstand in terms of the diatom assemblages. The diatom data presented here are among only a handful of such records for Pleistocene pluvial lakes in the American West and reveal that saline alkaline lakes can yield important information despite valve preservation issues.
Detrital zircon provenance and transport pathways of Pleistocene-Holocene eolian sediment in the Pampean Plains, Argentina
Austin Bruner; Andrew L. Leier; David L. Barbeau Jr.; Alex Pullen; Mary Kate Fidler ...
Abstract: The Pampas of Argentina contain a broad distribution of Pleistocene to Holocene loessic sediments and eolian dune deposits. Models describing the sediment provenance of this eolian system have, at times, conflicted. We address the provenance of these deposits through U-Pb detrital-zircon geochronology. Our results indicate broad similarity in age distributions between samples, with a dominant Permian-Triassic mode, and widespread but lesser Cenozoic, Devonian-Mississippian, Ediacaran-Cambrian, and Mesoproterozoic modes. These data are inconsistent with a large contribution of detritus from Patagonia as previously suggested. These data are consistent with very limited contribution of first cycle volcanogenic zircon to the Pampean eolian system, but abundances of older Neogene zircon indicate proto-sources in the Andes. The ríos Desaguadero, Colorado, and Negro contain populations that were likely within the dust production pathways of most of the loess, paleosol, and eolian dune deposits, but the derivation of the zircon ages in these sediments cannot be explained solely by these river systems. One statistical outlier, a loess sample from the Atlantic coast of the Pampa region, indicates quantitative similarity to the age spectra from the ríos Colorado and Negro, consistent with derivation from these subparallel rivers systems during subaerial exposure of the continental shelf under high global ice-volume. Another statistical outlier, a paleosol sample from the Río Paraná delta region, has zircon ages more closely associated with sediments in the Paraná region than in rivers south of the Pampa region. Collectively, these data point to the complexity of the Pampean eolian system and substantial spatial-temporal variation in this Pleistocene−Holocene eolian system.
Late Cretaceous time-transgressive onset of Laramide arch exhumation and basin subsidence across northern Arizona−New Mexico, USA, and the role of a dehydrating Farallon flat slab
Jacob O. Thacker; Karl E. Karlstrom; Shari A. Kelley; Ryan S. Crow; Jerry J. Kendall
Abstract: Spatiotemporal constraints for Late Cretaceous tectonism across the Colorado Plateau and southern Rocky Mountains (northern Arizona−New Mexico, USA) are interpreted in regards to Laramide orogenic mechanisms. Onset of Laramide arch development is estimated from cooling recorded in representative thermochronologic samples in a three-step process of initial forward models, secondary HeFTy inverse models with informed constraint boxes, and a custom script to statistically estimate timing of rapid cooling from inverse model results. Onset of Laramide basin development is interpreted from increased rates of tectonic subsidence. Onset estimates are compared to published estimates for Laramide timing, and together suggest tectonism commenced ca. 90 Ma in northwestern Arizona and progressed eastward with later onset in north-central New Mexico by ca. 75−70 Ma. The interpreted sweep of onset progressed at a rate of ∼50 km/m.y. and was approximately half the 100−150 km/m.y. rate estimated for Late Cretaceous Farallon-North America convergence during the same timeframe. Previous suggestions that the Laramide tectonic front progressed at a rate similar to convergence via basal traction are not supported by our results. We thereby suggest that (1) a plate margin end load established far field compression and that (2) sequential Laramide-style strain was facilitated by progressive weakening of North American lithosphere from the dehydrating Farallon flat slab. Results are compared to models of sweeping tectonism and magmatism in other parts of the Laramide foreland. Discussions of the utility of the custom script and the potential for stratigraphic constraints to represent only minimum onset estimates are also presented.
Cenozoic deformation in the eastern domain of the North Qaidam thrust belt, northern Tibetan Plateau
Bing Li; Yongchao Wang; Andrew V. Zuza; Xuanhua Chen; Zhaogang Shao ...
Abstract: The present topography of the northern Tibetan Plateau is characterized by the northwest-trending Eastern Kunlun Range, Qaidam Basin, and Qilian Shan, which figure importantly into the evolution and mechanism of Tibetan plateau development during Cenozoic Indo-Asian convergence. Understanding the Cenozoic deformation history and the source-to-sink relationship through time has significant implications for deciphering the growth history of the northern Tibetan Plateau. Despite decades of study, the timing, pattern, and mechanisms of deformation across the northern Tibetan Plateau are still vigorously debated. The North Qaidam thrust belt, located between the Qaidam Basin and Qilian Shan thrust belt, provides a valuable record of Cenozoic deformation in the northern Tibetan Plateau. Here, we present the results of new geologic mapping, structural and sedimentology analysis, and apatite fission track thermochronology to constrain the Cenozoic evolution history and reconstruct the paleogeomorphology of the eastern domain of the North Qaidam thrust belt and its foreland, the Wulan Basin. Our analyses reveal the North Qaidam thrust belt experienced multi-phase exhumation since the Cretaceous. A period of Eocene localized thrust-related uplift of the North Qaidam thrust belt initiated shortly after India-Asia collision, and lower erosion rates in the Oligocene allowed the thrust belt to expand along-strike eastward. Local uplift shed sediments to the southwest, directly into the Qaidam Basin. Reactivation of the proximal thrust faults and initiation of the northwest-striking right-slip Elashan fault at ca. 15−10 Ma drove the final accelerated mid-Miocene cooling and denudation to the surface. This phase of deformation established the overall framework morphology of the northeastern margin of the Tibetan Plateau, including the overall structure of the basins and ranges.
Ediacaran magmatism and rifting along the northern margin of the Tarim craton: Implications for the late Neoproterozoic Rodinia configuration and breakup
Hong-Xiang Wu; Yildirim Dilek; Feng-Qi Zhang; Han-Lin Chen; Huan Chen ...
Abstract: The Tarim craton in modern Central Asia was an important component of the supercontinent Rodinia in the Neoproterozoic, although its paleogeography in Rodinia during that era is still controversial. Here, we present new stratigraphic, geochemical, and geochronological data from the Neoproterozoic sedimentary and volcanic rock successions along the northwestern margin of the Tarim craton and discuss the significance of these data and our interpretations for its tectonic evolution and paleogeographic position within Rodinia. The Lower Ediacaran sedimentary sequence (Sugetbrak Formation) in northwest Tarim includes terrestrial and shallow-marine clastic rocks intercalated with two discrete basaltic lava flows near the top. The Upper Ediacaran sedimentary sequence conformably overlying the volcanic and clastic rocks consists mainly of stromatolitic dolomite (Chigebrak Formation), representing a transgressive shallow-marine environment. Previous U-Pb zircon dating of the basaltic lava flows has constrained the timing of their eruption in the early Ediacaran (615 Ma). Detrital zircon U-Pb dating of a feldspar-quartz-sandstone unit situated between the two lava flows revealed an oldest age of 2517 ± 18 Ma and a youngest age of 612 ± 6 Ma, with a majority of zircon grains (n = 42) dated at 891−754 Ma (Tonian). A quartz-sandstone unit above the upper lava flow revealed an oldest age of 2724 ± 15 Ma and a youngest age of 607 ± 8 Ma, with a missing age group of 891−800 Ma. These data and observations indicate: (1) a major switch in the depositional setting from a terrestrial (synrifting) to shallow-marine environment following the eruption of the upper lava unit; and (2) an abrupt disappearance of the source rocks of the 891−800 Ma zircons and sediments from the provenance of the post-615 Ma (postrifting) sedimentary sequence. The basaltic rocks have low SiO2 and MgO but high total Fe2O3 and TiO 2 contents (2.34−3.19 wt%), analogous to high-Ti basalts and continental flood basalts. Their Ti/V ratios (65−88), low Th/Nb ratios (∼0.1), and high TiO2/Yb ratios (∼1.1) are similar to those of ocean-island basalt (OIB). Combined with their Sm/Yb and La/Sm ratios and Sr-Nd-Pb-Hf isotope values, we infer that magmas of the Sugetbrak basalts were likely derived from partial melting of an enriched mantle source (EM I) in a transitional spinel-garnet lherzolite field. This petrogenetic evolution was a result of mantle plume−influenced rift magmatism during the Ediacaran breakup of Rodinia. The Central Tianshan terrane, which was attached to the northern Tarim craton until the Ediacaran Period, was the provenance of 891−800 Ma (Tonian) zircons in the synrift sedimentary succession. As the Central Tianshan terrane broke away from the Tarim craton after ca. 615 Ma, Tonian-aged zircons were no longer available to the depocenter of the postrift sedimentary sequence. The transition from rifting to drifting between the Tarim craton and the Central Tianshan terrane marked the final breakup of Rodinia, a global event that was possibly driven by mantle plume activities coeval with the development of the Central Iapetus magmatic province in Laurentia during the Ediacaran.
Featured Neoarchean granitoid association in the central North China Craton: An indicator of warm plate subduction
Guozheng Sun; Yalu Hu; Shuwen Liu; Sanzhong Li; Jinghao Fu ...
Abstract: Diverse Neoarchean granitoid assemblages, which generally include tonalites−trondhjemites−granodiorites (TTGs) and various K-rich granitoids, are prevalent in most basement terranes of the North China Craton. However, the Hengshan terrane is an exceptional case in the North China Craton; it is dominated by late Neoarchean sodic diorite-TTGs (DTTGs) and sanukitoids. These sanukitoids are the only high-K granitoids and show Mg-rich chemical features. The late Neoarchean DTTGs and sanukitoids were generated at ca. 2486−2537 Ma and show an intimate spatial association. The granitoid assemblages of the DTTGs and sanukitoids are characterized by high Mg# [100 × Mg/(Mg + Fetotal)] values (43−65) and enriched in light rare earth elements, large ion lithophile elements, heterogeneous zircon Lu−Hf (εHf = −1.6 to +7.4), and whole-rock Sm−Nd (εNd = +0.9 to +4.2) isotopic components, which indicates that they may be derived from varying degrees of interactions between mantle peridotite and subduction-related materials. Combined with the relatively high apparent geothermal gradient (∼17 ± 2 °C/km) and the relatively low basal heat flow of continental crust (∼25 ± 5 mW m−2), the crust-mantle interaction process indicates that the occurrence of late Neoarchean high-Mg magmatism was closely related to warm oceanic slab subduction in the Hengshan terrane, and the featured lithological association of DTTGs and sanukitoids most likely developed in the active continental margin at the end of the Archean.
Geochemical evidence for incorporation of subducting sediment-derived melts into the mantle source of Paleozoic high-Mg andesites from northwestern Tianshan in western China
Li-Tao Ma; Li-Qun Dai; Yong-Fei Zheng; Zi-Fu Zhao; Wei Fang ...
Abstract: Although high-Mg andesites (HMA) have attracted increasing attention due to their unique geochemical composition and important geological significance, there is no consensus on their petrogenesis. The present study indicates that the subducting terrigenous sediment-derived hydrous melts were incorporated into the mantle source of Paleozoic HMA in northwestern Tianshan, western China. These HMA are composed of basaltic andesite and andesite. They generally exhibit arc-type trace element distribution patterns and weakly enriched Sr-Nd-Hf isotope compositions. Some of them show remarkably higher Ba/La, Ba/Th, Rb/Nb, and U/Th ratios than normal mid-ocean ridge basalts (MORB). Furthermore, most of them are characterized by higher Th/Nb, Th/Yb, and Th/Nd ratios but lower Nb/U ratios relative to normal MORB, similar to those of terrigenous sediments. Whole-rock Nd isotopes covary with Rb/Nb and Th/Yb ratios, indicating contributions from both oceanic crust-derived aqueous solutions and terrigenous sediment-derived hydrous melts. Together with their high zircon δ 18O values, it appears that the mantle source of the target HMA contains terrigenous sediment-derived hydrous melts in addition to subducting oceanic crust-derived aqueous solutions. In the studied HMA, the andesite generally exhibits higher contents of large-ion lithophile elements and light rare earth elements and more enrichments in Sr-Nd-Hf isotopes than the basaltic andesite. These differences indicate that the mantle source of andesite would contain more subducting sediment-derived hydrous melts than that of basaltic andesite. As a consequence, relatively Si-rich and Si-poor pyroxenite sources were respectively generated as the mantle sources of the andesite and basaltic andesite. This qualitative interpretation is verified by quantitative modeling of the geochemical transfer from subducting oceanic crust into the mantle wedge. Model calculations indicate that the addition of ∼3% oceanic crust-derived aqueous solutions and 4%−12% terrigenous sediment-derived hydrous melts into the mantle wedge peridotite can account for the geochemical compositions of the target HMA. Therefore, the HMA in northwestern Tianshan provide the geochemical evidence for the crust-mantle interaction during the oceanic subduction in the Paleozoic. As such, the subducting terrigenous sediment-derived hydrous melts play a dominant role in the composition of the lithochemically fertile, geochemically enriched mantle sources and thus in the origin of HMA above oceanic subduction zones.
Correlation between South China and India and development of double rift systems in the South China−India Duo during late Neoproterozoic time
Bingbing Liu; Touping Peng; Weiming Fan; Guochun Zhao; Jianfeng Gao ...
Abstract: South China, India, and their derivative blocks preserve many similar magmatic and sedimentary records related to the tectonic transition from Rodinia to Gondwana. They provide crucial insights into not only the paleogeographic correlation between them but also the geodynamic mechanism for such a transition. Our new results, combined with published data from these blocks, reveal that South China remained linked with India at least from ca. 830 Ma to ca. 510 Ma and formed the South China−India Duo, which is located at the western margin of Rodinia. The identical magmatism and sedimentation reflect that double late Neoproterozoic rift systems in the South China−India Duo developed owing to the rollback of subducting oceanic slab beneath them. For example, an intracontinental rift developed along the Jiangnan−Aravalli−Delhi fold belt, which separated the Yangtze-Marwar block from the Cathaysia-Bundelkhand block. Another intra-arc rift developed contemporaneously along the northern and western margins of the Yangtze block, through the Marwar terrane of western India, and then into the Seychelles and Madagascar terranes. Such an intra-arc rift is the most feasible explanation for the common development of coeval arc-like and extension-related magmatic rocks and extensional sedimentary sequences on the western margin of the South China−India Duo, in Seychelles and Madagascar, and even at other subduction zones. South China was finally separated from Indian Gondwana at ca. 510 Ma due to the opening of the Proto-Tethys Ocean.
Fingerprinting the metal source and cycling of the world's largest antimony deposit in Xikuangshan, China
Zheng-Yu Long; Kun-Feng Qiu; M. Santosh; Hao-Cheng Yu; Xiang-Yong Jiang ...
Abstract: The Xikuangshan antimony (Sb) deposit is the largest Sb deposit in the world; however, the metal source and cycling and a holistic understanding of the deposit genesis remain equivocal. Sulfur isotope signatures offer a means of fingerprinting different sources in a hydrothermal deposit, although one must be careful to rule out subsequent isotope fractionation during fluid ascent, mixing, and ore precipitation. Here, we investigated the sulfur isotope composition of stibnite occurring at depth in the Xikuangshan deposit to distinguish the isotopic signals from the source and the superimposed imprint from near-surface−derived sulfur mixing or isotopic fractionation. All stibnites from the deep orebodies displayed limited δ34S variation from +6.8‰ to +8.4‰, despite their widely varying depths. These results provide direct evidence that δ34S values measured in deep orebodies are representative of the isotopic composition of initial fluids. The most likely factor controlling the variation of the sulfur isotopes in shallow stibnites (+3.5‰ to +16.3‰) is a series of hydrothermal processes, including Rayleigh fractionation during ore precipitation, fluid boiling induced by pressure release, and/or local input of pyrite from wall rocks via fluid-rock interaction. Accordingly, we conclude that the Neoproterozoic basement served as the metal source. We propose a holistic genetic model wherein we envisage that Sb and S were leached from the basement rocks, and the ore-bearing fluids ascended along a deep fault and eventually precipitated beneath the Devonian shale cap. Thus, sulfur isotopic systematics represent a powerful repository for interrogating the metal source and cycling in the hydrothermal ore system.
Strontium isotope stratigraphy and LA-ICP-MS U-Pb carbonate age constraints on the Cenozoic tectonic evolution of the southern South China Sea
Gang Li; Weihai Xu; Yun Luo; Jianguo Liu; Jianxin Zhao ...
Abstract: The tectonic evolution of the South China Sea is closely associated with multiple subduction-collision processes in Southeast Asia. When the collision of the Dangerous Grounds terrane with Sabah-Palawan Islands terminated is debated due to poor age constraints at the southern margin of South China Sea. A deep well drilled on Meiji Atoll penetrates Cenozoic carbonate strata in central Dangerous Grounds. Robust strontium isotope ages and laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb dates provide critical chronological constraints on the Cenozoic evolution of the southern South China Sea. A middle Miocene hiatus spanning 9 m.y. on Meiji Atoll is thought to be mainly caused by tectonic uplift in the central Dangerous Grounds. The uplift in the central Dangerous Grounds was accompanied by under-thrusting beneath the southern Palawan margin and orogenic uplifting in north Borneo during the middle Miocene. Data interpretation indicates an active collision in the southern South China Sea during the middle Miocene. The regrowth of the Meiji Atoll above the middle Miocene hiatus represents the end of this collision event in the southern South China Sea at ca. 11 Ma, after the cessation of seafloor spreading, which occurred at ca. 15 Ma.
Compositional and metamorphic controls on tectonic erosion along a continental subduction-collision zone: Implications from mafic granulites in the northern Sulu orogen
Lishuang Liu; Fulai Liu; Matthew J. Kohn; Jinghui Guo
Abstract: Subduction erosion has been reported recently in continental subduction-collision zones, and the response of the upper plate remains enigmatic. The Dabie-Sulu orogen is considered to have formed by deep northward subduction of the Yangtze block (lower plate) beneath the North China block (upper plate). However, within the northern Sulu ultrahigh-pressure (UHP) belt, the Haiyangsuo complex has intriguingly been accepted as Neoarchean−Paleoproterozoic metamorphic basement from the North China block, implying subduction-erosion could have occurred. Here, we examined new petrographic, mineral chemical, and published geochronological data from the Haiyangsuo mafic granulites to decipher their multiphase metamorphic evolution. Pressure-temperature (P-T) estimates from pseudosection modeling and geothermobarometry show two major metamorphic events: a medium-pressure granulite-facies event (7.3−8.3 kbar and 830−895 °C) and a later high-pressure granulite-facies event (12.2−16.6 kbar and 800−875 °C). Geochronology and mineral geochemistry link these events to Paleoproterozoic and Triassic metamorphism, respectively. Peak Triassic high-pressure granulite-facies metamorphism was followed by near-isothermal decompression and then near-isobaric cooling. These rocks derive from the upper plate and were metamorphosed at the same time as UHP eclogites, but maximum pressures were substantially lower, and P-T paths did not converge until late-stage exhumation. Evidently, metamorphic basement from the North China block was dragged to lower-crustal to upper-mantle depths (50−60 km) while that from the Yangtze block was subducted deeply, and then they were tectonically juxtaposed during exhumation. These data corroborate thermal-mechanical models that predict tectonic erosion as a major process during continental subduction and collision.
Spatially averaged stratigraphic data to inform watershed sediment routing: An example from the Mid-Atlantic United States
J.E. Pizzuto; K.J. Skalak; A. Benthem; S.A. Mahan; M. Sherif ...
Abstract: New and previously published stratigraphic data define Holocene to present sediment storage time scales for Mid-Atlantic river corridors. Empirical distributions of deposit ages and thicknesses were randomly sampled to create synthetic age-depth records. Deposits predating European settlement accumulated at a (median) rate of 0.06 cm yr−1, range from ∼18,000 to 225 yr old, and represent 39% (median) of the total accumulation. Sediments deposited from 1750 to 1950 ("legacy sediments") accumulated at a (median) rate of 0.39 cm yr−1 and comprise 47% (median) of the total, while "modern sediments" (1950−present) represent 11% of the total and accumulated at a (median) rate of 0.25 cm yr −1. Synthetic stratigraphic sequences, recast as age distributions for the presettlement period, in 1900 A.D., and at present, reflect rapid postsettlement alluviation, with enhanced preservation of younger sediments related to postsettlement watershed disturbance. An averaged present age distribution for vertically accreted sediment has modal, median, and mean ages of 190, 230, and 630 yr, reflecting the predominance of stored legacy sediments and the influence of relatively few, much older early Holocene deposits. The present age distribution, if represented by an exponential approximation (mean age ∼300 yr), and naively assumed to represent steady-state conditions, implies median sediment travel times on the order of centuries for travel distances greater than ∼100 km. The percentage of sediment reaching the watershed outlet in 30 yr (a reasonable time horizon to achieve watershed restoration efficacy) is ∼60% for a distance of 50 km, but this decreases to View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36282.1/613512/Spatially-averaged-stratigraphic-data-to-inform
Geological history and supercontinent cycles of the Arctic
Christopher Harrison; Marc R. St-Onge
Abstract: The geological history of the Arctic is constrained within the framework of the assembly and breakup of three supercontinents. The first of these was preceded by the crystallization of the oldest dated rocks on Earth and consolidation of the Arctic region's Archean cratons between 2.82 and 2.54 Ga. Following the emplacement of regional mafic dike swarms between 2.51 and 2.03 Ga, the cratons were amalgamated into the Nuna (Columbia) supercontinent between 2.0 and 1.6 Ga, and the distribution of low-thermal-gradient eclogite (indicative of continental subduction) and ophiolite (indicative of obduction of oceanic crust onto a continental margin) suggests that diagnostic plate-tectonic processes were well in place by the early Paleoproterozoic. Basin formation, flood basalts, and dike swarms are features of the partial(?) breakup of Nuna (Columbia) by 1.5−1.27 Ga. The extent to which specific dike swarms led to continental breakup and a rift-to-drift transition remains unclear. Assembly of the second supercontinent (Rodinia, 1.4−0.9 Ga) is recorded by a network of Grenvillian and Sveconorwegian collisional orogenic belts. Prominent features of Rodinia breakup (780−615 Ma) in the Arctic are extensive dike swarms and regional-scale glacial-periglacial deposits associated with the Sturtian (717−661 Ma) and Marinoan (ca. 645 ± 6 to ca. 635 Ma) snowball Earth glaciations. Assembly of the third supercontinent, Pangea, between 600 Ma and ca. 250 Ma, was accomplished through stitching of four orogens in the Arctic (Timan-Varanger, Caledonian, Ellesmerian, and Urals-Taymyr). Pangea breakup (rifting since 250 Ma and oceanic spreading since the Cretaceous) led to the emplacement of Cretaceous and Paleogene flood basalts, new oceanic crust in the Labrador Sea, North Atlantic Ocean, and Arctic Ocean, and orogens characterized by relatively small but far-traveled accreted terranes with provenance in Laurentia, Baltica, and Siberia. Paleogeographic similarities and geological correlations among Laurentia, Baltica, Siberia, and the North China craton suggest that Rodinia formed following incomplete breakup of Nuna (Columbia) and/or by introversion, whereas unique paleogeographic traits for Pangea within the Arctic region point to supercontinent formation by extroversion.
Modification of the lithospheric mantle induced by recycled crustal components: Insights from Early Cretaceous appinites from the Liaodong Peninsula, NE China
Xiang-Yu Yan; De-Bin Yang; Wen-Liang Xu; Hao-Tian Yang; Mao-Song Mu ...
Abstract: The term "appinites" refers to amphibole-rich mafic rocks, as one of the most typical products of partial melting of a metasomatic lithospheric mantle. This paper presents results of integrated geochemical and isotopic analyses for six Early Cretaceous (125−119 Ma) appinitic plutons from the Liaodong Peninsula, northeastern North China Craton (NCC), to not only investigate the properties of the metasomatic lithospheric mantle induced by variable recycled crustal components but also to discuss a hydrous mantle source. The appinites originated from partial melting of sub-continental lithospheric mantle (SCLM), and can be geochemically categorized into three groups distinguished by the input of different recycled crustal components (e.g., terrigenous sediments and delaminated lower continental crust) on the basis of two distinct Hf-O isotopic arrays. Based on the occurrence of voluminous amphiboles (50−60 vol%) and minor clinopyroxene remnants, a petrogenetic model was proposed to interpret the formation of appinites. Asthenospheric upwelling caused by the subduction and roll-back of the Paleo-Pacific plate firstly resulted in partial melting of the SCLM to yield basaltic magmas. Subsequently, numerous hydration reactions between early anhydrous minerals (e.g., olivine and pyroxene) of basaltic magmas and hydrous melts derived from terrigenous sediments and delaminated lower continental crust occurred to produce the studied appinites. Ultimately, the recycled crustal materials can serve as the metasomatic agent not only to transfer the enriched isotopic signatures into the studied appinites but also to represent hydrous components to play the part of hydrous reactions. The high abundance of amphibole in the studied appinites is indicative of a hydrous lithospheric mantle beneath the eastern NCC.
Recognition of late Paleoproterozoic gold mineralization in the North China craton: Evidence from multi-mineral U-Pb geochronology and stable isotopes of the Shanggong deposit
Shao-Rui Zhao; Jian-Wei Li; Christopher R.M. McFarlane; Paul T. Robinson; Zhan-Ke Li ...
Abstract: The North China craton was stabilized in the late Paleoproterozoic but experienced significant removal of ancient lithospheric keel in the late Mesozoic that resulted in the formation of numerous world-class gold deposits with combined reserves of more than 7000 t of gold. However, it remains uncertain whether the North China craton contains older gold deposits formed during generation and final stabilization of the craton. Here, we show that the Shanggong gold deposit (105 t Au at 5.31 g/t) on the southern margin of the North China craton formed in the late Paleoproterozoic during the collision between the Eastern and Western blocks that led to formation of the Trans−North China orogen and final stabilization of the craton. The Shanggong deposit is hosted in amphibolite-facies rocks of the Neoarchean to early Paleoproterozoic Taihua Group and overlying volcanic rocks of the late Paleoproterozoic Xiong'er Group. Gold mineralization is structurally controlled by NE-striking faults and occurs in four segments: the Liuxiugou, Hugou, Shanggong, and Qiliping segments. The ores consist mainly of quartz-ankerite-sulfide stockworks and sulfide disseminations in hydrothermally altered wall rocks. Gold is mostly contained in arsenian pyrite that is variably associated with minor sphalerite, galena, and chalcopyrite. Ore-related alteration assemblages comprise mainly quartz, ankerite, K-feldspar, sericite, and tourmaline. Both the stockworks and mineralized alteration assemblages contain hydrothermal accessory minerals, including monazite, apatite, and rutile. Paragenetic relations and textural data show that these accessory phases precipitated synchronously with gold-bearing sulfides. Laser ablation−inductively coupled plasma−mass spectrometry spot analyses of monazite and apatite from the Shanggong segment yielded reproducible U-Pb dates of 1747 ± 20 Ma (2σ, mean square of weighted deviates [MSWD] = 0.46) and 1788 ± 200 Ma (2σ, MSWD = 11.3), respectively. These dates are indistinguishable within errors from an apatite U-Pb date of 1743 ± 79 Ma (2σ, MSWD = 1.6) at the Liuxiugou segment and a rutile U-Pb date of 1804 ± 52 Ma (2σ, MSWD = 0.77) at the Hugou segment. These new dates suggest that the Shanggong deposit formed at ca. 1.80−1.74 Ga, coeval with or immediately after formation of the Trans−North China orogen and final stabilization of the North China craton. Sulfides from the Shanggong gold deposit have δ34S values ranging from −18.5‰ to −6.9‰, whereas the coexisting ankerite has δ13CPDB of −6.81‰ to −1.61‰ and δ18OSMOW of 15.70‰−17.62‰. The stable isotope data are distinctively different from values of the Early Cretaceous gold deposits in the southern North China craton, indicating contrasting hydrothermal systems responsible for these two categories of gold deposits. The results presented here, combined with independent geologic evidence, allow Shanggong to be the first confirmed Paleoproterozoic orogenic gold deposit in the North China craton. Recognition of Paleoproterozoic orogenic gold mineralization provides significant new insights into the gold metallogeny of the well-endowed North China craton and has implications for future gold exploration along the three Paleoproterozoic orogenic belts in this craton.
GSA BULLETIN articles published ahead of print are online at https://bulletin.geoscienceworld.org/content/early/recent
