Geological Society of America Bulletin最新文献

{"title":"Updating the Upper Cretaceous (Campanian) Two Medicine Formation of Montana: Lithostratigraphic revisions, new CA-ID-TIMS U-Pb ages, and a calibrated framework for dinosaur occurrences","authors":"Raymond R. Rogers, John R. Horner, J. Ramezani, Eric M. Roberts, D. Varricchio","doi":"10.1130/b37498.1","DOIUrl":"https://doi.org/10.1130/b37498.1","url":null,"abstract":"The Campanian Two Medicine Formation of northwestern Montana, USA, is richly fossiliferous, and discoveries made within the unit over the past century have greatly advanced our appreciation of dinosaur paleobiology and evolution. Previously undifferentiated from a lithostratigraphic perspective, the formation is now subdivided into four new members that include (from base to top) (1) the Rock City Member, (2) the Shields Crossing Member, (3) the Hagans Crossing Member, and (4) the Flag Butte Member. These new formal units and their associated fossil occurrences are also now included in an age model founded on eight high-resolution chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb ages. New age data confirm that the Two Medicine Formation accumulated during much of the Campanian, with deposition spanning ca. 82.4 Ma to 74.4 Ma. New age data further indicate that a major reorganization of depositional systems, marked by a shift from predominantly lacustrine to alluvial facies and accompanied by a dramatic increase in accommodation, transpired near the base of the new Flag Butte Member at ca. 76.3 Ma. This change in depositional regime correlates in age with the Judith River−Belly River discontinuity, which marks the contact between the McClelland Ferry and Coal Ridge Members in the Judith River Formation and coincides with the onset of the Bearpaw transgression in north-central Montana. The new lithostratigraphic and chronostratigraphic framework for the Two Medicine Formation serves to contextualize and calibrate the formation’s rich dinosaur fossil record, which can now be interrogated with increased clarity and precision. These results also provide ground truth for numerical models that explore the structure of the fossil record in relation to alluvial architecture and terrestrial sequence stratigraphy.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-07-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141829767","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Spatial−temporal variations in Mesozoic crustal thickness along the northeast Asian continental margin: Response to the subduction history of the Paleo-Pacific Plate","authors":"Jie Tang, Peng Guo, Feng Wang, Yi‐Ni Wang, Wenxian Xu","doi":"10.1130/b37237.1","DOIUrl":"https://doi.org/10.1130/b37237.1","url":null,"abstract":"The initial timing and history of subduction of the Paleo-Pacific Plate beneath Eurasia are controversial. The crustal thickness variations at a convergent margin typically reflect the convergent process between the two plates. This study used a recently proposed machine-learning model to estimate the crustal thickness variations along the northeast Asian continental margin during the Mesozoic. The northeast Asian continental margin, particularly the eastern North China Craton, had its thickest crust during the Early Triassic and underwent crustal thinning during the Middle−Late Triassic. The former reflects the subduction and collision between the South China Block and North China Craton, and the latter occurred in a post-orogenic extensional setting. From the Early to Middle Jurassic, sustained crustal thickening occurred along the northeast Asian continental margin, which coincided with initial subduction of the Paleo-Pacific Plate beneath Eurasia. From the Early to Late Cretaceous, the northeast Asian continental margin generally underwent crustal thinning, but crustal thickening events occurred at 130 Ma, 110 Ma, and 90 Ma, which is consistent with rollback of the subducted Paleo-Pacific Plate beneath Eurasia. The relationship between crustal thickness and mineralization suggests that thicker crust favored the formation of porphyry-type Cu-Mo deposits, whereas thinner crust in an extensional setting favored the formation of epithermal Au deposits related to magmatism.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-07-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141830603","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"New discovery of 3.84−3.64 Ga diverse granitoids in eastern Hebei, North China Craton: Petrogenesis and significance","authors":"C. Dong, Shoujie Liu, A. Nutman, Pengchuan Li, Hangqiang Xie, Yuan Li, Dunyi Liu, Yusheng Wan","doi":"10.1130/b37553.1","DOIUrl":"https://doi.org/10.1130/b37553.1","url":null,"abstract":"Eoarchean rocks are scarce worldwide, limiting our understanding of Earth’s early history. This contribution presents the field geology, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating, Nd-Hf-O isotopic analysis, and whole-rock geochemical study of Eoarchean granitoids newly discovered in the Labashan area, eastern Hebei, North China Craton. Eoarchean rocks, including 3838−3754 Ma trondhjemitic gneisses (4 samples), 3786−3773 Ma granodioritic gneisses (4 samples), 3775 Ma quartz monzonite gneiss (1 sample), and 3786−3640 Ma potassic granite gneisses (3 samples), are identified in a tract 1 km long. They are in tectonic contact with 3.4−3.1 Ga supracrustal rocks, with both occurring as enclaves in 2.5 Ga potassic granite. The trondhjemitic gneisses show low (La/Yb)N values (23.4−56.8) and insignificant Eu anomalies (Eu/Eu* = 0.86−1.24). Compared with the trondhjemitic gneisses, the granodioritic gneisses and quartz monzonite gneiss have lower (La/Yb)N ratios (9.2−11.3) and lower Eu/Eu* values (0.62−0.79). The potassic granites have (La/Yb)N ratios of 20.7−55.2 and Eu/Eu* values of 0.75−1.84. Zircon saturation temperatures of the granitoids range from 667 °C to 878 °C. They have whole-rock εNd(t) of −4.6 to 1.0 and tCHUR(Nd) of 4.3−3.6 Ga, and zircon εHf(t) values of −6.9 to 0.4 and tCHUR(Hf) ages of 4.1−3.5 Ga (mainly 4.1−3.8 Ga) and δ18O of 4.6‰−7.5‰ (CHUR—chondritic uniform reservoir). It is speculated that different amounts of low-K mafic rocks and tonalite-trondhjemite-granodiorite rocks with prior crustal residence times were the source of the spectrum of trondhjemitic to granitic compositions at Labashan. Two possible models, magma underplating and convergent plate boundary processes, are explored for the tectonic regime forming these Eoarchean granitoids.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-06-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141270966","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Tectonic inversion of an intracontinental rift basin: An example from the opening and closure of the Paleo-Tethys Ocean, northern Tibetan Plateau","authors":"Jie Li, Chen Wu, Xiaogang Li, A. Zuza, P. Haproff, Yonghui Zhao, Wentao Zhao, Yahui Yue, Lin Ding","doi":"10.1130/b37605.1","DOIUrl":"https://doi.org/10.1130/b37605.1","url":null,"abstract":"Suture zones located across the Tibetan region clearly demarcate the rift-and-drift and continental accretion history of the region. However, the intraplate responses to these marginal plate-tectonic events are rarely quantified. Our understanding of the Paleo-Tethyan orogenic system, which involved ocean opening and closing events to grow the central Asian continent, depends on the tectonic architecture and histories of major late Paleozoic−early Mesozoic orogenic belts. These opening and collision events were associated with coupled intracontinental deformation, which has been difficult to resolve due to subsequent overprinting deformation. The late Paleozoic−early Mesozoic Zongwulong Shan−Qinghai Nanshan belt in northern Tibet separates the Qilian and North Qaidam regions and is composed of Carboniferous−Triassic sedimentary materials and mantle-derived magmatic rocks. The tectonic setting and evolutional history of this belt provide important insight into the paleogeographic and tectonic relationships of the Paleo-Tethyan orogenic system located ∼200 km to the south. In this study, we integrated new and previous geological observations, detailed structural mapping, and zircon U-Pb geochronology data from the Zongwulong Shan−Qinghai Nanshan to document a complete tectonic inversion cycle from intraplate rifting to intracontinental shortening associated with the opening and closing of the Paleo-Tethyan Ocean. Carboniferous−Permian strata in the Zongwulong Shan were deposited in an intracontinental rift basin and sourced from both the north and the south. At the end of the Early−Middle Triassic, foreland molasse strata were deposited in the southern part of the Zongwulong Shan during tectonic inversion in the western part of the tectonic belt following the onset of regional contraction deformation. The Zongwulong Shan−Qinghai Nanshan system has experienced polyphase deformation since the late Paleozoic, including: (1) early Carboniferous intracontinental extension and (2) Early−Middle Triassic tectonic inversion involving reactivation of older normal faults as thrusts and folding of pre- and synrift strata. We interpret that the Zongwulong Shan−Qinghai Nanshan initiated as a Carboniferous−Early Triassic intracontinental rift basin related to the opening of the Paleo-Tethyan Ocean to the south, and it was then inverted during the Early−Middle Triassic closing of the Paleo-Tethyan Ocean. This work emphasizes that pre-Cenozoic intraplate structures related to the opening and closing of ocean basins in the Tethyan realm may be underappreciated across Tibet.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141098636","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Constraints on a long-lived Paleoproterozoic metamorphic process in a middle- to lower-grade metamorphic complex, Songshan area, southern North China craton: Evidence from minor- and trace-element thermometry and geochronology","authors":"Qiang Feng, Junsheng Lu, Xu Kong, Gang Liu, Yuting Li, Yiyi Zhang, Zengyin Duan, Juanjuan Li","doi":"10.1130/b37338.1","DOIUrl":"https://doi.org/10.1130/b37338.1","url":null,"abstract":"The Dengfeng Complex and Songshan Group comprise a middle- to lower-grade metamorphic complex with abundant lithological associations in the Songshan area (central Henan, China), representing an excellent opportunity to investigate the thermal history of the North China craton. The polymetamorphic Dengfeng Complex is a typical granite-greenstone belt that records a late Paleoproterozoic metamorphic event (ca. 1.95−1.80 Ga) overprinting on a late Neoarchean metamorphic event (ca. 2.51−2.41 Ga). In contrast, the Songshan Group preserves well-developed stratigraphic sequences and sedimentary structures with greenschist-facies metamorphism. Numerous studies have focused on the late Neoarchean metamorphic event from the Dengfeng Complex, whereas the thermal evolution of ca. 1.95−1.80 Ga units from the Dengfeng Complex and Songshan Group has been largely overlooked. Quartzite of the Songshan Group unconformably overlies schists of the Dengfeng Group, and they show coherent NNE-SSW−striking schistosity structures with dip angles of ∼50°−65°. Here, the metamorphic temperatures from the Dengfeng Group were constrained to be ∼510−550 °C by the Ti-in-biotite geothermometer. The Ti-in-quartz and Zr-in-rutile geothermometers record metamorphic temperatures of ∼480−550 °C or 520−555 °C for the Songshan Group, respectively. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating of monazite and rutile constrains the timing of metamorphism of the Songshan Group to be ca. 1.94−1.83 Ga, while zircon constrains the timing of metamorphism of the Dengfeng Complex to be ca. 2.53 Ga and ca. 1.95−1.87 Ga. By integrating geochronologic data from the Dengfeng Complex and Songshan Group, the ca. 2.53 Ga metamorphic age of amphibolite may be connected with Neoarchean subduction-accretion processes, whereas the consistent temperature obtained in this study ca. 1.95−1.83 Ga indicates that the Songshan area may have experienced a long-lived metamorphic event, which could have resulted from the final collision between the Western and Eastern blocks of the North China craton in the late Paleoproterozoic. The persistence of a Paleoproterozoic hot orogen for millions of years was probably the norm in the North China craton, which provides new insights into the tectonic-thermal evolution in the southern North China craton.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141102291","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Insights into the Phanerozoic evolution of the São Francisco Craton based on detrital zircon thermochronology and U-Pb-Hf geochronology","authors":"A. Dias, Marco Antonio C. Martins-Ferreira, V. Pereira, Antonio S.W. Sales, Farid Chemale Jr.","doi":"10.1130/b37281.1","DOIUrl":"https://doi.org/10.1130/b37281.1","url":null,"abstract":"Basins within cratonic environments offer valuable insights into the fragmentation, dispersion, and agglutination of the lithosphere within supercontinents. The southern São Francisco Craton is home to sedimentary basins that formed in both intraplate tectonic settings (late Paleoproterozoic to Neoproterozoic and Mesozoic) and convergent tectonic settings (Ediacaran to Cambrian). To elucidate the Phanerozoic thermal history of the craton and the long-term depositional history of the southern São Francisco Craton’s cover, a comprehensive study was conducted utilizing detrital zircons through zircon fission-track (ZFT) thermochronology and U-Pb-Hf geochronology. This study analyzed samples from the following units: (1) the intracratonic basin—Lower and Upper Espinhaço Supergroup, with depositional age intervals ranging 1800−1600 Ma and 1200−920 Ma, respectively; (2) the rift-related passive margin basin—Lower Macaúbas Group (720 Ma and 640 Ma) and Jequitaí Formation, which is likely of Cryogenian age (650−635 Ma); (3) and the foreland basin—Bambui Group and Três Marias Formation, with ages of ca. 600−520 Ma and 520−480 Ma, respectively. U-Pb-Hf detrital zircon geochronology revealed the complex depositional history of the southern São Francisco Craton, and indicated changing source rocks over time. Conversely, the ZFT analysis of Precambrian to Cambrian samples allowed for the identification and characterization of Phanerozoic events in the southern São Francisco Craton. These events include: (1) the exhumation and late orogenic uplift of the foreland at ca. 500−480 Ma (mean age of 499 ± 5.5 Ma) during the late stages of the Brasiliano−Pan-African Cycle, which was associated with the Araçuaí Belt and lithospheric rebound of the peripheral cratonic margin of the southern São Francisco Craton; and (2) an age of 330−280 Ma (mean age of 306.6 ± 4.8 Ma) that possibly resulted from ice retreat and isostatic rebound interior of the Gondwana Paleocontinent. Additionally, analysis of detrital zircons from the Areado Group shows a later tectonic event recorded at 128.1 ± 9.4 Ma, which indicates Mesozoic intracontinental rifting. Based on these results and interpretations, the timing and exhumation of the southern São Francisco Craton during the Phanerozoic can be attributed to the final stages of the continental collision of the southern São Francisco Craton and Congo Craton and large-scale continental uplift during the Permo-Carboniferous late Paleozoic Ice Age of Gondwana. Overall, the findings demonstrate a direct correlation between known tectonic events at the plate margins of Western Gondwana and periods of cratonic basin formation.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141104770","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Hydrochemical evolution and source mechanisms governing the unusual lithium and boron enrichment in salt lakes of northern Tibet","authors":"Zhiwei Shi, Hongbing Tan, Fei Xue, Yulong Li, Xiying Zhang, P. Cong, M. Santosh, Yu Zhang","doi":"10.1130/b37516.1","DOIUrl":"https://doi.org/10.1130/b37516.1","url":null,"abstract":"The large-scale salt lakes widely distributed in the Tibetan Plateau provide unique and potential resources for lithium (Li) and boron (B). The concentration and characteristics of elements in these salt lakes resemble those found in geothermal water in northern Tibet, which highlights both as crucial sources of rare elements. This study presents comprehensive analyses of the hydrochemical composition and isotopes of B, strontium (Sr), hydrogen (H), and oxygen (O) in typical salt lakes, along with samples from surrounding springs and rivers in the Bangong-Nujiang suture zone of northern Tibet. The results reveal an extremely negative and anomalous distribution pattern of B isotopes in Zabuye Salt Lake that is closely associated with geothermal water. The enrichment of these elements in other salt lakes in the region is attributed to concentration of evaporation and sediment adsorption. Given the very high elevation of the recharge for geothermal water, the infiltration of salt lakes obviously cannot feed geothermal springs. On the contrary, we correlate the unusual enrichment of Li and B and other resources in salt lakes to geothermal spring discharge. The ultimate origin of these elements lies in magmatic sources, with later water-rock interaction leading to significant enrichment of incompatible elements such as Li, rubidium (Rb), cesium (Cs), and B in the geothermal system. The geothermal springs directly or indirectly fed the salt lakes, and with further evaporation, they became super-scale brine deposits.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141109477","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Neoarchean granitoid magmatism and geodynamic process in the northeastern North China craton","authors":"Guozheng Sun, Shuwen Liu, Sanzhong Li, Han Bao, Wei Wang, Rongrong Guo, Jinghao Fu, Lei Gao, Yalu Hu, Xi Wang, Shengyao Yu, Liming Dai","doi":"10.1130/b37584.1","DOIUrl":"https://doi.org/10.1130/b37584.1","url":null,"abstract":"The composition of Archean granitoid rocks changed from predominantly tonalite-trondhjemite-granodiorite (TTG) gneisses in the early Archean (4−3 Ga) to diversified granitoid rock assemblages in the late Archean (3.0−2.5 Ga), marking a crucial transformation in the geodynamic processes of early Earth. However, the reason for this major transition remains enigmatic because the petrogenetic features of different granitoid assemblages and their crust-mantle interactions during different periods are poorly understood. We use variations in the spatial-temporal distribution, lithological association, chemical composition, and petrogenesis of Neoarchean (2.7−2.5 Ga) granitoids and inferred correlative crust-mantle interactions in the Eastern Liaoning Range (ELR) of the northeastern North China craton to explore this geodynamic transition. The early Neoarchean (ca. 2.7 Ga) ELR granitoids were dominated by TTG gneisses, and the late Neoarchean (2.6−2.5 Ga) ELR granitoid typology and compositions became more complex, changing into TTGs and more K2O-rich granitoid rocks. The TTGs can be subdivided into a high-Ca group and a low-Ca group: The 2.71−2.68 Ga high-Ca group TTG magma originated from partial melting of subducted juvenile oceanic crust, and the low-Ca group TTG magma was derived from fractionation crystallization of the high-Ca group TTG magma. The chemical composition of the magmatic sources played a dominant role on the 2.60−2.50 Ga TTG magmatism: the high-Ca and low-Ca group TTG magmas came from low-K mafic rocks and tonalites, respectively. The 2.58−2.49 Ga K2O-rich granitoids can be divided into three petrogenetic series: (1) The high-Ca-Mg group K2O-rich granitoid magma originated from partial melting of high-K mafic rocks, (2) the low-Ca-Mg group K2O-rich granitoid magma was derived from partial melting of sedimentary rocks, and (3) the transition group K2O-rich granitoid magma was sourced from metagreywackes. The 2.71−2.68 Ga TTGs were generated in an island arc belt, and subducted slab melting and subsequent magmatic differentiation were the dominant mechanisms of the TTG magmatism. The 2.60−2.50 Ga diversified granitoids were formed in the oceanic-continental subduction process under the active continental margin; the complicated oceanic slab subduction and arc-arc and arc-continent collisions contributed to the diversity of late Neoarchean granitoid magmatism.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141112936","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Final closure of the Paleo-Tethys Ocean: Insights from Triassic granitoids in the central Qiangtang area, northern Tibetan Plateau","authors":"Qing-guo Zhai, Pei‐yuan Hu, Yiming Liu, Yue Tang, Haoyang Lee","doi":"10.1130/b37682.1","DOIUrl":"https://doi.org/10.1130/b37682.1","url":null,"abstract":"The geodynamic evolution during the closure of the Paleo-Tethys Ocean in the Tibetan Plateau remains to be fully understood. The Longmu Co−Shuanghu suture zone in the northern Tibetan Plateau has usually been considered to represent the main ocean basin of the Paleo-Tethys Ocean, so it plays a key role in understanding the evolution of the Paleo-Tethys Ocean. In this study, we focused on the Gacuo and Bensong batholiths on the north and south sides of the Longmu Co−Shuanghu suture zone, respectively. We conducted detailed zircon geochronology and whole-rock geochemical and Sr-Nd isotopic analyses, as well as zircon Hf isotope studies. Zircon U-Pb dating indicates that the Gacuo batholith was formed ca. 223−209 Ma, and the age of the Bensong batholith is ca. 213−203 Ma. The Gacuo batholith is mainly composed of I-type granitoids, which are most likely attributed to partial melting of ancient sedimentary materials of the North Qiangtang terrane with a mixture of ∼0%−30% amounts of mantle-derived components. In contrast, the Bensong batholith has granitoids of A-type affinity, and it was probably generated by partial melting of Mesoproterozoic crust of the South Qiangtang terrane with limited mantle contribution (<5%). Finally, we suggest that the Gacuo batholith was probably generated by the break-off of the oceanic slab beneath the North Qiangtang terrane, while the Bensong batholith was related to a possible lithospheric delamination process of the South Qiangtang terrane after continental collision. Therefore, the Gacuo and Bensong batholiths both developed in a postcollisional tectonic setting, and they recorded the evolutionary process of the subduction and closure of the Paleo-Tethys Ocean during the Late Triassic.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141113804","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Timescales of mafic magmatic fractionation documented by paleosecular variation in basaltic drill core, Snake River Plain volcanic province, Idaho, USA","authors":"John Shervais, K. Potter","doi":"10.1130/b37413.1","DOIUrl":"https://doi.org/10.1130/b37413.1","url":null,"abstract":"The timescales over which fractional crystallization and recharge work in mafic volcano-plutonic provinces is subject to great uncertainty. Currently modeled processes are subject to the scale of measurement: monogenetic basaltic fields accumulate over hundreds of thousands of years, consistent with U-Th-Ra isotopic variations that imply 50% crystallization of basic magmas on timescales of 100,000 years or more, whereas crystal diffusion modeling implies phenocryst residence times of ∼1−1000 years.\u0000 Monogenetic basalts of the Snake River Plain in southern Idaho, USA, are up to 2 km thick and postdate passage over the Yellowstone−Snake River Plain hotspot. Detailed lithologic and geophysical logging of core from deep drill holes, along with chemical stratigraphy and high-resolution paleomagnetic inclination measurements, document individual eruptive units, compound lava flows, and basaltic flow groups that accumulated over 1−6 m.y. Hiatuses are commonly marked by loess or fluvial interbeds that vary from ∼0.1 m thick to 20 m thick. Radiometric (40Ar-39Ar, detrital zircon U-Pb) and paleomagnetic timescale ages show that the deepest hole (Kimama drill hole, 1912 m total depth) accumulated over ∼6 m.y. Cycles of fractional crystallization and recharge are recognized in the chemical stratigraphy as up-section shifts in major and trace elements; these fractionation cycles commonly represent 40%−50% fractionation. Individual fractionation cycles may comprise 20−40 eruptive units (8−17 lava flows) with little to no change in paleomagnetic inclination (0°−1°), whereas adjacent cycles may differ by several degrees from one another or reflect changes in polarity.\u0000 Rates of paleosecular variation in Holocene lavas and sediments dated using 14C document significant shifts in magnetic inclination over short timescales, ranging from ∼0.05° to 2°/decade, with an average of ∼0.5°/decade and a minimum rate of 0.05°/decade. This implies that fractionation cycles with ≤1° variation in magnetic inclination formed on timescales of a few decades up to a few centuries (20−200 years). Thus, the lavas collectively represent only a few thousand years of eruptive activity, with major flow groups separated in time by tens to hundreds of thousands of years. We suggest that the rates defined by paleosecular variation capture the timescales of magmatic chamber evolution (fractionation/recharge) in the seismically imaged mid-crustal sill complex; in contrast, we suggest that crystal diffusion modeling captures the residence times in shallow subvolcanic magmatic chambers that underlie individual monogenetic volcanoes.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":null,"pages":null},"PeriodicalIF":4.9,"publicationDate":"2024-03-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140377655","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}