Geological Society of America Bulletin最新文献

{"title":"Mammalian faunal change of the Miocene Dove Spring Formation, Mojave region, southern California, USA, in relation to tectonic history","authors":"Fabian Cerón Hardy, Catherine Badgley","doi":"10.1130/b37082.1","DOIUrl":"https://doi.org/10.1130/b37082.1","url":null,"abstract":"Tectonic processes drive the evolution of basins through local and regional changes in topographic relief, which have long-term effects on mammalian richness and distribution. Mammals respond to the resulting changes in landscape and climate through evolution, shifts in geographic range, and by altering their community composition. Here, we evaluate the relationship between tectonic episodes and the diversification history of fossil mammals in the Miocene Dove Spring Formation (12.5−8.5 Ma) of southern California, USA. This formation contains a rich fossil record of mammals and other vertebrates as well as structural and sedimentological evidence for tectonic episodes of basin extension, rotation, and translation. We used several methods to compare the fossil record to the tectonic history of the Dove Spring Formation. We updated the formation’s geochronology to incorporate current radiometric dating standards and measured additional stratigraphic sections to refine the temporal resolution of large mammal (>1 kg) fossil localities to 200-kyr (or shorter) intervals. Observed species richness over time follows the same trend as the number of localities and specimens, suggesting that richness reflects sampling intensity. Estimates of stratigraphic ranges with 80% confidence intervals were used to conduct per capita diversification analysis and a likelihood approach to changes in faunal composition from one time interval to the next. While edge effects influence time bins at the beginning and end of the study interval, we found changes in diversification rates and faunal composition that are not solely linked to preservation. Several rare species appear at 10.5 Ma and persist through the top of the formation despite variable preservation rates. Changes in faunal composition at 12.1 Ma and 10.5 Ma are not associated with elevated preservation rates, which indicates that some faunal changes are not primarily driven by sampling effort. The lower portion of the formation is characterized by high origination rates and long residence times. The upper portion has high per capita extinction rates that increased in magnitude as basin rotation and translation progressed from 10.5 Ma. The greatest change in faunal composition coincided with basin rotation and translation that interrupted a long-running extensional period. Tectonics played key roles in the diversity of mammals by determining fossil productivity and shaping the landscapes that they inhabited.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":"17 12","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135590055","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":"Timing of Izanagi-Pacific ridge subduction beneath Eurasia: Constraints from metamorphic soles in Hokkaido, Japan","authors":"Yu Dong, Wen-liang Xu, Jin-rui Zhang, Yi-bing Li, Kiyoaki Niida, Shinji Yamamoto, Yi-ni Wang, Zheng Ji","doi":"10.1130/b37093.1","DOIUrl":"https://doi.org/10.1130/b37093.1","url":null,"abstract":"The subduction of the Pacific Plate beneath Eurasia controls not only Cenozoic intraplate volcanism, but also deep-focus earthquakes along the continental margin of eastern Asia. However, the timing of subduction of the Paleo-Pacific (Izanagi)−Pacific ridge and the initial subduction of the Pacific Plate beneath Eurasia remain subjects of debate. Metamorphic soles provide key evidence for ridge subduction, and together with coeval igneous activity, they are widely used to constrain the timing of ridge subduction. Here, we present the results of a study of amphibolites from the Hidaka metamorphic belt, Hokkaido, northern Japan, which are interpreted to be metamorphic soles. Our integrated study involved secondary ion mass spectrometry (SIMS) U-Pb zircon dating, mineral chemistry, whole-rock geochemistry, and Sr-Nd-Pb-Hf isotopic analyses. SIMS U-Pb zircon dating indicates that the amphibolite-facies sole metamorphism occurred in the late Eocene. Phase equilibria modeling suggests that the peak P−T conditions of metamorphism were 8.0−9.5 kbar/700−730 °C. The amphibolites belong to the tholeiitic series, and they are relatively enriched in heavy rare earth elements relative to light rare earth elements, with depletions in Nb, Ta, P, and Ti. They have (87Sr/86Sr)i = 0.704207−0.704998, εNd(t) = +11.65 to +11.96, εHf(t) = +14.28 to +16.32, (206Pb/204Pb)i = 18.240−18.255, and (207Pb/204Pb)i = 15.522−15.525. The geochemistry of these amphibolites reveals that their protoliths had normal mid-oceanic-ridge basalt affinities, and their Pb isotopic signatures indicate Indian-type mantle rather than Pacific-type mantle. Considering the geochemistry of coeval igneous rocks, we conclude that the late Eocene metamorphism in the Hidaka metamorphic belt records the intraoceanic subduction that followed heat transfer from the incipient mantle wedge toward the top of the subducting plate, and it provides a key constraint on the timing of subduction of the Izanagi-Pacific ridge.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":"30 27","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135818442","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":"Mid-Mesoproterozoic (ca. 1.37 Ga) anorogenic magmatism in the Dabie orogen, northern Yangtze Craton: Response to the breakup of Columbia","authors":"Wei Jin, Jianhui Liu, Yang Tian, Xin Deng, Daliang Xu, Jing Wang, Xiaofei Qiu","doi":"10.1130/b37141.1","DOIUrl":"https://doi.org/10.1130/b37141.1","url":null,"abstract":"It is evident that the Yangtze Craton was involved in the formation and breakup of the Columbia supercontinent. However, due to the scarcity of Mesoproterozoic geological records and reliable paleomagnetic data, little is known about the timing, paleogeographic position, and geological processes of the Yangtze Craton. We conducted detailed geological mapping, petrographic, geochemical, and in situ zircon U-Pb and Lu-Hf isotopic investigations on newly recognized Mesoproterozoic quartz syenite and monzogranite in the Dabie orogen, northern Yangtze Craton. The results show that the quartz syenite and the monzogranite were emplaced at 1369 ± 12 Ma and 1372 ± 5 Ma, respectively. Both rocks are high in total alkali (K2O + Na2O) content and FeOt/(FeOt + MgO), with elevated diagnostic 10,000*Ga/Al ratios and Zr + Nb + Ce + Y concentrations, and enrichment in light rare earth elements. They are also depleted in heavy rare earth elements with significant negative Eu anomalies but remarkably low Sr, Cr, and Ni contents. These compositions define affinity to A1-type granite. In addition, the quartz syenite displays variable zircon Hf and homogeneous whole-rock Nd isotopic compositions with positive εHf(t) values of +0.7 to +5.7 (average +2.4) and εNd(t) values of −0.1 to +2.5 (average +1.0). In contrast, the monzogranite has homogeneous zircon Hf and whole-rock Nd isotopic compositions with negative εHf(t) values of −5.7 to −2.6 (average −4.8) and εNd(t) values of −3.5 to −1.5 (average −2.2). We propose that the quartz syenite was likely generated by the partial melting of juvenile, crust-derived melt with involvement of minor mantle-derived material, while the monzogranite was likely derived from the partial melting of ancient crust in an extensional tectonic regime (e.g., continental rift). Based on the newly recognized ca. 1.37 Ga granitic magmatism and previously reported magmatic events, we argue that the mid-Mesoproterozoic (ca. 1.37 Ga) magmatism in the Yangtze Craton occurred in response to the breakup of Columbia, and represents the separation of the Yangtze Craton from Columbia. Furthermore, according to comparable magmatic and sedimentary events, we propose that the Yangtze Craton, along with central Hainan Island, may have been linked to northwestern Laurentia, southwestern Siberia, and northeastern Australia during 1.6−1.4 Ga.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":"34 3","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135271021","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":"Methanogen-mediated dolomite precipitation in an early Permian lake in northwestern China","authors":"Funing Sun, Wenxuan Hu, Xiaolin Wang, Zhongya Hu, Haiguang Wu, Yangrui Guo, Gangjian Wei","doi":"10.1130/b37156.1","DOIUrl":"https://doi.org/10.1130/b37156.1","url":null,"abstract":"Microbes are known to mediate dolomite precipitation in laboratory experiments; however, the linkage of specific microbes to ancient dolomites remains poorly constrained due to scarce diagnostic biogeochemical signatures and mineralized microbial relics in the rock record. Here, we report the occurrence of methanogen-mediated dolomite in the Lower Permian lacustrine Lucaogou Formation in northwestern China. The clumped isotope (Δ47) temperature provides direct evidence of a low-temperature origin (typically <40 °C). The extremely positive δ26MgDSM3 (up to +0.44‰) and δ13CVPDB (up to +19‰) values in the dolomite indicate authigenic precipitation in methanogenic lake sediments. Micron-sized spheroidal bodies and filamentous and sheetlike structures are interpreted as mineralized coccoid methanogenic archaea and extracellular polymeric substances (EPSs), respectively. Dolomite nanoglobules (primarily 40−100 nm in diameter) are interpreted as mineralized viruses attached to the archaea and EPSs and between the cells. A combination of geochemical and microscale evidence confirms the microbial origin of the dolomite induced by methanogens and their associated bacteriophages. Furthermore, dolomite nanoglobules initially nucleated on the surfaces of methanogen cells, EPSs, and viruses and then merged into larger aggregates. The formation of microbial dolomite is characterized by a metabolic incubation, heterogeneous nucleation, and aggregative growth pathway. These findings provide valuable clues to decipher the biosignatures of these particular ancient dolomites.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":"41 8","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135271199","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":"Tectonics of the Paleoproterozoic Rinkian orogen, central West Greenland","authors":"Pierpaolo Guarnieri, Diogo Rosa, Kristine Thrane, Thomas F. Kokfelt, Erik V. Sørensen, Michelle Y. DeWolfe, Nigel Baker","doi":"10.1130/b36930.1","DOIUrl":"https://doi.org/10.1130/b36930.1","url":null,"abstract":"A new tectonic model is presented to explain the tectono-stratigraphic evolution of the Paleoproterozoic Karrat Group in central West Greenland and the polyphase deformation, magmatism, and metamorphism of the Rinkian orogen. Sedimentation of the Karrat Group initiated after ca. 2000 Ma in an intracratonic rift basin with basal quartzites overlying Archean gneisses of the Rae craton. Rift-related alkaline volcanic rocks and synrift siliciclastic sediments were deposited in the north while an evaporite-carbonate platform developed in the south. The rift basin evolved to a back-arc basin, with associated subalkaline volcanic rocks, concomitant with the intrusion of arc-related granitoids of the Prøven igneous complex along the basal contact of the Karrat Group between 1900 Ma and 1850 Ma. The Karrat Group and magmatic arc rocks underwent metamorphism ca. 1830−1800 Ma during the collisional phase of the Rinkian orogeny. The metamorphic grade of the Karrat Group increases from greenschist facies in the south to granulite facies in the north, where it is marked by migmatization and emplacement of S-type leucogranites. Extensive east-southeastward thrust emplacement and fold vergence characterize the Rinkian orogen south of the Prøven igneous complex magmatic arc, where the arc-continent collision is established along a top-to-the-ESE shear zone postdating the Rinkian metamorphism. In summary, the Karrat Basin developed on the upper plate above eastward-dipping subduction and, together with the Rinkian orogen, represents the result of arc-continent collision that initiated the structuring of a back-arc fold-and-thrust system antithetic to the subduction system.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136104617","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":"Two-phase kinematic evolution of the Qilian Shan, northern Tibetan Plateau: Initial Eocene−Oligocene deformation that accelerated in the mid-Miocene","authors":"Bing Li, Bangshen Qi, Xuanhua Chen, Andrew V. Zuza, Daogong Hu, Yujun Sun, Zeng-Zhen Wang, Yiping Zhang","doi":"10.1130/b37159.1","DOIUrl":"https://doi.org/10.1130/b37159.1","url":null,"abstract":"The Cenozoic growth of the Tibetan Plateau and the distribution of deformation across it are a consequence of India-Asia collision and continued convergence, which have implications for studies of continental tectonics. The spatio-temporal development of Cenozoic deformation along the northern margin of the plateau is an important issue that can be better understood by testing various models of plateau growth. The northern Tibetan Plateau is bounded by the Cenozoic Qilian Shan thrust belt and the Haiyuan left-slip fault. We conducted geologic mapping, field observations, electron spin resonance (ESR) dating, and apatite (U-Th)/He (AHe) and apatite fission-track (AFT) analysis in the Qilian Shan thrust belt to improve our understanding of the timing of brittle faulting and range exhumation in the northern Tibetan Plateau. We document the first direct age constraints for Oligocene deformation within the central Qilian Shan via ESR dating, which correlates with AHe-AFT cooling ages in adjacent ranges. We demonstrate that the Qilian Shan thrust belt experienced a two-phase growth history, including Eocene−Oligocene fault-related uplift shortly after the India-Asia convergence, and mid-Miocene regional overprinting deformation that reactivated the proximal thrust faults. This deformational pattern suggests that the Qilian Shan thrust belt has experienced out-of-sequence development since the Eocene−Oligocene and has persisted as the stationary northeastern boundary of the Himalayan-Tibetan Orogen throughout the Cenozoic. The Paleozoic Qilian suture systems acted as a pre-existing weakness and played a decisive role in controlling the lithospheric rheology, which therefore impacted the timing, pattern, and strain distribution of Cenozoic deformation across the northern Tibetan Plateau.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":"14 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136317054","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":"Origins and evolution of two types of Late Triassic granitic magmas in the Caolong-Xiangkariwa area of central-eastern Songpan-Ganze terrane, northern Tibet: Implications for pegmatite lithium mineralization","authors":"Jin-Heng Liu, Qiang Wang, Wu-Fu Li, Bing-Zhang Wang, Derek A. Wyman, Lin Ding, He Wang, Chuan-Bing Xu, Shan-Ping Li, Chun-Tao Wang, Jian-Dong Liu, Rong-Qing Zhang, Zi-Long Wang, Tong-Yu Huang, Xin-Yuan Zhang","doi":"10.1130/b37088.1","DOIUrl":"https://doi.org/10.1130/b37088.1","url":null,"abstract":"Pegmatite-type lithium (Li) deposits are an important source of the low-carbon energy metal, which is generally considered to be formed by the high degrees of differentiation of strongly peraluminous granitic magma. However, the question of whether the parental rocks of these peraluminous magmas are igneous or sedimentary has been widely debated. Recent studies have identified a world-class pegmatite-type Li deposit belt (up to 2800 km long) closely associated with the Late Triassic granitoids and (meta)sedimentary rocks along the West Kunlun and Hoh Xil-Songpan-Ganze terranes in northern Tibet. This study presents a comprehensive petrological, geochronological, and geochemical study on the two types of Late Triassic granitoids in the Caolong-Xiangkariwa area of the Yushu region of central-eastern Hoh Xil-Songpan-Ganze terrane in northern Tibet: type A includes the Tongtianhe-Zhenqin-Zhaduo-Zhaya-Xiangkariwa (TZX) diorites and granodiorites (218−212 Ma), and type B includes the Caolong two-mica and muscovite granites (213−205 Ma), as well as ore-barren, beryl-bearing, and spodumene-bearing pegmatite dikes (209−196 Ma). The TZX diorites and granodiorites contain variable amounts of amphibolite and biotite. They are metaluminous to slightly peraluminous with variable SiO2 (54.4−71.3 wt%), MgO (0.72−7.26 wt%) contents, and Mg# values (40−70). Some samples with low SiO2 (54.4−58.5 wt%) contents have geochemical features similar to high-Mg andesites: e.g., high MgO contents (5.24−7.26 wt%) and Mg# values (61−70). The TZX diorites and granodiorites exhibit less enriched (86Sr/87Sr)i (0.7080−0.7118) and εNd(t) (−4.8 to −8.0). By contrast, the Caolong two-mica and muscovite granites are characterized by the occurrence of abundant muscovite (plus tourmaline and garnet) and are strongly peraluminous. The Caolong granites and pegmatites are geochemically characterized by high SiO2 contents (68.8−79.8 wt%) and low MgO contents (0.01−0.10 wt%) and Mg# values (4−23). They have εNd(t) values (−8.8 to −11.4; granites: −8.8 to −10.8; pegmatites: −9.2 to −11.4) that are more enriched than the TZX diorites and granodiorites but similar to those of Songpan-Ganze (meta)sedimentary rocks (−7.4 to −12.9), and the Caolong two-mica granites have high zircon δ18O values (11.6−12.1). In addition, decreasing K/Rb ratios correspond to increasing Cs contents in K-feldspar and muscovite from the Caolong granites and pegmatites. Taking into account the Late Triassic granitoids and (meta)sedimentary rocks in the Hoh Xil-Songpan-Ganze terrane, we suggest that the TZX diorites and granodiorites were most probably formed by the partial melting of metasomatized lithospheric mantle that subsequently underwent extensive fractional crystallization. Conversely, the Caolong two-mica and muscovite granites were likely generated purely by the partial melting of (meta)sedimentary rocks rather than via the evolution of dioritic-granodioritic magmas. In summary, there were two kinds of granit","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":"74 3","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136317821","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":"Cross-basin chronostratigraphic correlation of carbonate succession (Llandovery, Michigan Basin, USA) using global carbon δ13Ccarb isotope excursions","authors":"Mohammed Al-Musawi, Jeffrey J. Kuglitsch, William B. Harrison III, Peter J. Voice, Elizabeth M. Griffith, Matthew R. Saltzman, Stephen E. Kaczmarek","doi":"10.1130/b36809.1","DOIUrl":"https://doi.org/10.1130/b36809.1","url":null,"abstract":"Correlating shallow shelf carbonates and their deep basin equivalents is a perennial challenge in the geosciences, with wide-ranging implications. This hurdle is well illustrated in the Llandovery succession of the Michigan Basin, USA, a 40- to 265-m-thick carbonate interval represented by three lithostratigraphic units: the Cataract, the Burnt Bluff, and the Manistique groups. Although extensively studied at various localities within the basin and across the region, the chronostratigraphic relationships between these units remain unknown. The current study presents a cross-basin chronostratigraphic framework for the Llandovery succession utilizing globally documented carbon (δ13Ccarb) isotope excursions (CIEs). From 10 drill cores and three quarry sites throughout the Michigan Basin, five CIEs were identified and chronostratigraphically constrained using conodont biostratigraphy and conodont 87Sr/86Sr data. The five excursions are interpreted to be the global CIEs: the (1) Hirnantian Isotope Carbon Excursion (HICE; Hirnantian Stage), (2) Early Aeronian, (3) Late Aeronian (Aeronian Stage), (4) Valgu (Telychian Stage), and (5) Ireviken (Sheinwoodian Stage). Most importantly, the HICE and the Ireviken CIEs bracket the Llandovery strata preserved in the basin. The new high-resolution δ13Ccarb data suggest that CIEs can be effectively used to correlate among shallow marine shelf carbonates and their deeper water equivalents. The new chronostratigraphic framework shows that CIE-based time horizons across the Michigan Basin cut across lithostratigraphic unit boundaries, which indicates that these lithostratigraphic units are diachronous in the Michigan Basin. In addition to refining the stratigraphy of the Llandovery succession of the Michigan Basin, particularly the timing of various key sedimentary deposits, the new chronostratigraphic framework can be used to: (1) constrain the timing of various regional tectonic phenomena, (2) identify multiple tectonically driven siliciclastic sediment pulses in the basin, and (3) predict various stratal relationships that may result in previously unknown stratigraphic traps and, therefore, new hydrocarbon plays within the basin. The results of the current study also show that δ13Ccarb trends across the shelf-to-basin transect are spatially and temporally variable and do not match those reported in Modern carbonate settings, which possibly suggests that such δ13Ccarb trends, to some extent, reflect variations in water circulation and water mass heterogeneity during deposition.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":"3 10","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135568066","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":"Petrogenesis of Paleocene−Eocene gabbros in the Gangdese belt: Geochemical tracking of transitioning from oceanic subduction to continental collision related magmatism in southern Tibet","authors":"Zhaoping Hu, Lingsen Zeng, Yildirim Dilek, Michael W. Förster, Li-E Gao, Yaying Wang, Nuerkanati Madayipu, Huan Li","doi":"10.1130/b37003.1","DOIUrl":"https://doi.org/10.1130/b37003.1","url":null,"abstract":"The composition of the sub-arc mantle and the mode and nature of geodynamic processes during the India-Asia collision that controlled the melt evolution beneath the Gangdese belt (southern Tibet) are still unclear. Here, we present new U-Pb ages and Hf isotopes of zircon, and whole-rock geochemical and Sr-Nd-Pb isotopic data of the Paleocene−Eocene Najinla gabbros from the East Gangdese magmatic belt, aiming to track the transitioning magmatism formed from oceanic subduction to continental collision in the region. Zircon U-Pb analyses of these mafic rocks yield emplacement ages of 54 ± 1 Ma and 63 ± 1 Ma. The gabbros are characterized by variable SiO2 (45.87−55.44 wt%), MgO (1.03−8.18 wt%), FeOT (3.74−12.33 wt%), and Al2O3 (13.45−25.45 wt%) contents. Most samples exhibit high Al2O3 (17.15−25.45 wt%) and relatively low MgO (1.03−6.11 wt%), similar to typical high-alumina basalts and high-alumina basaltic andesites. The Najinla gabbros show characteristic subduction-related signatures with enriched large-ion lithophile elements and depleted high field strength elements. They have depleted Sr-Nd isotopic compositions with low and relatively homogeneous initial 87Sr/86Sr isotopic ratios of 0.7045−0.7049 and positive εNd(t) ratios of +2.2 to +3.2. The Najinla gabbroic rocks also have positive zircon εHf(t) values, ranging from +5.6 to +10.9. These results collectively suggest that magmas of the gabbros formed by partial melting of the asthenosphere with negligible crustal contamination during their emplacement. We propose that the mantle source of the Najinla gabbros was strongly influenced and metasomatized by subducted Neotethyan oceanic crust-derived fluids in the mantle wedge. Rollback of the subducted Neotethyan slab in the early Eocene led to partial melting of the subduction-modified mantle and the formation of these gabbros.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":"11 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135728726","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":"Lateral and vertical characteristics of floodplain aggradation cycles in the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, USA","authors":"Youwei Wang, Timothy F. Baars, Joep E.A. Storms, Allard W. Martinius, Philip D. Gingerich, Magda Chmielewska, Simon J. Buckley, Hemmo A. Abels","doi":"10.1130/b36908.1","DOIUrl":"https://doi.org/10.1130/b36908.1","url":null,"abstract":"Sedimentation on river floodplains is a complex process that involves overbank flooding, crevasse splaying, and river avulsion. The resulting floodplain stratigraphy often exhibits floodplain aggradation cycles with alternating fine-grained overbank flooding deposits that underwent significant petrogenesis, and coarser-grained, avulsion-belt deposits largely devoid of pedogenic impact. These cycles are linked to lateral migration and avulsion of channels driven by internal dynamics, external factors, or a combination of both. To better understand the spatial and vertical variability of such floodplain aggradation cycles, we map these in three dimensions using a photogrammetric model of the lower Eocene Willwood Formation in the northern Bighorn Basin, Wyoming, USA. This allows identifying 44 floodplain aggradation cycles in ∼300 m of strata with an average thickness of 6.8 m and a standard deviation of 2.0 m. All the cycles are traceable over the entire model, pointing to their spatial consistency over the 10 km2 study area. At the same time, rapid lateral thickness changes of the floodplain aggradation cycles occur with changes up to 4 m over a lateral distance of 400 m. Variogram analyses of both field and numerical-model results reveal stronger consistency of floodplain aggradation cycle thicknesses along the paleoflow direction compared to perpendicular to paleoflow. Strong compensational stacking occurs at the vertical scale of 2−3 floodplain aggradation cycles (14−20 m), while full compensational stacking occurs at larger scales of more than six floodplain aggradation cycles (>41 m). The lateral and vertical thickness variability of the floodplain aggradation cycles, as well as their compensational stacking behavior, are interpreted to be dominantly driven by autogenic processes such as crevasse splaying and avulsing that preferentially fill topographic lows. External climate forcing may have interacted with these autogenic processes, producing the laterally persistent and vertically repetitive floodplain aggradation cycles. The spatial variability of floodplain aggradation cycles demonstrated in this study highlights again the need for three-dimensional data collection in alluvial floodplain settings rather than depending on one-dimensional records.","PeriodicalId":55104,"journal":{"name":"Geological Society of America Bulletin","volume":"15 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135824766","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}