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; the Lower Mississippi Valley, USA; and the polarity of Mesozoic arcs along the western margin of North America. You can find these articles at https://bulletin.geoscienceworld.org/content/early/recent .
Magmatic-hydrothermal evolution of long-lived Nb-Ta-(Sn) mineralization in Lianyunshan, NE Hunan, South China
Nuerkanati Madayipu; Huan Li; Safiyanu Muhammad Elatikpo; Michael W. Förster; Hou-Xiang Zhou …
The Lianyunshan Complex (LYSC) is a Nb-Ta-Li-Be rare metals mining area in northeastern Hunan, South China. The deposit is composed of a granite-type rare-metal pegmatite mineralization that occurs as dikes around the LYSC. The present study examines the genetic relationship, fluid evolution, and metallogeny of the co-developed ore-free pegmatite (OFP) and ore-bearing pegmatite (OBP) in granite-related pegmatite-type Nb-Ta rare-metal deposits in the LYSC. Four minerals: columbite-tantalite (coltan), zircon, cassiterite, and monazite were investigated for their chemical and isotopic compositions. The coltan grains range from homogeneous crystals to zoned varieties with growth rims that result from equilibrium and disequilibrium reactions due to local changes in the physical and chemical conditions. These structures were altered by later metasomatic replacement processes related to the hydrothermal fluid activity and produced structures such as alteration rims, patches, irregular zonations, and complex zonations, with a corresponding increase in Ta. Coltan in OBP yielded two weighted mean 206Pb/238U ages of 135.9 ± 2.1 Ma and 120.5 ± 1.2 Ma, corresponding to magmatic (early-stage) and hydrothermal (late-stage) Nb-Ta mineralization ages, respectively. The cassiterite gives a weighted mean U-Pb mineralization age of 130.5 ± 3.9 Ma, which is interpreted as the magmatic-hydrothermal age (transitional stage). Zircon grains in OBP give a lower weighted mean U-Pb age of 121.5 ± 0.8 Ma, resulting from metamictization by the later fluid-assisted alteration and recrystallization in the late-stage hydrothermal event. Monazite grains from OFP yielded a weighted mean 206Pb/238U age of 137.1 ± 0.5 Ma, while zircon gives a mean U-Pb age of 130.3 ± 0.6 Ma. The ages of 137 and 130 Ma obtained for the OFP represent the magmatic (early-stage) and magmatic-hydrothermal stage (transitional) of OFP crystallization in the LYSC. Calculated zircon εHf (t) values and T DM2 ages from OFP (−7.6 to −5.4 and 1663−1532 Ma, respectively) and OBP (−8.7 to −8.2 and 1735−1704 Ma, respectively) are akin to those of the schist and metasandstone strata of the metasedimentary Lengjiaxi metasedimentary group—a melt source material to the LYSC composite batholith. We propose a long-lived ∼15 m.y. Nb-Ta-(Sn) mineralization model for the Lianyunshan complex.
Provenance and evolution of East Asian large rivers recorded in the East and South China Seas: A review
Licheng Cao; Lei Shao; Douwe J.J. van Hinsbergen; Tao Jiang; Di Xu …
Large rivers are the arteries of continents. Those originating from the Tibetan Plateau and traversing East Asia have a relatively young history due to continuous Cenozoic perturbations. However, it has been a long journey to reconstruct their genesis and dynamic evolution, in which many puzzles and challenges remain. The river history is documented by provenance information in the ultimate sediment sinks in the East and South China Seas, but a regional-scale correlation of provenance data is still developing. Here, we explore the promise of this provenance perspective by reconstructing the evolution of three large rivers in China (the Yangtze, Pearl, and Red Rivers) by compiling and reevaluating a large volume of published provenance data (zircon U-Pb geochronology, K-feldspar Pb isotopes, and whole-rock Nd isotopes) from both Cenozoic strata and modern sediments from the East and South China Seas and the large river basins. Unlike traditional approaches that average provenance signatures, intersample variability was carefully evaluated. The general inheritance of zircon age spectral patterns and small fluctuations of Nd isotopes in the Neogene strata suggest provenance stabilization in the East and South China Seas and the establishment of near-modern drainage configurations. The paleodrainage basins before the Miocene are interpreted to have been smaller than their modern sizes, and drainage expansion likely occurred over the Oligocene. Our analysis suggests that the widely accepted models that link drainage between the ancient Yangtze and Red Rivers may be unlikely. The compiled provenance signatures and prior paleocurrent measurements of Paleogene strata distributed in the southeastern Tibetan Plateau margin show sediment supplied from local terranes instead of through-flowing river systems.
Climate and anthropogenic impacts on North American erosion and sediment transport since the Last Glacial Maximum: Evidence from the detrital zircon record of the Lower Mississippi Valley, USA
Nikki Neubeck; Andrew Carter; Tammy Rittenour; Peter D. Clift
The Mississippi River provides an opportunity to examine models of sediment transport in large alluviated floodplain systems. We test the idea that sources of sandy sediment in such settings are invariable on timescales <104 y because of storage and recycling in the floodplains. To reconstruct the development of the Mississippi sediment load over the past 2500 years we collected sediment from an abandoned point bar complex nearby at False River, Louisiana, USA. We also took annual samples from the lower reaches between 2015 and 2021 to assess changes on that timescale. Optically stimulated luminescence dating indicated that the point bar accreted between 2460 and 860 years ago. Detrital zircon U-Pb dating was used to assess sediment source and variability over time. We confirm a dominant sediment flux from the Rocky Mountain foreland but with higher relative erosion from the Superior Province during the Last Glacial Maximum (LGM) based on existing data from the Gulf of Mexico. There have been resolvable changes in the sources of sediment particularly since the LGM and after 860 years ago, but also over shorter, even sub-annual timescales in the recent past. These changes may reflect seasonal weather or storm events in the headwater regions and imply limited floodplain buffering of the sand load. In recent times this may reflect the installation of levees in the lower reaches, suppressing reworking. Changes over 102−10 3 y time periods may be related to changes in climate (e.g., the Medieval and Roman warm periods) and to the development of agriculture across North America after ∼2000 years ago. Detrital zircon dating is an effective provenance tool and does not appear to be strongly biased by the grain size of the sediment in this setting.
Accretion of an early Paleozoic Alaska-type arc onto northern North China: Implications for continental growth of the Central Asian orogenic belt
Hao Zeng; Dongfang Song; Wenjiao Xiao; Puqing Li
The accretionary orogenesis of the Central Asian orogenic belt is essential for understanding the reconstruction and growth of the Asian continent. The origin and accretion of the Bainaimiao arc along the northern margin of North China remain controversial. Here, a comprehensive study of field geology, geochemistry, and geochronology was performed on plutons and an ophiolite-bearing accretionary complex in the Ganqimaodu region, Inner Mongolia. Geochemical data indicate that ca. 445−431 Ma plutons were generated in an intra-oceanic arc and represent the western extent of the Bainaimiao arc. Field mapping and zircon U-Pb dating revealed that the ophiolitic mélange contains ca. 470−435 Ma tectonic blocks of ultramafic rocks, gabbros, plagiogranites, and metavolcanic and siliceous rocks in a matrix of intensely foliated quartz schist. The pillow basalt−limestone sequence might have originated from a seamount and then been incorporated into the accretionary complex. Northward subduction of the South Bainaimiao Ocean was responsible for the early Paleozoic tectonic evolution of the Bainaimiao arc, based on the spatial-temporal configuration of arc magmatism and accretionary complexes. Integration of new data with previous studies indicates that the Bainaimiao arc was an Alaska-type arc with various components of intra-oceanic and continental arcs. We suggest that a scissor-like arc-continent collision led to the accretion of the Bainaimiao arc onto the northern margin of North China, which played a fundamental role in the continental growth of the southern Central Asian orogenic belt.
Re-appearance of precipitated aragonite crystal fans as evidence for expansion of oceanic dissolved inorganic carbon reservoir in the aftermath of the Lomagundi-Jatuli Event
Guang Ouyang; Zhenbing She; Qian Xiao; Kenan Qian; Zongyuan Kenan …
The initial accumulation of atmospheric oxygen is marked by the unprecedented positive δ13Ccarb excursions of the Lomagundi-Jatuli Event (LJE) and records an interval of abnormal O 2 production through elevated rates of organic carbon burial. Emerging evidence suggests that the post-LJE atmosphere-ocean system might have suffered a significant deoxygenation. These dynamic perturbations in the oceanic redox state and biogeochemical cycles would have led to fundamental changes in carbonate precipitation dynamics. Here, we report the discovery of centimeter-sized crystal fans in the post-LJE Huaiyincun Formation, Hutuo Supergroup in the North China Craton. The hexagonal cross-sections and square terminations suggest that these fan-like dolomitic structures were originally aragonite crystal fans (ACF). Variations of stromatolite morphology and frequent occurrences of storm-related deposits in the Huaiyincun Formation point to repeated cycles of sea level changes. The bedding-parallel distribution of the ACF and the homogeneous δ13C values of the ACF-bearing dolostones are consistent with a primary depositional origin for the ACF. An updated compilation of published records of ACF throughout geological history highlights a clear absence of ACF from the initiation of the Paleoproterozoic Great Oxidation Event until the end of the LJE, and a global reappearance of ACF in the post-LJE late Paleoproterozoic. We propose that the reappearance of ACF is in agreement with the expansion of the oceanic dissolved inorganic carbon reservoir. At the same time, consumption of dissolved oxygen during the oxidation of organic matter might have been stimulated by ferruginous deep seawater, facilitating the formation of Huiayincun ACF.
Large-scale Permo-Triassic back-arc extensions of the Mongol-Okhotsk Ocean
Mingshuai Zhu; John Wakabayashi; Daniel Pastor-Galán; Fuqin Zhang; Ariuntsetseg Ganbat …
The Late Paleozoic−Mesozoic Mongol-Okhotsk orogenic belt marks the final aggregation of East Asia. The geodynamics of the Mongol-Okhotsk oceanic plate subduction are still poorly understood due to its curved orogenic architecture, complex kinematics, and the protracted active continent margin that developed during oceanic subduction. Here, we report the discovery of an ophiolite within the southern paleo-active margin of the Mongol-Okhotsk orogenic belt. The ophiolitic slices are composed of serpentinites, metagabbros, and metabasalts, and interleaved with deformed volcaniclastic rocks. Using zircon U-Pb dating, we determined an age of 253 ± 2 Ma for the metagabbro, which suggests that the ophiolite formed during the Late Permian. Geochemical data and geologic relationships indicate that this ophiolite formed in a back-arc setting. The contemporaneous granitic dikes (ca. 255 Ma) intruding the basalts of the ophiolite were likely formed by crustal anatexis during the back-arc extension. Detrital zircon in sandstone associated with the ophiolite shows a prominent peak age of ca. 273 Ma that probably represents the sediments of the back-arc basin. Combining our discovery with the available data from the literature, we suggest that a >5000-km-long back-arc extension belt existed in the southwestern segment of the Mongol-Okhotsk belt, which indicates a probable Western Pacific-type active margin rather than the previously proposed Andean-type margin that formed during the Permo-Triassic period.
New geochronological results from late Mesoproterozoic to early Neoproterozoic successions in the eastern North China Craton and implications for the reconstruction of Rodinia
Chaohui Liu; Guochun Zhao; Fulai Liu; Wang Xu; Xun Sun
Late Stenian to Tonian stratum of the Xihe and Jinxian groups, the Sangwon system, the Penglai Group, the Tumen Group, and the Huaibei and Langan groups located at the eastern margin of the North China Craton provide an excellent record of changes in sediment provenance related to the amalgamation and dispersal of Rodinia and therefore the paleogeographic position of the craton. To decipher their protosource changes over time, we evaluated 7510 U-Pb and 963 Lu-Hf analyses from 92 samples across the entire eastern North China Craton, of which 1746 U-Pb and 415 Lu-Hf analyses from 18 samples are newly reported here. The detrital zircon results indicate two stratigraphic intervals with internally consistent lithostratigraphy, event stratigraphy, biostratigraphy, depositional age, and detrital zircon age patterns: the late Stenian to early Tonian strata and the late Tonian strata, of which the former mainly consists of the Mesoproterozoic detrital zircons with juvenile Hf isotopic features and the latter’s detritus chiefly derived from the Neoarchean to late Paleoproterozoic basement of the North China Craton. In consideration of the paleomagnetic and geochronological data from the coeval strata in other cratons, the Mesoproterozoic detrital zircons of the late Stenian to early Tonian strata are interpreted to be derived from the Musgrave Province in central Australia during the Rodinia amalgamation, and the transition to the autochthonic protosources in the late Tonian possibly indicated the breakup of the North China Craton and North Australian Craton connection.
Relative importance of magmatic and hydrothermal processes for economic Nb-Ta-W-Sn mineralization in a peraluminous granite system: The Zhaojinggou rare-metal deposit, northern China
Lu Zhang; Shao-Yong Jiang; Rolf L. Romer; Hui-Min Su
Highly evolved granitic melts typically experience late-stage melt-melt and fluid-melt immiscibility as well as fluid-melt and fluid-rock interaction. These processes are particularly important in the formation of deposits of the rare metals Nb, Ta, W, and Sn. We document the relation between immiscibility and alteration processes and the partitioning behavior of rare metals for the Zhaojinggou rare-metal deposit of northern China. This deposit shows a systematic change from a magmatic to a hydrothermal system, including the reaction of the exsolved fluid with earlier crystallized granite and the formation of late-stage quartz veins. The magmatic stage (Stage I) includes biotite alkali-feldspar granite (BAG) with moderate Nb-Ta mineralization. Extreme fractional crystallization of BAG eventually resulted in melt-melt immiscibility and the separation of a hydrosaline melt. Fractional crystallization of this hydrosaline albite granite (AG) melt finally exsolved a magmatic fluid. Therefore, the magmatic-hydrothermal transition (Stage II) includes a melt-dominated Stage IIa with strong Nb-Ta-Sn mineralization in AG and a fluid-dominated Stage IIb with minor Nb-Ta-Sn mineralization in muscovite and biotite greisen. Late hydrothermal processes (Stage III) formed quartz veins with important W mineralization. There are several texturally and chemically distinct generations of cassiterite and columbite-group minerals (CGM) in BAG and AG reflecting crystallization from an evolving magma. The porous and patchy-zoned reaction rims of tantalite-(Mn) and wodginite on CGM in AG are the result of fluid-melt interaction. Texture and compositions show that wolframite in AG is hydrothermal and formed through interaction of early exsolved magmatic fluids with the host granite. CGM and cassiterite in the biotite greisen and Ta-rutile in the muscovite greisen, as well as wolframite and scheelite in quartz veins that formed when fluid-rock interaction reduced the availability of H+ or F− or the temperature of the fluid decreased. The distribution and importance of mineralization demonstrate that Nb, Ta, W, and Sn strongly partitioned into the hydrosaline melt during melt-melt immiscibility and that W partitioned into the magmatic fluid during fluid-melt immiscibility. Exsolved magmatic fluids interacted with earlier crystallized rocks mobilizing rare (Nb, Ta, and Sn) and base (Fe and Ti) metals from Li-Fe mica, providing the ore elements for subordinate Nb-Ta-Sn mineralization in AG and in biotite and muscovite greisen. Thus, magmatic processes (with later metal redistribution by magmatic fluids) dominantly control economic Nb-Ta-Sn mineralization, whereas hydrothermal processes mainly control the formation of economic W mineralization.
The good, the bad, and the ugly: Analysis of three arguments in the ongoing debate concerning the polarity of Mesozoic arcs along the western margin of North America
Grant W. Lowey
The debate concerning eastward versus westward subduction along the western margin of Mesozoic North America involves three main arguments: The tomography argument (westward) claims subducted slabs are observed by tomographic processing of seismic waves; the geologic evidence argument (eastward) claims subduction polarity is recorded by tripartite accretionary complex−forearc basin−magmatic arc assemblages; and the crucial geologic test argument (eastward) claims endemic terranes are characterized by detrital zircons of appropriate age and abundance, particularly Precambrian zircons. Reconstruction of the arguments in standard logic form as categorical syllogisms indicates that all three arguments are valid. However, evaluation of the truth-falsehood of the propositions supporting the arguments suggests: (1) propositions for the tomography argument seem more truthful than false; (2) propositions for the geologic test argument seem more false than truthful; and (3) propositions for the crucial geologic test argument also seem more false than truthful. Thus, the tomography argument appears to be a “good” argument (i.e., valid and sound); the geologic evidence argument appears to be a “bad” argument (i.e., valid and unsound); and the crucial geologic test argument appears to be an “ugly” argument—in the spirit that some mathematical proofs are ugly (i.e., valid and unsound, in addition to being convoluted and couched in vague terms such as “appropriate”). Proposed tests for falsification of the arguments include application of an alternate tomographic processing method to obtain non-wall-like structures in the mantle, demonstration that tripartite successions are not consanguineous, and procurement of additional evidence showing that endemic terranes do not contain Precambrian detrital zircons of appropriate age and abundance.
Tracing the protoliths of the garnet amphibolitic and retrogressed eclogitic slices and a conceptual tectonic model for their emplacement onto the Central Menderes Massif, Turkey: New geochemical data and laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon and rutile ages
Semih Gürsu; Andreas Möller; Jeremy D. Inglis; M. Cemal Göncüoğlu; Kevin Hefferan …
The age, emplacement, and metamorphic history of the garnet amphibolitic and retrogressed eclogitic slices in the Menderes Massif (Eastern Aegean) have been a matter of debate since the 1990’s. Late Cretaceous garnet amphibolitic and retrogressed eclogitic slices in the Bozdağ and Çine nappes show low-angle tectonic contacts with the surrounding Early Cambrian meta-siliciclastics in the Alaşehir-Yahyaalcı and late Neoproterozoic basement rocks in the Birgi and Tire-Çamlıca Klippe in the northern Menderes Massif. Lower and upper intercept ages on the discordia diagrams of the garnet amphibolitic slices are 82 ± 230 Ma and 554 ± 14 Ma, respectively, in the Yahyaalcı-Alaşehir area of the Bozdağ Nappe; and 96 ± 260 Ma and 550 ± 13 Ma, respectively, in the Camlica-Tire Klippe area of the Çine Nappe. Retrogressed eclogitic slices give dates of 81 ± 13 Ma lower intercept and 546 ± 13 Ma upper intercept discordant ages in the Yenişehir-Kiraz area of the Çine Nappe. Lower intercept age of 81 ± 13 Ma is supported by two spots dated on metamorphic zircons showing positive Eu patterns yielding 86.0 ± 1.3 Ma concordant age. Zircon U-Pb dating on the oscillatory zoning spots are dated as 537.5 ± 1.6 Ma (mean square weighted deviation [MSWD] = 1.5, n = 41) for the garnet amphibolic slices in the Yahyaalcı-Alaşehir area (Bozdağ Nappe), 539.0 ± 1.1 Ma (MSWD = 1.4, n = 56) for the garnet amphibolite slices in the Camlica-Tire Klippe area (Çine Nappe), and 536.6 ± 2.3 Ma (MSWD = 1.7, n = 32) for the retrogressed eclogitic slices (Çine Nappe). These ages combined with Eu negative anomalies on the dated spots are evaluated as representing their magmatic crystallization ages of the protoliths. Garnet isopleth intersections and pseudosections gave pressure-temperature conditions of 14 kbar and ∼680 °C for the retrogressed eclogites, whereas garnet amphibolites display disequilibrium in whole rock scale. We propose that the age of their protoliths likely correlate with the Cadomian rift-related Early Cambrian meta-mafic dykes that were subducted beneath the İzmir-Ankara-Erzincan Neotethyan lithosphere during the Late Cretaceous. The new rutile U-Pb age of 30.1 ± 2.0 Ma supports that the tectonic slices of the İzmir-Ankara oceanic lithosphere and metamorphosed Tauride-Anatolide continental margin were emplaced onto the Menderes Massif to generate the “main Menderes metamorphic terrane” during the latest Paleocene and early Eocene.
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