Despite its importance, only a few researchers have incorporated the effects of fracturing into models of reactive transport for rock weathering. Here we explore 2D simulations that describe weathering under conditions of diffusive and advective transport within heterogeneous media consisting of rocky blocks and fractures. In our simulations, the Darcy velocities vary in space and time and depend on weathering processes within the rock matrix. We explore simulations with saturated and unsaturated flow for weathering bedrock that consists of blocks separated by inert or weathered material. The simulations show that a simplified homogenized model can approximate exact solutions for some of the simulated columns and hills and can allow exploration of coupling between flow and reaction in fractured rock. These hillslope simulations document that, even in the presence of 2D water flow, i) an increase in fracture density results in faster weathering advance rates; and ii) the water table locates deeper for a rock system that is weathered and fractured rather than weathered and unfractured. Some of these patterns have also been observed for natural systems. But these simulations also highlight how simplified models that do not use appropriate averaging of heterogeneities can be inaccurate in predicting weathering rate for natural systems. For example, if water flows both vertically and laterally through the vadose zone of a hill, then a prediction of the depth of regolith that is based on modeling strictly unidirectional downward infiltration will be unrealistically large. Likewise, if the fracture density observed near the land surface is used in a model to predict depth of weathering for a system where the fracture density decreases downward, the model will overestimate regolith depth. Learning how to develop accurately homogenized models could thus enable better conceptual models and predictions of weathering advance in natural systems.
{"title":"Using Homogenized Models to Explore the Effect of Fracture Densities on Weathering","authors":"M. Lebedeva, S. Brantley","doi":"10.2475/001c.68308","DOIUrl":"https://doi.org/10.2475/001c.68308","url":null,"abstract":"Despite its importance, only a few researchers have incorporated the effects of fracturing into models of reactive transport for rock weathering. Here we explore 2D simulations that describe weathering under conditions of diffusive and advective transport within heterogeneous media consisting of rocky blocks and fractures. In our simulations, the Darcy velocities vary in space and time and depend on weathering processes within the rock matrix. We explore simulations with saturated and unsaturated flow for weathering bedrock that consists of blocks separated by inert or weathered material. The simulations show that a simplified homogenized model can approximate exact solutions for some of the simulated columns and hills and can allow exploration of coupling between flow and reaction in fractured rock. These hillslope simulations document that, even in the presence of 2D water flow, i) an increase in fracture density results in faster weathering advance rates; and ii) the water table locates deeper for a rock system that is weathered and fractured rather than weathered and unfractured. Some of these patterns have also been observed for natural systems. But these simulations also highlight how simplified models that do not use appropriate averaging of heterogeneities can be inaccurate in predicting weathering rate for natural systems. For example, if water flows both vertically and laterally through the vadose zone of a hill, then a prediction of the depth of regolith that is based on modeling strictly unidirectional downward infiltration will be unrealistically large. Likewise, if the fracture density observed near the land surface is used in a model to predict depth of weathering for a system where the fracture density decreases downward, the model will overestimate regolith depth. Learning how to develop accurately homogenized models could thus enable better conceptual models and predictions of weathering advance in natural systems.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2023-02-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43884486","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
C. Colleps, N. McKenzie, P. A. van der Beek, W. Guenthner, Mukund Sharma, A. Nordsvan, D. Stockli
Modern approaches in low-temperature thermochronometry are capable of extracting long-term thermal histories from cratonic settings that may elucidate potential drivers of deep-time phases of intracontinental burial and erosion. Here, we assess the utilization of the Radiation Damage Accumulation and Annealing Model for apatite (RDAAM) and zircon (ZRDAAM) to track the long-term low-temperature thermal evolution of the Archean Bundelkhand craton and the surrounding undeformed strata of the ∼1.7–0.9 Ga Vindhyan successions in central India. We correspondingly interpret a complex basement and detrital zircon and apatite (U-Th)/He (ZHe and AHe, respectively) dataset in light of observed model limitations and known geologic context. ZHe and AHe dates from across the craton reveal a significant (>300 Myr) date inversion between the two systems within grains with moderate to high effective uranium (eU) concentrations. Inverse thermal models utilizing current ZRDAAM and RDAAM parameters are not capable of reproducing observed coupled basement ZHe and AHe data for the same thermal history. However, meaningful thermal information can be extracted from AHe inverse models coupled with a forward modeling approach applied to detrital ZHe data from Vindhyan deposits, which have notably lower eU concentrations and yield significantly older ZHe dates (between ∼1,475 and 575 Ma) than basement zircon. Resulting thermal models indicate that the Bundelkhand craton experienced peak burial temperatures of ∼150°C between 850 and 475 Ma, followed by a major crustal cooling event at ∼350–310 Ma, possibly driven by late Paleozoic glaciations and/or epeirorogenic uplift. Inverse models including AHe data require a Deccan Traps related thermal perturbation between ∼66 and 65 Ma, and we suspect that this event overprinted basement zircon with moderate to high eU concentrations. Although the effects of zonation, grain morphology, and/or uncertainties in damage-annealing parameters contribute to disparities between predicted and observed AHe and ZHe dates, these factors alone cannot account for the major ZHe and AHe date inversion observed from the Bundelkhand craton. Instead, it is likely the case that current damage-dependent models for 4He diffusion are not adequately calibrated at the resolution necessary to predict short-lived thermal perturbations that occurred in a late phase relative to a prolonged period of extensive damage accumulation.
低温热年代学的现代方法能够从克拉通环境中提取长期热历史，这可能阐明陆内埋藏和侵蚀的深层时间阶段的潜在驱动因素。在这里，我们评估了磷灰石（RDAAM）和锆石（ZRDAAM）辐射损伤累积和退火模型的使用情况，以跟踪印度中部太古宙Bundelkhand克拉通和~1.7–0.9 Ga Vindhyan序列周围未变形地层的长期低温热演化。根据观察到的模型限制和已知的地质背景，我们相应地解释了复杂的基底和碎屑锆石和磷灰石（U-Th）/He（分别为ZHe和AHe）数据集。整个克拉通的ZHe和AHe日期显示，在具有中等至高有效铀（eU）浓度的颗粒内，两个系统之间存在显著的（>300 Myr）日期反演。利用当前ZRDAAM和RDAAM参数的逆热模型不能再现相同热历史的观测到的耦合基底ZHe和AHe数据。然而，可以从AHe反演模型中提取有意义的热信息，再加上应用于Vindhyan矿床碎屑ZHe数据的正演建模方法，这些矿床的eU浓度明显较低，产生的ZHe日期（约1475至575 Ma）明显早于基底锆石。由此产生的热模型表明，Bundelkhand克拉通在850至475 Ma之间经历了约150°C的峰值埋藏温度，随后在约350至310 Ma发生了一次主要的地壳冷却事件，可能是由晚古生代冰川作用和/或表生隆升驱动的。包括AHe数据在内的反演模型需要在～66和65Ma之间的Deccan Traps相关热扰动，我们怀疑这一事件叠加了具有中高eU浓度的基底锆石。尽管分区、晶粒形态和/或损伤退火参数的不确定性的影响导致了预测和观测到的AHe和ZHe日期之间的差异，但仅凭这些因素并不能解释从Bundelkhand克拉通观测到的主要ZHe和AHe日期反演。相反，可能的情况是，4He扩散的当前损伤相关模型没有以预测短时间热扰动所需的分辨率进行充分校准，该热扰动发生在相对于长时间的广泛损伤累积的后期阶段。
{"title":"Assessing the long-term low-temperature thermal evolution of the central Indian Bundelkhand craton with a complex apatite and zircon (U-Th)/He dataset","authors":"C. Colleps, N. McKenzie, P. A. van der Beek, W. Guenthner, Mukund Sharma, A. Nordsvan, D. Stockli","doi":"10.2475/10.2022.01","DOIUrl":"https://doi.org/10.2475/10.2022.01","url":null,"abstract":"Modern approaches in low-temperature thermochronometry are capable of extracting long-term thermal histories from cratonic settings that may elucidate potential drivers of deep-time phases of intracontinental burial and erosion. Here, we assess the utilization of the Radiation Damage Accumulation and Annealing Model for apatite (RDAAM) and zircon (ZRDAAM) to track the long-term low-temperature thermal evolution of the Archean Bundelkhand craton and the surrounding undeformed strata of the ∼1.7–0.9 Ga Vindhyan successions in central India. We correspondingly interpret a complex basement and detrital zircon and apatite (U-Th)/He (ZHe and AHe, respectively) dataset in light of observed model limitations and known geologic context. ZHe and AHe dates from across the craton reveal a significant (>300 Myr) date inversion between the two systems within grains with moderate to high effective uranium (eU) concentrations. Inverse thermal models utilizing current ZRDAAM and RDAAM parameters are not capable of reproducing observed coupled basement ZHe and AHe data for the same thermal history. However, meaningful thermal information can be extracted from AHe inverse models coupled with a forward modeling approach applied to detrital ZHe data from Vindhyan deposits, which have notably lower eU concentrations and yield significantly older ZHe dates (between ∼1,475 and 575 Ma) than basement zircon. Resulting thermal models indicate that the Bundelkhand craton experienced peak burial temperatures of ∼150°C between 850 and 475 Ma, followed by a major crustal cooling event at ∼350–310 Ma, possibly driven by late Paleozoic glaciations and/or epeirorogenic uplift. Inverse models including AHe data require a Deccan Traps related thermal perturbation between ∼66 and 65 Ma, and we suspect that this event overprinted basement zircon with moderate to high eU concentrations. Although the effects of zonation, grain morphology, and/or uncertainties in damage-annealing parameters contribute to disparities between predicted and observed AHe and ZHe dates, these factors alone cannot account for the major ZHe and AHe date inversion observed from the Bundelkhand craton. Instead, it is likely the case that current damage-dependent models for 4He diffusion are not adequately calibrated at the resolution necessary to predict short-lived thermal perturbations that occurred in a late phase relative to a prolonged period of extensive damage accumulation.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42882126","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Stratigraphic variations in the carbon isotope composition of marine limestones (δ13Ccarb) have been widely reported from the Guadalupian, particularly in the uppermost Capitanian, but the extent to which they reflect global carbon cycle dynamics remains in dispute. Resolving these uncertainties is critical for interpreting the dynamics of the carbon cycle during the Guadalupian as well as for testing hypothesized circumstances of the marine extinction event during the Capitanian (latest Guadalupian). To assess the local versus global causes of stratigraphic variations in δ13Ccarb, here we report δ13Ccarb values from Guadalupian strata in the Hambast Valley, Abadeh region, southwest Iran, in two sections. We then compare the data from the two sections (Abadeh-1 and Abadeh-2) to data from numerous other well-studied sections that were distant from central Iran at the time of deposition. In the Hambast sections, there are broad excursions of δ13Ccarb, of up to 3‰. However, the δ13Ccarb values do not show any remarkable shifts across the Wordian/Capitanian or Capitanian/Wuchiapingian boundaries in the Abadeh-1 section. An excursion occurs near the Capitanian/Wuchiapingian boundary in the Abadeh-2 section but is associated with elemental and oxygen-isotope evidence for diagenetic resetting. The lack of reproducibility in the δ13Ccarb shifts between the two studied sections at Abadeh and evidence for diagenetic resetting in association with negative excursions are consistent with evidence from other, previously reported, study sites that negative excursions can be accounted for by local primary or diagenetic factors and do not require any global perturbation of the carbon cycle through this interval. When carbonate values are compared with previously published organic carbon isotope data and carbonate sections are compared for shared isotope features, the primary global signal that is supported by the data is one of stable and relatively heavy values of δ13Ccarb in marine limestone during the Guadalupian.
{"title":"Guadalupian carbon isotope stratigraphy indicates extended interval of carbon cycle stability","authors":"S. Arefifard, J. Payne, M. Rizzi","doi":"10.2475/09.2022.01","DOIUrl":"https://doi.org/10.2475/09.2022.01","url":null,"abstract":"Stratigraphic variations in the carbon isotope composition of marine limestones (δ13Ccarb) have been widely reported from the Guadalupian, particularly in the uppermost Capitanian, but the extent to which they reflect global carbon cycle dynamics remains in dispute. Resolving these uncertainties is critical for interpreting the dynamics of the carbon cycle during the Guadalupian as well as for testing hypothesized circumstances of the marine extinction event during the Capitanian (latest Guadalupian). To assess the local versus global causes of stratigraphic variations in δ13Ccarb, here we report δ13Ccarb values from Guadalupian strata in the Hambast Valley, Abadeh region, southwest Iran, in two sections. We then compare the data from the two sections (Abadeh-1 and Abadeh-2) to data from numerous other well-studied sections that were distant from central Iran at the time of deposition. In the Hambast sections, there are broad excursions of δ13Ccarb, of up to 3‰. However, the δ13Ccarb values do not show any remarkable shifts across the Wordian/Capitanian or Capitanian/Wuchiapingian boundaries in the Abadeh-1 section. An excursion occurs near the Capitanian/Wuchiapingian boundary in the Abadeh-2 section but is associated with elemental and oxygen-isotope evidence for diagenetic resetting. The lack of reproducibility in the δ13Ccarb shifts between the two studied sections at Abadeh and evidence for diagenetic resetting in association with negative excursions are consistent with evidence from other, previously reported, study sites that negative excursions can be accounted for by local primary or diagenetic factors and do not require any global perturbation of the carbon cycle through this interval. When carbonate values are compared with previously published organic carbon isotope data and carbonate sections are compared for shared isotope features, the primary global signal that is supported by the data is one of stable and relatively heavy values of δ13Ccarb in marine limestone during the Guadalupian.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46381319","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. Davis, L. Reeher, G. Jepson, B. Carrapa, P. DeCelles, Kayla M. Chaudoir
Interpretation of the late Cretaceous to Eocene Laramide tectonic evolution of the Colorado Plateau is hampered by the difficulty of placing precise temporal constraints on fault-induced basement uplift, for there is a paucity of exposed basement from which thermochronologic ages might be obtained. The Defiance and Zuni uplifts, located on the southeasternmost Colorado Plateau, offer rare basement exposure that provide additional temporal insights. The Zuni uplift exposes abundant Precambrian basement rock, which has been the subject of previous apatite thermochronologic study to interpret low-temperature tectonic/exhumation history. In the Defiance uplift region, which is the main focus of this study, there are two outcrop areas of Precambrian-basement rock along the trace of the East Defiance monocline. Both sites are quarries. Uplift/erosion consequences of Pennsylvanian-Permian Ancestral Rocky Mountains (ARM) deformation are explicit in the geology of these quarries, for the contact between the Supai Group (Permian) and underlying basement (1,703 ± 1.3 Ma, zircon U-Pb) is a nonconformity. Here, we apply a combination of structural analysis of Laramide fold/fault relations and multi-method thermochronology to the exposed granitic basement of both the Defiance and Zuni uplifts. Zircon U-Pb, zircon (U-Th)/He, apatite fission-track, apatite (U-Th-Sm)/He, and hematite (U-Th)/He reveal a poly-phase thermo-tectonic history. Initially, the Paleo-Proterozoic 1.7 Ga basement cooled to ∼400 °C by at least 1.4 Ga, followed by 1.4–1.0 Ga unroofing to depths of ∼8 km. Following cooling, the Defiance-Zuni granitic basement experienced protracted residence at temperatures ≤200 °C (∼8 km) between ca. 900 and 600 Ma. Sedimentary evidence and hematite (U-Th)/He dates bracket the ARM event (400–200 Ma), which may suggest fluid-rock interaction or near-surface exposure associated with the ARM. Following the ARM, the Defiance uplift experienced heating (>120 °C) associated with burial consistent with the stratigraphic overburden, until approximately 80 Ma. Finally, the Defiance-Zuni region experienced initial cooling at <70 Ma, with the main phase of exhumation to the upper crust (<2 km) at ca. 60–40 Ma. Detailed structural modeling along 15 normal-profile cross-sections across the east margin of the Defiance uplift reveals that Laramide trishear monoclinal folding was generated by an oblique-slip master fault that partitioned ∼8 km of strike-slip and ∼1.5 km of reverse-slip displacement. Inferred strike-slip compartmental faulting in the Zuni uplift appears to fit coherently within this overall kinematic model in relation to Laramide loading direction. In the context of geodynamic models for flat-slab subduction during the Laramide tectonic event, consideration of new data from the Defiance uplift (along with other recently reported thermochronology data) provides support for models that bring the flat slab beneath North America along an east-northeastward trajec
{"title":"Structure and thermochronology of basement/cover relations along the Defiance uplift (AZ and NM), and implications regarding Laramide tectonic evolution of the Colorado Plateau","authors":"G. Davis, L. Reeher, G. Jepson, B. Carrapa, P. DeCelles, Kayla M. Chaudoir","doi":"10.2475/09.2022.02","DOIUrl":"https://doi.org/10.2475/09.2022.02","url":null,"abstract":"Interpretation of the late Cretaceous to Eocene Laramide tectonic evolution of the Colorado Plateau is hampered by the difficulty of placing precise temporal constraints on fault-induced basement uplift, for there is a paucity of exposed basement from which thermochronologic ages might be obtained. The Defiance and Zuni uplifts, located on the southeasternmost Colorado Plateau, offer rare basement exposure that provide additional temporal insights. The Zuni uplift exposes abundant Precambrian basement rock, which has been the subject of previous apatite thermochronologic study to interpret low-temperature tectonic/exhumation history. In the Defiance uplift region, which is the main focus of this study, there are two outcrop areas of Precambrian-basement rock along the trace of the East Defiance monocline. Both sites are quarries. Uplift/erosion consequences of Pennsylvanian-Permian Ancestral Rocky Mountains (ARM) deformation are explicit in the geology of these quarries, for the contact between the Supai Group (Permian) and underlying basement (1,703 ± 1.3 Ma, zircon U-Pb) is a nonconformity. Here, we apply a combination of structural analysis of Laramide fold/fault relations and multi-method thermochronology to the exposed granitic basement of both the Defiance and Zuni uplifts. Zircon U-Pb, zircon (U-Th)/He, apatite fission-track, apatite (U-Th-Sm)/He, and hematite (U-Th)/He reveal a poly-phase thermo-tectonic history. Initially, the Paleo-Proterozoic 1.7 Ga basement cooled to ∼400 °C by at least 1.4 Ga, followed by 1.4–1.0 Ga unroofing to depths of ∼8 km. Following cooling, the Defiance-Zuni granitic basement experienced protracted residence at temperatures ≤200 °C (∼8 km) between ca. 900 and 600 Ma. Sedimentary evidence and hematite (U-Th)/He dates bracket the ARM event (400–200 Ma), which may suggest fluid-rock interaction or near-surface exposure associated with the ARM. Following the ARM, the Defiance uplift experienced heating (>120 °C) associated with burial consistent with the stratigraphic overburden, until approximately 80 Ma. Finally, the Defiance-Zuni region experienced initial cooling at <70 Ma, with the main phase of exhumation to the upper crust (<2 km) at ca. 60–40 Ma. Detailed structural modeling along 15 normal-profile cross-sections across the east margin of the Defiance uplift reveals that Laramide trishear monoclinal folding was generated by an oblique-slip master fault that partitioned ∼8 km of strike-slip and ∼1.5 km of reverse-slip displacement. Inferred strike-slip compartmental faulting in the Zuni uplift appears to fit coherently within this overall kinematic model in relation to Laramide loading direction. In the context of geodynamic models for flat-slab subduction during the Laramide tectonic event, consideration of new data from the Defiance uplift (along with other recently reported thermochronology data) provides support for models that bring the flat slab beneath North America along an east-northeastward trajec","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42896222","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Yanning Wang, Shengchao Xue, R. Klemd, Lin Yang, F. Zhao, Qingfei Wang
Geochronological investigations of mafic dikes along the southwestern margin of the Emeishan large igneous province (ELIP) in the South China block display a restricted range of U-Pb zircon, baddeleyite, and apatite isotopic ages ranging from 263 to 257 Ma, which overlaps with that of previously studied ELIP basalts and mafic intrusions. The dikes are divided into high-Ti and low-Ti groups, whereby the latter is further divided into two subgroups (low-Ti group-1 and -2). The high-Ti group rocks (Ti/Y > 500) are characterized by ocean island basalt-like trace element patterns with mantle-like zircon δ18O of 5.0 ± 0.10‰ and slightly enriched εNd(t) values of −1.0 to +1.0. The low-Ti group-1 rocks (Ti/Y < 500) have trace element patterns similar to those of the high-Ti group, yet generally with weak negative Nb-Ta anomalies, lower (Sm/Yb)N ratios, elevated zircon δ18O (6.6 ± 0.33 ‰), and highly variable εNd(t) values (−3.9 to +3.2). The low-Ti group-2 rocks (Ti/Y < 500) are characterized by pronounced negative Nb-Ta anomalies, more negative εNd(t) (−8.4 to −6.6) values, and higher initial 87Sr/86Sr ratios than those of the other two groups. The compositional variations of the high-Ti group and the low-Ti group-1 rocks, in conjunction with the negative correlation between the εNd(t) values and the (Th/Nb)N ratios, suggest that the two groups were generated from an isotopically heterogeneous mantle plume at different depths, and experienced varying degrees of crustal contamination (but < 20 wt.%). The high-Ti group rocks are considered to have originated from a deeper garnet-stable source, and the low-Ti group-1 rocks from a shallower source. Mixing calculations indicate that the highly enriched Sr-Nd isotopes of the low-Ti group-2 rocks cannot be explained by crustal contamination. A subduction-modified mantle source is required to account for the arc-like geochemical characteristics of this group. This is consistent with the spatial overlap of the low-Ti group-2 rocks and previously studied geochemically similar samples with rocks from the Neoproterozoic subduction zone along the western margin of the South China block. Furthermore, a fertilized mantle is also consistent with the variable δ18O values of various mafic-ultramafic rocks of the western and central ELIP due to the involvement of recycled oceanic crustal materials. Our results are in accordance with the model that the western ELIP late Permian magmatism was generated by the interaction of two distinct sources, that is, an isotopically heterogenous mantle plume and a Neoproterozoic subduction-modified, Nd isotope-enriched lithospheric mantle with distinct heterogenous oxygen isotope characteristics.
{"title":"Late Permian plume and Neoproterozoic subduction-modified mantle interaction: Insights from geochronology and Sr-Nd-O isotopes of mafic dikes of the western Emeishan large igneous province","authors":"Yanning Wang, Shengchao Xue, R. Klemd, Lin Yang, F. Zhao, Qingfei Wang","doi":"10.2475/08.2022.02","DOIUrl":"https://doi.org/10.2475/08.2022.02","url":null,"abstract":"Geochronological investigations of mafic dikes along the southwestern margin of the Emeishan large igneous province (ELIP) in the South China block display a restricted range of U-Pb zircon, baddeleyite, and apatite isotopic ages ranging from 263 to 257 Ma, which overlaps with that of previously studied ELIP basalts and mafic intrusions. The dikes are divided into high-Ti and low-Ti groups, whereby the latter is further divided into two subgroups (low-Ti group-1 and -2). The high-Ti group rocks (Ti/Y > 500) are characterized by ocean island basalt-like trace element patterns with mantle-like zircon δ18O of 5.0 ± 0.10‰ and slightly enriched εNd(t) values of −1.0 to +1.0. The low-Ti group-1 rocks (Ti/Y < 500) have trace element patterns similar to those of the high-Ti group, yet generally with weak negative Nb-Ta anomalies, lower (Sm/Yb)N ratios, elevated zircon δ18O (6.6 ± 0.33 ‰), and highly variable εNd(t) values (−3.9 to +3.2). The low-Ti group-2 rocks (Ti/Y < 500) are characterized by pronounced negative Nb-Ta anomalies, more negative εNd(t) (−8.4 to −6.6) values, and higher initial 87Sr/86Sr ratios than those of the other two groups. The compositional variations of the high-Ti group and the low-Ti group-1 rocks, in conjunction with the negative correlation between the εNd(t) values and the (Th/Nb)N ratios, suggest that the two groups were generated from an isotopically heterogeneous mantle plume at different depths, and experienced varying degrees of crustal contamination (but < 20 wt.%). The high-Ti group rocks are considered to have originated from a deeper garnet-stable source, and the low-Ti group-1 rocks from a shallower source. Mixing calculations indicate that the highly enriched Sr-Nd isotopes of the low-Ti group-2 rocks cannot be explained by crustal contamination. A subduction-modified mantle source is required to account for the arc-like geochemical characteristics of this group. This is consistent with the spatial overlap of the low-Ti group-2 rocks and previously studied geochemically similar samples with rocks from the Neoproterozoic subduction zone along the western margin of the South China block. Furthermore, a fertilized mantle is also consistent with the variable δ18O values of various mafic-ultramafic rocks of the western and central ELIP due to the involvement of recycled oceanic crustal materials. Our results are in accordance with the model that the western ELIP late Permian magmatism was generated by the interaction of two distinct sources, that is, an isotopically heterogenous mantle plume and a Neoproterozoic subduction-modified, Nd isotope-enriched lithospheric mantle with distinct heterogenous oxygen isotope characteristics.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47673724","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
D. Bradley, D. Evans, P. O’Sullivan, Cliff D. Taylor, B. Eglington
Detrital zircon data are reported from Mesoproterozoic to Ordovician strata from two tectonic domains in Mauritania: 14 samples from the Taoudeni Basin of the West African Craton and 15 samples from the Mauritanide orogen. Taoudeni Basin samples show four sequential, distinctive detrital zircon age distributions, which we refer to as “barcodes”. From old to young these are the Char, Assabet, Téniagouri, and Oujeft barcodes, each named for a constituent stratigraphic unit. Zircon age maxima are as follows, with the dominant ones in italics. The Char barcode, from Mesoproterozoic (ca. 1100 Ma) strata including the Char Group, yielded zircon age maxima at 2941, 2871, 2703, 2447, 2076, and 2041 Ma, all potentially traceable to sources in the West African Craton. The Assabet barcode is from strata, including the eponymous Assabet el Hassiane Group, that were deposited between ca. 883 and ca. 570 Ma; it has age maxima at 2137, 2053, 1769, 1510, 1212, 1021, and 936 Ma and a pronounced minimum during Geon 16 (1699–1600 Ma). The Assabet's Mesoproterozoic to early Neoproterozoic zircons cannot have come from the West African Craton or any of its surrounding orogens. The Téniagouri barcode, which takes its name from the Téniagouri Group, was deposited at ca. 569 Ma; it has dominant maxima at 1983, 1872, 1522, 1215, 1109, 988, and 601 Ma and resembles the Assabet barcode but with the addition of the youngest population. The Oujeft barcode, named for the Oujeft Group, is from strata deposited between 541 or slightly earlier and 444 Ma or younger, has age maxima at 2124, 2053, 1197, 624 and 579 Ma. The Téniagouri and Oujeft barcodes record input from Pan-African orogens. In the Mauritanide orogen, most of the metasedimentary rock units that were sampled yielded detrital zircon age spectra that match one of the Taoudeni Basin barcodes. These results imply new depositional age constraints based on barcode correlation and suggest affinities between Mauritanide strata and the West African Craton. Detrital zircon age distributions that broadly resemble the Assabet barcode occur in the Neoproterozoic of Morocco, Ghana, Greece, Russia, Brazil, and, in the Appalachian orogen of Canada and the United States, Avalonia and Ganderia. The recent Rodinia reconstruction of Evans (2021) restores these far-flung localities to a more compact area, with Avalonia, Ganderia, and other peri-Gondwanan terranes occupying an oblong area between Amazonia, Laurentia, Baltica, and West Africa. Our preferred explanation is that most of these places received detritus via the same continent-scale fluvial system as the West African craton. Among the craton's nearest Rodinia neighbors in the Evans (2021) reconstruction for 900 Ma, Amazonia has known igneous rocks corresponding to all of the major Assabet age populations, and also a lull, though not a complete magmatic gap, during Geon 16. This is consistent with overall north-directed paleocurrents in the Assabet El Hassiane Group and its cor
本文报道了毛里塔尼亚两个构造域中元古代至奥陶系地层的碎屑锆石资料:西非克拉通Taoudeni盆地14份，毛里塔尼亚造山带15份。陶德尼盆地样品显示出4个顺序分明的碎屑锆石年龄分布，我们称之为“条形码”。从老到年轻分别是Char, Assabet, tsamniagouri和Oujeft条形码，每个条形码都以一个组成地层单位命名。锆石年龄最大值如下，以斜体为优势值。中元古代(约1100 Ma)地层(包括Char群)的Char条形码测得的锆石年龄最大值分别为2941、2871、2703、2447、2076和2041 Ma，均可能溯源于西非克拉通。Assabet条形码来自地层，包括同名的Assabet el Hassiane组，沉积于约883至约570 Ma之间;其年龄最大值在2137、2053、1769、1510、1212、1021和936 Ma，最小值在1699 ~ 1600 Ma。阿萨比特的中元古代至新元古代早期锆石不可能来自西非克拉通或其周围的任何造山带。tsamiagouri条形码，它的名字来自tsamiagouri组，大约在569 Ma;它在1983年、1872年、1522年、1215年、1109年、988年和601年有优势的最大值，类似于Assabet条形码，但增加了最年轻的人口。以Oujeft群命名的Oujeft条形码来自于541或稍早至444 Ma或更年轻的地层，其年龄最大值为2124、2053、1197、624和579 Ma。tsamniagouri和Oujeft条形码记录了来自泛非造山带的输入。在茅里塔尼造山带，大多数变质沉积岩单元的碎屑锆石年龄谱与陶德尼盆地的一个条形码相匹配。这些结果暗示了基于条形码对比的新的沉积时代约束，并表明毛里塔尼地层与西非克拉通之间存在亲缘关系。与Assabet条形码大致相似的碎屑锆石年龄分布出现在摩洛哥、加纳、希腊、俄罗斯、巴西的新元古代，以及加拿大和美国的阿巴拉契亚造山带Avalonia和Ganderia。最近的Rodinia重建Evans(2021)将这些遥远的地区恢复到一个更紧凑的区域，Avalonia, Ganderia和其他周边冈瓦南地区占据了亚马逊，劳伦西亚，波罗的海和西非之间的长方形区域。我们更喜欢的解释是，这些地方中的大多数都是通过与西非克拉通相同的大陆尺度河流系统接收碎屑的。在埃文斯(2021)900 Ma重建中，在克拉通最近的Rodinia邻居中，亚马逊地区已知的火成岩与所有主要的Assabet时代人口相对应，并且在Geon 16期间也有一个平静期，尽管不是完整的岩浆缺口。这与Assabet El Hassiane群及其在西非克拉通上的同类群的整体北向古流一致。
{"title":"The Assabet barcode: Mesoproterozoic detrital zircons in Neoproterozoic strata from Mauritania, West Africa","authors":"D. Bradley, D. Evans, P. O’Sullivan, Cliff D. Taylor, B. Eglington","doi":"10.2475/08.2022.01","DOIUrl":"https://doi.org/10.2475/08.2022.01","url":null,"abstract":"Detrital zircon data are reported from Mesoproterozoic to Ordovician strata from two tectonic domains in Mauritania: 14 samples from the Taoudeni Basin of the West African Craton and 15 samples from the Mauritanide orogen. Taoudeni Basin samples show four sequential, distinctive detrital zircon age distributions, which we refer to as “barcodes”. From old to young these are the Char, Assabet, Téniagouri, and Oujeft barcodes, each named for a constituent stratigraphic unit. Zircon age maxima are as follows, with the dominant ones in italics. The Char barcode, from Mesoproterozoic (ca. 1100 Ma) strata including the Char Group, yielded zircon age maxima at 2941, 2871, 2703, 2447, 2076, and 2041 Ma, all potentially traceable to sources in the West African Craton. The Assabet barcode is from strata, including the eponymous Assabet el Hassiane Group, that were deposited between ca. 883 and ca. 570 Ma; it has age maxima at 2137, 2053, 1769, 1510, 1212, 1021, and 936 Ma and a pronounced minimum during Geon 16 (1699–1600 Ma). The Assabet's Mesoproterozoic to early Neoproterozoic zircons cannot have come from the West African Craton or any of its surrounding orogens. The Téniagouri barcode, which takes its name from the Téniagouri Group, was deposited at ca. 569 Ma; it has dominant maxima at 1983, 1872, 1522, 1215, 1109, 988, and 601 Ma and resembles the Assabet barcode but with the addition of the youngest population. The Oujeft barcode, named for the Oujeft Group, is from strata deposited between 541 or slightly earlier and 444 Ma or younger, has age maxima at 2124, 2053, 1197, 624 and 579 Ma. The Téniagouri and Oujeft barcodes record input from Pan-African orogens. In the Mauritanide orogen, most of the metasedimentary rock units that were sampled yielded detrital zircon age spectra that match one of the Taoudeni Basin barcodes. These results imply new depositional age constraints based on barcode correlation and suggest affinities between Mauritanide strata and the West African Craton. Detrital zircon age distributions that broadly resemble the Assabet barcode occur in the Neoproterozoic of Morocco, Ghana, Greece, Russia, Brazil, and, in the Appalachian orogen of Canada and the United States, Avalonia and Ganderia. The recent Rodinia reconstruction of Evans (2021) restores these far-flung localities to a more compact area, with Avalonia, Ganderia, and other peri-Gondwanan terranes occupying an oblong area between Amazonia, Laurentia, Baltica, and West Africa. Our preferred explanation is that most of these places received detritus via the same continent-scale fluvial system as the West African craton. Among the craton's nearest Rodinia neighbors in the Evans (2021) reconstruction for 900 Ma, Amazonia has known igneous rocks corresponding to all of the major Assabet age populations, and also a lull, though not a complete magmatic gap, during Geon 16. This is consistent with overall north-directed paleocurrents in the Assabet El Hassiane Group and its cor","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46463940","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Mohamed Ghnahalla, A. El Albani, Ahmed Abd Elmola, O. Bankole, C. Fontaine, Mohamed Salem Sabar, A. Trentesaux, Claude Laforest, A. Meunier, Céline Boissard, Chenyi Tu, T. Lyons
Understanding paleoenvironmental dynamics linked to biological evolution in Earth history is a major goal within the geological community. The difficulty of achieving this goal, at least in part, lies with the secondary transformations experienced by a majority of ancient rocks, especially through metamorphism and hydrothermal activity. The Mesoproterozoic (∼1.1 Ga) shallow-marine deposits from the Taoudeni Basin, Mauritania, have suffered a complex, multiphase tectonic, and thermal evolutionary history. Representative samples from two drill cores (a background site [S2] and a dolerite intrusion-bearing drill core [S1]) from the El Mreiti Group were evaluated for transformations and overprints of original mineralogies and geochemical compositions. Our results show that the drill core hosting the dolerite intrusion (S1) is characterized by a suite of minerals (that is, pyroxene, graphite, pyrrhotite, garnet, zeolite, and authigenic clay minerals) resulting from contact metamorphism and associated hydrothermal activity. However, compared to the S1, the S2 core shows no evidence of post-depositional transformation. The geochemical data obtained from S1 reveal a striking elevation of iron contents likely delivered from the hydrothermal fluids. Moreover, concentrations of redox-sensitive trace elements (molybdenum, uranium, and vanadium) increased dramatically during hydrothermal and metamorphic activity. This study demonstrates that need for caution when assessing paleoenvironmental conditions in ancient sedimentary rocks, particularly for iron and trace metal approaches commonly used in reconstructions of paleo-redox.
{"title":"Post-depositional transformations in sedimentary rocks and implications for paleoenvironmental studies: evidence from the Mesoproterozoic (∼1.1 Ga) of the Taoudeni Basin, Mauritania","authors":"Mohamed Ghnahalla, A. El Albani, Ahmed Abd Elmola, O. Bankole, C. Fontaine, Mohamed Salem Sabar, A. Trentesaux, Claude Laforest, A. Meunier, Céline Boissard, Chenyi Tu, T. Lyons","doi":"10.2475/07.2022.02","DOIUrl":"https://doi.org/10.2475/07.2022.02","url":null,"abstract":"Understanding paleoenvironmental dynamics linked to biological evolution in Earth history is a major goal within the geological community. The difficulty of achieving this goal, at least in part, lies with the secondary transformations experienced by a majority of ancient rocks, especially through metamorphism and hydrothermal activity. The Mesoproterozoic (∼1.1 Ga) shallow-marine deposits from the Taoudeni Basin, Mauritania, have suffered a complex, multiphase tectonic, and thermal evolutionary history. Representative samples from two drill cores (a background site [S2] and a dolerite intrusion-bearing drill core [S1]) from the El Mreiti Group were evaluated for transformations and overprints of original mineralogies and geochemical compositions. Our results show that the drill core hosting the dolerite intrusion (S1) is characterized by a suite of minerals (that is, pyroxene, graphite, pyrrhotite, garnet, zeolite, and authigenic clay minerals) resulting from contact metamorphism and associated hydrothermal activity. However, compared to the S1, the S2 core shows no evidence of post-depositional transformation. The geochemical data obtained from S1 reveal a striking elevation of iron contents likely delivered from the hydrothermal fluids. Moreover, concentrations of redox-sensitive trace elements (molybdenum, uranium, and vanadium) increased dramatically during hydrothermal and metamorphic activity. This study demonstrates that need for caution when assessing paleoenvironmental conditions in ancient sedimentary rocks, particularly for iron and trace metal approaches commonly used in reconstructions of paleo-redox.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43682764","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jin Zhang, J. Qu, Beihang Zhang, Heng Zhao, Ronggou Zheng, Jianfeng Liu, Jie Hui, Pengfei Niu, Long Yun, Shuo Zhao, Yiping Zhang
Intracontinental transform structures are important forms of continental deformation, such as the Altyn Tagh fault on Tibetan Plateau. Although many intracontinental transform structures have developed throughout geological history, their identification is relatively difficult due to later deformation and sedimentary covering. Strike-slip faults played an important role in the formation and subsequent transformation of the Central Asian orogenic belt (CAOB). In this study, a group of nearly EW-trending dextral shear zones along the southern CAOB in the Beishan, Alxa, northern margin of the North China Craton and the Great Xing'an Mountains to the east, is reported. Regional strike-slip duplex systems were developed and strongly superimposed on the CAOB in the Beishan and Alxa regions. Meanwhile, to the west of the Beishan, coeval ductile shear zones with the same kinematics also developed along the CAOB. The ages of the shear zones range from 280 Ma to 230 Ma and become younger to the east. This megashear system may also connect with the shortening in the Ural Orogenic belt to the west and the convergence along the eastern margin of the Eurasian continent, which is approximately more than 9000 km long in the Asian continent and consists of an intracontinental transform structure in the central Pangea continent. Further west, the dextral shear system may also connect with the coeval shear zones with the same kinematics along the southern Variscan orogenic belt in Europe and even the South Appalachian Orogenic Belt in the southeastern North America, which we call the Intra-Pangean Megashear (IPM) after Irving (2004). The rotation and approach of the Baltic Craton and Siberian Craton and the northern Pangean lithosphere heated by mantle plumes and its lat.eral (eastward) spreading may have caused the development of the IPM and intracontinental deformation from Pangea B to Pangea A.
{"title":"Determination of an intracontinental transform system along the southern Central Asian orogenic belt in the latest Paleozoic","authors":"Jin Zhang, J. Qu, Beihang Zhang, Heng Zhao, Ronggou Zheng, Jianfeng Liu, Jie Hui, Pengfei Niu, Long Yun, Shuo Zhao, Yiping Zhang","doi":"10.2475/07.2022.01","DOIUrl":"https://doi.org/10.2475/07.2022.01","url":null,"abstract":"Intracontinental transform structures are important forms of continental deformation, such as the Altyn Tagh fault on Tibetan Plateau. Although many intracontinental transform structures have developed throughout geological history, their identification is relatively difficult due to later deformation and sedimentary covering. Strike-slip faults played an important role in the formation and subsequent transformation of the Central Asian orogenic belt (CAOB). In this study, a group of nearly EW-trending dextral shear zones along the southern CAOB in the Beishan, Alxa, northern margin of the North China Craton and the Great Xing'an Mountains to the east, is reported. Regional strike-slip duplex systems were developed and strongly superimposed on the CAOB in the Beishan and Alxa regions. Meanwhile, to the west of the Beishan, coeval ductile shear zones with the same kinematics also developed along the CAOB. The ages of the shear zones range from 280 Ma to 230 Ma and become younger to the east. This megashear system may also connect with the shortening in the Ural Orogenic belt to the west and the convergence along the eastern margin of the Eurasian continent, which is approximately more than 9000 km long in the Asian continent and consists of an intracontinental transform structure in the central Pangea continent. Further west, the dextral shear system may also connect with the coeval shear zones with the same kinematics along the southern Variscan orogenic belt in Europe and even the South Appalachian Orogenic Belt in the southeastern North America, which we call the Intra-Pangean Megashear (IPM) after Irving (2004). The rotation and approach of the Baltic Craton and Siberian Craton and the northern Pangean lithosphere heated by mantle plumes and its lat.eral (eastward) spreading may have caused the development of the IPM and intracontinental deformation from Pangea B to Pangea A.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"69325513","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
B. Ellwood, S. Warny, Rebecca A. Hackworth, Suzanne H. Ellwood, Jonathan H. Tomkin, S. Bentley, D. Braud, G. Clayton
Radiocarbon dating of the two LSU Campus Mounds (16EBR6) indicates that the construction of one, Mound B, began at ∼11,000 BP, making Mound B the oldest known and intact manmade structure in the Americas. The age analyses presented here are based on thirty one 14C dates. The older (deeper) parts of both of the LSU Campus Mounds contain many thin, burned ash lenses, suggesting that the Mounds may have been used for ceremonial or cremation purposes. These ash layers are composed mainly of phytoliths, bio-silicate (SiO2) structural compounds in plants that remained after burning of these plants. Analysis of the abundant ash lenses indicates that the plants burned were mainly C4 hydrophilic grasses that are dominated by 90-98% reed and cane plants. The ash layers also contain microscopic fragments of burned, large mammal osteons (bone). The layers of reed and cane phytoliths, containing very small numbers of osteons, are indicative of very hot fires. This finding supports the argument that the fires were used for ceremonials or cremations. No ash beds later than 5,000 BP are known from either LSU Campus Mound A or B, although at ∼800 calBP, a wooden post (now charcoal) was planted and burned on the top of Mound B. It appears that construction of Mound B began during the climate amelioration that followed the Younger Dryas climate event, which ended at ∼11,700 BP. Construction of Mound A appears to have begun at ∼9,500 calBP. Building of the LSU Campus Mounds shows a hiatus when climate deteriorated during the 8200 Climate Event, which defined the end of the Holocene Greenlandian Stage and the beginning of the Northgrippian Stage. Construction began again at ∼7,500 BP, when both mounds continued construction until ∼6,000 BP, with one apparently anomalous date in Mound A at ∼5,100 calBP.
{"title":"The LSU campus mounds, with construction beginning at ∼11,000 BP, are the oldest known extant man-made structures in the Americas","authors":"B. Ellwood, S. Warny, Rebecca A. Hackworth, Suzanne H. Ellwood, Jonathan H. Tomkin, S. Bentley, D. Braud, G. Clayton","doi":"10.2475/06.2022.02","DOIUrl":"https://doi.org/10.2475/06.2022.02","url":null,"abstract":"Radiocarbon dating of the two LSU Campus Mounds (16EBR6) indicates that the construction of one, Mound B, began at ∼11,000 BP, making Mound B the oldest known and intact manmade structure in the Americas. The age analyses presented here are based on thirty one 14C dates. The older (deeper) parts of both of the LSU Campus Mounds contain many thin, burned ash lenses, suggesting that the Mounds may have been used for ceremonial or cremation purposes. These ash layers are composed mainly of phytoliths, bio-silicate (SiO2) structural compounds in plants that remained after burning of these plants. Analysis of the abundant ash lenses indicates that the plants burned were mainly C4 hydrophilic grasses that are dominated by 90-98% reed and cane plants. The ash layers also contain microscopic fragments of burned, large mammal osteons (bone). The layers of reed and cane phytoliths, containing very small numbers of osteons, are indicative of very hot fires. This finding supports the argument that the fires were used for ceremonials or cremations. No ash beds later than 5,000 BP are known from either LSU Campus Mound A or B, although at ∼800 calBP, a wooden post (now charcoal) was planted and burned on the top of Mound B. It appears that construction of Mound B began during the climate amelioration that followed the Younger Dryas climate event, which ended at ∼11,700 BP. Construction of Mound A appears to have begun at ∼9,500 calBP. Building of the LSU Campus Mounds shows a hiatus when climate deteriorated during the 8200 Climate Event, which defined the end of the Holocene Greenlandian Stage and the beginning of the Northgrippian Stage. Construction began again at ∼7,500 BP, when both mounds continued construction until ∼6,000 BP, with one apparently anomalous date in Mound A at ∼5,100 calBP.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42365039","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Characterized by high Sr/Y (> 40) and La/Yb (> 20) ratios, the adakitic features are, generally, attributed to a garnet-bearing residue. However, adakitic features may be inherited from an adakitic source. In this contribution, we take the Buya granite as an example to demonstrate the effect of source on adakitic features through petrology, zircon U-Pb geochronology and Lu-Hf isotopes, whole-rock geochemistry, and Sr-Nd-Pb isotopes. The 445 Ma Buya granite contains many coeval microgranular magmatic enclaves (MMEs), and some late fine-grained granitic dikes. The host granite and the MMEs display sub-parallel to parallel trace-element patterns characterized by enrichments in LREE and LILE, and depletions in HREE and HFSE, typical of adakitc features. Importantly, the two lithologies possess comparable zircon Hf and whole rock Sr-Nd-Pb isotopic compositions that plot along a mantle array. The MMEs are interpreted as cognate fragments of early-formed crystals and the parental magma was sourced from overriding mantle wedge metasomatized by fluids derived from the subducted Proto-Tethys slab. Comparatively, the dikes, with positive εNd(t) (∼2) values and lower (87Sr/86Sr)t (∼0.705), were products of the partial melting of a plagioclase-rich crustal source. Although the host granite and the MMEs possess adakitic features, the latter have higher La/Yb ratios and relatively lower Sr/Y ratios, which are inconsistent with garnet effect because of similarly high DY/DSr (∼1900) and high DYb/DLa ratios (∼2300) for garnet/melt. Instead, this feature is attributed to the fractional crystallization of hornblende and allanite. Therefore, the host granite and the MMEs may inherit adakitic features from the source. Likewise, the adakitic features of the dikes are attributed to a plagioclase-rich source due to the enrichment in Sr element. The early fractional crystallization of amphibole and biotite in the MMEs can elevate Zr/Sm and lower Nb/Ta ratios, respectively, in the residual liquid to form evolved magma similar to adakites and TTGs. The Buya granite displays an arc affinity, demonstrating the northward subduction of the Proto-Tethys oceanic plate.
以高Sr/Y（>40）和La/Yb（>20）比率为特征的阿达基岩特征通常归因于含石榴石的残留物。然而，阿达基特的特征可能是从阿达基特来源遗传来的。本文以布亚花岗岩为例，通过岩石学、锆石U-Pb地质年代学和Lu-Hf同位素、全岩地球化学和Sr-Nd-Pb同位素，论证了源岩对adakitic特征的影响。445 Ma Buya花岗岩含有许多同时代的细粒岩浆包体（MME）和一些晚期细粒花岗岩脉。寄主花岗岩和MME显示出亚平行至平行的微量元素模式，其特征是LREE和LILE富集，HREE和HFSE贫化，这是典型的阿达基特特征。重要的是，这两种岩性具有相当的锆石Hf和全岩Sr-Nd-Pb同位素组成，沿地幔阵列绘制。MME被解释为早期形成晶体的同源碎片，母岩浆来源于俯冲的原特提斯板块流体交代的覆盖地幔楔。相比之下，具有正εNd（t）（～2）值和较低（87Sr/86Sr）t（～0.705）值的岩脉是富含斜长石的地壳源部分熔融的产物。尽管主花岗岩和MME具有adakitic特征，但后者具有较高的La/Yb比率和相对较低的Sr/Y比率，这与石榴石效应不一致，因为石榴石/熔体的DY/DSr（～1900）和DYb/DLa比率（～2300）同样高。相反，这种特征归因于角闪石和尿囊石的分离结晶。因此，寄主花岗岩和MME可能从源头上继承了adakitic特征。同样，由于Sr元素的富集，岩墙的adakitic特征被归因于富含斜长石的来源。MME中角闪石和黑云母的早期分级结晶可以分别提高残余液体中的Zr/Sm和降低Nb/Ta比率，形成类似于埃达克岩和TTG的演化岩浆。布亚花岗岩表现出弧形亲和性，表明原特提斯洋板块向北俯冲。
{"title":"Effect of source compositions on adakitic features: A case study from the Buya granite, in western Kunlun, NW China","authors":"Peng Wang, Guochun Zhao, Qian Liu, Jinlong Yao, Yigui Han, Jianhua Li","doi":"10.2475/06.2022.03","DOIUrl":"https://doi.org/10.2475/06.2022.03","url":null,"abstract":"Characterized by high Sr/Y (> 40) and La/Yb (> 20) ratios, the adakitic features are, generally, attributed to a garnet-bearing residue. However, adakitic features may be inherited from an adakitic source. In this contribution, we take the Buya granite as an example to demonstrate the effect of source on adakitic features through petrology, zircon U-Pb geochronology and Lu-Hf isotopes, whole-rock geochemistry, and Sr-Nd-Pb isotopes. The 445 Ma Buya granite contains many coeval microgranular magmatic enclaves (MMEs), and some late fine-grained granitic dikes. The host granite and the MMEs display sub-parallel to parallel trace-element patterns characterized by enrichments in LREE and LILE, and depletions in HREE and HFSE, typical of adakitc features. Importantly, the two lithologies possess comparable zircon Hf and whole rock Sr-Nd-Pb isotopic compositions that plot along a mantle array. The MMEs are interpreted as cognate fragments of early-formed crystals and the parental magma was sourced from overriding mantle wedge metasomatized by fluids derived from the subducted Proto-Tethys slab. Comparatively, the dikes, with positive εNd(t) (∼2) values and lower (87Sr/86Sr)t (∼0.705), were products of the partial melting of a plagioclase-rich crustal source. Although the host granite and the MMEs possess adakitic features, the latter have higher La/Yb ratios and relatively lower Sr/Y ratios, which are inconsistent with garnet effect because of similarly high DY/DSr (∼1900) and high DYb/DLa ratios (∼2300) for garnet/melt. Instead, this feature is attributed to the fractional crystallization of hornblende and allanite. Therefore, the host granite and the MMEs may inherit adakitic features from the source. Likewise, the adakitic features of the dikes are attributed to a plagioclase-rich source due to the enrichment in Sr element. The early fractional crystallization of amphibole and biotite in the MMEs can elevate Zr/Sm and lower Nb/Ta ratios, respectively, in the residual liquid to form evolved magma similar to adakites and TTGs. The Buya granite displays an arc affinity, demonstrating the northward subduction of the Proto-Tethys oceanic plate.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47036038","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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