R. McAleer, D. Bish, M. Kunk, P. Valley, G. Walsh, R. Wintsch
A combination of modal estimates from powder X-ray diffraction (XRD) experiments and argon isotopic data shows that muscovite 40Ar/39Ar total gas age correlates with muscovite composition near the retrograde Bald Mountain shear zone (BMSZ) in Claremont, New Hampshire, and that the shear zone was active at ∼245 Ma. Petrologic study demonstrates that chemical disequilibrium is preserved in muscovite grains in these samples. The recognition of this preservation is critical to the interpretation of the 40Ar/39Ar step-heating experiments, which never produce age plateaus and yield spectra with steps that range in age by ∼20 Ma. Petrographic, compositional, and crystallographic data all indicate that the age spectra reflect dissolution of metastable Na-rich muscovite and precipitation of stable Na-poor muscovite associated with deformation in the BMSZ.Comparison of whole rock and muscovite concentrate XRD patterns from individual samples demonstrates that the mineral separation process can fractionate these muscovite populations. Therefore, four muscovite concentrates of varying magnetic susceptibility were prepared from a single hand sample, analyzed by XRD, and dated. These four splits define a mixing line that resolves end-member ages of 244.5 ± 4.2 Ma and 302.5 ± 12.5 Ma (1σ). Although the ages are imprecise, the petrologically supported conclusion that these schists preserve two discrete ages is distinct from an interpretation that the spectra reflect cooling through closure at ∼270 Ma, as might be concluded in the absence of petrologic characterization. The XRD results also demonstrate that, even well above anchizone conditions, petrologic information relevant to 40Ar/39Ar dating is observable in subtle variations in the crystallography of muscovite grains.
{"title":"Unmixing multiple metamorphic muscovite age populations with powder X-ray diffraction and 40Ar/39Ar analysis","authors":"R. McAleer, D. Bish, M. Kunk, P. Valley, G. Walsh, R. Wintsch","doi":"10.2475/03.2021.02","DOIUrl":"https://doi.org/10.2475/03.2021.02","url":null,"abstract":"A combination of modal estimates from powder X-ray diffraction (XRD) experiments and argon isotopic data shows that muscovite 40Ar/39Ar total gas age correlates with muscovite composition near the retrograde Bald Mountain shear zone (BMSZ) in Claremont, New Hampshire, and that the shear zone was active at ∼245 Ma. Petrologic study demonstrates that chemical disequilibrium is preserved in muscovite grains in these samples. The recognition of this preservation is critical to the interpretation of the 40Ar/39Ar step-heating experiments, which never produce age plateaus and yield spectra with steps that range in age by ∼20 Ma. Petrographic, compositional, and crystallographic data all indicate that the age spectra reflect dissolution of metastable Na-rich muscovite and precipitation of stable Na-poor muscovite associated with deformation in the BMSZ.Comparison of whole rock and muscovite concentrate XRD patterns from individual samples demonstrates that the mineral separation process can fractionate these muscovite populations. Therefore, four muscovite concentrates of varying magnetic susceptibility were prepared from a single hand sample, analyzed by XRD, and dated. These four splits define a mixing line that resolves end-member ages of 244.5 ± 4.2 Ma and 302.5 ± 12.5 Ma (1σ). Although the ages are imprecise, the petrologically supported conclusion that these schists preserve two discrete ages is distinct from an interpretation that the spectra reflect cooling through closure at ∼270 Ma, as might be concluded in the absence of petrologic characterization. The XRD results also demonstrate that, even well above anchizone conditions, petrologic information relevant to 40Ar/39Ar dating is observable in subtle variations in the crystallography of muscovite grains.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"332 - 364"},"PeriodicalIF":2.9,"publicationDate":"2021-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46304722","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}
The Early Cretaceous New England-Quebec igneous province is a classic example of postrift magmatism along the eastern North American passive margin. Although a suite of 40Ar/39Ar ages has been available for the Monteregian Hills lobe in the Quebec portion of the New England-Quebec igneous province for many years, only a single high accuracy radiometric age has been published for the Burlington lobe and none for the Taconic lobe in the New England portion of the province. As a result, the timing of and driving mechanisms behind the magmatism have remained unresolved, and a hotspot origin for the entire province persists in the literature. We have dated four dikes and one pluton in the Burlington and Taconic lobes using 40Ar/39Ar and U–Pb geochronology to improve understanding of the age of magmatism in the New England portion of the province. In the Burlington lobe, 40Ar/39Ar plateau ages include a 137.55 ± 1.80 Ma biotite age and a 136.9 ± 4.2 Ma amphibole age for a lamprophyre dike from Charlotte, Vermont, and a 133.6 ± 2.2 Ma biotite age for a lamprophyre dike from Colchester, Vermont. In the Taconic lobe, ages include an 40Ar/39Ar plateau amphibole age of 107.09 ± 1.32 Ma for a lamprophyre dike from Castleton, Vermont, a 122 Ma minimum 40Ar/39Ar biotite age for a lamprophyre dike from Poultney, Vermont, and a 103.13 ± 0.53 Ma LA-ICP-MS U–Pb zircon age from the quartz syenite of the Cuttingsville complex. These results show that magmatism spanned at least 35 Ma, from ∼138 to 103 Ma, which is broadly consistent with the span of magmatism suggested by workers in the 1970s and 1980s based on K–Ar and Rb–Sr ages. This extended span of magmatism for the Burlington and Taconic lobes is in contrast to the brief 1 to 2 Ma episode of magmatism at ∼124 Ma inferred for the Monteregian Hills lobe. The New England-Quebec igneous province has traditionally been attributed to passage of the Great Meteor hotspot. However, given the close proximity of the Burlington and Taconic lobes, the magmatism in these lobes should span only a few Ma if the product of a hotspot. The age data are also difficult to reconcile with a more complex expression of hotspot magmatism in continental lithosphere related to either plume head magmatism or long-distance migration of plume material. Instead, the extended duration of Early Cretaceous New England-Quebec igneous province magmatism in New England may represent an expression of edge-driven convection, a process known to occur along passive margins and inferred to be operating beneath the eastern North American margin today.
{"title":"40Ar/39Ar and LA-ICP-MS U–Pb geochronology for the New England portion of the Early Cretaceous New England-Quebec igneous province: Implications for the postrift evolution of the eastern North American Margin","authors":"J. C. Boemmels, J. Crespi, L. Webb, J. Fosdick","doi":"10.2475/03.2021.03","DOIUrl":"https://doi.org/10.2475/03.2021.03","url":null,"abstract":"The Early Cretaceous New England-Quebec igneous province is a classic example of postrift magmatism along the eastern North American passive margin. Although a suite of 40Ar/39Ar ages has been available for the Monteregian Hills lobe in the Quebec portion of the New England-Quebec igneous province for many years, only a single high accuracy radiometric age has been published for the Burlington lobe and none for the Taconic lobe in the New England portion of the province. As a result, the timing of and driving mechanisms behind the magmatism have remained unresolved, and a hotspot origin for the entire province persists in the literature. We have dated four dikes and one pluton in the Burlington and Taconic lobes using 40Ar/39Ar and U–Pb geochronology to improve understanding of the age of magmatism in the New England portion of the province. In the Burlington lobe, 40Ar/39Ar plateau ages include a 137.55 ± 1.80 Ma biotite age and a 136.9 ± 4.2 Ma amphibole age for a lamprophyre dike from Charlotte, Vermont, and a 133.6 ± 2.2 Ma biotite age for a lamprophyre dike from Colchester, Vermont. In the Taconic lobe, ages include an 40Ar/39Ar plateau amphibole age of 107.09 ± 1.32 Ma for a lamprophyre dike from Castleton, Vermont, a 122 Ma minimum 40Ar/39Ar biotite age for a lamprophyre dike from Poultney, Vermont, and a 103.13 ± 0.53 Ma LA-ICP-MS U–Pb zircon age from the quartz syenite of the Cuttingsville complex. These results show that magmatism spanned at least 35 Ma, from ∼138 to 103 Ma, which is broadly consistent with the span of magmatism suggested by workers in the 1970s and 1980s based on K–Ar and Rb–Sr ages. This extended span of magmatism for the Burlington and Taconic lobes is in contrast to the brief 1 to 2 Ma episode of magmatism at ∼124 Ma inferred for the Monteregian Hills lobe. The New England-Quebec igneous province has traditionally been attributed to passage of the Great Meteor hotspot. However, given the close proximity of the Burlington and Taconic lobes, the magmatism in these lobes should span only a few Ma if the product of a hotspot. The age data are also difficult to reconcile with a more complex expression of hotspot magmatism in continental lithosphere related to either plume head magmatism or long-distance migration of plume material. Instead, the extended duration of Early Cretaceous New England-Quebec igneous province magmatism in New England may represent an expression of edge-driven convection, a process known to occur along passive margins and inferred to be operating beneath the eastern North American margin today.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"365 - 391"},"PeriodicalIF":2.9,"publicationDate":"2021-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48453777","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}
The rock record of organic carbon abundance and its isotopic composition is consistent with the evolution of life more than 3800 million years ago (Ma). Despite this, there are very few insights as to the ecology of this ancient biosphere or to its level of activity. One possible insight, however, comes from the isotopic composition of inorganic and organic carbon in ancient rocks. This isotope record can be used, in principle, to determine the proportion of inorganic carbon entering the oceans that was buried in sediments as organic matter, and thus it helps constrain the activity level of the ancient biosphere. A quantitative analysis of this isotope record, however, requires that we understand how the Earth-surface carbon reservoir has evolved over time, as burial rates of organic matter in marine sediments depend on the input rates of carbon to the oceans. We must also know how organic matter is weathered as a function of atmospheric oxygen concentrations, thus indicating how much of the organic matter in sediments is newly formed or recycled. To explore these issues, a carbon cycle model is developed here that includes an evolving Earth-surface carbon reservoir as well as the oxygen dependency of the organic matter weathering in rocks. The model also allows for the release of CO2 from organic matter during metamorphism and it contains a rock cycle with young and old reservoirs with appropriate transfer fluxes between them. The model shows that before the Great Oxidation Event (GOE) about 2400 Ma, only about 5 percent to 10 percent as much organic matter was buried into marine sediments as compared with today. Such low rates of organic matter burial would be consistent with a subdued marine biosphere. Such a subdued biosphere could possibly be consistent with primary production driven by anoxygenic photosynthesis coupled to an iron cycle. In association with, and in the aftermath of, the GOE, the biosphere likely increased its activity level by an order of magnitude. This large increase would have completely transformed the biology of the Earth and could have resulted from either the evolution and/or expansion of oxygen-producing cyanobacteria or a dramatic increase in the availability of nutrients to fuel oxygenic phototrophs.
{"title":"Carbon cycle evolution before and after the Great Oxidation of the atmosphere","authors":"D. Canfield","doi":"10.2475/03.2021.01","DOIUrl":"https://doi.org/10.2475/03.2021.01","url":null,"abstract":"The rock record of organic carbon abundance and its isotopic composition is consistent with the evolution of life more than 3800 million years ago (Ma). Despite this, there are very few insights as to the ecology of this ancient biosphere or to its level of activity. One possible insight, however, comes from the isotopic composition of inorganic and organic carbon in ancient rocks. This isotope record can be used, in principle, to determine the proportion of inorganic carbon entering the oceans that was buried in sediments as organic matter, and thus it helps constrain the activity level of the ancient biosphere. A quantitative analysis of this isotope record, however, requires that we understand how the Earth-surface carbon reservoir has evolved over time, as burial rates of organic matter in marine sediments depend on the input rates of carbon to the oceans. We must also know how organic matter is weathered as a function of atmospheric oxygen concentrations, thus indicating how much of the organic matter in sediments is newly formed or recycled. To explore these issues, a carbon cycle model is developed here that includes an evolving Earth-surface carbon reservoir as well as the oxygen dependency of the organic matter weathering in rocks. The model also allows for the release of CO2 from organic matter during metamorphism and it contains a rock cycle with young and old reservoirs with appropriate transfer fluxes between them. The model shows that before the Great Oxidation Event (GOE) about 2400 Ma, only about 5 percent to 10 percent as much organic matter was buried into marine sediments as compared with today. Such low rates of organic matter burial would be consistent with a subdued marine biosphere. Such a subdued biosphere could possibly be consistent with primary production driven by anoxygenic photosynthesis coupled to an iron cycle. In association with, and in the aftermath of, the GOE, the biosphere likely increased its activity level by an order of magnitude. This large increase would have completely transformed the biology of the Earth and could have resulted from either the evolution and/or expansion of oxygen-producing cyanobacteria or a dramatic increase in the availability of nutrients to fuel oxygenic phototrophs.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"297 - 331"},"PeriodicalIF":2.9,"publicationDate":"2021-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46622541","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}
Orogens that form at convergent plate boundaries typically consist of accreted rock units that form an incomplete archive of subducted oceanic and continental lithosphere, as well as of deformed lithosphere of the former upper plate. Reading the construction of orogenic architecture forms the key to decipher the pre-orogenic paleogeographic distribution of oceans and continents, as well as bathymetric and topographic features that existed thereon such as igneous plateaus, seamounts, microcontinents, or magmatic arcs. Current classification schemes of orogens divide between settings associated with termination of subduction [continent-continent collision, continent-ocean collision (obduction)] and with ongoing subduction (accretionary orogenesis), alongside intraplate orogens. Perceived diagnostic features for such classifications, particularly of collisional orogenesis, hinge on dynamic interpretations linking downgoing plate paleogeography to upper plate deformation, plate motion changes, or magmatism. Here, we show, however, that Mesozoic-Cenozoic orogens that undergo collision almost all defy these proposed diagnostic features and behave as accretionary orogens instead. To reconstruct paleogeography of subducted and upper plates, we therefore propose an alternative approach to navigating through orogenic architecture: subducted plate units comprise nappes (or mélanges) with Ocean Plate Stratigraphy (OPS) and Continental Plate Stratigraphy (CPS) stripped from their now-subducted or otherwise underthrust lower crustal and mantle lithospheric underpinnings. Upper plate deformation and paleogeography respond to the competition between absolute motions of the upper plate and the subducting slab. Our navigation approach through orogenic architecture aims to avoid a priori dynamic interpretations that link downgoing plate paleogeography to deformation or magmatic responses in the upper plate, to provide an independent basis for geodynamic analysis. From our analysis we identify ‘rules of orogenesis' that link the rules of rigid plate tectonics with the reality of plate deformation. We use these rules for a thought experiment, in which we predict orogenic architecture that will result from subducting the present-day Indian Ocean and colliding the Somali, Madagascar, and Indian margins using a published continental drift scenario for a future supercontinent as basis. We illustrate that our inferred rules (of thumb) generate orogenic architecture that is analogous to elements of modern orogens, unlocking the well-known modern geography as inspiration for developing testable hypotheses that aid interpreting paleogeography from orogens that formed since the birth of plate tectonics.
{"title":"Deciphering paleogeography from orogenic architecture: Constructing orogens in a future supercontinent as thought experiment","authors":"D. V. Hinsbergen, Thomas L. A. Schouten","doi":"10.31223/x5m895","DOIUrl":"https://doi.org/10.31223/x5m895","url":null,"abstract":"Orogens that form at convergent plate boundaries typically consist of accreted rock units that form an incomplete archive of subducted oceanic and continental lithosphere, as well as of deformed lithosphere of the former upper plate. Reading the construction of orogenic architecture forms the key to decipher the pre-orogenic paleogeographic distribution of oceans and continents, as well as bathymetric and topographic features that existed thereon such as igneous plateaus, seamounts, microcontinents, or magmatic arcs. Current classification schemes of orogens divide between settings associated with termination of subduction [continent-continent collision, continent-ocean collision (obduction)] and with ongoing subduction (accretionary orogenesis), alongside intraplate orogens. Perceived diagnostic features for such classifications, particularly of collisional orogenesis, hinge on dynamic interpretations linking downgoing plate paleogeography to upper plate deformation, plate motion changes, or magmatism. Here, we show, however, that Mesozoic-Cenozoic orogens that undergo collision almost all defy these proposed diagnostic features and behave as accretionary orogens instead. To reconstruct paleogeography of subducted and upper plates, we therefore propose an alternative approach to navigating through orogenic architecture: subducted plate units comprise nappes (or mélanges) with Ocean Plate Stratigraphy (OPS) and Continental Plate Stratigraphy (CPS) stripped from their now-subducted or otherwise underthrust lower crustal and mantle lithospheric underpinnings. Upper plate deformation and paleogeography respond to the competition between absolute motions of the upper plate and the subducting slab. Our navigation approach through orogenic architecture aims to avoid a priori dynamic interpretations that link downgoing plate paleogeography to deformation or magmatic responses in the upper plate, to provide an independent basis for geodynamic analysis. From our analysis we identify ‘rules of orogenesis' that link the rules of rigid plate tectonics with the reality of plate deformation. We use these rules for a thought experiment, in which we predict orogenic architecture that will result from subducting the present-day Indian Ocean and colliding the Somali, Madagascar, and Indian margins using a published continental drift scenario for a future supercontinent as basis. We illustrate that our inferred rules (of thumb) generate orogenic architecture that is analogous to elements of modern orogens, unlocking the well-known modern geography as inspiration for developing testable hypotheses that aid interpreting paleogeography from orogens that formed since the birth of plate tectonics.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"955 - 1031"},"PeriodicalIF":2.9,"publicationDate":"2021-01-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46749578","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}
Y. Wan, Shoujie Liu, Zhiyong Song, S. Wilde, Laiming Wang, C. Dong, H. Xie, S. Xie, Jianhua Hou, Wenqian Bai, Dunyi Liu
Qixia is a typical area of early Precambrian basement in eastern Shandong Province, eastern North China Craton. Many TTG (tonalite-trondhjemite-granodiorite) assemblages were once considered to be supracrustal rocks (the Jiaodong Group), and the formation ages of the rocks have only been determined in a few outcrops as shown on the early geological map of the area. We carried out geological mapping, geochemical study and SHRIMP U-Pb zircon dating in order to determine the temporal and spatial distribution and origins of the TTG rocks. In the newly compiled geological map (1:50,000), the main rock types of the Archean basement are ∼2.9 Ga, ∼2.7 Ga and ∼2.5 Ga tonalitic gneisses with local occurrences of trondhjemitic gneisses, granodioritic gneisses, (quartz) dioritic gneisses and meta-gabbro showing the same age range. Supracrustal rocks with ages of ∼2.9 Ga and ∼2.5 Ga are locally identified. All rocks broadly extend in a NW-SE direction as a result of strong tectonothermal events of the late Neoarchean and late Paleoproterozoic. Although the late Paleoproterozoic tectonothermal event strongly influenced all 2.7 to 2.9 Ga rocks in the area, metamorphic zircon ages are not widely recorded in these rocks because the high-grade metamorphism at ca. 2.5 Ga caused the older 2.7 to 2.9 Ga rocks to become relatively dry systems. The three generations of TTG rocks are similar in major element composition, characterized by high Na2O and low K2O, except for the late Neoarchean granodioritic gneisses, which locally occur and are relatively high in K2O. All the TTG rocks of different ages commonly have zircon O isotopic compositions within the range determined by Valley and others (2005) for Archean magmatic zircon. The ∼2.9 Ga TTG rocks show large Sr/Y and La/Yb variations and depletion in whole-rock Nd and zircon Hf isotopic compositions. The ∼2.7 Ga TTG rocks are similar in Nd-Hf isotopic compositions to the ∼2.9 Ga TTG rocks but have low Sr/Y and La/Yb ratios. The ∼2.5 Ga TTG rocks are similar in trace element composition to the ∼2.9 Ga TTG rocks, showing large variations in Sr/Y and La/Yb ratios. They can be further subdivided into two types in terms of Nd-Hf isotopic compositions with the depleted type mainly including tonalitic gneisses [εNd(t) = +1.86 to +4.59, εHf(t) = +1.0 to +8.7] and the enriched type including trondhjemitic and granodioritic gneisses [εNd(t) = −2.38 to −0.06, εHf(t) = −1.6 to −2.9]. It is concluded that the ∼2.9 Ga TTG rocks were formed in an oceanic environment (oceanic plateau or intra-ocean subduction), and the 2.7 Ga TTG rocks were formed by mantle underplating that resulted in partial melting of lower crustal mafic rocks under relatively low pressure conditions. More ancient continental materials played a role in the ∼2.5 Ga magmatic process, but more work is required to determine the tectonic environment (underplating or arc magmatism).
栖霞是华北克拉通东部鲁东早前寒武纪基底的典型地区。许多TTG(闪长岩-闪长岩-花岗闪长岩)组合曾被认为是表壳岩(胶东群)，其形成时代仅在该地区早期地质图的少数露头中确定。通过地质填图、地球化学研究和SHRIMP U-Pb锆石定年，确定了TTG岩石的时空分布和成因。在新编制的1:5万地质图中，太古宙基底的主要岩石类型为~ 2.9 Ga、~ 2.7 Ga和~ 2.5 Ga的调性片麻岩，局部有长闪长片麻岩、花岗闪长片麻岩、(石英)闪长片麻岩和变质辉长岩，具有相同的年龄范围。局部鉴定出年龄为~ 2.9 Ga和~ 2.5 Ga的上地壳岩石。由于新太古代晚期和古元古代晚期强烈的构造热事件，所有岩石均向北西-东南方向广泛伸展。虽然晚古元古代构造热事件强烈影响了该区所有2.7 ~ 2.9 Ga的岩石，但由于2.5 Ga左右的高变质作用使较老的2.7 ~ 2.9 Ga岩石成为相对干燥的体系，变质锆石在这些岩石中没有广泛记录。三代TTG岩石除新太古代晚期花岗闪长片麻岩局部发育且K2O含量相对较高外，主要元素组成相似，均表现为高Na2O、低K2O。所有不同年龄的TTG岩石的锆石O同位素组成都在Valley等(2005)对太古宙岩浆锆石确定的范围内。~ 2.9 Ga TTG岩石表现出较大的Sr/Y和La/Yb变化和全岩Nd和锆石Hf同位素组成的耗损。~ 2.7 Ga TTG岩石的Nd-Hf同位素组成与~ 2.9 Ga TTG岩石相似，但Sr/Y和La/Yb比值较低。~ 2.5 Ga TTG岩石的微量元素组成与~ 2.9 Ga TTG岩石相似，Sr/Y和La/Yb比值变化较大。根据Nd-Hf同位素组成可进一步分为两类，贫型主要为调性片麻岩[εNd(t) = +1.86 ~ +4.59， εHf(t) = +1.0 ~ +8.7]，富型主要为长闪质和花岗闪长片麻岩[εNd(t) = - 2.38 ~ - 0.06， εHf(t) = - 1.6 ~ - 2.9]。结果表明，~ 2.9 Ga TTG岩形成于海洋环境(海洋高原或洋内俯冲)，2.7 Ga TTG岩形成于相对低压条件下地幔底沉降导致下地壳基性岩部分熔融作用。更古老的大陆物质在~ 2.5 Ga岩浆过程中发挥了作用，但需要更多的工作来确定构造环境(底板或弧岩浆作用)。
{"title":"The complexities of Mesoarchean to late Paleoproterozoic magmatism and metamorphism in the Qixia area, eastern North China Craton: Geology, geochemistry and SHRIMP U-Pb zircon dating","authors":"Y. Wan, Shoujie Liu, Zhiyong Song, S. Wilde, Laiming Wang, C. Dong, H. Xie, S. Xie, Jianhua Hou, Wenqian Bai, Dunyi Liu","doi":"10.2475/01.2021.01","DOIUrl":"https://doi.org/10.2475/01.2021.01","url":null,"abstract":"Qixia is a typical area of early Precambrian basement in eastern Shandong Province, eastern North China Craton. Many TTG (tonalite-trondhjemite-granodiorite) assemblages were once considered to be supracrustal rocks (the Jiaodong Group), and the formation ages of the rocks have only been determined in a few outcrops as shown on the early geological map of the area. We carried out geological mapping, geochemical study and SHRIMP U-Pb zircon dating in order to determine the temporal and spatial distribution and origins of the TTG rocks. In the newly compiled geological map (1:50,000), the main rock types of the Archean basement are ∼2.9 Ga, ∼2.7 Ga and ∼2.5 Ga tonalitic gneisses with local occurrences of trondhjemitic gneisses, granodioritic gneisses, (quartz) dioritic gneisses and meta-gabbro showing the same age range. Supracrustal rocks with ages of ∼2.9 Ga and ∼2.5 Ga are locally identified. All rocks broadly extend in a NW-SE direction as a result of strong tectonothermal events of the late Neoarchean and late Paleoproterozoic. Although the late Paleoproterozoic tectonothermal event strongly influenced all 2.7 to 2.9 Ga rocks in the area, metamorphic zircon ages are not widely recorded in these rocks because the high-grade metamorphism at ca. 2.5 Ga caused the older 2.7 to 2.9 Ga rocks to become relatively dry systems. The three generations of TTG rocks are similar in major element composition, characterized by high Na2O and low K2O, except for the late Neoarchean granodioritic gneisses, which locally occur and are relatively high in K2O. All the TTG rocks of different ages commonly have zircon O isotopic compositions within the range determined by Valley and others (2005) for Archean magmatic zircon. The ∼2.9 Ga TTG rocks show large Sr/Y and La/Yb variations and depletion in whole-rock Nd and zircon Hf isotopic compositions. The ∼2.7 Ga TTG rocks are similar in Nd-Hf isotopic compositions to the ∼2.9 Ga TTG rocks but have low Sr/Y and La/Yb ratios. The ∼2.5 Ga TTG rocks are similar in trace element composition to the ∼2.9 Ga TTG rocks, showing large variations in Sr/Y and La/Yb ratios. They can be further subdivided into two types in terms of Nd-Hf isotopic compositions with the depleted type mainly including tonalitic gneisses [εNd(t) = +1.86 to +4.59, εHf(t) = +1.0 to +8.7] and the enriched type including trondhjemitic and granodioritic gneisses [εNd(t) = −2.38 to −0.06, εHf(t) = −1.6 to −2.9]. It is concluded that the ∼2.9 Ga TTG rocks were formed in an oceanic environment (oceanic plateau or intra-ocean subduction), and the 2.7 Ga TTG rocks were formed by mantle underplating that resulted in partial melting of lower crustal mafic rocks under relatively low pressure conditions. More ancient continental materials played a role in the ∼2.5 Ga magmatic process, but more work is required to determine the tectonic environment (underplating or arc magmatism).","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"1 - 82"},"PeriodicalIF":2.9,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"69322075","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}
Xiaoxia Wang, Tao Wang, C. Ke, Yang Yang, Yongfei Tian
The Qinling orogen, one of the most important orogens in Asia, belongs to the northeastern part of the Tethyan orogen. The architecture and processes of the Qinling orogen remain controversial. In this study, we present 15 new zircon U–Pb ages, 20 whole-rock geochemical and 46 Sm-Nd isotopic analyses, and 30 zircon Lu–Hf isotopic data for early Mesozoic granitoids in this orogen, combining with data from literature, to delineate the crustal architecture and processes of the orogen. A total of 181 zircon U–Pb ages show three phase (252–230, 230–198, and 190–185 Ma) of granitoids. The first-phase granitoids occur mainly in the westernmost segment of the orogen and formed in a subduction setting during the closure of the Mianlue Ocean (a northern branch of the Paleo-Tethyan Ocean). The second- and third-phase granitoids, distributed in the middle to eastern parts of the Qinling orogen, were generated in late syn-collisional and post-collisional tectonic settings, respectively. Whole-rock Nd and zircon Hf isotopic mapping of these granitoids yield six and seven isotopic provinces, respectively. These provinces display that the southern margin of the North China Block and the northern margin of the South China Block are dominated by ancient deep crust, that is, early Paleoproterozoic (2.3–1.8 Ga) and late Paleoproterozoic (∼1.7 Ga) components, respectively. By way of camparison, the North Qinling contains younger Mesoproterozoic [εNd(t) = −10.7 to −0.2; TDM = 1.4–1.0 Ga] basement, evidencing that it is an independent terrane different from the North China Block. The isotopic mapping also reveals a deep-seated NNE–SSW-trending zoned architecture that is approximately perpendicular to the WNW–ENE-trending of the orogen. This provides new evidence for the “Spaghetti Junction model” for the Qinling orogen. The old Nd (2.2–1.0 Ga, mostly 2.0–1.2 Ga) and Hf (2.3–0.8 Ga, mostly 2.0–1.2 Ga) model ages indicate that the continental growth in this orogen occurred mainly during the Paleoproterozoic and Mesoproterozoic, with only minor amounts of juvenile [εNd(t) = ∼0, TDM = ∼0.1 Ga] continental growth along the Shangdan and Mianlue sutures. These characteristics suggest that the Qinling orogen is dominantly formed by the collision of ancient continental blocks, distinct from some typical accretionary orogens, such as the Central Asian Orogenic Belt with voluminous juvenile crust.
{"title":"Timing and Nd-Hf isotopic mapping of early Mesozoic granitoids in the Qinling Orogen, central China: Implication for architecture, nature and processes of the orogen","authors":"Xiaoxia Wang, Tao Wang, C. Ke, Yang Yang, Yongfei Tian","doi":"10.2475/01.2021.03","DOIUrl":"https://doi.org/10.2475/01.2021.03","url":null,"abstract":"The Qinling orogen, one of the most important orogens in Asia, belongs to the northeastern part of the Tethyan orogen. The architecture and processes of the Qinling orogen remain controversial. In this study, we present 15 new zircon U–Pb ages, 20 whole-rock geochemical and 46 Sm-Nd isotopic analyses, and 30 zircon Lu–Hf isotopic data for early Mesozoic granitoids in this orogen, combining with data from literature, to delineate the crustal architecture and processes of the orogen. A total of 181 zircon U–Pb ages show three phase (252–230, 230–198, and 190–185 Ma) of granitoids. The first-phase granitoids occur mainly in the westernmost segment of the orogen and formed in a subduction setting during the closure of the Mianlue Ocean (a northern branch of the Paleo-Tethyan Ocean). The second- and third-phase granitoids, distributed in the middle to eastern parts of the Qinling orogen, were generated in late syn-collisional and post-collisional tectonic settings, respectively. Whole-rock Nd and zircon Hf isotopic mapping of these granitoids yield six and seven isotopic provinces, respectively. These provinces display that the southern margin of the North China Block and the northern margin of the South China Block are dominated by ancient deep crust, that is, early Paleoproterozoic (2.3–1.8 Ga) and late Paleoproterozoic (∼1.7 Ga) components, respectively. By way of camparison, the North Qinling contains younger Mesoproterozoic [εNd(t) = −10.7 to −0.2; TDM = 1.4–1.0 Ga] basement, evidencing that it is an independent terrane different from the North China Block. The isotopic mapping also reveals a deep-seated NNE–SSW-trending zoned architecture that is approximately perpendicular to the WNW–ENE-trending of the orogen. This provides new evidence for the “Spaghetti Junction model” for the Qinling orogen. The old Nd (2.2–1.0 Ga, mostly 2.0–1.2 Ga) and Hf (2.3–0.8 Ga, mostly 2.0–1.2 Ga) model ages indicate that the continental growth in this orogen occurred mainly during the Paleoproterozoic and Mesoproterozoic, with only minor amounts of juvenile [εNd(t) = ∼0, TDM = ∼0.1 Ga] continental growth along the Shangdan and Mianlue sutures. These characteristics suggest that the Qinling orogen is dominantly formed by the collision of ancient continental blocks, distinct from some typical accretionary orogens, such as the Central Asian Orogenic Belt with voluminous juvenile crust.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"118 - 151"},"PeriodicalIF":2.9,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"69322090","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}
S. Wilde, Shoujie Liu, Y. Rojas‐Agramonte, Guochun Zhao
{"title":"This Issue Is Dedicated To The Memory Of Distinguished Scientist Alfred Kroner Who Sadly Passed Away On 22 May 2019","authors":"S. Wilde, Shoujie Liu, Y. Rojas‐Agramonte, Guochun Zhao","doi":"10.2475/01.2021.08","DOIUrl":"https://doi.org/10.2475/01.2021.08","url":null,"abstract":"","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"vii - xi"},"PeriodicalIF":2.9,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"69322232","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}
The Epupa Complex of northern Namibia constitutes the south-western margin of the Archean Congo Craton of central Africa. We present new petrological and geochronological data for metasedimentary migmatites that decode the poorly-known Paleoproterozoic to Mesoproterozoic evolution of this remote part of the craton. Detrital magmatic zircons with concordant 207Pb/206Pb ages between 1898 and 1774 Ma are interpreted to demonstrate the formation of the metasedimentary protoliths through reworking of Paleoproterozoic granitoids of a magmatic arc in a back-arc basin. SHRIMP U-Pb zircon analyses of metamorphic rims around the detrital grains linked with constraints from pseudosection modelling revealed three distinct high-grade metamorphic events in the migmatites. Late Paleoproterozoic regional HT-LP metamorphism between 1740 and 1720 Ma constitutes the oldest event and only affected the rocks of the northernmost part of the Epupa Complex (Eyao Unit). Heating to mid-amphibolite facies peak P-T conditions of 720 °C and 4 kbar caused the partial replacement of early biotite-sillimanite by garnet-cordierite assemblages during melt-producing reactions in metapelites. The near-isobaric heating-cooling P-T paths and the high geothermal gradient (60 °C/km) are consistent with HP-LP metamorphism in a back-arc basin setting with up-rising basic melt as heat source. Early Mesoproterozoic (1530 Ma) HT-LP metamorphism is recorded by rare Mg-rich Opx-Crd rocks that are restricted to the westernmost part of the Eyao Unit. Near-isobaric P-T paths culminate at granulite facies peak-conditions of 830 °C and 2 kbar. This local HP-LP event is interpreted to record contact metamorphism related to the coeval emplacement of early-Mesoproterozoic granitoids or the emplacement of intimately associated gabbros. Metapelitic Grt-Bt-Sil migmatites record the third high-grade metamorphic event that was active at ca. 1330 Ma and is restricted to the southern part of the Epupa Complex (Orue Unit). Sillimanite pseudomorphs after kyanite and late cordierite coronas around garnet indicate a clockwise P-T path during regional upper-amphibolite facies metamorphism that peaks at lower crustal conditions of 770 °C and 7.5 kbar. The clockwise P-T path is interpreted to reflect crustal thickening through magmatic accretion of basic melts that produced the coeval Kunene Intrusive Complex, the largest massif-type anorthosite in the world. The metamorphic events detected in the migmatites record a change of the geotectonic position of the Epupa Complex from a convergent continental margin during the late Paleoproterozoic to an intracratonic position in the Mesoproterozoic. The migmatites of the Eyao Unit were affected by an intense hydrothermal alteration during the Pan-African orogeny (ca. 520 Ma) that also caused the partial re-setting of the U-Pb zircon isotope system.
{"title":"Unravelling the P-T-t history of three high-grade metamorphic events in the Epupa Complex, NW Namibia: Implications for the Paleoproterozoic to Mesoproterozoic evolution of the Congo Craton","authors":"S. Brandt, R. Klemd, H. Xie, Patrick Bobek","doi":"10.2475/01.2021.07","DOIUrl":"https://doi.org/10.2475/01.2021.07","url":null,"abstract":"The Epupa Complex of northern Namibia constitutes the south-western margin of the Archean Congo Craton of central Africa. We present new petrological and geochronological data for metasedimentary migmatites that decode the poorly-known Paleoproterozoic to Mesoproterozoic evolution of this remote part of the craton. Detrital magmatic zircons with concordant 207Pb/206Pb ages between 1898 and 1774 Ma are interpreted to demonstrate the formation of the metasedimentary protoliths through reworking of Paleoproterozoic granitoids of a magmatic arc in a back-arc basin. SHRIMP U-Pb zircon analyses of metamorphic rims around the detrital grains linked with constraints from pseudosection modelling revealed three distinct high-grade metamorphic events in the migmatites. Late Paleoproterozoic regional HT-LP metamorphism between 1740 and 1720 Ma constitutes the oldest event and only affected the rocks of the northernmost part of the Epupa Complex (Eyao Unit). Heating to mid-amphibolite facies peak P-T conditions of 720 °C and 4 kbar caused the partial replacement of early biotite-sillimanite by garnet-cordierite assemblages during melt-producing reactions in metapelites. The near-isobaric heating-cooling P-T paths and the high geothermal gradient (60 °C/km) are consistent with HP-LP metamorphism in a back-arc basin setting with up-rising basic melt as heat source. Early Mesoproterozoic (1530 Ma) HT-LP metamorphism is recorded by rare Mg-rich Opx-Crd rocks that are restricted to the westernmost part of the Eyao Unit. Near-isobaric P-T paths culminate at granulite facies peak-conditions of 830 °C and 2 kbar. This local HP-LP event is interpreted to record contact metamorphism related to the coeval emplacement of early-Mesoproterozoic granitoids or the emplacement of intimately associated gabbros. Metapelitic Grt-Bt-Sil migmatites record the third high-grade metamorphic event that was active at ca. 1330 Ma and is restricted to the southern part of the Epupa Complex (Orue Unit). Sillimanite pseudomorphs after kyanite and late cordierite coronas around garnet indicate a clockwise P-T path during regional upper-amphibolite facies metamorphism that peaks at lower crustal conditions of 770 °C and 7.5 kbar. The clockwise P-T path is interpreted to reflect crustal thickening through magmatic accretion of basic melts that produced the coeval Kunene Intrusive Complex, the largest massif-type anorthosite in the world. The metamorphic events detected in the migmatites record a change of the geotectonic position of the Epupa Complex from a convergent continental margin during the late Paleoproterozoic to an intracratonic position in the Mesoproterozoic. The migmatites of the Eyao Unit were affected by an intense hydrothermal alteration during the Pan-African orogeny (ca. 520 Ma) that also caused the partial re-setting of the U-Pb zircon isotope system.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"235 - 296"},"PeriodicalIF":2.9,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"69322225","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}
The Xar Moron River fault zone, located in the eastern segment of the Central Asian Orogenic Belt (CAOB), represents the intensely debated final collision zone of the Siberian Craton (SC) and North China Craton (NCC). To determine the tectonic evolution of the eastern segment of the CAOB, we undertook petrography and zircon U-Pb dating of the Huanggangliang and Linxi formations in the Wufendi and Xingfuzhilu areas along the Xar Moron River. Petrographic analysis of Permian sandstones revealed a close relationship between the sedimentary and orogenic sources suggesting short transport distances. A sample from the Huanggangliang Formation yielded detrital zircon U-Pb ages ranging from 2653 Ma to 265 Ma, with three age populations: at 2653 to 2443 Ma, 1935 to 1764 Ma, and 482 to 265 Ma, whereas samples from the Linxi Formation yielded detrital zircon U-Pb ages ranging from 3363 Ma to 257 Ma, with four age populations: at 2705 to 2403 Ma, 2011 to 1203 Ma, 571 to 375 Ma, and 356 to 257 Ma. The age spectrum differences of sandstones on both banks indicate that the Xar Moron River fault zone is the final collision zone of the eastern segment of the CAOB. The sandstone of Huanggangliang Formation yielded a weighted mean age of 265.7 ± 1.5 Ma, suggesting that the main deposition of the Huanggangliang Formation was during the Middle Permian. In addition, a comparison of the youngest age in the sedimentary rocks with U-Pb ages obtained for pyroclastic rock implies that the Linxi Formation formed in the late Permian. The results of our study support the view that the final closure of the eastern segment of Paleo-Asian Ocean (PAO) occurred during late Permian to earliest Triassic times.
{"title":"SHRIMP U-Pb dating of detrital zircons from the Permian sandstones along the southern and northern margins of Xar Moron River, central inner Mogolia: Implications for provenance and the tectonic evolution of the eastern segment of the Central Asian Orogenic Belt","authors":"Xiancang Wu, Yuruo Shi, J. Anderson","doi":"10.2475/01.2021.04","DOIUrl":"https://doi.org/10.2475/01.2021.04","url":null,"abstract":"The Xar Moron River fault zone, located in the eastern segment of the Central Asian Orogenic Belt (CAOB), represents the intensely debated final collision zone of the Siberian Craton (SC) and North China Craton (NCC). To determine the tectonic evolution of the eastern segment of the CAOB, we undertook petrography and zircon U-Pb dating of the Huanggangliang and Linxi formations in the Wufendi and Xingfuzhilu areas along the Xar Moron River. Petrographic analysis of Permian sandstones revealed a close relationship between the sedimentary and orogenic sources suggesting short transport distances. A sample from the Huanggangliang Formation yielded detrital zircon U-Pb ages ranging from 2653 Ma to 265 Ma, with three age populations: at 2653 to 2443 Ma, 1935 to 1764 Ma, and 482 to 265 Ma, whereas samples from the Linxi Formation yielded detrital zircon U-Pb ages ranging from 3363 Ma to 257 Ma, with four age populations: at 2705 to 2403 Ma, 2011 to 1203 Ma, 571 to 375 Ma, and 356 to 257 Ma. The age spectrum differences of sandstones on both banks indicate that the Xar Moron River fault zone is the final collision zone of the eastern segment of the CAOB. The sandstone of Huanggangliang Formation yielded a weighted mean age of 265.7 ± 1.5 Ma, suggesting that the main deposition of the Huanggangliang Formation was during the Middle Permian. In addition, a comparison of the youngest age in the sedimentary rocks with U-Pb ages obtained for pyroclastic rock implies that the Linxi Formation formed in the late Permian. The results of our study support the view that the final closure of the eastern segment of Paleo-Asian Ocean (PAO) occurred during late Permian to earliest Triassic times.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"152 - 177"},"PeriodicalIF":2.9,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"69322139","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}
Pengfei Li, M. Sun, C. Yuan, F. Jourdan, Wan-Long Hu, Yingde Jiang
The evolution of the largest accretionary orogen in the world, the Central Asian Orogenic Belt (CAOB), involved a prolonged accretion history since the Neoproterozoic, followed by a collisional phase in response to the closure of the Paleo-Asian Ocean in the latest Paleozoic. The exact process for the tectonic transition from subduction to collision is still poorly constrained. Here we address this issue by investigating the late Paleozoic tectonic evolution of the Chinese Altai and Tianshan orogens in the western CAOB. We provide new geochronological data from two areas of the Chinese Altai and Tianshan orogens, which allow us to link polyphase deformation with orogenic processes. In the Fuyun area of the Chinese Altai Orogen, we conducted monazite U-Pb dating on four samples that show pervasive foliations with the originally sub-horizontal orientation (DS2/DQ2). The monazite U-Pb ages cluster at ∼284 to 281 Ma, which interpreted to represent the time of sub-horizontal foliations (DS2/DQ2) that may result from orogen-parallel extension related to the collision of the Chinese Altai Orogen with the East Junggar Terrane. Farther south, in the Gangou area of the Chinese Tianshan Orogen, we obtained a muscovite 40Ar/39Ar plateau age of 256.6 ± 0.6 Ma for a mica schist from the dextral South Central Tianshan Shear Zone. This age confirms the Permian activity of dextral strike-slip deformation (DCT4) in the Chinese Tianshan Orogen. In contrast, three mylonitic schist/granitoid samples from the dextral Main Tianshan Shear Zone are characterized by 40Ar/39Ar plateau ages of 353.9 ± 1.9 Ma (biotite), 353.9 ± 1.5 Ma (biotite) and 352.1 ± 0.7 Ma (muscovite). We interpret these early Carboniferous ages to either represent a pre-Permian dextral shearing event, or to record an early Carboniferous tectono-thermal event with recrystallized micas not reset during the Permian strike-slip deformation (DCT4). An additional 40Ar/39Ar plateau age of 280.9 ± 0.5 Ma (hornblende) from a mafic dike (dolerite) that crosscuts macroscopic folds (DST2) in the southern Chinese Tianshan Orogen, provides a minimum time constraint for these folds. This age supports the simultaneous folding deformation (DST2) with dextral shearing (DCT4) in the Chinese Tianshan Orogen. Combined with a comprehensive synthesis of available geological and geochronological data, we argue that orogen-parallel extension and transpressional tectonics might have played a significant role in the late Paleozoic arc/continental amalgamation of the western CAOB.
{"title":"Late Paleozoic tectonic transition from subduction to collision in the Chinese Altai and Tianshan (Central Asia): New geochronological constraints","authors":"Pengfei Li, M. Sun, C. Yuan, F. Jourdan, Wan-Long Hu, Yingde Jiang","doi":"10.2475/01.2021.05","DOIUrl":"https://doi.org/10.2475/01.2021.05","url":null,"abstract":"The evolution of the largest accretionary orogen in the world, the Central Asian Orogenic Belt (CAOB), involved a prolonged accretion history since the Neoproterozoic, followed by a collisional phase in response to the closure of the Paleo-Asian Ocean in the latest Paleozoic. The exact process for the tectonic transition from subduction to collision is still poorly constrained. Here we address this issue by investigating the late Paleozoic tectonic evolution of the Chinese Altai and Tianshan orogens in the western CAOB. We provide new geochronological data from two areas of the Chinese Altai and Tianshan orogens, which allow us to link polyphase deformation with orogenic processes. In the Fuyun area of the Chinese Altai Orogen, we conducted monazite U-Pb dating on four samples that show pervasive foliations with the originally sub-horizontal orientation (DS2/DQ2). The monazite U-Pb ages cluster at ∼284 to 281 Ma, which interpreted to represent the time of sub-horizontal foliations (DS2/DQ2) that may result from orogen-parallel extension related to the collision of the Chinese Altai Orogen with the East Junggar Terrane. Farther south, in the Gangou area of the Chinese Tianshan Orogen, we obtained a muscovite 40Ar/39Ar plateau age of 256.6 ± 0.6 Ma for a mica schist from the dextral South Central Tianshan Shear Zone. This age confirms the Permian activity of dextral strike-slip deformation (DCT4) in the Chinese Tianshan Orogen. In contrast, three mylonitic schist/granitoid samples from the dextral Main Tianshan Shear Zone are characterized by 40Ar/39Ar plateau ages of 353.9 ± 1.9 Ma (biotite), 353.9 ± 1.5 Ma (biotite) and 352.1 ± 0.7 Ma (muscovite). We interpret these early Carboniferous ages to either represent a pre-Permian dextral shearing event, or to record an early Carboniferous tectono-thermal event with recrystallized micas not reset during the Permian strike-slip deformation (DCT4). An additional 40Ar/39Ar plateau age of 280.9 ± 0.5 Ma (hornblende) from a mafic dike (dolerite) that crosscuts macroscopic folds (DST2) in the southern Chinese Tianshan Orogen, provides a minimum time constraint for these folds. This age supports the simultaneous folding deformation (DST2) with dextral shearing (DCT4) in the Chinese Tianshan Orogen. Combined with a comprehensive synthesis of available geological and geochronological data, we argue that orogen-parallel extension and transpressional tectonics might have played a significant role in the late Paleozoic arc/continental amalgamation of the western CAOB.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"178 - 205"},"PeriodicalIF":2.9,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"69322155","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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