The Baoyintu uplift is located at the northwestern margin of the North China Craton(NCC). The affiliation and evolution history of the uplift have been unresolved until now. Here we present LA-ICP-MS and SHRIMP U-Pb d...The Baoyintu uplift is located at the northwestern margin of the North China Craton(NCC). The affiliation and evolution history of the uplift have been unresolved until now. Here we present LA-ICP-MS and SHRIMP U-Pb data for zircons extracted from samples of monzogranitic orthogneiss and the Baoyintu Group in the uplift. The 1 679 ± 13 Ma date for the orthogneiss demonstrates that Late Palaeoproterozoic rocks do exist in the uplift. The ca. 1 413 Ma minimum age of detrital zircons in metasedimentary rocks shows that the maximum age of the Baoyintu Group is Mesoproterozoic. The age distribution of detrital zircons in the first unit of the Group form clusters at ca. 2 680, 2 450, 1 800 and 1 560 Ma, which partly correspond to tectonic-magmatic-metamorphic events previously recognised in the NCC, and show that the metasedimentary rocks are largely sourced from different parts of the NCC. These confirm that the Baoyintu uplift is part of the NCC. Amphibolite with a dolerite protolith from the third unit of Baoyintu Group shows evidence for metamorphism during ca. 877–851 Ma. This combined with published geochronological data indicate the presence of Neoproterozoic magmatic and metamorphic events in the NCC, which significantly increases our understanding of the Proterozoic tectonic evolution of the region and its possible correlation with the Rodinian Supercontinent.展开更多
Garnet amphibolite in the Baoyintu Group,Inner Mongolia, is mainly composed of garnet, hornblende,plagioclase, quartz and minor rutile/ilmenite. Garnet occurs as porphyroblasts surrounded by plagioclase, forming typic...Garnet amphibolite in the Baoyintu Group,Inner Mongolia, is mainly composed of garnet, hornblende,plagioclase, quartz and minor rutile/ilmenite. Garnet occurs as porphyroblasts surrounded by plagioclase, forming typical ‘‘white-eye socket'' texture. Garnet shows grossular content of 0.26–0.28 and pyrope of 0.11–0.13, without significant zoning. Plagioclase is generally zoned with anorthite(An) increasing from core to rim. The P–T pseudosection calculated using THERMOCALC in the system Mn NCKFMASHTO shows that the garnet amphibolite experienced a clockwise P–T path with a peak at*1.3 GPa/725 °C defined from the minimum An content in plagioclase and maximum pyrope content in garnet,followed by an isothermal decompression. LA-ICP-MS zircon dating for the garnet amphibolite indicates metamorphic ages of 399 ± 6 Ma. The peak P–T condition corresponds to a thermal gradient of *18 °C/km, indicating typical medium-pressure type that commonly occurs in orogenic process with crustal thickening. We prefer to interpret this orogenic event to be a result of collision,following the closure of the Paleo-Asian Ocean before theearly Devonian although there is a popular view that the Paleo-Asian Ocean may have lasted to the early Mesozoic.展开更多
Although porphyroblast microstructures play an important role in structural and metamorphic studies, there are still controversies in the interpretation. The focus is how porphyroblasts grow during deformation and met...Although porphyroblast microstructures play an important role in structural and metamorphic studies, there are still controversies in the interpretation. The focus is how porphyroblasts grow during deformation and metamorphism. In this paper, we introduce a new approach, the Synchrotron Radiation X-Ray Fluorescence, to a hemi-quantitative interpretation of the growth mode of porphyroblasts. The analysis was done at the Beijing Synchrotron Radiation Facility. The specimens were sampled from metapelite of the Baoyintu Group, northern Urad Middle Banner, Inner Mongolia. The new method is successful for determining the microscopic distribution of trace elements in porphybroblasts. The results support the theory of deformation partition, which has been brought forth by Bell and his colleagues, and demonstrate the existence of porphyroblast growth phases and the growth mode of porphyroblasts by hemi-quantitative mineral chemical analysis. The porphyroblast grows stage by stage in the manner of the distribution of a roseleaf and is controlled by deformation. We call the growth stage of porphyroblast a growth phase.展开更多
基金financially supported by the National Natural Science Foundation of China (No.42002099)the Natural Science Foundation of Shandong Province (Nos.ZR2018QD002,ZR2020QD031)the Key Technology Research and Development Program of the Shandong Province (No.2019GSF109101)。
文摘The Baoyintu uplift is located at the northwestern margin of the North China Craton(NCC). The affiliation and evolution history of the uplift have been unresolved until now. Here we present LA-ICP-MS and SHRIMP U-Pb data for zircons extracted from samples of monzogranitic orthogneiss and the Baoyintu Group in the uplift. The 1 679 ± 13 Ma date for the orthogneiss demonstrates that Late Palaeoproterozoic rocks do exist in the uplift. The ca. 1 413 Ma minimum age of detrital zircons in metasedimentary rocks shows that the maximum age of the Baoyintu Group is Mesoproterozoic. The age distribution of detrital zircons in the first unit of the Group form clusters at ca. 2 680, 2 450, 1 800 and 1 560 Ma, which partly correspond to tectonic-magmatic-metamorphic events previously recognised in the NCC, and show that the metasedimentary rocks are largely sourced from different parts of the NCC. These confirm that the Baoyintu uplift is part of the NCC. Amphibolite with a dolerite protolith from the third unit of Baoyintu Group shows evidence for metamorphism during ca. 877–851 Ma. This combined with published geochronological data indicate the presence of Neoproterozoic magmatic and metamorphic events in the NCC, which significantly increases our understanding of the Proterozoic tectonic evolution of the region and its possible correlation with the Rodinian Supercontinent.
基金supported by the National Basic Research Program of China(‘‘973’’Program)(2013CB429801)the China Survey of Geology(1212011121077)
文摘Garnet amphibolite in the Baoyintu Group,Inner Mongolia, is mainly composed of garnet, hornblende,plagioclase, quartz and minor rutile/ilmenite. Garnet occurs as porphyroblasts surrounded by plagioclase, forming typical ‘‘white-eye socket'' texture. Garnet shows grossular content of 0.26–0.28 and pyrope of 0.11–0.13, without significant zoning. Plagioclase is generally zoned with anorthite(An) increasing from core to rim. The P–T pseudosection calculated using THERMOCALC in the system Mn NCKFMASHTO shows that the garnet amphibolite experienced a clockwise P–T path with a peak at*1.3 GPa/725 °C defined from the minimum An content in plagioclase and maximum pyrope content in garnet,followed by an isothermal decompression. LA-ICP-MS zircon dating for the garnet amphibolite indicates metamorphic ages of 399 ± 6 Ma. The peak P–T condition corresponds to a thermal gradient of *18 °C/km, indicating typical medium-pressure type that commonly occurs in orogenic process with crustal thickening. We prefer to interpret this orogenic event to be a result of collision,following the closure of the Paleo-Asian Ocean before theearly Devonian although there is a popular view that the Paleo-Asian Ocean may have lasted to the early Mesozoic.
基金supported by the National Natural Science Foundation of China(grant 40102020).
文摘Although porphyroblast microstructures play an important role in structural and metamorphic studies, there are still controversies in the interpretation. The focus is how porphyroblasts grow during deformation and metamorphism. In this paper, we introduce a new approach, the Synchrotron Radiation X-Ray Fluorescence, to a hemi-quantitative interpretation of the growth mode of porphyroblasts. The analysis was done at the Beijing Synchrotron Radiation Facility. The specimens were sampled from metapelite of the Baoyintu Group, northern Urad Middle Banner, Inner Mongolia. The new method is successful for determining the microscopic distribution of trace elements in porphybroblasts. The results support the theory of deformation partition, which has been brought forth by Bell and his colleagues, and demonstrate the existence of porphyroblast growth phases and the growth mode of porphyroblasts by hemi-quantitative mineral chemical analysis. The porphyroblast grows stage by stage in the manner of the distribution of a roseleaf and is controlled by deformation. We call the growth stage of porphyroblast a growth phase.
