A study of REE compositions of Sinian to Triassic sedimentary strata from Guichi stratigraphic region located in northern margin of the Yangtze block indicates that average total REE concentration is 127μg/g which is...A study of REE compositions of Sinian to Triassic sedimentary strata from Guichi stratigraphic region located in northern margin of the Yangtze block indicates that average total REE concentration is 127μg/g which is lower than that of sedimentary rocks from the whole world(230 μg/g) and approximately resembles to that of sedimentary rocks from the platforms(129 μg/g). Among different lithologies, average total REE concentration of shales is the highest(230 μg/g), that of limestones is the lowest(18 μg/g) and that of sandstones and siltstones is in between(213 μg/g) .REE concentrations of strata of different geologic ages are controlled by lithology.REE concentrations of Ordovician and Triasaic strata consisting mostly of carbonate rocks are the lowest(40 and 44 μg/g, respectively), and those of Silulian and Devonian strata consisting mainly of terrestrial detrital rocks are the highest (231 and 248 μg/g, respectively). Sedimentary rocks studied show very similar REE patterns with LREE enrichment and negative Eu anomaly regardless of lithology and stratigraphic age. Samples are plotted in a confined area in Σ(La～Nd)-Σ(Sm～Ho)-Σ(Er～Lu) ternary plot, suggesting homogeneity of REE composition and a stable environment for the studied region. Similarity between REE compositions of sedimentary rocks from the studied region and that from typical platforms of the world suggests the platform character of the studied region during Sinica to Triassic. Results of this study suggests that the provenance of the studied sedimentary rocks is not the Jiangnanian old land mass, but the old continent(s) from the north.展开更多
The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known.In contrast,accretionary orogenic processes occurring at the supercontinent periphe...The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known.In contrast,accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood.To fill this gap,we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt,where numerous data testify for Meso-to Neoproterozoic magmatic reworking.The tectono-metamorphic evolution of the periSiberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic–early Cambrian(Baikalian)cycle.However,we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block,previously considered as a typical example of the Baikalian metamorphic belt.This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist.Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at 720℃ and 6.0 kbar followed by further burial to the kyanite stability field at 750℃ and 9 kbar and decompression to 650℃ and 8 kbar.The Grt-St schist records initial burial to the staurolite stability field at 620℃ and 6 kbar,followed by further burial to 590℃ and 8.5 kbar.The monazite data yield a continuum of ^(207)Pb-corrected ^(238)U/^(206)Pb dates of ca.926–768 Ma in the Grt-Sil-Ky gneiss,and ca.937–754 Ma in the Grt-St schist.Based on monazite textural positon,internal zoning,and REE patterns,the time of prograde burial to 6.0 kbar under a thermal gradient of 27–32℃/km is estimated at ca.890–853 Ma.It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22℃/km dated at ca.835–815 Ma.The late monazite recrystallization at ca.790 Ma is related to decompression.Additionally,monazite with dates of ca.568–515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint.Metamorphic zircon rims with Th/U ratios of 0.01–0.06 in Grt-Sil-Ky gneiss with 877±7 Ma age,together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism.The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice–probably of back-arc or arc type.This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean.The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly,followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.展开更多
文摘A study of REE compositions of Sinian to Triassic sedimentary strata from Guichi stratigraphic region located in northern margin of the Yangtze block indicates that average total REE concentration is 127μg/g which is lower than that of sedimentary rocks from the whole world(230 μg/g) and approximately resembles to that of sedimentary rocks from the platforms(129 μg/g). Among different lithologies, average total REE concentration of shales is the highest(230 μg/g), that of limestones is the lowest(18 μg/g) and that of sandstones and siltstones is in between(213 μg/g) .REE concentrations of strata of different geologic ages are controlled by lithology.REE concentrations of Ordovician and Triasaic strata consisting mostly of carbonate rocks are the lowest(40 and 44 μg/g, respectively), and those of Silulian and Devonian strata consisting mainly of terrestrial detrital rocks are the highest (231 and 248 μg/g, respectively). Sedimentary rocks studied show very similar REE patterns with LREE enrichment and negative Eu anomaly regardless of lithology and stratigraphic age. Samples are plotted in a confined area in Σ(La～Nd)-Σ(Sm～Ho)-Σ(Er～Lu) ternary plot, suggesting homogeneity of REE composition and a stable environment for the studied region. Similarity between REE compositions of sedimentary rocks from the studied region and that from typical platforms of the world suggests the platform character of the studied region during Sinica to Triassic. Results of this study suggests that the provenance of the studied sedimentary rocks is not the Jiangnanian old land mass, but the old continent(s) from the north.
基金supported by the Czech Science Foundation,Czechia(grant number 19-27682X to K.S.)。
文摘The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known.In contrast,accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood.To fill this gap,we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt,where numerous data testify for Meso-to Neoproterozoic magmatic reworking.The tectono-metamorphic evolution of the periSiberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic–early Cambrian(Baikalian)cycle.However,we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block,previously considered as a typical example of the Baikalian metamorphic belt.This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist.Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at 720℃ and 6.0 kbar followed by further burial to the kyanite stability field at 750℃ and 9 kbar and decompression to 650℃ and 8 kbar.The Grt-St schist records initial burial to the staurolite stability field at 620℃ and 6 kbar,followed by further burial to 590℃ and 8.5 kbar.The monazite data yield a continuum of ^(207)Pb-corrected ^(238)U/^(206)Pb dates of ca.926–768 Ma in the Grt-Sil-Ky gneiss,and ca.937–754 Ma in the Grt-St schist.Based on monazite textural positon,internal zoning,and REE patterns,the time of prograde burial to 6.0 kbar under a thermal gradient of 27–32℃/km is estimated at ca.890–853 Ma.It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22℃/km dated at ca.835–815 Ma.The late monazite recrystallization at ca.790 Ma is related to decompression.Additionally,monazite with dates of ca.568–515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint.Metamorphic zircon rims with Th/U ratios of 0.01–0.06 in Grt-Sil-Ky gneiss with 877±7 Ma age,together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism.The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice–probably of back-arc or arc type.This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean.The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly,followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.
