Rock Assemblage And Geochronology Of "Neoproterozoic" Yimianpo And Huangsong Groups In Eastern Heilongjiang Province:Implications For Regional Tectonic Evolution

This thesis studies the Yimianpo and Huangsong groups,previously believed as Neoproterozoic,in the eastern Songnen-Zhangguangcai Range Massif.Based on the field investigations,detailed petrographical study,zircon U-Pb and biotite ⁴⁰Ar/³⁹Ar geochronology,we constrain the rock assemblage,formation time and tectonic nature of the Yimianpo and Huangsong groups.Researches on the sedimentary formations,igneous rock associations and their geochemistry,combined with the regional deposition formations and tectonic thermal events,reveals the Paleozoic-Early Mesozoic tectonic evolution of the eastern Heilongjiang Province.1.Formation time and tectonic nature of the"Neoproterozoic"Yimianpo GroupThe"Neoproterozoic"Yimianpo Group,from bottom to top,is subdivided into the Tangjiatun and Yangmugang formations,and was covered by volcanic rocks from the Early Mesozoic Taiantun Formation.The lower part of the Tangjiatun Formation is composed mainly of the rhyolites.The magmatic zircon U-Pb dating results indicate that the thyolites formed in 295 Ma,i.e.,Early Permian.The youngest weighted average zircon ²⁰⁶Pb/²³⁸U age for the feldspar quartz sandstone from upper part of the Tangjiatun Formation is 234±3 Ma?n=12?,suggesting that the deposition of its protolith began after 234±3 Ma.The youngest zircon ²⁰⁶Pb/²³⁸U age for the silty mudstone from the Yangmugang Formation is 226±3 Ma?n=2?,implying that it was deposited after 226±3Ma.The formatiom age of the overlying rhyolites from the Taiantun Formation is 189±1Ma?n=23?.According to zircon U-Pb dating results and the conformable contact between the Tangjiatun and Yangmugang formations,we conclude that the Yimianpo Group was formed in the Early Permian-Late Triassic?295¹89 Ma?,and is a set of consecutive sedimentary formation.2.Formation time and tectonic nature of the"Neoproterozoic"Huangsong GroupThe"Neoproterozoic"Huangsong Group,from bottom to top,is subdivided into the Yangmu and Yanwangdian formations,and was covered by the Early Mesozoic Tuopangou Formation.The bottom of standard section of the Yangmu Formation:1)The youngest ²⁰⁶Pb/²³⁸U ages for the biotite-plagioclase gneisss,biotite quartz schist and two-mica quartz schist are 263 Ma,258 Ma,and 248 Ma,respectively,suggesting that they were deposited after 263 Ma.Additionally,the middle part of the Yangmu Formation was intruded by the gneissic two-mica granite,whose zircon U-Pb age is221±3 Ma,indicating that the bottom of the Yangmu Formation formed in the late Late Paleozoic-Early Mesozoic?263²21 Ma?;2)The youngest detrital zircon U-Pb age for the garnet-bearing two-mica schist from upper part of the Yangmu Formation is 509±2Ma?n=17?,implying that it was deposited after 509 Ma.Combined with detrital zircons of⁴85 Ma commonly found in the early Late Paleozoic sedimentary rocks in the study area,we conclude that the garnet-bearing two-mica schist formed between 509 and 485Ma.Taken together,we concluded that the bottom of the Yangmu Formation of the Huangsong Group formed in 263²21 Ma,and its upper part was formed in 509⁴85Ma,rather than Neoproterozoic previously reported.The overlapping relationship that the 263²21 Ma strata were covered by the 509⁴85 Ma strata indicates that the Yangmu Formation is not a normal sedimentary sequence,but is a tectonic mélange consisting of geological bodies with different ages.The metamorphic zircon with the age of 196 Ma from biotite-plagioclase gneiss,together with Ar-Ar age?193 Ma?of biotites from mica schist,reveals that the exhumation of the tectonic mélange took place in the Early Jurassic.The Yanwangdian Formation is composed of two-mica quartz schist and muscovite schist which was intruded by the metagabbro.1)The youngest detrital zircon U-Pb age for muscovite schist is 426±3 Ma?n=10?.Combined with detrital zircons of³90 Ma commonly found in the early Late Paleozoic strata from the study area,we conclude that the muscovite schist was formed in the early Late Paleozoic?426³90 Ma?;2)The zircon dating results reveal that the metagabbro?amphibolite?intruding into muscovite schist formed in 280 Ma;3)The muscovite schist and metagabbro were covered by the two-mica quartz schist,whose youngest zircon U-Pb age is 232±2 Ma?n=11?.Combined with the zircon U-Pb dating results?198 Ma?for overlying Early Jurassic Tuopangou Formation,we suggest that the two-mica quartz schist formed in Late Triassic?232¹98 Ma?.The above dating results indicate that the Yanwangdian Formation of the Huangsong Group formed in the early Late Paleozoic and Early Mesozoic,rather than Neoproterozoic previously believed.Taken stratum assemblage,formation time and contact relationship of the Yangmu and Yanwangdian formations of the Huangsong Group together,we conclude that the Huangsong Group is not a continous stratigraphic sequence,but is actually a suite of tectonic mélanges that include not only the early Early Paleozoic,early Late Paleozoic and Early Mesozoic sedimentary formations,but also the deformed intrusion?Early Permian metagabbro?.3.Provenance of"Neoproterozoic"Yimianpo and Huangsong groups and block attributeThe sandstone from upper part of the Tangjiatun Formation and the silty mudstone from the Yangmugang Formation have similar detrital zircon ages.About 92%of detrital zircon ages range from 329 Ma to 230 Ma and 8%of detrital zircon ages range from 783 Ma to 357 Ma,implying that the late Late Paleozoic-Early Mesozoic geological units provided a large proportion of the clastic material in the Yimianpo Group,whereas the Early Paleozoic and Neoproterozoic ones provided minor clastic material.This interpretation is consistent with the geological bodies exposed in the study area.Furthermore,the detrital zircons are euhedral-subhedral in shape and have low degrees of roundness and separation,implying a rapid sedimentation.Additionally,the existence of the Neoproterozoic detrital zircons suggests that a Neoproterozoic magmatic event could occur on the eastern margin of the Songnen-Zhangguangcai Range Massif.The statistical results of 260 detrital zircons from the Yangmu Formation of Huangsong Group show that about 14.6%of detrital zircon ages range from 195 Ma to251 Ma,40%range from 253 Ma to 418 Ma,15.4%range from 423 Ma to 514 Ma,11%range from 551 Ma to 978 Ma,and 19%range from 1237 Ma to 2499 Ma.These age populations indicate that the Yangmu Formation sediments were sourced mainly from the Paleozoic geological units exposed within the study area and adjacent areas,and a little from the Early Paleozoic and Proterozoic geological units;The statistical results of 118 detrital zircons from the Yanwangdian Formation show that about 10%of detrital zircon ages range from 228 Ma to 250 Ma,21%range from 253 Ma to 363 Ma,50%range from 421 Ma to 542 Ma,11%range from 547 Ma to 843 Ma,and 8%range from 1293 Ma to 2171 Ma,with a peak age of 1750 Ma.These age populations indicate that 71%of Yanwangdian Formation sediments were sourced from the Paleozoic geological units,which is consistent with the geological bodies exposed in the study area.The detrital zircons in the Huangsong Group are euhedral-subhedral in shape and exhibit low degrees of roundness and separation,implying a near-source rapid sedimentation.Additionally,about 19%of sediments were sourced from the Neoproterozoic and Paleoproterozoic geological units,suggesting the existence of the Precambrian magmatic event on the eastern margin of the Songnen-Zhangguangcai Range Massif.4.Sedimentary formation and igneous rock association of the Yimianpo andHuangsong groups and regional tectonic evolutionThe Early Paleozoic igneous rock associations in the eastern Songnen-Zhangguangcai Range Massif reveal an active continental margin setting.The existence of the late Early Paleozoic collisional granites and the early Late Paleozoic?Early-Middle Devonian?sedimentary formation in the passive continental margin from the Huangsong Group indicates that the collision between the Songnen-Zhangguangcai Range and Jiamusi massifs took place in the late Early Paleozoic.The early Late Paleozoic sedimentary formation in the passive continental margin,Carboniferous A-type granites,and Early Permian bimodal igneous rocks reveal that they formed in a post-collisional extensional setting.A scissor-like closure of the Paleo-Asian Ocean from west to east during the middle-late Permian to Early-Middle Triassic resulted in the rapid sedimentation of the Yimianpo and Huangsong groups,which was accompanied by the open of the Mudanjiang Ocean?240²00 Ma?.In the Early Jurassic,the onset of subduction of the Paleo-Pacific Plate beneath Eurasia led to the formation of the Early Jurassic volcanic rocks of the Taiantun and Tuopangou formations,the low-temperature thermodynamic metamorphism of the Huangsong Group,the closure of the Mudanjiang Ocean,and the emplacement of the Heilongjiang Complex.