| The Appalachian orogenic belt in North America is one of the most typical accretion orogenic belts globally.Newfoundland is the area with the best rock exposure,the largest scale,the most complete preservation,and the highest degree of research in the orogenic belt.It is an excellent place to study the early evolution of the Appalachian orogenic belt by documenting the complex collage process of several microcontinents between Laurentia and Gondwana during the early and middle Paleozoic.The granitic rocks are widely exposed on the island,but the current understanding of the petrogenesis of granitic rocks and the tectonic evolution of related magmatic rocks is relatively lacking.This study aims to understand the petrographic assemblages and geochemical and isotopic compositions of granitoids in different periods of the Appalachian orogenic belt in Newfoundland,discussing the tectonic environment granitoid and providing new evidence for the intra-zone block assemblage from the perspective of magmatic rock.In addition,as one of the most typical accretive orogenic belts,the deep crustal structures and juvenile crust amount of the orogenic belt have been concerned.In this study,multiple isotope geochemical tracing technology and NdHf isotope mapping were used to explore the composition characteristics of juvenile and ancient compositions in the deep crust of the orogenic belt and provide constraints for the determination of block attributes in the island,the origin of microcontinents and their metallogenic regularity.The primary achievements are as follows.1.Further delineate the chronology framework and temporal-spatial evolution of granitic magma in the Newfoundland Appalachian orogenic beltBased on the 57 samples from 38 granitic rocks by LA-ICP-MS U-Pb ages and previous studies of magmatic rocks,the Appalachian orogenic belt in Newfoundland is divided into six stages: CambrianOrdovician(~510-472 Ma?~468-459 Ma?~446-435 Ma)?Silurian(~437-421 Ma)?Devonian(~420-407 Ma?~395-372 Ma).The temporal and spatial evolution characteristics of granitic magma in this area were determined by digital mapping,revealing that the regular migration and evolution of magmatic events from northwest to southeast in the middle and late Cambrian-Devonian.2.Characterize the juvenile and ancient compositions of various tectonic units through Nd isotopic mapping,evidencing for the division of tectonic units.Through the Nd isotope mapping of the Paleozoic granites in the island,it is roughly divided into five isotopic provinces with different composition characteristics of juvenile and old materials in the deep,namely the Humber zone,the Notre Dame subzone,the Exploits subzone,the Gander zone,and the Avalon zone.They are consistent with the previous tectonic belts based on tectonic elements.Among them,the area of the juvenile crust(?Nd(t)value>0)on the island accounts for about 40% of the total crustal content,mainly distributed on both sides of the Notre Dame subzone and the Exploits subzone suture(RIL-Red Indus line)and the eastern Avalon belt.In addition,although the Exploits subzone and the Gander zone have different compositional characteristics of juvenile and old materials as a whole,there is a high degree of similarity in the central area,and they should belong to the same block(Ganderia microcontinent).The differential distribution of the core and edge of the block is attributed to the addition of juvenile crustal material in the orogenic event,which is also consistent with the rule revealed by previous geophysical deep reflection profiles.This study shows that the deep material composition features can provide a more direct deep material basis for dividing the island blocks.3.Provide new constraints on the properties and affiliation of the Dashwoods microcontinent.The question of the properties of the Dashwoods microcontinent has been concerned.Most scholars believe that the micro-continent was split from the Laurentia continent during the opening of the ancient Atlantic Ocean,and some scholars believe it originated from Gondwana.Under the condition that the microcontinent basement is not exposed,and the surface information cannot be restrained,the current research still cannot provide adequate information for the source of the Dashwoods microcontinent.Based on the Lu-Hf isotope tracing of the early-middle Ordovician granitic plutons in the Notre Dame subzone and their inherited zircon/captured zircon mapping,this research concludes that the Dashwoods microcontinent and the adjacent Laurentia continental margin have markedly different basement properties.Instead of breaking up from the Laurentia continental margin,the micro-continent represents a composite tectonic belt composed of ancient and new crustal materials.4.Provide magmatic evidence for the assembly processes of tectonic blocks of the Newfoundland Appalachian orogenic belt.This paper systematically studies the rock assemblages and their geochemical characteristics of igneous rocks of different stages in the Appalachian orogenic belt of Newfoundland.Furthermore,compared with the igneous rock assemblages formed in different tectonic environments to reveal the history of block assembly related to the Early-Mesozoic Paleozoic and orogenic events in Newfoundland from the perspective of magmatic rocks.It is revealed that the Paleozoic Laurentia experienced the Taconic orogenic event(490~459Ma),the Salinic orogenic event(446~421 Ma),and the Acadian orogenic event(437~372 Ma),which occurred in Newfoundland,respectively—completed the splicing process of Humber zone-Notre Dame subzone,Exploits subzone-Gander zone,and Gander zone-Avalon zone. |
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