The Early Paleozoic Magmatic Events In The Eastern North Qilian Orogenic Belt And Its Geological Significance

The studied areas are located in the Qingshui-Zhangjiachuan region,Gansu province.It belongs to the junction area between the North Qilian Orogenic Belt and the North Qinling Orogenic Belt,traversing the north-south direction tectonic belt.As the the intersection zone of Paleo-Asia oceanic,Thethyan,and pacific tectonic domains,it is of great concern for the study on the geodynamics in China.Due to the strongly tectonic construction,the studied areas developed multi-stage magmatism.This work focuses on the diorite rocks and granitic gneisses outcropping in the Qingshui-Zhangjiachuan region.Base on the existed work,petrology,geochemistry,zircon geochronology,and Sr-Nd isotopic geochemistry have been carried out in order to explore the early Paleozoic magmatic evolution of the eastern part of North Qilian Orogenic Belt.The main achievements are following:(1)The major rock-forming minerals of diorites in the Malu section include plagioclase,alkali feldspar,quartz,amphibole,and biotite.It mainly crops out in the Gongmen-Yanjiadian-Malu and Baijia-Lijiahe-Changgouhe areas,which is the largest early Palaeozoic dioriteic pluton in the junction of Qinling-Qilian Orogen,and termed the Yanjiadian Diorite.This pluton gradually develops the orientated structure with the mylonitic texture and gneissic structure from north to south resulting from the effect of the Maojiamo NWW-trending faults.In this study,The southern mylonitic diorites have been sampled and dated by the LA-ICP-MS with a formation age of 418�3.2Ma.Major and trace element analysis displays high SiO2,Al2O3,Na2O and Sr,low Yb and Y,high ratios of Sr/Y and(La/Yb)N,and no or weakly Eu anomalies,similar to the geochemical characteristics of the adakites.Low ratios of Na/K,Nb/U and Ce/Pb,and nagtive ?Hf(t)values suggest that these adakitic rocks result from the partial melting of the continental crust.However,samples with high MgO and Mg#indicate the addition of mantle materials,which could be the result of interaction between mantle peridotite and delaminated thicken basic lower crust under collision.Meanwhile,the southern of the Yanjiadian diorite have low ratios of Y/Yb,(Ho/Y)N,and Nb/Ta than northern of the Yanjiadian diorite,indicate the resource residual phase are mainly garnet and hornblende in southern of the Yanjiadian diorite,hornblende in the northern of the Yanjiadian diorite,the depth of source of southern samples are greater than northern samples.In conclusion,the crust thickness in the Continuity of Qinling-Qilian was progressively increasing during the period of?440Ma to?418Ma,and Yanjiadian?418Ma diorite formed in the post-subuduction and collision orogenic stage.(2)Granitic gneiss in Malu section composed of hornblende biotite plagioclase gneiss,gneissic diorite,amphibolite.It is termed as Longshan group which was thought to be in the Paleo-Mesoproterozoic time(1:250000 geological map).all samples fall in the volcanic rock area in the Si-(al+fm)-(c+alk)discrimination diagram,suggest that are orthometamorphic rocks.Major and trace element analysis displays SiO2>55.46%,Na2O>2.85%,Na2O/K20>1.1,A/CNK<1.1,low ratios of 87Sr/86Sr(0.704913?0.705627)<0.708,similar to the geochemical characteristics of the I type granite.Hornblende Biotite plagioclase gneiss and gneissic diorite dated by the LA-ICP-MS with a formation age of 444.1�1.0Ma and 420.6�1.5Ma respectively and metamorphic age of 418.6�3.1Ma.Hornblende biotite plagioclase show enrichment of LREE and LILE and depletion of HREE and HFSE,similar to the characterized of arc magama.?Hf(t)(-8.17?9.66)values with larger changing range recorded the message of delepted mantle and composition of continental crust at the same time,indicate that it was formed in the continental marginal arc.Gneissic diorite(?420Ma)display low SiO2 and high Mg#,trace elements have high ratios of Ce/Pb,Nb/U compared with hornblende Biotite plagioclase gneiss,indicate the source with addition of mantel materials.Plagioclase reverse zones indicate Longshan complex formed an open magmatic system,which probably experienced multi-stage magmatic activities.Low ratios of 87Sr/86Sr(0.704913?0.705627),positive ?Hf(t)values and young model age recorded a strong event,which new crust was extracted from the mantle.Combining with the regional heologic characters,collision compression have stopped in 420Ma,lithosphere stretching caused a lot of mantle-derived basic magma rising,the upwelling mantle not only make the thicken basic lower crust was delaminated,but also may be the main reasons for the early metamorphism.(3)The eastern of the North Qilian Orogenic Belt has experienced complex tectonic evolution process since the Neoproterozoic,start to subduction,collision,and then stretching in the Early Paleozoic.Research by early paleozoic magmatism(northern Yanjiadian diorite and southern Longshan complexes)in the Qingshui-Zhangjiachuan region,Gansu province,we have discussed the early paleozoic process of structural evolution and tectonic setting of the eastern of North Qilian Orogenic Belt.The formation age of biotite hornblende plagioclase gneiss(-444Ma)enrichment of LREE and LILE(Rb?Th?Ba)and depletion of HREE and HFSE(Nb?Ta?Ti),similar to the characterized of Arc magama.And ?Hf(t)(-8.17?9.66)values with larger changing range suggest it was formed in the continental marginal arc,probably related to the subduction of North Qilian ocean.gneissic diorite(?420Ma)display low SiO2 and high Mg#,trace elements have high ratios of Ce/Pb,Nb/U compared with hornblende biotite plagioclase gneiss,indicate the source with addition of mantel materials.Low initial ratios of 87Sr/86Sr,positive ?Hf(t)values,imply muther the new crust was formed by asthenosphere mantle upwilling,and the tetonic setting transform from compression to extension.The Yanjiadian diorite(420Ma)display low ratios of Na/K,Nb/U and Ce/Pb,and nagtive ?Hf(t)values suggest that these adakitic rocks result from the partial melting of the continental crust.But,samples with high MgO and Mg#indicate the addition of mantle materials,which was the production of delaminated thicken basic lower crust,indicating that the time of tectonic setting of the North Qilian Orogenic Belt changed from compression to extension in-420 Ma.