Genesis,Geodynamic Background And It’s Metallogenic Enlightenment Of Early Paleozoic Basic Dykes In Ziyang Area,Shaanxi Province

The The research area of Ziyang is located in the central part of the South Qinling Block,which is a composite tectonic zone composed of the Shangdan suture zone,the Mianlue suture zone,the southern edge of the North China Block,the South Qinling Block,and the northern edge of the Yangtze Block.It has undergone a series of tectonic processes such as the growth of the Archean continental crust,the cracking of the Paleoproterozoic,the contraction of the Late Paleozoic crust,and the collision orogeny of the Mesozoic;In addition to the development of pre Cambrian basement metamorphic rock series（crustal rocks/strata and ancient plutonic rocks）,there are also widespread development of Paleozoic strata,Mesozoic strata,and intrusive rocks;Among them,the development of Paleozoic mafic dykes has attracted the attention and research of scholars both domestically and internationally.At present,previous studies have revealed that the mafic dykes in the early Paleozoic period of the South Qinling Mountains were mainly concentrated in the Silurian period（410～440 Ma）,and pointed out that from the Cambrian to the Silurian period,the South Qinling Mountains were in a crustal extensional background.However,there is controversy among previous researchers about the origin and geodynamic background of the mafic dykes during this period,and there is a lack of research on their ore-forming potential.In view of this,this paper chooses the exposed mafic dykes group in the Daba Village area of Ziyang as the main research object.Based on field geological surveys,petrology and petrology,as well as LA-ICP-MS zircon U-Pb isotope dating,selective whole rock main and trace element and Sr-Nd isotope measurements,as well as in-situ Hf isotope analysis of zircon minerals and in-situ main and trace element analysis of rock forming minerals were carried out;This paper explores the genesis and geodynamic background of rocks from the perspective of element multi isotope geochemistry,and analyzes the possibility of vanadium titanium magnetite mineralization.The results and progress achieved are as follows:（1）The mafic dykes developed in this area are NWW oriented,with diabase as the main lithology.The main minerals are plagioclase（An50-70）（45%-55%）,common pyroxene（35%-45%）,and common hornblende（5%）,ilmenite（10%）,and accessory minerals such as apatite and sphene（<1%）.（2）The zircon U-Pb isotope dating results reveal that the range of diagenetic age variation is 475.8±3.5 Ma～487.2±3.8 Ma,which is a product of early Ordovician magmatic activity.Based on this study and previous research,it is reasonable to constrain the diagenetic age of the mafic dykes in the study area to the early Ordovician to late Silurian,as a typical response to tectonic magmatic activity in the middle and late Caledonian periods.（3）The geochemical characteristics of the elements reveal that the Si O₂ content is41.72～45.27 wt.%,Al₂O₃ content is 12.26～18.52 wt.%,Ti O₂ content is 3.02～5.52 wt.%,Fe₂O₃content is 12.13～15.56 wt.%,Mg O content（3.27～5.98 wt.%<7.0 wt.%）and Mg^#value（36～48,average value 42）indicate that the magma does not have the characteristics of primary magma and is generally a sodium alkaline series.The Huck diagram indicates that clinopyroxene is the main separated crystalline phase in the process of separation and crystallization.Ca O shows an upward trend with the increase of Si O₂,but due to the almost absence of negative anomalies in Eu（δEu>1.0）indicates that although there is separation crystallization of plagioclase,the separation crystallization effect is not strong.Enriched with elements such as Rb,Ba,Sr,Ti,Nb,Ta,and depleted with elements such as Zr,Hf,Th,and U.The Ti/Y average is 1297.9,which is similar to the geochemical characteristics of high titanium basalts formed by the Emeishan mantle plume;In the original mantle standardized trace element spider web map and chondrite standardized rare earth element distribution map,the mafic dykes samples in the study area show a right leaning spectral pattern that is almost consistent with the OIB,indicating that they come from the same source area as the OIB and fall into the high Ti basalt area of the Emeishan mantle plume,indicating that the magmatic activity that formed the mafic dykes in the study area is related to the mantle plume.（4）The Sr-Nd isotope composition reveals that the variation range of(⁸⁷Sr/⁸⁶Sr)i is0.7044～0.7050,TheεNd（t）values range from 3.1 to 3.6,which are similar to the Sr Nd isotope composition and mantle source characteristics of oceanic island basalt（OIB）,indicating that the mafic dykes in the study area may have come from the products of partially melted magma in the same mantle source area as OIB,similar to other early Paleozoic mafic dykes in the South Qinling Mountains;The range ofεHf（t）variation is4.44～12.16,¹⁷⁶Hf/¹⁷⁷Hf variation range is 0.282634～0.282846.The mafic dykes samples in this article indicate that the isotopes in the source area are depleted,while trace elements are actually enriched.This indicates that mantle metasomatism occurred not long ago,and isotopes have not had enough time to evolve to display enriched characteristics.（5）The mafic dykes in the study area were not affected by crustal contamination,and magma with a melting degree of about 10%to 20%experienced separate crystallization mainly composed of clinopyroxene during the process of upwelling magma chambers.In the Ta/Hf-Th/Hf and Nb/Zr-Th/Zr discrimination diagrams,all sample points were located in the basalt area of the mantle plume,further indicating that the diagenesis process of the mafic dykes is controlled by the tail column or hot spot system of the mantle plume,Its NWW oriented diagenesis parallel to the main faults in the region is consistent with the mantle plume activity in the early Paleozoic era of the region.（6）Previous studies have identified a series of Silurian titanium magnetite deposits（points）closely related to the genesis of diabase in the surrounding area.These types of mineralization are closely related to the genesis of diabase,and the mafic dykes diagenetic age,geochemical characteristics of major and trace ore-forming elements,lower partial melting degree,temperature and pressure conditions,and degree of crustal contamination in the study area are similar to the South Qinling vanadium titanium magnetite ore-forming rock mass and have the same mineralization alteration,Only the positioning depth is different.In addition,the mafic dykes samples in the study area contain about 10%of ilmenite,and the analysis of ore-forming elements reveals high V,low Cr,Ni and other iron loving elements,as well as relatively high associated elements Ga and Sc of ilmenite.Based on the ore-forming characteristics of the diabase and vanadium titanium magnetite ore-forming system,it is speculated that the deep and intermediate plutons of the mafic dykes diagenetic system in the study area should have the potential to search for vanadium titanium magnetite deposits.