A Geochemical Study Of Mesozoic Magmatic Rocks From The Xiaoqinling-Xiong'ershan Area In The Southern Margin Of The North China Block

Post-collisional magmatic rocks are widespread in continental collision zones,and their petrogenesis is temporally and spatially linked to reworking and recycling of crustal rocks at convergent plate boudaries.Post-collisional felsic magmatic rocks not only record reworking of the orogenic crust,but also register the tectonic evolution of collisional orogens.Post-collisional mafic magmatic rocks may document recycling of different crustal components to form heterogeneous mineral assemblages and geochemical characteristics in metasomatic mantle domains.The geochemical characteristics of post-collisional magmatic rocks not only depend on the nature of their magma sources,but also reflect the tectonic evolution of continental collision zones with respect to chemical geodynamics.The Qinling orogen is located in central China,and many key scientific issues on its formation mechanism and tectonic evolution remain to be resolved.The Xiaoqinling-Xiong'crshan region in the southern margin of the North China Block lies to the northern part of the Qinling orogen(North Qinling),where Mesozoic granites and intermediate-basic dykes are widely exposed but their petrogenesis is still controversial.For this reason,this dissertation presents an integrated study of petrography,geochronology and geochemistry for Late Mesozoic granites and intermediate-basic dykes in this region.The results not only place constraints on their formation ages and the nature of their magma sources,but also provide geochemical clues to understand the tectonic evolution of the North Qinling-Xiaoqinling continental collision zone and the recycling of different crustal components in subduction channels.Four representative Late Mesozoic granite plutons in the Xiaoqinling-Xiong'ershan area were studied in the dissertation.According to their trace element and radiogenic isotope characteristics as well as the relict zircon U-Pb ages,these granites can be generally subdivided into two groups.Group I granites from the Huashan,Wenyu and Heyu plutons arc high in Sr/Y and(La/Yb)N ratios without significant negative Eu anomalies and thus similar to adakitic rocks.Group ? granites represented by the Funiushan pluton have low Sr/Y and(La/Yb)N ratios and significant negative Eu anomalies.Group I granites have consistent emplacement ages of 137-130 Ma,whereas Group II granites show variable formation ages from 136 to 120 Ma.Group I granites were derived from partial melting of the thickened orogenic crust,whereas Group II granites were produced by partial melting of the thinned orogenic crust.Therefore,the Late Mesozoic granites in the sourthern margin of the North China Block witness lagre-scale thinning of the orogenic lithosphere in the early Cretaceous.Group I granites have enriched Sr-Nd-Hf isotope compositions and contain the relict zircons with Neoarchean and Paleoproterozoic U-Pb ages.The whole-rock Sr-Nd and zircon Hf-O isotope compositions of Group I granites are similar to those of the ancient crystalline basement in the southern margin of the North China Block,and the Hf-O isotope compositions of the Neoarchean and Paleoproterozoic relict zircons are also consistent with those in the Taihua and Xiong'er Groups.This indicates that Group I granites were mainly derived from partial melting of the ancient crystalline basement in the southern margin of the North China Block,The Palcoproterozoic relict zircons and some syn-magmatic zircons of Group II granites have low ?Hf(t)values,suggesting that their magma source contains similar ancient crustal components.Compared to Group I granites,nevertheless,Group II granites have relatively less enriched radiogenic Sr-Nd-Hf isotope compositions,indicating material contribution from the juvenile crust.In addition,Group ? granites contain not only the Paleoproterozoic relict zircons but also the early Neoproterozoic and early Paleozoic relict zircons,and the Hf-O isotope compositions of the early Neoproterozoic and early Paleozoic relict zircons are similar to those of the eclogite/gneiss protoliths and the Paleozoic granites in the North Qinling orogen.Therefore,Group ? granites were produced by partial melting of a mixed source composed of the ancient crystalline basement in the southern margin of the North China Block and the juvenile crust of the North Qinling microcontinent in the Late Mesozoic.In the Cambrian,the Proto-Tethys oceanic slab had pulled the North Qinling microcontinent to subduct northwards beneath the southern margin of the North China Block and underwent ultrahigh pressure metamorphism in the stability fields of coesite and diamond.Subsequently,the deeply subducted continental crust was detached into slices for thrusting exhumation,whereas part of the North Qinling juvenile crust would have remained beneath the southern margin of the North China Block.This part would then become partially melted together with the ancient crystalline basement in the southern margin of the North China Block in the Late Mesozoic,giving rise to Group ? granites in the post-collisional extensional stage.Aluminous A1-type granites were generally emplaced at the end of post-collisional felsic magmatism in continental collision zones,and they show close petrogenetic linkage to post-collsional I-type granites.The petrogenesis and geological implications of aluminous A1-type granite have been still hotly debated.The Taishanmiao syenogranite in the Xiong'ershan region provides a good opportunity for us to study the nature of magma source and the formation mechanism of aluminous A 1-type granites.This dissertation has performed a detailed comparsion of petrography,geochronology and geochemistry between the Taishanmiao syenogranite and the adjacent Funiushan granite.The Taishanmiao syenogranite was intruded into the Xiong'er Group volcanic rocks at 119±1 Ma,consistent with the emplacement age of the late-stage non-adakitic granite in the Funiushan pluton.The whole-rock Nd and zircon Hf-O isotope compositions of the Taishanmiao syenogranite are also similar to those of the Funiushan granite,indicating that the Taishanmiao syenogranite was derived from a mixing source composed of the ancient crystalline basement in the southern margin of the North China Block and the juvenile crust of the North Qinling microcontinent.According to their petrographical and geochemical features,the Taishanmiao syenogranite belongs to aluminum A1-type,whereas the Funiushan granite is ?-type.The Taishanmiao syenogranite was formed through fractional crystallization of hornblende and plagioclase from the late-stage non-adakitic granitic magma of the Funiushan pluton.Therefore,we propose a petrogenetic mechanism for aluminum A1-type granites,in that crust-derived ?-type non-adakitic granitic magmas would produce aluminum A1-type granites through fractional crystallization at shallow crustal levels.Both magmatic process and the nature of magma source play important roles in the petrogenesis of aluminum A1-type granites,for example,A-type geochemical characteristics and low Y/Nb ratios are controlled by fractional crystallization of amphibole and plagioclase,whereas the aluminum characteristics depend on the crustal affinity of their source rocks.Aluminum A1-type granites are formed at the end of the post-collisional felsic magmatism,and mark the termination of post-collisional extension.The Y/Nb ratios of A-type granites should be carefully used to judge the nature of their magam sources and tectonic settings.There are two types of mafic magmatic rocks(IAB-like and OIB-like)with different geochemical features in the continental collision zone.They record dehydration melting of the subducted continental and oceanic crust materials at different depths,respectively.Different crustal components may be involved in recycling at the convergent plate boundary,leading to the generation of mantle metasomatites with distinctive mineral assemblages and geochemical characteristics.This dissertation presents a geochemical study of Late Mesozoic intermediate-basic dykes in the Xiaoqinling-Xiong'ershan region.The results are used to constrain their formation ages and the nature of magma sources,and to explore their petrogenesis and the effects of different recycled crustal components on the composition of their mantle sources.According to their trace element characteristics,the Late Mesozoic intermediate-basic dykes can be generally subdivided into two groups:Group ? ones are dolerites,show low TiO2/AlO3 and Nb/U ratios,IAB-like trace element characteristics and enriched radiogenic Sr-Nd-Hf isotope compositions;Group ? ones comprise gabbro,pyroxene-bearing diorite and lamprophyre,exhibit high TiO2/Al2O3 and Nb/U ratios,OIB-like trace element characteristics and relatively depleted Sr-Nd-Hf isotope compositions.The two groups of intermediate-basic dykes were mainly intruded into the Taihua Group ancient crystalline basement in the southern margin of the North China Block at 118-120 Ma and 122-134 Ma,respectively.Their mantle sources were generated through chemical metasomatism by two properties of felsic melts,respectively,derived from the deeply subducted North Qinling microconintent and previously subducted Proto-Tethys oceanic crust.This gave rise to mafic-ultramafic mantle metasomatites with different mineral assemblages and geochemical characteristics.The two groups of intermediate-basic dykes show large ranges in Sr-Nd-Hf isotope compostions,suggesting that their mantle sources would contain recycled crustal components in different properties and proprotions.The whole-rock geochemistry and zircon Lu-Hf isotope compositions indicate that the recycled crustal components in the mantle source of Group ? intermediate-basic dykes are mainly the ancient crystalline basement in the southern margin of the North China Block and the North Qinling juvenile crust,and phlogopite is abundant in their mantle source with a small amount of amphibole and pyroxene;the ancient crustal material in the mantle source of lamprophyre dykes is the seafloor sediment derived from the ancient crystalline basement in the southern margin of the North China Block,and the mantle metasomatite is rich in clinopyroxene,amphibole and garnet;the mantle source of gabbro and pyroxene-bearing diorite dykes is dominated by recycled juvenile oceanic basalts,and is mainly composed of pyroxene and olivine.The intermediate-basic dykes in the southern margin of the North China Block witness recycling of the different crustal components and the heterogeneous mineral assemblages and geochemical characteristics of their mantle sources.In summary,the Late Mesozoic magmatic rocks in the Xiaoqinling-Xiong'ershan region not only record the crustal reworking and recycling,but also register the tectonic evolution of the continental collision zone in the North Qinling and Xiaoqinling orogens.Both the ancient crystalline basement in the southern margin of the North China Block and the North Qinling juvenile crust would have participated in the crustal reworking and recycling,respectively,generating the magma sources of post-collisional granites and intermediate-basic dykes.The aluminum A1-type granites developed at the end of post-collisional felsic magmatism and represent the termination of post-collisional extension.The mantle metasomatites generated by the recycling of different crustal components have different mineral assemblages and geochemical characteristics,and then could produce mafic rocks with distinctive geochemical characteristics.