| The orogenic mafic igneous rocks are of great significance in revealing the recycling of the subducted crustal materials,the geochemical characteristics of the mantle source and the mantle evolution process in the orogenic belt.The Qinling orogenic belt in central China was subducted by the Paleo-tethys Ocean and South China Block during the late Paleozoic to Mesozoic.During this process,subducted crustal materials with different properties may be recycled into the mantle,resulting in the heterogeneity of the lithospheric mantle in the Orogenic Belt,and finally be recorded by the orogenic magmatic rocks.At present,the research on the subduction and dynamical mechanism of the Mianlue oceanic crust is still very weak due to the lack of early Mesozoic arc magmatic record in west Qinling orogenic belt.Therefore,this master’s thesis has made a detailed petrological and geochemistry study on the early Triassic Guyi complex outcropped in the west Qinling,which provides new constraints for the recycling of crustal materials,the process and mechanism of crust-mantle interaction,the nature of the orogenic mantle source and the tectonic evolution of the orogenic belt.Zircon in situ U-Pb dating,Lu-Hf isotopes,whole rock major and trace elements,Sr-Nd-Hf isotopes and Fe isotope analysis of the early Mesozoic mafic magmatic rocks(Guyi complex)in western Qinling orogenic belt have been carried out in this study.The rock types of Guyi complex mainly include gabbro,gabbro-diorite,diorite and granite.Zircon U-Pb dating shows that the Guyi complex was formed in the early Triassic(248-249Ma),corresponding to the oceanic subduction stage and before the initial collision between the South China Block and North China Block.Meanwhile,these mafic magmatic rocks have arc-type trace element distribution characteristics:enrichment of large ion lithophile elements(LILE)and light rare earth elements(LREE),and depletion of high field strength elements(HFSE).They also enriched in whole-rock radiogenic Sr-Nd-Hf isotopes,with(87Sr/86Sr)i ratios of 0.7085～0.7109,εNd(t)values of-9.3 to-7.2 and εHf(t)values of-3.5 to-7.4.They generally display constant δ56Fe values of-0.06 to 0.09‰,with an average value of 0.03±0.02‰,slightly lower than those of MORB(δ56Fe=0.11±0.04‰),but consistent with the average arc volcanic rock composition(δ56Fe=0.05±0.05‰).The element and isotope compositions of the studied samples indicate that the Guyi complex is from the enriched mantle source generated by the subduction oceanic crust-derived fluid metasomatism.Compared with MORB,the higher ratios of Th/Yb(0.56～4.29),La/Sm(3.87～7.33),(La/Yb)N(4.84.～8.76)and Th/Nd(0.08～0.53)in the samples may indicate the sediment melt in the source region,the higher ratios of Rb/Nb(1.20～9.00),Ba/Th(42.83～194.19)and Ba/La(14.84～38.68)in the samples may indicate the existence of aqueous solutions metasomatism in the source region.The high contents of K2O(0.36-2.58),the low ratios of Ba/Rb(6.48～25.8)but high ratios of Rb/Sr(0.02-0.24)in the rocks may indicate that the present of phlogopite in the mantle source.Partial melting of such mantle source may explain why the Fe isotope composition of arc magmatic rocks is lower than that of MORB.At the same time,the Sr-Nd isotopic quantitative simulation results further indicate that about 5-15%aqueous solution from subducted basaltic oceanic crust and 2-8%hydrous melt from overlying sediments participate in mantle source metasomatism.Based on the early Mesozoic subduction tectonic setting in the orogenic belt,it is suggested that the melt/fluids from the oceanic crust metasomatized the mantle wedge peridotite,resulting in the enrichment of Sr-Nd isotopes and arc-type trace element features for the the mantle source.Considering that the Guyi complex was formed during the tectonic transition from oceanic subduction to continental subduction,the early Mesozoic plate roll-back of the subducting oceanic slab may be the geodynamics mechanism for partial melting of the mantle source.The Early Triassic mafic magmatic rocks in the west Qinling orogenic belt region were further influenced by the magmatic differentiation during the ascent of the mafic magma.Therefore,the mafic magmatic rocks in the Guyi complex not only provide geochemistry evidence for paleooceanic crust recycling in the collisional orogenic belt,but also record the tectonic evolution from oceanic subduction to continental subduction. |
