Petrogenesis And Geodynamic Process For The Early Paleozoic Mafic Intrusive Rocks At Hualong-Lajishan Area,South Central Qilian

The Wilson cycle,as the key idea of the plate tectonic theory,means a cycle of a complete continent from rifting of continental lithosphere,to opening of an ocean,then to subduction of the oceanic slab,and finally to closure of the ocean with susquently continent-continent collision.Orogens are capable of recording the magmatism during whole the stages of a Wilson cycle.The mafic rocks,as an important part of the magmatism at orogens,carry key informations from the deep.They hold different features at different stages,thus being able to provide deep understanding on evolution of an orogen as well as their related deep geodynamic processes.As a representative Caledonian orogen at western China,the Qilian Orogen is divided into the North Qilian belt,the Central Qilian block,and the South Qilian belt from north to south.Many previous studies on the North Qilian belt have been carried out in establishing evolution of this belt from opening the North Qilian Ocean to northward subduction of the oceanic slab,and finally to continent-continent collision.By contrast,research on central-south Qilian is relatively limited.It is reported that there are many Paleozoic mafic rocks in the southern Central Qilian block.Carrying integrated studies of field work,petrography,zircon U-Pb geochronology and Hf isotope,and whole-rock major and trace elements and Sr-Nd isotopes on these rocks are benefit for establishing geochronologic framework of mafic magmatism,and discussing their petrogenesis,and making constraints on regional tectonic evolution.In this study,we do research on mafic rocks from the Jianliangju ophiolite,Lashuixia pluton,Duojielaka pluton,Yishenchun pluton,Liushendong pluton,Jinyuan pluton,and Keqianyakou pluton in Hualong-Lajishan area at the southern Central Qilian block.The main results gained in this study are listed as follow:1.The zircon U-Pb isotopic study of the seven mafic plutons show their magmatic crystallization ages of 454–517 Ma.They are divided into three groups by their field occurrence,age,and geochemical composition.Group one includes the Jianliangju SSZ-type ophiolite.It is located in the Lajishan tectonic belt with magmatic crystallization age of 517 Ma.Group two contains the Lashuixia MORB-like gabbro,which generated at 506 Ma and intruded into the Precambrian Hualong Group.Group three are arc-type mafic intrusions,consisting of the Duojielaka,Yishenchun,Liushendong,Jinyuan,and Keqianyakou plutons.They emplaced at 454–470 Ma and crop out at both the Hualong Microblock and the Lajishan tectonic belt.2.The gabbro and pyroxenite in the Jianliangju SSZ-type ophiolite are cumulates with their calculated parent magmatic compositions similar to those of the basalt and dolerite,which show E-MORB features with overprinting of arc signature.The lithologies are hence regarded as evolved partial melts from enriched asthenospheric mantle with incorporation of slab materials at a subduction zone.3.Samples from the Lashuixia pluton have pronouncedly depleted Nd isotopic composition with?_Nd?t?of+5.8?+7.0.In primitive mantle-normalized trace element spider diagram,they overally show transitional compositions between the N-MORB and E-MORB with additionally enrichment of Rb,Ba,Sr,and Pb and depletion of Nb.These observations imply involvement of all the depleted asthenospheric mantle,subcontinental lithospheric mantle,and the slab fluid.Trace elemental calculation indicates?7–17%partial melting of 90%depleted asthenospheric mantle plus 10%subcontinental lithospheric mantle at spinel-stable depth accords well with most incompatible trace element compositions of our studied samples.4.The generation of the 454–470 Ma arc-type mafic intrusions varies from one to another with their spatial and temporal distribution.Comparing to the 454–465 Ma intrusions,the 470 Ma Duojielaka pluton displays more depleted Sr-Nd-Hf isotopic compositions and lower incompatiable elemental contents.The Duojielaka pluton is proposed to be derived from partial melting of mantle wedge materials that had been metasomatized by both slab fluid and sediment melt.All the Yishenchun,Liushendong,Jinyuan,and Keqianyakou plutons have relatively enriched Sr-Nd-Hf isotopic compositions,but the Yishenchun and Liushendong plutons from the Hualong Microblock are obviously more enriched in isotopic compositions than those of the Jinyuan and Keqianyakou plutons from the Lajishan tectonic belt.The geochemical and isotopical characteristics of the four plutons indicate their mantle sources had been metasomatized by sediment melt.Further isotopic and geochemical calculations show mantle sources of the Yishenchun and Liushendong plutons had been metasomatized by?4.5%sediment melt with the former and the later experienced?9%partial melting at garnet-and spinel-stable depth,respectively,while mantle sources of the Jinyuan and Keqianyakou plutons underwent?5%partial melting at garnet-stable depth.5.Based on available data,we made a simulation to account for generation of the Middle-Late Ordovician arc-type magmatism at the southern Central Qilian block.The result turns out that degree of sediment melt that incorporated into the mantle wedge decreases from the southwest to northeast.This phenomenon proves the proposal that the tectonic line from the North Qaidam belt to the north margin of the West Qinling belt represents the site of oceanic closure and the ocean?i.e.North Qaidam-West Qinling Ocean?suffered from northward subduction.6.The evolution of Hualong-Lajishan area during the Early Paleozoic is mainly divided into the following four stages according to regional data:?1?Early stage of northward subduction of the North Qaidam-West Qinling oceanic slab?ca.535–520Ma?;?2?On-going oceanic subduction with evolution of the Lajishan back-arc basin?ca.520–470 Ma?.At the early period,rolling back of the subducted oceanic slab took place,which gave rise to generation of the Jianliangju SSZ-type ophiolite and the Lashuixia MORB-like gabbro.During the late period,the oceanic slab subducted at a normal angle to generate the Duojielaka pluton at a continental arc setting;?3?Closure of the Lajishan back-arc basin?ca.470–455 Ma?.At this stage,arc-continent or/and continent-continent collision occurred at back-arc site,which caused rolling back of the oceanic slab and the partial melting of slab sediment.The resultant sediment melt metasomatized the mantle wedge to produce the Yishenchun,Liushendong,Keqianyakou,and Jinyuan plutons;and?4?Closure of the North Qaidam-West Qinling Ocean and subsequently continent-continent collision?<ca.455 Ma?.