The Petrographic Composition And Early Paleozoic Tectonic Evolution Of The Subductionaccretion Complex In The Central Segment Of Northern Qilian

The Qilian Orogen is located in the northern rim of the Tibetan Plateau,and preserves detailed records of the tectonic evolution of the northern Tethyan realm since early Paleozoic.The Northern Qilian Belt（NQB）has been considered as a subduction-accretion complex formed by long term subduction of the North Qilian Ocean between the Alashan（North）and Central Qilian Block（South）in the early Paleozoic.However,debate still exists regarding to the temporal and spatial framework for the subduction-accretion process in the NQB,such as subduction polarity,mechanism of crustal growth,and closure time of the North Qilian Ocean.A better understanding of the oceanic subduction process can be obtained by studying the petrographic composition and tectonic affinity of rock units in the subduction-accretion complex which may contain critical‘fossils’of various tectonic dynamics.Ophiolites in the NQB distribute generally along two parallel belts:the southern and northern ophiolite belt.While,the linkage and relationship between the west and east of the northern ophiolite belt remain enigmatic,mainly due to the lack of available data from the central segment of the northern belt.A typical subduction-accretion complex corridor is well exposed along Suyougou-Sugou-Niuxinshan areas,in the central segment of the NQB,which consists of ophiolite mélanges（northern belt）,flysch formations,arc-volcanic rocks,granitoids,high-grade and high-pressure metamorphic rocks.This study aims,firstly to identify litho-tectonic units in this corridor based on large-scale geological mapping and structural analysis;then to obtain the structural attribute,formation age and tectonic setting of the newly identified ophiolite mélange and to constrain the tempo-spatial evolution and detailed tectonic setting of subduction-related igneous rocks by carrying out geochronological and geochemical analysis;finally to provide crucial constrains for constituting framework of the subduction-accretion process in the NQB on the basis of a comprehensive analysis of available data.The main achievements of this doctoral dissertation are as follows:（1）The newly investigated Sugou ophiolitic mélange is characterized by the“block-in-matrix”structure（the“matrix”is mainly composed by green schists,sericite phyllites and foliated tuff;the“blocks”include serpentinite,basalt,gabbro,chert,meta-sandstone,limestone,volcanic breccia,andesite,olistostrome,and migmatite）.The rocks in the Sugou ophiolitic mélange are strongly deformed and characterized by compressive and shear deformation recorded by imbricate trusts,isoclinal folds,foliations,and locally ductile shear zones.The mafic rocks in the ophiolitic mélange exhibit two types of geochemical characteristics.The basalt samples from the northern part show clear OIB affinity which is diagnostic of hot spot or plume related magmatism associated with oceanic islands,while the basalt and gabbro samples from the southern part exhibit MORB-like signatures with a diagnostic trend of SSZ-type ophiolite.The chert in the mélange is non-hydrothermal origin,and may contains some terrestrial components as indicated by the negative Ce anomalies,suggesting that the chert is far away from the mid-oceanic ridge.While,geochemical features show that the sedimentary environment of the chert is neither typical pelagic basin nor continental margin.The spinels in the serpentinite are characterized by high Cr^#values,and have the mineral chemical features resembling that of the spinels from the forearc peridotite.Above evidences suggest that the Sugou ophiolitic mélange may contain imbricated fragments of oceanic islands and seamounts.The basalt and gabbro in the southern part formed in forearc setting within an intra-oceanic subduction zone.LA-ICP-MS U–Pb zircon dating indicates that the gabbro formed at 519±3 Ma,which constrains the emplaced time of the Sugou ophiolite mélange.（2）The andesite in the Sugou ophiolitic mélange is characterized by considerable high Mg O contents and Mg#values,as well as high Cr（274–424 ppm）,Ni（264–402 ppm）,Y（22.5–27.1 ppm）,and Yb（2.6–3.1 ppm）contents,while relative low Sr/Y ratios（3.9–5.6）,which resemble that of the typical sanukite（a subtype of high-Mg andesites）.The sanukitic high-Mg andesites is generally considered to be produced by equilibrium reaction of silicic liquids derived from partial melting of subducted sediments and mantle peridotites associated with subduction oceanic slab/sediments.U–Pb zircon dating indicates that the Sugou sanukite formed at 504±5 Ma.So,the formation of the sanukite would be related to the subduction of the North Qilian oceanic slab during the Cambrian.Besides,detrital zircon age components of the meta-sandstone imbricated in the Sugou ophiolitic mélange show an affinity to the Central Qilian Block to the south,suggesting a southward polarity of subduction.（3）In this study,a suit of boninite was newly identified in the Qingyanggou area,to the south of the Sugou ophiolitic mélange,which cropped out together with pillow basalt,intermediate-mafic tuff,volcanic breccia,chert and meta-sandstone.Two types of texture of boninites can be observed（porphyritic and massive）,both of which bear clino-enstatite,a diagnostic mineral of typical boninite.Acicular pyroxene skeletal crystals or uralite microcrystals are clustered and distributed in the matrix,which formed the spiny texture similar to that of komatiite.The Qingyanggou boninites have high contents of Si O₂,Mg O,and Al₂O₃,High Mg#values and Cr,Ni contents,while have very low contents of Ti O₂,TFe O,and Ca O.In the chondrite-normalized REE diagram,the Qingyanggou boninites display an“U”shaped pattern,which are similar to that of typical boninites in the Bonin Island.Considering the relationships between the boninite and surrounding rock assemblages,we proposed that the boninite was formed in a forearc setting during intra-oceanic subduction.The Qingyanggou boninite yielded zircon U–Pb age of 520±3 Ma,which indicate that the intra-oceanic subduction in the North Qilian Ocean initiated in early Cambrian.（4）Based on geological mapping,several intermediate-acid intrusive plutons were identified in the Suyougou-Sugou-Niuxinshan areas in the central NQB,and new zircon U–Pb ages,Hf isotopic and whole-rock elemental data of these intrusive rocks have been conducted to constrain the tectonic evolution of NQB.Zircon dating results revealed that the Lujiaodaban monzonitic pluton,Ladonggou,Suyougou granitic plutons and Xichagou dioritic pluton were emplaced at 434±2 Ma,505±3 Ma,471±3 Ma,and 425±3 Ma,respectively.The Lujiaodaban monzonite is characterized by high Mg O,Sr,Cr and Ni contents,while low Y contents,in accordance with that of the classic adakite.The monzonite exhibits mantle-derived positive zircon Hf(ε_Hf（t）=+2.5～+4.8)isotopic characteristics.The original magma of the monzonite was likely sourced from partial melting of the subducting oceanic slab,and was metasomatized by the mantle wedge during ascending.The Ladonggou monzogranite displays negative anomalies of Nb,Ta,Ti,P and negativeε_Hf（t）values（-6.5 to-0.3）,and old Hf crustal model ages（1.5–1.9 Ga）,suggesting generation from melting of ancient crust above subducted North Qilian oceanic slab.The Suyougou granite possesses geochemistry of I-type granite.The relative enrichment of LILEs,depletion of HFSEs and negative Eu anomalies,as well as prominent Hf isotope heterogeneities and old Hf crustal model ages（2.1–2.5 Ga）suggest a magma source of ancient crust mixed with minor mantle-derived melts in a subduction-related setting.The diorites from the Xichagou pluton are characterized by high Mg#values,enrichments in Cr,Ni,Y,Yb and high Sr/Y ratios,with geochemical features analogous to those of sanukite of the Setouchi volcanic belt,Japan.The convincing subduction-related features suggest the diorites generating from partial melting of subducted sediments mixed with mantle components.（5）Combined our results with the published data from previous literatures,the subduction process of the North Qilian Ocean in the Early Paleozoic can be reconstructed as the following three stages:1)Intra-oceanic subduction initiated in early Cambrian（ca.520 Ma）;2)South subduction of the North Qilian oceanic plate beneath the Central Qilian terrane between ca.520 and 490 Ma.As the subduction ongoing,some tectonic units in the ocean,such as oceanic islands,seamounts,and intra-oceanic arcs,were scraped off from the down-going plate and accreted to the north margin of the Central Qilian terrane;3)During the Ordovician–Silurian（ca.470 to 425 Ma）the bi-polarity（north and south）model characterized the subduction process.