The Ocean-Continet Evolution Process In Shule River,Qilian:Constraints From The Early Paleozoic Geological Records

As an important part of the Qilian Mountain,the South Qilian was located in the connecting position between the Qilian Orogenic belt and North Qaidam,which plays a key role in the geologic evolution.All the time,there were many disputes on its tectonic attribute and evolution history,attracting more and more researchers.The Paleozoic geological bodies had become a significant research object,which widely distributed in the South Qilian area.Compared with the high research-level of the North Qilian,the South Qilian had relatively less researching materials,which restricted the understandings on the crucial geological problems,including La Ji Mountain-Southern Danghe Mountain Ocean's existence,nature of the ocean basin and its evolution model.In this thesis,as the selected area in Shule River,Qilian,in which it chose the Paleozoic geological bodies,involving the Wuligou Formatin?O₁w?,Yanchiwan Formation?O₂y?,Balonggonggaer Formation?Sb?,intermediate-acid rocks and ophiolite m�langes.Intensive studies,including fieldwork,sedimentology,petrology,petrography geochemistry,zircon U-Pb geochronology and Lu-Hf isope,were conducted on these geological bodies.Combined with the previous studies,it investigated the nature of the La Ji Mountain-Southern Danghe Mountain Ocean,geological record responsing on the ocean to land evolution process.Basing on the recognition before,it established the evolution model of the Paleozonic geological bodies of the researching area.It redefined three unites of Palaeozoic Stratigraphy,Wuligou Formation?O₁w?,Yanchiwan Formation?O₂y?and Balonggonggaer Formation?Sb?.The rock assemblages of the O₁w were silicon argillaceous rocks and flysch sediments,which indicated pelagic deposit,with ocean island formation of limestone and basalt.These rock assemblages showed oceanic formation.The main rock assemblages were mudstones intercalating with siltstones,which representing flysch formation of turbidite facies and Bathypelagic-sea facies.The Sb rock assemblages were mainly molasses formation with prunosus thick-layer conglomerate,sand-conglomerate,showing foreland basin characteristics.The basalt of ocean island facies and andesite of subduction background indicated that the Lajishan-Danghenanshan ocea'phase transferred from basin construction to subduction background.The Neo-Mesoproterz-oic detrital zircon U-Pb dating indicated that the Qilian Terrane was affinity with Yangtze Terrane.The Gangou Ophiolite m�lange mainly consisted of peridotite,Mafic-ultramafic rocks,gabbro,volcanic rocks and siliceous rock.By zircon U-Pb dating,the ultramafic rock and gabbro relatively had ages of 483.4�5.0Ma and 486.2�2.7Ma,attributing to the Late Cambrian to Early Ordovician.Zircon?_Hf?t?and T_DM ^C of ultramafic rock suggested a depleted mantle origin.The gabbro had the characteristics of the oceanic ridge basalt and arc island volcanic rocks.With the zircon?_Hf?t?and T_DM ^C,it indicated that the gabbro had the mantle magma origin and the parental magma of gabbro derived probably from the mantle influenced by the subduction.The parental magma was developed by the lower crustal assimilation contamination during the magma evolution and experience a certain extent of separation and crystallization as well.Furthermore,it showed that the Gangou Ophiolite m�lange was formed in a back-arc basin setting and similar to the SSZ type ophiolite.Basing on the ages on the Lajishan-Danghenanshan tectonic settings,it indicated a characteristics of East eaely,West late.The Halunwusu Granites consisted of adamellite-granodiorite with gabbro xenolith,granite.Zircon U-Pb ages relatively were 479.8�3.3Ma?482.8�1.8Ma and460.6�1.8Ma,belonging to Early to Middle Ordovician,which were consistent with the contact relation among the three rocks.The?_Hf?t?values and T_DM^C ages of the three lithologies were respectively-1.94³.18,8.08¹1.67 and-17.28².46,1254Ma¹574Ma,627Ma⁷73Ma and 1282Ma²519Ma.The petrography geochemistry of the adamellite-granodiorite and granite showed that they were genetically of calc-alkaline arc-island I-type granites.The integrated petrology and elemental and isotopic compositions suggest that during the Early Ordovician,the southern margin of the researching area was under the North Qaidam ocea subduction setting.The granites were originated from partial melting of ancient crustal materials.The mafic enclaves were produced by binary mixing between the gabbroic and granitic magmas.The mantle magma origined from the subducting oceanic lithosphere provided the ancient crust with the heat source and made it remelting.The mantle magma mixing with the crust melt formed the highly fractionated I-type granites.It inferred that the late granites was formed in an magmatic arc tectonic environment during the Lajishan-Danghenanshan ocean southward subduction.With the splitting of the Rodinia supercontinent,the Qilian Block came into the stage of ocean-cotinent evolution,in the Early Paleozoic.The evolution of this ocean could be divided into four stages:1,It generated a series of arc island I-type granites,at the setting of the North Qaidam oceanic plate northward subducting under the Central Qilian blocks,during Middle to Late Cambrian.2.The subduction angle became steep,which generated the back-arc basin.The Lajishan-Danghenanshan ocean formed at this stage with the characteristic of East Early,West Late,during Late Cambrian to Early Ordovician.3.During the Middle to Late Ordovician,the North Qaidam ocean closed and the Lajishan-Danghenanshan ocean was at the tectonic setting of southward subduction.The flysch sediment of the O₂y represented the compression stress environment.4.During the Silurian,the closure of the Lajishan-Danghenanshan ocean happened.The molasse sedimentary characteristics of the Sb indicated the continental collision.The North Qaidam and Qilian Block were amalgamated during this time.