Magmatic Response To The Early Paleozoic Tectonic Transition In The South Qilian And North Qaidam And Its Constraints On Orogenic Process

The Qilian Orogen,lying at the north margin of the Tibet Plateau,is a NWW trending long-lived composite orogenic belt,which is bounded by the Qaidam Block to the south,Alax Block to the north,North China Block to the east and Altyn Tagh Fault to the west.Several subduction-accretion/subduction-collision suture zones related to the closure of Proto-Tethys Ocean occurred in this orogenic belt,which recorded the whole Early Paleozoic orogenesis from oceanic subduction,continental subduction/collision,followed orogen building and later orogen collapse,making it a classical region to study the plate tectonic theory.Focusing on the magmatism response to the Early Paleozoic tectonic transition in the South Qilian Belt(SQB)and North Qaidam Belt(NQB),this thesis conducted detained petrology,geochemistry,whole-rock Sr-Nd isotopic composition,and zircon U-Pb and Lu-Hf isotopic composition analyses on Cambrian-Early Silurian volcanic rocks in Dangjinshang,Danghenanshan and Naerzhashan regions in the western segment of the north margin of the SQB,and Yuka-Luofengpo region in the western segment of the NQB,as well as the Late Silurian bimodal magmatism in the Qaidam Mountain,the south margin of the SQB,and reconstructed the Early Paleozoic evolution history of SQB and NQB in the terms of magmatism,which provide new constraints on the evolution of the Proto-Tethys Ocean in the Qilian Orogen.The main achievements and new understandings of this thesis can sum up as follows:1.Systematic study on volcanic sequences in Dangjinshan,Danghenanshan and Naerzhashan regions,western segment of the north margin of the SQB show that these volcanics are related to intra-oceanic subduction,oceanic-continental subduction and arc-continent collision,respectively,and they are magmatism response to the closure of the South Qilian Ocean.(1)E-MORB-type volcanics,adakitic dacite porphyry,and arc-type volcanics occur from south to north in Dangjinshan area.The E-MORB-type volcanics displays similar rare earth elements(REE)patterns with enriched-mid-ocean ridge basalt.But except for water-soluble elements such as Rb,Ba and Sr,the E-MORB-type volcanics have obviously lower high field strength elements(HFSE)contents than typical E-MORB,shows slightly negative anomalies of Nb,Zr,Hf and Ti,and has whole-rock ?Nd(t)values of 4.0 to 6.5.These results show that the E-MROB-type volcanics were generated by partial melting of upwelling enriched asthenosphere mantle associated with little slab-derived aqueous fluid input;The adakitic dacite porphyry,belonging to metaluminous calc-alkaline series,is enriched in large ion lithophile elements(LILE)and LREE,depleted in HREE and HFSE,and has high Sr/Y ratio,which was derived from partial melting of oceanic crust at the early stage of subduction.The adakitic dacite porphyry has decoupled Nd-Hf isotopic compositions with positive zircon ?Hf(t)values of 11.1 to 13.5 and negative whole-rock ?Nd(t)values of-1.6 to-0.8,indicating that the parent magma was derived from both oceanic crust and sediment;The arc-type volcanic rocks include basalt,andesite,and dacite,which are all enriched in LREE and LILE,but depleted in HREE and HFSE and show typical characters of island arc volcanic rocks.Of which,the basalt was derived from the metasomatized mantle wedge;The dacite has depleted Nd-Hf isotopic composition,indicating that its source rock is mainly altered basic oceanic crust,thus probably derived from partial melting of altered oceanic crust triggered by the underplating of basic magma;The andesite is a mixture of basic magma and acid magma.Zircon U-Pb dating obtained formation ages of 500�3Ma and 497�4Ma for the adakitic dacite porphyry and arc-type dacite,respectively.Based on the field distribution and formation ages of the intra-oceanic arc volcanic sequence,we proposed that the north-dip intra-oceanic subduction occurred at?500 Ma and the mature island arc formed at?497 Ma.(2)High-Mg andesite and Mg-andesite occur in Danghenanshan area.The high-Mg andesite has high MgO(6.02wt.%-9.95wt.%),Cr(346-663ppm),Ni(83.9-197ppm)contents and Mg#(69?76),showing characters of primitive andesitic magma in equilibrium with mantle peridotite.It is enriched in LREE and LILE,but depleted in HREE and HFSE,which is similar to the sanukite,thus is complete reaction product of melts generated by partial melting of the oceanic crust with mantle peridotite;The Mg-andesite has relatively lower MgO(2.63wt.%?5.43wt.%),Cr(51.3?116ppm),Ni(20.9?50.8ppm)contents and Mg#(54?64).It is formed by the same melts as high-Mg andesite ascending via early magma channel.Its parent magma is not in equilibrium with mantle peridotite,but contaminated by the mantle material.Zircon U-Pb dating yielded formation age of 441 �5Ma for the high-Mg andesite.In combination with the previous studies on the 470?440Ma continental arc magmatism on the north margin of the SQB,we proposed that the south dipping oceanic-continental subduction occurred during Early Ordovician-Early Silurian.(3)The rhyolite in Naerzhashan area has high Si,ALK and Al,but low Ti,Fe,Mg and Ca,and is enriched in LREE and LILE and depleted in HREE and HFSE,similar to the syn-collisional S-type granite.It was formed by biotite dehydration melting of greywacke under granulite facies conditions of 7>810?,P<9kbar.Zircon U-Pb dating obtained its formation age of 443�2Ma,indicating the arc-continent collision event during Early Silurian.2.The complete magmatic record associated with subduction initiation and the subsequent evolution of an oceanic island arc was found in Yuka-Luofengpo region,western segment of the NQB,which are magmatism response to the intra-oceanic subduction of the North Qaidam Ocean during Early Paleozoic.The intra-oceanic arc magmatic rocks in the Yuka-Luofengpo area are composed of MOR-type/MORB-like basalts,Mg-andesite/boninite,arc-type volcanics/Nb-enriched basalt and plagiogranite.MOR-type basalt has similar characters with mid-ocean-ridge basalt and is generated by decompression melting product of ascending asthenosphere after fragmentation of oceanic lithosphere;The MORB-like basalt has similar REE distribution pattern and positive Nd-Hf isotopic composition with MORB,but is relatively enriched in water-soluble elements(e.g.Rb,Ba,U and Sr)and obviously depleted in other incompatible elements and show negative HFSE anomalies(e.g.Nb,Zr and Hf).Therefore,it is generated by partial melting of ascending asthenosphere mantle affected by slab-derived aqueous fluid.Both the MOR-type and MORB-like basalts were formed during near-trench spreading after subduction began.Mg-andesite/boninite have high Mg#and MgO,Cr and Ni,but low REE contents.It is obviously depleted in HFSE(e.g.Nb,Ta and Ti)and enriched in water-soluble elements(e.g.Rb,Ba,U,Sr,LREE)and Th.These features indicate that the Mg-andesite/boninite were generated when the residual,highly depleted mantle melted at shallow levels after fluxing by aqueous fluid derived from subduction plate;The arc-type volcanics/Nb-enriched basalt are both enriched in LREE and LILE,depleted in HREE and HFSE.The Nb-enriched basalt has high Nb content(9.54?14.3ppm)and negative ?Nd(t).Both the arc-type volcanics and Nb-enriched basalt were derived from the enriched metasomatized mantle wedge,but the mantle source of arc-type volcanics was metasomatized by aqueous fluid whereas the mantle source of Nb-enriched basalt was metasomatized by hydrous melt;The plagiogranite have high Si,Na and Ca,and low K and compatible elements contents.It shows obvious positive Eu anomaly and has depleted Nd-Hf isotopic composition with zircon?Hf(t)values of 13.1 to 15.3 and whole-rock ?Nd(t)values of 3.5 to 4.1.These characters indicate that the plagiogranite was generated by partial melting of cumulate gabbro in lower oceanic crust triggered by the underplating of arc magma.Zircon U-Pb dating obtained the formation ages of 511?509 Ma and 498?497 Ma for the MORB-like basalt and plagiogranite.3.Late Silurian bimodal magmatic rock assemblage occurs in the Qaidam Mountain,the south margin of the SQB,which is the response to the slab breakoff.The dolerite and dacite porphyry assemblage occurs as rock stocks or dykes in the Qaidam Mountain,the south margin of the SQB.The dolerite has similar trace elements patterns with OIB,which is enriched in LREE,LILE and HFSE,and crystalized at 425�7Ma.Combined with previous studies,we proposed that the fertile mantle source of the OIB-like dolerite was likely formed by the crust-mantle interaction between the oceanic crust and juvenile lithospheric mantle at the instability field of rutile above the subduction zone;The dacite porphyry is enriched in Al,Fe,LILE and LREE,depleted in Ca,Mg,Ba,Sr,Eu and HFSE,and is characterized by high Zr concentrations(231?271ppm),10000*Ga/Al ratios(2.51?2.73)and crystallization temperatures(738?836?),which are similar to A-type granite.Zircon U-Pb and Lu-Hf isotopic data revealed the dacite porphyry crystalized at 423�5Ma with ?Hf(t)values ranging from-8.9 to-3.3;The inherited magmatic zircons gave a formation age of 472�8 Ma with ?Hf(t)values ranging from-3.9 to-1.9,contemporaneous with the subduction-related magmatism in the SQB,indicating the dacite porphyry may be derived from the Early Paleozoic arc.The occurrence of OIB-type dolerite and A-type dacite porphyry in the Qaidam Mountain indicate that the tectonic regime had transformed from compression to extensition at 425?423Ma,which is earlier than the appearance of post-collisional magmatism but consistent with the youngest HP-UHP peak metamorphic age in the NQB.Therefore,we proposed that this bimodal magmatisam is the response to the slab breakoff,which triggered the thermal perturbation from the asthenosphere upwelling at ca.425 Ma and the subsequent partial melting of the fertile metasomatized mantle and Early Paleozoic arc,forming the OIB-like dolerite and A-type dacite porphyry in the upper plate of the subduction zone.