Petrogenesis And Tectonic Significance Of The Early Paleozoic Mafic-ultramafic Rocks In The Northern Segment Of The Central Qilian

The Qilian Orogen is located along the northeastern margin of the Tibetan Plateau and considered as a typical early Paleozoic accretionary orogenic belt.The Central Qilian is an important component of the Qilian Orogen.Understanding geological structural attributes of the Central Qilian is essential for improved structural background of the early Paleozoic Qilian orogen.There are different types of mafic-ultramafic rocks in the Delenuoer area in the western segment of the Central Qilian.However,these rocks remain under-researched.The Delenuoer mafic-ultramafic intrusions,in the western segment of the Central Qilian,is composed of the Delenuoer ophiolite,the Shibandun cumulates,and the sub-volcanic rocks in bimodal volcanic rocks of the north of Shibandun.In this thesis,the Delenuoer mafic-ultramafic intrusions are regarded as the study object and systemic work of field geology,petrology,geochemistry,geochronology,and plate tectonics are used to ascertain its petrogenesis and tectonic setting.Based on the integrated regional geological studies,we established the evolution of the western segment of the North and Central Qilian.The newly discovered Delenuoer ophiolite is composed of serpentinized peridotite,gabbro?including massive gabbro and cumulate gabbro?,diabase,massive basalt,and pillow basalt.LA–ICP–MS U–Pb dating of zircons from the cumulate gabbro yielded a magmatic crystallization age of 472±4 Ma?MSWD=1.5?.The massive gabbro show SiO₂?42.42 wt.%?,Al₂O₃?12.82 wt.%?,and CaO?9.08 wt.%?.The cumulate gabbros have SiO₂ of 49.78–61.93 wt.%,Al₂O₃ of 0.50–21.63 wt.%,and CaO of 4.70–13.04wt.%.MgO content of the massive gabbro and cumulated gabbros varies from 7.39 wt.%to 22.24 wt.%,and the Mg#values varies from 76 to 81,which are subalkaline series?Nb/Y=0.05–0.10?.The chondrite normalized REE distribution pattern of the massive gabbro is weak left-dipping,relatively depleted LREE and extremely weak negative Eu anomaly?Eu/Eu*=0.9?.In the primitive mantle-normalized trace element diagram,the massive gabbro is relatively enriched in U,depleted in Pb and Sr,and have no obvious anomalies of Nb,Ta,Zr,and Hf.The massive gabbro is consistent with N-MORB in term of REE and trace element.The REE distribution patterns of the cumulate gabbros are relatively flat,with obvious positive Eu anomalies?Eu/Eu*=1.4–1.8?.In the primitive mantle-normalized trace element diagram,the cumulate gabbros are characterized by enrichment of Sr,Pb,and U,depletion of high field strength elements?Nb,Ta,Zr,HF,and Ti?.The basalts have SiO₂?44.9–50.9 wt.%?contents,TiO₂?0.9–1.1 wt.%?contents,Al₂O₃?12.64–14.33 wt.%?contents,CaO?7.66–14.7 wt.%?contents,K₂O?0.00–0.29 wt.%?contents,and MgO?6.03–9.16 wt.%?,giving Mg#values of 50–63 and subalkaline characteristics?Nb/Y=0.11–0.15?.All basalts belong to the oceanic tholeiite.The chondrite normalized REE distribution pattern of the basalts is flat,with slight negative Eu anomaly?Eu/Eu*=0.7–1.2?,and has geochemical characteristics of N-MORB.The massive gabbro and basalts have_?Nd?t?values?+4.5 to+5.3?,indicating that they were derived from a depleted mantle source and that ophiolite is a typical MORB type ophiolite.On the basis of regional geological constraints,it is proposed that the Delenuoer ophiolite,along with the Gulangxia–Delenuoer fault,defines the westernmost part of the tectonic boundary between the North and Central Qilian Shan,which is the oceanic remnant of southward subduction of the North Qilian Ocean slab during the early Paleozoic.The Shibandun cumulates,which intruded into the Precambrian Changcheng metamorphic rock,consist of cumulate olivine pyroxene,cumulate gabbro,and diorite,and has a well multicycle crystallization.Zircons from the cumulate gabbro gave a weighted mean ²⁰⁶Pb/²³⁸U age of 487±3?MSWD=2.6?.The cumulate olivine pyroxene contain 48.06–56.90 wt.%SiO₂,0.64–6.03 wt.%Al₂O₃,2.32–6.82 wt.%CaO,0.08–0.19 wt.%TiO₂,and 24.27–32.75 wt.%MgO,with Mg^#values of 83–87.The cumulate gabbros have SiO₂ of 48.13–49.83 wt.%,Al₂O₃ of 17.01–19.21 wt.%,CaO of9.71–14.11 wt.%,TiO₂ of 0.08–0.30 wt.%,and MgO of 9.71–18.50 wt.%,with Mg^#values of 80–83.Geochemical analysis results show that cumulate olivine pyroxene have similar compositions to the cumulate gabbro,which are subalkaline series?Nb/Y=0.06–0.35?.Both cumulate olivine pyroxene and cumulate gabbro show relatively enriched in LREE,flat patterns of HREE,and positive Eu anomalies?Eu/Eu*=1.0–3.0?.In the trace element diagram,all cumulates display abundant in Sr,Pb,and U,depleted in high field strength elements?Nb,Ta,Ti,Zr,Hf?.This research shows that these rocks with multicycle crystallization distribution are the result of cumulation of basaltic magma.Therefore,the Shibandun cumulates are product of partial melting of asthenosphere mantle,which has been metasomatized by southward subduction fluid of the North Qilian Ocean during the early Paleozoic.They formed in the tectonic environment of continental margin arc.The bimodal volcanic rocks in the north of the Shibandun are composed of sub-volcanic rocks and decites.The SiO₂ content of sub-volcanic rocks is 48.00-52.12 wt.%and that of dacites is 61.84-65.03 wt.%.There are obvious SiO₂ content discontinuities between them.The sub-volcanic rocks have TiO₂?2.13–3.37 wt.%?,Al₂O₃?11.26–16.47 wt.%?,CaO?3.48–7.23 wt.%?,and MgO?4.70–8.84 wt.%?,giving Mg#values of 35-57.These samples show Na₂O?2.88–4.80 wt.%?and K₂O?0.07–0.75 wt.%?,with Na₂O+K₂O contents?2.52–4.93 wt.%?and Na₂O/K₂O ratios?<1?,which are subalkaline series?Nb/Y=0.24–0.43?.The decites have TiO₂?0.75–1.15 wt.%?contents,Al₂O₃?9.91–13.44 wt.%?contents,CaO?3.00–12.32 wt.%?contents,and MgO?3.02–4.92 wt.%?contents,with Mg#values of 48–52.They have Na₂O of 0.18–3.35 wt.%,K₂O of 0.57–2.80 wt.%,with A/CNK values?0.43–1.27?,Na₂O+K₂O contents?0.75–4.86 wt.%?and Na₂O/K₂O ratios?0.31–2.21?.The sub-volcanic rocks show the REE distribution patterns of LREE enrichment,relatively flat HREE,and slight negative Eu anomaly?Eu/Eu*=0.7–0.9?.Th,U,Nb-Ta-La-Ce,and Zr-Hf of these rocks are slightly enriched to flat distribution with strong depleted Sr.The sub-volcanic rocks have geochemical characteristics of intraplate basalts,which may be derived from partial melting of lithospheric mantle and formed in extensional tectonic environment.The dacites are obviously rich in LREE,relatively flat HREE,and negative Eu anomalies?Eu/Eu*=0.6–0.8?.In the primitive mantle normalized trace elements,Th,U,La,Ce,and Pb are enriched,Nb,Ta,Ti,P,and Sr are negative,which are originated from partial melting of mafic materials in the lower crust and formed in the tectonic environment of the continental margin of the post orogenic extension.According to the new date of this study,the continental margin arc?active continental margin?developed in the western segment of the Central Qilian,followed by the post-orogenic tectonic magma evolution stage during the Paleozoic.This provides important petrological and chronological evidence for the southward subduction of the North Qilian oceanic plate under the Central Qilian-Qaidam.On the basis of the regional geological results and previous references,the Qilian Orogen has undergone 5 tectonic events during the Neoproterozoic and Paleozoic.?1?Late Neoproterozoic?about 710–540 Ma?,the opening of the North Qilian ocean.?2?Early-middle Cambrian?about 550–520 Ma?,the continuous expansion of the North Qilian ocean.?3?Late Cambrian-late Ordovician?about 520–445 Ma?,the continuous North Qilian Ocean expansion and subduction with corresponding island arc,back-arc basin,and continental margin arc evolution stages.?4?Late Ordovician-early Silurian?about445–400 Ma?,the closing of the North Qilian Ocean and orogeny.?5?Silurian-Devonian?<400 Ma?,the tectonic evolution stage of post-orogenic extension.