The Paleo-mesoproterozoic Tectono-magmatism Of The Western North Qilian And Its Geological Significance

The North Qilian Orogenic Belt?NQOB?which is a typical Early Palaeozoic plate suture zone is embeded by many Precambrian continental crust remnants composed of Beidahe Group?BDHG?and Zhulongguan Group?ZLGG?.These crust remnants record the Late Paleoproterozoicto Mesoproterozoic ectono-magmatism of continental convergence-splitting and the formation of oceanic basin,providing important information for the analysis of tectonic evolution and tectonic framework of the North Qilian area?NQ?during Paleo-Mesoproterozoic.Thus,we conducted systematic petrology,mineralogy,isotope chronology,geochemistry and Sr-Nd-Hf isotope analyses on Paleoproterozoic BDHG plagioclase amphibolites andbiotite amphibole gneisses and Mesoproterozoic ZLGG basalts from the western NQOB to unravel their petrogenesis and tectonic significance.Our results,combined with previous study,suggest the following conclusions:?1?The most ancient metamorphic basement existing in the NQOB is the BDHG and the magmatic zircon ²⁰⁷Pb/²³⁵U weighted average age of plagioclase amphibolites is 1740±16 Ma,which represents the crystallization age of their protoliths and indicates that NQ have experienced intense tectono-magmatism in the late Paleoproterozoic.The geochemical characteristics of metamorphic rocks in different strata of BDHG are similar.The plagioclase amphibolites and the biotite amphibole gneisses,located in lower and upper section of BDHG,respectively,have similar contents of SiO₂?45.1-50.5 wt.%,45.8-51.5 wt.%?,variable MgO(3.52-9.39 wt.%,1.43-6.47 wt.%;Mg^#=29.7-52.8,38.4-47.1;Mg^#=100×Mg²⁺/(Mg²⁺+TFe²⁺)),and TiO₂?1.02-3.55 wt.%,1.78-3.57 wt.%?,high TFe₂O₃?16.0-18.7 wt.%,13.7-17.3 wt.%?,enriched in Na₂O?1.36-3.92 wt.%,1.67-2.91 wt.%?and depleted in K₂O?0.14-1.61 wt.%,0.63-1.24 wt.%?.The plagioclase amphibole schists in middle part of BDHG have stable SiO₂?46.4-48.6 wt.%?,MgO?8.41-8.96 wt.%;Mg^#=56.6-58.0?,TiO₂?0.71-3.57 wt.%?and Na₂O?2.45-2.62 wt.%?and slightly variable K₂O?0.93-1.98 wt.%?.It is found that the protoliths of these rocks are calc-alkaline basaltic rocks.The plagioclase amphibolites,plagioclase amphibole schists and biotite amphibole gneisses have characteristics of enriched in LREE??La/Yb?_N=2.02-3.86,1.99-2.35 and 1.15-6.80,respectively?and U and Pb,however,depleted in HFSEs?Nb,Ta,Zr,Hf and Ti?,indicating affinities with island arc basalt?IAB?.Partial of samples show positive Ti anomaly which are accounted of the accumulation of hornblende which are enriched in Ti.The Nd and Hf isotope characteristic(?_Nd?t?=-7.74--0.59 and-5.03--0.62;?_Hf?t?=-9.85-+2.17 and-7.11-+2.01)show evolution trend from slightly depleted to enriched mantle souce,combined with the characteristics of trace elements,indicating that the souce of protoliths of these rocks were metasomatized by subduction fluid and sediment melt,but the source of protoliths of plagioclase amphibole schists were metasomatized only by subduction fluid.Moreover,through the research of apatites in plagioclase amphibolites of BDHG,it is found that the apatites in plagioclase amphibolites were formed in magnetite-haematite buffer oxygen fugacity environmen which is related to subduction environment.Comprehensive studies show that the protoliths of BDHG metamorphic rock series are the prducts of partial melting of asthenospheric mantle metasomatized by subduction fluid and sediments melt and formed in continental arc setting.The protoliths of these rocks have experiedced the crystallization differentiation of olivine,clinopyroxene and partial of them have experiedced the crystallization differentiation of plagioclase and accumulation of Amphibole.?2?The ZLGG are composed of Aoyougou and Huashugou Formation.The magmatic zircon ²⁰⁷Pb/²³⁵U weighted average age of quartz mica schist in the AYGF,ZLGG is 1555±9 Ma,indicating that the tectono-magmatism that generated the AYGF was happened during early Mesoproterozoic.The AYGF can be divided into Lower Aoyougou?LAYGF?and Upper Aoyougou Formation?UAYGF?.The basalts of the LAYGF have relatively low contents of SiO₂?42.6-47.5 wt.%?,MgO?3.74-5.79 wt.%,Mg^#=31.4-42.4?and TiO₂?1.74-2.27 wt.%?,variable CaO?3.62-13.2 wt.%?and low K₂O but high Na₂O?K₂O/Na₂O=0.16-0.43?,high Al₂O₃?14.9-18.0 wt.%?and TFe₂O₃?12.7-16.2 wt.%?and these basalts are sodium alkaline basalts.As for the UAYGF basalts have characteristics of relatively high SiO₂?48.2-51.4 wt.%?,TiO₂?2.10-3.12 wt.%?,MgO?5.75-7.21 wt.%,Mg^#=49.9-53.4?,enriched in K₂O and depleted in Na₂O?K₂O/Na₂O=1.66-5.62?,similar Al₂O₃?15.4-16.3 wt.%?and relatively low Fe₂O₃?11.2-14.0 wt.%?,CaO?2.58-5.98 wt.%?.Thus,these basalts are ultrapotassic alkaline basalts.The LAYGF and UAYGF basatlts are enricned in LREE?La/Yb?_N=5.76-14.49 and 6.92-9.44),LILEs?Rb?Ba?and HFSEs?Nb?Ta?Ti?and coupled with ignorable Eu anomalies.The geochemical characteristics of these rocks reveal that they are OIB-like basalts.Moreover,the Sr,Nd and Hf isotope characteristic((⁸⁷Sr/⁸⁶Sr)_i=0.70464-0.70699 and 0.70490-0.70791,?_Nd?t?=-1.02-+2.56 and+2.15-+5.84;?Hf?t?=-1.49-+5.70 and+4.89-+9.89)show feature of plume-related OIB mantle source and Nd-Hf isotope lower than mantle array indicate that the mantle source are involved in recycled oceanic crust.Fruthermore,the Ce/Pb?10.77-16.13 and 10.60-16.93?)is lower than OIB?²5?and show positive correlation with?_Nd?t?and?_Hf?t?are on account of the involvement of subducted sediments.The above research indicate that The LAYGF and UAYGF basalts were the products of mantle plume and formed in intracontinental rift setting.Furthermore,the highest crystallization temperature of clinopyroxene phenocrysts in AYGF basalts is1470?,which reflects the extremely high potential temperature of the mantle and further attest that the AYGF formed by the mantle plume magmatism.The mantle sources of lower and upper basalts are“garnet+spinel lherzolite”which underwent partial melting with degree of about 5-10%and garnet>spinel lherzolite”with partial melting degree of about 10-15%,respectively.The parental magmas possibly underwent crystallization differentiation of olivine and clinopyroxene,without crustal contamination during the magmatic evolution.?3?The HSGF basalts could be divided into lower and upper basalts.They have relatively low SiO₂?41.7-45.3 wt.%?,Al₂O₃?13.5-14.4 wt.%?,medium TFe₂O₃?10.1-12.8 wt.%?,MgO?6.90-8.76 wt.%,Mg^#=54.5-65.9?.The lower basalt can are alkaline basalts with relatilvely high K₂O?3.15-3.41 wt.%?,total alkali?5.96-6.32wt.%?and TiO₂?2.65-3.18 wt.%?but relatilvely low CaO?7.22-7.96 wt.%?and the upper basalts are tholeiites with relatilvely low K₂O?0.06-0.08 wt.%?,total alkali?0.29-0.31 wt.%?and TiO₂?1.28-1.31 wt.%?but relatilvely high CaO?17.2-18.2 wt.%?.The HSGF alkali basalts are enriched in LREE??La/Yb?_N=27.6-31.2?,LILEs?Rb and Ba?and HFSEs?Nb,Ta and Zr?and without Eu anomaly??Eu=1.01-1.04?.The geochemical characteristics of REE and trace elements show that they are OIB-like basalts.Moreover,the Sr-Nd-Hf isotope characteristics((⁸⁷Sr/⁸⁶Sr)_i=0.70434-0.70637,?_Nd?t?=+4.60-+8.57,?_Hf?t?=+2.12-+8.60)show features of plume-related and HIMU mantle source and Nd-Hf isotope lower than mantle array indicate that the mantle source are involved in recycled oceanic crust.Fruthermore,the Ce/Pb?17.28-22.28?is slightly lower than OIB?²5?and show positive correlation with?_Nd?t?and?_Hf?t?are on account of the involvement of subduction sediments.The above research indicate that the HSGF alkaline basalts were the products of mantle plume and formed in intracontinental rift setting.The mantle sources are“garnet>spinel lherzolite”which underwent partial melting with degree of<5%.The parental magmas possibly underwent crystallization differentiation of clinopyroxene and without crustal contamination during the magmatic evolution.The HSGF tholeiites have geochemical characteristics of slightly enriched LREEs??La/Yb?_N=1.33-1.42?,depleted in LILE?Rb and Pb?and negligible Nb,Ta and Ti anomalies without Eu anomaly??Eu=0.94-1.07?and show N-MORB affinities.The Sr-Nd-Hf((⁸⁷Sr/⁸⁶Sr)_i=0.70131-0.70236,?_Nd?t?=+8.57-+12.64,?_Hf?t?=+14.73-+15.89)isotope characteristics indicate that they were originate from depleted asthenospheric mantle.The comprehensive researches indicate that they were formed in the extensional tectonic environment of initial finite oceanic basin.The mantle sources are“spinel lherzolite”which underwent partial melting with degree of about10%.The parental magmas possibly underwent crystallization differentiation of clinopyroxene without crustal contamination during the magmatic evolution.?4?Pre-Cambrian crustal remnants embedded in the NQOB record the Paleo-Mesoproterozoic tectono-magmatism,which is a response to the convergence-splitting events of Columbia Supercontinent.Based on the regional geological background study,it is concluded that the NQ underwent the tectonic evolutions of paleo-ocean subduction during the late Paleoproterozoic?¹.7 Ga?,plates convergence during the final Paleoproterozoic?1.7-1.6 Ga??,intracontinental rifting and the formation of initial finite ocean basin during Mesoproterozoic?1.6-1.2 Ga?.