| The North Qilian orogenic belt,located between the Middle Qilian block and Alashan block,is a typical Early Paleozoic suture zone,which contains many Precambrian metamorphic volcanic rocks and sedimentary rocks in addition to the Early Paleozoic accretive complex.The tectonic evolution of these crustal remnants is closely related to the convergence and cracking of the Columbia supercontinent.The study of these crustal remnants is helpful to clarify the tectonic evolution history of the Qilian Alashan area and its response to the convergence and cracking of the Columbia supercontinent in the Proterozoic.This article through to the north qilian orogenic belt for the north river rocks change rhyolite,pillow basalts of feldspar and biotite quartz schist of zircon chronology,rock geochemistry and isotope geochemistry study,combining previous research results,reveals the change rhyolite,pillow basalts feldspar and biotite quartz schist background of genesis and formation of rock structural dynamics,The Mesoproterozoic tectonic framework of the western Qilian region is clarified,and the research content of the evolution of the Columbia supercontinent in the Qilian region is enriched.This research mainly obtains the following understanding:(1)The LA-ICP-MS zircon U-Pb ages of the Tudaban metamorphic rhyolite can be divided into three groups.The first group is 1761±17Ma(n=10,the end of Paleoproterozoic),which represents the age of rock formation.The other two groups are 1400-1600Ma and 1100-1200Ma respectively,representing two tectonic thermal events in the study area.The content of SiO2varies greatly from 60.76%-68.25%,the content of Al2O3is 12.69%-15.72,the content of TFe2O3is higher from 5.06%-6.86%,the content of Mg O is higher from 1.82%-2.98%,the average value of Mg#is 37.23,and the content of TiO2 is lower.Is 0.14%-0.32%,the rock is rich in potassium and poor in sodium,the content of Na2O is 0.99%-3.19%,the content of K2O is2.29%-7.33%,the ratio of K2O/Na2O is 0.75-7.39,A/CNK is 0.79-1.32,The Ritman index(?=(Na2O+K2O)2/(SiO2-43))is 1.23-3.90.The sample falls into calc-alkaline-high potassium calc-alkaline-potassium basalt series in SiO2-K2O diagram,calc-alkaline series in AFM diagram,and quasi-aluminous-peraluminous series in A/CNK-A/NK diagram.The total amount of REE in the rocks is81.02×10-6-128.17×10-6,the fractionation of heavy and light REE is obvious,and the LREE is moderately enriched(LREE/HREE=2.63-3.70),(La/Yb)N=7.5-10.5 in the normalized chondrite partition map of REE.There are moderate negative Eu anomalies(?Eu=0.59-0.61),and high field strength elements(Nb,Ta,Ti,Ce,P,etc.)are relatively depleted in the primitive mantle-normalized cobweb of trace elements,while large ion lithophile elements(Rb,Th,U,REE,etc.)are enriched.The geochemical characteristics of the rocks show that 10000Ga/Al=1.1-1.6,Nb/U=3.3-4.8,Nd/Th=2.2-3.3,Ce/Pb=2.8-6.3,Nb/Ta=15.6-16.6,Zr/Hf=35.3-39.7,?Nd(t)=2.07-3.98,It is suggested that the tudaban rhyolite originated from the partial melting of crustal material and was subjected to the mixing of mantle material to some extent.Formed in an active continental margin environment.(2)The SiO2 content of Binggou pillow basalt is 46.39%-48.56%,high Ca O(9.23%-10.44%),Al2O3(14.40%-14.95%)and Mg O(8.41%-8.79%,Mg#=54.02-55.01),and low TiO2(0.73%-0.92%).Relatively rich in sodium and poor in potassium(Na2O=2.45%-2.51%,K2O=0.93%-1.98%,(Na2O+K2O)=3.42-4.49%,K2O/Na2O=0.38-0.79),A/CNK=0.74-0.82,The Rittman index(?=(Na2O+K2O)2/(SiO2-43))is2.20-5.95,and it is calc-alkaline basalt.The total content of rare earth elements is29.72×10-6-52.61×10-6,the content of light rare earth elements is 15.51×10-6-26.64×10-6,and the content of heavy rare earth elements is 14.21×10-6-26.00×10-6.LREE is slightly enriched(LREE/HREE=1.03-1.22),(La/Yb)N=2.1-2.4,and the fractionation of LREE is not obvious.The weak negative Eu anomaly to no anomaly(?Eu=0.85-1.03)is characterized by both N-MORB and IAB geochemistry.In the original mantle-normalized cobweb map of trace elements,Ba,Th,Nb,Ta,La,Ce are relatively depleted,while Rb,U,Pb,P,Ti are enriched.(La/Sm)N=1.7-1.9,Dy/Yb=1.5-1.6,Y/Tb=37.5-45.1,Ba/Th=56.3-228.6,Nb/U=8.2-16.4,Ce/Pb=1.8-3.9,Ta/U=0.4-0.9,The results show that the pillow basalt originated from the asthenosphere mantle and was metasomatized by subduction fluid in the source area.It is the product of partial melting of spinel peridotite and mixed by the crust,and was formed in the backarc extensional tectonic environment.(3)Tudaban biotite quartz feldspar schist has high SiO2content(73.48%-82.86%),low Ca O content(0.23%-0.57%),Al2O3content(6.09%-12.85%),Mg O content of 0.77%-1.23%,Mg#=18-23;The ratio of TFe2O3/Mg O is high with an average of 6.12-8.26.TiO2was 0.31%-0.66%,Na2O was low(0.02%-0.17%),and K2O content varied greatly from 0.86%-3.90%.The total content of rare earth elements is 92.62×10-6-192.26×10-6,the total content of light rare earth elements is73.12×10-6-160.16×10-6,and the total content of heavy rare earth elements is18.90×10-6-39.27×10-6,In the normalized chondrite fragmentation map,LREE is moderately enriched(LREE/HREE=3.39-4.99),(La/Yb)N=8.63-16.93,and has moderate negative Eu anomaly(?Eu=0.55-0.84).In the original mantle-normalized cobweb map of trace elements,Ba,Nb,Ta,Sr,P,Ti are relatively depleted,while Th,U,La,HF,Zr,Gd are enriched.The biotite quartzite feldspar schist is a metamorphic rock,and the protolith is a lithic sandstone.The detrital zircon ages of the biotite quartzite and feldspar schist are mainly distributed in four intervals of 1150-1200Ma,1450-1500Ma,1600-1700Ma and 1800-2000Ma,respectively,indicating that this region has experienced at least four periods of tectonic thermal events.Among them,the tectonic thermal events of 1600-1700Ma,1450-1500Ma and 1150-1200Ma are consistent with the ages of metamorphic rhyolite zircons.In the discriminant map of the tectonic background of sedimentary rocks,the samples all fall into the environment of active continental margin.Combined with the maturity analysis,it is believed that the biotite quartz feldspar schist protolith was formed in the environment of active continental margin.(4)Combined with the study of regional geological background,the Qilian block and the North China craton have tectonic affinity and are located in an active continental margin environment from late Paleoproterozoic to Mesoproterozoic.The Columbia supercontinent converging event ended approximately in the late Paleoproterozoic to early Mesoproterozoic(1.7-1.6Ga),and then the supercontinent entered the stage of tectonic cracking evolution(e.g.,the tectono-magmatic event during 1.4-1.5Ga).The Late Mesoproterozoic(1.1-1.2Ga)tectono-magmatic event was a response to the Rodinia supercontinent convergence event. |
PDF Full Text Request