Geochemical Characteristics,Geochronology And Tectonic Implications Of Mafic-ultramafic Rocks From Dashantou,Beishan,Gansu

Beishan lies in the south of CAOB,and is an important part of CAOB.Dashantou locates in the rift zone of Beishan.By exploring the rock masses of Dashantou,it is of significant importance for establishing tectonic framework,providing a reliable geochemical and chronological basis for magmatic evolution and also enriching the tectonic evolution history in Beishan.The study focuses on mafic-ultramafic intrusion on the Dashantou rock masses: Hongliugou and Miaomiaojing.Based on the field inspection,we firstly confirmed the invasion stage and then deeply investigated the procedure and the cause of tectonic dynamics.By means of the data analysis from the onsite study and geochemistry,it shows that the samples of Hongliugou and Miaomiaojing from composite rock mass are basically mafic-ultramafic rocks.The lithology of Hongliugou is composed of Gabbro,Olivine Gabbro,Pyroxenite and Serpentinized Peridotite.Moreover,the lithology of Miaomiaojing has characteristic of two-stage invasion which consists of Pyroxene Peridotite,Olivine Pyroxenolite,Pyroxenite,Hornblende Gabbro,Diorite.Detailed geochronological study shows that,the age of LA-ICP-MS zircon U-Pb in Hongliugou rock mass is 401.6±2.2Ma which means that it is formed in the early Devonia.The age of LA-ICP-MS zircon U-Pb in Miaomiaojing gabbro rock mass is356.8 ± 0.92 Ma.Combined with previous researches,it means that it is formed in the stage of late intrusion on gabbro.The amount of SiO₂?45.6%-51.9%?and TFe?TFe₂O₃=6.38%-12.59%?in Hongliugou gabbro varies entirely.K₂O?0.07%-1.32%?and Na₂O?0.55%-2.39%?stay in low level.The amount of relative K-poor?Na₂O/K₂O= 1.5-16.8?,Al2O3?7.68 %-18.85%?,MgO?7.68%-18.75%?,Mg#=68-78,CaO?6.0%-10.2%?and TiO₂?0.28%-1.6%?has been measured.At the same time,Gabbro has positive anomaly in Eu and Sr.It is also full of large ion lithophile elements?Rb,Sr,Ba,etc.?,light rare earth elements?LREE?,high depletion of high field-strength elements?Nb,Th,Ce,etc.?and heavy rare earth elements?HREE?.Augite peridotite has low SiO₂?38.8% 38.95%?,Al2O3?4.05% ⁴.6%?,Ca O?2.71% 2.81%?,TiO₂?0.1%⁰.14%?,low level of K₂O?0.05% 0.14%?,Na₂O?0.21%⁰.47%?and K-poor?Na₂O/K₂O=3.4⁴.2?.It also has high level of MgO?27.9%³0%?,TFe₂O₃?15.06%¹6.44%?,and Mg#=85⁸8.Besides,it is full of large ion lithophile elements,light rare earth elements?LREE?,depletion of high field-strength elements?Nb,Ti?,heavy rare earth elements?HREE?and depletion of large ion lithophile element Sr.Rock geochemistry study shows that,the geochemical features of both two kinds of rocks in Hongliugou rock mass are similar to arc magma.The different characteristics of Th between augite peridotite and gabbro show that two kinds of rocks were mixed with low degree of crustal substances during the magma migration process and subduction metasomatic fluids are involved in the rock producing procedure.The amount of gabbro which invades the Miaomiaojing at the early stage varies from 48.78% to 55.47%.Compared with gabbro formed in the later period,it has high level of Al2O3?10.62%²0.7%?,K₂O?0.39%¹.3%?,Na₂O?1.52%³.45%?and TiO₂?0.67%².21%?.Conversely,it has low amount of MgO?4.96%¹1.7%?.Moreover,TFe₂O₃?6.73%¹1.49%?,Mg#?55⁷0?,relative depletion of high field-strength elements?Nb,Ba?,large ion lithophile elements?Rb,Th,etc.?,light rare earth elements?LREE?,high depletion of high field-strength elements?Nb,P,Ba?and heavy rare earth elements?HREE?has been detected and measured.There is also mild negative anomaly of Eu inside of it.In the lately formed gabbro,the amount of SiO₂ varies from 50.26% to 52.32%.It has high amount of MgO?21.5%²3.6%?.TFe₂O₃?8.64%¹2.34%?,Mg#?82⁸8?,Al2O3?2.51% 3.7%?,K₂O?0.03%⁰.06%?,Na₂O?0.25%⁰.42%?,Na₂O&K₂O?0.28%⁰.47%?and K-poor?6.8¹2.3?have been measured.TiO₂?0.3%⁰.42%?and Na₂O?0.25%⁰.42%?has relatively low amount.The content of CaO?2.82%¹1.05%,average amount: 7.53%?varies a lot.Large iron lithophile elements and depletion of high field-strength elements are gathering in the gabbro.High and light rare earth elements do not have obvious differentiation.Two-stage geochemistry study on Miaomiaojing rock bass shows that crustal contamination affect magma.However,magmatic source had been influenced by subduction metasomatic fluids which creates beneficiation effect on the mantle source.Miaomiaojing rock masses are the products of depleted mantle melting.Hongliugou rock and the earlier formed Dashantou rock,formed in the active continental margin of the post-collisional extensional tectonic setting.This responses that limited marine basin located in the south of Beishan was in the transformational period from extrusion to extension tectonic regime after subduction-collision in early Devonian and early Permain.Accordingly,this zone extended the structure to a new stage of evolution: epicontinental arc turns into rift environment under the action of the extensional tectonics and mantle uplift in which the lately Miaomiaojing gabbro formed.