Determination And Geological Significance Of Late Paleozoic Alkali-rich Intrusions Belt At The Northern Foot Of Yinshan-Yanshan

This paper is based on previous research results,by means of petrology,mineralogy,petrogeochemistry,high-precision SHRIMP zircon U-Pb dating and isotopes,to study the alkali-rich intrusions in the northern margin of North China Craton systematically in terms of space-time distribution,source of magmatic materials and tectonic environment,which can provide more evidences for the study of magmatic activity and the tectonic pattern in the northern margin of the Paleozoic North China Cration.The Huangheshao,Gucheng,and Badaogou intrusions studied in this paper are located in the northern foot of Yinshan-Yanshan in the northern margin of North China Craton,south of the Solon suture zone,and spread along the East-West direction.The main rock category are syenite,monzonite and quartz monzonite.The experimental data shows that the SiO2 content of these whole intrusions accout for 59.08%?66.52%,signifying that these alkali-rich intrusions belong to intermediate rock with quasi-aluminous.Geochemical characteristics of trace elements shows that the rocks enriches lithophile elements(LILE);on the contrast,it's relatively poor in high field strength elements(HFSE).The rare earth elements geochemistry shows distact fractionation of LREE and HREE,reveals a deficit of heavy rare earth elements and enriched the light one;with strong negative Eu anomaly.High precision dating shows that SHRIMP zircon U-Pb ages of Gucheng Pyroxene syenite(G1-2),Hornblende syenite(G5-1),Hornblende syenite(G6-1)are 414.6±5.OMa,415.0±5.9Ma and 413.2±3.3Ma respectively;SHRIMP zircon U-Pb ages of Badaogou monzonite(B2-4)and quartz monzonite(B2-1)are 372.3±4.7Ma and 366.8±5.2Ma respectively.It shows that they are all products of Paleozoic alkali-rich intrusive magmatism.Combined with previous research results on other alkali-rich intrusions(Gaojiacun alkaline syenite in the east of Guyang,Inner Mongolia;Huangheshao alkaline syenite;Shuiquangou alkaline complex in Shangyi county,Heibei province)in the northern foot of Yinshan-Yanshan,inferring that there may be a late Paleozoic alkali-rich intrusions belt in the northern foot of Yinshan-Yanshan,in the northern margin of the North China Craton.The isotopes data of alkali-rich intrusions obtained in this paper shows that the initial values of 87Sr/86Sr(87Sr/86Sr)i and ?Sr(t)change range 0.70412?0.70796 and 1.4?55.3,all the values of ?sr(t)are positive;(143Nd/144Nd)i and ?Nd(t)range 0.510785?0.511539 and-12.2?-12.6,all the values of ?Nd(t)are negative.The geochemical and isotope characteristics analyses indicate the magmas came from the mantle depth,relating to enriched mantle.This coincides with the enrichment characteristics of the mantle in the northern margin of the North China Craton during the Paleozoic.And the magmas may be contaminated by crustal materials in the process of its ascending emplacement.It is speculated that the corresponding diagenetic temperature and the pressure depths of Huangheshao Gucheng and Badaogou intrusions are roughly from the bottom to the middle of the thickend continental crus.The petrogeochemical discrimination based on the tectonic environment indicates the the Paleozoic alkali-rich intrusions belt in the northern foot of Yinshan and Yanshan was mainly formed in the extensional tectonic environment and was the product of collision-postorogenic magmatism.Combined with previous studies on late Paleozoic tectonic and magmatism in the study area,it is concluded that the tectonic environment formed by the Paleozoic alkali-rich intrusions in the northern foot of Yinshan and Yanshan mountains may be closely related to the extensional tectonic background after the arc-continent collision between the northern margin of the North China Craton and the Bainaimiao island arc on its northern side in the late Paleozoic from late Silurian to early Devonian,it proves conclusion of tectonic environment discrimination further.