Characteristics Of Middle Permian Granitoids In Jining, Inner Mongolia And Their Geological Significance

Central Asian Orogenic Belt has undergone intense tectonic and magmatic activities,developing a number of magmatic rocks, which record the history of the Central AsianOrogenic Belt evolution. Formation age, geochemical characteristics, rock type, rockcauses and nature of the source region of these igneous rocks have crucial indicationsignificance for the regional tectonic evolution. The study area is located in Jining, thecentral of Inner Mongolia. In the tectonic position, it is in the north of Jining-Longhuafault, the south of Solon-Linxi fault, and across two tectonic units, the Central AsianOrogenic Belt and the northern margin of the North China Craton. The main object of thearticle is Middle Permian Granitoids. In the study area, it is exposed a large area, widelydistributed, and mainly located in the south of Butumuji, the north of Qahar Right WingRear Banner, Shangdu County, and the west of Huade County. It is mainly distributedalong Hadahashao-Honggeertu-Dakulun-the northwest of Xianghuang Banner, andoverall extending northeast. Through the detailed study of petrology, petrography,geochronology, geochemistry and other aspects, the paper discusses their rock types,petrogenesis, source characteristics and tectonic significance, and finally explores thetectonic evolution history of Central Asian Orogenic Belt in the Late Paleozoic especiallythe Middle Permian.The main results of this paper are obtained as follows: 1. The Honggeertu granite of Middle Permian is mainly syenogranite and monzoniticgranite. It is rich in Si and alkali, poor in Mg, Fe, Ti et al. The value of A/CNK is in0.95~1.41, Alkali aluminum index (AKI) is in0.68~0.97, Alkalinity (AR) is in3.30~6.68,and the granite belongs to weakly peraluminous-peraluminous alkaline granite. Thegranite intrusion has the total REE varing in a large range, and is enriched in LREE anddepleted in HREE with negative Eu anomaly(δEu=0.03~0.89). It is relatively enrichedin high field strength elements (Th, U, Hf, Ta and Y) and depleted in large ion lithophileelements (Ba Eu and Sr). There is a negative correlation between P2O5and SiO2, apositive correlation between Th and Rb, and the sum of high field strength elements(Zr+Nb+Ce+Y) is loss than350×10-6. The rock belongs to highly fractionated I-typegranites and formed in the post orogenic tectonic environment. By zircon LA-ICP-MSdating, we got3zircon206Pb/238U age weighted average which are267.2Ma±1.4Ma(MSWD=1.4),269.2Ma±1.6Ma (MSWD=1.7) and272.1Ma±1.2Ma (MSWD=0.38).This indicates that the rock formed in the Middle Permian.2. The Wulanhada granite of Middle Permian is mainly alkali feldspar granite. It isrich in Si and alkali, poor in H2O, Mg, Fe, Ca et al. The value of A/CNK is in1.00~1.03,Alkali aluminum index (AKI) is in0.93~0.95, Alkalinity (AR) is in5.30~5.67, and thegranite belongs to weakly peraluminous alkaline granite. The granite intrusion has thehigh total REE, and is enriched in LREE and depleted in HREE with negative Euanomaly significantly (δEu=0.22~0.38). It is relatively enriched in high field strengthelements (Nb, Zr, and Hf) and depleted in the elements (Ba, Sr, P, Nb, Ta and Ti). Thegranite has high sum of high field strength elements (Zr+Nb+Ce+Y=402×10-6~640×10-6), and high ratio of10000*Ga/Al(2.36~2.79). The rock belongs to A-type granites andformed in the post orogenic tectonic environment. By zircon LA-ICP-MS dating, thezircon206Pb/238U age weighted average is265.1Ma±1.5Ma (MSWD=1.5). This indicatesthat the rock formed in the Middle Permian.3. After two ancient plate completing collision suture basically and entering the postcollision stage, under the effect of thermal events, Crustal extended and cracked, themagma which came from the partial melting of lower crust started emplacement andconsolidation, so the Honggeertu granite and the Wulanhada granite were formed. In the study area, the collide suture time of two plates should be at least as early as theformation of these two rock masses, that should be at least as early as267.2Ma~272.1Ma.4. Combined with previous research results and research data of other rock in thestudy area, we got the tectonic evolution characteristics of the middle of the northernmargin of the North China plate as follows: Devonian ago(Subduction of oceanic crustand arc-continent collision)â†'Devonian(Continental marginal extensional deformation)â†'Carboniferous(Subduction of oceanic crust and continental arc environment)â†'EarlyPermian(Continental arc environment ended gradually, and enter the syn collision stage)â†'Middle Permian(Two ancient plate completing collision suture, the paleo Asian oceandie, and enter the post orogenic stage).