Age, Geochemistry And Petrogenesis Of The Weihe Granite Batholith In The Northern Zhangguangcai Range

The Zhangguangcai Range, located in the eastern of the Mongolian Orogenic belt, is akey tectonic component unit in Northeast China. There’s large area of granitic rockscharacterized by immense volumes of granites batholith in this region. Many comprehensivestudies on granitic rocks that are regular and conclusive have been proved effective in thisregion, however, there’s quite limited detailed research work regarding huge granites batholith.Due to the lack of detailed research work on batholith inside, it’s difficult for the researcherswho are engaged in regional geological reconnaissance and science research to accept that theage confirmation of the immense volumes of granites batholith is only based on1-2isotopicages from this region. Moreover, one pending issue is whether such immense and complexgranites batholith is the product of magmatic evolution of the same provenance or not. Third,the granites batholith tectonic setting is controlled by the Paleo-Asian Ocean tectonic domainor by the Pacific tectonic domain. Therefore, this article focuses on Weihe granite batholith,one of the largest granites batholith from the north of Zhangguangcai Range. By detailedstudies of geology, petrography, chronology and geochemistry, we identify the rockassociation of the batholith and the age of the different rock types, analysis the geochemicalcharacteristics, discuss the cause of the rocks and do detailed research on this.1. Rock association and the age of the different rock typesThe Weihe granite batholith is composed of granodiorite, monzogranite, syenograniteand small amount of aklali-feldspar granite, quartz diorite and tonalite. The result of zirconU-Pb isotopic dating indicates that the ages of different rock types in the granites batholith areduring the early Yanshanian of early Jurassic: the age of the quartz diorite and tonalite is190Ma and197Ma respectively, the ages of the granodiorite are195¹90Ma; the ages of themonzogranite are183¹79Ma,196Ma and202²01Ma; the ages of the syenogranite are197Ma and202Ma. The zircon U-Pb isotopic dating of the granites can be dvided into two phases of183¹79Ma and202¹90Ma. The magma crystallization age of202¹90can bedvided into quartz diorite-tonalite-granodiorite and monzogranite-syenogranite, and themagma crystallization age of quartz diorite-tonalite-granodiorite are later than monzogranite-syenogranite.2. Geochemical characteristics of the granitePetrogeochemistry characteristics of the granitic rocks show that quartz diorite-tonalite-granodiorite in Weihe granite batholith has63.25%⁷4.45%SiO₂, low Mg [w（MgO）=0.20%⁰.72%], and comparatively rich sodium（Na₂O/K₂O=1.16³.32）. The total amount ofrare earth elements is not high with rich light rare earth elements and depleted heavy ones,and are enriched in LILE, such as Rb, K, etc., depleted in HFSE, such as Nb, Ta, P, Ti, etc.and with minor negative Eu anomalies or positive Eu anomalies（δEu=0.59¹.18）.The monzogranite has70.37%⁷5.65%SiO₂, low Mg [w（MgO）0.20%⁰.72%], andalso presented with rich Na （Na₂O/K₂O=0.86¹.67）. The total amount of rare earth elementsis not high too, with rich light rare earth elements and depleted heavy ones, and are enrichedin LILE, such as Rb, Ba, K, etc., depleted in HFSE such as Nb, Ta, P, Ti, etc. and withintermediate negative e Eu anomalies or minor negative Eu anomalies（δEu=0.60⁰.81）.The syenogranite has75.20%⁷6.63%SiO₂,0.07%⁰.24%MgO, and high K（Na₂O/K₂O=0.53⁰.99）. The total amount of rare earth elements is lower, with rich light rareearth elements and depleted heavy ones, and are enriched in LILE, such as Rb, K, etc.,butdifferent degrees of depleted in negative Ba anomalies, and depleted in HFSE such as Nb, Ta,P, Ti, etc. and obviously negative Eu anomalies or minor negative Eu anomalies（δEu=0.34⁰.82）.As a conclusion, the main and minor elements in different rock types of Weihe granitebatholith have similar characteristics and change rules: rich silicon and aluminum contents,metaluminous-wealy prealuminous （A/CNK=0.97¹.03）, and I-type granite series genetictype.3. Petrogenesis of the granite batholithThe result of Zircon Lu-Hf isotopic test shows that except the only granodiorite sample（No:1115-1） with negative εHf（t） values （-0.2^-6.7） and higher Hf modal ages （TDM2=1657¹246Ma）, the other samples have positive εHf（t） values of+0.82⁺12.1with Hf modal ages of1184⁴64Ma, including tonalite sample （No:1087-4）, monzogranite sample （No:1094-1and1106-1） and syenogranite sample （No:1094-2）. Combined with the research results from theSr-Nd and Sr-Nd-Pb isotopic compostion in this region, we come to the conclusion that Weihe granite batholith is generated by the partial melting of lower crust during Neoproterozoic andPhanerozoic, and some extent fractional crystallization took place during the phases ofmagma crystallization, and the rock with negative εHf（t） values are products of old crustalmaterials during the elevation and emplacement of magma.Comprehensive research shows that Weihe granite batholith is formed in the tectonicsetting of active continental margin related to the oblique subduction of the Pacific platetowards the Eurasia plate in early Jurassic.