Mesozoic Low Temperature Thermochronology And Geodynamics Of The Heigoubei Granitic Pluton In The Middle Segment Of Hengshan Mountain

The north-south gravity gradient zone running through the Da Hinggan MountainsTaihang Mountains-Wuling Mountains is one of the most remarkable geological and geomorphological features in eastern China,it is also an important basis for studying the difference between the East and west of the continental lithosphere in mainland China.The Heigoubei pulton is located in the middle part of Hengshan mountain in the northeast of Shanxi Province,it is also located in the Taihang Mountain gravity gradient zone in the central part of North China Craton,and outcropping large scale Mesozoic granitic intrusions.Thus the petrogenesis of Heigoubei granitic pulton is closely related to he formation of the North-South gravity gradient zone and the destruction of the North China Craton.According to the main elements discrimination diagram and Petrography,the Heigoubei pulton is divided into three facies belts,it is mainly composed of fine-grained monzonitic granite,medium grained monzogranite and porphyry granite.With the analysis of geochemistry of major,trace and rare earth element features,the Heigoubei granitic pulton have high Si O2(66.46%~ 76.72%)and total-alkaki(8.47% ~9.47%),and low P2O5(0.01%~0.17%)and Mg(0.05%~0.93%),belonging to the peraluminous high potassium calc-alkaline rocks.Relative enrichment of light rare earth elements(LREE)and depletion of heavy rare earth elements(HREE),with a “seagull-style” pattern slight REE fractionation and strong negative Eu anomaly(?Eu=0.01~1.07),showing the feature of A-type granite.Further,it can be divided into A1 type granite and Formed in the extensional extension environment after orogeny.Genetic mineralogy study and electron probe analysis shows that biotites is Iron mica and iron biotite;Hornblende is iron hornblende;Plagioclase is mostly feldspar and albite;Estimation of hornblende and biotites mineral thermobarometer exhibits that the Heigoubei granitic pulon formed at P=746~857?,T=83~143MPa,corresponding to crustal depth of approximately 3.1~5.4km,it illustrate that Heigoubei granitic pulon is a medium-deep phase intrusive plution,with ?O2 of ?NNO ~ ?NNO+0.7 and Characteristics of crust mantle mixed source with near shell source,and water content in the melt of 5.6~6wt.%.Combined with the geological background,the emplacement mechanism can be summarized as follows: in late Mesozoic dynamic effects of subduction of paleo Pacific plate to Eurasia,regional tectonic transformation has occurred in the eastern and central parts of the North China Craton.From compressional orogeny to extensional environment,partial melting of lower crust resulted from delamination of the lower crust and mantle lithosphere.The magma subsequently invaded and mixed with the underlying mantlederived material.Finally,the Migmatite emplaced and crystallized under certain temperature and pressure conditions to form the Heigoubei granite pulton composed of fine-grained monzonite,medium-grained monzonite and porphyry granite.By using apatite fission track and(U+Th)/He low-temperature thermochronolgy technology,the Heigoubei granitic pulton is completely crystallized in 100~110Ma.We divided the uplift process into four stages,there is a slow structural uplift at 100 ~ 90.1Ma;In the 90.1 ~ 66.4Ma stage,the Heigoubei pulton entered the rapid uplift stage,and reached the peak of structural uplift at 78.8Ma;In 19.4Ma stage,it begins to rapidly cool down and uplift again.The Late Mesozoic-Cenozoic uplift process in this area may be influenced by the large-scale basin rifting in the middle and eastern part of the North China Craton,which also reflects the regional tectonic events in Hengshan and its adjacent areas.KEY WORDS: North China Craton destruction Taihang Mountain gravity gradient.