Geochemistry And Petrogenesis Of Cenozoic Hannuoba Period Of Basalt In Chifeng Region

In the past20years, the development of geochemical makes scholars have evenmore depth-the study of the Cenozoic basalts in eastern China （East Asia）, and obtain awealth of scientific research. China ’s volcano activity, a lot of volcanic rock in thenortheast china, north china and other regions during the Cenozoic. The Cenozoiccontinental rift type Hannuoba basalt was for many years the Chinese and foreign scholarsfocus of common concern. Cenozoic basalts from Chifeng area, tectonic position located atthe juncture of North China Craton and the Da hinggan MTS.-Mongolia orogenic belt, asthe northern margin of the early Paleozoic North China Craton accretion-type Orogen.Chifeng Cenozoic basalts are belongs to the generalized Hannuoba basalt, Previous studiesof large amounts of data are from the narrow sense of the Hannuoba basalt, ignoring theInner Mongolia and the surrounding areas of broad sense of the Hannuoba basalt research,Especially in Chifeng Cenozoic basalt research.How the magmatism in the process of thesebasaltic rocks? What the magma source and Geodynamic environment of these basalticrocks? What is the relationship between the the Chifeng Cenozoic basalts and theneighboring regions of Cenozoic basalts? And the research base of the eastern InnerMongolia region of Cenozoic basalts and Hannuoba basalt, while the Chifeng Hannuobabasalt compared to the gap significantly. Based on the above idea, this paper choose inNorth of Chifeng City, Inner Mongolia, E118°00′—118°30′，N42°50′－43°10′Hannuobabasalt as the object of study. Through detailed field geological survey line,systematic sampling, on the petrology,geochemistry and whole rock Sr-Nd-Hf isotope research tomake sure the genesis and magma source of the chifeng hanhuoba basalt, then through thecontrast of the surrounding Cenozoic basalts to discuss on northern margin of North Chinalithosphere mantle composition.1、petrology of the Chifeng Cenozoic Hannuoba basaltPetrological characteristics show that there exist such basalt rock types asiddingsite olivine basalt, iddingsite olivine pyroxene basalt and pyroxene basalt. Thehand specimen colour is generally black, Porphyritic texture Matrix, is cryptocrystallinetexture, massive structure. Phenocrysts mainly about olivine, followed by small amountsof plagioclase and clinopyroxene.2、Geochemistry of Chifeng Cenozoic Hannuoba basalt and Sr-Nd-Hf isotope of WholerockMajor elements of Chifeng Cenozoic Hannuoba basalt are translated into CIPWand then divided into two groups.1group is olivine tholeiite and2group is quartz tholeiite.Two groups of basalt has quite similar to the major element characteristics. SiO₂contentsranging from46wt%to52wt%,Na₂O+K₂O ratios being between3.1%～5.3%.MgOcontents and Mg^#values ranging from4.14%to10.57%and from49.1to62.1,. In theHarker diagram,it can be seen that the contents of SiO₂、Al₂O₃、Na₂O decrease linearlyfollowing the increase of MgO contents,but contents of K₂O、TiO₂、P₂O₅No correlationlinearly following the increase of MgO contents showing No correlation with fractionalcrystallization..The Chifeng Cenozoic Hannuoba basalt rare display was light REE enrichmentcharacteristics,obvious Eu positive anomaly. Quartz tholeiite has a low content of rare earthelements The ΣREE=44.4¹45.6μg/g, an average of70.5μg/g, REE fractionation （La/Yb）_N=3.2to10.3, an average of5.2; olive tholeiitic basalt has a high content of rare earthelements the ΣREE=63.2¹36.5μg/g, the average is85.7μg/g, light, REE fractionation （La/Yb） N=5.1⁷.5, an average of6.7. From the olives tholeiite to quartz tholeiite ofthe Chifeng Hannuoba basalt shows a LREE contents lower but HREE content is slightlyhigher trend. The trace elements are displayed with a relatively consistent patterns, showssimilar right-dipping forms, enriched in large ion lithophile elements （LILE）such asRb,Ba,and Sr, Nb and Ta elements for the basalt show obvious positive anomalies in thespider diagram. Similar to the distribution patterns of oceanic island basaltsWe selected7samples for Sr-Nd-Hf analysis test. The Chifeng Cenozoic Hannuobabasalt have high¹⁴³Nd/¹⁴⁴Nd ratios（0.512822-0.512934） and (⁸⁷Sr/⁸⁶Sr)iratios（0.703695-0.704315）, ε_Nd（t） values（3.59-5.77）. Samples of Lu-Hf isotope results show thatChifeng Hannuoba basalt with similar Hf isotopic composition, And have a good positivecorrelation with Nd isotopes.176Hf/177Hf ratios,ranging from0.282969to0.283063,εHf（t） values（+6.89-+10.3）3、Petrogenesis of Chifeng Cenozoic Hannuoba basaltChifeng Hannuoba basalt has the primary magma attribute, its Nb/U and La/Nbratio in the OIB range, Nb and Ta elements for the basalt show obvious positive anomaliesin the spider diagram, Sr-Nd-Hf isotope with OIB characteristics, indicating that the theChifeng Hannuoba basalts from the sources of the asthenosphere. which reflects invisiblecrustal contamination and represents the isotope composition of mantle source. La-La/Smdiagrams showed significantly positive correlation,which infer that partial melting is themain factors affecting the changes in basalt composition. The voted diagram of the Ni-Mg^#and CaO/Al₂O₃-CaO, showing a positive correlation illustrate that basaltic magmaExperienced clinopyroxene and olivine fractionation crystallization during magma ascent.4、Magma source and Source characteristics of Chifeng Cenozoic Hannuoba basaltChifeng Hannuoba basalt has a high Rb/Nb and Ba/Th ratio similar to EM I typeoceanic island basalt.. In the Zr/Nb-Ba/Nb Figure also fall into the EM I type of oceanicisland basalt. Implies that there exist a lot of lithospheric mantle material in the reservoir. Sr-Nd-Hf isotope studies have shown that two types of basalt was the continuous changesin the characteristics between EMI and DMM mantle end-members and have the samethrend. The difference between two type basalt may be just due to the EMI and DMM-typemantle melt in a mixture of different proportions. According to the CIPW standard mineralsimulated basalt depth of origin: olivine tholeiite formed about the depth of about70km,and quartz tholeiite formation depth at about60km.5、Deep process of Chifeng Cenozoic Hannuoba basaltThe two groups of the Chifeng Hannuoba basalts may be from the same or similarsource region, But not comagmatic products. According to the simulation may berepresented by a column under different pressure of molten magma product or pressuremelting column product. However, due to the Chifeng Hannuoba basalt need thelithosphere-asthenosphere interaction, we adopted the Perry, et al （1987） proposed theso-called "pudding" model to explain the deep dynamics mechanism. Finally from thepetrology, geochemistry and Sr-Nd-Hf isotopic Perspective. Through contrast ChifengHannuoba basalt and the surrounding area of Cenozoic basalts, we get the followingconclusions: line of Chifeng-Hannuoba-Jining-tianheyong source region component ofthe northern margin of the North China Cenozoic basalt area. should be a component ofDMM-EM I mixed product, form different basalt is the result of mantle two end-membermixing ratio induced by different, and lithospheric thinning intensity decrease from east toWest. Inner Mongolia Abaga-Mongolia Dariganga Cenozoic basalt area sourcecomponent is the DMM-EM II mixed product, which may mean that the Chifeng andAbaga between West LA, Mulun River fault as the boundary, on both sides of thelithospheric mantle with different attributes...