Chronology And Geochemistry Of Jingshan "Migmatitic" Granite In Bengbu

The relationship between origin of granite and evolution of continental crusthas been one of the issues in the field of earth sciences. The Bengbu uplift islocated in the southeastern edge of the North China craton (NCC), with theTan-Lu fault zone and Sulu orogenic belts as its eastern margin and adjacent toDabie orogenic belt in the south. Chronology and petrogenesis of granites inBengbu uplift, and their relationship with the formation and evolution ofDabie-Sulu orogenic belts have been controversial. However, it is of greatsignificance for us to solve above-mentioned problems not only to understandthe regional tectonic evolution, but also to reveal the effects of Dabie-Suluorogenic belts on adjacent areas. Therefore, the thesis takes the Jingshan granitein Bengbu as a example, reports its chronology, mineralogy, petrology andgeochemistry, as well as Hf isotope of zircons, and discusses the nature ofmagma sources and its tectonic background.1. Chronology of Jingshan graniteZircon U-Pb isotopic chronology was determined for Jingshan granitesusing SHRIMP method. The dating results for magmatic zircons with oscillatoryzoning rims indicate that they range from 158 to164Ma on the concordia, andyield a mean 206Pb/238U age of 160.2±1.3 Ma, suggesting the Jingshan graniteformed in early stage of the Late Jurassic.2. Geochemistry of Jingshan granite and Hf isotope of zirconJingshan granite shows similar geochemical characteristics with the lategranitic veins, i.e. SiO2 contents ranging from 72.82 wt % to 75.07 wt %,K2O/Na2O ratios being between 0.75 and 0.98, Al2O3 contents and A/CNKvalues ranging from 13.88% to 15.05% and from 0.96 to 1.06, respectively. Theybelong to sub-aluminous to weak peraluminous series. In the Harker diagram, itcan be seen that the contents of Al2O3, Fe2O3, FeO, MgO and CaO decreaselinearly following the increase of SiO2 contents, showing a trend of fractionalcrystallization.Jingshan granites have low ∑REE abundances, ranging from 23.16μg/g to45.41μg/g, average being 31.58 μg/g, and are characterized by relativeenrichment in light rare earth elements (LREE), and depletion in heavy rare earthelements (HREE), and obvious positive Eu anomalies (1.23~2.54). TheirLREE/HREE ratios and (La/Yb)N values range from 1.33 to 11.57 and from 0.71to 10.17, respectively.Chondrite-normalized REE distribution patterns of Jingshan granites show"U" type in shape, which is similar to the high-degree fractionation granites. Therelative HREE enrichment can be related to the existence of garnet and zircon.In addition, Jingshan granites are enriched in large ion lithophile elements(LILE) such as Rb, Ba, and Sr, and depleted in high field strength elements(HFSE) such as Zr, Hf, Nb and Ta. Ba, U, Sr, Eu and Y elements for the graniteshow obvious positive anomalies in the spider diagram. In contrast, Th, Nd, andSm elements display obvious negative anomalies in the spider diagram.Jingshan granites have high initial 87Sr/86Sr ratios (ISr) (0.7082 ~ 0.7090)and low εNd(t) values (-15.30 ~ -16.20), corresponding Nd model ages (TDM) of2.47 ~ 2.94Ga. They have relatively average Pb isotopic compositions, i.e.,206Pb/204Pb ratios ranging from 17.030 to 17.095, 207Pb/204Pb ratios ranging from15.404 to 15.443, and 208Pb/204Pb ratios ranging from 37.399 to 37.535.The inherited cores and magmatic zircon rims with oscillatory zoning havesimilar initial 176Hf/177Hf ratios, ranging from 0.282060 to 0.282252 andfrom0.282158 to 0.282258, respectively. Inherited cores show relatively low εHf(t)values (-0.71 ~ -7.33), corresponding Hf model ages is 1.44~1.70Ga. Comparedwith inherited cores, the magmatic zircons have lower εHf(t) values (-15.05~-18.39), being similar to the granite, corresponding Hf model age being1.51~1.64Ga.3. Petrogenesis of Jingshan graniteThe field relationship, petrography, the magmatic zircon rims withoscillatory zoning and geochemical data for the Jingshan granite indicate that itshould be derived from the crystallization of granitic magma. Jingshan granitesare characterized by low SiO2 content (<76%), low K2O/Na2O ratios (<1.0), andlow A/CNK values (<1.1), suggesting it belongs to weak peraluminous rocks.Most of the (La/Yb)N values are <5, and Eu/Eu* ratios >0.7. The above lines ofevidence, combined with the presence of epidote, garnet and sphene, and theabsence of cordierite together, suggests that the Jingshan granite is ofcharacteristic of "I" type granite.4. Magma source and its nature of Jingshan graniteSr, Nd and Pb isotopic compositions of granite, Hf isotope of zircons andthe SHRIMP U-Pb dating results indicate that Jingshan granites have affinitieswith the Yangtze craton basement. The primary magma for the granite should bederived from the partial melting of lower continental crust of the Yangtze craton.5. Tectonic setting of formation for Jingshan graniteThe Bengbu uplift is located in southeastern margin of the North Chinacraton (NCC), i.e., belonging to the NCC. The geochemical data for the granitehave indicated that the magma source for the granite should be the Yangtzecraton basement, implying that the Yangtze craton basement could exist withindeep crust of the NCC. It is consistent with the tectonic features that Yangtzecraton subducted beneath the NCC in north-west direction.Combined with the inherited zircon chronology on the Mesozoic granites ineastern NCC, the discovery of eclogites xenoliths in the Mesozoic adakitic rocksfrom the Xu-Huai area the direction of the Xu-Huai nappe, and geochemicalfeatures on the Mesozoic high-Mg diorites and Fangcheng -Feixian basalts inwestern Shandong together, it is suggested that the collision of the Yangtzecraton with the NCC could take place along the Tan-Lu fault zone in NW or nearW-E direction.It can be concluded that the Jingshan granites in Bengbu uplift could formunder the extensional environment after the rapid exhumation of the Dabie-Suluorogenic belt based on petrogenetic type and petro-geochemical characteristicsof the granite, as well as the analysis for regional tectonic history, which can alsobe demonstrated by the simultaneous volcanic rocks formed under riftenvironment.