| The Feidong section of the Tan-lu fault zone is located in the southern part of the Zhangbaling uplift between the Dabie and Sulu orogen.Mesozoic granitoids are extensively developed in this area and contemporaneous with the Lower Yangtze River Belt?LYRB?igneous rocks which have shown similar geochemical characters,while there are some differences between them.In order to further define the origins of these rocks and their tectonic settings,a number of granite rocks revealed in the Feidong area were selected for this work.Comprehensive petrography,zircon chronology,and elemental isotopic geochemistry were developed.The zircon LA-ICP-MS dating results show that the ages of the formation of the Sidingshan pluton,the Jimingshan pluton,the Zhannongcun pluton,the Shankouling disk,the Shankouling pluton,and the Zhuyuanzhang pluton were 131.0±1.7 Ma,130.4±4.2 Ma,126.6±2.1 Ma,123.1±1.0 Ma,128.8±3.6 Ma and 128.7±1.5 Ma,which ranged from 131123 Ma,and belonged to the products of Early Cretaceous magmatism.The SiO2 content of these rocks is 57.99%76.90%,rich in Al?12.50%15.78%?and rich in potassium?Na2O/K2O=0.551.40?and alkali?Na2O+K2O=6.53%9.00%?,which is high-K calc-alkaline series.The rocks are enriched with Ba,U,K and other large ion lithophile elements and light rare earth elements,and relatively deplete high field strength elements such as Nb,Ta,and Ti,and heavy rare earth elements.The samples all have high Sr(406×10-61375×10-6)low Y(2.6×10-615×10-6)and low Yb(0.26×10-61.21×10-6)and High Sr/Y(53.5×10-6 to240×10-6),high?La/Yb?N(17.1×10-6 to 71.88×10-6)and other characteristics.These results indicate that they have some similar geochemical characters with adakitic rocks which are prefer with those adakites originated from Dabie Orogen thickened low continent crust.These data features are more biased toward Dabie orogenic thicker lower crust-derived adakitic rocks.The contents of the samples from the Zhuyuanzhang Dioritic rock were higher?4.45%6.31%?and Mg#?62.5464.28?,while those of the remaining granitic MgO?0.26%0.91%?and Mg#?25.2844.47?values are relatively low and can be divided into two types:high-Mg adakites and low-Mg adakites.The87Sr/86Sr?t?values of these rocks ranged from 0.7061 to 0.7073,?Nd?t?ranged from-16.64 to-25.21,and the Pb isotopic composition varied from 206Pb/204Pb?t?=15.943 to16.974,207Pb/204Pb?t?=15.318 to 15.427,208Pb/204Pb?t?=36.435 to 37.392,showing strong isotopic composition and low radiogenic Pb isotopic composition,similar to the early stage Yanshanian granites in the Dabie orogen.Comprehensive studies have shown that adakitic rocks in the southern part of the Tan-Lu fault zone are not formed by the partial melting of the young oceanic crust or the assimilation and separation of crystals?AFC?or magma mixing.The source may be similar to the lower crust partial melting of the Dabie orogen.in which the high-Mg adakite rocks are the cause of the de-subsiding,while the low-Mg rocks are the causes of partial melting of the lower crust.At the same time,there are significant differences in the geochemical characteristics of the Yanshanian granite in the Feidong area and the Lower Yangtze area in the contemporary period.Therefore,we believe that the Zhangbailing area should be part of the Dabie-Sulu orogen,which is part of the Dabie orogen.It may have originated from the Indosinian period and the collisional orogenic period. |
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