Characteristics Of Zirconology And The Research Significance About The Late Mesozoic Alkaline Rocks In Eastern China

In this paper,a systematic research about zircons was carried out focusing on five alkaline intrusive rocks which distributed on different tectonic units in eastern China, including Nianzishan alkaline granite in Heilongjang province and Baishilazi alkaline granite in Jilin province on the Xingmeng-Jihei Orogenic Belt, Tashan-Huanglongnao alkaline complex rock in Henan province、Xishan alkaline complex rock in Shandong procince which are on the North China Craton and Xianghongdian alkline syenite rock Anhui province of the Dabie Orogenic Belt.The zircons about the late Mesozoic alkaline rocks in eastern China is studied by cathodoluminescence(CL)、high-precision SHRIMP U-Pb analysis and LA-ICP-MS anslysis. On this basis we discuss the genesis of the host rocks and the characteristics and evolution of lithosphere in different tectonic units. The results are as follows:1. The late Mesozoic alkaline rocks in eastern China mainly formed in 125 Ma to 111 Ma of the early cretaceous. The age of alkaline rock masses is respectively 112 Ma, 111 Ma, 123 Ma and 125 Ma in different tectonic units from north to south,which is including the Xingmeng-Jihei Orogenic Belt, the North China Craton and the Dabie Orogenic Belt.2. Zircon crystals of the late Mesozoic alkaline rocks in eastern China are generally a prism in shape. CL images of zircon show that zircon crystals preserved growth zoning. Located in the Dabie Orogenic Belt, zircon grains of alkaline rock, whose particle size is the largest about 300-500μm, are pure. Zircon crystals in the North China Craton are 150-350μm with alteration obviously, while on the Dabie Orogenic Belt the particle size are just 100-300μm and part of zircon grains are slightly alterated.3. According to REE characteristics, zircons of the late Mesozoic alkaline rocks in eastern China are characterized by depletion of light rare elements(LREE), enrichment of heavy rare elements(HREE), the content of high Hf and low Y, and loss of Nb and Ta. In addition to the Nianzishan alkaline rocks with strong negative Ce anomaly and other tectonic units are characterized by positive anomaly of Ce, reflect that the source area of magma is not only related to the mantle, but also have the participation of crustal materials. Zircons distributed on the Xingmeng-Jihei Orogenic Belt perform a significant negative anomaly of Eu(0~0.06), Yb/Gd =5.78~12.64, Nb/Ta=3.38~6.72;the North China Craton is as same as the Dabie Orogenic Belt which has a weak negative Eu anomaly,and they are respectively δE=0.63~1.18, Yb/Gd=0.11~9.36 and δE=0.80~1.08, Yb/Gd=0.93~3.28. These reveal that the magma have experienced high fractional crystallization and the magmatic emplacement is low on the Xingmeng-Jihei Orogenic Belt, speculating the thickness of lithosphere may be thinner; the source magma on the North China Craton has a lowest degree of differentiation and evolution, and the lithosphere thickness is the largest; the Dabie Orogenic Belt takes second place. The magmatic crystallization temperature calculated by Ti zircon thermometry mainly lie between 600℃and 800℃, indicating the diagenetic temperature was relatively high.4. The characteristics of Lu-Hf isotope and trace elements geochemistry show that the late Mesozoic alkaliine rocks on the Xingmeng- Jihei Orogenic Belt are mainly formed by the partial melting of materials from young lithosphere crust related to the depleted mantle( εHf(t)=7.5~33.3, Nb/Ta=3.38~6.72), and the source of rocks on the North China Craton and the Dabie Orogenic Belt is originated from the enriched mantle contaminated by some crustal material in the process of rising invation(Nb/Ta=10~82.80).