| The indosinian "Five Golden Flower" granitic rock masses, which were developed in Lixian-Minxian ore concentrated area, Western Qinling, consist of Baijiazhuang, Zhongchuan,Lvjing, Luchuba and Jiaochangba granitic bodies, among which mafic enclaves were extensively developed, except for Baijiazhuang body. Some gold,copper,lead,zinc,tungsten-tin deposits, Which have direct genetic relationship with granites are distributed around the "Five Golden Flower" granitic bodies. Consequently research on granite magmatism and rock-forming tectonic background of "Five Golden Flower" granitic bodies are of great significance to limit the metallogenic epoch and metallogenic tectonic setting of these deposits. The petrography systematic study and minerals electron probe, rock geochemistry, Zircon U-Pb geochronology and Zircon Lu-Hf isotopic testing and analysis were conducted on the host granites and mafic enclaves of "Five Golden Flower". Meanwhile, petrogenesis, rock-forming material source and diagenetic dynamic background of "Five Golden Flower" granitic rock masses are discussed in this paper. We get some conclusions as follows:The Zircon U-Pb age of 215± 1.1 Ma to 222±2.9Ma of "Five Golden Flower" host granite and 217.1±2.7Ma to 219.8±1.1Ma of mafic enclaves are obtained.Roughly the same age of host granite and mafic enclaves indicated that both of them were the products of Indosinian magmatism.Based on the research result of this paper,combining other granite chronological data of Qinling orogenic belt,we devided the Indosinian granite magmatism of Qinling into three phases:The first phases lies between 264Ma and 240Ma and the granites were mainly distributed in Western Qinling,such as Tianzishan and Xianggou granitic bodies.Both the second phase of 236Ma to 225Ma and the third phase of 225Ma to 190Ma have the granite distribution in whole Qinling orogenic belt,such as "Five Golden Flower" granitic bodies of Western Qinling and Guangtoushan and Dongjiangkou granitic rock masses of Eastern Qinling.The host granites of "Five Golden Flower" are generally belong to metaluminous to peraluminous and high-k calc-alkaline series rocks,on the granite type discrimination(K2O-Na2O、SiO2-P2O5、Rb-Th、Rb-Y),host granites all belong to I-type granites.The host granites have similar features of rare earth and trace elements,the value of LREE/HREE are between 2.74 and 25.32,the value of (La/Yb)N are between 2.21 and 74.31, Eu exhibits weak depletion; relative enrichment LILE(Rb、Th、U、Pb), relative depletion HFSE(Nb、Ta、P、Ti).The mafic enclaves in the each rock mass are granodioritic rocks,comparing with host granite,it depletes Si,enriches Ti、Fe、Mg and so on,enclaves are are generally belong to peraluminous and shoshonite series rocks,it has similar features of rare earth and trace elements with host granite,but the total REE is higher than host granite, on Hake diagrams,the main oxides vs SiO2 of host granites and enclaves have good linear relationship,such evidence indicates the five rock mass are the product of magma mixing.The Lu-Hf isotopic of host granites and enclaves of the "Five Golden Flower" show that these five rock masses have similar magma source,the host granites and enclaves are all crust-mantle mixture,the crust-derived material may be basement materials of the South Qinling with some old continental materials.The "Five Golden Flower" formed in the third stage(225Ma-190Ma) of indosinian granite magmatism of Qinling,they were under the tectonic setting changed from compression to extension after post collision of Qinling orogenic belt,the break-off of subducted slab and thickened lower crust delamination which induced asthenosphere uplifting,then it led to the partial melting of the lower crust,mantle magma and crustal magma mixed incompletely,Finally,the host granites and mafic enclaves emplaced. |
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