Zircon LA-MC-ICP-MS U-Pb analysis was carried out for the Xitian granite in east Hunan Province.Two samples from the first period of medium-(fine)grained porphyritic biotite monzonitic granite show ages of (220.9 ± 0.6) Ma and (220.7± 0.7) Ma,respectively.The age of the second period of medium-fine grained two-mica monzonitic granite is (154.4 ± 0.7) Ma.Granites of both periods are high potassium,rich in alkali and weakly peraluminous.Their compositions are characterized by high ∑REE,rich U and Th,depleted HFSE such as Ti,P and LILE such as Ba,Sr,and high 104 Ga/Al.These characteristics indicate that they are A-type granites.The Indosinian granite is rich in LREE,with rather weak Eu negative anomaly.The Yanshanian granite shows no obvious fractionation between LREE and HREE,with significant Eu negative anomaly.Hf isotopic analysis indicates that the two periods of granite have low εHf(t) (-4.91 ～-11.04),and the two stage Hf model ages of the depleted mantle are concentrated in 1.6～ 1.8 Ga,in accordance with characteristics of the old metamorphic basement of Cathaysian block.Therefore,the Xitian granite is a product from partial melting of Proterozoic crust materials of Cathaysian block under the extensional setting.The confirmation of Xitian Atype granite has important significance for understanding the tectonic background of the east part of Hunan Province during Indosinian and Yanshanian movements.It also provides more evidence that there is W-Sn mineralization related to A-type granite during Indosinian period in South China.%对湘东锡田岩体进行的LA-ICP-MS定年结果表明,2个第一期中(细)粒斑状黑云母二长花岗岩样品的年龄分别为(220.9±0.6) Ma及(220.7±0.7)Ma；第二期中细粒二云母二长花岗岩的年龄为(154.4±0.7)Ma.两期花岗岩均为高钾、富碱、弱过铝质岩石.稀土元素总量较高,富集U、Th,亏损Ti、P等高场强元素和Ba、Sr等大离子亲石元素,具高的104 Ga/Al,显示A型花岗岩特征.印支期花岗岩略富集轻稀土元素,Eu异常较弱；燕山期花岗岩轻、重稀土元素分异不明显,Eu异常显著.Hf同位素研究显示,两期次花岗岩均具有较低的εHf(t)(-4.91～-11.04),亏损地幔二阶段模式年龄集中在1.6～1.8 Ga,与华夏地块古老的变质基底年龄一致.因此,锡田岩体是华夏地块古元古代地壳物质在伸展的构造背景下部分熔融的产物.锡田A型花岗岩复式岩体的确定,对于研究湖南东部在中生代印支、燕山两期构造事件中所处的构造背景具有重要意义,同时也为华南地区存在印支期A型花岗岩钨锡成矿作用增加了证据.
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