外北山碱长花岗岩主要由条纹长石、更钠长石、石英和少量铁质黑云母组成,发育晶洞构造,晶洞由石英晶簇、钾长石充填.在岩石化学上,岩石富SiO2、K2O、FeO、F,贫Al2 O3、MgO、CaO、P2 O5;稀土元素组成上,富集REE(∑REE变化范围为107.4×10-6～330.71×10-6),轻稀土分馏程度明显强于重稀土分馏,Eu负异常明显,稀土元素球粒陨石标准化配分曲线呈现“W”型四分组效应;微量元素上,富Rb、Th、Cs、U、Nb,贫Sr、Ba、P、Eu和Ti,显示出典型的铝质A型花岗岩特征.锆石LA-ICP-MS U-Pb同位素定年表明,该岩体形成时代为96.2±0.8Ma.与东南沿海其它晚中生代A型花岗岩体相比,外北山碱长花岗岩具有明显偏高的Nb/Ta和较低的Y/Nb比值,属A1型花岗岩.综合分析表明,晚白垩世早期东南沿海岩石圈强烈伸展及多组断裂活化,引发铁镁质下地壳在高温低压贫水的条件下部分熔融产生花岗质岩浆,并在结晶分异过程中受到富F流体的交代作用,形成外北山富Nb的铝质A型花岗岩.%Waibeishan alkali-feldspar granite is mainly composed of perthite,albite,quartz,with minor amounts of iron biotite.There developed miarolitic structure,filled by quartz druse and potassium feldspar.In petrochemistry,the rock mass is rich in SiO2,K2 O,FeO,F,while poor in Al2 O3,MgO,CaO,P2 O5.Riching in REE [the variation range of REE is (107.4～330.71) × 10-6],with obviously stronger fractionation of light REE than that of heavy REE and conspicuous negative Eu anomalies.Besides,the "W"-type tetrad effect is present in the granites' REE distribution diagrams.In the aspect of trace element,the rock is rich in Rb,Th,Cs,U,Nb and poor in Sr,Ba,P,Eu and Ti,which displays the typical aluminous A-type granite characteristics.Zircon U-Pb LA-ICP-MS isotopic dating shows that the age of the rock mass is 96.2 ±0.8Ma.Compared with other Southeast Coastal Late Mesozoic A-type granite,Waibeishan alkali-feldspar granite bears significantly higher Nb/Ta and lower Y/Nb,which categorizes it as A1-type granite.Comprehensively,the analysis indicats that the strong extension and fracture activation of early Late Cretaceous lithosphere in the southeast coastal area of China caused partial melting of mafic lower crust in poor water under high temperature and low pressure,which generated granitic magma.The Waibeishan Nb-rich aluminous A-type granite was formed by the metasomatism of F-rich fluid during the fractional crystallization process of the granitic magma.
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