北山地区岩浆岩分布广泛,这些侵入岩的形成年龄时间可从早古生代跨至中生代.尽管前人对北山地区岩浆岩形成年龄开展了大量研究,然而多集中于对古生代岩浆岩的研究,而较少关注中生代岩浆.对甘肃北山红旗山井斑状二长花岗岩的年龄做了高精度的激光剥蚀电感耦合等离子体质谱(laser ablation inductively coupled plasma mass spectrometry,LA-ICP-MS)锆石U-Pb测年,获得其年龄加权平均值为(237.6±1.1)Ma(平均标准权重偏差MSWD=0.44),为中三叠世岩浆活动的产物.岩石地球化学数据表明,该岩体具高硅(SiO2=67.17％～68.33％),高铝(Al2O3=15.02％～15.82％),贫镁(MgO=1.07％～1.18％),以及富钠、钾(Na2O=3.35％～3.75％,K2O=4.2％～4.38％,Na2O/K2O=0.80～0.85)等特征,TiO2含量为0.57％～0.64％,CaO含量2.39％～2.6％.轻、重稀土明显分馏,LREE/HREE为36.86～44.52,轻稀土明显富集,具轻微的Eu负异常.微量元素分析发现岩石中明显富集大离子亲石元素Ba及强不相容元素Th、U,亏损Nb、Ta、Ti、Y、Yb、Lu等高场强元素,表现出I型花岗岩的特征.因此,结合区域构造特征,认为在中三叠世时期,北山洋盆尚未闭合,而北山南带处于大洋板块俯冲的大陆边缘岛弧环境.
Zircon U-Pb Dating,Geochemical Characteristics and Geological Significance of Hongqishanjing Rock Mass from Beishan Area,Gansu Province
Magmatic rocks are widely distributed in Beishan area,and the formation age of these intrusive rocks can span from early Paleozoic to Mesozoic.Although many studies have been carried out on the formation age of magmatic rocks in the Beishan area,most of them focus on the Paleozoic magmatic rocks but little on the Mesozoic ones.Laser ablation inductively coupled plasma mass spec-trometry(LA-ICP-MS)zircon U-Pb dating was applied to determine the age of porphyritic monzonitic granite in Hongqishanjing,Beishan,in Gansu Province,which revealed its average age at(237.6±1.1)Ma(MSWD=0.44)during Middle Triassic.The geo-chemical analysis shows that the rock is characterized with high silicon(SiO2=67.17％～68.33％),high alumina(Al2O3=15.02％～15.82％),poor magnesium(MgO=1.07％～1.18％),and rich sodium and potassium(Na2 O=3.35％～3.75％,K2O=4.2％～4.38％,Na2O/K2O=0.80％～0.85％).The contents of TiO2 and CaO are 0.57％～0.64％and 2.39％～2.6％.The LREE/HREE ranged from 36.86 to 44.52,showing obvious enrichment of light rare earth and slightly negative anomaly of Eu.Trace element analysis reveals the enrichment of the large ionic lithophilic elements Ba and strongly incompatible elements Th and U and depletion of the high field strength elements such as Nb,Ta,Ti,Y,Yb and Lu,showing the characteristics of I-type granite.Therefore,combined with the regional tectonic characteristics,it suggests that the Beishan Ocean Basin has not been closed until the Middle Trias-sic,and the southern Beishan Belt is in the continental margin island arc environment under the subduction of the oceanic plate.
porphyritic monzonitic granitezircon U-Pblithogeochemistrysubduction of oceanic platesBeishan area
万方数据
维普
