内蒙古东乌旗碱性侵入岩的时代、成因及地质意义

Geochronology and Petrogenesis of the Alkaline Pluton in Dong Ujimqi,Inner Mongolia and Its Tectonic Implications

内蒙古东乌旗西部的碱性侵入岩为碱性花岗岩和石英正长岩组合,含钠铁闪石.采用LA-MC-ICP-MS锆石U-Pb法对2件碱性花岗岩样品进行年代学研究.定年结果分别为272.3±0.7 Ma和271.7±0.7 Ma,属早二叠世晚期.13件样品的地球化学数据显示：碱性花岗岩具高SiO2（72.62％～78.46％）,富碱（K2O＋Na2O=7.54％～9.80％）,贫CaO（0.66％～0.91％）、MgO（0.07％～0.31％）的特征,铝饱和指数A/CNK为0.87～0.99,碱性指数NK/A为0.91～1.04,属准铝质-过碱质碱性岩系列;稀土总量中等（∑REE=146.4×10^-6～389.0×10^-6）,轻重稀土分馏均较弱-（La/Yb）N 3.74～6.83],强烈的负Eu异常（δEu为0.12～0.36）;贫Ba、Sr、P、Ti,富Rb、Th、U、Hf等元素,显示“A1”型花岗岩的特征.石英正长岩与碱性花岗岩相比较,Ba的含量（554×10^-6～1230×10^-6）显著高于后者（Ba=74×10^-6～207×10^-6）,Eu负异常缺失（δEu=0.94～1.24）,可能由岩浆源区深度的差异造成的.二者均具非常低Sr（27.20×10^-6～157.00×10^-6,2个样品大于100×10^-6）和高Yb（2.08×10^-6～7.26×10^-6,除P24-11-1为1.36×10^-6）的特征,与板内花岗岩特征一致.综上,我们认为东乌旗碱性侵入岩可能为造山后-非造山伸展体制岩浆作用的产物,标志着大约在272Ma兴蒙造山带部分造山演化可能处于造山后向非造山构造转换阶段.

Alkaline pluton in Dong Ujimqi, Inner Mongolia is an association of alkaline granite and quartz syenite, with minor arfvedsonite. Two zircon samples were collected for LA-MC-ICPMS U-Pb dating and 13 samples for geochemical analysis. Geochronological analysis yields two ages of 272. 3±0. 7 Ma and 271.7±0.7 Ma, suggesting the alkaline granites formed in Early Permian. Geochemical study indicates that the alkaline granites are rich in silica （SiO2 = 72.62% -78.46%）, alkali （K2O＋Na20= 7. 54%-9.80%） and low CaO （0.66%-0.91%）, MgO （0.07-0.31%）, with the character of metaluminous- peralkaline （A/CNK=0.87-0.99, NK/A=0.91-1.04）. The rocks are characterized by moderate REE （146.4×10^-6-389.0×10^-6）, strong Eu depletion （δEu=0. 12-0.36） and low grade fractionation of LREE/HREE[（La/Yb）N 3.74-6.83], with enriched in Rb, Th, U, Hf and depleted in Ba, St, P and Ti, which indicates the Al-type granites. Compared with alkaline granites （Ba=74 × 10^-6 -207 × 10^-6）, quartz-syenites have high Ba （554 × 10^-6 -1230 × 10^-6） and slightly Eu anomalies （δEu=0.94-1.24）, which may be induced by the differences of residual minerals as a result of partial melting. Both of them have very low Sr （27.20×10^-6-157.00×10^-6, 2 samples〉100×10^-6） and high Yb （2.08×10^-6-7.26 ×10^-6, except P24-11-1 = 1.36×10^-6）, similar to within-plate granites. Thus, we consider that the alkaline plutons in the east of Dong Ujimqi had been formed in extensional tectonics, which indicates that -272 Ma may be the tectonic evolution from Post-Orogenic to Anorogenic in the Xing＇anling-Mongolia Orogenic Belt.