华北克拉通北缘隆化地区蓝旗镇古元古代石榴石花岗岩的成因及地质意义

Petrogenesis of Lanqizhen Paleoproterozoic Garnet Granite at Longhua Area in the Northern Margin of the North China Craton and Its Geological Significance

隆化地区蓝旗镇石榴石花岗岩位于华北克拉通北缘中部，与古元古代红旗营子杂岩密切共生。岩石主要由碱性长石、斜长石、石英、石榴石以及少量黑云母等矿物组成。锆石UPb定年结果表明，石榴石花岗岩的形成时代为1894~1878Ma，属于古元古代中晚期。岩石的SiO2和K2O含量分别介于64.09%~70.80%和3.64%~5.73%之间，属于高钾钙碱性到钾玄岩过渡系列；岩石具有较高的Al2O3（14.50%~16.13%）和较低的CaO（1.25%~3.27%）和Na2O（1.80%~2.57%）含量，铝饱和指数（ACNK）介于1.07~1.34之间，表明该岩体属于S型花岗岩。岩石具有较高的CaO/Na2O比值（〉0.3），以及较低的Al2O3/Ti2O、Rb/Sr和Rb/Ba比值，指示岩浆源区为贫粘土、富斜长石的砂质岩。锆石Hf同位素分析进一步说明其物源可能是单塔子和红旗营子杂岩经风化作用的产物。蓝旗镇石榴石花岗岩具有与华北克拉通孔兹岩带内S型花岗岩相似的年代学和地球化学特征，结合区域变质岩和岩浆岩资料，本文认为孔兹岩带在空间上可能从内蒙古集宁地区向东延伸到了冀北地区。

The Lanqizhen garnet granite in the Longhua area is situated at the middle part of the northern margin of the North China Craton （NCC）, which is associated with the Paleoproterozoic Hongqiyingzi complex closely in space. The rocks are composed of alkali feldspar, plagioclase, quartz, garnet and less biotite. Zircon U Pb dating indicates that the Lanqizhen garnet granite was formed in 1894-1878 Ma, belonging to mid late Paleoproterozoic. The contents of SiO2 and K2O of the pluton are 64.09-70.80 % and 3.64-5.73 %, respectively, which suggests that the rocks belong to the transitional series between high K calc alkaline and shoshonite series. The rocks have higher Al2O3 （14.50-16.13%） and lower CaO （1.25-3.27%） and Na2O （1.80-2.57%） contents, with the aluminum saturation index （ACNK） being from 1.07 to 1.34. All these petrological and geochemical characteristics suggest that the pluton belongs to S type granite. The rocks have higher CaO/Na2O （〉0.3） and lower Al2O3/Ti2O, Rb/Sr and Rb/Ba ratios, which suggest that the magmatic sources are clay poor, plagioclase rich psammitic rocks. Zircon Hf isotopic analysis further indicates that the psammitic rocks might be the weathering products of the Dantazi and Hongqiyingzi metamorphic complexes. The geochronological and geochemical characteristics of Lanqizhen garnet granite are similar to those of the S types granites in the khondalite belt to the west of the study area. Considering regional metamorphic and magmatic date on the northern margin of the NCC, it is suggested that the khondalite belt might extend eastward from Jining area of Inner Mongolia to north Hebei province.