华北地块西南缘陇县保家山古元古代花岗岩及其构造意义研究

Paleoproterozoic Granite and Its Tectonic Significances in Southwestern Margin of North China Plate,Baojiashan Section in Longxian County

在华北地块西南缘陇县保家山西部铁马河剖面出露钾长花岗斑岩,与其伴生的有蚀变辉绿玢岩,整体沿六盘山断裂带分布.保家山钾长花岗斑岩富钾（Na2O/K2O=1.66～ 3.36）,具钙碱性和准铝—弱过铝质特征（A/CNK =0.85 ～ 1.18）.富集轻稀土元素且轻重稀土元素分异明显（LREE/HREE=4.08～12.03）,Eu负异常明显（δEu =0.50～0.53）.富集大离子亲石元素K、Rb、Ba、Th,亏损高场强元素Nb、Ta、Ti、Hf,强烈亏损P、Sr,且具有较高的Rb/Sr和Ga/Al比值,表现出A型花岗质岩石的特征.对两个样品进行锆石U-Pb LA-ICP-MS测年,分别获得n（206Pb）/n（207Pb）加权平均年龄1814±12Ma和1781±12Ma,说明岩体属于古元古代岩体.结合区域地质资料的研究表明,华北地块西南缘在古元古代期间处于强烈伸展背景,推测应该与古元古代贺兰坳拉槽的向南延伸有关.

Proterozoic magmatic rocks are seldom reported in the southwestern margin of the North China craton,but the K-feldspar granite porphyry and altered diabase distributing along the Liupan fault belt in the Tiemahe section in Baojiashan of Longxian County in southwestern margin of the North China Craton are formed in Early Proterozoic.The K-feldspar granite porphyry is characterized by rich potassium （Na2 O/K2O =1.66 ～ 3.36）,Calc-alkaline and metaluminous to weakly peraluminous （A/CNK =0.85 ～ 1.18）series.It shows a LREE enrichment pattern,a obvious differentiation between LREE and HREE （LREE/HREE =4.08 ～ 12.03） and significantly negative Eu anomalies （δEu =0.50 ～ 0.53）.The trace element geochemistry is characterized by enrichment of large ion lithophile elements K,Rb,Ba,Th and depletion of high strength field elements Nb,Ta,Ti,Hf; P and Sr depleted extremely.The K-feldspar granite porphyry has higher ratios of Rb/Sr and Ga/Al,and exhibits characteristics of A-type granitic rocks.Zircon U-Pb LA-ICP-MS ages of two samples of the granite porphyry are 1814 ± 12Ma and 1781 ± 12Ma respectively.According to regional geological data,the Southwestern Margin of North China Plate in Paleoprotozoic is under the strongly extensional setting and probably related to the Helan aulacogen＇s southwards.