华北地块西南缘古元古代花岗斑岩及其构造意义

Paleoproterozoic Granite Porphyry in Southwestern Margin of North China Craton and its Geological Significance

在六盘山南段陇县固关镇东部出露多个原认为属于中一新生代的花岗斑岩岩体，其中的自家沟花岗斑岩显示古元古代壳源岩浆活动的记录，属于钙碱性花岗岩系。该花岗斑岩高SiO2（71．14％～73．33％），高碱（Na20＋K20=7．61％～8．70％），富钾（K2O／Na2O=1．21～2．52，平均为2．03），具有准铝一弱过铝质特点（铝饱和指数A／CNK=0．93～1．09，平均1．00）。稀土元素配分曲线呈轻稀土强烈富集的典型“海鸥型”样式，具有明显的Eu负异常（δEu=0．44～0．47）。岩石富集大离子亲石元素K，Rb，Ba，Th，亏损高场强元素Ti，Nb，Ta，具有壳源A型花岗岩的地球化学特征。对该花岗斑岩进行的锆石U-PbLA-ICP-MS同位素年代测定获得了1846±32Ma的上交点年龄，说明花岗斑岩形成于古元古代而不是前人认为的中-新生代，上奥陶统背锅山组砾屑灰岩实际上是沉积于古元古代花岗斑岩之上。结合区域地质资料研究结果表明华北地块西南缘在古元古代时期处于强烈的伸展环境，这可能为古元古代贺兰坳拉槽向南延伸至六盘山南段提供了直接的岩石学证据。

The granite porphyry in southern section of Liupanshan and eastern section of Longxian Guguan Zhen was considered as Mesozoic-Cenozoic high-K calc-alkaline granites that record Paleoproterozoic magma activities. The granite porphyry contains high level of SiO2 （71.14%～73.33%）, alkaline-rich （Na20＋K20=7.61～8.70）, K-high （K20/Na20=l.21～2.52, with an average of 2.03） , falling into the range of metaluminous to weakly peraluminous （A/CNK=0.93～1.09, with an average of 1.0）. The intense enrichment in LREE asymmetrical right-inclined rare earth element pattern, and the obvious negative Eu anomaly （δEu=0.44～0.47） suggest that the rocks are enriched in LILE （K, Rb, Ba, Th） and depleted in the high field strength elements （Ti, Nb, Ta）. They are characterized by crustal derived A-type granites in geochemistry. Zircon U-Pb LA-ICP-MS dating of the magmatic genetic zircon yields an age of 1846±32 Ma, which should represent the emplacement age of the granite porphyry, suggesting that the granite porphyry was formed in Paleoproterozoic, not Mesozoic-Cenozoic as previously thought. The calcirndites of Beiguoshan Formation in Upper Ordovician were actually deposited on the granite porphyry of Paleoproterozoic. This study shows that the southwestern margin of North China Craton in Paleoproterozoic was in an extensional tectonic environment. It may provide direct petrological evidence for Paleoproterozoic aulacogen of Helan mountain that extends southward to the southern section of Liupanshan.