辽河盆地欧利坨子富钾质火山岩特征与成因探讨

The Characteristics and Genesis of Potassic Volcanic Rocks from the Oulituozi Area, Liaohe Basin

欧利坨子位于辽河盆地东部凹陷中段,是新生代古近纪火山活动非常强烈的地区,主要岩性为玄武岩、粗面岩和火山碎屑岩。对该区富钾质火山岩(粗面岩)的喷发特征、喷发年代、地球化学和成因进行了研究。火山口位于该区的中部,属于裂隙式喷发,弧形裂隙的两端喷发强度大,并在晚期形成了破火山口。粗面岩的KAr年代学和地球化学表明,它们形成于沙三时期(38.0～43.0Ma),具有富碱(K2O+Na2O=10.22%～12.47%)、富钾(K2O/Na2O=0.69～1.88)、高铝(Al2O3=17.82%～19.44%)、低钛(TiO2=0.19%～0.67%)的特点,属于典型的钾玄岩系。粗面岩的分布受火山裂隙控制,同时也受区域的断裂活动影响。在该区富钾质岩浆喷发前后,华北东部中、新生代构造体制存在由EW向挤压构造向NNE向以伸展为主的构造格局、由板缘向板内构造环境的重大转折,郯庐断裂由左旋走滑变为右旋走滑,辽河盆地在沙三期进入大幅度张裂期。欧利坨子富钾质火山岩的形成是裂谷期岩石圈伸展作用的产物,是玄武质岩浆经过大比例分离结晶作用的产物,可能与晚中生代太平洋板块俯冲残片的部分熔融有密切关系。

Oulituozi, where the fierce volcanic activities occurred in Eocene of Cenozoic, is located iri the middle eastern sag of Liaohe Basin. The main litholigies are basalt, trachyte and tephra. It is shown in this study that the crater is in the central part of the area, and belongs to fissure eruption type. The eruptional intensities were strong at the two ends that turned to calderas at a later time. The techyte, formed during the time of ES3 （ 38.0 - 43.0 Ma） and with better stored property for oil and gas, has the characters of high alkalies （ K2O ＋ Na2O = 10.22% -12.47% ）, high potassium （ K2O/Na2O = 0.69 - 1.88 ）, high aluminium （ AL2 O3 = 17.82% - 1 9.44% ） and low titanium （TiO2 =0.19% -0.67% ）, and belongs to the typical shoshonite series. The distribution of techyte was controlled by the volcanic fissures and affected by regional structures and faults. During the time of the techyte eruption, some important events took place in East China, such as Pacific Plate changed its moving direction from NNW to NWW, Tanlu fault shifted its movement from left-lateral slip to right-lateral slip, Liaohe rift valley formed to its greatest extent. During these events, the potassic volcanic rocks of Oulituozi were produced during extension stage of the rift with subsequent fractional crystallization and derived from basaltic magma. The genesis of the volcanic rocks possibly had a close relation with partial melting of the subducted slab of Pacific Plate oceanic crust.