胶莱盆地南缘白垩系青山群火山岩地球化学特征

Geochemical Characteristics of Volcanic Rocks in Cretaceous Qingshan Group from Southern Margin of Jiaolai Basin,Shandong Province

在胶莱盆地南缘开展1：5万区调工作的基础上，对胶莱盆地南缘的白垩纪青山群火山岩进行了层型剖面实测及系统采样，经过化学分析及数据整理，得出胶莱盆地南缘白垩纪青山群火山岩SiO2的含量为51．08％～74．86％，全碱（Na2O＋K2O）含量的变化范围为3．2％～8．8％，Al2O3的含量集中于13．18％～17．3％之间，全铁（TFe2O3）含量的变化范围在0．86％～10．48％之间；火山岩微量元素强烈富集大离子亲石元素La和Nd，亏损高场强元素Nb、P和Ti，轻稀土元素（LREE）相对富集，重稀土元素（HREE）相对亏损。后夼组与石前庄组火山岩的地球化学特征较相似，以酸性火山岩为主，而八亩地组和方戈庄组火山岩的地球化学特征较相似，以中一中偏基性火山岩为主；后期喷发的石前庄组与方戈庄组火山岩较前期的后夼组、八亩地组火山岩碱性物质含量增加。通过综合分析得出，胶莱盆地南缘白垩纪青山群火山岩是一套中一酸性过铝质、富钾钙碱性为主的火山岩，其四个不同喷发期次的岩浆应来源于同一岩浆源——下地壳，可能与扬子板块的俯冲作用有关，岩浆形成演化过程中表现出强烈的结晶分异作用，基于青山群火山岩岩性及岩浆源分析，认为其不属于典型的双峰式火山喷发。

On the basis of the work that the regional geological survey of JiaolaiBasin southern margin was carried, the stratotype section for the volcanic rocks of Cretaceous Qingshan Group in study area was measured with system sampling. It was showed that the content of SiO2 in the volcanic rocks of Cretaceous Qingshan Group is about 51.08%-74.86%, total alkalis （Na2O＋K2O） is about 3.2%-8.8%, Al2O3 is about 13. 18% ~ 17. 3%, and TFe2O3 is about 0. 86%- 10. 48%, after chemical analysis and data processing. The character of trace and rare earth element is that La and Nd which are LILE and LREE are relatively enriched, while Nb, P and Ti which are HFSE and HREE are relatively loss. Houkuang and Shiqianzhuang Formation have the similar geochemical characteristics that the volcanic rocks are mainly composed of acidic rocks, while Bamudi and Fanggezhuang Formation have the similar geochemical characteristics that the volcanic rocks are mainly composed of intermediate rocks. The volcanic rocks of Bamudi and Fanggezhuang Formation that erupted lately have more total alkalis than that of Houkuang and Shiqianzhuang Formation. The result of comprehensive analysis was that the volcanic rocks of Cretaceous Qingshan Group in Jiaolai Basin southern margin were intermediate-acidic rocks with peraluminous, K-rich and calc-alkaline characteristic. The magma source of the volcanic rocks that erupted in four different episodes was the same--the lower crust, the reason of which may be related with subduction of Yangtze plate. Magma evolution process of the volcanic rocks showed strong crystal fractionation. Combined with the lithological character and magma source analysis, volcanic rocks of Qingshan Group were not typical bimodal volcanic eruption.