鲁西中生代富钾火山岩及煌斑岩微量元素和Nd—Sr同位素地球化学

STUDY ON TRACE ELEMENT AND Nd-Sr ISOTOPIC GEOCHEMISTRY OF MESOZOIC POTASH-RICH VOLCANIC ROCKS AND LAMPROPHYRES IN WESTERN SHANDONG PROVINCE

要鲁西中生代富钾火山岩产于断陷型陆相火山岩盆地中，煌斑岩多位于火山岩盆地的外围，二者形成时代相近，产出的构造背景相同，在主量元素、微量元素和Nd-Sr同位素等特征上极为相似．均表现出富碱富钾、富轻稀土及大离子亲石元素和贫高场强元素及亲铁元素的特点，并具有显著偏低的εNd值和明显偏高的I_(Sr)值。研究结果表明，富钾火山岩和煌斑岩均起源于富集型地幔的部分熔融。

The Mesozoic potash- rich volcanic rocks in western Shandong occurred mainly in terriestrial depression volcanic basins. Their distrbution were controlled obviously by deep faults, and the typical localities of them are Zouping volcanic basin, Linqu volcanic basin, Yiyuan volcanic basin, Mengyin volcanic basin and Pingyi- Feixian volcanic basin. Lamprophyres in western Shandong often occurred as dyke swarms outside the volcanic basins, and mainly distributed in two areas, one is in the vicinity of Zaozhuang, such as Xuecheng, Shagou, Tengxian etc., the other is in Xinzhuang. Both potash - rich volcanic rocks and lamprophyres were formed in early Cretaceous (Rb-Sr isochron age of potashrich volcanic rocks in Mengyin is 119. 6± 3. 7 Ma and K- Ar ages of lamprophyres in Zaozhuang and Xinzhuang are 125 - 126Ma and 113Ma respectively ). According to their mineral constitutions and chemical compositions, the lithological association of potash - rich volcanic rocks in western Shandong belongs to a set of trachybasalt-basaltic trachyandesite -trachyandesite - latite - trachyte, of which the latite is the predominant rock type. Lamprophyres in Zaozhuang arc of alkaline- rich ultramafic categories, such as cascadite, olivine and pyroxene- bearing kersantite etc., and those in Xinzhuang belong to a set of alkaline-rich intermediate to basic types,such as leucite- bearing lamprophyres, kersantite and spessartite etc. Geochemical investigation shows that both potash - rich volcanic rocks and lamprophyres have similar characteristics on major clements, trace elements and Nd -Sr isotopic compositions, i. c., they are all rich in alkali, potassium, LREE and LILE, but poor in HFSE and siderophile elements, and have quite low ENd values and obviously higher Isr ratios. The 87 Sr/86Sr ratios and Nd values of potash-rich volcanic rocks and lamprophyres are 0. 71103-0. 71322,- 11. 02 -17. 54 and 0. 70953 - 0. 71109,-11. 57-19. 54 respectively, similar to the carbonatite in this district. Based on detailed geochemical investigations and comprehensive studies of their tectonic setting, it is concluded that both potash-rich volcanic rocks and lamprophyres in western Shandong were originated mainly from the partial melting of enriched mantle.