藏北羌塘荣玛地区古近纪火山岩地球化学特征及其构造意义

Geochemical and Tectonic Significance of the Paleogenic Volcanic Rocks from Rongma Area, Qiangtang Terrane, Northern Tibet, China

藏北羌塘荣玛地区古近纪火山岩主要为玄武岩、粗面玄武岩、玄武安山岩、粗安岩、英安岩,SiO2含量为41.56%~68.35%,Al2O3含量为11.63%~15.73%;Na2O与K2O含量分别为0.20%~5.06%、0.26%~4.68%,K2O/Na2O=0.06~23.40。岩石富集轻稀土元素、亏损重稀土元素,没有明显的铕正、负异常;总体富集Rb、K、Ba、U、La等元素,亏损Ta、Nb、Sr、P、Ti等元素。地球化学特征表明其形成于板内拉张环境,岩浆来源于青藏高原特殊的壳幔过渡带及受俯冲地壳交代的过渡型地幔和富集型地幔。青藏高原北部新生代可能不仅存在一条特殊的壳幔过渡带,还存在与陆内俯冲作用导致的软流圈上涌有关的过渡型和富集型地幔。

The Paleogenic volcanic rocks from the Rongma area, Qiangtang terrane, northern Tibet, China, consist of basalt, trachybasalt, basaltic andesite, trachyandesite and dacite. SiO2 content ranges from 41.56% to 68.35%, Na2O from 0.20% to 5.06%, K2O from 0.26% to 4.68% and K2O/Na2O ratio from 0.06 to 23.40. The rocks are characterized by an enrichment of LREE, a depletion of HREE and there is little positive or negative Eu anomaly. They are enriched in Rb, K, Ba, U and La, and depleted in Ta, Nb, Sr, P, Ti and other elements. Geochemical features indicate they formed in the tectonic extensional environment within the plate. Some lavas were derived from the special crust-mantle transition zone in the Qinghai-Tibetan Plateau. Others were derived from transitional mantle and enriched mantle sources that were affected by metasomatism of the subducting crust. There may be, not only a special crust-mantle transition zone, but also transitional mantle and enriched mantle related to the upwelling of the asthenosphere caused by intracontinental subduction during the Cenozoic.