相山流纹英安岩和流纹英安斑岩地球化学特征及意义

Geochemical Characteristics of Rhyodacite and Rhyodacitic Porphyry in Xiangshan Volcanic Basin in the Late Jurassic and Their Significance

关于相山火山盆地中打鼓顶组的流纹英安岩与流纹英安斑岩的成岩期次、成岩作用争论已久,通过相山火山盆地流纹英安岩和流纹英安斑岩元素和同位素地球化学特征研究,为两个岩石类型成岩期次和成岩作用对比提供依据,以及更好的深入研究岩浆演化史。结果表明,流纹英安岩和流纹英安斑岩均为过铝质、碱性、钾玄质酸性火山岩系,具有富硅、高碱、富钙、低钛等特点;他们的微量元素与稀土元素地球化学特征也高度相似,均具有Sr,Ba,P,Ti的负异常和Rb,Th,K,Ce的正异常,稀土总含量较高,稀土元素配分曲线为右倾型,轻稀土富集,重稀土亏损,轻稀土分馏程度稍高于重稀土元素,具有明显负Eu异常;Sr-Nd同位素组成相近,均具有相近的ISr值、相近的(^(143)Nd/^(144)Nd)i值、以及范围相近的εNd值。地球化学特征显示出二者在岩浆过程中均主要受到部分熔融作用的制约,其源岩均可能为成熟度较高的上地壳物质。流纹英安岩和流纹英安斑岩具有相似的地球化学组成与特征、相似Sr-Nd同位素组成、高度相似的岩浆过程,表明相山火山盆地早白垩世流纹英安岩和流纹英安斑岩应为同期同源产物,而并非来源于不同期次岩浆过程。

The diagenesis and its stage of rhyodacite and rhyodacitic porphyry in Xiangshan volcanic basin Da gu ding group has long been debated. The purpose of the paper is to provide evidence for diagenesis of the rhyodacite and rhyodacitic porphyry two types of rock, also provide a better study of the history of magma evolution, based on systematical studying on the geochemical characteristics of elements and isotopes about rhyodacite and rhyodacitic porphyry in Xiangshan volcanic basin by comparing its similarities and differences. The studies show that rhyodacite and rhyodacitic porphyry belong to peraluminous, alkalinous, shoshonitic and acidic volcanic rock series, with high content of SiO2 and CaO,high alkali ,poor in titanium;Their rare earth and trace elements geochemical characteristics are highly identical, showing relative negative Sr, Ba, P, Ti elements anomalies, positive Rb, Th, K, Ce elements anomalies, high total REE, and the distribution pattern of REE shows right-inclining type, enrichment of LREE, depleted HREE, fractionation degree of LREE slightly higher than that of HREE, with significant negative europium anomalies;Rhyodacite and rhyodacitic porphyry have similar Sr-Nd isotopic composition, similar ISr content and （143Nd/144Nd）i value and similar eNd content. The geochemical characteristics indicate that rhyodacite and rhyodacitic porphyry are mainly restricted by partial melting during the process of magmatic processes, and de- rived from the continental upper crust material with high maturity probably. The study believe that rhyodacite and rhyodacitic porphyry have similar geochemical compositions and characteristics, similar Sr-Nd isotopic composition, also have highly similar magmatic processes, therefore rhyodacite and rhyodacitic porphyry in Xiangshan volcanic basin should be the production of the same period and source rather than the different stages of magmatism