歧口凹陷火成岩的地球化学特征、形成时代及构造意义

歧口凹陷侏罗系火成岩主要以中-酸性的流纹岩、粗面岩和粗面安山岩为主,白垩系主要为中-基性的粗面玄武岩和玄武质粗面安山岩,第三系火成岩以粗面玄武岩、碱性玄武岩及辉绿岩为主。同位素定年结果获得白垩系玄武岩的年龄为133±20 Ma,安山岩和粗安岩年龄分别为111.8±0.9 Ma和122.1±3.1 Ma,第三系辉绿岩年龄为16.57±0.23 Ma。地球化学特征显示,侏罗系火成岩相对低钛、高碱和钠、低铁镁,轻重稀土元素强烈分馏、微量元素相对富集LILE、贫化HFSE、亏损Nb和Ti、P,部分高Sr、低Yb,总体指示为挤压环境下加厚地壳物质的熔融产物,白垩系火成岩中等钛和碱的含量,高钠、铁和镁,稀土和微量元素显示其来自于伸展背景的富集地幔源,但受到陆壳物质的混染,第三系火成岩高钛含量、中等碱含量但变化大、高钠、富铁镁,轻重稀土元素分馏弱到中等、微量元素富集LILE和HFSE,同样来自板内伸展环境的富集地幔源的产物。表明歧口凹陷构造转换发生在晚侏罗世和早白垩世之间。

Geochemical characteristics, forming age and tectonic implications of the igneous rocks in Qikou depression

Petrology and geochemistry study based upon core observation, slice observation and major elements analysis indicates that the main rock types of the Jurassic igneous rocks are intermediate-acid rhyolite, trachyte and trachyandesite, while the Cretaceous volcanic rocks mostly consist of alkali basalt and basaltic trachyandesite.The trachybasalt, alkali basalt and diabase are dominated among the Tertiary igneous rocks.The U-Pb and Ar-Ar isotope studies indicates that the Jurassic basalt formed at 133±20 Ma and the age of trachyte and andesite are 122.1±3.1 Ma and 111.8±0.9 Ma, respectively.The age of diabase is 16.57±0.23 Ma.Geochemical research shows that the Jurassic volcanic rocks are formed in a compression environment while the Tertiary intraplate basalt and diabase are formed in extension setting, and the magma comes from mantle plume-related enriched mantle and was contaminated by crustal material.Therefore, the tectonic setting transformation occurred between the Late Jurassic and the Early Cretaceous.This research is of great importance in the theory for understanding volcanic formation and origin, tectonic setting and its evolution under which the volcanic activity occurs.