北京房山侵入岩体:微量元素、稀土元素及Sr同位素组成对其成因的制约

Fangshan Intrusion, Beijing: Constraints from Trace and Rare Earth Elements as well as Sr-Isotope Composition

房山岩体是由 3次脉动式侵位形成的复式岩体 ,不同期次侵位的岩石及其中的包体来自相同的岩浆源区 ,为下地壳的中、基性火成岩。全岩 矿物Rb Sr等时线年龄为 12 6± 3Ma。第 1期至第 2期 ,主要是由以斜长石为主的矿物平衡结晶造成的 ;第 2期至第 3期 ,则可能是批式熔融的结果。包体的成因机制可能不止一种 :英安玢岩质包体和石英粗安玢岩质包体可能分别代表第 2期和第 3期脉动式侵入体的冷凝壳 ;石英正长岩包体则可能代表该侵入体演化到最后的产物 ;

Fangshan intrusion is a complex pluton consisting of three pulsative intrusive stages.Rocks of the three stages and the enclaves are of intermediate basic igneous origin in lower crust.The Rb Sr isochron age of whole rock minerals is 126±3 Ma. The intrusion from the first stage to the second stage mainly resulted form the mineral equilibrium crystallization of plagioclase, whereas that from the second stage to the third stage was probably caused by batch melting. There exists more than one mechanism for forming enclaves: dacitic porphyrite and quartz trachyandesitic porphyrite enclaves may be chilled crusts of the second and the third pulsative intrusive stages respectively, quartz syenitic enclave represents the end product of magmatic evolution, and quartz dioritic and quartz monzodioritic enclaves are probably the early product of the cognate magma.