房山岩体的岩浆来源、形成机制及动力学意义

房山岩体是华北东部典型的燕山期中酸性侵入杂岩体,其地球化学特征为富Si、Na、Al、Sr,亏损高场强元素(Nb、Ta),轻重稀土分异强烈,Sr-Nd同位素具有类似EMI型富集地幔的特点,表明其岩浆来自于加厚下地壳的部分熔融,并且中生代华北东部下地壳已经被早先的富集岩石圈地幔置换.不同基性程度岩浆的不完全混合是形成微粒闪长质包体的原因.侵入杂岩的角闪石压力计显示下地壳部分熔融形成的中酸性岩浆先后在19~8 km里的深度范围内结晶就位.古太平洋板块在地幔过渡带深度于早白垩世水平俯冲至太行山一线,洋壳脱水导致上覆地幔部分熔融,后者成为加热下地壳部分熔融的热源.

Fangshan Pluton: Magma Source, Generation Mechanism and Dynamic Significance

Fangshan pluton is a typical Yanshanian middle-to-acid intrusive complex in the east of North China.It has the geochemical characteristics of high contents of Si,Na,Al and Sr,and lack in high field strength elements(HFSE),such as Nb,Ta,and with a high ratio of the LREE/HREE.The composition of Sr-Nd isotope is similar to that of EMI.It is indicated from the geochemical evidences that,the intrusive magma was generated by the partial melting of thickened lower-crust,and the later had been replaced by the former enriched mantle.The incomplete mixing of magma with different basic degrees might have formed the micro-dioritic enclaves.The amphibole geobarometer reveals that the magma crystallized in the depth of 19 to 8 km.The paleo-Pacific slab "flat-subducted" beneath the NCC to the Taihang Mountains line in the earlier Cretaceous epoch,which resulted in the dehydration of oceanic crust and the partial melting of its superior mantle,and the later would be the heat source to melt the lower crust.