Geochemical Characteristics And Genetic Mechanism Of The Mafic-ultramafic Rock Of Yaogou In The Northern Taihang Mountains

Mesozoic magmatic activity has been a focus for the study in northern Taihang Mountains.Previous studies have focused on the causation and mineralization of acid rocks in the area,but the study of mafic-ultramafic rocks has been done little.However,the mafic-ultramafic rock is the product of crystallization of the mantle.It is one of the important researchs in understanding the nature of mantle magmatic activity and the exploration of regional tectono-magmatic events.Here,based on the field geological observation,the geology,petrology,geochemistry and isotope chronology of the mafic-ultramafic rock of Yaogou' rock mass has been studied in detail,and then the petrogenic and tectonic setting are discussed.The results are as follows:The mafic-ultramafic rock of Yaogou' rock mass include olive hornblendite and gabbro diorite.The amphiboid of the olive hornblendite presents generally wide clintheriform and retains the characteristics of the pyrolyte cleavage,which indicate the amphibolite is a product of the degradation of the pyroxene and the original rock of olive hornblendite should be olivine pyrostone.Olive hornblendite is a small stock that invades gabbro diorite.The boundary between olive hornblendite and gabbro diorite is distinct not gradual,which indicates a result of two magma crystallization.Olive hornblendite,having the character of high Mg,Fe and low Al,Na,K,belongs to tholeiitic series.Gabbro diorite,having the character of high Al,Na,K and low Mg,Fe,belongs to calc-alkali series.The olivine hornblendite is LREE enrichment,the gabbro diorite is LREE obvious enrichment.The rocks are characterized by depleted high field strength elements of Ta,Nb,Th,U,P,Zr,Hf and enriched large ion lithophile elements of Rb,Ba,K,Pb,Sr.Furthermore,the large ion-stone elements of the gabbro diorite are obvious enrichment.Based on the synthetical analyses,the magma of two rocks may originate from the same mantle source,but they are two phase magma crystallization products which are form different degrees of partial melting.Zircon U-Pb dating results show that the age of olive hosrnblendite is 129.53±0.95 Ma,quartz diorite is 129.5±1.0Ma.It indicates that there the ultra-basic and the intermediate-basic magma emplacement are almost simultaneously in Yaogou area during the early cretaceous.Through the study of elemental geochemical analysis of the mafic-ultramafic rock of Yaogou' rock mass,we think the rock is formed in extension environment,Wulonggou deep fault provides channels for magma rising,and magma is likely to suffer from the influence of crustal contamination during rising process and is controlled by certain crystallization during cooling process.