Lithogeochemical Characteristics Of Zhangyingzi Ultrabasic Rock Bodies In North Hebei Province

This Paper has completed scientific and detailed study of the Zhangyingzi ultrabasic rocks(dunite,pyroxene peridotite,pyroxenite olive,pyroxenite and amphibolite)based on the geological survey and prospecting research made by predecessors on the Zhangyingzi ultrabasic rocks and relying on the project“General Survey on Vanadium Titanium Magnetite of Zhangyingzi Mining Area in Chengde County of Hebei Province”.Moreover,detailed description and analysis has been made targeting the petrological and geochemical characteristics of the Zhangyingzi ultrabasic rock,with the analogy method adopted to have researched the formation age of the intrusive rocks in this area and simultaneously discussed the petrogenesis of the intrusive rock and its formation tectonic environment.The Zhangyingzi ultrabasic rock mass is dominated by pyroxenite and combined with peridotite and hornblendite as well as the transitional rock types among the three,in which the hornblendite is iron-rich ultrabasic rock while pyroxenite,olive pyroxenite and dunite are iron-contained ultrabasic rocks,so it can be determined that the pyroxenite,olive pyroxenite,pyroxene peridotite and dunite should be the products of the magmatic crystallization and differentiation by means of their field features,while hornblendite should be the product of late intrusion and have an intrusive contact relationship with the former.The M/F value is 1.66~3.98(which is smaller than 6 ~ 7)for each lithology of the Zhangyingzi ultrabasic rock mass and the crystallization temperature is about within the range of 1574.37?~1243.52?,so it should be tholeiite in the plate.In all kinds of rocks of the Zhangyingzi ultrabasic rock mass,only the dunite features slight Eu loss while the europium(Eu)loss is not obvious in all other types of rocks.In the standardized diagram for distribution of the rare earth elements in the chondrite of each type of rock,the distribution curves are supremely similar,so they should be comagma products and all are characterized by relative steepness on the left,slowdown rightwards andrelatively obvious right-leaning(except for Y),showing the distribution characteristics of the right-leaning light rare earth enrichment,which is similar to the distribution characteristics of the rare-earth elements in intraplate alkaline basalt.The standardized spider diagram of the primitive mantle shows that the contents are slightly higher for large ion lithophile elements as well as Sc and La elements in the rock while Rb,Sr,Sc and La are slightly enriched;Nb and Ta feature obvious negative anomaly,Ti features slight content loss and Co is characterized by obvious loss,which conforms to the rock characteristics of the intraplate tholeiite.The Zhangyingzi ultrabasic rock mass is located in the Hongshila-Tougou fault zone together with the Hongshila and Tiemahabaqin rock masses and they have identical or similar lithologic associations,mineral components and geological structure and alteration features;besides,the age test results show that the diagenetic age is about 380~400Ma during the Early-Middle Devonian for Hongshila and Tiemahabaqin rock masses,such rock masses were formed at the extension stage after the arc-continent collision and it is inferred that the Zhangyingzi ultrabasic rock mass is related to such a tectonic environment.The Zhangyingzi ultrabasic rock has the characteristics of the intraplate basalt and at the same time,the discrimination diagram for analyzing and researching the geotectonic environment of the high field-strength elements subject to inactive waves and the standardized distribution curves of the rare-earth and trace elements show that the Zhangyingzi ultrabasic rock mass was formed in the intraplate extensional environment.Moreover,its magma is the product of basaltic magmatic differentiation in the upper mantle and the magmatic evolution experienced the magma fractional crystallization and assimilation hybridism at the early and later stages respectively.