Identification Of The "Phenocryst" Crystal Populations Of The Yangyuan Porphyry-rich Basalt:Petrogenesis And Deep Process

The Yangyuan area is located in Zhangjiakou City,Hebei Province,where a large number of Cenozoic intra-continental small-volume alkaline basalts are erupted.In the field survey and sampling process,we found a new type of Cenozoic alkaline basalt rich in"phenocryst"crystal populations.According to the euhedral degree and mineral composition characteristic,the olivine is divided into four types:low-Fo olivine xenocrysts?core Fo=79.6-80.1,CaO=0.07-0.09wt.%?,high-Fo olivine xenocrysts?core Fo=89-90.7,CaO=0.06-0.09wt.%?,high-Fo olivine euhedral crystals?core Fo=88-8-85.9,NiO=0.19-0.21wt.%?,low-Fo olivine euhedral crystal;at the same time low-Fo olivine euhedral crystals are subdivided into two subclasses:high-nickel olivine euhedral crystals?core Fo=78.5-79.7,NiO=0.13-0.15 wt.%?and low-nickel olivine euhedral crystals?core Fo=78.7-79.3,NiO=0.06-0.09 wt.%?.The clinopyroxenes are divided into three types:the core-colorless clinopyroxene xenocrysts?core Mg#=86.4-90.9,FeO=2-4wt.%?,and the core-green clinopyroxene xenocrysts?core Mg#⁶0,FeO=10.8-12.6wt.%?,phenocrysts phase pyroxenes?the core is light brown Mg#=68.9-79.2,TiO₂=1.56-5.30wt.%?.The orthopyroxenes are divided into two types:The iron-rich orthopyroxene xenocrysts?Mg#=52.5?and the ordinary orthopyroxene xenocrysts?Mg#=90.6-90.9?.The spinels are divided into three types:aluminum-rich spinel?Al₂O₃=56.73-57.53wt.%,Cr₂O₃=10.40-10.53wt.%?,transitional spinel?Al₂O₃=50.86-52.01wt.%,Cr₂O₃=16.20-19.11wt.%?and chrome-rich spinel?Al₂O₃=42.17-45.30wt.%,Cr₂O₃=22.83-25.86 wt.%?.Using high-resolution mineral-scale petrographic observation and electron microprobe analysis,we found that there are many so-called"phenocryst",which are actually xenocrysts and forming a variety of complex crystal population types.This basalt rich in"phenocryst"crystals populations also produces unique whole rock composition and complex mineral structure.The content of the porphyry-rich basalt crystal populations were obtained by crystal size distribution analysis.The olivine phenocryst and the xenocrysts content of the three samples were between 0.49-0.51%and 7.38-9.50%,respectively.The content of pyroxene phenocrysts and xenocrysts is between 0.84-1.23%and 2.97-3.69%,respectively.The grayscale analysis is used to obtain the more accurate Ca and Fe-Mg diffusion band widths,and the magma rise scale is estimated to be between dozen hours and several days.Calculate the crystallization temperature of olivine and pyroxene at a temperature of about 1300-1340°C and a pressure of about 10-11.5Kbar using a thermometer and pressure gauge.Only using geological chemistry to study the magma rise process will make us ignore many real details,such as complex and diverse crystal population types and mineral structures.Moreover,the whole rock composition of the Yangyuan porphyry-rich basalt is not only caused by the partial melting of the deep rock mass and the end-members mixing.The melt had also been modified by a large number of xenocrysts.In the study samples,the content of the xenoliths and xenocrysts from the crust were much less than that from the mantle,probably due to changes in the driving force and type of rise of the magma during the ascent.