Petrology Characteristics Of The Zhenanbu Porphyroid Granite In The Chicheng, North Hebei Area And Its Geological Implications

The Zhenanbu porphyritic granite is located at Houlou-Zhenanbu-Changgoumen area, in Chicheng, North Hebei area, which was also named as Zhenanbu gneiss and Changgoumen gneiss before. Through field geological survey, mineralogical chemistry and geochemical analysis, this granite is identified with characteristics of exceedingly low metamorphic degree but not to metamorphic rocks, and deformation predominated. For this reason, it is granite instead of metamorphic gneiss. According to phenocryst amount, the Zhenanbu porphyritic granite is divided into two phases, which are Transitional facies to the original Zhenanbu gneiss, and marginal facies to the original Changgoumen gneiss.The transitional facies and marginal facies of the Zhenanbu porphyritic granite have similar mineralogical chemistry and petrological geochemistry, based on the petrographic and geochemical evidences. But edge phase has less phenocryst, moreplagioclase, higher plagioclase grades, more dark minerals, and lower content of Mg O in biotite than transitional phase.Geochemistry evidences show that the Zhenanbu porphyritic granite is characterized by S-type granite, calcalkaline, aluminium saturation index > 1.0, peraluminous granite, corundum molecules over 1%, and low Ga/Al, Ti O2 / Zr ratio. Rare earth element distribution pattern is the right type, characterized by light rare earth elements enrichment and the heavy rare earth elements depletion. Zhenanbu porphyritic granite has similar trace element distribution pattern, enriched in Rb, Nd, La and Ce, depleted in Nb, Ta, Sr and Y. According to the tectonic discrimination diagrams, Zhenanbu porphyritic granite resulted from caused by collision.Zircon U-Pb geochronology show that the Zhenanbu porphyritic granite(marginal facies) zircon aged 1850±20 Ma, and Zhenanbu porphyritic granite(transitional facies) zircon U-Pb age was 1848.6±7.5 Ma. This suggest that edge phase and transitional phase formed contemporaneous in Paleoproterozoic within reasonable errors. Thus there is Paleoprotero-zoic magmatic event within Chicheng region, Northern Hebei, which is response to the widespread contemporary magmatic activity in Northern China Craton. Therefore, Chicheng region belongs to part of the North China Craton, and Jining-Shangyi-Chongli-Chicheng fault zone is not the northern boundary of North China Craton, the Yunzhou supracrustal rock(including its internal degeneration eclogite, serpentinization harzburgite and serpentinization olive marble lenticular tectonic terrane) is not be the product of “indigenous” metamorphism, but the “alien” rock mass.