Study On The Late Archean Zanqi Rocks In The Northwest Of Hebei Province And Its Tectonic Significance

Sanukitoids are formed from a special tectonic environment with special geochemical characteristics. Sanukite, as a type of black vitreous volcanic rock, was firstly recognized in the Miocene in Japan, and applied to some Archaean intrusive rocks like the geochemical characteristics of sanukite. Sanukitoids are widespread in Archaean and concentrated after the3.0Ga. The sanukitoids formed in the late Archaean have small volumes. Geologists generally believe that they are formed in the late stages of tectonic evolution and related to ancient subduction. The results indicate that sanukitoids may be an existing evidence of plate tectonic mechanism.On the basis of fieldwork of outcrop of sanukitoids from Wayaokou, Huaian County, Shuangmiao, Xuanhua County, Xiliang, Shangyi County and Wayaokou, Tianzhen County, we systematically gathered rock samples. By polarizing microscope, electron probe analysis, XRF and ICP-MS, we do some researches for sanukitoids on petrography, mineral chemistry and geochemistry. Preliminary conclusions are summarized as follows:(a) The intrusive rocks similar to sanukitoids from the Archaean in Northwest Hebei can be divided into two categories:(1) Granodiorite and monzonite intrusions are metamorphosed to the amphibolite-granulite facies;(2)Metamorphic gabbro and diabase with dike occurrence are metamorphosed to the amphibolite-granulite facies and keep gabbro-diabasic textures.(b)The mineral assemblage of diorite, granodiorite and monzonite is plagioclase+clinopyroxene+orthopyroxene+amphibole+quartz+biotite+magnetite+ilmenite. The mineral assemblage of gabbro-diabase is plagioclase+clinopyroxene+magnetite+hypersthenes+ilmenite. Data of mineral chemistry show:(1) An of plagioclase in diorite is the range of41-43, and belongs to high andesine; An of plagioclase in gabbro is the range of50-54, and belongs to high labradorite.(2) Augite elements in diorite are Wo=45, En=31.5-34and Fs=19.7-21.4, and orthopyroxenes in diorite belong low-Al hypersthenes with Al2O3=0.77wt%; Augite elements in gabbro are Wo=34.8-44.2, Fs=32.5-43.5and En=19.9-21, and orthopyroxenes belong to low-Al hypersthene, Al2O3=1.38wt%. Plagioclases and hypersthenes in these rocks have the characteristics of mantle-derived magma.(c) Geochemical characteristics of intrusive rocks of sanukitoids from the Archaean in Northwest Hebei has SiO2=46wt%-69wt%and enriched Cr, Ni, Sr and Ba. There is a slight negative Eu anomaly or no negative anomalies, and high value of HREE. It has the obvious characteristics of crust and mantle contamination. In contrast to the typical sanukitoids, these intrusive rocks belong to sanukitoids.(d) Sanukitoids include both basic and intermediate-acidic rocks. There is an evolutionary tendency from acidic to basic magma. The evolution from granodiorite to diorite and then to gabbro is composed of a sanukitoid series. Sanukitoids are deduced from melting material of lower crust, which is caused by the failure of oceanic crust subduction and the rising of mantle-derived melt.