河北涞源岩基中的超镁铁质岩研究

RESEARCH ON THE ULTRAMAFIC ROCKS IN THE LAIYUAN BATHOLITH,HEBEI PROVINCE

以窑沟橄榄辉石角闪石岩岩体为代表 ,研究了涞源岩基中超镁铁质岩石。超镁铁质岩石是涞源岩基最早的一次独立岩浆活动形成的 ,角闪石岩岩浆来源于上地幔 ,沿深大断裂上升侵位。角闪石岩与之后的第二阶段闪长岩—花岗闪长岩—二长花岗岩—斑状花岗岩—白岗岩有很大差别。涞源岩基在晚三叠纪就开始形成 ,岩浆活动持续时间在 12 0Ma以上。

The Laiyuan batholith is in the Zijingguan deep fault belt, and is composed of 3 series of intrusive rocks. This paper chooses the Yaogou hornblendite body, one of the two major ultramafic outcrops in the Laiyuan batholith, as a representative to carry out detailed research. Though the hornblendite outcrops are very small, and is cut by later magma, they are concentrated in a small area. Immediately adjacent to the hornblendite is the pyroxene monzodiorite. Further away is the biotite quartz diorite, which is fully gradational to the monzodiorite. There are hornblendite inclusions in the monzodiorite and the quartz diorite, mostly near the hornblendite body. The further from the hornblendite body, the smaller and less the hornblendite inclusions.The hornblendite features poikilitic texture. It consists mainly of amphibole(53%), olivine(13%), bronzite (18%), and biotite(10%). The amphiboles include brown ones and green ones, both magnesiohastingite , with no sharp boundaries between them. But the brown ones contain more Ti and Al(IV), which indicates their deep origin. The chrysolites are generally included in amphiboles and orthopyroxenes. There are small sulfide balls composed of chalcopyrite, cubanite, and triolite in brown amphiboles.Mineralogical and textural characteristics of the 3 kinds of rocks indicate that the monzodiorite and the quartz diorite derived from the same magma. At the contact, there are more early precipitated amphiboles and the monzodiorite undergone more strongly late-magmatic alteration, such as K-feldsparization, sericitization, etc. The hornblendite was emplaced earlier than the quartz diorite. The major and trace element characteristics of the hornblendite and the quartz diorite rocks differ greatly. The REE pattern indicate that the hornblendite came from the mantle. The amphibole K-Ar age of the hornblendite is 223 Ma, and the quartz diorite, 187Ma. The above facts indicates that the Yaogou hornblendite crystalize from a earliest, independent magma which came from the mantle. The magma elevated along the deep fault, and formed the Ⅰ series rocks. Later at the same spot but shallower depth, magma generated, and also emplaced along the deep fault at the beginning. Quartz diorite crystalized firstly. When more magma was emplaced, they occupied the whole deep fault belt. Although the major part of the Laiyuan batholith crystalized in the Late Jurassic- Early Cretaceous Epoch, the earliest part began to form as early as in the Late triassic Epoch. Totally the magmatism lasted over than 120Ma.