福建邵武附近下侏罗系的变质

THE METAMORPHISM OF LOWER JURASSIC IN THE VICINITY OF SHAOWU, FUKIEN

解放前,福建地质工作者有这样一种趋向:把本省南靖系以外的千枚岩归入罗峰溪千枚岩系;把所有片岩、片麻岩归入片岩片麻岩系.解放以来,地质工作大力展开之后,前人的把它们当作古老地层的看法,尚普遍地被沿用着.从1955年以来,王宠对此,抱着怀疑.这是福建地质上的一个主要问题.我们试找个具有代表性的地区,对某些变质岩进行观察,来追寻一个解决这个问题的途径.从邵武县城之南的焦坑直至其北的漠口这一段。

Various schistosed, metamorphosed rocks, such as phyilites and schists, in Fukien have been widely considered as Pre-Cambrian in age.The writers investigated and have studied the metamorphosed and un-metamorphosed portions of the Lower Jurassic along the river valley between Shaikou and Mokou near Shaowu. They have also studied several typical rock specimens under microscope.There are four important intrusive rocks, in the order of their intrusion, namely, diorite, biotite granite, pink granite and pegmatite. The intrusion of the pink granite is effective in raising the temperature which is an important factor in the metamorphism of the Lower urassic. The writers have found out the features which are characteristic of the metamorphism of the Lower Jurassic. The formation is only partially but conspicuously metamorphosed, which could be easily distinguished from the un-metamorphosed layer, and it is almost as easy as the un-metamorphosed rocks to establish and sub-divide the stratigraphic colume.Though the die-out of the beds existes, but the lithological characters and rock facies, both of the sedimentary facies and the metamorphosed facies, are still stable.Along some profile, the change from metamorphosed to un-metamorphosed layers is gradual and transitional. Moreover, there are original mineral phase and texture left as a relict in the metamorphosed rocks. The blastopsammitic texture is prevailingly seen thru out this area. Only within some narrow contact-metamorphic zone, there happens to have some injection-gneiss and pthgmatic injection-dikes, but no contact-aureole has been met. In these zones, to compare with that of another place, there is no geological feature that can amount for the metamorphosing influence strengthened by temperature, and there is nothing more than the metamorphosed sandstones and the schists of low-grade metamorphism. The schistocity unexceptionally coincides with the stratification, and their trends are altogether always stable. As to the mineral facies, the mineral assemblages are confined to the green schist facies, especially chlorite-muscovite sub-facies of low grade metamorphism.The features mentioned above bear out the conclusion that only a part of the Lower Jurassic is metamorphosed, and the metamorphism is not of high grade. But some one has taken it's metamorphic grade to be high, and more unfortunately this misconception has guided him to another misconception, that these rocks are Pre-Cambrian in age. What leads to these two misconceptions is that he has been puzzled by the attractive banded structure and schistosity, but has neglected the blastopsammitic texture un-conspicuous to naked eye.By the aid of the lithological data, it can be certain thet the important factors in the metamorphism of the Lower Jurassic are low temperature raised by the post-Jurassic granites and the stress caused by the tangential compressive force in the upmost part of the crust. And, the stress is of more significance. The writers have also found out many features which can be explained by the principle of stress ellipsoid, and that certifies the significance of the stress in the respect of regional geology.There is indeed an old metamorphic series nearby. But it is far less widely distributed than the Lower Jurassic.Finally, the writers are sure that to study both tectonic fracture (with or without displacement) and metamorphism combined is the way to effectively solve the geological problems of Fukien. They are also sure that North Fukien is not a 'Shield' but an activiticing platform, and its overlying beds are more or less metamorphosed.