武夷地块中古元古代镁铁质-超镁铁质岩石的发现——剖面介绍及岩石学、岩相学、年代学特征

The discovery of Paleoproterozoic mafic-ultramafic rocks in the Wuyishan Block:Description of profile and characteristics of petrology,petrography and isotope geochronology

通过1:250 000衢州区域地质调查,在武夷地块浙西南地区发现古元古代镁铁质-超镁铁质岩石。该套岩石在金华、龙游-带呈面状分布,金华张村出露最为完整,主要由辉石角闪石岩、角闪石岩、斜长阳起石岩、斜长辉石岩和斜长角闪岩等组成。对辉石角闪石岩与阳起石岩开展了LA-ICP-MS锆石U-Pb测年分析,分别获得其成岩年龄为1 834±14 Ma(MSWD=0.23,N=16)、1 839±17 Ma(MSWD=0.22,N=11),暗示这套镁铁质-超镁铁质岩石为古元古代岩浆活动的产物。这一发现表明,约1 830 Ma武夷地块处于板内伸展构造环境,同时该时期武夷地块已经具备足够的刚性,以致产生大规模的脆性破裂以及可能由地幔对流作用引发的基性岩浆活动,推测在约1.83 Ga武夷地块已经具有克拉通的性质。

A suite of Paleoproterozoic maficultramafic rocks was discovered in the Wuyishan Block in southwest Zhejiang Province through 1：250 000 regional geological survey of Quzhou Sheet. These maficuhramafic rocks comprising mainly pyroxene hornblendite, hornblendite, plagioclase actinolite, plagioclase pyroxenite and amphibolite are exposed perfectly in Zhangcun Village of Jinhua County and spatially exhibit planar-shaped distribu- tion in ]inhua County and Longyou County. Zircon U-Pb dating with LA-ICP-MS shows that the formation age of pyroxene hornblendite is 1 834±14 Ma （MSWD= 0.23, N = 16）, and that of actinolite is 1 839± 17 Ma （MSWD = 0.22, N = 11）, which suggest that these maficuhramafic rocks formed during the Paleoproterozoic magmatism. This discovery reveals that the Wuyishan Block was experiencing intraplate extension process at about 1 830 Ma and that the Wuyishan Block might have been a rigid continent at that time so that largesized brittle fractures could be formed in the interior of the Wuyishan Block and induced basic magmatism resulting from mantle convection. Based on these facts and inference, the authors hold that the Wuyishan Block already had been cratonized at about 1.83 Ga.