韩国Hadong-Sanchung地区斜长岩Sr-Nd同位素初步研究——成因和前寒武纪构造意义

斜长岩呈长条带出露于朝鲜半岛南部,侵入到年代约为2.0Ga的Yeongnam前寒武纪基底岩石中,虽然岩石类型简单(斜长岩和辉长岩质斜长岩),但可以同世界已知块状类型斜长岩相对比。这些斜长岩具有几个重要的差别,例如呈层状构造,镁铁相成分是角闪石而不是辉石,并且不具斜方辉石巨晶。应用Rb-Sr和Sm-Nd同位素系统研究这些岩石的年龄和成因,测定出一种页理化辉长岩质斜长岩矿物的Sm-Nd等时线年龄为1678±90Ma,推断其为侵位年龄,因为中生代绿岩相变质期间这些岩石的Sm-Nd同位素体系呈封闭状态。这一年龄和过去曾报道的元古宙块状斜长岩的年龄范围(1.1~1.7Ga)相吻合。认为斜长岩成因可以用所谓元古宙斜长岩事件来解释。斜长岩的岩浆活动对朝鲜半岛南部前寒武纪基底岩石的构造历史有重要意义。全岩εNd(t)值范围-1.6~-5.2,而87Sr/86Sr初始值变化于0.704~0.706之间,据此可解释地幔成因的斜长岩岩浆是在其结晶作用期间吸收了地壳物质的结果。然而不能排除是下地壳源的可能性。

Preliminary Sr-Nd isotope study of the Hadong-Sanchung anorthositic rocks in Korea: Implication for their origin and for the Precambrian tectonics

The anorthositic rocks in southern part of Korean peninsula occur as a long belt, intruding the ca. 2.0 Ga old Precambrianbasement rocks of the Yeongnam massif. Although they have simple rock types (anorthosite and gabbroic anorthosite) comparable towell known massif-type anorthosites worldwide, they possess several important differences such as layered structure, amphibole ratherthan pyroxene as a mafic phase, and no orthopyroxene megacrysts. The age of intrusion was not available previously. We have appliedRb-Sr and Sm-Nd isotope systematics to investigate the age and origin of these rocks. One foliated gabbroic anorthosite defines a Sm-Nd mineral isochron age of 1678 ± 90 Ma. This age is tentatively interpreted as the emplacement age because of apparently closed-sys-tem behavior of Sm-Nd system in these rocks during greenschist-facies metamorphism of presumably Mesozoic age. This age agreeswith the age range (1.1 to 1.7 Ga) reported for the occurrence of Proterozoic massif-type anorthosites, suggesting that origin of theanorthositic rocks might be explained in the context of so-called Proterozoic anorthosite event. Significance of the anorthositic magma-tism to the tectonic history of the Precambrian basement rocks in southern Korea Peninsula is discussed. εNd( t ) values of whole rocksrange - 1.6 to - 5.2, while initial 87Sr/86Sr ratios vary from 0. 704 to 0. 706. These data are interpreted as the result of assimilationof crustal material during crystallization of presumed anorthositic magma of mantle origin, although possible lower crustal source cannot be excluded.