北秦岭双龙-夏馆地区大面积榴闪岩的发现及锆石U-Pb年代学研究

Discovery and Zircon U-Pb Dating of Garnet Amphibolite in Shuanglong-Xiaguan Area, North Qinling

通过详细的野外地质调查,在北秦岭双龙-夏馆地区精确圈定出大面积榴闪岩,榴闪岩常以似层状、条带状、豆荚状和透镜状产于秦岭岩群郭庄岩组混合片麻岩内,空间上可划分三个带:南带与松树沟一带的麻粒岩构成一个较宽的高压-超高压变质带,北带则与官坡-双槐树一带的榴辉岩构成一线,而中带向西可能延伸至丹凤大寺沟一带。区内榴闪岩主体受秦岭岩群郭庄岩组的展布形态控制,呈面状遍布于整个郭庄岩组内,而非局限于某一侧或两侧,暗示秦岭岩群可能整体卷入早古生代的俯冲和碰撞作用。野外宏观特征、岩相学及锆石U-Pb年代学研究表明榴闪岩原岩为辉长岩,成岩年龄为654.1±5.6Ma、656.1±6.7Ma,获得峰期高压麻粒岩相变质年龄为472.9±6.0Ma、485.0±33.0Ma,其后分别在455.4±9.0Ma、416.7±1.9Ma和401.1±6.2Ma遭受了多期次退变作用的叠加。结合区域研究成果,初步认为北秦岭高压-超高压基性岩石的原岩至少存在于新元古代中期和晚期,其成岩时代分别与扬子地块北缘新元古代中、晚期的镁铁-超镁铁质岩床群一致,并具有相似的地球化学特征,而两者是否为同一大地构造背景下的同期侵位的产物,尚待进一步研究。

Based on detailed field geological survey, garnet amphibolite was recognized in the Shuanglong-Xiaguan area of the Northern Qinling Mountain. Garnet amphibolite is widespread in the study area, and distributed mainly in the melanosome of migmatitic gneiss in the Guozhuang Formation of Qinling Complex exhibiting stratoid, podiform, and lentoid shapes. The garnet amphibolite can be divided into three SE-NW trending belts. The southern belt is located north of the Shangxian-Danfeng fault, along the southern margin of the Qinling Complex. In this belt, the garnet amphibolite and granulite constitute a broad high pressure-ultrahigh pressure (HP-UHP) metamorphic belt in the Songshugou Area. The northern belt is located south of the Zhuyangguan-Xiaguan fault, outcropping along the northern margin of Qinling Complex, and forming a narrow HP-UHP metamorphic belt with eclogite in the Guanpo- Shuanghuaishu Area. The middle belt, located in the central part of the Qinling Complex, constitutes the third HP-UHP metamorphic belt that extend westward to the north of Dagousi Area in Danfeng, together with the garnet pyroxenite in the Zhaigen area and the coesite-bearing amphibolites in the Dagousi Area. The garnet amphibolite is primarily controlled by Guozhuang Formation of the Qinling Complex, which suggests that the Qinling Complex was involved in the early Paleozoic deep subduction and collision. The overall field features, petrography and zircon U-Pb chronology demonstrate that the protolith of the garnet amphibolite is gabbro with crystallization age of 654.1±5.6 Ma to 656.1± 6.7 Ma, which experienced peak metamorphism of high-pressure granulite facies at 472.9±6.0 Ma to 485.0±33.0 Ma, and subsequently multistage retrogressive metamorphism of amphibolite facies at 455.4±9.0 Ma, 416.7±1.9 Ma and 401.1±6.2 Ma, respectively. Combined with the results of regional studies, we preliminarily suggest that the protoliths of the HP-UHP rocks in the Northern Qinling Mountain consist mid- and late Neoproterozoic basic rocks. The age and geochemical characteristics of the basic rocks are similar to these of the mid- to late Neoproterozoic mafic-ultramafic sills in the northern margin of the Yangtze Block, suggesting that they might have been formed under similar geotectonic background.