黑龙江依兰地区蓝片岩的形成与就位过程研究

The high-pressure metamorphic rocks exposed in the Yilan area, Heilongjiang Province, are characterized by alternating of blueschists, mica schists and felsic mylonites, with small amounts of ultramafic rocks and marbles that were tectonically interleaved mélange, representing the rock associates derived from the suture zone between the Jiamusi and Songnen massifs. Therefore, the study on the formation and metamorphic evolution of the blueschists is helpful to consider the tectonic evolution related to the suturing and collision between the Jiamusi and Songnen massifs. In this project, the systematic field investigation will be done, in order to recognizing the spacial distribution and the tectonic deformation pattern of the blueschists. Further, the study of petrography, mineral chemistry, analysis of rock microstructure and geochronology, including U-Pb dating for zircon and 40Ar/39Ar dating for phengite, will be carried out on the representative blueschists, respectively. The results obtained will contribute toward constraining the P-T conditions and formation timing of the high-pressure metamorphism for the Yilan blueschists, as well as their precise pressure-temperature-time paths. Moreover, this study is also helpful to reveal the microstructural characters of the deformed minerals developed in the progressive metamorphic stages, to consider the processes of formation and emplacement of the blueschists and its geodynamic background, and final to reconstruct the tectonic model for the oceanic subduction and subsequent continental collision between the Jiamusi and Songnen massifs.

黑龙江省依兰地区出露了一套经高压变质形成的蓝片岩、云母片岩、长英质糜棱岩和少量超基性岩及大理岩并经历了强变形作用改造，一直被认为是佳木斯地块与其西侧松嫩地块碰撞拼贴所形成的缝合带产物。探讨蓝片岩的成因与演化将为揭示两地块构造属性及其碰撞拼合的演化过程提供重要的突破口。故此，本项目以该区域内出露的蓝片岩作为主要研究对象，通过系统的野外调查，查明蓝片岩的空间展布规律及其构造变形特征。通过对关健性蓝片岩样品的岩相学研究、矿物化学分析、显微变形组构分析、锆石U-Pb和多硅白云母40Ar/39Ar测年，限定蓝片岩的形成时代及其高压变质温压条件，建立精确的变质演化P-T-t轨迹，查明不同变质期次参与变形矿物的微观变形特征。进而恢复蓝片岩形成与就位的演化过程及其动力学背景，建立佳木斯地块与松嫩地块碰撞拼贴的构造演化模式。

黑龙江依兰地区出露了一套经高压变质形成的蓝片岩、云母片岩、长英质糜棱岩和少量超基性岩及大理岩并经历了强变形作用改造，被认为是佳木斯地块与松嫩地块碰撞拼贴所形成的缝合带产物。本项目通过详细的野外调查、岩相学与矿物化学分析，查明了蓝片岩的分布，并识别出3种不同类型的蓝片岩：羊角沟地区主要发育石榴石冻蓝闪石片岩，矿物组合为Ab-Chl-Phn- Brs-Qtz-Act-Ep-Grt-Gln-Ttn-Mag-Stp；涌泉地区发育绿帘蓝闪石片岩，矿物合为Ab-Ep-Chl-Phn-Gln-Qtz-Wnc-Ttn-Cal和蓝闪霓辉石片岩，矿物组合为Ab-Chl-Ep-Agt-Gln-Ttn-Brs-Wnc-Qtz-Bt-Phn。利用常规矿物温压计和变质视剖面图，估算了绿帘蓝闪石片岩（T = 350~550℃，P = 10~14 kbar）、蓝闪霓辉石片岩（T = 350~550℃，P = 10~14 kbar）和石榴石冻蓝闪石片岩（T = 500~540℃，P = 10~12 kbar）的峰期变质温压条件。同时也获得了蓝片岩围岩—石榴石云母片岩的峰期变质温压条件为P = 9-11 kbar，T = 420-460℃，证实了其围岩也经历了高压变质作用。绿帘蓝闪石片岩中定向排定的蓝闪石EBSD组构分析指示了低温条件下的位错蠕变。地球化学分析结果显示依兰地区蓝片岩具有OIB洋岛玄武岩特征。黑龙江杂岩中碎屑锆石LA-ICP-MS U-Pb年龄谱中包含捕获的古老锆石年龄（>1000 Ma）、来自佳木斯基底麻山杂岩的锆石年龄（~500 Ma）以及佳木斯地块周边晚二叠-早三叠世的花岗质岩石年龄（~250 Ma）。其中，最年轻的锆石U-Pb年龄（199-231 Ma）被认为是揭示了晚三叠世佳木斯地块与松嫩地块之间古洋盆的存在。蓝片岩中多硅白云母40Ar/39Ar年龄，指示蓝片岩相变质作用发生于侏罗纪（190-145 Ma）。综上，建立了变质演化P-T-t轨迹。其中，绿帘蓝闪石片岩具“发卡式”变质演化 P-T-t轨迹；而蓝闪霓辉石片岩与石榴石冻蓝闪石片岩具顺时针的演化轨迹。我们推测其成因可能为不同类型的蓝片岩形成于不同的俯冲阶段，由动力学机制的变化所造成。该研究成果为进一步探索佳木斯地块与松嫩地块碰撞拼贴的构造演化过程提供重要的依据。