中国大陆科学钻探(CCSD)榴辉岩磷灰石脉体中铁的氧化物、重晶石和独居石出溶物的发现及其意义

Discovery of iron oxide, monazite and barite exsolutions in apatite veins in eclogite from the Chinese Continental Scientific Drilling (CCSD) Project and its significance.

CCSD主孔榴辉岩等UHP岩石中存在团块状到不规则脉体状的磷灰石集合体,它们多与石英脉共生,显微镜下观察发现这些磷灰石脉体存在大量出溶物,电子探针和激光拉曼光谱仪联合测定显示磷灰石主要为氟磷灰石,其中出溶物主要有四类:A．不规则状的磁铁矿和赤铁矿的连生体;B．针状赤铁矿;C．板状到菱型的独居石;D．针状锶重晶石。A类出溶物的延长方向平行于磷灰石的C轴,长度介于10～50μm;B类主要沿磷灰石的C轴方向排列,宽仅为0．5～2μm,多数在1．5μm左右,长度变化较大,为6～50μm;C类独居石出溶体宽约6～10μm,多为6μm,长约50～75μm;D类锶重晶石主要与B类赤铁矿出溶物共生,宽多1．5μm左右,长约60～70μm。其中A,B和D类出溶物是在磷灰石中首次发现。四类出溶物的长轴和生长方向均基本平行于磷灰石之C轴,显示它们可能是基本同时出溶的。CCSDUHP岩石中氟磷灰石脉体的存在及其铁氧化物、磷酸盐和硫酸盐出溶体的发现,显示UHP岩石在折返的过程中,快速减压曾造成矿物的分解和大量出溶体的形成,伴随脱水和SiO2等硅酸盐熔体的出溶,还存在少量的磷酸盐熔体,同时可能曾经有铁氧化物的熔浆的加入。磷灰石中铁氧化物等出溶物的出溶是在氧逸度较高的环境下进行的．

Massive to irregular veins apatites were recognized in quartz veins of CCSD UHP rocks such as eclogite. Abundant exsolutions in the apatite veins were found under microscope. Electron microscope and laser raman spectroscopy analyses show that the apatite is fluorapatite, and the exsolutions can be divided into four types： A. irregular magnetite and hematite intergrowth; B. needle-like hematite exsolution; C. platy to rhombic monazite exsolution; D. needle-like strontium barite exsolution. Type A exsolutions are parallel with the C axis of apatite and with a length of 10 -50μm; Type B are also parallel with the C axis of apatite with a width of only 0. 5 -2μm, mostly about 1.5μm, and a greatly-varied length of 6 -50μm; The width and length of type C monazite exsolutions are about 6 - 10μm, mostly 6μm, and about 50 -75μm, respectively; Type D strontium barite exsolutions coexist mostly with type B hematite exsolution, with a width of about 1.5μm and a length of about 60 -70μm. Type A, B and D exsolutions in apatite are firstly reported. Most of the exsolutions are parallel with C axis of the apatite, implying that they were probably exsolved synchronously. Appearance of apatite veins in CCSD UHP rocks and discovery of large amount of iron oxides, phosphate and sulfate exsolutions in the apatite suggest that rapid exhumation of the CCSD UHP rocks resulted in large-scale minerals decomposition and formation of large amount of H2O-SiO2 melt, as well as a little amount of phosphate melt. Meanwhile, iron oxide melt might have been added. The exsolutions in the CCSD apatite veins were formed under relative oxidation environment.