大别山双河超高压榴辉岩中变质锆石:离子探针和微区结构研究

Metamorphic zircon from Shuanghe ultra-high pressure eclogite,Dabieshan: ion microprobe and internal micro-structure study.

对南大别双河超高压岩板榴辉岩中锆石进行了阴极发光、喇曼光谱、U- Pb和氧同位素离子探针微区分析。发现大部分锆石具有核边结构 ,其核和边有明显不同的 U ,Th,Pb含量、Th/ U比、2 0 6 Pb/ 2 38U年龄和δ1 8O值。锆石边部具有变质锆石的结构和化学特征 ,核部具有岩浆锆石向变质锆石变化的过渡特征。 17个点 U - Pb同位素分析中大多数为不一致年龄 ,由 14个正向不一致点所构成的不一致线与一致曲线的上、下交点年龄分别为 2 4 89± 2 5和 2 4 8± 16 Ma,它们代表了榴辉岩原岩形成时间和前进变质至峰期变质过程中原岩岩浆锆石重结晶和变质增生作用的时间 ,大别山变质锆石的成因是复杂的 ,它既可以是变质新生锆石 ,呈独立的颗粒或原岩岩浆锆石的增生边 ,也可以是由原岩锆石在固相或流体存在下的重结晶作用形成。不同程度的重结晶作用是超高压条件下双河榴辉岩原岩岩浆锆石发生的主要的物理化学变化 ,它导致锆石中 U、Th、Pb和Th/

Zircon from Shuanghe UHP eclogite, Southern Dabieshan was analysed, for observation and determination of cathodoluminscence image, Raman spectrometer, SIMS U-Pb and oxygen isotopes. Most of zircon grains show core-mantle-ring structure with different U, Th and Pb concentrations, Th/U ratios, Pb-206/U-238 ages and delta(18)O values. The rings show characteristics of metamorphic zircon structure and chemical compositions. Whereas the cores show transition from magmatic to metamorphic zircon. Most of 17 measured SIMS ages are discordant, and discordia line obtained from 14 positive discordant data points yield their lower and upper intercept ages of 2489+/-25 Ma and 248+/-16 Ma, respectively. The upper intercept age represents the protolith formation and lower intercept represents the time of re-crystallization of magmatic zircon and metamorphic zircon overgrowth during prograde to peak UHP metamorphism. The genesis of metamorphic zircon of Dabieshan is quite complicated. It could be new zircon growth of either forming an independent grain or as a ring of protoloth zircon, and also a product of re-crystallization of magmatic zircon under solid state and/or fluid existence. Different degree of re-crystallization is a major physico-chemical phenomenon happened for protolith zircon of Shuanghe eclogite during the whole UHP stage. It caused reduce of U, Th, Pb content and Th/U ratio, and removing of magmatic oscillatory zoning.