大别山双河和碧溪岭超高压变质岩流体包裹体研究

Fluid inclusions in ultrahigh pressure metamorphic rocks from Shuanghe and Bixiling in Dabie Shan.

对大别山双河和碧溪岭含柯石英榴辉岩和硬玉石英岩进行了详细的流体包裹体研究。根据流体包裹体的成分和盐度的不同 ,可以划分出至少五种类型不同的气液包裹体 :( 1) N2 包裹体 ;( 2 )高盐度流体包裹体 ;( 3) CO2 包裹体 ;( 4) CO2 -H2 O包裹体 ;( 5 )低盐度流体包裹体。N2 包裹体仅见于含柯石英榴辉岩 ,而高盐度流体包裹体则几乎存在于所有的榴辉岩和硬玉石英岩中。 CO2 包裹体沿榴辉岩中微剪切带分布 ,或存在于强变形的硬玉石英岩中 ,而低盐度流体包裹体分布较为局限。N2 和高盐度流体被认为是超高压变质作用平衡的产物 ,来源于变质前的古孔隙流体。而且榴辉岩中含有一系列盐度不同的流体显微域 ,其大小与矿物颗粒相当。在板块折返过程中 ,包裹体中流体可能发生迁移 ,但是不会超出寄主晶体的范围。CO2流体明显是外来的 ,可能来自共生大理岩或含碳酸盐的变质沉积岩。这与菱镁矿和柯石英等共生组合所指示的进变质过程中大理岩不存在脱碳反应的结论相一致。而低盐度流体则可能系顶峰变质矿物分解而成 。

Fluid inclusions in coesite bearing eclogites and jadeite quartzite from Shuanghe and Bixiling in Dabie Shan have been studied in details. At least five types of fluid inclusions can be identified by microthermometry and Raman analysis: (1) pure N 2; (2) highly brine; (3) CO 2 (±N 2±CH 4); (4) mixed CO 2 H 2O; (5) low salinity aqueous. Pure N 2 inclusions have been observed only in fresh coesite bearing eclogites, whereas highly brine inclusions exist almost throughout the ultrahigh pressure (UHP) metamorphic rocks. Carbonic inclusions occur mainly along and/or near micro shear zones, or in strongly deformed jadeite quartzite. Low salinity aqueous inclusions are limited within retrograded UHP rocks. The N 2 and highly brines, first equilibrated at eclogite conditions, are far remnants of pre UHP metamorphic volatiles, re equilibrated with peak metamorphic minerals at a local scale, corresponding to size of crystal(s). The carbonic fluids are externally derived, possibly being infiltrated from neighbouring marble or carbonate metasediments. The low salinity aqueous fluids may be locally derived from the breakdown of hydrous UHP minerals and exsolution of hydroxyls from nominally anhydrous minerals, or directly introduced into the rocks from the country rocks.