阿尔泰晚古生代超高温麻粒岩流体特征及其意义

Fluid characteristics of Late Paleozoic ultrahigh-temperature granulites from the Altay orogenic belt,northwestern China and its significance

北疆阿尔泰造山带南缘晚古生代超高温麻粒岩存在高铝斜方辉石＋夕线石＋石英和尖晶石＋石英等超高温特征矿物组合，其退变质作用主要以典型的降压反应矿物堇青石的形成为标志。对其中新发现的石榴子石及石英流体包裹体开展显微测温和激光拉曼探针分析，结果表明：基质中的石英以及被石榴子石包裹的石英中孤立分布的原生流体包裹体成分为近纯CO2（部分含少量的N2），均一温度在10．1～29℃之间，其对应的密度在0．631～0．861g／cm^3。石榴子石中的原生和假次生CO2包裹体费米峰差在104．1cm^-1左右，低于石英中CO2包裹体的费米峰差（～104．7cm^-1），暗示其密度低于石英中的CO2包裹体。由CO2流体包裹体密度所得到的等容线从P—T轨迹退变质阶段的下方穿过结合区域地质背景，我们初步认为阿尔泰超高温麻粒岩的形成与幔源岩浆底侵有关。基性岩浆的底侵不但提供了异常高的地温梯度，同时其所释放的大量CO2降低了水活度，使得变质作用峰期的特征矿物得以保存。低密度的CO2流体指示了阿尔泰超高温麻粒岩沿顺时针P—T轨迹紧随超高温峰期变质之后快速抬升降压，捕获的同变质流体包裹体因腔体体积增大而密度降低的一种退变质过程。

The Late Paleozoic ultrahigh-temperature （UHT） granulite from the Altay orogenic belt, northwestern China contains UHT diagnostic mineral assemblages of spinel ＋ quartz and orthopyroxene ＋ sillimanite ＋ quartz. Its retrograde metamorphism is characterized by the formation of cordierite through typical decompression reaction. Newly discovered primary and pseudosecondary fluid inclusions in garnet together with primary fluid inclusions in quartz were studied in terms of petrography and microthermometry as well as laser Raman spectroscopy analyses in detail. Melting temperatures of inclusions obtained indicated that the trapped fluid phase is dominantly carbonic. Raman probe confirmed a near pure CO2 composition with only minor dilutant of N2. The homogenized temperatures of fluid inclusions both from the quartz in matrix and enclosed in garnet porphyroblasts range from 10. 1℃ to 29. 0℃with the corresponding densities of 0. 631 ～0. 861g/cm^3. The distance values of Fermi doublet from the pseudosecondary and secondary fluid inclusions trapped in garnets are around 104. lcm^-1 , smaller than those from quartz （ ～ 104. 7cm^-1 ）, indicating the lower density nature of the fluids in garnets. The range of CO2 isochores computed from density measurements in fluid inclusions passed below the retrograde stage of the P-T trajectory. We inferred that the Altay UHT granulite probably formed in an extensional setting, with the underplating of mantle-derived magma, which provided not only heat, but also sufficient CO2 to dilute the H2O activity. The consistent low-density carbonic fluids are probably a reflection of significant density reversal owing to modification of inclusion cavity volume by rapid decompression of the Ahay UHT granufite along a clockwise P-T path.