摘要
高度演化花岗岩类多为富F的熔体溶液体系 ,具有鲜明的、不同于其他体系的地球化学行为。富F岩浆固相线和液相线的降低和岩浆寿命的延长 ,使残余熔体与热水热液的性状差异减小 ,模糊了岩浆与热液之间的界线。最近对于富F、B和P伟晶岩中熔融包裹体的研究获得了新的进展。在约 70 0～ 5 0 0℃的温度和 1 0 0 0× 1 0 5Pa的压力下 ,在伟晶岩石英中发现两种不同类型的熔体包裹体 ,一种是富硅酸盐、贫水的熔体包裹体 ,另一种是贫硅酸盐、富水的熔体包裹体。两种熔体在硅酸盐 (+F +B +P) 水体系的溶离线边界上同时被圈闭。这表明 ,在地壳浅部侵位的侵入体 ,当温度≥ 70 0℃时 ,水在富F、B和P的熔体中可以无限混溶 ;而一旦温度降低 ,就会分离为两种共存的熔体并伴随强烈的元素分异作用。在溶离线的富水一侧形成与正常硅酸盐熔体有很大不同的高度富挥发份的熔体 ,这种致密、高粘度、高扩散性以及高活动性的超富水 (hyper aqueousmelt)熔体 ,可以与水溶液流体相类比。这为岩浆热液过渡性流体的假说提供了新的有利的证据。此外 ,在这种具有超富水和熔体特征的过渡性流体中 ,微迹元素可能具有特殊的地球化学行为 ,如在许多晚期花岗岩包括淡色花岗岩和伟晶岩中稀土元素配分模式所显示的四分组效应等。富F熔体溶液体?
Most of the highly evolved granites may belong to F-rich system. The F-rich silicate melt-hydrous fluid system shows distinctive geochemical behaviors. The fluorine lowers the liquidus and solidus temperature of hydrous granitic melts and can extend appreciably the magmatic “lifespan” of crystallizing granitic magmas by thermally depressing the liquidus-solidus phase relations. This reduces and blurs the distinction between residual magma and magmatic hydrothermal solution. Recently, some new advances in researches on fluorine-, boron- and phosphorus-rich pegmatites had been made. Over a temperature range from about 700 to 500 ℃ at a pressure of about 1 000×10~5 Pa, pegmatite quartz crystals continuously trapped two different types of melt inclusions during cooling and growth: a silicate-rich H_2O-poor melt and a silicate-poor H_2O-rich melt. Both melts were simultaneously trapped on the solvus boundaries of the silicate(+fluorine+boron+phosphorus)-water system. This demonstrates that at shallow intrusion levels of the crustal conditions, complete miscibility between silicate melts and hydrous fluids is attained. Simple cooling induces a liquid-phase separation into two coexisting melts, accompanied with strong element fractionation. On the water-rich side of the solvus, very volatile-rich melts are produced, which have vastly different physical properties as compared to “normal” silicate melts. The density, viscosity, and mobility of such hyper-aqueous melts under these conditions are more comparable to an aqueous fluid. This provides a new and convincing argument for the hypothesis of magmatic-hydrothermal transitional fluid. In addition, the microelements in the transitional ore-forming fluid characterized by hyper-aqueous and melt may possess particular geochemical behaviors during rock-forming and ore-forming processes, such as “tetrad” or “double-double” effect in rare earth element distribution patterns observed in REE pattern of a number of late-stage granitic rocks including leucogranites and pegmatites. The silicate melt-hydrous fluid system played a critical role for the zoning of pegmatite, the extreme enrichment of the late-stage melt in HFS(high field strength) incompatible elements, and the formation of rare metal and REE granite, pegmatite and quartz vein deposits. Contrast with Cl-rich system , the fundamental questions of F-rich system remain unanswered, further careful studies will be required. The W-Sn granitoid and the ore fluid related with it in South China are the F-rich silicate melt-hydrous fluid system. The geochemical behaviors and metallogeny of the system are important topics worthy of further research.
出处
《地学前缘》
EI
CAS
CSCD
2004年第2期479-490,共12页
Earth Science Frontiers
基金
国家自然科学基金资助项目 ( 4 0 173 0 2 1)
国家重点基础研究项目 ( 973项目 :2 0 0 2CB412 6)
关键词
富F熔体-溶液体系
成矿流体地球化学
成矿效应
现状
问题
F-rich system
silicate melt-hydrous fluid system
ore-forming fluid geochemistry
metallogeny
current status
problems
