山东省沂水汞丹山地块与深熔作用有关的变质流体

Metamorphic Fluids Related to Anatexis in Gongdanshan Block, Yishui County, Shandong Province

沂水汞丹山地块为一片麻岩－花岗岩穹隆，穹隆中心出露大片紫苏花岗岩和麻粒岩相表壳岩残留地层（或包体），向北部、东部和南部外围过渡为角闪岩相片麻岩、斜长角闪岩和花岗质岩石，其中常见深熔脉体和伟晶岩脉。麻粒岩相表壳岩围岩很少含或几乎不含流体包裹体、而在深熔脉体、紫苏花岗岩和伟晶岩脉中含大量富ＣＯ２流体包裹体，少量Ｈ２Ｏ包裹体、ＣＯ２－Ｈ２Ｏ包裹体，偶见含固相（石盐、碳酸盐）的多相包裹体与富ＣＯ２包裹体伴生。早期富ＣＯ２包裹体（Ｉａ型）具有较高流体密度，而沿石英（石榴石）中穿颗粒裂隙分布的富ＣＯ２包裹体流体密度降低。富Ｈ２Ｏ包裹体大都沿裂隙分布，也属峰期变质后的退变质晚期产物。低密度富气相包裹体则多与ＣＯ２（Ｈ２Ｏ）包裹体组分的渗漏或部分爆裂有关。对深熔作用期间流体的作用进行了探讨，认为在中部地壳压力条件下，一种富ＣＯ２－Ｈ２Ｏ流体沿岩石页理面或裂隙渗流能产生石榴石／斜方辉石的部分熔融，Ｈ２Ｏ优先进入熔体，ＣＯ２进入共存的气相而分离，并可作为流体包裹体被捕获于石榴石或辉石包裹的石英和随后由硅酸盐熔体冷却结晶的长英质脉体石英中。根据矿物平衡和流体包裹体资料推断了峰期变质后岩石经历的Ｐ－Ｔ条件。

The Gongdanshan block in Yishui County, Shandong Province, is a gneiss-granite dome. In the center of the dome, charnockites and supercrustal rock sequences (restites) of granulite facies are exposed widely; outwards to the northern, eastern and southern margins of the dome, there occur gneisses, amphibolite and granitoids in which leucocratic felsic veins and pegmatites are common. The granulite facies supercrustal rocks contain very few or almost no fluid inclusions; in contrast, the anatectic veins contain abundant CO 2_rich fluid inclusions, a small number of H 2O and CO 2-H 2O inclusions and occassionally saline aqueous inclusions with solid(s). Microthermometry and laser Raman microprobe analyses have shown that the CO 2_rich inclusions contain 70～85 mol.% CO 2 with minor H 2S, SO 2, CH 4 and H 2O. The early CO 2_rich inclusions have higher densities while the CO 2_rich inclusions distributed along the transgranular fractures have lower densities. H 2O (aqueous) inclusions belong to the products of late retrograde metamorphism after peak metamorphism. The vapor_rich inclusions of low_density are probably related to leakage or partial decrepitation of CO 2 (±H 2O) inclusions. The role of fluid during anatexis is tentatively discussed. It is suggested that under the pressure of the middle crust the infiltration of a CO 2-H 2O fluid along foliations of fissures of rocks would cause partial melting that led to the formation of garnet or orthopyroxene, with H 2O entering the melt preferentially and CO 2 entering the coexisting vapor garnet and/or pyroxene porphyroblasts and in quartz within felsic veins crystallized subsequently from the silication melt. The P-T conditions the rocks experienced after peak metamorphism are deduced from the mineral equilibria and the microthermometry of the fluid inclusions.