Changes in Mineral Assemblages with Metamorphism of Some Banded Precambrian Iron-Formations
Changes in Mineral Assemblages with Metamorphism of Some Banded Precambrian Iron-Formations
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DOI：
10.2113/gsecongeo.68.7.1075
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发表时间：
1973-11
期刊：
影响因子：
5.8
通讯作者：
C. Klein
中科院分区：
文献类型：
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作者：
C. Klein
未变质的前寒武纪铁组的脱溶沉积氧化物相通常由燧石、碧玉或石英、赤铁矿或磁铁矿或两者的细带状组合组成，局部还有一些水合氧化铁。在区域变质为蓝晶石级或硅线石级后，燧石和氧化铁发生再结晶，并伴随着组成相（尤其是石英）晶粒尺寸的显著增加。这些矿物之间通常不发生反应，在重结晶组合中往往保留了原始的沉积结构。由此产生的带状变质石英-镜铁矿-磁铁矿层，石英-镜铁矿和石英-磁铁矿带在小于1毫米的尺度上交替，表明在变质作用期间，氧在这些矿床中表现为内部控制（缓冲）成分。带状沉积碳酸盐相通常由一种或多种碳酸盐（方解石、铁白云石-铁白云石系列的成员和菱铁矿）、少量的磁铁矿、燧石或石英组成，局部还有一些铁硅酸盐。如果原始碳酸盐相仅由碳酸盐、磁铁矿和石英组成，并且在随后的变质作用中，如果co2的化学势在局部保持足够高，以防止碳酸盐的分解，则共存相之间不会发生反应。只注意到再结晶和晶粒尺寸增大。另一方面，如果二氧化碳的化学势降低，燧石（或石英）将与碳酸盐反应形成新的硅酸盐。例如:Ca(铁、镁)(公司3)2 + 2 sio 2 = Ca(铁、镁)如果2 O 6 + 2 CO 2铁白云石clinopyroxeneIf H 2 O的化学势高,虽然CO 2的化学势低变质期间,以下类型的反应发生:5 Ca(铁、镁)(公司3)2 + 8 sio 2 + H 2 O = Ca 2(铁、镁)5如果8点22 (OH) 2 + 3 caco 3 + 7 CO 2铁白云石actinoliteAs中期高级变质条件的结果(蓝晶石硅线石区),大部分带状碳酸盐相含有变质硅酸盐，表明在变质作用过程中co2的普遍损失。然而，在同一变质区内，同一碳酸盐相的某些部分可能仍然由原始的石英-碳酸盐-磁铁矿组合组成，尽管是再结晶的。这意味着co2的化学势在富含铁的碳酸盐岩中是局部可变的，因此co2不能一直被认为是完全可移动的成分。这种碳酸盐-石英-磁铁矿的再结晶是在一个基本上与二氧化碳封闭的体系中发生的。原始硅酸盐相由含水铁硅酸盐（绿绿岩、菱辉岩和辉灰岩）、碳酸盐（白云石-铁白云石系列、菱铁矿和方解石的成员）、燧石或石英和氧化铁的复杂混合物组成。这部分铁的形成最符合渐进变质过程中一般脱碳脱水的结果。碳酸盐和燧石不仅反应形成丰富的硅酸盐，如明石-绿辉石系列的成员，而且原来的硅酸盐也被斜角闪石和正角闪石、石榴石和正斜辉石所取代。橄榄石（辉橄榄岩）的生产只被描述为接触变质矿床，例如沿着明尼苏达州德卢斯辉长岩与Biwabik铁组的接触。Fayalite组合代表了o2相对化学势较低的条件。富含锰的地平线经常出现。最初的锰氧化物、氢氧化物和碳酸盐（菱铁矿和钾锰矿）的混合物，在变质作用后，让位给含有菱铁矿、锰石榴石（闪辉石或calderite）、锰矿明辉石、富锰辉石（如锰矿绿辉石和钙辉石）、铜石和菱辉石的组合物。在拉布拉多海槽的变质组合中，许多富锰层的铁组也富钠，如锰-锌-菱铁矿-镜铁矿-菱铁矿等组合。
The unleached sedimentary oxide facies of unmetamorphosed Precambrian iron-formation generally consists of a finely banded assemblage of chert, jasper or quartz, hematite or magnetite, or both, and locally some hydrous iron oxides. Upon regional metamorphism to kyanite or sillimanite grade, recrystallization of the chert and iron oxides takes place, accompanied by a marked increase in grain size of the constituent phases, especially quartz. Generally no reactions take place among these minerals and original sedimentary textures are frequently preserved in the recrystallized assemblage. The resulting, banded, metamorphic quartz-specularite-magnetite horizons, with quartz-specularite and quartz-magnetite bands alternating on a scale of less than 1 mm, indicate that oxygen has behaved as an internally controlled (buffered) component in these occurrences during metamorphism.The banded sedimentary carbonate facies generally consists of one or more carbonates (calcite, members of the ferroan dolomite-ankerite series, and siderite), lesser amounts of magnetite, chert, or quartz, and locally some iron silicates. If the original carbonate facies consists only of carbonate, magnetite, and quartz, and if the chemical potential of CO 2 during subsequent metamorphism remains locally high enough to prevent the breakdown of the carbonates, no reactions occur among the coexisting phases. Only recrystallization and increased grain size are noted. If, on the other hand, the chemical potential of CO 2 is reduced, chert (or quartz) will react with the carbonates to form new silicates. For example :Ca(Fe,Mg) (CO 3 ) 2 + 2SiO 2 = Ca(Fe,Mg)Si 2 O 6 + 2CO 2 ferroan dolomite clinopyroxeneIf the chemical potential of H 2 O is high, while the chemical potential of CO 2 is low during metamorphism, the following type of reaction occurs:5Ca(Fe, Mg) (CO 3 ) 2 + 8SiO 2 + H 2 O = Ca 2 (Fe,Mg) 5 Si 8 O 22 (OH) 2 + 3CaCO 3 + 7CO 2 ferroan dolomite actinoliteAs a result of medium- to high-grade metamorphic conditions (kyanite to sillimanite zones), much of the banded carbonate facies contains metamorphic silicates, indicating a generai loss of CO 2 during metamorphism. Some parts of the same carbonate facies in the same metamorphic area may, however, still consist of the original, although recrystallized, quartz-carbonate-magnetite assemblage. This implies that the chemical potential of CO 2 has been locally variable in the iron-rich carbonate rocks and that CO 2 , therefore, cannot at all times be considered as a perfectly mobile component. Such carbonate-quartz-magnetite recrystallization has taken place in a system essentially closed to CO 2 .The original silicate facies consists of a complex mixture of hydrous Fe-silicates (greenalite, stilpnomelane, and minnesotaite), carbonates (members of the dolomite-ankerite series, siderite, and calcite), chert or quartz, and iron oxides. This part of the iron-formation shows most consistently the results of general decarbonatization and dehydration during progressive metamorphism. Not only do the carbonates and chert react to form abundant silicates, such as members of the cummingtonite-grunerite series, but also the original silicates give way to clino- and orthoamphiboles, garnet, and ortho-and clinopyroxenes. The production of olivine (fayalite) has been described only from contact metamorphic occurrences such as along the contact of the Duluth Gabbro with the Biwabik Iron Formation in Minnesota. Fayalite assemblages represent conditions of low relative chemical potential of O 2 . Manganese-rich horizons are frequently present. Original mixtures of Mn-oxides, hydroxides, and carbonates (rhodochrosite and kutnahorite) give way, upon metamorphism, to assemblages containing rhodonite, Mn-garnet (spessartite or calderite), manganoan cummingtonite, Mn-rich pyroxenes (such as manganoan aegrine-augite and hedenbergite), bustamite, and rhodochrosite. In the metamorphic assemblages of the Labrador Trough, many Mn-rich horizons of the iron-formation are also Na-rich, as shown by assemblages such as Mn-aegirine-rhodonite-specularite-rhodochrosite.