Isograds and P–T evolution in the eastern Lepontine Alps (Graubünden, Switzerland)
Isograds and P–T evolution in the eastern Lepontine Alps (Graubünden, Switzerland)
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Lepontine 阿尔卑斯山东部的等梯度和 P-T 演化（瑞士格劳宾登州）
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发表时间：
2002
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通讯作者：
M. Frey
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作者：
Thorsten J. Nagel;C. Capitani;M. Frey
使用矿物分布图、微观结构观察和平衡相图研究了 Lepontine Dome（瑞士中部阿尔卑斯山）东南部产生富铝指数矿物的反应。明显的十字石矿物带边界对应于钠石分解反应 Pg + Grt + Qtz = Pl + Al2O3 + W。平衡相图显示，只要单价阳离子主要容纳在白云母中，大多数天然变辉石不含十字石或铝硅酸盐。对于各种变泥岩成分，钠石分解会释放出足够的铝来形成这些矿物。在先前绘制的矿区边界以北，十字石和蓝晶石的罕见出现与钠长石共存，并且仅限于极其富含铝的散装成分。在 660°C 以上形成蓝晶石的钠晶石分解反应的稳定分支会产生额外的可映射等梯度。第二组释放铝的反应是产生黑云母的多硅白云母分解。然而，这些反应不太适合在现场产生明确的反应等梯度，因为它们更加连续并且它们的进展强烈依赖于本体组成。直到十字石开始分解后，变泥岩中发育良好的纤维岩才会出现。我们得出的结论是，研究区域的角闪岩相条件是通过减压获得的，无需在低压下进行大量加热。
Reactions producing Al‐rich index minerals in the south‐eastern part of the Lepontine Dome (Central Alps, Switzerland) are investigated using mineral distribution maps, microstructural observations and equilibrium phase diagrams. The apparent staurolite mineral zone boundary corresponds to the paragonite breakdown reaction Pg + Grt + Qtz = Pl + Al2O3 + W. Equilibrium phase diagrams show that most natural metapelites do not contain staurolite or alumosilicates as long as univalent cations are predominantly accommodated in white mica. For a wide range of metapelitic compositions the paragonite breakdown releases sufficient Al for the formation of these minerals. Rare occurrences of staurolite and kyanite, north of the formerly mapped mineral zone boundaries, coexist with paragonite and are restricted to extremely Al‐rich bulk compositions. The stable branch of the kyanite‐forming paragonite breakdown reaction above 660 °C yields an additional mapable isograd. The second set of Al‐releasing reactions is biotite‐producing phengite breakdown. However, these reactions are less suitable to produce well defined reaction isograds in the field as they are more continuous and their progress is strongly dependent on bulk composition. Well developed fibrolite in metapelites does not appear until staurolite starts to breakdown. We conclude that amphibolite facies conditions in the study area were attained by decompression, without substantial heating at low pressures.