Monticellite marble at Cascade Mountain. Adirondack Mountains. New York
Monticellite marble at Cascade Mountain. Adirondack Mountains. New York
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喀斯喀特山的蒙蒂塞利特大理石。
DOI：
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发表时间：
1978
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通讯作者：
Howeno W. Jerne
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作者：
Howeno W. Jerne
阿迪朗达克大理石含有方解石-透辉石-镁橄榄石-钙钛榴石组合，并含有微量的锌尖晶石、钛钙铁榴石、伊达克石和闪锌矿。微探针分析表明，镁橄榄石和镁橄榄石的Mg/（Mg+ Fe）均大于0.9，且镁橄榄石比镁橄榄石更富铁。镁橄榄石的寄主颗粒含有镁橄榄石的罕见的显微出溶层片，这一特征只在另一个陆地镁橄榄石中发现。透辉石是强烈的地带性，并含有高达6%的重量AlrOr和大量的三价铁，表明典型的钙硅酸盐单斜辉石的fassaitic字符。Fe和Mg在硅酸盐矿物中的分布表明，生成蒙地沸石的反应是一个Fe-Mg连续反应，并结合了端元反应MgrSiOn * CaMgSiRO。* 2 CaCO_4：3 CaMgSiO_4 + 2 CO_2，具有两个Fe-Mg交换平衡：2 CaMgSiO_4 * Fe_2 SiO_2。：2 CaFeSiO，* Mg，锡永CaMgSiRO。* CaFeSiO。：CaFeSiOu * CaMgSiO。计算表明，Fe不会显著降低反应温度。在方解石-透辉石-镁橄榄石-钙磷榴石组合中，在相同的露头和不同的矿物成分中出现无钙磷榴石的大理石，这意味着这些岩石形成于aCO中，与aCO呈梯度，可能从大理石层的中心到边缘逐渐减少。计算了无铁反应的P-T-XCO关系式，表明在8-10 kbar，700 - 800 ℃的Adirondack区域变质条件下，上述反应中钙镁橄榄石的形成需要变质流体中H_2O对CO的大量稀释，或在无流体条件下低aCO。另一种可能的解释是，较早的低压接触变质作用被后来的格伦维尔时代高压区域变质作用所掩盖，在这种区域变质作用中，先前存在的高级矿物组合没有受到实质性的影响。
An Adirondack marble contains the assemblage calcite-diopside-forsterite-monticellite, with trace amounts of zincian spinel, titanian andradite, idocrase, and sphalerite. Microprobe analyses indicate that the monticellite and forsterite both have Mg/(Mg+ Fe) greater than 0.9, and that monticellite is more iron-rich than coexisting forsterite. Monticellite host grains contain rare microscopic exsolution lamellae of forsterite, a feature noted in only one other terrestrial monticellite occurrence. Diopside is strongly zoned and contains up to 6 weight percent AlrOr and substantial ferric iron, indicating a fassaitic character typical ofcalc-silicate clinopyroxenes. Distribution of Fe and Mg among the major silicate minerals indicates that the monticelliteproducing reaction must be an Fe-Mg continuous reaction combining the end-member reactlon, MgrSiOn * CaMgSirO. * 2 CaCO, : 3 CaMgSiO4 + 2CO2 with two Fe-Mg exchange equilibria: 2 CaMgSiOn * Fe2SiO. : 2 CaFeSiO, * Mg,SiOn CaMgSirO. * CaFeSiO. : CaFeSirOu * CaMgSiO. Calculations suggest that Fe does not lower the reaction temperature significantly. The occurrence of monticellite-free marble in the same outcrops and variable mineral compositions in the calcite-diopside-forsterite-monticellite assemblage implies that these rocks formed in an aCO, gradient with aCO, probably decreasing from center to edge of the marble layer. P-T-XCO, relations for the iron-free reaction have been calculated, and suggest that formation of monticellite in the above reaction, under presumed conditions of Adirondack regional metamorphism of 8-10 kbar and about 700'-800oC, requires either great dilution of CO, by HzO in metamorphic fluid, or low aCO, under fluid-absent conditions. A possible alternative explanation suggests earlier contact metamorphism at low pressures lollowed by a later Grenville-age high-pressure regional metamorphism in which the pre-existing high-grade mineral assemblages were not substantially affected.