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首页> 外文期刊>Journal of Metamorphic Geology >Constraining the P-T path of a MORB-type eclogite using pseudosections, garnet zoning and garnet-clinopyroxene thermometry: an example from the Bohemian Massif
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Constraining the P-T path of a MORB-type eclogite using pseudosections, garnet zoning and garnet-clinopyroxene thermometry: an example from the Bohemian Massif

机译：使用伪剖面，石榴石分区和石榴石-斜茂铁测温法约束MORB型榴辉岩的P-T路径：以波西米亚地块为例

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A mid-ocean ridge basalt (MORB)-type eclogite from the Moldanubian domain in the Bohemian Massif retains evidence of its prograde path in the form of inclusions of hornblende, plagioclase, clinopyroxene, titanite, ilmenite and rutile preserved in zoned garnet. Prograde zoning involves a flat grossular core followed by a grossular spike and decrease at the rim, whereas Fe/(Fe + Mg) is also flat in the core and then decreases at the rim. In a pseudosection for H2O-saturated conditions, garnet with such a zoning grows along an isothermal burial path at c. 750 degrees C from 10 kbar in the assemblage plagioclase-hornblende-diopsidic clinopyroxene-quartz, then in hornblende-diopsidic clinopyroxene-quartz, and ends its growth at 17-18 kbar. From this point, there is no pseudosection-based information on further increase in pressure or temperature. Then, with garnet-clinopyroxene thermometry, the focus is on the dependence on, and the uncertainties stemming from the unknown Fe3+ content in clinopyroxene. Assuming no Fe3+ in the clinopyroxene gives a serious and unwarranted upward bias to calculated temperatures. A Fe3+-contributed uncertainty of +/- 40 degrees C combined with a calibration and other uncertainties gives a peak temperature of 760 +/- 90 degrees C at 18 kbar, consistent with no further heating following burial to eclogite facies conditions. Further pseudosection modelling suggests that decompression to c. 12 kbar occurred essentially isothermally from the metamorphic peak under H2O-undersaturated conditions (c. 1.3 mol.% H2O) that allowed the preservation of the majority of garnet with symplectitic as well as relict clinopyroxene. The modelling also shows that a MORB-type eclogite decompressed to c. 8 kbar ends as an amphibolite if it is H2O saturated, but if it is H2O-undersaturated it contains assemblages with orthopyroxene. Increasing H2O undersaturation causes an earlier transition to SiO2 undersaturation on decompression, leading to the appearance of spinel-bearing assemblages. Granulite facies-looking overprints of eclogites may develop at amphibolite facies conditions.
机译：来自波希米亚地块摩尔达努比亚地区的洋中脊玄武岩（MORB）型榴辉岩以角闪石，斜长石，斜辉石，钛矿，钛铁矿，钛铁矿和金红石形式保存在分区石榴石中，保留了其前进路径的证据。前进分区涉及一个扁平的粗大芯，然后是粗大的尖峰并在边缘处减少，而Fe /（Fe + Mg）在芯中也是平坦的，然后在边缘处减少。在H2O饱和条件的假剖面中，具有这种分区的石榴石沿着c处的等温埋藏路径生长。在组合斜长石-角闪石-二叠系斜ino石-石英中从10 kbar开始在750摄氏度，然后在角闪石-二齿的斜ino木-石英中，以17-18 kbar结束生长。从这一点出发，没有关于压力或温度进一步升高的基于伪截面的信息。然后，使用石榴石-斜茂铁测温法，重点是对斜茂铁的依赖性和不确定性，其中不明的Fe3 +含量。假设在斜茂铁中没有Fe3 +会给计算温度带来严重且毫无根据的向上偏差。由Fe3 +引起的不确定性+/- 40摄氏度，加上校准和其他不确定性，在18 kbar时出现760 +/- 90摄氏度的峰值温度，与埋藏到榴辉岩相条件下不再加热有关。进一步的假切片建模表明减压至c。在水不足的条件下（约1.3 mol。％H2O），从变质峰基本上等温地出现了12 kbar，这使得大多数石榴石都具有象征性的和残基的斜辉石。建模还显示，将MORB型榴辉岩解压缩为c。如果饱和的H2O，则8 kbar以闪石结尾，但如果饱和的H2O不饱和，则其包含邻苯二甲酚。 H2O欠饱和度的增加会导致减压时更早过渡到SiO2欠饱和度，从而导致出现尖晶石轴承组件。在角闪岩相的条件下，可能会看到颗粒状的榴辉岩叠印物。

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