榴辉岩中传统地质温压计新解:来自PT视剖面图的证据

A new interpretation of the conventional thermobarometry in eclogite: Evidence from the calculated PT pseudosections.

石榴石-单斜辉石(GC)温度计和石榴石-单斜辉石-多硅白云母(GCP)压力计是确定榴辉岩形成温压条件的最常用方法,二者主要依据石榴石、绿辉石和多硅白云母中相组分之间的交换和转换变质反应。依据MORB成分计算的PT视剖面图表明,在不同榴辉岩矿物组合中,控制3个矿物相成分变化的相组分之间的变质反应不同。在低温含绿泥石、滑石和蓝闪石榴辉岩组合中,石榴石和绿辉石的镁含量主要受到含水矿物脱水反应的控制,并都随温度升高而升高,二者之间的铁镁交换反应并不起主要作用。因此,在自然界含有蓝闪石等含水矿物的低温榴辉岩中,由于绿辉石相对富镁而常常导致GC温度计结果偏低。在含有硬柱石的高压-超高压榴辉岩中,石榴石中的钙含量受到硬柱石的控制,随着压力升高或温度降低,硬柱石含量增加,使石榴石中钙降低,此时石榴石-绿辉石-多硅白云母之间的转换反应对石榴石成分的影响会很微弱,由于石榴石相对贫钙而导致GCP压力计结果偏低。在含有蓝晶石的中温高压-超高压榴辉岩中,矿物成分的变化受到石榴石-绿辉石之间的铁镁交换反应和石榴石-绿辉石-多硅白云母-蓝晶石-石英/柯石英之间的一系列转换反应控制,因此,GC和GCP温压计都能给出相对合理的结果。在低压普通角闪石榴辉岩中,石榴石和绿辉石中的镁含量主要反应压力变化,有时并不指示变质作用温度。在含有蓝闪石等含水矿物的低温榴辉岩中,Thermocalc程序中的平均温压(avPT)方法可以给出比较合适的温度,但压力结果与GCP压力计一样也会偏低一些。在蓝闪石和绿帘石等含水矿物消失后的中温蓝晶石榴辉岩中,avPT方法难以给出合理的PT信息。相对来说,视剖面图方法能够给出最多的PT信息,是目前确定变质岩PT条件的最好方法。

The garnet-clinopyroxene ( GC) thermometer and garnet-clinopyroxene-phengite(GCP) barometer are commonly used for determining the PT conditions of eclogite metamorphism. These thermobarometries are calibrated from the exchange and net transfer reactions among the phase components in garnet, clinopyroxene and phengite. The calculated PT pseudosections from a MORB composition indicate that in different eclogite assemblages there are different exchange and net transfer reactions among the phase components to control the compositions of the three minerals: In the chlorite-, talc- and glaucophane-bearing low-T eclogites, the Mg-contents in garnet and omphacite are dependent on the dehydrations of hydrous minerals, increasing as temperature rise, and the Fe-Mg exchange reaction between garnet and omphacite plays a trivial role. Thus, in the natural low-T eclogites which contain glaucophane and other hydrous minerals, omphacite is usually rich in Mg, which may result in the GC thermometer giving lower temperature results, In the lawsonite-bearing HP-UHP eclogites, the Ca-contents in garnet are dependent on the proportions of lawsonite, decreasing as pressure rise or temperature decrease. In this case, the net transfer reactions among garnet, omphacite and phengite may have a minor effect on the garnet compositions, an(] the GCP barometer may give lower pressure results due to garnet poor-in Ca. In the medium-T kyanite-bearing UP-UHP eclogites, the garnet, omphacite and phengite compositions are largely controlled by the Fe-Mg exchange reactions between garnet and omphacite, and the net transfer reactions in garnet- omphacite- phengite- kyanite- quartz / coesite. Thus, both GC thermometer and GCP barometer can give available PT results. In the lower-P hornblende-bearing eclogite, the Mg-contents in garnet and omphacite mainly reflect pressure changes, and commonly do not reflect temperature variations. The average PT mode (avPT) in Thermocalc could give available temperature results, and relatively lower pressures similar to those from the GCP barometer for the glaucophane-bearing low-T eclogites, but could not yield reasonable PT formation for the medium-T kyanite eclogites without the hydrous minerals such as glaucophane and epidote etc. Comparatively, the pseudosection thermobarometry can provide much more PT formation, and is the most optimal method for determining the metamorphic PT conditions.