Attached is a photomicrograph of gabbro. In the center a plagioclase phenocryst occurs that includes many clinopyroxene and hornblende minerals. The phenocryst was albitized. The matrix around the plagioclase phenocryst consists of clinopyroxene, hornblende and plagioclase. Some clinopyroxene and hornblende were altered to chlorite. Many of the matrix plagioclases consist of a dark or clouded core, with fine ferromagnesian minerals included, and a clear rim. My question is: how were these phenocryst and matrix plagioclases (core and rim) formed?
Dear Dr. Liu,
It should be noted that a rock with an average matrix grain size of 0.5 mm and a clearly porphyritic structure with coarse phenocrysts in a fine-grained ground mass can not be called 'gabbro'.
The picture actually shows a section of an altered basaltic rock (dolerite).
The entire rock fabric is strongly altered, the plagioclase show clear signs of saussuritization, this means that the plagioclase is replaced by fine-grained aggregates of zoisite, epidote, albite, calcite, and/or zeolites. This may have occurred during a metamorphic or deuteric process and is frequently accompanied by chloritization of the ferromagnesian minerals.
Best regards,
G. Grundmann
Thank Prof. Grundmann. I agree that this is a sign of saussuritization. However, electron microprobe analyses gave data that suggest presence of albite and zeolites but not zoisite. Please check in the photomicrograph, the plagioclase phenocryst includes many minerals with colorful interference colors. If these minerals are epidote, then it is a sign of saussuritization; if the minerals are clinopyroxene, how shall one explain.
Here I attach another photomicrograph of granite that shows a zoned plagioclase. The core is dark and contains fine-grained minerals, whereas the rim is clear. If the core has been saussuritized during a deuteric (late magmatic or post-magmatic) process, then how do one explain crystallization of the rim that must have followed crystallization and alteration of the core. This means the magmatic process was interrupted by a hydrothermal process: the magmatic process when the core crystallized occurred before the saussuritization; whereas the magmatic process when the rim crystallized occurred after the saussuritization. This is unthinkable.
More likely, the core-rim structure can be explained with mixing of a more mafic magma, where the plagioclase core crystallized, with a felsic magma. The rim grew from the mixed magma. The problem is when did the saussuritization occur. If it was deuteric, why the rim was not altered? If the saussuritization occurred in the plagioclase core before the mafic magma was mixed with the felsic magma, then another problem occurred. A mafic magma after deuteric alteration of plagioclase mixed with the felsic magma. However, deuteric process usually means the magma was essentially crystallized. In this case how could the nearly solidified mafic magma be mixed with felsic magma?
Dear Dr. José Ribeiro Aires:
Thank you for the explanation. What do you mean by magnetism influence during crystallization. Do you mean the ferromagnesian minerals were attracted into the plagioclase phenocryst? Please see the second photomicrograph that I attached after I responded to Prof. Grundmann's explanation. Your answer is very interesting. Can you explain in more detail?
Dear Dr. Liu,
Many thanks for the second image.
During the cooling of a basaltic or granitic magma, the anorthite-rich plagioclase crystallize first. The adjoining plagioclase zones are increasingly albite-rich (Na), since the anorthite component (Ca) was incorporated into the inner (early) plagioclase zones.
A subsequent (low temperature) deuteric alteration accesses the anorthite-rich zones at first because anorthite is only stable at high temperatures and unstable at low temperatures. The albite-rich zones, however, are hardly altered since they are stable at low temperatures. The water rich solutions separated from the same magmatic body and reacted with the primary magmatic minerals. Since the hydrothermal fluids can easily penetrate into the innermost regions of the plagioclase, the anorthite-rich core zones are selectively replaced by secondary minerals.
Best regards,
G. Grundmann
Dear Prof. Grundmann:
Thank you for the second answer. There are still two problems. Please check in the plagioclase phenocryst in the first photomicrograph, Some clinopyroxene grains, at least, are included in the phenocryst. You can judge from the reference color. Just compare the inclusion clinopyroxene with the matrix pyroxene. I mean that they are not epidote or zoisite. So, the plagioclase phenocryst or its dark core cannot be fully explained with saussuritization. Second, the variation from the core to the rim is abrupt instead of gradual (in this case, your explanation is more convincing). It is also true that the content of mineral inclusions, darkness and intensity of alteration of the core and rim present dichotomy. The core and the rim of the plagioclase more likely were formed in two contrasting environments: one is Fe-, Mg-, Ca- and H2O-rich, whereas the other is felsic and relatively H2O-deficient.
Best regards,
Liu Wei
Dear Dr. Liu,
Thank you for your second response.
In my answer to the inclusions in the zoned plagioclase phenocryst of course I did not mean that all the above mentioned types of minerals should be present, but optional for all species 'and / or' might be present.
The clinopyroxene inclusions may also be spared relics preserved after the alteration . You know very well that the highly visible alteration with saussuritization of feldspar, and / or serpentinization, and / or chloritization point clear to the changes in an open system with supply and removal of various elements (micro metasomatism). Therefore, it is generally very difficult to determine the original chemical composition and mineral components. Sorry, from the distance and without further details of the geological environment (crystallization and deformation history) I am not able to give better answers.
Best regards,
G. Grundmann