Dec
19

Eclogitization in the Lofoten Islands: Petrology-based Hypothesis

As I continue with my analyses, I have had to split my time between my structural analyses and my petrologic analyses. In the case of the structural component, as a reminder, I am trying to quantify the kinematics of my shear zones’ burial and exhumation. In regards to the petrology, I am looking for distinct chemical changes and mineralogical assemblage changes that point me towards how much of a volume change there was over time as well as indicators for physical metamorphic conditions of eclogitization. As of late, I have developed a petrologic hypothesis for how my rocks went through assemblage changes in order to produce the intriguing mineral composition I see today. My hypothesis can be tested via microprobe analyses of particular mineral grains and I look forward to completing that analyses in the coming months.

My hypothesis is based on the evidence that I have collected so far that my protoliths outside and inside the shear zones are olivine-normative gabbronorites. This means that they consisted of orthopyroxene, clinopyroxene, plagioclase, and olivine as the primary constituents. I also know that they were metaluminous based on the A/CNK plots I made which plotted the molar concentrations of aluminum oxide, calcium oxide, sodium oxide, and potassium oxide molar concentrations determined by the geochemical analysis completed over the summer.

As the gabbronorites form intrusively in a pluton at around mid-crustal levels (this is known based on past studies) and begins to sink deeper under higher pressures, olivine ceases to be stable in the presence of calcic plagioclase (anorthite), and is consumed by this chemical reaction: forsterite + anorthite = enstatite (orthopyroxene variety) + diopside (clinopyroxene variety) + spinel (2Mg2SiO4 + CaAl2Si2O8 = 2MgSiO3 + CaMgSi2O6 + MgAl2O4). There is also a discontinuous reaction that breaks the tie-line between plagioclase and Al-rich orthopyroxene (essentially a way of saying a metaphorical connection between two that allows them to subsist in equilibrium together) and forms a tie-line between garnet and clinopyroxene. This can happen at granulite-facies metamorphic conditions, which is the phase directly before one would reach eclogite facies as is represented by this chemical formula: CaAl2Si2O8 + 2(Mg,Fe)SiO3 = Ca(Mg,Fe)2Al2Si3O12 + SiO2. These chemical components represent the mineral transformation of anorthite + orthopyroxene = garnet + quartz. Thus, under granulite-facies, you can have garnet form between plagioclase and orthopyroxene as elemental abundances of particular atoms between to move around and recrystallize. This could be a reason why I see garnet rimming between plagioclase and orthopyroxene grains in my outer boundary rock samples.

Next, as my rocks get buried deeper and deeper into the crust and reach eclogite-facies conditions, the plagioclase becomes unstable and dissolves via a terminal disappearance reaction. This allows for the calcium and aluminum sequestered in the plagioclase to be lodged in garnet and the sodium, some of the aluminum, some of the calcium, the magnesium, and the iron to form omphacite (a variety of clinopyroxene characteristic of eclogites). If some of the relict pyroxene grains in my outer boundary samples are in fact omphacite (I will determine this from the microprobe analysis that will be completed in coming months), this would support my hypothesis.

Whilst at these incredible depths, there would have had to be a kickstarter for the localized shearing that occurred to form the shear zones, and it seems to make the most sense that this would be some sort of fluid-mediated localization. This means that a hydrothermal fluid percolated through my rocks and caused strain softening to occur, sometimes called strain weakening, which allowed for there to be shearing movement in the particular spot with fluid injection. This could also explain why there was no garnet rimming in the samples from the center of my shear zone, and why I am only finding garnet rimming on the outer boundaries, where the fluid was not as channeled.

Next, the rocks, now also containing a shear zone, would have had to be exhumed an incredible distance back up to the surface for me to find them in the Lofoten Islands today. During this exhumation, it is known that there was an amphibolite-facies metamorphic overprinting. However, before this happened, I hypothesize that a reverse reaction occurred consisting of this transformation:  garnet + quartz = anorthite + orthopyroxene. One piece of evidence that can confirm whether this could have happened would be the formation of a symplectite (an intergrowth) of plagioclase and pyroxene, which I do indeed find in some of my rock samples. This also would have caused many garnets to partially dissolve and completely dissolve back into plagioclase, which would explain why I see plagioclase in my rocks today after they were at depths where they would have been unstable.

Finally, these rocks and these shear zones would have passed through amphibolite-facies conditions, which would have led to the transformation of the remaining pyroxene into green amphibole (most likely hornblende variety). The garnet rims would have remained between the plagioclase and pyroxene, with the garnet being deteriorated from the recrystallization of plagioclase and orthopyroxene from its elemental constituents. Thus, this is why I see the amalgamation of recrystallized plagioclase, amphibole, the occasional pyroxene grain in my boundary samples specifically, and partially eaten-away garnets rimming boundaries between plagioclase matrices and amphibole matrices.

This is just a hypothesis that I have developed from the evidence I see in my samples as well as past studies and textbook reports. I will be able to support or reject this hypothesis with more information from the microprobe analysis. There are some structural indications that this hypothesis may not have occurred the way I have predicted. For example, how did the plagioclase ribbons from if they recrystallized after shearing? Additionally, how are there still intact, not-recrystallized grains of plagioclase on the outsides of the shear zones if all of the rocks containing the shear zones went down to plagioclase-unstable, eclogite-facies depths? I definitely have some tweaking to do to my hypothesis and some more questions that need answering, but it certainly has been comforting to know that there most likely is a chemical explanation for why my rocks look so peculiar mineralogically.

 

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