Eclogitization in the Lofoten Islands: Thin Section Arrival

My thin sections have finally arrived and the wait is over! I sent off 20 small blocks of rock (each no more than 3 inches by 1 inch and approximately half an inch thick) and they came back to me in the form of microscope slides. I can now view my rocks samples using a petrographic microscope and this will help me gather a plethora of information.

Firstly and quite importantly, viewing my thin section will help me confirm the actual rock types I collected from across Shear Zone 1 and Shear Zone 2. My geochemistry data gave me some clues and hints as to what I should be calling them, but actually seeing the minerals present in each sample will help me finally assign a name. Whilst collecting the samples in the field I could get some ideas, but a fine grained green pyroxene and a fine grained green amphibole can look remarkably similar in the field and make it almost impossible to aptly determine. Thus, my thin sections will be the key to deciphering my rock types.

So far I have come across some surprises. Most of the sheared, deformed samples that I had been calling eclogites while in the field are actually amphibolites; there are very little to no clinopyroxene grains left in most of the thin sections of deformed samples from inside the shear zones, and omphacite, a greenish clinopyroxene, is a defining mineral in eclogites. This makes sense however, due to the reports from previous studies citing an amphibolite-facies metamorphic event that overprinted these rocks at around 469-461 Ma. The fact that I see no clinopyroxene grains and therefore no eclogitic remains inside my shear zones is a tad surprising. The amphbolite overprinting was clearly quite severe. There has been one sample that was collected from the shear zone boundary of Shear Zone 2 that does include some clinopyroxene grains. I will eventually send off this sample again in order to use a microprobe to determine which variety of clinopyroxene I have, and if it is omphacite, then this will be some relic evidence of a once-eclogitic rock.

Another surprise that I have encountered is the presence of garnet and biotite in one of the gabbronorite samples from outside of Shear Zone 2 that I originally thought was igneous. Garnet and biotite are metamorphic index minerals, meaning that if you see them in a rock sample, it means that rock experienced metamorphic conditions up to at least medium grade in order for garnet to form. Biotite forms at slightly lower metamorphic conditions.

As of now, the mineral compositions of my thin sections have been my focus. I am consistently finding plagioclase, amphibole (most likely hornblende), garnet, biotite, and opaque minerals, while also finding minor amounts of clinopyroxene (specific variety unknown without further microprobe analysis), epidote, scapolite, and rutile. My next steps will be to start measuring the major and minor axes of various mineral grains as well as the angles at which they rotated during shearing in order to begin quantifying the kinematics using vorticity numbers. This will eventually tell me the sense of shear these rocks endured.

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