My samples were collected from a suite of metamorphic basic intrusive rocks that include plagioclase amphibolite shist, troctolite and anorthosite. These rocks consist of hornblende, plagioclase and olivine. I checked about ten thin sections and did not find pyroxene. In the plagioclase amphibolite shist, hornblende predominates over plagioclase. In the troctolite, plagioclase predominates over olivine. But, in some samples, olivine equates plagioclase is more abundant than the latter. These rocks have been metamorphosed. Amphibole and elongate plagioclase are orientated along the shistosity (Photo 1.1, 1.2). However, hornblende and plagioclase from some samples do not exhibit strong shistosity but show hypidiomorphic or granular texture (Phtoto 2.1, 2.2).
The troctolite occur as bands within the shist or alternate with the latter or grades into diorite, showing magmatic differentiation. Some troctolites were fragmented and are enclosed by plagioclase-rich rock. It appears that the plagioclase was recrystallized and re-enriched along teb troctolite.
I want to know what the protolith of the plagioclase amphibolite shist, gabbro or diorite is? If the protolith is gabbro, why were no pyroxene grains or remnants found? In the latter case, why does diorite instead of gabbro coexist with troctolite?
Central to the question is how to distinguish igneous from metamorphic amphiboles.
Attached are two sets of photomicrographs. Photos1.1 and 1.2 for plagioclase amphibolite shist, and 2.1 and 2.2 for diorite, in both plane- and cross-polarized lights.
Offhand, I think of a couple things that may help distinguishing the different amphiboles (but may not be too helpful):
1. Often different phases of amphibole will have slightly different pleochroism due to slight differences in composition.
2. Looking at grain boundary relationships may help reveal amphibole that overgrew pre-existing features.
3. I'd bet that there is a different geochemical signature to the magmatic vs metamorphic amphibole.
As for the lack of pyroxene in the metamorphic rocks... with a little bit of H2O and deformation, it is not too difficult to imagine that the pyroxene might not survive, giving way to more hydrous phases like amphibole.
If your rocks belong to an ophiolite suite I can suggest the following explanation:
Pyroxene becomes hornblende when there is infiltration of water into the gabbro at high T ° (with or without tectonic stress).
Seawater can enrich plagioclase in Na and transform a gabbro to diorite.
I am a bit surprised that olivine would survive intact if the amphibole would be formed as a result of a metamorphic reaction reflecting the ingress of water. It would likely affect the olivine as well, I'd think. But if the amphibole is primary, then it is indeed interesting that you do not find any pyroxene whatsoever in your association...
Dear Marlina Elburg: you clearly got to the point. Olivine remain unaltered except on the surface of outcrop or the samples where iddingsite occur. But on the fresh surface of the rocks, fresh olivine instead of iddingsite occurs. We checked about ten thin sections and did not find pyroxene. We think the 'amphibolite' likely is gabbro or even hornblendite.
Of course you can get hornblendites in arc settings, but to have no pyroxenes at all surprises me. If you look at descriptions of the Talkeetna arc section, there are plenty of px-bearing rocks around (and no trocolites). Do you have the chance to do any microprobe analyses? Perhaps that could help...
Yes, this is quite interesting. We are just going to do electron microprobe analysis of minerals (Ol, hbl, px, plg.). After that, we will do trace element analysis using ion or laser microprobe. In the gabbroic or dioritic rocks, the pyroxene is absent or in a little amount. But, in the troctolite, pyroxene may be present significantly. Anorthosite also occurs. Yes, this was once an arc setting, but I am not sure the place was still in arc setting at the time these rocks were formed. Precambrian basement likely is present.
I would think that the most telling trace element analysis for determining whether it still was an arc setting would be Sr compared to its neighbouring elements in mantle-normalised diagrams (of course taking distribution coefficients into account). If the troctolites contain the 'missing' pyroxene, then my first bet would be an arc setting, with the plagioclase-hbl samples being (the cumulates from) the more evolved magmas. Seem like you found yourself quite an interesting set of rocks!
Thank Marlina Elburg for very helpful suggestions. Actually, we have seen magmatic layering and cumulate structure with troctolite alternating with gabbro or hornblendite (depending on the proportion of plagioclase to hornblende). Can you introduce some references with field, handspecimen photos and photomicrographs.
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