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Gigapan Comments (2)

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  1. Ron Schott

    Ron Schott (September 08, 2011, 12:51PM )

    In this case, the plagioclase feldspars (off white and/or bluish) and the pyroxene (dark green) are primary igneous minerals and the garnet (red) is metamorphic. The plagioclase crystals almost certainly formed first based on their subhedral to euhedral shapes (crystallizing without impediment in the parental magma) and the pyroxenes crystallized at the end of the original igneous crystallization sequence as seen by their anhedral shapes (filling in the gaps between the well formed plagioclase crystals). In fact, the abundance of plagioclase in anorthosites (>90% by definition) is, in this instance, thought to be the result of early formed plagioclase crystals accumulating by floatation in a relatively dense ferrodioritic magma chamber. The pyroxenes are basically the interstitial last remanents of the slow crystallizing magma. The garnet is definitely a metamorphic mineral and thus formed post-igneous crystallization, though in this case my recollection is that the metamorphism was relatively soon after the magmatic event. Here's the official geochronology: the anorthosite crystallized ~1135Ma as part of the AMCG intrusions (anorthosite-mangerite-charnockite-gabbro) and was subsequently metamorphosed to granulite facies during the 1080-1050Ma Ottawan Orogeny of the Grenville Orogenic cycle (McLelland and Chiarenzelli, 1990).

  2. Chris Fastie

    Chris Fastie (September 08, 2011, 06:47AM )

    Is it possible to know which of the crystals grew when the intrusion first cooled a billion years ago, and which were formed a bit later during the events that metamorphosed this pluton?