Exploring the Grand Canyon, from the bottom up.

Metasedimentary rock = Metamorphic rock that was formed from the metamorphism of sedimentary rock.Conglomerate - Sedimentary rock made up of gravel or larger sized clasts. There are also commonly found "hybrid" sediments - Conglomeratic sandstones. The conclomeratic compontent are lag deposits, with the sand being deposited around and above the larger clasts.The pre-existing sedimentary rocks are called the protoliths. An earlier form that became a later form.In the case of the Vishnu protoliths, the sediments show no evidence anywhere of being from the processes that form conglomerates.Bottom line - For whatever reason, the protolith sediments did not contain gravel or larger sized clasts.Moose

Okay, but still a few questions.You say: Metamorphic rock. Does that mean that the form of the rock has changed?What I get from all this is:The Vishnu Schist began life as a sandstone, but was changed to what you guys call schist over time by temperature and pressure. That schist is a coarse grained rock and since it is formed at a higher temperature and pressure than say, slate, it is also harder.Is that correct?

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Aslan is not a Tame Lion

From reading the technical paper I linked to earlier, it seems the lack of a coarser grained component (i.e., the conglomeratic sequence), points to a marine depositional setting rather than a terrestrial setting for the original proto-sediments; which is consistent with previous theories regarding the geologic history of that particular terrane.In another thread, I briefly touched on how the U.S. grew via island arc accretion south from about the Wyoming/Montana area. Wyoming is located on the Archean Wyoming craton and everything south and basically west are progressively younger accreted terranes - generally considered to be island arc terranes (volcanic island chains similar to Japan and Indonesia that develop along subduction zones).According to the linked paper (Ilg et al., 1996), the protosediments which later became the Vishnu Schist, are remants of one of those island arc terranes that collided with the continent back in the Precambrian (early Proterozoic time?). Evidence for this is in the relict (i.e., original unmetamorphosed rock with primary texture) volcanic and fine-grained sedimentary textures/rocks still visible in some portions of the entire Vishnu schist sequence, which Ilg et al. (1996) renames to Granite Gorge Metamorphic Suite.

That's basically correct, Jar.What metamorphism also does, because of increased heat and/or pressure, is cause recrystallization and secondary mineral growth. Subjecting slate to higher temps and pressures can result in a schist.So with increased temperature and/or pressure, you will get new [secondary] minerals forming from the destruction of the primary minerals due to mineral instability. The new minerals form because they are stable within the new pressure/temperature regime and the original ones were not (because they were formed and stable under other temp/pressure conditions).What metamorphic petrologists can do is look at the minerals comprising metamorphic rocks and pretty much tell you what the original protolith was and often how much water it contained. Clays, such as in shale, when metamorphosed become chlorite during low-grade or green schist metamorphism and will alter to mica under higher temps (and pressure?).Each metamorphic grade from low to high is represented by specific suites of minerals, which are indicative of ranges of temperature and/or pressure.

Okay, sticking to the Vishnu Schist.It began life as sandstone. In the post to Moose you hint at the origin of the sandstone, but for now I'd like to put that off beyond simply saying that the Vishnu Schist was once a mountain that got eroded down into sandstone that later got buried and under pressure turned into schist.Is that correct so far?

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Aslan is not a Tame Lion

The general sequence of events as I understand them from reading around the internet:1. Formation of island arc at subduction zone located off the coast of the U.S. (in Utah or Arizona at the time??) which is slowly migrating toward the continent;2. Deposition of volcanic and marine sedimentary rock (protosediments) at the island arc;3. Collision (accretion)of island arc onto North American continent, metamorphism, and possibly igneous intrusions soon after;4. Some time later 'mountain building'/uplift (more metamorphism?) began in the vicinity of the arc accretion (possibly due to deep seated structural sutures developed during accretion);5. Erosion of mountain range which cuts across both metaseds and igneous intrusions;6. Deposition of Grand Canyon supergroup.Ilg et al. (1996) appear to suggest that metamorphism occurred while the island arc was still in the marine environment and possibly due to collision. Uplift of the island arc system to sea level exposed the surficial volcanic environment to marine (wave?) erosion and continued uplift eventually exposed the metamorphosed core. They don't mention any mountain-building episode. When the island arc system was eroded far enough, eventually marine sediments were able to be deposited atop the erosional surface.

Okay, but that seems to be talking about the Grand Canyon supergroup and I'm not sure we are there yet. The Vishnu Schist is below the Super Group IIRC. We'll get to it soon.

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Aslan is not a Tame Lion

Well, I did mention it, but the Supergroup does not make an appearance until #6, after the major erosive event.The Vishnu Schist, apparently now known collectively as the Granite Gorge Metamorphic Suite, is formed from the metamorphism of the volcanic rocks and marine sediments deposited by the island arc system. This package, which we've been calling the Vishnu Schist, was later uplifted and eroded prior to deposition of the Grand Canyon Supergroup.Is that more clear? It's a bit difficult to keep all this stuff straight and I am probably doing a terrible job of explaining it.

I think I have it now.So from the bottom so far we have one level that started out as sandstone but later was transformed into schist. The sandstone was a second generation product that had been produced by wearing down some earlier formation.If that's okay, I'd like to move on to the question about Zoroaster granite.The Zoroaster granite is both below and within the Vishnu schist.Let me ask three questions to start this off.First, what is the difference between schist and granite?How did the Zoroaster granite get below and within the Vishnu schist?Is the Zoroaster granite then younger than teh Vishnu schist?edited to fix subtitleThis message has been edited by jar, 03-14-2006 09:09 PM

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Aslan is not a Tame Lion

Gramite is an igneous rock - it's formed directly from magma. From the posted diagram it intrudes into the Vishnu schist. So veins of magma have worked their way through the schist (or, IMHO more likely, the rocks that became the schist).This must hav e happened after the rocks that became the Vishnu Schist were deposited. From the diagram, it looks as if it happened before the fault, and I would guess before the Bass limestone was deposited (but that latter is just a guess). If the magma reached the surface it has been eroded away.THe intruison of igneous rck is probably part of the event - or one of the events - that produced the metamorphism that transformed the surrounding rock to schist.To answer yor questions directly.1) Schist is matamorphic and grantie is igneous - they are very different rocks2) The granite was formed from magma that welled up underneath, and broke through, the schist.3) The granite has to be younger thna the schist- and we know that because it intrudes into the schist.eThis message has been edited by PaulK, 03-15-2006 02:56 AM

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