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Author Topic:   Exploring the Grand Canyon, from the bottom up.
jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 46 of 282 (295524)
03-15-2006 11:52 AM
Reply to: Message 45 by Jazzns
03-15-2006 11:44 AM


Calling for geologist input on aisle 11
So before we move on, we need an answer to two more questions.

Is the Vishnu Schist an example of contact metamorphos or compression metamorphos and how can the difference be told?


Aslan is not a Tame Lion
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IrishRockhound
Member (Idle past 2415 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 47 of 282 (295538)
03-15-2006 12:27 PM
Reply to: Message 45 by Jazzns
03-15-2006 11:44 AM


Re: Can we head back towards the topic.
Hey Jazz

Tough question there - why have only one kind of metamorphism? To be honest without examining the schist around the igneous intrusion we won't know how extensive the contact metamorphism is. Considering the size of the schist I would expect that most of it was formed by regular metamorphism.

So let's see:

1. Some silicate rock somewhere gets eroded into sand.
2. That sand is deposited which later becomes the Vishnu Sandstone.
3? 4? A magma body intrudes into the sandstone causing contact metamorphism.
3? 4? The formation is subjected to pressures and temperatures causing it to metamorphose further.
5. The magma never makes it to the surface and therefore cools as granite.

The timing of 3, 4, and 5 depend on information we don't have at the moment. Any help Rox?

IRH


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Jazzns
Member (Idle past 1890 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 48 of 282 (295544)
03-15-2006 12:37 PM
Reply to: Message 47 by IrishRockhound
03-15-2006 12:27 PM


Re: Can we head back towards the topic.
But really there is also another scenario depending on the evidence where no contact metamorphism is necessary. The granite could have formed and been exposed before the sandstone was even deposited.

This case should be easy to identify though if there is an erosional unconformity between the granite and the schist. If there is then probably all the metamorphism is compressional.

So really the first question we have to ask is if the granite is an intrusion or not.


Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)
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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 49 of 282 (295546)
03-15-2006 12:43 PM
Reply to: Message 47 by IrishRockhound
03-15-2006 12:27 PM


on the ordering
Okay. From this I gather that it's possible to have metamorphic rocks that were formed by two different processes, and to determine which process was involved we'd need to look at an idividual sample.

Overall though, it's possible to assert a general order, that goes like this so far.

Rock is created.

Rock is eroded into sandstone.

Magma intrudes and becomes granite.

Sandstone is put under heat (the heat could come from the magma or just the pressure of being buried under other levels or a combination of both) and becomes schist.

Is that a reasonable summary of what is found in the Vishnu Schist and Zoroaster Granite levels.


Aslan is not a Tame Lion
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Jazzns
Member (Idle past 1890 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 50 of 282 (295551)
03-15-2006 12:48 PM
Reply to: Message 49 by jar
03-15-2006 12:43 PM


Re: on the ordering
Unless, like I said in my most recent post, there is an erosional unconformity between the granite and the schist. The granite really has to be an intrustion for that scenario to work. This might have been cleared up before but I don't recall. Sorry if it has.


Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)
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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


(1)
Message 51 of 282 (295552)
03-15-2006 12:55 PM
Reply to: Message 50 by Jazzns
03-15-2006 12:48 PM


Re: on the ordering
I don't want to get too far ahead of where we are now, but this site may help. Please don't use the site to jump beyond the two layers and four events we are dealing with so far.

I really want this thread to move slowly through the layers.


Aslan is not a Tame Lion
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PaulK
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Posts: 14562
Joined: 01-10-2003
Member Rating: 2.1


Message 52 of 282 (295566)
03-15-2006 1:26 PM
Reply to: Message 48 by Jazzns
03-15-2006 12:37 PM


Re: Can we head back towards the topic.
quote:

The granite could have formed and been exposed before the sandstone was even deposited.

I don't think that this is possible. The diagram in Message 8 shows veins of granite running through the schist, and according to another post it couldn't have reached the surface.


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Jazzns
Member (Idle past 1890 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 53 of 282 (295569)
03-15-2006 1:40 PM
Reply to: Message 52 by PaulK
03-15-2006 1:26 PM


Re: Can we head back towards the topic.
That is the evidence for an intrusion that we need. If there is a dike then there is no doubt. Then we are back to what IRH said which is that there was probably a combination of the types of metamorphism. We just may need someone to show us what to look for if this was the case.

I seem to recall someone mentioning the grade of the metamorphism and that part of the schist was more metamorphosed (is that even the right word?) than other parts. If that scales with distance from the granite then it seems like contact metamorphism is our most likely and probably sole canidate. This is just my armchar knowledge speaking though and I hope rox or IRH or someone else can confirm or correct.


Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)
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roxrkool
Member (Idle past 875 days)
Posts: 1493
From: Nevada
Joined: 03-23-2003


Message 54 of 282 (295595)
03-15-2006 2:57 PM
Reply to: Message 53 by Jazzns
03-15-2006 1:40 PM


Re: Can we head back towards the topic.
Boy, would it be wonderful if there was a geologist here for each geologic topic that came up. There have been a lot lately.

The degree of metamorphism seen in the Vishnu and other schists is quite pervasive over long distances and very little of it is the result of igneous intrusions as far as I can tell. The reason for this conclusion is that the degree of metamorphism, which includes both deformation fabric/structure and mineralogy, is present throughout the entire body of schist regardless of the presence of intrusive bodies.

Contact metamorphism is a point source for the heat and so heat is radiated out from the dike/intrusion through the rock. This sort of thermal conductivity results in minralogic and textural changes in the country rock (rock being intruded), which, depending on it's depth in the crust, can be cold, warm, or hot. The higher in the crust, the colder the country rock, the more water it typically has, and (I think) the more pronounced the reaction of the country rock to the intrusion. The presence of water or other fluids is easily detected by the resultant metamorphic minerals and this water can come from the country rock or the intrusion. We can tell the difference using isotopic signatures.

So in contact metamorphic zones, the highest degree of alteration is typically located immediately adjacent to the intrusion (again especially if the rock is cold) and the alteration becomes progressively less pronounced the further away from the heat source we move.

Because I don't see such a pronounced zoning (an alteration halo) around each intrusion, and the degree to which the Vishnu Schist is deformed, the metamorphism event that formed the schists in the Grand Canyon area are not primarily the result of contact-metamorphism.

That doesn't mean there is not any contact metamorphism present in the schists, there very likely is, and this would probably result in increasing the grade of metamorphism in the vicinity of the intrusions. But again, it would be like a halo radiating away from the intrusive bodies unless there was a large concentration of intrusive bodies in one location. The resulting minerals found in a contact metamorphic halo would be different that what is found in the rest of the schist simply because by intruding the schist with an igneous body, you are introducing new fluids and minerals (in other words, new elements/chemistry) into the system.

From what I've read about these intrusions in the paper I linked to on the first page (Ilg et al, 1996), there are two separate types of intrusions that occurred at different times and are not genetically related to each other.

One is an earlier and more mafic group of gabbroic to granoditoritic intrusions probably related to arc magmatism, which may have been the source of the volcanic material (pillow basalts, volcanic seds, flows, etc.) erupted by the island arc. The mafic bodies are highly deformed, though not too alteres as to obliterate primary mineralogy and structure apparently. But the fact that they are deformed suggests they preceded the major deformation event and were deformed during the same event that created the schist.

The other group of intrusives is a later granitic phase that may have been the result of partial melting of the schist or sourced by other processes, but these appear to have intruded the schist at the same time the schist was forming. This is based on various things such as lack of deformation and how dikes tend to follow folds and fraacture/jointing patterns as the schist later cooled and hardened. Basically, they took advantage of incipient weaknesses in the schist while it was forming and after it started cooling.

This message has been edited by roxrkool, 03-15-2006 03:00 PM


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IrishRockhound
Member (Idle past 2415 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 55 of 282 (295609)
03-15-2006 3:29 PM
Reply to: Message 54 by roxrkool
03-15-2006 2:57 PM


Re: Can we head back towards the topic.
*bows to Rox*

You explained it better than I did. :)


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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 56 of 282 (295614)
03-15-2006 3:38 PM


Okay, last call for questions on
the Vishnu Schist and Zoroaster Granite.


Aslan is not a Tame Lion
  
Jazzns
Member (Idle past 1890 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 57 of 282 (295617)
03-15-2006 3:45 PM
Reply to: Message 54 by roxrkool
03-15-2006 2:57 PM


Re: Can we head back towards the topic.
Awesome! Thanks Rox!

the highest degree of alteration is typically located immediately adjacent to the intrusion (again especially if the rock is cold) and the alteration becomes progressively less pronounced the further away from the heat source we move.

I had just remember you posting something earlier about the difference is grade of the schist. I wasn't sure how far out the heat from a contact metamorphism scenario would go or it if was uniform. Sounds like it is not.

Thanks again for all the great info!


Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)
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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 58 of 282 (295625)
03-15-2006 4:05 PM


Moving on to the Grand Canyon Supergroup.
Please folk.

I would like to take this one very slow, layer by layer. Can we hold off ALL discussion of what happened to the Grand Canyon Supergroup until we get it laid out like a layer cake.

I have heard that the lowest layer of the Grand Canyon Supergroup is the Bass Limestone layer.

Limestone is yet another rock that hasn't been mentioned so far. Sticking to the Bass Limestone layer (leave out the white cliffs of dover) what can you good folk tell me about this layer?


Aslan is not a Tame Lion
  
Ratel
Inactive Member


Message 59 of 282 (295642)
03-15-2006 4:58 PM
Reply to: Message 54 by roxrkool
03-15-2006 2:57 PM


Dumb questions
Hi, sorry if this is stupid, but is there any way that these intrusions could have happened while the strata was still uh, muddy?

(edited out timeframe question)

This message has been edited by Ratel, 03-15-2006 05:14 PM


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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004
Member Rating: 1.6


Message 60 of 282 (295645)
03-15-2006 5:07 PM
Reply to: Message 59 by Ratel
03-15-2006 4:58 PM


Holding off on time for the moment
I don't think the time really matters much right now. Let's just go nice and slowly and stick to things we can definitely point to.

This message has been edited by jar, 03-15-2006 04:07 PM


Aslan is not a Tame Lion
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