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Author Topic: Ophiolitic exposures in Jamaica... Can anyone help me with a few more questions?  (Read 25217 times)

Offline susanshirleyjamaica

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Hi... This is Susan in Jamaica again.  I received so much great information in my last posts that it has taken a long time to digest it.  Much still to learn but I believe I am leaps and bounds from my initial puzzelment over the rocks I am finding.

I understand that Ophiolitic exposures provide most of the very most interesting samples.  But where in Jamaica might that exposure be?

I am still a bit confused by cherts and their relationship to chalcedony.  In the first picture I show just a few examples of the plentiful "banded chert".  Can it be related to contact metamorphism?  Maybe it means nothing, but the rock on the upper left appears to have a piece of volcanic rock still attached to it.

The second group I have guessed is chalcedony, but why does it sometimes appear as softly shaped agates and "opals", and at other times appear as tightly compacted horizontally stacked crystals?  None of the samples can be scratched by a knife



« Last Edit: 09/12/2008 18:58:13 by susanshirleyjamaica »


 

Offline JimBob

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Were I capable of rational thought I would answer forthwith - but I am so tired and worn out from my bout with bronchitis I can only come up with a poorly framed insult here and there. So have a snotty day - do nothing.
 

Offline susanshirleyjamaica

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Hi JimBob,

I am sorry you feel so crappy, but I don't understand the way you chose to express it as reply to my post.  Was it directed at me (hurt, hurt), or the world in general?

Hope you are feeling better soon,
Susan
 

Offline dentstudent

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JimBob manages to offend another member. Well done JimBob. Who are you after next?
 

Offline frethack

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No worries susan, I know JimBob very well.  He is just not feeling well and was attempting to explain (in his own roundabout way) that he is sick and will answer you as soon as he has the energy to do so.  His sarcasm was an attempt at humor.  ;D

Trust me...he loves to flex his brain muscles, especially about sedimentary rocks, and will have an answer for you that will leave you pondering for some time!

« Last Edit: 10/12/2008 18:52:18 by frethack »
 

Offline susanshirleyjamaica

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Thanks Frethack... I anxiously await JimBob's return to the land of the living.  ;)
 

Offline Bass

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Hi... This is Susan in Jamaica again.  I received so much great information in my last posts that it has taken a long time to digest it.  Much still to learn but I believe I am leaps and bounds from my initial puzzelment over the rocks I am finding.

I understand that Ophiolitic exposures provide most of the very most interesting samples.  But where in Jamaica might that exposure be?

The following quote comes from Scott, Jackson & Dunham, Economic potential of the ultrama®c rocks of Jamaica and Tobago:two contrasting geological settings in the Caribbean, Mineralium Deposita (1999) 34: 718±723

 In Jamaica, ultramaic rocks are found in one small area (ap-
proximately 2 km2) between Cedar Valley and Arntully in the Blue
Mountains to the east of Kingston (Fig. 1a). They are considered
to be part of a dismembered ophiolite suite (Wadge et al. 1982,
1984) of Upper Cretaceous age (Robinson 1994) and are in tectonic
contact with other strata of the Blue Mountain Inlier. The field
relations have been described by Scott et al. (1992).


Quote
I am still a bit confused by cherts and their relationship to chalcedony.  In the first picture I show just a few examples of the plentiful "banded chert".  Can it be related to contact metamorphism?  Maybe it means nothing, but the rock on the upper left appears to have a piece of volcanic rock still attached to it.

The second group I have guessed is chalcedony, but why does it sometimes appear as softly shaped agates and "opals", and at other times appear as tightly compacted horizontally stacked crystals?  None of the samples can be scratched by a knife

Chert is cryptocrystalline quartz that forms by sedimentary (or diagenetic- immediately after sedimentary deposition) processes.  Chert may be laminated or be found as irregular masses, commonly dark to light gray- though colors vary, and is opaque.  Chalcedony forms from hydrothermal processes, is commonly white, and may be opaque or translucent. 

[quote



[/quote]

Rocks in the bottom photo are probably all chalcedony- or opal if the mineral structure contains water.  Rocks in the upper photo are probably chert, with the exception of the top center (looks like some sort of vein), and the far right, which I would call a banded jasper.
 

Offline susanshirleyjamaica

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Hi Bass

Thanks alot.. This information is very exciting... I need to look into this using a map or possibly google earth.

Above you quoted:

"In Jamaica, ultramaic rocks are found in one small area (ap-
proximately 2 km2) between Cedar Valley and Arntully in the Blue
Mountains to the east of Kingston (Fig. 1a)."

Do you have access to that (Fig. 1a)?  I tried to google the document, but no luck. 
 

Offline Bass

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Sorry, I don't have the map.  Here's the link to the abstract  http://www.springerlink.com/content/31hf2058qlyyw4tq/

You can purchase the article for download.
 

Offline susanshirleyjamaica

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Hi Bass... Now I am so confused that my computer froze, twice, and then my brain froze.  I found the area between Cedar Valley and Arntully. Problem is it is on the other side of the blue mountains from us.  So it must not be responsible for providing the "grand trinity of ophiolites" that I am collecting so much of.

I guess I'll just have to be happy knowing that my rocks had an exciting "birth" way down deep and a great adventure rising again and finding themselves on my beach.

Didn't you say breccias are your favourite? Now that I know what I am looking for, I have found several new specimens... some quite wild.


 

Offline Bass

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BRECCIAS!!?? :o    drool, pant, pant...

Post more pictures!!!

I just finished working on a 5000+ foot diameter breccia pipe.  Even JimBob would like it- the rocks used to be calcareous siltstones and quartzites.  Cool stuff!
 

Offline susanshirleyjamaica

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Hi Bass... What do you mean by a 5000 diameter breccia pipe? Is this one solid mass?

Here are two breccias.  The one on the left has a grainy textured matrix.  Both the clasts and the matrix fizz in vinegar.

The one on the right has a finer matrix.  The clasts fizz but the matrix does not.



I have more
 

Offline Bass

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my first thought when I saw these was "solution collapse breccia", and your mention that they fizzed confirmed my suspicion.  These form in karst topography (caves), where rainwater has dissolved limestones underground.  Eventually, the surrounding rocks collapse into the open space of the cave, creating these sorts of breccias.

As to my breccia pipe, the surface expression is roughly circular- approximately one mile in diameter.  The rocks are a mixture of tectonic (caused by faulting) and hydrothermal breccias with subvolcanic granitic dikes.  Pretty well mashed and smashed!

great picture, by the way.
« Last Edit: 11/12/2008 16:17:50 by Bass »
 

Offline susanshirleyjamaica

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Hi Bass... do you have any pictures of that monster breccia?

Here are two more breccias for you.

 

Offline Bass

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Clast support, angular clasts, no rotation of he clasts and clasts are all the same rock.  picture on the right shows reaction rims around the clasts.  My guess is, once again, solution collapse breccia.  Specimen on the right has had some sort of solution go through the breccia after formation, which explains the reaction rims.
 

Offline susanshirleyjamaica

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ok... Just one more breccia (if it is breccia)... I promise!  The dark masses in this one look volcanic.



Now... there is something else I REALLY need to know. That is how chert nodules are formed.  Should ask this question as a new post?
 

Offline JimBob

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S
Were I capable of rational thought I would answer forthwith - but I am so tired and worn out from my bout with bronchitis I can only come up with a poorly framed insult here and there. So have a snotty day - do nothing.

GAG !!! THE SHAME OF IT ALL!

Susan - My deepest apologies. I didn't even realize what I was typing at that moment and cannot decipher the post myself right now. I have no idea what I was thinking.

I believe I was trying to make light of my sickly condition. Obviously it did not come off well.

Again, NO offense was meant and I am so very sorry that it was offensive.

Jim
 

Offline susanshirleyjamaica

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Thanks alot JimBob... Apology accepted and no hard feelings!  You really did give me a bit of a shock though.  Imagine me, naive newbie that I am, excitingly opening up the first responce to my question and being wished a "snotty day" in which I should "do nothing.

What is your speciality?  I have another mystery that I need solved. 

How are chert nodules formed (from start to finish)?  I'm reading that a silica rich sponge died and eventually became chert.  I'd love to see what that sponge looked like

Thanks again,
Susan



 

Offline Bass

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Your last breccia picture- clasts are volcanic/igneous in quartz matrix.  I'm guessing quartz instead of calcite based on the fact that the matrix material is less worn down than the fragments.
That much quartz indicates hydrothermal fluids- not uncommon in volcanic terraines- suggesting ore deposits may be near by the source area.

As to your chert question, my understanding (this is more JimBob's specialty) is that they most commonly form during diagenesis- which are changes that occur in sediments after their deposition but before they are totally lithified as rock.  Cold, silica-rich solutions replace limestone or dolomite material resulting in cryptocrystalline silica- chert.  As to where the silica originates and why it nucleates to form the nodules, that's a whole different discussion.
 

Offline susanshirleyjamaica

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Thanks Bass...

I did a lot of internet searching over the weekend and I found what seems like a good explanation in that the chert nodules began with sponge skeletons (which are apparently silica)that were deposited with limestone sediments.

Sounds plausible to me... I tried to find out what the living sponge looked like, but no luck there.  You think JimBob might have more detailed information?

Thanks lots,
Susan
 

Offline Bass

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Thanks Bass...

You think JimBob might have more detailed information?

I'm sure he'll weigh in with his opinion whether or not he has more detailed information
 

Offline JimBob

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UPDATE

Still infected - feeling much better but struggling to just get a little bit of work out the door each day.

 

paul.fr

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  You think JimBob might have more detailed information?

Classic! You are right to question that.

I'm sure he'll weigh in with his opinion whether or not he has more detailed information

That made my day.
 

Offline frethack

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Quote
I did a lot of internet searching over the weekend and I found what seems like a good explanation in that the chert nodules began with sponge skeletons (which are apparently silica)that were deposited with limestone sediments.

For the most part, sponges decompose into microscopic spicules (needles) of silica or carbonate material (usually aragonite).  There are a few species of sponges whose spicules are made from proteins instead of inorganic minerals.  Chert can also come from plankton such as radiolarians and diatoms, who build their tests from silica as well.  Im sure there are many other sources, including hydrothermal activity (Bass?), but the main sources that I know of are organic in origin.

Here is a diagram on sponge structure...the part labelled "glasslike structural elements" are the spicules:


Here are a few living sponges:
 

Offline Bass

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Chert can also come from plankton such as radiolarians and diatoms, who build their tests from silica as well.  Im sure there are many other sources, including hydrothermal activity (Bass?), but the main sources that I know of are organic in origin.

Hydrothermal activity is unlikely as a source of silica for chert nodules- hydrothermal silica would probably manifest itself in other ways, and the rocks would show more signs of alteration and/or thermal effects.  Even "white smokers" would leave different chemical signature, such as Ba, F, Be, etc.

The more likely source is your organic suggestion- radiolarians and diatoms, especially in or downwind from volcanically active areas.  Or the silica could be provided by nearby more silica-rich sediments during diagenesis.

Deep ocean chert layers (ie top of ophiolite sequences)are derived from radiolarian and diatom tests as they filter to the ocean bottom (plus lots of time!)
 

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