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Thread: igneous petrology

  1. #1 igneous petrology 
    ilg
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    hi !

    I have a few question on petrology

    Porphyritic texture : I dont understand why can some minerals precipitated at higher Temperature can be surrounded by oikocrysts in some granitic rocks ? (if the minerals are precipitated at higher temperature, they should be precipitated earlier than the oikocrysts?)

    Partial melting: It seems to me that partial melting of lower crust occurs in divergent zone where plates are pulled apart. In this case, granitic magma should be formed (seems some website agree on this). Yet, there are illustration indicates rising of BASALTIC magma in divergent boundary. So... there should be partial melting of LOWER CRUST / UPPER MANTLE in this divergent boundary ?



    also, i dont understand why alkaline rocks does not contain hypersthene ?!
    and is it because the source of Cr is limited, so Cr mineralization is an indication for ophiolitic rocks ?

    Can the mistakes be pointed out (if any) ? (i dropped it down in lecture)

    intergranular texture of diabase (dolerite) " plagioclase grow faster and plagioclase crystals are included in spaces between pyroxene minerals''
    again , I don't understand why plagioclase grow faster ( why shouldn't be pyroxene crystallise first ?)
    also, in this case, the intergranular texture and ophitic texture (big pyroxene surround plagioclase)seem contradict to each other ?


    Last edited by ilg; April 30th, 2012 at 02:11 AM.
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    Quote Originally Posted by ilg View Post
    Porphyritic texture : I dont understand why can some minerals precipitated at higher Temperature can be surrounded by oikocrysts in some granitic rocks ? (if the minerals are precipitated at higher temperature, they should be precipitated earlier than the oikocrysts?)

    yes they should... what do you exactly think of?

    Partial melting: It seems to me that partial melting of lower crust occurs in divergent zone where plates are pulled apart. In this case, granitic magma should be formed (seems some website agree on this). Yet, there are illustration indicates rising of BASALTIC magma in divergent boundary. So... there should be partial melting of LOWER CRUST / UPPER MANTLE in this divergent boundary ?
    melting up the mantle directly you can get very hard granits, at divergent boundaries you've got bimodal volcanism basaltic melting up the upper mantle and granitic from melting up the crust



    also, i dont understand why alkaline rocks does not contain hypersthene ?!

    hypersthene needs mg and fe, doesn't it? alkaline magmas are resulting out of very little melting up ratios so I would guess mg and fe stay in the restite

    and is it because the source of Cr is limited, so Cr mineralization is an indication for ophiolitic rocks ?
    in the bowen series chromit comes right behind olivine, its at the bottom of every oceanic crust, so also at ophioliths


    intergranular texture of diabase (dolerite) " plagioclase grow faster and plagioclase crystals are included in spaces between pyroxene minerals''
    again , I don't understand why plagioclase grow faster ( why shouldn't be pyroxene crystallise first ?)
    also, in this case, the intergranular texture and ophitic texture (big pyroxene surround plagioclase)seem contradict to each other ?
    I think there is a range of temperature at which both are crystallizing, maybe thats the solution


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    ilg
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    thanks...
    is it opposite things occur in ophitic texture and intergranular texture ? ( in ophitic , plagioclase is surrounded by other minerals that forms before plagioclase ; while in intergranular, plgioclase forms first and other minerals grow in between the spaces ?) Also, what makes the differences between ophitic and poikilitic texture?

    In addition, I am a bit confused with the differences between the formation of Granitic Magma and Andesitic Magma ? I thought both of them are involved in subduction zone, but is it there will be different in the magma formed between continental + oceanic plate & oceanic + oceanic plate & Continental + continental plate?



    Thanks
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  5. #4  
    ilg
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    also, i dont understand why alkaline rocks does not contain hypersthene ?!

    hypersthene needs mg and fe, doesn't it? alkaline magmas are resulting out of very little melting up ratios so I would guess mg and fe stay in the restite



    I am sorry but I don't quite understand this. Alkaline magmas are resulting out of very little melting up ?
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    Quote Originally Posted by ilg View Post

    also, i dont understand why alkaline rocks does not contain hypersthene ?!

    hypersthene needs mg and fe, doesn't it? alkaline magmas are resulting out of very little melting up ratios so I would guess mg and fe stay in the restite

    I am sorry but I don't quite understand this. Alkaline magmas are resulting out of very little melting up ?
    yes
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    Quote Originally Posted by ilg View Post
    thanks...
    is it opposite things occur in ophitic texture and intergranular texture ? ( in ophitic , plagioclase is surrounded by other minerals that forms before plagioclase ; while in intergranular, plgioclase forms first and other minerals grow in between the spaces ?) Also, what makes the differences between ophitic and poikilitic texture?
    I didn't find a adequat translation I just can guess that it might be something like the one is for SiO2 rich (like pegmatites) stones the other for SiO2 poor.

    In addition, I am a bit confused with the differences between the formation of Granitic Magma and Andesitic Magma ? I thought both of them are involved in subduction zone, but is it there will be different in the magma formed between continental + oceanic plate & oceanic + oceanic plate & Continental + continental plate?
    Keep the bowen series in mind and look what's melting up. Normally an oceanic plate gets subducted the upper mantle is melting up so we've got in the very beginning a basaltic magma, it gets assimilated differentiated... other parts of the crust are melting up, so there can be also existing both magmas. Andesitic magmas seem only to exist out of magma mixing (this is called Daly Gap). If you've got just continental crust melting up you just can get granitic magmas. Perhaps you also should take a look at things like I-Type, M-Type, A-Type, S-Type Granites
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    ilg
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    [QUOTE=The Geographer;322453][QUOTE=ilg;322425]

    also, i dont understand why alkaline rocks does not contain hypersthene ?!

    hypersthene needs mg and fe, doesn't it? alkaline magmas are resulting out of very little melting up ratios so I would guess mg and fe stay in the restite
    can u explain more on the formation of alkaline magmas? Because I haven't leant about this ->alkaline magmas are resulting out of very little melting up ratios


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  9. #8  
    ilg
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    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by ilg View Post
    thanks...
    is it opposite things occur in ophitic texture and intergranular texture ? ( in ophitic , plagioclase is surrounded by other minerals that forms before plagioclase ; while in intergranular, plgioclase forms first and other minerals grow in between the spaces ?) Also, what makes the differences between ophitic and poikilitic texture?
    I didn't find a adequat translation I just can guess that it might be something like the one is for SiO2 rich (like pegmatites) stones the other for SiO2 poor.

    In addition, I am a bit confused with the differences between the formation of Granitic Magma and Andesitic Magma ? I thought both of them are involved in subduction zone, but is it there will be different in the magma formed between continental + oceanic plate & oceanic + oceanic plate & Continental + continental plate?
    Keep the bowen series in mind and look what's melting up. Normally an oceanic plate gets subducted the upper mantle is melting up so we've got in the very beginning a basaltic magma, it gets assimilated differentiated... other parts of the crust are melting up, so there can be also existing both magmas. Andesitic magmas seem only to exist out of magma mixing (this is called Daly Gap). If you've got just continental crust melting up you just can get granitic magmas. Perhaps you also should take a look at things like I-Type, M-Type, A-Type, S-Type Granites
    andesite magma comes from magma mixing ? em... but it seems andesite magma does comes from subduction zone (oceanic + oceanic plate) [ from a diagram of my notes]
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    Quote Originally Posted by ilg View Post
    Quote Originally Posted by The Geographer View Post
    Quote Originally Posted by ilg View Post
    thanks...
    is it opposite things occur in ophitic texture and intergranular texture ? ( in ophitic , plagioclase is surrounded by other minerals that forms before plagioclase ; while in intergranular, plgioclase forms first and other minerals grow in between the spaces ?) Also, what makes the differences between ophitic and poikilitic texture?
    I didn't find a adequat translation I just can guess that it might be something like the one is for SiO2 rich (like pegmatites) stones the other for SiO2 poor.

    In addition, I am a bit confused with the differences between the formation of Granitic Magma and Andesitic Magma ? I thought both of them are involved in subduction zone, but is it there will be different in the magma formed between continental + oceanic plate & oceanic + oceanic plate & Continental + continental plate?
    Keep the bowen series in mind and look what's melting up. Normally an oceanic plate gets subducted the upper mantle is melting up so we've got in the very beginning a basaltic magma, it gets assimilated differentiated... other parts of the crust are melting up, so there can be also existing both magmas. Andesitic magmas seem only to exist out of magma mixing (this is called Daly Gap). If you've got just continental crust melting up you just can get granitic magmas. Perhaps you also should take a look at things like I-Type, M-Type, A-Type, S-Type Granites
    andesite magma comes from magma mixing ? em... but it seems andesite magma does comes from subduction zone (oceanic + oceanic plate) [ from a diagram of my notes]
    Ok, ocean-ocean collision is a special case, normally you've got andesits at continental-ocean collision (the name andesit comes from the andes). I guess there are not so many andesits at most island-arcs. Those andesits can be created when parts of the accretionary wedge or subducted oceanic islands are melting up and maybe sometimes even by differentiation of mantle magmas.
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    Quote Originally Posted by ilg View Post

    also, i dont understand why alkaline rocks does not contain hypersthene ?!

    hypersthene needs mg and fe, doesn't it? alkaline magmas are resulting out of very little melting up ratios so I would guess mg and fe stay in the restite
    can u explain more on the formation of alkaline magmas? Because I haven't leant about this ->alkaline magmas are resulting out of very little melting up ratios


    I would guess 3-5% of melting up the mantle with little fluid (you know the complete series can be seen in the silica vs alkali diagram), most times they are located within a plate and not at plate boundaries so these magmas are typical for the so called hot spots, wet spots, plumes...
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