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Thread: Origin of rapakivi granite

  1. #1 Origin of rapakivi granite 
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    Hello.
    I need help with the origin of rapakivi.
    How was formed rounded shape of orthoclase?
    what was chemical condition of this origin?
    Thanks.


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  3. #2  
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    Quote Originally Posted by petitmx View Post
    Hello.
    I need help with the origin of rapakivi.
    How was formed rounded shape of orthoclase?
    what was chemical condition of this origin?
    Thanks.
    I read on Wiki that these granites may have been formed by fractional crystallisation of a magma body, but, like you, I don't quite see why this would explain the rounded shapes. But there seem to be a few articles on the subject on the web - why not try looking at those? .....and then you can explain it to me.....


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    Looks like they may have commonly formed in volcanic sills. Their shape, meaning the shape of ther crystals, form due to the crystalline makeup of the minerals within the rock.
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    Quote Originally Posted by iDevonian View Post
    Looks like they may have commonly formed in volcanic sills. Their shape, meaning the shape of ther crystals, form due to the crystalline makeup of the minerals within the rock.
    Sure, but why the rounded crystals, specifically? Seems to me one gets rounded crystals when a crystal partially redissolves, as the vertices are slightly less stable than the rest of the crystal and so tend to melt first. I suppose if these magma bodies underwent some gentle heating and cooling cycles, around the crystallisation temperature, one might get rounded crystals. But I'm only speculating.....
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    The rapakivi granites have complex thermal and pressure histories. One of the pointers to this is related to their definition: rocks containing "ovoid megacrysts of alkali-feldspar, which are surrounded, although not always, by oligoclase-andesine rims."1 Further evidence lies in the apparent different P-T regimes for phenocryst and groundmass2. So, a provisional conclusion, based on a very cursory examination of the literature, confirms that this is simply a mineralogical adjustment to different equilibrium conditions.

    Here is the abstract of the second reference:

    The rapakivi texture (plagioclase-mantled K-feldspar ovoids, rounded quartz megacrysts and euhedral plagioclase megacrysts in a more fine-grained granitic matrix) has been studied in five Proterozoic (1.641.55 Ga) anorogenic rapakivi granite batholiths in the Fennoscandian Shield with emphasis on mineral stability and inherited cores. K-feldspar ovoids and rounded quartz megacrysts with deep embayments consist principally of a core zone and an outer melted and recrystallized zone. When the K-feldspar ovoids are mantled by plagioclase, the outermost part of the mantle is homogeneous, but towards the K-feldspar, a skeletal texture develops. Intensive parameters obtained from different textural positions show that mineral inclusions in cores of the K-feldspar ovoids and quartz megacrysts were formed at low T (∼680720C) and high P (56 kbar) conditions, while inclusions in the periphery of ovoids and the plagioclase mantles display high T (∼780C) and intermediate to low P (3.51 kbar). The lowest P is comparable to that during solidification of the matrix. The total water equivalent of the volatile content in the magma has been calculated as ∼2.5%. The amount of solids in the magma at ∼1 kbar has been estimated as ∼40%. Theoretical calculations and experimental data for mineral stabilities in granitic systems suggest that the texture formed when a crystal-saturated (Kfsp+Qtz+Pl ∼60%) and volatile-undersaturated A-type granite magma was transported under approximately constant temperature (760780C) from the lower-middle crust (56 kbar) to upper crustal levels. According to phase stabilities in the eutectoid granite system, quartz and K-feldspar were resorbed but plagioclase remained stable and precipitated during this sub-isothermal rise of magma. Textural (presence of disequilibrium textures) and mineralogical (presence of different mineral assemblages, including relics) evidence of a sub-isothermal rise of the rapakivi magmas is better preserved in subvolcanic and contact varieties of rapakivi granites than in the more deep-seated rapakivi granites formed by slow cooling.


    1. Larin, A.M. Rapakivi granites in the geological history of the earth. Part 1, magmatic associations with rapakivi granites: Age, geochemistry, and tectonic setting. Stratigraphy and Geological Correlation 2009 Vol 17 Issue 3 Paper available here
    2. Eklund, O. & Shebanov, A. D. 1999. The origin of rapakivi texture by sub-isothermal decompression. Precambrian Research 1999 95
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  7. #6  
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    Quote Originally Posted by John Galt View Post
    The rapakivi granites have complex thermal and pressure histories. One of the pointers to this is related to their definition: rocks containing "ovoid megacrysts of alkali-feldspar, which are surrounded, although not always, by oligoclase-andesine rims."1 Further evidence lies in the apparent different P-T regimes for phenocryst and groundmass2. So, a provisional conclusion, based on a very cursory examination of the literature, confirms that this is simply a mineralogical adjustment to different equilibrium conditions.

    Here is the abstract of the second reference:

    The rapakivi texture (plagioclase-mantled K-feldspar ovoids, rounded quartz megacrysts and euhedral plagioclase megacrysts in a more fine-grained granitic matrix) has been studied in five Proterozoic (1.641.55 Ga) anorogenic rapakivi granite batholiths in the Fennoscandian Shield with emphasis on mineral stability and inherited cores. K-feldspar ovoids and rounded quartz megacrysts with deep embayments consist principally of a core zone and an outer melted and recrystallized zone. When the K-feldspar ovoids are mantled by plagioclase, the outermost part of the mantle is homogeneous, but towards the K-feldspar, a skeletal texture develops. Intensive parameters obtained from different textural positions show that mineral inclusions in cores of the K-feldspar ovoids and quartz megacrysts were formed at low T (∼680720C) and high P (56 kbar) conditions, while inclusions in the periphery of ovoids and the plagioclase mantles display high T (∼780C) and intermediate to low P (3.51 kbar). The lowest P is comparable to that during solidification of the matrix. The total water equivalent of the volatile content in the magma has been calculated as ∼2.5%. The amount of solids in the magma at ∼1 kbar has been estimated as ∼40%. Theoretical calculations and experimental data for mineral stabilities in granitic systems suggest that the texture formed when a crystal-saturated (Kfsp+Qtz+Pl ∼60%) and volatile-undersaturated A-type granite magma was transported under approximately constant temperature (760780C) from the lower-middle crust (56 kbar) to upper crustal levels. According to phase stabilities in the eutectoid granite system, quartz and K-feldspar were resorbed but plagioclase remained stable and precipitated during this sub-isothermal rise of magma. Textural (presence of disequilibrium textures) and mineralogical (presence of different mineral assemblages, including relics) evidence of a sub-isothermal rise of the rapakivi magmas is better preserved in subvolcanic and contact varieties of rapakivi granites than in the more deep-seated rapakivi granites formed by slow cooling.


    1. Larin, A.M. Rapakivi granites in the geological history of the earth. Part 1, magmatic associations with rapakivi granites: Age, geochemistry, and tectonic setting. Stratigraphy and Geological Correlation 2009 Vol 17 Issue 3 Paper available here
    2. Eklund, O. & Shebanov, A. D. 1999. The origin of rapakivi texture by sub-isothermal decompression. Precambrian Research 1999 95
    Ha! Thanks, that's very interesting. So I was half right and half wrong: the rounded shapes are indeed due to partial remelting and refreezing, but more likely due to variations in pressure than to those of temperature. I should have considered this possibility.
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    Be carefull about assuming melting. Some of these changes may well have been sub-solidus. H.H.Read's gordian knot observation "there are granites and there are granites" remains true today, more than half a century later.
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    Quote Originally Posted by John Galt View Post
    Be carefull about assuming melting. Some of these changes may well have been sub-solidus. H.H.Read's gordian knot observation "there are granites and there are granites" remains true today, more than half a century later.
    Yes indeed, good point. What I had in mind is that the vertices of crystals have slightly lower stability than the body of the crystal and will be the first to remelt - or, I suppose, to diffuse slowly away even in a solid close to, but below, its bulk MP. I take the broad point that one has to envisage processes that can occur over geological timescales, even though they may be negligibly slow under lab observation conditions.
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