Abstract Details
(2020) Cordilleran Granitoids and Restite Entrainment: A Thermodynamic Modelling
García-Arias M, Puerto-León JS & Ronderos-Almeida CD
https://doi.org/10.46427/gold2020.797
The author has not provided any additional details.
04j: Room 1, Saturday 27th June 00:36 - 00:39
Marcos García-Arias
View abstracts at 4 conferences in series
Juan Sebastián Puerto-León
Carlos Daniel Ronderos-Almeida
Juan Sebastián Puerto-León
Carlos Daniel Ronderos-Almeida
Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.
Submitted by Changqian Ma on Saturday 20th June 09:15
Thank you for showing a very important study! What is the difference between the Peritectic assemblage entrainment of I-type and S-type granites?
Dear Ma, First of all, thanks for your question. The difference in the peritectic assemblage between I- and S-type granites is the nature of the mineral that are produced in the melting reaction. Metasedimentary rocks are strongly peraluminous (they contain muscovite and Al-rich biotite), but the melt cannot take up much of this aluminium, so the peritectic minerals become the sink for this element: cordierite, garnet, Al-silicate, etc. Only during biotite breakdown, once Al-silicate has been consumed, less-aluminous phases can form: orthopyroxene. Extra K not taken by the melt produces K-feldspar and extra Ti forms ilmenite. In the case of I-type granites, the source rocks (whatever they are) are metaluminous or, as much, slightly peraluminous (they contain Al-poor biotite and amphibole, or even pyroxenes). The melting of such rocks makes the melt the main sink of Al and thus the peritectic minerals are Al-poor: orthopyroxene and clinopyroxene. This is the reason why the entrainment of large amounts of peritectic phases in the melting of metasedimentary rocks produce horizontal to positive slopes in A/CNK vs maficity diagrams, but negtaive to strongly negative slopes in the melting of metaluminous sources. Kind regards, Marcos
Thank you for showing a very important study! What is the difference between the Peritectic assemblage entrainment of I-type and S-type granites?
Dear Ma, First of all, thanks for your question. The difference in the peritectic assemblage between I- and S-type granites is the nature of the mineral that are produced in the melting reaction. Metasedimentary rocks are strongly peraluminous (they contain muscovite and Al-rich biotite), but the melt cannot take up much of this aluminium, so the peritectic minerals become the sink for this element: cordierite, garnet, Al-silicate, etc. Only during biotite breakdown, once Al-silicate has been consumed, less-aluminous phases can form: orthopyroxene. Extra K not taken by the melt produces K-feldspar and extra Ti forms ilmenite. In the case of I-type granites, the source rocks (whatever they are) are metaluminous or, as much, slightly peraluminous (they contain Al-poor biotite and amphibole, or even pyroxenes). The melting of such rocks makes the melt the main sink of Al and thus the peritectic minerals are Al-poor: orthopyroxene and clinopyroxene. This is the reason why the entrainment of large amounts of peritectic phases in the melting of metasedimentary rocks produce horizontal to positive slopes in A/CNK vs maficity diagrams, but negtaive to strongly negative slopes in the melting of metaluminous sources. Kind regards, Marcos
Submitted by Félix Gervais on Friday 26th June 15:21
Hi. Could you provide more info about your modeling. You write "Holland and Powell database" for modeling, but what did you use exactly? The older Tds55 database and all the suggested TC solution models; the 2014 tds62 database and models or the HGP solution models?
Hi. Could you provide more info about your modeling. You write "Holland and Powell database" for modeling, but what did you use exactly? The older Tds55 database and all the suggested TC solution models; the 2014 tds62 database and models or the HGP solution models?
Submitted by Félix Gervais on Friday 26th June 16:28
Hi. Could you provide more info about your modeling. You write "Holland and Powell database" for modeling, but what did you use exactly? The older Tds55 database and all the suggested TC solution models; the 2014 tds62 database and models or the HGP solution models?
Dear Dr. Gervais. My undergraduate students (Juan Sebastián Puerto and Carlos Daniel Ronderos) have used the following thermodynamic databases and solution models: * Thermodynamic database: hp633ver.dat, based on the tc633 database of Holland and Powell, 2011, updated in 2018. * Solution models: melt, biotite, garnet, orthopyroxene, clinopyroxene, olivine and spinel from Holland et al. (2018), white mica from White et al. (2014), ternary feldspar from Fuhrman and Lindsley (1988), clinoamphibole from Green et al. (2016), ilmenite from White et al. (2000) modified by White et al. (2014) and epidote from Holland and Powell (2011).
Hi. Could you provide more info about your modeling. You write "Holland and Powell database" for modeling, but what did you use exactly? The older Tds55 database and all the suggested TC solution models; the 2014 tds62 database and models or the HGP solution models?
Dear Dr. Gervais. My undergraduate students (Juan Sebastián Puerto and Carlos Daniel Ronderos) have used the following thermodynamic databases and solution models: * Thermodynamic database: hp633ver.dat, based on the tc633 database of Holland and Powell, 2011, updated in 2018. * Solution models: melt, biotite, garnet, orthopyroxene, clinopyroxene, olivine and spinel from Holland et al. (2018), white mica from White et al. (2014), ternary feldspar from Fuhrman and Lindsley (1988), clinoamphibole from Green et al. (2016), ilmenite from White et al. (2000) modified by White et al. (2014) and epidote from Holland and Powell (2011).
Submitted by Ke Gao on Friday 26th June 21:35
In No. 12 page of your keynote, you mentioned that the amount of granites decreases with increasing maficity (i.e. entrained minerals) because the magmas become denser and more viscous, but I think felsic rocks are more viscous in general? How do you deal with the sampling bias in your dataset? Thank you!
Dear Gao, Thanks for your question. Felsic melts are viscous, but imagine that you add minerals to an already viscous liquid (i.e. you form a magma): the viscosity increases, making the more mafic magmas more viscous and thus less mobile. We tend to relate higher maficity to low viscosity, but that only applies to melts. For a constant melt composition and temperature, viscosity increases when minerals are present in the melt (when they become magmas). Kind regards,
In No. 12 page of your keynote, you mentioned that the amount of granites decreases with increasing maficity (i.e. entrained minerals) because the magmas become denser and more viscous, but I think felsic rocks are more viscous in general? How do you deal with the sampling bias in your dataset? Thank you!
Dear Gao, Thanks for your question. Felsic melts are viscous, but imagine that you add minerals to an already viscous liquid (i.e. you form a magma): the viscosity increases, making the more mafic magmas more viscous and thus less mobile. We tend to relate higher maficity to low viscosity, but that only applies to melts. For a constant melt composition and temperature, viscosity increases when minerals are present in the melt (when they become magmas). Kind regards,
Submitted by Calvin Miller on Friday 26th June 21:35
Very interesting work! A couple of questions: (1) If I understand correctly, you are saying that, in the MASH model, only fractional crystallization can produce compositional variability. Is that what you meant? (2) In your opinion, how common are S-type granites in the American (N&S) Cordillera?
Dear Prof. Miller, 1) What I say in my presentation is that, in the classic model of generation of magmas in continental arcs, fractional crystallization seems to be the main process that can change the composition of the mantellic melts to intermediate ones to overcome the density barrier of the continental crust. Some assimilation of the lower crust can also happen, but restite entrainment does not. As far as I know, and I am learning about continental arcs bit a bit, restite entrainment was not proposed to play a role in the compositional variation of cordilleran granitoids. This is what I mean. However, in the current model of magmatism, in which diapirs of intermediate composition relaminate the base of the lower continental crust, restite entrainment may also contribute to the compositional variability. It does not rule out fractional crystallization or other processes. What I mean is that, in this new magmatic scenario, restite entrainment is another player in the game, an unrecognized player until now. 2) I do not know how frequent S-type granites can be in continental arcs, but I guess they are not very common. The first magma pulses coming from the mantle may melt rocks during their ascent or emplacement, and if these rocks happen to be metasedimentary, some s-type granites may form. However, once the first Cordilleran magmas emplace, the next ones encounter the previous cordilleran granitoids, so these will melt and produce metaluminous magmas to slightly peraluminous magmas. I am currently in Colombia and I was to start a field campaign to take samples of the Ibagué and Antioquia batholiths, but the coronavirus emergency halted those plans. I will ask my colleagues here if they know about S-type granites in the magmatic arc, but Colombian geology is not well known and understood. There are lots of things to do here. Kind regards, Marcos
Very interesting work! A couple of questions: (1) If I understand correctly, you are saying that, in the MASH model, only fractional crystallization can produce compositional variability. Is that what you meant? (2) In your opinion, how common are S-type granites in the American (N&S) Cordillera?
Dear Prof. Miller, 1) What I say in my presentation is that, in the classic model of generation of magmas in continental arcs, fractional crystallization seems to be the main process that can change the composition of the mantellic melts to intermediate ones to overcome the density barrier of the continental crust. Some assimilation of the lower crust can also happen, but restite entrainment does not. As far as I know, and I am learning about continental arcs bit a bit, restite entrainment was not proposed to play a role in the compositional variation of cordilleran granitoids. This is what I mean. However, in the current model of magmatism, in which diapirs of intermediate composition relaminate the base of the lower continental crust, restite entrainment may also contribute to the compositional variability. It does not rule out fractional crystallization or other processes. What I mean is that, in this new magmatic scenario, restite entrainment is another player in the game, an unrecognized player until now. 2) I do not know how frequent S-type granites can be in continental arcs, but I guess they are not very common. The first magma pulses coming from the mantle may melt rocks during their ascent or emplacement, and if these rocks happen to be metasedimentary, some s-type granites may form. However, once the first Cordilleran magmas emplace, the next ones encounter the previous cordilleran granitoids, so these will melt and produce metaluminous magmas to slightly peraluminous magmas. I am currently in Colombia and I was to start a field campaign to take samples of the Ibagué and Antioquia batholiths, but the coronavirus emergency halted those plans. I will ask my colleagues here if they know about S-type granites in the magmatic arc, but Colombian geology is not well known and understood. There are lots of things to do here. Kind regards, Marcos
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