Abstract Details
(2020) The 2 Ga Eclogites of Central Tanzania: Directly Linking Age and Metamorphism
Tamblyn R, Brown D, Hand M, Morrissey L & Anczkiewicz R
https://doi.org/10.46427/gold2020.2541
The author has not provided any additional details.
04h: Room 5, Thursday 25th June 22:21 - 22:24
Renee Tamblyn
View all 2 abstracts at Goldschmidt2020
View abstracts at 2 conferences in series
Dillon Brown
Martin Hand View all 4 abstracts at Goldschmidt2020
Laura Morrissey View all 2 abstracts at Goldschmidt2020 View abstracts at 3 conferences in series
Robert Anczkiewicz View abstracts at 11 conferences in series
Dillon Brown
Martin Hand View all 4 abstracts at Goldschmidt2020
Laura Morrissey View all 2 abstracts at Goldschmidt2020 View abstracts at 3 conferences in series
Robert Anczkiewicz View abstracts at 11 conferences in series
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 Taehwan Kim on Tuesday 16th June 07:36
Thank you for your guidance to ancient eclogite! I'd like to give you several questions below. 1) Is it safe to say your "warm subduction zone gradients consistent with modern-style subduction (in Conclusions)"? Cold and deep nature are essential for the modern-style one reflected by blueschist and UHP rocks, respectively, as far as I know. 2) Both of your pseudosections have "L" as equilibrium phase at the P-T conditions of interest. If "L" stands for melt, do you see any field and/or petrographic evidence (including Opx you mentioned) for partial melting in your eclogite samples? Any comments on the bulk-rock XFe3+ (ferric) and H2O assumption in calculating pseudosection? 3) I'd like to know the protolith information, if you have, such as tectonic setting. MORB, rift basalt, etc. Thanks a lot!
Thank you for your questions and comments Taehwan. 1) I guess it depends how you define moderns style subduction, and where you draw the line between ultra-cold geotherms during deep subduction (which create blueschists and UHP rocks) or moderate ones that can create warm eclogite-facies rocks (like in Tanzania). I would suggest that modern-style subduction incorporates all of these styles, because warm eclogite-facies rocks still form in Earths recent history, as well as blueschists or UHP rocks created by cold, deep subduction. 2) Yes, L represents melt. There is no obvious petrological evidence that the eclogites experienced partial melting. Modal proportions of melt in these fields are low (<3%), so it seems unlikely that they would preserve petrological evidence. There are some quartz-feldspar veins which cross cut one of the samples, but it is unclear if this is a result of melting or simply a fluid. Bulk rock XFe ratios were constrained from P-X diagrams. H2O was calculated from the modal abundance of amphibole (the only hydrous mineral in the rock), however we also calculated some diagrams with water in excess to investigate the prograde path of the eclogites. 3) Moller et al. (1995) suggested a MORB-like affinity for the eclogites. I hope this answers your questions :) Ref: Mo?ller, A., Appel, P., Mezger, K., & Schenk, V. (1995). Evidence for a 2 Ga subduction zone: eclogites in the Usagaran belt of Tanzania. Geology, 23(12), 1067-1070.
Thank you for your guidance to ancient eclogite! I'd like to give you several questions below. 1) Is it safe to say your "warm subduction zone gradients consistent with modern-style subduction (in Conclusions)"? Cold and deep nature are essential for the modern-style one reflected by blueschist and UHP rocks, respectively, as far as I know. 2) Both of your pseudosections have "L" as equilibrium phase at the P-T conditions of interest. If "L" stands for melt, do you see any field and/or petrographic evidence (including Opx you mentioned) for partial melting in your eclogite samples? Any comments on the bulk-rock XFe3+ (ferric) and H2O assumption in calculating pseudosection? 3) I'd like to know the protolith information, if you have, such as tectonic setting. MORB, rift basalt, etc. Thanks a lot!
Thank you for your questions and comments Taehwan. 1) I guess it depends how you define moderns style subduction, and where you draw the line between ultra-cold geotherms during deep subduction (which create blueschists and UHP rocks) or moderate ones that can create warm eclogite-facies rocks (like in Tanzania). I would suggest that modern-style subduction incorporates all of these styles, because warm eclogite-facies rocks still form in Earths recent history, as well as blueschists or UHP rocks created by cold, deep subduction. 2) Yes, L represents melt. There is no obvious petrological evidence that the eclogites experienced partial melting. Modal proportions of melt in these fields are low (<3%), so it seems unlikely that they would preserve petrological evidence. There are some quartz-feldspar veins which cross cut one of the samples, but it is unclear if this is a result of melting or simply a fluid. Bulk rock XFe ratios were constrained from P-X diagrams. H2O was calculated from the modal abundance of amphibole (the only hydrous mineral in the rock), however we also calculated some diagrams with water in excess to investigate the prograde path of the eclogites. 3) Moller et al. (1995) suggested a MORB-like affinity for the eclogites. I hope this answers your questions :) Ref: Mo?ller, A., Appel, P., Mezger, K., & Schenk, V. (1995). Evidence for a 2 Ga subduction zone: eclogites in the Usagaran belt of Tanzania. Geology, 23(12), 1067-1070.
Submitted by Christopher Lambert on Wednesday 17th June 16:38
Nice talk Renee
Thank you Christopher :)
Nice talk Renee
Thank you Christopher :)
Submitted by Shah Wali Faryad on Tuesday 23rd June 15:16
The 2 Ga Eclogites of Central Tanzania: Directly Linking Age and Metamorphism Tamblyn R, Brown D, Hand M, Morrissey L & Anczkiewicz R I like your conclusion that the Paleoarchean subduction had similar or almost style as the modern. Your rutile in zircon T range from 600-800 oC. You also assume that the calculated P are minimum. You have also show that eclogite are strongly reequilibrated at low-pressure conditions. Why you assume that it belongs to HT eclogite?
Thanks for your question. We interpret that the Zr in rutile temperatures reflect eclogite facies metamorphism, as rutile only occurs as inclusions inside prograde-peak garnet (and rutile is stable above 12 kbar on the phase diagrams). As such, we think the range of Zr in rutile reflects prograde to peak conditions on the rock's way to eclogite facies. The Ti in zircon is less known, as zircon commonly occurs in garnet and less commonly in the matrix. I am happy to discuss this more in the question time :)
The 2 Ga Eclogites of Central Tanzania: Directly Linking Age and Metamorphism Tamblyn R, Brown D, Hand M, Morrissey L & Anczkiewicz R I like your conclusion that the Paleoarchean subduction had similar or almost style as the modern. Your rutile in zircon T range from 600-800 oC. You also assume that the calculated P are minimum. You have also show that eclogite are strongly reequilibrated at low-pressure conditions. Why you assume that it belongs to HT eclogite?
Thanks for your question. We interpret that the Zr in rutile temperatures reflect eclogite facies metamorphism, as rutile only occurs as inclusions inside prograde-peak garnet (and rutile is stable above 12 kbar on the phase diagrams). As such, we think the range of Zr in rutile reflects prograde to peak conditions on the rock's way to eclogite facies. The Ti in zircon is less known, as zircon commonly occurs in garnet and less commonly in the matrix. I am happy to discuss this more in the question time :)
Submitted by Ziyi Zhu on Thursday 25th June 09:07
Great presentation! I really enjoyed it. May I ask two questions: (1) Apart from subduction, are there any other tectonic settings to form eclogites? (2) You mentioned that zircon (and some other minerals) are inclusions in garnet, suggesting they were in equilibrium during growth. Could you please explain a bit more why this shows the equilibrium? And are there any other ways indicating zircon growing in equilibrium with garnet? Thanks!
Thank you Ziyi :) (1) There are certainly other tectonic setting that form eclogites, such as collisional orogens and also the base of crustal roots. The key is that subduction-related eclogites record colder thermal gradients than these, as the downgoing oceanic crust is cold and 'drags' geotherms down into the mantle, creating low-temperature high-pressure conditions. However, the exact thermal gradient that decides if an eclogite is subduction-driven or not is debated, and you often need more information from the surrounding geology to decide the tectonic setting. (2) Zircon inclusions in garnet could suggest that zircon and garnet were both growing at the same time, allowing the garnet to entrap the zircon as it grew, or, it could mean that the zircon formed first and then the garnet overgrew it. A good way to tell is to look at the REE composition of the zircons. If the zircons grew with garnet, then they will have a flat HREE profile (like some of the ones in the Tanzanian eclogites do!). I hope this answers your questions. Feel free to email me if it doesn't (renee.tamblyn@adelaide.edu.au).
Great presentation! I really enjoyed it. May I ask two questions: (1) Apart from subduction, are there any other tectonic settings to form eclogites? (2) You mentioned that zircon (and some other minerals) are inclusions in garnet, suggesting they were in equilibrium during growth. Could you please explain a bit more why this shows the equilibrium? And are there any other ways indicating zircon growing in equilibrium with garnet? Thanks!
Thank you Ziyi :) (1) There are certainly other tectonic setting that form eclogites, such as collisional orogens and also the base of crustal roots. The key is that subduction-related eclogites record colder thermal gradients than these, as the downgoing oceanic crust is cold and 'drags' geotherms down into the mantle, creating low-temperature high-pressure conditions. However, the exact thermal gradient that decides if an eclogite is subduction-driven or not is debated, and you often need more information from the surrounding geology to decide the tectonic setting. (2) Zircon inclusions in garnet could suggest that zircon and garnet were both growing at the same time, allowing the garnet to entrap the zircon as it grew, or, it could mean that the zircon formed first and then the garnet overgrew it. A good way to tell is to look at the REE composition of the zircons. If the zircons grew with garnet, then they will have a flat HREE profile (like some of the ones in the Tanzanian eclogites do!). I hope this answers your questions. Feel free to email me if it doesn't (renee.tamblyn@adelaide.edu.au).
Submitted by Andreas Moeller on Friday 26th June 00:10
Sorry I missed the session. Very nice presentation video. Some comments on the question about melting etc: The associated metapelites of Yalumba Hill show evidence of melting, they are migmatitic, and garnet rims contain multiphase melt inclusions (Herms et al. ECROFI abstract 2015; Herms et al. Geobonn abstract 2018). Large garnets in both mafic and metapelitic eclogite-facies rocks from Yalumba show evidence for prograde garnet growth and isothermal decompression (Möller et al., Geology, 1995) at ca. 750C. That is prossible when metling occurred. We did not find zircon in those melt pockets in the metapelites, but that would have been ideal. However, we did date the titanite overgrowing rutile as result of decompression and the 7-6 age of these titanites is 1996±2 Ma, a lower age limit on the age of the HP conditions. Monazite from two metapelites have 7/6 ages of 1999±2 and 2000±2. Since Tc of monazite is higher than the max T, we interpreted these as prograde or max. T conditions. In-situ analysis of rutile inclusions and matrix grains in mafic and metapelitic samples (Möller et al., AGU abstract, 2014) gave slightly higher peakT (810ºC) and partially reset U-Pb results, even for inclusions in garnet that looked totally shielded.
Thank you for the comments and the information, Andreas. The encasing metapelites are certainly migmatitic, there is a paper about to be published by Brown et al. in Precambrian Research titled "Thermobarometric constraints on burial and exhumation of 2-billion-year-old eclogites and their metapelitic hosts", which also investigates the metapelites in detail. Interestingly, the U-Pb system in the rutiles that we dated from garnet inclusions in the mafic eclogites was also disturbed.. it is interesting that Pb must be lost to the surrounding garnet or perhaps the grain boundary (?). However it seems that Zr concentrations in those same rutiles were not disturbed, as Zr concentration shows no correlation to discordance.
Sorry I missed the session. Very nice presentation video. Some comments on the question about melting etc: The associated metapelites of Yalumba Hill show evidence of melting, they are migmatitic, and garnet rims contain multiphase melt inclusions (Herms et al. ECROFI abstract 2015; Herms et al. Geobonn abstract 2018). Large garnets in both mafic and metapelitic eclogite-facies rocks from Yalumba show evidence for prograde garnet growth and isothermal decompression (Möller et al., Geology, 1995) at ca. 750C. That is prossible when metling occurred. We did not find zircon in those melt pockets in the metapelites, but that would have been ideal. However, we did date the titanite overgrowing rutile as result of decompression and the 7-6 age of these titanites is 1996±2 Ma, a lower age limit on the age of the HP conditions. Monazite from two metapelites have 7/6 ages of 1999±2 and 2000±2. Since Tc of monazite is higher than the max T, we interpreted these as prograde or max. T conditions. In-situ analysis of rutile inclusions and matrix grains in mafic and metapelitic samples (Möller et al., AGU abstract, 2014) gave slightly higher peakT (810ºC) and partially reset U-Pb results, even for inclusions in garnet that looked totally shielded.
Thank you for the comments and the information, Andreas. The encasing metapelites are certainly migmatitic, there is a paper about to be published by Brown et al. in Precambrian Research titled "Thermobarometric constraints on burial and exhumation of 2-billion-year-old eclogites and their metapelitic hosts", which also investigates the metapelites in detail. Interestingly, the U-Pb system in the rutiles that we dated from garnet inclusions in the mafic eclogites was also disturbed.. it is interesting that Pb must be lost to the surrounding garnet or perhaps the grain boundary (?). However it seems that Zr concentrations in those same rutiles were not disturbed, as Zr concentration shows no correlation to discordance.
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