In the Monte Duria area (Adula-Cima Lunga unit, Central Alps, N Italy) garnet peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both crustal and ultramafic rocks share a common high pressure (HP) peak at 2.8 GPa and 750 °C and post-peak static equilibration at 0.8-1.0 GPa and 850 °C. Garnet peridotites show abundant amphibole, dolomite, phlogopite and orthopyroxene after olivine, suggesting that they experienced metasomatism by crust-derived agents enriched in SiO2, K2O, CO2 and H2O. Peridotites also display LREE fractionation (La/Nd = 2.4) related to LREE-rich amphibole and clinopyroxene grown in equilibrium with garnet, indicating that metasomatism occurred at HP conditions. Kfs+Pl+Qz+Cpx interstitial pocket aggregates and Cpx+Kfs thin films around symplectites after omphacite parallel to the Zo+Omp+Grt foliation in the eclogites suggest that they underwent partial melting at HP.The contact between garnet peridotites and associated eclogites is marked by a tremolitite layer, which also occurs as layers within the peridotite lens, showing a boudinage parallel to the garnet layering of peridotites, flowing in the boudin necks. This clearly indicates that the tremolitite boudins formed when peridotites were in the garnet stability field. Tremolitites also show Phl+Tc+Chl+Tr pseudomorphs after garnet, both crystallised in a static regime postdating the boudins formation, suggesting that they derive from a garnet-bearing precursor. Tremolitites have Mg# > 0.90 and Al2O3 = 2.75 wt.% pointing to ultramafic compositions but also show enrichments in SiO2, CaO, and LREE suggesting that they formed after the reaction between the eclogite-derived melt and the garnet peridotite at HP. To test this hypothesis, we performed a thermodynamic modelling at fixed P = 3 GPa and T = 750 °C to model the chemical interaction between the garnet peridotite and the eclogite-derived melt. Our results show that this interaction produces a Opx+Cpx+Grt assemblage + Amp+Phl, depending on the water activity in the melt, suggesting that tremolitites likely derive from a previous garnet websterite with amphibole and phlogopite. In the Ulten Zone (Tonale nappe, Eastern Alps, N Italy), peridotite bodies occur within high-grade crustal rocks. Peridotites show a transition from coarse spinel-lherzolites to mylonitic garnet-amphibole peridotites. Pyroxenites veins and dikes, transposed along the peridotite foliation, show a similar evolution from coarse garnet-free websterites to fine-grained garnet + amphibole clinopyroxenites. This coupled evolution has been interpreted to reflect cooling and pressure increase of pyroxenites and host peridotites from spinel- (1200 °C, 1.3-1.6 Gpa) to garnet-facies conditions (850 °C and 2.8 Gpa) likely induced by mantle corner flow. As a consequence, garnet formed coronas around spinel and exsolved from porphyroclastic, high-T pyroxenes, and finally crystallised along the pyroxenite and peridotite foliations. Textural evidences and CPO data indicate that the transition from spinel- to garnet-facies conditions was assisted by intense shearing and deformation. Pyroxene porphyroclasts in garnet clinopyroxenites show well-developed CPOs, high frequencies of low-angle misorientations, and non-random distribution of the low-angle misorientation axes, indicating that pyroxene porphyroclasts primarily deform by dislocation creep. Dislocation creep is accompanied by reaction-induced dynamic recrystallisation during the spinel to garnet phase transition, which promotes a sudden reduction of the grain size and a shift from dislocation creep in the porphyroclast to grain-size sensitive creep (GSS) in the recrystallised grains. This results in a dramatic rheological weakening of pyroxenites at HP peak conditions when pyroxenites and host peridotites were coupled with crustal rocks.

L’obiettivo di questo studio è quello di caratterizzare il trasferimento di massa tra crosta e mantello. A questo scopo sono stati considerati due terreni metamorfici di alta pressione (HP) dove peridotiti a granato affiorano all’interno di rocce crostali di alto grado, i.e. l’area del Monte Duria (falda Adula-Cima Lunga, Alpi centrali, N Italia) e la zona d’ Ultimo (falda del Tonale, Alpi orientali, N Italia). Nell’area del Monte Duria, peridotiti a granato affiorano in contatto diretto con eclogiti migmatitiche (Borgo). Sia le peridotiti che le eclogiti registrano condizioni di picco in HP a 2.8 GPa e 750 ° C e un riequilibratura statica a 1.0 GPa e 850 ° C. Le peridotiti mostrano abbondanti anfibolo, dolomite, flogopite e ortopirosseno (su olivina), suggerendo che le peridotiti registrano metasomatismo ad opera di agenti crostali arrichiti in SiO2, K2O, CO2 e H2O. Le peridotiti mostrano anche un frazionamento in LREE (La/Nd = 2.4) legato alla presenza di anfibolo e clinopirosseno. Questi minerali sono equilibrio con il granato, indicando che il metasomatismo è avvenuto in HP. Nelle eclogiti, microstrutture di fusione come aggregati microcristallini a Kfs+Pl+Qz+Cpx e Cpx+Kfs sono allineate lungo la foliazione a Zo+Omp+Grt, indicando che le eclogiti hanno subito un evento di fusione parziale in HP. Il contatto tra le peridotiti e le eclogiti di Borgo è marcato dalla presenza di un livello di tremolitite. Boudins di tremolititi si ritrovano anche trasposti lungo la foliazione a granato della peridotite, indicando che il boudinage delle tremolititi è avvento in alta pressione. Le tremolititi mostrano aggregati a Phl+Tc+Chl+Tr interpretati come psudomorfi su granato. Tali pseudomorfi si sviluppano in condizioni statiche post-datando la formazione dei boudins, suggerendo che le tremolititi derivano da precursori a granato. Le tremolititi mostrano Mg# > 0.90 e Al2O3 = 2.75 wt.% tipici di composizioni ultramafiche ma allo stesso tempo presentano arricchimenti in SiO2, CaO, e LREE, indicando che esse rappresentano il prodotto dell’interazione in alta pressione tra le peridotiti e i fusi derivati dalle eclogiti. Per testare questa ipotesi abbiamo sviluppato un modello termodinamico a P = 3 GPa e T = 750 °C. I nostri risultati indicano che l’interazione fuso-peridotite produce una paragenesi a Opx+Cpx+Grt, suggerendo che le tremolititi rappresentano il prodotto di retrocessione di una westerite a granato. Nella zona d’Ultimo, numerose lenti di peridotite affiorano all’interno di rocce crostali di alto grado. Le peridotiti mostrano una transizione da lherzoliti a spinello protogranulari a peridotiti milonitiche a granato e anfibolo. Le pirosseniti trasposte lungo la foliazione della peridotite mostrano un’evoluzione simile, da pirosseniti a spinello a pirosseniti a granato. Questa evoluzione riflette il passaggio indotto dal corner flow del mantello da condizioni in facies a spinello a a granato. Come consguenza, il granato forma corone intorno allo spinello ed essoluzioni all’interno dei porfiroclasti di pirosseno, e cristallizza lungo la foliazione delle pirosseniti e delle peridotiti Evidenze tessiturali e dati cristallografici indicano che la transizione spinello-granato avviene in un contesto deformativo. I porfiroclasti di pirosseno mostrano evidente CPO, alte frequenze delle misorientazioni a basso angolo, e distribuzione non-random degli assi di misorientazione per misorientazioni a basso angolo, indicando che i pirosseni si deformano per dislocation creep. Il dislocation creep è contemporaneo a processi di ricristallizzazione dinamica e alla transizione spinello-granato. Ciò induce una riduzione della grana e una transizione permanente da disclocation creep nei porfiroclasti a grain-size sensitive creep nei grani ricristallizzati che risulta in un forte indebolimento delle pirosseniti e delle peridotiti quando queste vengono tettonicamente accoppiate alle rocce crostali.

(2020). Modelling of mechanical mixing and chemical interaction between the subducting crust and the overlying mantle at (ultra)high pressures: implications for the slab-to-mantle mass transfer. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2020).

Modelling of mechanical mixing and chemical interaction between the subducting crust and the overlying mantle at (ultra)high pressures: implications for the slab-to-mantle mass transfer

PELLEGRINO, LUCA
2020

Abstract

In the Monte Duria area (Adula-Cima Lunga unit, Central Alps, N Italy) garnet peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both crustal and ultramafic rocks share a common high pressure (HP) peak at 2.8 GPa and 750 °C and post-peak static equilibration at 0.8-1.0 GPa and 850 °C. Garnet peridotites show abundant amphibole, dolomite, phlogopite and orthopyroxene after olivine, suggesting that they experienced metasomatism by crust-derived agents enriched in SiO2, K2O, CO2 and H2O. Peridotites also display LREE fractionation (La/Nd = 2.4) related to LREE-rich amphibole and clinopyroxene grown in equilibrium with garnet, indicating that metasomatism occurred at HP conditions. Kfs+Pl+Qz+Cpx interstitial pocket aggregates and Cpx+Kfs thin films around symplectites after omphacite parallel to the Zo+Omp+Grt foliation in the eclogites suggest that they underwent partial melting at HP.The contact between garnet peridotites and associated eclogites is marked by a tremolitite layer, which also occurs as layers within the peridotite lens, showing a boudinage parallel to the garnet layering of peridotites, flowing in the boudin necks. This clearly indicates that the tremolitite boudins formed when peridotites were in the garnet stability field. Tremolitites also show Phl+Tc+Chl+Tr pseudomorphs after garnet, both crystallised in a static regime postdating the boudins formation, suggesting that they derive from a garnet-bearing precursor. Tremolitites have Mg# > 0.90 and Al2O3 = 2.75 wt.% pointing to ultramafic compositions but also show enrichments in SiO2, CaO, and LREE suggesting that they formed after the reaction between the eclogite-derived melt and the garnet peridotite at HP. To test this hypothesis, we performed a thermodynamic modelling at fixed P = 3 GPa and T = 750 °C to model the chemical interaction between the garnet peridotite and the eclogite-derived melt. Our results show that this interaction produces a Opx+Cpx+Grt assemblage + Amp+Phl, depending on the water activity in the melt, suggesting that tremolitites likely derive from a previous garnet websterite with amphibole and phlogopite. In the Ulten Zone (Tonale nappe, Eastern Alps, N Italy), peridotite bodies occur within high-grade crustal rocks. Peridotites show a transition from coarse spinel-lherzolites to mylonitic garnet-amphibole peridotites. Pyroxenites veins and dikes, transposed along the peridotite foliation, show a similar evolution from coarse garnet-free websterites to fine-grained garnet + amphibole clinopyroxenites. This coupled evolution has been interpreted to reflect cooling and pressure increase of pyroxenites and host peridotites from spinel- (1200 °C, 1.3-1.6 Gpa) to garnet-facies conditions (850 °C and 2.8 Gpa) likely induced by mantle corner flow. As a consequence, garnet formed coronas around spinel and exsolved from porphyroclastic, high-T pyroxenes, and finally crystallised along the pyroxenite and peridotite foliations. Textural evidences and CPO data indicate that the transition from spinel- to garnet-facies conditions was assisted by intense shearing and deformation. Pyroxene porphyroclasts in garnet clinopyroxenites show well-developed CPOs, high frequencies of low-angle misorientations, and non-random distribution of the low-angle misorientation axes, indicating that pyroxene porphyroclasts primarily deform by dislocation creep. Dislocation creep is accompanied by reaction-induced dynamic recrystallisation during the spinel to garnet phase transition, which promotes a sudden reduction of the grain size and a shift from dislocation creep in the porphyroclast to grain-size sensitive creep (GSS) in the recrystallised grains. This results in a dramatic rheological weakening of pyroxenites at HP peak conditions when pyroxenites and host peridotites were coupled with crustal rocks.
MALASPINA, NADIA
ZANCHETTA, STEFANO
subduzione calda; fusione eclogiti; websteriti; weakening mantello; corner flow
warm subduction; eclogite melting; websterites; mantle weakening; corner flow
GEO/07 - PETROLOGIA E PETROGRAFIA
Italian
10-mar-2020
SCIENZE CHIMICHE, GEOLOGICHE E AMBIENTALI
32
2018/2019
open
(2020). Modelling of mechanical mixing and chemical interaction between the subducting crust and the overlying mantle at (ultra)high pressures: implications for the slab-to-mantle mass transfer. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2020).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/271024
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