We have studied the following topics: 1) metasomatism of the lithospheric mantle atop a plume (NE Brazil and Fernando de Noronha); 2) metasomatism of the mantle wedge by slab-derived components (Cerro de los Chenques, Patagonia); 3) origin of dunite and wehrlite regions in the subcontinental lherzolitic mantle (Balmuccia); 4) reactions between diorite dikes and subcontinental mantle and consequent geodynamic implications (Baldissero massif).Isotopic and geochemical results indicate that in the case study 1 the mantle lithosphere underwent thermochemical and mechanical erosion by infiltration of alkali basalts during Cenozoic and that this process affected a mantle recording previous, Mesozoic, mantle processes. The similarity of processes and geochemical characteristics in NE Brazil and Fernando de Noronha supports the hypothesis that the lithosphere under the Island is a detached portion of the continental one and that the respective lithospheres interfered with the same plume.Geochemical and isotope evidence indicate that the wedge beneath Chenques (case 2) interfered with slab-derived fluids rather than melts. Since the study locality is 400 km east from the trench and the slab beneath Chenques is about 200 km deep, a main problem is how to explain the long travel of slab-released fluids in order to reach the observed spinel-facies region of the wedge. We have modelled this process as break-down of phengite in the slab and percolation of the derived fluids in the wedge under increasing temperature and water activity. Reaction during the long fluid migration produces phlogopite, K-richterite, garnet, and, in the lower-P region, amphibole. The geochemical effects of these reaction products are recorded in the fluids reaching the observed spinel-facies region. Besides already known decimetric dunite at websterite dyke contacts, decametric dunites regions have been discovered into the subcontinental Balmuccia Peridotite (case 3). The latter dunites (± wehrlites) are of two types: a) very high Mg# values in bulk-rock and minerals and characterised by massive Cr-spinel layers; b) Mg# values in bulk rock and minerals lower than the ambient lherzolite and characterised by amphibole, phlogopite and plagioclase pods. As discussed in more detail in a specific abstract of this session, these two dunite types are related with infiltration of different melts and different processes, of increasing and decreasing melt mass, respectively, during reactive flow.The reaction between diorite dykes and the Baldissero peridotite (topic 3) has already been previously discussed. However, new isotopic data provide constraints on the age of dyke emplacement which have important geodynamic implications, specifically illustrated in a work presented in this session. Finally, the mafic-ultramafic Niquelandia complex have been re-interpreted in the light of isotopic evidence in terms of energy-controlled assimilation and fractional crystallisation. Thermal balance indicate that the complex underwent large crystallisation before heating the crust to melting. Heat production was related to the intrusion volume and resulted higher in the largest lower sequence with respect to the smaller upper sequence. The results were production of different anatectic melts, different residual metasediments, and different interference results between anatectic melts and mafic magmas.

Results of melt migration in the lithospheric mantle and in continental mafic-ultramafic layered complexes / Rivalenti, Giorgio; Mazzucchelli, Maurizio. - In: EPITOME. - ISSN 1972-1552. - STAMPA. - 2:(2007), pp. 111-111. ((Intervento presentato al convegno GeoItalia 2007 "VI Forum Italiano di Scienze della Terra" tenutosi a Rimini nel 12-14/09/2007.

Results of melt migration in the lithospheric mantle and in continental mafic-ultramafic layered complexes.

RIVALENTI, Giorgio;MAZZUCCHELLI, Maurizio
2007

Abstract

We have studied the following topics: 1) metasomatism of the lithospheric mantle atop a plume (NE Brazil and Fernando de Noronha); 2) metasomatism of the mantle wedge by slab-derived components (Cerro de los Chenques, Patagonia); 3) origin of dunite and wehrlite regions in the subcontinental lherzolitic mantle (Balmuccia); 4) reactions between diorite dikes and subcontinental mantle and consequent geodynamic implications (Baldissero massif).Isotopic and geochemical results indicate that in the case study 1 the mantle lithosphere underwent thermochemical and mechanical erosion by infiltration of alkali basalts during Cenozoic and that this process affected a mantle recording previous, Mesozoic, mantle processes. The similarity of processes and geochemical characteristics in NE Brazil and Fernando de Noronha supports the hypothesis that the lithosphere under the Island is a detached portion of the continental one and that the respective lithospheres interfered with the same plume.Geochemical and isotope evidence indicate that the wedge beneath Chenques (case 2) interfered with slab-derived fluids rather than melts. Since the study locality is 400 km east from the trench and the slab beneath Chenques is about 200 km deep, a main problem is how to explain the long travel of slab-released fluids in order to reach the observed spinel-facies region of the wedge. We have modelled this process as break-down of phengite in the slab and percolation of the derived fluids in the wedge under increasing temperature and water activity. Reaction during the long fluid migration produces phlogopite, K-richterite, garnet, and, in the lower-P region, amphibole. The geochemical effects of these reaction products are recorded in the fluids reaching the observed spinel-facies region. Besides already known decimetric dunite at websterite dyke contacts, decametric dunites regions have been discovered into the subcontinental Balmuccia Peridotite (case 3). The latter dunites (± wehrlites) are of two types: a) very high Mg# values in bulk-rock and minerals and characterised by massive Cr-spinel layers; b) Mg# values in bulk rock and minerals lower than the ambient lherzolite and characterised by amphibole, phlogopite and plagioclase pods. As discussed in more detail in a specific abstract of this session, these two dunite types are related with infiltration of different melts and different processes, of increasing and decreasing melt mass, respectively, during reactive flow.The reaction between diorite dykes and the Baldissero peridotite (topic 3) has already been previously discussed. However, new isotopic data provide constraints on the age of dyke emplacement which have important geodynamic implications, specifically illustrated in a work presented in this session. Finally, the mafic-ultramafic Niquelandia complex have been re-interpreted in the light of isotopic evidence in terms of energy-controlled assimilation and fractional crystallisation. Thermal balance indicate that the complex underwent large crystallisation before heating the crust to melting. Heat production was related to the intrusion volume and resulted higher in the largest lower sequence with respect to the smaller upper sequence. The results were production of different anatectic melts, different residual metasediments, and different interference results between anatectic melts and mafic magmas.
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Rivalenti, Giorgio; Mazzucchelli, Maurizio
Results of melt migration in the lithospheric mantle and in continental mafic-ultramafic layered complexes / Rivalenti, Giorgio; Mazzucchelli, Maurizio. - In: EPITOME. - ISSN 1972-1552. - STAMPA. - 2:(2007), pp. 111-111. ((Intervento presentato al convegno GeoItalia 2007 "VI Forum Italiano di Scienze della Terra" tenutosi a Rimini nel 12-14/09/2007.
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