Project

magmatic processes; adakite-like; geochronology; radiogenic isotopes; subduction; Ecuador; Peru; porphyry systems

Lead
Lay summary
In the past years (SNF Projects N. 200021-109636 and 200020-117616, first year of the current project 200020-126896) we have investigated the link between changes in magma chemistry (in particular the development of adakite-like signatures) and geodynamic setting at convergent continental margins of Ecuador and Peru, and how these could influence the formation of porphyry-type deposits. The main results of our previous and current investigations can be summarized as follows: (i) in the investigated cases adakite-like rocks are formed by magmatic evolution at variably deep crustal levels and under hydrous conditions, favoring the stability of amphibole ± garnet but not that of plagioclase; (ii) porphyry-type deposits form in association with adakite-like rocks during a few Ma (~1-3 Ma) towards the end of a several Ma (~6-15 Ma) long transition from normal to adakite-like magmatism; (iii) increased compression with or without attending crustal thickening is held responsible to have caused a shift of the locus of magmatic evolution from shallow crustal levels (normal arc rocks) to deeper ones (adakite-like signatures); (iv) increased compression might be related to subduction of aseismic ridges and/or any other topographic high (e.g., oceanic plateau) which increase the coupling between subducting and overriding plates, although other changes at the subduction zone (e.g., subduction obliquity, plate relative velocity) may also play a role. After having verified in the previous years the occurrence of such an “empirical” link between porphyry-type deposit and transition from normal to adakite-like signatures, in the current project (N. 200020-126896) and its extension we want to investigate in detail three processes which can potentially play a role in this connection: (1) longevity of magmatic reservoirs with adakite-like signatures; (2) temporal frequency of magmatic pulses in mineralized and non-mineralized magmatic systems with adakite-like signatures; (3) specialization of adakite-like magmas in terms of volatiles (S, Cl) and metals. To do this we study both ancient mineralized systems (subproject A: PhD project of Mr. Cyril Chelle-Michou) and recent or active non-mineralized volcanic systems (subproject B: carried out by the applicant), both characterized by adakite-like signatures, using a range of state-of-the-art analytical tools (whole rock major and trace element geochemistry, whole rock isotope (Sr, Nd, Pb) geochemistry, mineral chemistry by microprobe and LA-ICP-MS, high-precision ID-TIMS and LA-ICP-MS U/Pb dating of zircons, Hf isotope geochemistry of zircons). The ancient mineralized system investigated is the Coroccohuayco magmatic system which displays a magmatic evolution through time (from about 42 to 35 Ma) characterized by a transition from normal to adakite-like rocks. Coroccohuayco belongs to the world-class Tintaya skarn and porphyry district and therefore is suitable to try to answer the above questions. The recent to active volcanic systems investigated are the Quaternary adakite-like rocks of Ecuador. The advantage of studying the Quaternary volcanoes of Ecuador in the frame of this project is twofold: (i) well-constrained knowledge of the geodynamic setting; (ii) opportunity to work on fresh volcanic rocks and glasses on which study the behaviour of metals and volatiles under the adakitic regime. The combination of the information obtained on the young rocks with that obtained on the ancient mineralized system will be a powerful tool to elucidate the effective role of adakite-like magmas in the generation of porphyry-type deposits.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

The present proposal is for a 2-year extension of the SNF project. N. 200020-126896 “Magmas, ore deposits and geodynamic evolution at a convergent margin: the case of Ecuador”. Funds are asked for a 2-year extension of the PhD student salary, fieldwork and attendance to conferences by the PhD student and the applicant, as well as for analytical work. The PhD student, Mr. Cyril Chelle-Michou, was engaged for this project from 1 January 2010.In the past years (SNF Projects N. 200021-109636 and 200020-117616, first year of the current project 200020-126896) we have investigated the link between changes in magma chemistry (in particular the development of adakite-like signatures) and geodynamic setting at convergent continental margins in Ecuador and Peru, and how these could influence the formation of porphyry-type deposits. The main results of our previous and current investigations can be summarized as follows: (i) in the investigated cases adakite-like rocks are formed by magmatic evolution at variably deep crustal levels and under hydrous conditions, favoring the stability of amphibole ± garnet but not that of plagioclase; (ii) porphyry-type deposits form in association with adakite-like rocks during a few Ma (~1-3 Ma) towards the end of a several Ma (~6-15 Ma) long transition from normal to adakite-like magmatism; (iii) increased compression with or without attending crustal thickening is held responsible to have caused a shift of the locus of magmatic evolution from shallow crustal levels (normal arc rocks) to deeper ones (adakite-like signatures); (iv) increased compression might be related to subduction of aseismic ridges and/or any other topographic high (e.g., oceanic plateau) which increase the coupling between subducting and overriding plates, although other changes at the subduction zone (e.g., subduction obliquity, plate relative velocity) may also play a role.After having verified in the previous years the occurrence of such an “empirical” link between porphyry-type deposit and transition from normal to adakite-like signatures, in the current project (N. 200020-126896) and its extension we want to investigate in detail three processes which can potentially play a role in this connection: (1) longevity of magmatic reservoirs with adakite-like signatures; (2) temporal frequency of magmatic pulses in mineralized and non-mineralized magmatic systems with adakite-like signatures; (3) specialization of adakite-like magmas in terms of volatiles (S, Cl) and metals.To do this we study both ancient mineralized systems (subproject A: PhD project of Mr. Cyril Chelle-Michou) and recent or active non-mineralized volcanic systems (subproject B: carried out by the applicant), both characterized by adakite-like signatures. The ancient mineralized system investigated is the Coroccohuayco magmatic system which displays a magmatic evolution through time (from about 42 to 35 Ma) characterized by a transition from normal to adakite-like rocks. Coroccohuayco belongs to the world-class Tintaya skarn and porphyry district and therefore is suitable to try to answer the above questions.The recent to active volcanic systems investigated are the Quaternary adakite-like rocks of Ecuador. The advantage of studying the Quaternary volcanoes of Ecuador in the frame of this project is twofold: (i) well-constrained knowledge of the geodynamic setting; (ii) opportunity to work on fresh volcanic rocks and glasses on which study the behaviour of metals and volatiles under the adakitic regime. The combination of the information obtained on the young rocks with that obtained on the ancient mineralized system will be a powerful tool to elucidate the effective role of adakite-like magmas in the generation of porphyry-type deposits.