Project

zircon; U-Pb geochronology; Permian; Triassic; Andes; Peru; magmatism; geochemistry; U-Pb dating; Ar-Ar dating; subduction; Jurassic; intracontinental rifting; geochronology; South America; volcanic arc; extension; back-arc; U-Pb

Lead
Lay summary
This proposal seeks funding to continue research on the pre-Andean evolution of the South American active margin in Peru using geochemical and geochronological tools. We have gained new ideas about the evolution of the Peruvian segment of the Andean margin, from the first traces of orogenic magmatism in the mid-Proterozoic, to Jurassic peralkaline magmatism at the onset of modern Andean orogenic dynamics. Detrital zircons in metasedimentary sequences as well as the magmatic record of the Eastern Cordillera testify the existence of different mobile belts along the proto-Andean margin: Grenvilian/Sunsás (1.3-0.8 Ga), pre-Braziliano (0.77-0.66 Ga), Braziliano-Panafrican (0.65-0.55 Ga) and Famatinian/Pampean (0.65-0.45 Ma), and the Gondwanide orogenies (0.35-0.20 Ga). Initial Hf isotopic compositions of dated zircons allow quantification of crustal growth over time during different geotectonic settings: the results indicate that during continental arc stages reworking of the pre-existing crust is the main process, and only during the post-tectonic, extensional stages (Neo-Proterozoic, Permo-Triassic, Late Triassic, Early Jurassic) subordinate volumes of juvenile melts were accreted at the base of the crust. The latter were enough to compensate for the periods of crustal reworking, but not important enough to produce significant crustal growth along the proto-Andean margin.With this proposal we intend to focus on two well-defined topics of general interest:Subproject A The birth and death of an intracontinental rift: the Mitu Group in southern Peru: The southern part of the Eastern Cordillera is characterized by the development of a large Triassic to early Jurassic intracontinental rift. With a PhD project, we plan to study the timing and rates of crustal thinning, basin subsidence and magma emplacement. Rifting terminated in the early Jurassic after emplacement of an anorogenic peralkaline volcanic and plutonic suite, offering insight into the contribution of crustal vs. mantle sources over time in an evolving and eventually aborted continental rift.Subproject B The Jurassic magmatic arc in southwestern Peru: The Triassic-Jurassic transition is characterized by the abortion of the Mitu rift and renewed subduction-related magmatism in a magmatic arc along the present-day coastline. We intend to study, with a PhD project, the transition from the extensional geodynamic scenario forming the Mitu Rift to renewed subduction and subsequent magma production, in terms of timing and sources of magmatic liquids. Understanding the timing and nature of Jurassic arc magmatism will also explain the lack of Jurassic rocks north of Lima.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

This proposal seeks funding to continue research on the pre-Andean evolution of the South American active margin in Peru using geochemical and geochronological tools. During the five previous years of this research project, we have gained new ideas about the evolution of the Peruvian segment of the Andean margin, from the first traces of orogenic magmatism in the mid-Proterozoic, to Jurassic peralkaline magmatism at the onset of modern Andean orogenic dynamics. Detrital zircons in metasedimentary sequences as well as the magmatic record of the Eastern Cordillera testify the existence of different mobile belts along the proto-Andean margin: Grenvilian/Sunsás (1.3-0.8 Ga), pre-Braziliano (0.77-0.66 Ga), Braziliano-Panafrican (0.65-0.55 Ga) and Famatinian/Pampean (0.65-0.45 Ma), and the Gondwanide orogenies (0.35-0.20 Ga). The latter gave rise to a 800 km long magmatic belt in the Eastern Cordillera, with a compositional evolution from Pennsylvanian arc-related intrusions, followed by Permo-Triassic post-collisional, extension-related granites and late Triassic S-type, rift-related granites in the southernmost Cordillera de Carabaya. Initial Hf isotopic compositions of dated zircons allow quantification of crustal growth over time during different geotectonic settings: the results indicate that during continental arc stages reworking of the pre-existing crust is the main process, and only during the post-tectonic, extensional stages (Neo-Proterozoic, Permo-Triassic, Late Triassic, Early Jurassic) subordinate volumes of juvenile melts were accreted at the base of the crust. The latter were enough to compensate for the periods of crustal reworking, but not important enough to produce significant crustal growth along the proto-Andean margin. Our conclusion is that the proto-Andean margin stayed during more than 1 billion years in a supra-subduction position at the leading edge of Western Gondwana. Neither supra-subduction magmatism nor vertical accretion of residual mantle melts at the base of the crust, but only lateral accretion of juvenile terranes (arcs, plateaus or juvenile crust) are proposed to be suitable processes to lead to significant crustal growth.The successful approach using isotopic tracing and dating motivates us to continue research on the pre-Andean evolution of the South American margin. With this proposal we intend to focus on two well-defined topics of general interest:Subproject A The birth and death of an intracontinental rift: the Mitu Group in southern Peru: The southern part of the Eastern Cordillera is characterized by the development of a large Triassic to early Jurassic intracontinental rift (Mitu Group). With a PhD project, we plan to study the timing and rates of crustal thinning, basin subsidence and magma emplacement. We hope to provide answers to the questions of when and how transtension and finally rifting replaced Carboniferous orthogonal subduction and magmatism, was the crustal thinning caused by a highly oblique direction of the subducting plate or by the response of the South American continent to Pangea breakup. Rifting terminated in the early Jurassic after emplacement of an anorogenic peralkaline volcanic and plutonic suite, offering insight into the contribution of crustal vs. mantle sources over time in an evolving and eventually aborted continental rift.Subproject B The Jurassic magmatic arc in southwestern Peru: The Triassic-Jurassic transition is characterized by the abortion of the Mitu rift and renewed subduction-related magmatism in a magmatic arc along the present-day coastline, and in the interior of the Arequipa massif south of 12°S. Numerous continuous sections indicate that an early bimodal volcanic suite may be transitional from extensional/transtensional tectonics and that calc-alkaline arc magmatism resumes in the middle Jurassic. We intend to study, with a PhD project, the transition from the extensional geodynamic scenario forming the Mitu Rift to renewed (orthogonal) subduction and subsequent magma production, in terms of timing and sources of magmatic liquids. Understanding the timing and nature of Jurassic arc magmatism will also explain the lack of Jurassic rocks in the enigmatic “craton-free zone”, north of 12°S (north of Lima).To carry out the proposed research, we are requesting 30 months of salary to complete the PhD project of M. Reitsma (project A), and 27 months for the PhD project of F. Boekhout (project B).