Project

Discipline

Lower crust; thermal evolution; timescales of layer formation; high precision dating; intraplutonic contacts; igneous petrology

Lead
Lay summary
I propose a combinedfield (Ivrea, Italy and Sierra Nevada, California), detailed analytical and modelling study on the formation of layeredcomplexes and igneous layering formation on plutonic rocks, to get constraintson the emplacement mechanisms and rates of magma injection in the deep crustand along interplutonic contacts. Since the thermal budget added to the crustby crystallizing plutonic rocks depends on the emplacement mechanisms andvolumes of magmas, understanding how magmatic layering along interplutoniccontacts forms and how external parameters such as density, fertility, crystalcargo and viscosity influence the accretion mechanics of dikes and sills, thestudy of deep seated intrusions close to the Moho and subsequent extractionchannels are crucial to the understanding of the magmatic construction of the continental crust.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

This proposal requests funding for three years of combined field, analytical and modeling work on layered plutonic rocks. The grant will support Othmar Müntener, and the field and analytical expenses of two PhD students.I propose a combined field, detailed analytical and modelling study on the formation of layered complexes and igneous layering formation on plutonic rocks, to get constraints on the emplacement mechanisms and rates of magma injection in the deep crust and along interplutonic contacts. Since the thermal budget added to the crust by crystallizing plutonic rocks depends on the emplacement mechanisms and volumes of magmas, understanding how magmatic layering along interplutonic contacts forms and how external parameters such as density, fertility, crystal cargo and viscosity influence the accretion mechanics of dikes and sills, the study of deep seated intrusions close to the Moho and subsequent extraction channels are crucial to the understanding of the magmatic construction of the crust.Ongoing studies in our group on the shallow emplacement of sill complexes in Torres del Paine and the formation of plutons in the Adamello massif (Italy) have shown that emplacement of mafic to granitic magmas preferentially occurs in small volumes and mostly in sills - and in exceptional cases the temporal evolution can be deciphered. One of the largest and well studied layered complexes of mafic magmas (the Ivrea zone) is thought to be formed by crystallization from a small magma chamber and subsequent high-temperature flow of partially crystallized mafic rocks away from the chamber (Gabbro glacier model, Quick and Denlinger 1992). Yet the time integrated geochemical evolution and the available age data indicate high temperature conditions prevailing for several 10’s of millions of year. This model is different from the one proposed for shallower systems. Recent thermal models that requires repetitive sill intrusions to produce substantial heating and partial melting of the lower crust are an alternative explanation for the emplacement of large plutonic bodies at the crust mantle boundary.We propose to study in detail the phase petrology and geochemistry of lower crustal mafic rocks and its intercalated crustal rocks in well-defined natural system (Ivrea zone, Italy), to identify thermal constraints from within the intrusion and on its borders. These data will be used to develop a thermal model that in turns allow to provide quantitative constraints on the melt fraction that might be extracted from such bodies. These constraints can then be tested by using existing field data. The intermediate to silicic magmas rise to construct plutons in intermediate to upper crustal levels. How they rise, via diapirs, dikes or a combination of both is debatable. We propose to study in detail such melt extraction channels (schlieren and comb-layering) along intra-plutonic contacts. In particular, we evaluate the potential of H2O saturation during a late stage of crystallization, as a mechanism for the formation of such magma extraction channelsIn order to achieve this, we will sample mafic rocks and intercalated metasedimentary units in the Ivrea zone [Italy], and layered diorititc to gabbroic rocks from the Sierra Nevada [California, USA], and Adamello [Alps] to establish: (a) the potential of novel trace element thermometers (Zr-in rutile, Ti in zircon) and chronometers (U-Pb lead in rutile, zircon) to obtain peak temperatures of crustal rocks within mafic magmas and in the immediate surrounding rocks. (b) use these data, together with phase relations of mafic rocks to build a thermal model that provides quantitative constraints on the amount of residual liquids and liquids that might be produced by partial melting of surrounding rock (PhD 1). These studies will be complemented by studies on intra-plutonic schlieren and comb-layering, as spectacularly exposed in Adamello (Italy) and the Sierra Nevada (California). Phase petrology and crystal size distributions will be used to identify the key parameters that generate such intraplutonic contacts. We will test current models that such extraction channels are H2O-rich or even H2O saturated (PhD 2). During the current project we will focus on detailed field work, phase petrology, geochemistry, dating and thermal modeling. I propose to start this project with SNF grants for 3 years, with two Ph.D. students, in collaboration with international and national co-workers (Prof. J. Blundy, Bristol, Dr. C. Annen, Bristol, Prof. U. Schaltegger, Geneva, Dr. T. Müller, Bochum) and several colleagues from Lausanne. Two Ph.D. students should make an integral part of the project.