HR: 11:20h
AN: V32A-05    [Abstracts]
TI: What are the differences between preserved vs. delaminated lower crust? Evidences from the Kohistan arc
AU: * Jagoutz, O E
EM: Oliver.Jagoutz@geo.unibe.ch
AF: Institute for Geology, University of Bern, Baltzerstrasse 1+3, Bern, 3012, Switzerland
AU: Müntener, O
EM: Othmar.Muntener@unil.ch
AF: Institute of Mineralogy, University of Lausanne, Anthropole, Lausanne, 1015, Switzerland
AU: Burg, J
EM: Burg@erdwethz.ch
AF: Department of Earth Science, ETH Zurich, Clausiusstr, Zurich, 8092, Switzerland
AU: Schmidt, M
EM: max.schmidt@erdw.ethz.ch
AF: Department of Earth Science, ETH Zurich, Clausiusstr, Zurich, 8092, Switzerland
AU: Ulmer, P
EM: peter.ulmer@erdw.ethz.ch
AF: Department of Earth Science, ETH Zurich, Clausiusstr, Zurich, 8092, Switzerland
AB: Delamination and foundering of the deep continental lithosphere into the mantle is an important crust forming mechanisms and might have implication for the formation of mantle heterogeneities. For example it is generally assumed that delamination of lower crust forms an enriched component within the mantle. However, the preserved lower continental crust is not complement to the upper crust. Therefore, if delamination is a significant crust forming process for the upper continental crust the delaminating crust must differ in mineralogy and composition from the preserved lower crust. A natural but unfortunate consequence of delamination is that the foundered crust is generally lost and therefore direct observations are scare. Consequently our knowledge of the composition and mineralogy of the preserved vs delaminated lower crust remains rather illusive. However, within the Kohistan arc (NE Pakistan) two mantle-crust sections (the Jijal and the Chilas section) are preserved which are markedly different in mineralogy. We describe lithologies of these sections and argue that fractionation mechanisms that produced them document two liquid lines of descent with largely differing initial water contents. We propose that the upper, non-sedimentary CC is dominantly formed by hydrous high-pressure fractionation of complementary garnet-pyroxene-amphibole-rich cumulates preserved in the Jijal section. In contrast, the lower crust of the Chilas section is formed by "less-hydrous" parental melts. Density considerations indicate that whereas the "Chilas lower crust" will be preserved over geological timescale the garnet-rich "Jijal lower crust" can delaminate and sink back into the mantle. The Chilas lower crust has a bulk composition remarkably similar to the bulk lower crust estimates. Contrasting the high density Jijal lower crust is strongly depleted in incompatible elements but has positive anomalies of Sr, Pb, and Eu and high Ba/Nb. If such a hydrous composition is transferred back into the depleted upper mantle its isotopic evolution can explain certain characteristics of intra plate magmatism.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting