HR: 0800h
AN: V51D-1519    [Abstracts]
TI: Maturation of Island Arcs: Pb-Sr-Nd-Hf Isotopic Evidence for Continuous Mantle Source Enrichment During ~120 Ma of Magmatic Activity of the Kohistan Paleo-Island Arc
AU: * Jagoutz, O
EM: Oliver.Jagoutz@geo.unibe.ch
AF: Institute of Geological Sciences, University of Bern, Erlachstrasse 9a , Bern, 3012 Switzerland
AU: Schaltegger, U
EM: urs.schaltegger@terre.unige.ch
AF: Department for Mineralogy, University of Geneva, Rue des Maraichers 13, Geneve, 1205 Switzerland
AU: Muentener, O
EM: othmar.muntener@geo.unibe.ch
AF: Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3 , Bern, 3012 Switzerland
AU: Burg, J
EM: jean-pierre.burg@erdw.ethz.ch
AF: Institute for Geology, ETH Zurich, Sonneggstr. 5, Zurich, 8092 Switzerland
AB: Understanding the long-term geochemical trends of arc magmas can yield important constraints on the thermal and chemical evolution of convergent margins. It is generally assumed that the flux from the subducted slab into the subarc-mantle wedge strongly controls the light rare earth (LREE) and incompatible trace element (LILE) composition of arc magmas. Accordingly, the ratio between the fluxes from the subducted slab compared to the degree of depletion due to partial melting of the sub-arc mantle wedge exerts a strong control on the temporal evolution of the LREE and LILE of primary arc magmas. Temporal maturation of island arc magmas has been proposed in the early seventies our understanding of the long-term evolution of Island arcs however, is still limited. Previous studies mainly focused on the high resolution tephra record of present-day active Island arcs (e.g. Mariana). These studies however, covered only a limited part (e.g. ~50 Ma in the case of the Marianas) of the potentially long (>100 Ma) temporal evolution of magmatic arcs. In this study we focused on the Kohistan paleo-Island arc, an accreted and uplifted cretaceous Island arc exposed in NW Pakistan. We present the results of combined high precision U-Pb ages and Hf isotopic data from zircons and Pb-Sr-Nd whole rock isotopic data from the same samples. The samples document a ~120 Ma long period of magmatic activity in the Kohistan arc. Our results indicate that the isotopic composition of the studied mafic to granitic rocks may be explained by a mixture of a depleted upper mantle component (DM) and two different types of enriched sources (EM 1&2). The temporal trends display a steady decrease in εNd and εHf and an increase in ^87/86Sr and ^{208,207,206/204}Pb. Older samples are generally more depleted whereas younger ones tend towards more enriched values. These internally consistent trends might reflect mantle source evolution or increasing influence of crustal assimilation associated with growth of the arc. To distinguish between these two processes we studied a single large scale mafic intrusion (the so called Chilas Complex) in more detail. The Chilas complex is one of the largest intrusions within the Kohistan arc and provides field evidence for assimilation of crustal material. However, the isotopic composition of different mafic whole rock samples from various locations within the intrusion is extremely homogeneous. It thus seems that assimilation of crustal material has no significant control on the isotopic composition of the host rock. Accordingly, we argue that the observed temporal trends represent a long-term metasomatic enrichment of the Kohistan sub-arc mantle source by a slab derived component.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005