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Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea-Verbano Zone

Journal article
Authors G. L. Luvizotto
Thomas Zack
Published in Chemical Geology
Volume 261
Issue 3-4
Pages 303-317
ISSN 0009-2541
Publication year 2009
Published at Department of Earth Sciences
Pages 303-317
Language en
Keywords accessory phases, trace element, partitioning, slow cooling, diffusion, fluids, granulite-facies metamorphism, u-pb zircon, southern-alps, lower crust, cooling history, monazite growth, detrital rutile, trace-elements, mafic complex, strona-ceneri
Subject categories Earth and Related Environmental Sciences, Geochemistry, Geology, Solid earth geology and petrology, Mineralogy

Abstract

Detailed textural observations and in situ analyses (EMP, SIMS and LA-ICP-MS) are used to characterize trace element behavior during prograde and retrograde metamorphic reactions involving rutile. The Ivrea-Verbano Zone is a classic granulite area and rocks from the Strona and d'Ossola valleys are an example of the amphibolite to granulite facies transition. Although different rock types occur in the area, detailed sampling and petrographic work show that rutile only occurs in granulite facies paragneisses. These rocks show a rich inventory of textures that allow not only for the investigation of trace element behavior in response to prograde rutile growth, but also for the effect of post-peak processes on rutile chemistry. Nb concentrations in rutile from lower grade samples show a larger spread (from 500 to 5000 ppm within one sample) when compared to those from higher grades. This pattern can be modeled using prograde rutile growth formed from biotite breakdown. Zr concentrations in rutile are characterized by an anomalously large spread and a bimodal distribution. Maximum Zr concentrations increase according to the general metamorphic gradient known for this area. Temperatures (from Zr-in-rutile thermometry), although feasible, are considerably higher than previous calculations (increasing from ca. 850 to 930 degrees C). A second cluster of Zr concentrations in rutile occurs at rather constant concentrations (ca. 1000 ppm) for all localities and is interpreted to be related to intense fluid influx at high-temperature and/or to post-peak diffusional resetting favored by slow cooling rates. Alteration textures, characterized by a complex network of micro-veins, are evidence for the late fluid influx The fluid strongly affected the rutiles, which is evidenced by corrosion of older rutile grains and formation of rutile veinlets. (C) 2008 Elsevier B.V. All rights reserved.

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