Zircon geochronology of the Koraput alkaline complex: Insights from combined geochemical and U–Pb–Hf isotope analyses, and implications for the timing of alkaline magmatism in the Eastern Ghats Belt, India K. Hippe a,b, ⁎, A. Möller c , A. von Quadt a , I. Peytcheva a,d , K. Hammerschmidt b a Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland b Institute of Geological Sciences, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany c Department of Geology, The University of Kansas, 1475 Jayhawk Blvd., Lawrence, KS 66045, USA d Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev st. bl. 24, 1113 Sofia, Bulgaria abstract article info Article history: Received 25 October 2014 Received in revised form 11 February 2015 Accepted 22 February 2015 Available online xxxx Handling Editor: A. Kröner Keywords: Metamorphic zircon U–Pb geochronology Lu–Hf Eastern Ghats Belt Alkaline magmatism Zircon formation and modification during magmatic crystallization and high-grade metamorphism are explored using TIMS and LA-ICP-MS U–Pb geochronology, Lu–Hf isotope chemistry, trace element analysis and textural clues on zircons from the Koraput alkaline intrusion, Eastern Ghats Belt (EGB), India. The zircon host-rock is a granulite-facies nepheline syenite gneiss with an exceptionally low Zr concentration, prohibiting early magmatic Zr saturation. With zircon formation occurring at a late stage of advanced magmatic cooling, significant amounts of Zr were incorporated into biotite, nearly the only other Zr-bearing phase in the nepheline syenite gneisses. In- vestigated zircons experienced a multi-stage history of magmatic and metamorphic zircon growth with repeated solid-state recrystallization and partial dissolution–precipitation. These processes are recorded by complex pat- terns of internal zircon structures and a wide range of apparently concordant U–Pb ages between 869 ± 7 Ma and 690 ± 1 Ma. The oldest ages are interpreted to represent the timing of the emplacement of the Koraput alkaline complex, which significantly postdates the intrusion ages of most of the alkaline intrusion in the western EGB. However, Hf model ages of TDM = 1.5 to 1.0 Ga suggest an earlier separation of the nepheline syenite magma from its depleted mantle source, overlapping with the widespread Mesoproterozoic, rift-related alkaline magmatism in the EGB. Zircons yielding ages younger than 860 Ma have most probably experienced partial re- setting of their U–Pb ages during repeated and variable recrystallization events. Consistent youngest LA-ICP- MS and CA-TIMS U–Pb ages of 700–690 Ma reflect a final pulse of high-grade metamorphism in the Koraput area and underline the recurrence of considerable orogenic activity in the western EGB during the Neoproterozoic. Within the nepheline syenite gneisses this final high-grade metamorphic event caused biotite breakdown, releasing sufficient Zr for local saturation and new subsolidus zircon growth along the biotite grain boundaries. © 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction Defining the conditions of zircon crystallization in magmatic and metamorphic environments is one of the main challenges in zircon U– Pb geochronology in order to understand which geological processes are being dated. Particularly in rocks with a high-grade metamorphic overprint, the obtained age information often reflects a complex history of polyphase zircon growth, dissolution and/or recrystallization (e.g., Harley et al., 2007). With the development of high-precision mass spectrometry and small-diameter spot measurements the analysis of single grains or specific domains within single crystals has become possible. These techniques allow us to date multiple stages of crystal growth or modification and, thus, to shed light on detailed regional tectonometamorphic histories. In this context, any geologic interpreta- tion of zircon U–Pb data requires a thorough evaluation of zircon textures and morphology as obtained by high-resolution imagery, e.g., cathodoluminescence (CL), electron backscattering (BSE) or sec- ondary electron microscopy (cf. Corfu et al., 2003). Additionally, crucial information about zircon growth conditions and the source of the mag- matic host rock can be drawn from trace element distribution as well as the Hf isotope composition (e.g., Watson and Harrison, 1983; Belousova et al., 2002, 2006; Watson et al., 2006; Scherer et al., 2007; Kemp et al., 2009). In this study, we discuss the processes of zircon formation and mod- ification in granulite facies, alkaline rocks from the Indian Eastern Ghats Belt (EGB). This work combines U–Pb zircon dating with analyses of Gondwana Research xxx (2015) xxx–xxx ⁎ Corresponding author at: Laboratory of Ion Beam Physics, ETH Zürich, Otto-Stern-Weg 5, 8093 Zürich, Switzerland. Tel.: +41 44 633 06 24; fax: +41 44 633 10 67. E-mail address: hippe@phys.ethz.ch (K. Hippe). GR-01421; No of Pages 16 http://dx.doi.org/10.1016/j.gr.2015.02.021 1342-937X/© 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr Please cite this article as: Hippe, K., et al., Zircon geochronology of the Koraput alkaline complex: Insights from combined geochemical and U–Pb– Hf isotope analyses, and implications..., Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2015.02.021