whole-rock and electron probe microanalysis mineral composition data) and sample .... the magma compositions from which detrital igneous zircons crystal- ..... Zircon forms clear and colourless grains, usually with rounded or fractured edges.
Characterization of magma from inclusions in zircon: Apatite and biotite work well, feldspar less so E.S. Jennings1, H.R. Marschall1,2, C.J. Hawkesworth3, and C.D. Storey4 1
Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 3 Department of Earth Sciences, University of St. Andrews, North Street, St. Andrews KY16 9AL, UK 4 School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK 2
ABSTRACT Detrital zircon grains are employed to decipher sediment provenances and crustal evolution, and they provide unique evidence of Hadean crust-mantle differentiation processes. We demonstrate that mineral inclusions in zircon provide valuable information on the conditions under which zircon crystallized. Zircon grains from selected plutonic rocks from Dronning Maud Land, Antarctica, contain inclusions of apatite, biotite, amphibole, and pyroxenes that accurately reflect the chemical compositions of the equivalent phases in the host-rock matrix, and the compositions of the whole rocks. High concentrations of Y and low concentrations of Sr in apatite inclusions in zircon are diagnostic of evolved, felsic granitoid host rocks. In contrast, the relative abundances and compositions of plagioclase and alkali feldspar inclusions in zircon are decoupled from the composition of the whole rock, and they are generally indicative of granitic melts regardless of the bulk rock. This is best explained by the late crystallization of zircon relative to the bulk of the feldspars. We conclude that inclusions of apatite and mafic phases in zircon constrain the potential source rocks of detrital zircon, whereas feldspar inclusions do not. INTRODUCTION Sediments can access broad areas of exposed crust, and they carry detrital grains from a number of rock types. Detrital zircon is used in provenance studies because it is resistant and yields precise age information for the rock in which it crystallized. Studies of the large-scale evolution of the continental crust have therefore successfully employed detrital zircon separated from sedimentary rocks (e.g., Veevers, 2007). However, interpretations remain difficult, because of a significant overlap in geochemical features recorded in zircon derived from a wide variety of host rocks. Low crystallization temperatures and steep rare earth element (REE) patterns with characteristic anomalies are not restricted to zircon from granite, but may be typical of igneous zircon in general (e.g., Hoskin et al., 2000; Coogan and Hinton, 2006). The geology of the Hadean is only documented through detrital zircon grains in Archean metasediments from the Jack Hills, Australia (e.g., Maas et al., 1992). These grains are indicative of growth from cool, wet, evolved melts (Harrison, 2009, and references therein). They have sparked an ongoing discussion on the existence of differentiated continental crust and significantly depleted mantle before 4 Ga, and the possible onset of plate tectonics in the Hadean (e.g., Wilde et al., 2001; Harrison, 2009; Kemp et al., 2010). Inclusion assemblages in Hadean zircons (Maas et al., 1992; Cavosie et al., 2004; Hopkins et al., 2008) and in zircons separated from younger granitoid rocks have been identified as largely granitic, even in mafic and intermediate granitoids (Nutman and Hiess, 2009; Darling et al., 2009). Hence, a tool is needed that is diagnostic for detrital zircon derived from mafic or intermediate igneous rocks rather than evolved granites. This study demonstrates that the compositions of apatite and mafic mineral inclusions in zircon can be linked to matrix phases and to the bulk composition of the host rock; in contrast, inclusions of feldspar indicate that they crystallized from more evolved melts.
SAMPLES AND METHODS Samples were collected from ca. 520 Ma plutonic rocks from Dronning Maud Land, East Antarctica. Six samples (the Z7.10 series) were taken from a single outcrop at Hoggestabben Peak, which forms a complex intrusive body of mafic to evolved metaluminous compositions. They include a monzodiorite enclave (Z7.10.3) and a larger mafic domain (Z7.10.8), a mingling zone of quartz monzonitic composition (Z7.10.11) between a mafic dike and the monzonite, a granite dike (Z7.10.7), and a titanite-granite body (Z7.10.4), all of which intrude a large monzonite body (Z7.10.1). Additional samples were examined from an adjacent orthopyroxene (Opx) monzonite intrusion (Vedkosten Peak; Z7.3.1), and from the base of the Stabben Peak (monzonite Z7.41.1; ~50 km west of Hoggestabben). Whole-rock SiO2 = 53%–72% and all samples contain accessory apatite and zircon (see the GSA Data Repository1). Normalized REE plots (see the Data Repository) show negative or no Eu anomalies for all investigated samples, demonstrating that they do not represent cumulates, consistent with petrographic evidence. Inclusion-rich zircon grains were hand-picked from several magnetic fractions, polished and examined through cathodoluminescence and backscattered electron imaging, facilitating the discrimination against secondary inclusions and inherited cores. Each sample consisted of 18–32 grains (Table DR1 in the Data Repository), with a focus on inclusion-rich zircons. Suitable inclusions >5 μm in diameter with no (or occasionally minor) fractures were identified, and analyzed using a Cameca SX100 electron probe microanalyzer (EPMA), at 15 kV for silicate and oxide inclusions and 20 kV (1 μm beam) for apatite inclusions. Matrix minerals were analyzed from thin sections and mineral separates at 20 kV (defocused beam for apatite). Previous studies have suggested that halogen concentrations in apatite are characteristic of certain bulk rock compositions (Belousova et al., 2001; Piccoli and Candela, 2002). In this study, all apatites were identified as fluorapatite, but halogen concentrations are not quantitative due to analytical problems in measuring small apatite grains included in zircon (cf. Stormer et al., 1993). Wholerock major and trace element compositions were determined by X-ray fluorescence analysis using fused glass tablets and powder tablets on a Siemens SRS303 spectrometer. RESULTS Inclusion Assemblages By far the most abundant inclusion phase observed in zircon is apatite, which accounts for 20%–88% (mean 70%) of all monophase inclusions, and is also common in polyphase inclusions. In contrast, apatite crystals constitute 4 Ga zircons: Annual Review of Earth and Planetary Sciences, v. 37, p. 479–505, doi:10.1146/ annurev.earth.031208.100151. Hopkins, M., Harrison, T.M., and Manning, C.E., 2008, Low heat flow inferred from >4 Gyr zircons suggests Hadean plate boundary interactions: Nature, v. 456, p. 493–496, doi:10.1038/nature07465. Hopkins, M., Harrison, T.M., and Manning, C.E., 2010, Constraints on Hadean geodynamics from mineral inclusions in >4 Ga zircons: Earth and Planetary Science Letters, v. 298, p. 367–376, doi:10.1016/j.epsl.2010.08.010. Hoskin, P.W.O., Kinny, P.D., Wyborn, D., and Chappell, B.W., 2000, Identifying accessory mineral saturation during differentiation in granitoid magmas: An integrated approach: Journal of Petrology, v. 41, p. 1365–1396, doi:10.1093/petrology/41.9.1365. Hsieh, P.-S., Chen, C.-H., Yang, H.-J., and Lee, C.-Y., 2008, Petrogenesis of the Nanling Mountains granites from South China: Constraints from systematic apatite geochemistry and whole-rock geochemical and Sr-Nd isotope compositions: Journal of Asian Earth Sciences, v. 33, p. 428–451, doi:10.1016/ j.jseaes.2008.02.002. Kemp, A.I.S., Wilde, S.A., Hawkesworth, C.J., Coath, C.D., Nemchin, A., Pidgeon, R.T., Veervoort, J.D., and DuFrane, S.A., 2010, Hadean crustal evolution revisited: New constraints from Pb-Hf isotope systematics of the Jack Hills zircons: Earth and Planetary Science Letters, v. 296, p. 45–56, doi:10.1016/j.epsl.2010.04.043. Maas, R., Kinny, P.D., Williams, I.S., Froude, D.O., and Compston, W., 1992, The Earth’s oldest known crust—A geochronological and geochemical study of 3900–4200 Ma old detrital zircons from Mt Narryer and Jack Hills, Western Australia: Geochimica et Cosmochimica Acta, v. 56, p. 1281–1300, doi:10.1016/0016-7037(92)90062-N. Nutman, A.P., and Hiess, J., 2009, A granitic inclusion suite within igneous zircons from a 3.81 Ga tonalite (W. Greenland): Restrictions for Hadean crustal evolution studies using detrital zircons: Chemical Geology, v. 261, p. 77–82, doi:10.1016/j.chemgeo.2008.09.005. Piccoli, P.M., and Candela, P.A., 2002, Apatite in igneous systems: Reviews in Mineralogy and Geochemistry, v. 48, p. 255–292, doi:10.2138/ rmg.2002.48.6. Prowatke, S., and Klemme, S., 2006, Trace element partitioning between apatite and silicate melts: Geochimica et Cosmochimica Acta, v. 70, p. 4513–4527, doi:10.1016/j.gca.2006.06.162. Sha, L.K., and Chappell, B.W., 1999, Apatite chemical composition, determined by electron microprobe and laser-ablation inductively coupled plasma mass spectrometry, as a probe into granite petrogenesis: Geochimica et Cosmochimica Acta, v. 63, p. 3861–3881, doi:10.1016/S0016-7037(99)00210-0. Stormer, J.C., Pierson, M.L., and Tacker, R.C., 1993, Variation of F and Cl X-ray intensity due to anisotropic diffusion in apatite during electron-microprobe analysis: American Mineralogist, v. 78, p. 641–648. Veevers, J.J., 2007, Pan-Gondwanaland post-collisional extension marked by 650–500 Ma alkaline rocks and carbonatites and related detrital zircons: A review: Earth-Science Reviews, v. 83, p. 1–47, doi:10.1016/j.earscirev .2007.03.001. Watson, E.B., and Harrison, T.M., 1983, Zircon saturation revisited—Temperature and composition effects in a variety of crustal magma types: Earth and Planetary Science Letters, v. 64, p. 295–304, doi:10.1016/0012-821X (83)90211-X. Wilde, S.A., Valley, J.W., Peck, W.H., and Graham, C.M., 2001, Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago: Nature, v. 409, p. 175–178, doi:10.1038/35051550. Manuscript received 31 December 2010 Revised manuscript received 8 April 2011 Manuscript accepted 13 April 2011 Printed in USA
GEOLOGY, September 2011
GSA DATA REPOSITORY 2011250
Jennings et al.
Sample descriptions
Z7.3.1
Opx-bearing monzonite
Coarse grained rock with large (≈12 mm) euhedral perthite, plagioclase (An17) and amphibole crystals and smaller ‘messy’ mixtures of quartz, amphibole, magnetite, biotite (Mg# = 0.13) and Opx (En 10), showing complex reaction textures. Zircon forms clear and colourless grains, usually with rounded or fractured edges. Grains themselves are moderately fractured with occasional secondary in-fill material. Concentric zoning is seen in transmitted light; concentric and occasional sector zoning is seen in CL images. In some grains, irregularly zoned enclaves are seen, indicating localised recrystallisation. Distinct cores are rare. Z7.10.1 Monzonite Sample 10.1 is a coarse grained rock with large (10 mm) perthite phenocrysts and smaller plagioclase (An23), biotite (Mg# = 0.32) and amphibole grains (ferroedenite-ferropargasite, Mg# = 0.29) in the coarse-grained matrix. Interstitial sites contain anhedral biotite (X Mg = 0.33), amphibole, plagioclase (An03), K-feldspar, ilmenite, apatite, titanite and zircon. Zircon grains are clear, colourless and euhedral. Fractures are fairly common; these often contain abundant secondary material. CL images reveal that many grains contain distinct cores, although the majority of each grain volume is occupied by the most recent igneous growth zone, characterised by concentric and sector zoning. Some grains contain enclaves of blotchy, irregular zoning patterns. Z7.10.3 Monzodiorite This is a medium grained equigranular rock with 50% plagioclase (An34; occasionally zoned). Other matrix phases are biotite (Mg# = 0.44), amphibole (ferroedenite, Mg# = 0.44), augite (Di60) and orthoclase. A reactionary texture is seen; Cpx cores exist within replacement biotite and amphibole and Cpx crystals have a ragged appearance. Apatite and ilmenite are abundant throughout; zircon is rare. Zircons: Grains are clear, colourless and very fractured, resulting in most being small (5mm) surrounding feldspar and quartz. Zircon is occasionally found included in quartz and feldspar in the interstitial titanite. Zircon tends to be small and elongate. They are clear and colourless with occasional fracturing, sometimes accompanied by brownish secondary in-fill material. CL activity is very low so zoning is
difficult to determine, but appears sometimes concentric, sometimes irregular, implying some recrystallisation or replacement. Cores are present in some grains. Z7.10.7 Amphibole-bearing granite This sample consists of coarse grained microcline perthite (with occasional myrmekitic replacement at grain boundaries), quartz and plagioclase (An20), with a small proportion (
