Slow cooling of deep crustal granulites and Pb-loss in zircon

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We attempted to obtain the magmatic crystallization age of a metamorphosed (granulite facies, T = 800 ± 90°C; P = 8 ± 1 kbar) massif-type anorthosite from the Ankafotia body of southwest Madagascar. The sample studied is a coarse-grained leuconorite with good preservation of igneous texture and mineralogy, although plagioclase, which contains abundant rutile and zircon inclusions, has been slightly recrystallized. Thirty three isotope dilution U-Pb analyses of zircons representing single-grain fragments (29 analyses) and multi-grain fractions (4 analyses) yield a spectrum of concordant ages from 631 to 549 Ma, a time span of more than 80 myr. Back-scattered electron and cathodoluminescence images show that most grains are either homogeneous, structureless fragments (35%), or are permeated to a variable degree by anastamosing cracks occupied by relatively U- and/or Th-enriched zircon (45%); a smaller percentage of grains show relict magmatic zoning (20%). Thin, U- and/or Th-rich overgrowths occur on about 25% of grains. Raman spot analyses demonstrate that all fragments are highly crystalline and non-metamict. There are marked correlations between zircon grain size and internal features, such that the oldest grains are larger, and show relict magmatic zoning; the youngest grains are small fragments containing high-U crack networks. Ion microprobe spot analyses show that each zircon grain preserves a distinct trace element signature; rare earth element patterns show heavy REE-enrichment, with negative Eu anomalies and positive Ce anomalies. We suggest that the ca. 80 myr spread in concordant U-Pb ages in this sample is indicative of high-temperature Pb-loss during one or more protracted periods of granulite facies metamorphism, with only minor episodic or continuous metamorphic zircon growth. Volume diffusion and/or fracture-assisted diffusion seems to be the dominant mechanism of Pb-loss. Cooling curves, calculated using recently-measured Pb diffusion parameters, conform to the age-size relationship, and imply very slow cooling rates (1–2°C/myr or less), as might be expected for a terrane in which granulite conditions were maintained for an extended period of time. Our results, therefore, suggest a note of caution for interpretation of concordant zircon ages in meta-igneous rocks affected by high-grade metamorphism of long duration.