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Journal of Petrology | Volume 43 | Number 6 | Pages 1089-1104 | 2002
© Oxford University Press 2002
Inherited and Magmatic Zircon from Neogene Hoyazo Cordierite Dacite, SE Spain—Anatectic Source Rock Provenance and Magmatic Evolution
In Memoriam Professor Chris Powell, 
2001.07.21
1TECTONICS SPECIAL RESEARCH CENTRE, UNIVERSITY OF WESTERN AUSTRALIA, PERTH, AUSTRALIA
2RESEARCH SCHOOL OF EARTH SCIENCES, INSTITUTE OF ADVANCED STUDIES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, ACT 0200, AUSTRALIA
Received February 28, 2001; Revised typescript accepted January 14, 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGALMANDINE-GRAPHITE-BEARING...ZIRCON U-Th-Pb ANALYSESINTERPRETATIONCONCLUSIONSREFERENCES |
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Almandine–graphite-bearing biotite–cordierite dacite from Cerro del Hoyazo, southern Spain, represents erupted magma produced by Late Alpine anatexis of high-grade pelitic/quartzo-feldspathic paragneisses. U–Th–Pb ion microprobe analysis of inherited/detrital zircon cores from the dacite reveals five principal age groups, 2·8–2·5 Ga, 2·1–1·9 Ga, 1·1–0·9 Ga, 650–550 Ma and 350–320 Ma, indicating a Gondwana domain provenance. Some of the cores are surrounded by a thin euhedral overgrowth which has the same age as a suite of sharply euhedral new-grown zircons in the dacite, 6·33 ± 0·15 (t
) Ma. This zircon precipitated from the melt during or just before eruption of the magma. A foliated graphite-bearing cordierite–plagioclase–almandine–sillimanite–biotite restite rock inclusion contains inherited zircon cores similar in age to those in the dacite, consistent with the syngenetic melt–restite relationship between the magmatic melt and its Al-rich rock inclusions. Overgrowths on the enclave zircon cores have an age of 8·34 ± 0·45(t) Ma, which probably records an early stage of the ultra-metamorphism leading to the generation of the anatectic magma body. The inheritance age patterns of both dacite and restitic enclave, and zircon ages from the basement below the Cerro del Hoyazo, suggest that the anatectic source complex was probably a high-grade amphibolite facies equivalent of either the Palaeozoic schist complex of the basal nappe section or, more likely, its Permo-Triassic cover section. KEY WORDS: crustal anatexis; post-collisional magmatism; zircon inheritance; Alpine Betic–Rif belt
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGALMANDINE-GRAPHITE-BEARING...ZIRCON U-Th-Pb ANALYSESINTERPRETATIONCONCLUSIONSREFERENCES |
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Cerro del Hoyazo cordierite dacite is a classic S-type magmatic rock, well known for its abundant, extremely Al-rich rock inclusions. Initially the rock was thought to have originated from a country rock contaminated andesitic magma (e.g. Burri & Parga-Pondal, 1936
). Current modelling, however, indicates that it represents an erupted anatectic magma generated by partial melting of a pelitic/quartzo-feldspathic high-grade gneiss complex (Zeck, 1968, 1970, 1992; Munksgaard, 1984; Zeck et al., 1999); the Al-rich enclaves represent the complementary syngenetic restite material trapped within the anatectic dacitic melt.
The present paper reports the use of ion microprobe U–Pb zircon dating to test the model of anatectic magma production, and to infer the anatectic source rock complex and the timing of the magmatic evolution from anatexis to eruption. The zircon U–Pb isotopic system has a very high closure temperature (Lee et al., 1997) and has been shown to survive anatexis (e.g. Köppel & Grünenfelder, 1971). The inherited/detrital zircon age spectra for the dacite and one of its syngenetic restite enclaves are compared with those from Alpine nappe core complexes (Zeck & Whitehouse, 1999; Zeck & Williams, 2001), to try and identify the anatectic source rock complex. Dating the zircon precipitated from the dacitic melt provides information on the timing of the Neogene magmatic evolution.
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGALMANDINE-GRAPHITE-BEARING...ZIRCON U-Th-Pb ANALYSESINTERPRETATIONCONCLUSIONSREFERENCES |
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The Cerro del Hoyazo dacite occurrence is part of the Alborán volcanic province (Fig. 1), which encompasses the Neogene volcanic and minor plutonic rocks in southeastern Spain and northern Africa, and on the sea floor and a few islands in the intervening Alborán region. Magmatism post-dates the continental collision in the tectonic evolution of the Alpine Betic–Rif belt (Zeck, 1996
, 1997, 1999; Zeck et al., 1998, 1999), the main volcanic activity having taken place within the short period between 12 and 10·5 Ma (Zeck et al., 2000), long after final allochthon emplacement by extrusional/extensional tectonics through the period 22–18 Ma (Zeck, 1996; Zeck & Williams, 2001).
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Cerro del Hoyazo forms a small hill (X/Y UTM = 573350/4091070) at the foot of the southern slope of the Sierra Alhamilla, isolated 10–20 km inland of the major calc-alkaline volcanic range of Sierra de Gata. Its name (from Spanish hoya –pit and -azo –large) refers to the peculiar morphology of the outcrop, which consists of an almost perfectly round, 
1 km2 central depression floored by well-exposed volcanic rocks, surrounded by an escarpment of overlying Pliocene limestones. The volcanic body consists of a small central pipe with vertical flow-structured massive rock surrounded by its eruptive equivalents in block and ash facies (Zeck, 1968, 1970). The Pliocene limestones represent reef and reef-debris material deposited 6 Myr ago on the eroded remnants of the hill formed earlier by the volcanic eruption (Dabrio et al., 1981).
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ALMANDINE–GRAPHITE-BEARING BIOTITE–CORDIERITE DACITE OF CERRO DEL HOYAZO |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGALMANDINE-GRAPHITE-BEARING...ZIRCON U-Th-Pb ANALYSESINTERPRETATIONCONCLUSIONSREFERENCES |
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The petrography, and chemical and isotopic compositions of the Hoyazo dacite have been documented in detail by Zeck (1968
, 1970, 1992), Munksgaard (1984) and Zeck et al. (1999). Where unaltered, the dacite consists of 50 vol. % glass matrix, 27% crystallization products from the melt (10% plagioclase, 9% cordierite, 8% biotite, plus small amounts of sillimanite, zircon and apatite) and 20–25% rock inclusions and isolated crystals derived from them. The rock inclusions consist of Al-rich restite (13%) and intermediate igneous rocks (8%). The latter may reach a size of 0·5 m and represent magmatic material enclosed/mingled into the dacitic melt. The Al-rich (20–35% Al2O3) restitic enclaves (Fig. 2) range in diameter from a few millimetres up to 1 m. Most are foliated and have highest amphibolite facies parageneses (quartz or plagioclase—never the combination—with almandine, biotite, cordierite, graphite, sillimanite and spinel; muscovite and K-feldspar are notably absent). Crystals of graphite (100–200 µm diameter), cordierite (0·5–20 mm), garnet (2–10 mm) and quartz (0·1–8 mm) found within the dacitic glass matrix have been derived from these restitic enclaves.
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The dacite has whole-rock 
18O values in the range 13·1–15·6
, and the Al-rich restitic enclaves are very similar, 13·0–16·2 (Munksgaard, 1984). Whole-rock Sr isotopic relations in this Neogene volcanic rock are remarkable, defining an isochron age of 210 ± 17 (2) Ma [mean square weighted deviation (MSWD) = 2·4] and an initial 87Sr/86Sr of 0·7100 ± 3 (Munksgaard, 1984). The 210 Ma age is thought to represent a diagenetic event in the sedimentary protogenetic complex that much later underwent Alpine high-grade metamorphism and anatexis (Zeck et al., 1999). Preservation of the high-age Rb–Sr isochron age suggests that effective homogenization, including restite–melt repartitioning on a larger than sample scale, has not taken place during subsequent migration of the Hoyazo dacitic magma.
The mineralogical, chemical and isotopic observations summarized above argue convincingly against country rock contamination and in favour of magma genesis through anatexis of a high-grade metamorphic protolith of pelitic to quartzo-feldspathic composition (Zeck, 1968, 1970, 1992, 1996; Munksgaard, 1984; Zeck et al., 1999), producing a dacitic melt and complementary, syngenetic restite. The high abundance of restite material in the rock indicates that (a major part of) the restitic material was not separated from the melt phase of the magma during magma extraction and transport.
Three samples were selected for zircon analysis, two of the dacite (samples 95Z96 and 96Z46) and one of a foliated, graphite-bearing almandine–cordierite–biotite–sillimanite rock enclave (sample 95Z97), the most common type of restite (Fig. 2). As restitic material on all scales, down to individual crystals, is widely distributed throughout the dacite itself, the dacite samples included not only glass and precipitation products from the melt, but also large amounts (15–20 vol. %) of restite.
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ZIRCON U–Th–Pb ANALYSES |
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Sample preparation and analytical procedures
The two dacite whole-rock samples (95Z96 and 96Z46) represent 60–80 kg blocks, which were crushed and cleaned by hammer chipping in the field. The resulting
6 kg samples consisted of 2–10 cm fragments, which were cleaned in an ultrasonic bath with de-ionized water, dried and crushed in a jaw crusher to a size of 3–10 mm. Approximately 1 kg of this material was crushed in a swing mill in several steps interspersed with screening to <0·3 mm. The restitic enclave (sample 95Z97) was collected in toto (30 kg) and a slice of 4 kg was cut off, cleaned and crushed as for the two dacite samples. The <0·3 mm crushings were panned, and the resulting heavy fraction was purified by heavy liquid (methylene iodide) and magnetic separation. Care was taken not to bias the zircon samples by removing the more magnetic grains. Final purification was by hand sorting.
Zircon yields ranged from several hundred grains from each dacite sample to several thousand grains from the enclave. Samples were mounted separately for SHRIMP analysis; 300 grains from each dacite sample and >1000 grains from the enclave were cast in epoxy, along with crystals of reference zircons AS3 and SL13, then polished to their centres. Before ion microprobe analysis the zircon was documented by optical photomicrography and SEM cathodoluminescence (CL) imaging, and the mounts were coated with 5 nm of high-purity gold.
Zircons were analysed for U–Th–Pb isotopes on the ANU SHRIMP II ion microprobe according to procedures described by Williams & Claesson (1987) and Williams et al. (1996). A 10 kV primary beam of negative O2 ions was focused to a 25 µm diameter spot on the sample surface. Positive secondary ions were extracted at 10 kV and analysed at 5000R by pulse counting using a single electron multiplier and cyclic magnet field switching. Each five-scan analysis took about 13 min. Pb isotopic ratios were measured without fractionation correction. Interelement fractionation was calibrated against zircon standard AS3 (radiogenic 206Pb/238U = 0·1859: Paces & Miller, 1993) or SL13 (radiogenic 206Pb/238U = 0·0928: Claoué-Long et al., 1995).The coefficient of variation for multiple measurements of the standard Pb/U was <0·5% for each analytical session. This uncertainty is included in the errors given on the reported ages. Common Pb contents were mostly very low, consistent with minor laboratory contamination of the sample surface. The few analyses that showed significantly higher common Pb were corrected assuming a common Pb composition commensurate with the age of the target zircon (Cumming & Richards 1975). Most common Pb corrections were made using the measured 204Pb, but for many analyses, particularly those with low Th/U, significantly better precision could be achieved using 208Pb and Th/U. In those cases, radiogenic 208Pb/206Pb and 208Pb/232Th were not determined independently. Calculated U, Th and Pb concentrations assumed 238 ppm U in SL13.
The U–Th–Pb analyses are listed in Table 1 and plotted on Tera–Wasserburg concordia diagrams in Figs 4, 6, 7 and 8 (see below). Uncertainties listed and plotted are 1 SE precision estimates, calculated from counting errors and the propagation of uncertainties introduced by the common Pb correction. Inferred ages are the best estimate of the age from each analysis, taking into account the age, degree of discordance, and size of the common Pb correction. For zircons with concordant ages <1·5 Ga, the inferred age is based on radiogenic 206Pb/238U and assumes concordance. For discordant zircons, and those older than 1·5 Ga, the inferred age is based on radiogenic 207Pb/206Pb. Inaccuracies introduced by discordance and imprecise common Pb corrections have thereby been minimized. Ages were calculated using the constants recommended by the IUGS Subcommission on Geochronology (Steiger & Jäger, 1977) and, except in the data table, the uncertainties cited are 95% confidence limits, namely t, where t is Student’s t.
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, before correction; 
, after), see text and Williams & Hergt (2000)
, all other analyses. The outlier at 238U/206Pb
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Cordierite dacite
Zircon morphology and zoning
The range of zircon crystal morphologies and growth structures in the two dacite samples 95Z96 and 96Z46 (Fig. 3) is the same. Most grains are clear, small to medium sized (50–100 µm diameter) and weakly coloured. Many have a dusting of fine inclusions, others are relatively inclusion-free. Only in a few grains is growth zoning visible in transmitted light. Zircon grains are either subhedral and equant or sharply euhedral and prismatic.
Subhedral, equant grains comprise 80% of the zircon population. The crystal terminations may appear rounded, but some grains in fact have multiple, although commonly poor quality, facets. CL imaging reveals a wide variety of internal growth structures. Most of the grains consist of a very large equant core (>90% of the grain diameter) surrounded by a thin composite rim (Fig. 3E–R). Many of the cores have well-developed, simple, fine euhedral zonation, usually with moderate luminescence (Fig. 3F, J, K and M). Core–rim boundaries are commonly embayed (Fig. 3E, F and J–Q). Sharp truncations of the core zoning indicate that most of the cores are broken or abraded fragments of originally much larger grains (Fig. 3J, K, M, P and Q). The same is true of the rare grains without rims (Fig. 3S). As abrasion cannot account for the embayed surfaces, many of the cores must have been partially dissolved or marginally recrystallized subsequent to the rounding.
The rims on the cores are structurally complex (Fig. 3E–M). Most consist mainly of a very thin (<10 µm) layer of strongly luminescent, structureless zircon (Fig. 3J, K and O–R); locally its thickness may reach 40 µm (Fig. 3N). In some crystals this layer abuts the core (Fig. 3E, F and R), but in most the two are separated by a very thin (<5 µm) discontinuous, weakly luminescent intermediate zone (Fig. 3G–K and M–Q). The strongly luminescent layer fills embayments into the cores and intermediate layers, and its outer surface is commonly itself embayed (Fig. 3J–L, P and Q). In about 10% of the subhedral, equant grains the composite rim is completed by an additional, different overgrowth. It ranges in thickness from a few microns to tens of microns, is discontinuous, weakly luminescent and usually shows some euhedral oscillatory zoning (Fig. 3E–I and L–M). In contrast to the earlier layers/rims, this final overgrowth forms sharply euhedral crystal faces. This youngest zircon in places contains small glass-filled embayments (Fig. 3 E and G).
Sharply euhedral, commonly highly prismatic grains comprise the remaining 20% of the zircon population (Fig. 3A–D). Some have aspect ratios greater than 10:1, but it is difficult to assess the original size distribution because most grains have been broken, presumably during sample preparation. The terminations on the euhedral grains range from blunt {011} faces (Fig. 3A) to sharply pointed {211} faces (Fig. 3B and C). CL imaging reveals internal structures that are totally different from those in the subhedral, equant grains. More than 95% of the elongate grains have uniformly very weak luminescence and show almost no internal zoning (Fig. 3A). Where zoning is visible, it is very broad, simple and prism-parallel. A few grains with slightly stronger luminescence contrast show weak sector zoning (Fig. 3D). One crystal (46-3) is distinctive in being finely euhedrally zoned (Fig. 3C).
Analytical results
Sixty-four analyses from 60 crystals, representing 54 cores (both from rimmed and rimless grains), one strongly luminescent marginal layer and three euhedral overgrowths on subhedral, equant grains, and six sharply euhedral, prismatic crystals from the two dacite samples are listed in Table 1 and plotted on Tera–Wasserburg concordia diagrams in Figs 4 and 6. One analysis (96-16.1), an attempt to analyse a thin strongly luminescent rim, straddled a core–rim boundary and is not considered further.
The subhedral, equant crystals have a wide range of U and Th contents, Th/U, and isotopic compositions, as would be expected from their variable, complex structures and wide range in luminescence response. U in the cores ranges from very low to high (40–2200 ppm), as does Th (10–700 ppm) and Th/U (0·02–2·8). The one strongly luminescent layer/rim analysed (96-14.1) has moderately low U (280 ppm ) and Th/U (0·25). Three weakly luminescent, euhedral overgrowths have consistently high U contents (1490–4520 ppm), and low to moderate Th/U (0·07–0·23).
Figure 4 shows the 54 core analyses from both dacite samples plotted on a modified Tera–Wasserburg concordia diagram—natural logs of the isotopic ratios are plotted instead of the ratios themselves to make the dispersion on each axis per unit time more uniform. The main features of the dataset are the wide range of isotopic compositions, the near concordance of most of the analyses and the tendency of the data to fall into clusters. The cluster pattern is displayed more clearly in a relative probability diagram (Fig. 5a). In this type of diagram the age inferred from each analysis (Table 1) is allocated a Gaussian curve of unit area, which is scaled to reflect the uncertainty on the analysis; then the curves are summed.
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Six cores have ages in the range 2·8–2·5 Ga; two grains have the same radiogenic 207Pb/206Pb within analytical uncertainty, 0·1638 ± 0·0012 (), equivalent to an age of 2495 ± 25(2) Ma, whereas four grains have dispersed apparent ages in the range 2·80–2·65 Ga. The prominent bimodal cluster at 2·0 Ga is composed of 13 analyses, nearly a quarter of the total sample. Five of the six analyses in the older group are equal within analytical uncertainty, the weighted mean radiogenic 207Pb/206Pb, 0·12682 ± 0·00042() giving an age of 2054 ± 16(t) Ma. One of the analyses in the younger group (46-12.1) is >10% discordant, and might be part of the older group. The remaining six analyses, dispersed just slightly more than can be explained by the analytical uncertainties, have a mean radiogenic 207Pb/206Pb of 0·11465 ± 0·00061(), equivalent to an age of 1874 ± 24(tobs) Ma.
The prominent cluster at 1·0 Ga is composed of 10 analyses, nearly 20% of the total sample. The range of ages is far greater than can be explained by analytical error on a central value. There are not really enough analyses to assess this population statistically, but nevertheless, modelling the mixture using the Galbraith method (Sambridge & Compston, 1994) suggests that it is composed of two dominant groups with 206Pb/238U ages of 1037 ± 35(t) Ma and 990 ± 32(t) Ma, plus a minor group at 930 Ma. The complex cluster at 600 Ma, composed of 17 analyses (nearly one-third of the dataset), also must be composite. Three analyses fall outside the main group (one above, two below). The distribution of the remainder is bimodal, consisting of 625 ± 12(t) Ma and 555 ± 10(t) Ma components.
The six core analyses within the range 350–320 Ma also show a greater variation in radiogenic 206Pb/238U than can be explained by analytical error alone. Mixture modelling suggests that two of the grains have a mean age of 346 Ma, whereas the remainder are equal within uncertainty at 324 ± 7(t) Ma. Two core analyses yield much younger ages, 295 ± 12(t) Ma and 267 ± 30(t) Ma.
Only one attempt to analyse the thin, strongly luminescent part of the composite rims was successful (96-14.1, Fig. 3N), yielding a near-concordant age of 1058 ± 14() Ma, compared with 1848 ± 12() Ma for the core of the grain.
The sharply euhedral acicular grains (six analyses) show <10 Ma Neogene ages, similar to those of the euhedral, weakly luminescent overgrowths (three analyses) on the subhedral, equant grains. Therefore these nine analyses can be considered together. The U and Th contents and Th/U ratios vary more than expected, considering the similarity in morphology and CL zoning. Of the six acicular grains four show a similar range of compositions with rather high U (750–4320 ppm), Th (410–3920 ppm) and Th/U (>0·55). The remaining two grains are different. One grain (46-3; Fig. 3C), unlike the rest, has well-developed oscillatory growth zoning and relatively low U and Th (415 and 150 ppm, respectively). The other grain (46-2; Fig. 3A) appears unzoned and has extremely high U and Th contents (16 400 and 23 500 ppm, respectively). Compared with the bulk of the acicular grains the three euhedral overgrowths on subhedral, equant grains show generally higher U contents, lower Th contents and distinctly lower Th/U (0·07–0·23).
Plotted on a Tera–Wasserburg concordia diagram before correction for common Pb, the analyses of the Neogene zircon do not simply fall on a mixing line between common and radiogenic Pb (Fig. 6). First, the strongly oscillatory zoned grain (sample 46-3), at a corrected age of 9·5 ± 0·5 () Ma, is significantly older than the others. Second, the eight analyses of the other grains and overgrowths, although more nearly collinear, still scatter beyond error. The main outliers are the two analyses with the highest U contents (samples 46-2.1 and 46-10.2). These determinations are probably affected by a matrix-related bias in the measurement of Pb/U, similar to that documented in SHRIMP analyses of very U-rich zircon from Tasmanian dolerites (Williams & Hergt, 2000). That study found Pb/U to be overestimated by a factor that increased at the rate of between 1·5 and 2·0% per 1000 ppm for U contents over 2500 ppm. Correction of the present analyses for this effect eliminates the scatter (Table 1, Fig. 6). Using the expected radiogenic 207Pb/206Pb to calculate each common Pb correction accordingly yields eight estimates of radiogenic 206Pb/238U that are equal within analytical uncertainty with a weighted mean of 0·000982 ± 0·000008(). This is equivalent to an age of 6·33 ± 0·15(t) Ma, which we regard as the best estimate of the age of the zircon precipitated from the dacitic melt.
Restitic enclave
Zircon morphology and zoning
The large zircon yield from the foliated graphitic almandine–plagioclase–sillimanite–biotite restitic rock inclusion sample 95Z97 consists mainly of 50–180 µm, equant (length/width <3), subhedral (to euhedral), clear grains with few inclusions and very little zoning visible in transmitted light. CL imaging reveals that virtually all grains consist of a core, which ranges in size from 20 to 90% of the grain diameter, surrounded by a rim, which ranges in width from a few to tens of microns. Some very rare (<1%), small zircon crystals are acicular and weakly luminescent throughout.
The zircon cores of the subhedral crystals mostly have complex internal growth structures (Fig. 3a–l) with abrupt truncations at the rim interface, indicating that the cores represent fragments of originally much larger grains. Their luminescence ranges from very weak to very strong, indicating a wide range of chemical compositions. The outer surfaces of the cores can be angular or rounded, but in nearly all cases are convex.
The rims mainly consist of strongly luminescent zircon, which is either homogeneous or nebulously zoned (Fig. 3a–l). In many crystals the inner part of the rim consists of a very thin, irregular layer of weakly luminescent zircon. Commonly the outer surface of this layer is serrated or embayed, suggesting a history of dissolution, replacement and/or recrystallization. The outer surface of the strongly luminescent rim is locally irregular (e.g. Fig. 3c, f and g), but more commonly has well-developed crystal faces. Many grains on which this overgrowth is thick have a sharply euhedral shape (Fig. 3a, b, d, h, k and l).
This zoning pattern has some marked similarities to that seen in the subhedral, equant zircon grains from the dacite samples (Fig. 3). The difference is that the final euhedral overgrowth on the zircon grains from the restitic enclave is strongly luminescent, whereas in the dacite zircon it is weakly luminescent. Also different from the dacite is the rarity of zircon grains without rims.
Analytical results
Thirty-seven zircon grains from the restitic enclave were analysed, five of them in two places (Table 1). Thirty-two of the analyses were of cores and nine of strongly luminescent overgrowths. One analysis (sample 97-23.1) straddled a core–rim boundary and will not be considered further. None of the small, weakly luminescent, acicular crystals was analysed. Analytical results are listed in Table 1 and plotted on Tera–Wasserburg concordia diagrams in Figs 7 and 8.
The zircon cores show a wide range of chemical and isotopic compositions. U contents range from 70 to 1590 ppm, Th from 3 to 835 ppm, and Th/U from 0·007 to 1·16. This reflects the diversity of origin of the cores, which is confirmed by their wide range of isotopic compositions (Fig. 7). With one notable exception (97-9.1), the analyses are broadly concordant and distributed over an age range similar to that for the cores of the zircon crystals from the dacite samples. The age distribution is best demonstrated in the relative probability plot of inferred ages (Fig. 5b). The older ages range from 2·9 to 1·8 Ga with no apparent clustering. There is some clustering at 1·0 Ga (five ages). Four of those ages are statistically indistinguishable, defining a weighted mean age of 1072 ± 42(t) Ma; the other is 992 ± 44(2) Ma. The age cluster at 600–500 Ma is the most prominent in the dataset, comprising 15 analyses, or about 50% of the analysed cores. Mixture modelling (Sambridge & Compston, 1994) shows that two age groups dominate, one (five grains) at 615 ± 20(t) Ma, and the other (seven grains) at 568 ± 10(t) Ma. There is only one Permo-Carboniferous zircon core age in the dataset (350 Ma; 97-13·1).
The strongly luminescent zircon rims (nine analyses) show very little compositional variation. U contents are all low (200–270 ppm), and Th contents are very low (mostly 
1 ppm), resulting in very low Th/U ratios (mostly 0·005). The resultant low radiogenic Pb contents (0·3 ppm) mean that, although the amounts of common Pb measured are small, two of the corrections for common Pb are relatively large (27% and 46%, respectively). Plotted on a concordia diagram before common Pb correction, the data fall within analytical uncertainty on a simple mixing line between common and radiogenic Pb (Fig. 8). Correction for common Pb, assuming concordance (namely using 207Pb/206Pb), yields estimates of radiogenic 206Pb/238U that are equal within analytical uncertainty, with a weighted mean of 0·001294 ± 0·000030 (). This is equivalent to an age of 8·34 ± 0·45(t) Ma, which represents the best estimate for the age of Alpine zircon crystallization in the Hoyazo restitic material.
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Inherited/detrital zircon
Crystal morphology and internal zoning patterns of the subhedral, equant zircon crystals in both the dacite samples and the restitic enclave show that their cores—or the crystals in toto, where unrimmed—represent clastic, abraded material inherited from the sedimentary precursor of the anatectic source rocks from which the dacitic magma was derived. Therefore, the inherited zircon provides direct information on the provenance of that precursor and also places a limit on its age of deposition.
Two main factors determine the apparent ages found for the individual shot points: (1) the magmatic/metamorphic age of the zircon before its sedimentary/clastic history; (2) any (partial) isotopic resetting during subsequent tectono-metamorphic events. Recent investigations of zircon from Hercynian basement rocks from the Alps (Vavra et al., 1999) indicate that isotopic resetting (see Pidgeon, 1992) may be widespread. Commonly the resetting is associated with annealing/recrystallization, which causes both the localized loss of trace elements, such as radiogenic Pb, and growth zoning from the affected parts of the zircon grains (Zeck & Whitehouse, 2002). However, these features are not prominent in the Hoyazo material, where they are restricted to thin zones within the zircon crystals (Fig. 3). Such a zone was wide enough to be analysed only in one crystal (Fig. 3N, 96-14) and the resulting ages indeed suggest that the strongly luminescent zone represents a Grenvillian (1060 Ma) recrystallization effect in a Palaeoproterozoic (1850 Ma) crystal.
It thus seems likely that the five main age groups—Late Archaean, Palaeoproterozoic, Grenvillian, Pan-African and Hercynian—found in the inherited zircon from the two dacite samples and the restitic enclave (Fig. 5a and b) represent discrete metamorphic/magmatic episodes in the region from which the protolithic sediment ultimately was derived (more than one cycle of sedimentation is possible). Delimiting the timing of these episodes precisely, however, is not an easy task—even more so for the discrete events within these episodes. Effectively screening out the partially reset individual ages is difficult in a dataset as small as the present one.
Zircon inheritance pattern in the Hoyazo dacite and its restitic enclave
The inherited zircon age patterns in the two dacite samples are very similar (Fig. 4). There are differences, however, such as a greater relative abundance of 2·8–2·5 Ga and 1·1–0·9 Ga zircon in sample 95Z46. The small size of the two zircon age samples does not allow a firm statistical decision on the null hypothesis for the two mixed zircon age populations. Therefore it cannot be excluded that the differences are real, implying that inherited zircon of different ages is not uniformly distributed throughout the dacitic body. The latter would be consistent with a recent study of 143Nd/144Nd and 87Sr/86Sr isotopic systems in Neogene volcanic rocks of SE Spain (Zeck et al., 1999), which concluded that the 210 Ma Rb–Sr whole-rock isochron for the Neogene Hoyazo dacite was best explained by the anatectic dacitic magma having preserved chemical and isotopic heterogeneity inherited from a lithologically heterogeneous source rock complex. The implication is that the anatectic magma was not homogenized, either before or during its eruption. The small differences we find in the inherited zircon from the two dacite samples thus might, or might not, reflect lithological variation within the protogenetic sedimentary rock complex.
Whichever of the two hypotheses is correct, the inheritance pattern for the combined dacite samples (Fig. 5a) provides a picture of the zircon age spectrum of the source rock complex as a whole. The inheritance pattern for the restitic enclave shows a close similarity to that of the dacite, but there are also minor differences (Fig. 5a and b). Grenvillian ages in both age samples form prominent clusters, both of which are bimodal, namely 1037 ± 35/1072 ± 42(t) Ma and 990 ± 32/992 ± 44(t) Ma for dacite/restite, respectively. The two events in the Pan-African cluster are even more similar, 625 ± 12/615 ± 20(t) Ma and 555 ± 10/568 ± 10(t) Ma for dacite/restite, respectively. However, the Pan-African age group in the restite is much more prominent (50% of all core ages) than in the dacite (30%). Other differences are the absence of any Late Archaean and Palaeoproterozoic clustering in the restitic enclave (eight ages are evenly distributed through the range of 2·9–1·8 Ga) and the scarcity of Hercynian ages in the restite—only one (350 Ma) age, compared with eight ages in the range 350–270 Ma in the dacite.
The interpretation of these similarities and differences in the zircon inheritance patterns for the dacite and the restitic enclave must take into account the limitations imposed by the small sample sizes. We consider it likely that the differences are indeed a sampling effect and thus do not negate the possibility that the zircon populations in dacite and restite are identical. However, the alternative that the differences are real cannot be excluded. This alternative interpretation would imply that the parts of the source rock complex that partially melted to yield the bulk of the melt fraction of the magma had a slightly different provenance from those that grew more restitic.
Possible anatectic source rock complexes in the basement underlying the Hoyazo dacite—zircon provenance analysis
The Hercynian detrital zircon content of the Hoyazo dacite/restite and its pelitic/arenaceous anatectic derivation preclude rock complexes from the deeper crustal, pre-Hercynian basement as anatectic source rocks. That leaves rock complexes from the Alpine nappe pile, or perhaps the upper part of the sub-nappe basement, which may harbour similar lithologies. The well-known lithostratigraphy of the Alpine collisional nappes (e.g. Nijhuis, 1964; Zeck, 1968; Egeler & Simon, 1969; Fontboté & Vera, 1983; Sanz de Galdeano, 1997) features two major rock complexes that have the quartzo-feldspathic/pelitic composition required for the Hoyazo-type anatexis: the basal nappe section and the pelitic/arenaceous part of its cover section. The basal nappe section consists mainly of medium-high grade pelitic and quartzitic schists. Its zircon age spectrum, comprising inherited/detrital, metamorphic as well as magmatic ages (Zeck & Whitehouse, 1999; Zeck & Williams, 2001), shows that these schists are Hercynian–Alpine polymetamorphic and that their sedimentary parent material had a Palaeozoic (600–300 Ma) age of deposition. The zircon age spectra from these two studies are illustrated in two relative probability histograms (Fig. 5c and d). The cover section within the Alpine nappe stratigraphy consists predominantly of Mesozoic sedimentary rocks subjected to variable degrees/grades of Alpine metamorphism. Its basal part has a Permo-Triassic (290–230 Ma) deposition age (e.g. Egeler & Simon, 1969; Fontboté & Vera, 1983; Sanz de Galdeano, 1997) and consists mainly of phyllites, pelitic schists and quartzites; no zircon age information is available.
The zircon age distributions in the Hoyazo dacite/restite and the two basement samples (Zeck & Whitehouse, 1999; Zeck & Williams, 2001) have close similarities (Fig. 5). The minor differences—a few basement ages in the range 1·5–1·4 Ga that are not found in the dacite/restite and the larger abundance of Late Archaean ages in the dacite/restite—could very well reflect a sampling effect. This suggests that the basal schist complex of the Alpine nappes is a feasible anatectic source rock complex for the Hoyazo dacite. However, the data do not exclude the Permo-Triassic cover complex, the sedimentary parent material of which was probably derived mainly from the Hercynian basement, inheriting the major trends in its zircon inventory in the process. The youngest inherited/detrital cores in the Hoyazo dacite/restite could provide a means of choosing between these two options, as the deposition of the complex’s sedimentary precursor must post-date the youngest detrital zircon. The youngest inherited zircon age from the Hoyazo dacite/restite is 267 ± 30(2) Ma. The next youngest grain has an age of 295 ± 12(2) Ma. The youngest cluster is at 324 ± 7(t) Ma. Youngest detrital zircon crystals/cores from the schist complex range in age from 350 to 300 Ma (n = 7; Zeck & Whitehouse, 1999; Zeck & Williams, 2001). Thus, among the rounded/equant, detrital grains from the three reservoirs there is only a single grain, moreover with a large error, which argues for rejecting the basement option. Zeck & Williams (2001) found also Hercynian, in situ formed zircon, 305 ± 3(t) Ma (n = 13), occurring as 50 µm, euhedral metamorphic grains, in basement schists only a few kilometres north of Cerro del Hoyazo. Such grains are absent from the Hoyazo dacite/restite, which argues for rejection of the basement option and at the same time favours the Permo-Triassic as source rock complex, as these delicate grains hardly could survive a clastic-, clay- and sand-producing sedimentary cycle.
We conclude that the zircon age evidence at present available to discriminate between two possible source rock complexes for the Hoyazo anatectic dacite is not conclusive. The combined evidence seems to favour an amphibolite facies equivalent of the Permo-Triassic complex found in the cover section of the Alpine nappe stratigraphy. However, rock complexes corresponding to its Palaeozoic basal schist complex remain a feasible alternative.
Newly formed, Neogene magmatic zircon
Melt-precipitated zircon in the dacite comprises both whole new grains and overgrowths. Although yielding statistically indistinguishable ages with a mean of 6·33 ± 0·15(t) Ma, these two forms of melt-precipitated zircon are chemically distinct—Th/U in the overgrowths is <0·23, whereas in the large new-grown grains it is >0·55. Apparently two zircon types have formed at about the same time, but not in the same environment. Possibly the overgrowths precipitated on restitic cores slightly earlier than the whole new grains were formed from the melt phase as the magma became zircon saturated during cooling. Glass-filled embayments in the overgrowths indicate latest stage zircon corrosion within the dacitic melt.
The 8·34 ± 0·45(t) Ma age of the zircon overgrowths from the restitic enclave, significantly older than the zircon precipitated in the dacite itself, provides further evidence for episodic zircon growth. Their distinctive, extremely low Th/U (most 0·005), suggests growth under high-grade metamorphic conditions (Williams & Claesson, 1987), probably in the presence of a Th-rich mineral such as monazite. These zircon overgrowths may record an early stage of the ultrametamorphism producing the anatectic melt, with magmatic eruption following 2 Myr later.
The age of 9·5 ± 1·0(2) Ma measured on a single grain (46-3) from the dacite is older still than the restitic zircon precipitation. The difference is not due to any known analytical bias (U = 414 ppm), and the grain also has a growth structure (Fig. 3C) and composition (Table 1) unlike any of the 6·3 or 8·3 Ma grains/overgrowths. Its fine euhedral zoning, low U and intermediate Th/U suggest that it grew under igneous, not metamorphic conditions. We propose that the grain was produced during an earlier magmatic event, possibly the same as is represented by abundant enclaves of gabbroic/dioritic/basaltic material found in the Hoyazo dacite. It has been suggested (Zeck, 1968, 1970, 1992) that such enclaves derive from basic–intermediate magmas intruded into the source region for the dacite, possibly inducing anatexis.
The combined evidence from the three stages of Neogene magmatic zircon growth arrested by the present study is that the process from anatectic melting to magmatic eruption (>900°C) took at least 2–3 Myr. Volcanism in the nearby Sierra de Gata spanned the period 12–10·5 Ma (Zeck et al., 2000), and in that case also ion microprobe zircon ages provide evidence of magmatic activity at depth at least 5 Myr before eruption commenced (Zeck et al., 1998).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGALMANDINE-GRAPHITE-BEARING...ZIRCON U-Th-Pb ANALYSESINTERPRETATIONCONCLUSIONSREFERENCES |
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The close similarity in grain morphology, internal zoning and age of the inherited zircon in the Hoyazo dacite and one of its complementary, syngenetic restitic enclaves supports the interpretation that the dacitic magma is of anatectic origin (Zeck, 1968,
1970, 1992). Minor differences in zircon inheritance patterns for each of the two dacite samples and the aluminous enclave may be sampling effects, but alternatively may reflect heterogeneities in the anatectic source rock complex, such as those previously inferred from Nd- and Sr-isotopic studies of the Hoyazo dacite and other Neogene volcanics in SE Spain (Zeck et al., 1999).
The study shows that the age range of detrital zircon in the anatectic source rock complex was very large, comprising Late Archaean, Palaeoproterozoic, Grenvillian, Pan-African and Hercynian age groups, indicating a Gondwana domain provenance. Comparison with zircon age inventories of the basal section of the Alpine nappe stratigraphy (Zeck & Whitehouse, 1999; Zeck & Williams, 2001) suggests that the source rock complex for the magma corresponds to an amphibolite facies equivalent of either the Palaeozoic or the Permo-Triassic series of the Alpine nappes, with some preference for the latter option. The study also shows that in the case of composite zircon inheritance spectra, very large numbers of ion microprobe zircon U–Pb analyses are needed to screen out resetting effects and to precisely delimit distinct age groups and their component age events.
The ages measured on magmatic zircon grains and overgrowths in the dacite and restitic enclave indicate that anatexis commenced at least as early as 8·3 Ma, 2 Myr before eruption of the dacite at 6·3 Ma. The presence of rare igneous zircon as old as 9·5 Ma suggests that magmatism at depth might have preceded the Hoyazo eruption by at least 3 Myr. Such an extended history is consistent with recent findings in the nearby Sierra de Gata, where volcanism (12–10·5 Ma; Zeck et al., 2000) followed deep-seated magmatism over the preceding 5 Myr (Zeck et al., 1998).
Crustal anatexis features prominently in the petrogenesis of the Neogene Alborán volcanic province (Zeck, 1968, 1970, 1992, 1996, 1999; Zeck et al., 1998, 1999). The anatectic source rock complexes for the various volcanic units are highly variable. For the Hoyazo cordierite dacite they consist of pelitic/quartzo-feldspathic lithologies. For the pyroxene andesitic magmas of the Hernández Formation (Zeck et al., 1998) amphibolitic complexes are called for. Anatexis post-dates orogenic collision and final, extrusional/extensional tectonics, and was probably controlled by the inflow of high-temperature asthenosphere following the detachment of the sinking lithospheric slab beneath Iberia and the Alborán sea (Fig. 1; Zeck, 1996, 1999; Zeck et al., 1998, 1999).
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ACKNOWLEDGEMENTS |
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Careful reviews by Bob Pidgeon (Perth) and Ron Vernon (Macquarie) improved the paper considerably. H.P.Z. was supported by the Japanese Ministry of Education (Monbusho fellowship), Carlsberg Foundation (grant 45/10-800), Gledden Foundation (University of Western Australia) and the Danish Research Council (SNF grant 9800918). ISEI directors Ikuo Kushiro and Masaru Kono, and TSRC director John Dodson are thanked for hospitality and gracious support at the Misasa and Perth institutes, respectively. We thank Shane Paxton and John Mya, RSES, for their careful mineral separations, and Neil Gabbitas for assistance with photography and CL imaging.
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FOOTNOTES |
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*Corresponding author. Present address: Geological Institute, Copenhagen University, OesterVoldgade 10, 1350K Copenhagen, Denmark. e-mail: zeck.frappier{at}wanadoo.fr
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