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THE COMPOSITION OF ZIRCON IN THE PERALUMINOUS HERCYNIAN GRANITES OF THE SPANISH CENTRAL SYSTEM BATHOLITH

Cecilia Pérez-Soba^1,, Carlos Villaseca¹, José González Del Tánago¹ and Lutz Nasdala²

¹ Departamento de Petrología y Geoquímica, Faculdad de Ciencias Geológicas, Universidad Complutense, E–28040 Madrid, Spain
² Institut für Mineralogie und Kristallographie, Universität Wien, A–1090 Wien, Austria

View larger version (44K): [in this window] [in a new window]   FIG. 1. Geological map of the Sierra de Guadarrama, in the Spanish Central System, showing the distribution and type of granite plutons considered in this study (modified after Villaseca et al. 1998).

View larger version (49K): [in this window] [in a new window]   FIG. 2. Back-scattered-electron images of different types and subtypes of zircon distinguished in this work. Type-1 zircon: A) Elongate crystal showing oscillatory zoning ( Atalaya Real granodiorite, s. 95922). B) Common crystal of type 1 ( La Pedriza leucogranite, s. 67065). C) Euhedral transversal section of type-1 zircon, with characteristic oscillatory zoning. D) Subhedral section of type-1 zircon, showing oscillatory zoning, weakly contrasted. Type-2 zircon: E) Subtype 2b ( alveolar): common crystal of alveolar subtype, in this case with irregular inclusions of plagioclase. The arrow points to the spot of the composition with SiO2 = 31.65 wt.% ( Table 3); ( La Pedriza leucogranite, s. 87059) F) Subtype 2c ( complex): crystal with alveolar center showing an eccentric nucleus ( arrow) with a composition SiO2 = 24.83 wt.% in Table 3, in a wide unzoned central sector ( Atalaya Real aplite, s. 96922). G) Subtype 2b ( alveolar): zoned crystal with an alveolar center ( rich in bright and opaque inclusions, composition with SiO2 = 28.84 wt.% in Table 3) and a homogeneous rim ( SiO2 = 31.35 wt.% in Table 3) ( La Pedriza leucogranite, s. 87225). H) Subtype 2b ( alveolar): euhedral transversal section of zircon rich in irregular inclusions; area ( marked by the arrow) with a deficient analytical total ( composition with SiO2 = 28.76 wt.% in Table 3) ( La Pedriza leucogranite, s. 87166). I) Subtype 2c ( complex): zoned crystal with a small alveolar sector in a mainly homogeneous inner zone and a rim with subtle oscillatory growth-zoning; the bright crystal is xenotime ( La Pedriza leucogranite).

View larger version (17K): [in this window] [in a new window]   FIG. 3. Histograms of length (Lmax) and elongation (width/length) in zircon of type 1 and 2. For each category, the percentage of the main rocks to the total of the population is compared.

View larger version (27K): [in this window] [in a new window]   FIG. 4. Compositional comparison between zircon of types 1 and 2 from representative SCS granitic plutons. Bar shows the complete range of zircon composition (vertical line), including the median (short horizontal line). Oxides expressed in wt.%.

View larger version (30K): [in this window] [in a new window]   FIG. 5. Variation in amount of U versus Th in zircon crystals. Diagonal lines representing Th:U ratios of 0.1, 0.6 and 2 are shown. The compositional range of zircon of type 1 and type 2 from monzogranites and granodiorites (MzGr), and zircon of type-1 from leucogranites and aplites (LAP) are enclosed in areas. Arrows define the compositional zoning in a single crystal. Symbols for type-2 zircon are as follows: La Cabrera pegmatite, + La Cabrera aplite, x Atalaya Real aplite, Atalaya Real leucogranite, La Pedriza leucogranite, Cabeza Mediana leucogranite.

View larger version (19K): [in this window] [in a new window]   FIG. 6. Raman spectrum of a micro-area in zircon in aplite from the Atalaya Real pluton, whose electron-microprobe analysis gave a deficient total (96027c in Table 3). It is shown in comparison with spectra of the moderately radiation- damaged CZ3 zircon (Nasdala et al. 2001b) and well-crystallized, synthetic zircon (Nasdala et al. 2002). Spectra are stacked for clarity. Broadening and extremely low intensities of Raman bands characterize sample 96027c as strongly radiation-damaged.

View larger version (19K): [in this window] [in a new window]   FIG. 7. Diagrams showing the extent of solid solution toward xenotime in zircon crystals in SCS granites (in atoms per formula unit: apfu). A) A-site substitution: (Zr,Hf,Th,U)4+ versus (Y,REE)3+ diagram. B) B-site substitution: Si4+ versus (P5+ + Al) plot. Area encloses zircon crystals of types-1 and 2 from monzogranites and granodiorites, and the diagonal line indicates the 1:1 substitution line. Same symbols as in Figure 5.

View larger version (19K): [in this window] [in a new window]   FIG. 8. Zircon plots. A) (Ca2+ + Fe2+) versus REE plot. B) 3(Zr,Hf,Th,U)4+ versus (Ca,Fe)2+ + 3(Y,REE)3+ substitution diagram in zircon of types 1 and 2. The diagonal line indicates 1:1 substitution line. The compositions are expressed in atoms per formula unit (apfu). Same symbols as in Figure 5.

View larger version (16K): [in this window] [in a new window]   FIG. 9. Plot of Th in whole rock versus amount of ThO2 in zircon from granites of the Atalaya Real and La Pedriza plutons. Data points represent the compositional range of ThO2 contents in zircon crystals from a single thin-section.

View larger version (22K): [in this window] [in a new window]   FIG. 10. A) Plot of (Th + U) versus (Y+HREE) for zircon of types-1 and 2 from all the plutons studied. B) Analytical total versus (Y2O3 + HREE2O3) + (CaO + FeO) in wt.% in zircon crystals. See text for further explanations. Same symbols as in Figure 5.

View larger version (19K): [in this window] [in a new window]   FIG. 11. The composition of zircon in terms of Hf (wt.%) versus Zr:Hf. In both diagrams, the enclosed area is the compositional field of zircon crystals in granodiorites and monzogranites. Compositional growth-zoning (core to rim) is also marked for some zircon crystals of type 2. Same symbols as in Figure 5.

View larger version (28K): [in this window] [in a new window]   FIG. 12. Comparison of the Zr:Hf values in zircon of different crustal sources and types of granite. Box plots show the 25th and 75th percentiles (limits of the box), the median (thin line), the mean (coarse line) and the maximum and minimum values excluding outliers. Values for SCS rocks are taken from this study and from Villaseca et al.(2003); other values are taken from Pupin (2000).