serie NOVA TERRA nº 49

74 ( Ludwig, 2002, 2003 ). Ages younger than 1 Ga are reported based on 206 Pb/ 238 U ratios corrected from common Pb using the 207 Pb method. Older ages are reported based on the 204 Pb-corrected 206 Pb/ 207 Pb isotopic ratio. The Pb composition used for initial Pb corrections ( 204 Pb/ 206 Pb=0.0554, 207 Pb/ 206 Pb=0.864 and 208 Pb/ 206 Pb=2.097) was estimated using the Stacey and Kramers (1975) model. Analytical results are presented in Table 1 and plotted in Figs. 5 and 6 . La to Yb and Hf were measured concurrently with U – Th – Pb analysis as additional masses on each pass through the mass range. The concentrations of these elements were calibrated against an in-house standard (MAD) and are reproducible at 2 – 4% (1 σ ), except for La (15%) because of its typical very low concentration (30 ppb). U, Th, Hf and rare earth element (REE) analyses are listed in Table 2 . 4.2. Zircon description and U – Pb results Theanalysed zircongrainsarecolorlesswithvariablelengthtowidth ratios (between 1:1 and 3:1). Most are rounded prisms with pitted surfaces, although a few are less rounded with smooth faces. Inclusions are common in all the zircon crystals. Under CL, the zircon grains show an assortment of textures suggesting for most an inherited origin ( Fig. 4 ). Common textures include non-luminescent homogeneous zir- con crystals, variably luminescent oscillatory and sector zoning, and complex grains with xenocryst cores mantled by oscillatory zoning and occasionally surrounded by an irregular non-luminescent rim. Thirty-two analyses were obtained on 31 zircon grains ( Fig. 5 ). The data can be roughly sorted in four age groups, <400 Ma, 480 – 530 Ma, 545 – 735 Ma and >2000 Ma ( Table 1 ). In the fi rst group, three scat- tered ages around 320 Ma could represent metamorphic ages or ex- treme Pb-loss from older zircon grains during the Variscan orogeny (analyses 20, 21 and 25). Since both the CL images and Th/U ratios are typically magmatic, we favour the latter interpretation. This is also in agreement with the general absence of important tectonothermal events of this age in the Órdenes Complex. From the second group, the best age estimate is obtained by pooling six analyses that yield an age of 510.53 (+8.39, − 2.29)Ma using the TuffZirc algorithm ( Ludwig and Mundil, 2002 ). This age is the median obtained from a group of 11 analyses ranging from 480 to 530 Ma, which represent the largest set of internally concordant dates that are statistically coherent, and is interpreted to date the crystallization of the Ares dyke. Ages calculated using this method are reliable provided the data are cogenetic and unaffected by Pb-loss. In this case, we argue for the validity of these assumptions based on the zircon features, namely their homogeneity and sector zoning, which are consistent with zircon growth in ma fi c Table 1 SHRIMP Th – U – Pb zircon data from sample GCH-06-2 (dyke Ares-1). Spot number and description a Common 206 Pb (%) U (ppm) Th (ppm) Th/U 238 U/ 206 Pb b (%) 207 Pb/ 206 Pb b (%) 238 U/ 206 Pb ⁎ c (%) 207 Pb ⁎ / 206 Pb ⁎ c (%) 206 Pb ⁎ / 238 U d 206 Pb/ 238 U age e,f GCH-06-2: Ares dyke Variscan ages 25 m 0.12 521 813 1.61 20.27±0.4 0.0535±1.3 20.25±0.4 0.0541±1.4 0.0493±0.0002 310.1±1.4 21 c 0.13 833 810 1 .00 19.32±0.4 0.0540±1.0 19.34±0.4 0.0532±1.2 0.0517±0.0002 324.9±1.2 20 r <0.01 706 252 0.37 17.76±0.4 0.0528±1.2 17.77±0.4 0.0524±1.3 0.0564±0.0002 353.5±1.5 Magmatic ages 6 m 0.14 57 48 0.87 12.90±1.3 0.0579±3.2 12.93±1.3 0.0556±3.9 0.0774±0.0010 480.8±6.0 31 m <0.01 55 41 0.76 12.52±1.2 0.0562±3.4 12.52±1.2 0.0562±3.4 0.0800±0.0010 496.0±6.0 13 m 0.09 200 34 0.17 12.44±0.6 0.0579±1.6 12.46±0.6 0.0567±1.9 0.0803±0.0005 498.1±3.0 18 m 0.01 1163 638 0.57 12.35±0.3 0.0574±0.7 12.35±0.3 0.0577±0.7 0.0809±0.0002 501.7±1.3 22 m <0.01 37 28 0.78 12.23±1.5 0.0550±3.7 12.23±1.5 0.0550±3.7 0.0820±0.0012 508.2±7.3 3.1 m, r 0.06 157 72 0.47 12.18±0.7 0.0580±2.3 12.16±0.7 0.0595±2.6 0.0821±0.0006 508.4±3.7 11 m 0.06 150 47 0.32 12.15±0.7 0.0579±1.8 12.16±0.7 0.0574±1.9 0.0823±0.0006 509.6±3.8 5 m <0.01 169 108 0.66 12.11±0.8 0.0574±1.9 12.11±0.8 0.0574±1.9 0.0826±0.0007 511.5±3.9 10 m <0.01 110 49 0.46 12.10±0.8 0.0544±2.1 12.11±0.8 0.0537±2.3 0.0829±0.0007 513.7±4.3 1 m 0.16 126 66 0.54 11.91±0.8 0.0590±2.0 11.92±0.8 0.0584±2.2 0.0838±0.0007 518.9±4.0 28 m <0.01 151 93 0.64 11.73±0.8 0.0555±1.9 11.73±0.8 0.0556±1.9 0.0855±0.0007 528.8±4.0 Inherited ages 29 m <0.01 195 79 0.42 11.34±0.6 0.0581±1.5 11.35±0.6 0.0571±1.9 0.0882±0.0006 545.1±3.4 17 m <0.01 118 70 0.61 11.14±0.8 0.0585±1.9 11.14±0.8 0.0585±1.9 0.0898±0.0007 554.3±4.4 27 m 0.13 104 73 0.72 10.98±1.0 0.0599±3.4 11.01±1.0 0.0572±4.2 0.0910±0.0009 561.4±5.5 26 m <0.01 36 29 0.83 10.96±1.7 0.0588±4.1 10.99±1.7 0.0563±5.1 0.0913±0.0016 563.0±9.4 30 m 0.25 168 61 0.38 10.80±0.7 0.0611±1.6 10.82±0.7 0.0598±1.8 0.0924±0.0006 569.6±3.8 19 m <0.01 264 103 0.40 10.84±0.5 0.0578±1.3 10.86±0.5 0.0561±1.8 0.0924±0.0005 569.6±3.0 9 m 0.22 69 37 0.55 10.78±1.1 0.0609±2.5 10.78±1.1 0.0609±2.5 0.0926±0.0010 570.9±5.9 32 m <0.01 207 88 0.44 9.85±0.7 0.0592±1.5 9.83±0.7 0.0607±1.9 0.1017±0.0007 624.5±4.0 16 m <0.01 302 212 0.73 9.65±0.5 0.0599±1.1 9.65±0.5 0.0599±1.1 0.1037±0.0005 636.3±3.0 4 m <0.01 185 131 0.73 9.38±0.6 0.0605±1.4 9.37±0.6 0.0612±1.5 0.1067±0.0007 653.8±3.9 7 m <0.01 178 73 0.43 8.38±0.7 0.0634±1.4 8.38±0.7 0.0634±1.4 0.1194±0.0008 727.1±4.7 3.2 c <0.01 244 170 0.72 8.27±0.5 0.0636±1.1 8.28±0.5 0.0633±1.1 0.1209±0.0007 735.7±3.8 2 m 0.05 74 59 0.82 2.76±0.8 0.1229±0.8 2.76±0.8 0.1227±0.8 0.3621±0.0034 1996±15 23 m, r 3.14 481 174 0.37 3.50±0.4 0.1244±0.8 3.50±0.4 0.1242±0.8 0.2766±0.0012 2018±14 24 c 1.18 192 75 0.40 2.93±0.6 0.1248±1.9 2.93±0.6 0.1247±2.0 0.3370±0.0025 2024±35 12 m <0.01 108 51 0.49 2.54±0.7 0.1289±0.7 2.54±0.7 0.1286±0.7 0.3960±0.0034 2079±13 14 m <0.01 45 21 0.47 2.57±1.1 0.1297±1.1 2.57±1.1 0.1289±1.1 0.3901±0.0051 2083±20 15 m 0.93 142 33 0.24 1.90±0.7 0.1937±0.6 1.90±0.7 0.1936±0.6 0.5207±0.0049 2773±9 Bold ages represent the analyses considered to date the crystallization age of the dyke. All errors are 1 σ . a Zircon characterization: m = magmatic; zoned: c = core, r = rim. b Uncorrected ratios. c Radiogenic lead 204 Pb corrected for common lead. d Radiogenic lead 207 Pb corrected for common lead. e 207 Pb corrected for common lead. f Except analyses 2, 23, 24, 12, 14 and 15 ( 207 Pb/ 206 Pb age, 204 Pb corrected for common lead). 357 F. Díaz García et al. / Gondwana Research 17 (2010) 352 – 362