U-Pb zircon geochronology of the porphyritic charnockite

ronment, but porphyritic granites and the reddish granites, both or at least the latter formed in a syn-collisional setting during continent – continent collision that also produced the main thrusting event. The new geochemical data based on our new subdivision of the suite also indicate that its Palaeoproterozoic granitoids have a juvenile char- acter. This conclusion is in good agreement with new Sm – Nd data on the Umba enderbite – charnockite – porphyritic granite suite yielding TDM model ages ranging between ca 1.9 and 2.1 Ga Timmerman and Balagansky, 1994.

6. U-Pb zircon geochronology of the porphyritic charnockite

6 . 1 . Analytical techniques Conventional analyses of zircons from sample 101249 were carried out at the Institute of Pre- cambrian Geology and Geochronology, Russian Academy of Sciences IPGG RAS, St Petersburg, Russia on a Finnigan MAT 261 8-collector mass- spectrometer in static mode. Zircons were extracted from the crushed rock sample with heavy liquid and magnetic separation techniques. Hand picked aliquots of zircon were analyzed following the method of Krogh 1993. An air-abrasion treatment of the zircon was per- formed by the Krogh 1982 technique. All sam- ples were spiked with a 235U-208Pb mixed tracer. The total blanks were 0.05 – 0.1 ng Pb and 0.005 ng U. The PbDat and ISOPLOT programs by Ludwig 1991a,b were used for uncertainties and correlations of UPb. Ages were determined using the decay constants given by Steiger and Jaeger 1977. All errors are reported at the 2w level. The error of the UPb ratios is 0.6. Corrections for common Pb were made using values of Stacey and Kramers 1975. The back-scattered electron imaging BSE was taken at the Institute of Precambrian Geology and Geochronology with a SEM model ABT 55 using an accelerating voltage of 20 – 30 kV. The BSE images were recorded using the image-analy- ses system VideoTest-Morpho. 6 . 2 . Analytical results Inspection of the zircon grain concentrate from sample 101249 led us to distinguish four groups of magmatic zircons that are characterized by the different degrees of progressive recrystallization under high-grade conditions Type I — Brown-pink turbid subhedral crystals of prismatic shape. The range of crystal sizes is 80 – 550 mm with a lengthwidth ratio of 2.0 – 4.0. Growth zoning is visible. The outer parts of the crystals are unzoned and fractured and represent apparently irregular thin overgrowths Fig. 12 a, b, c. This type of zircon represents about 15 of the bulk population and appears to be of primary igneous origin. Type IIa — Brown-pink transparent or translucent prismatic or short prismatic zircons, 70 – 500 mm long with lengthwidth ratio of 2.0 – 3.0. These zircons reveal complex internal rela- tions that were recognized by using the back-scattered electron imaging techniques Fig. 12 d, e. They consist of an igneous core with multiple growth zones and an unzoned fractured rim. This zircon type forms about 30 of the population. Type IIb — Brown-pink transparent or translucent more elongated zircon crystals than the type I crystals 70 – 500 mm long with length width ratio of 5.0 – 6.0 that are characterized similar internal structure Fig. 12 f, g. The oscil- latory zonation of igneous cores is weak. The total amount of these zircon grains is about 30. Type III — Pale pink zircons that are transpar- ent and subhedral with an isometric or short-pris- matic habit. Generally these zircons do not reveal any internal features, except for the rare traces of zoning Fig. 12 h, i. It seems that these grains are recrystallized to different degrees from type I and type II zircons. The range of crystal sizes is 60 – 350 mm with a lengthwidth ratio of 1.0 – 1.7. Four multigrain fractions of all recognized zir- con types were analyzed Nos. 1 – 4, Table 3. Then zircons from the distinct types were sub- jected to air-abrasion treatment whereby different amount of zircon material was removed Nos. 5 – 8, Table 3. On a concordia plot all data points are discordant Fig. 13. The results for V . Glebo 6 itsky et al . Precambrian Research 105 2000 247 – 267 Table 3 U-Pb isotopic data for the zircon from sample 101249 from Umba massif Rho c Fraction size, mm and zircon type Age, Ma Fraction weight, mg Concentrations ppm Isotopic ratios corrected for blank and common Pb b NN 207 Pb 206 Pb 208 Pb 206 Pb 207 Pb 235 U 206 Pb 238 U 207 Pb 235 U 206 Pb 238 U 207 Pb 206 Pb Pb U 206 Pb 204 Pb a 0.1181 9 3 0.09703 9 3 5.524 9 23 0.3392 9 11 0.82 0.67 1904 9 8.0 16.8 1883 9 6.0 1928 9 4.3 46.0 \ 150, I 1 1366 281 3601 0.1168 9 1 0.11304 9 1 5.418 9 18 0.3364 9 10 0.95 1888 9 6.2 1869 9 5.8 1908 9 1.8 0.44 2 101 \ 100, IIa 0.1169 9 1 0.09781 9 1 5.338 9 17 0.3313 9 10 0.97 1875 9 6.0 3 1845 9 5.7 \ 100, IIb 1909 9 1.5 0.89 94.1 270 6362 0.1171 9 1 0.08982 9 1 5.448 9 18 0.3375 9 10 0.94 1893 9 6.2 5714 1875 9 5.8 4 1912 9 2.0 291 103 0.52 100-70,III 0.1196 9 1 0.07495 9 1 5.579 9 17 0.3385 9 10 0.99 1913 9 5.9 5 1879 9 5.8 \ 150, I,A 80 1949 9 0.7 0.67 136 389 7536 0.1207 9 1 0.08861 9 1 5.731 9 18 0.3444 9 11 0.99 1936 9 6.2 780.7 1908 9 5.9 6 1966 9 0.9 208 79.8 0.52 \ 100, IIa,A 50 368 6141 0.1169 9 1 0.09988 9 1 5.399 9 17 0.3350 9 10 0.99 1885 9 5.9 1862 9 5.8 1909 9 0.8 7 \ 100, IIb,A 40 0.80 130 0.1177 9 1 0.10051 9 1 5.547 9 17 0.3418 9 10 0.99 1908 9 6.0 6125 1895 9 5.8 310 1922 9 0.8 8 100+70,III, A 40 0.43 112 a Measured ratio. b Uncertainties 95 confidence level refer to last digits of corresponding ratios. c Correlation coefficients of 207 Pb 235 U vs. 206 Pb 238 U ratios; 50 of zircon was removed during the air-abrasion. Fig. 12. BSE images of zircons from the Umba porphyritic granite sample 101249. a, b and c — type I zircons; d and e — type IIa zircons; f and g — type IIb zircons; h and I — type III zircons. unabraded zircons of types IIa, IIb, III were abraded up to 40 that means 40 of the zircon was removed by abrasion. Type IIb zircons define a discordia which intersects the concordia at 1912.5 9 7.7 Ma and 187 9 388 Ma MSWD = 1.2. The unabraded and strongly abraded zircons of type I and abraded zircons of types IIa and III cluster to the right of the discordia and have older 207 Pb 206 Pb ages 1922 Ma – 1966 Ma. The abraded type I and IIa zircons Nos. 5 and 6, Table 3 are characterized by the oldest 207 Pb 206 Pb ages at 1949 9 7 Ma and 1966 9 9 Ma. That is in a good agreement with the morphological features of types I and IIa zircons that show evidence of being the least recrystallized in the population. We prefer an interpretation of the U – Pb zircon data for the porphyritic charnockite in which the 1912.5 9 7.7 Ma age, obtained for unabraded zir- cons of types IIa, IIb, III and abraded type IIb zircons, represents the intrusion age. The 1949 9 7 Ma and 1966 9 9 Ma ages, obtained for abraded type I and IIa zircons, are considered as repre- senting ages of inherited zircons in the magma. This agrees well with field observations that the porphyritic charnockite is vastly contaminated by sedimentary material from the Umba block metasediments. A second possibility for interpre- tation is that the younger age represents the age of the metamorphism while the intrusion age was older. However, the syn-metamorphic emplace- ment of the charnockites and the peak metamor- phic episode would then have occurred over a period of 40 Ma, as judged from the difference in ages obtained for the igneous cores abraded type IIa zircons; No. 6 in Table 3 and the upper discordia intercept for recrystallized zircons. This age difference seems too large when compared to the duration of about 20 Ma which is normal for regional metamorphic episodes in areas of poly- cyclic deformation Kotov et al., 1995. We there- fore strongly prefer that the somewhat older 207 Pb 206 Pb ages of abraded type I and IIa zircons should be explained by the presence of an inher- ited zircon component that is older than 1920 – 1930 Ma in the porphyritic granite magma.

7. Discussion and conclusions