HadeanâArchean Detrital Zircons from Jatulian Quartzites ... - Springer

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HADEAN–ARCHEAN DETRITAL ZIRCONS 321Isotope U–Th–Pb data for several representative grains of detrital zircons from Jatulian rocksIsotope ratioAge, MaPoint no. U, ppmU/Th206Pb*-----------207Pb*207Pb*206Pb*% ----------- % ----------- %235238U*U206Pb*-----------238Sample S 3976, Jatulian quartzite, Woloma structure90 38 0.7 5.6775 ±2.3 11.9084 ±2.4 0.4904 ±0.6 2572.2 ± 13.4 2616.8 ± 38.0 98.356 48 0.6 5.5673 ±2.6 12.8283 ±2.9 0.5180 ±1.3 2690.6 ± 28.6 2649.4 ± 42.5 101.619 207 1.4 5.3759 ±2.2 13.2621 ±3.2 0.5171 ±2.3 2686.8 ± 50.5 2707.2 ± 35.8 99.221 30 0.5 5.2359 ±0.9 14.0563 ±1.0 0.5338 ±0.5 2757.4 ± 11.2 2750.7 ± 14.8 100.223 181 1.9 5.1575 ±1.5 14.3475 ±3.3 0.5367 ±2.9 2769.5 ± 65.3 2775.5 ± 24.4 99.864 256 1.3 5.0394 ±2.6 15.1104 ±3.2 0.5523 ±1.9 2834.6 ± 42.4 2813.4 ± 43.0 100.870 28 1.2 5.0278 ±2.9 15.2377 ±3.2 0.5556 ±1.3 2848.6 ± 30.9 2817.1 ± 48.0 101.188 110 0.9 4.5708 ±2.0 17.7313 ±2.9 0.5878 ±2.1 2980.5 ± 49.6 2971.7 ± 31.9 100.32 40 0.7 4.5608 ±1.4 17.8442 ±1.5 0.5903 ±0.6 2990.4 ± 13.4 2975.2 ± 23.2 100.563 93 1.4 4.2743 ±1.7 19.8042 ±2.0 0.6139 ±1.1 3085.8 ± 27.2 3079.3 ± 27.1 100.277 69 2.1 4.1478 ±1.6 20.7384 ±1.7 0.6239 ±0.5 3125.3 ± 12.6 3127.2 ± 25.5 99.927 84 1.6 3.7634 ±2.3 24.6163 ±3.1 0.6719 ±2.1 3313.2 ± 55.2 3280.9 ± 36.3 101.068 101 2.1 3.7174 ±4.0 25.1330 ±4.3 0.6776 ±1.7 3335.2 ± 44.0 3300.2 ± 62.8 101.1100 51 2.3 3.4717 ±2.5 27.8663 ±2.9 0.7017 ±1.4 3426.9 ± 37.2 3407.1 ± 39.1 100.61 123 1.5 2.9648 ±1.4 35.5125 ±1.6 0.7636 ±0.8 3657.5 ± 23.2 3650.5 ± 21.7 100.299 116 2.1 2.6220 ±2.8 43.1147 ±3.0 0.8199 ±1.0 3860.0 ± 29.3 3837.2 ± 42.1 100.6Sample 5883, cement of Jatulian conglomerate, Onega syncline22 89 1.1 5.3794 ±2.0 12.1443 ±2.9 0.4738 ±2.1 2500.3 ± 43.1 2706.2 ± 32.2 92.446 20 1.3 5.2089 ±2.4 13.9498 ±2.7 0.5270 ±1.2 2728.8 ± 26.3 2759.2 ± 39.7 98.957 20 0.9 5.1158 ±3.0 14.5726 ±3.1 0.5407 ±0.5 2786.3 ± 12.0 2788.8 ± 49.5 99.920 114 1.1 4.8181 ±2.9 15.8609 ±3.1 0.5542 ±1.1 2842.8 ± 24.4 2886.5 ± 46.9 98.518 71 1.5 4.7686 ±2.3 16.5862 ±2.6 0.5736 ±1.2 2922.7 ± 28.9 2903.2 ± 37.4 100.729 115 1.1 4.4560 ±1.5 17.7840 ±2.6 0.5747 ±2.1 2927.3 ± 49.6 3012.6 ± 24.3 97.219 141 1.1 4.4145 ±1.0 18.5633 ±1.8 0.5943 ±1.5 3007.0 ± 35.6 3027.7 ± 15.4 99.348 7 1.0 4.3379 ±3.7 19.2363 ±3.8 0.6052 ±0.5 3050.8 ± 12.2 3055.7 ± 59.7 99.825 177 1.0 4.1743 ±1.9 20.4015 ±4.0 0.6177 ±3.5 3100.6 ± 85.2 3117.0 ± 30.7 99.549 57 1.3 4.0014 ±2.8 21.7602 ±3.0 0.6315 ±1.2 3155.5 ± 30.2 3184.2 ± 43.7 99.123 174 0.3 3.9035 ±4.0 23.1427 ±4.1 0.6552 ±0.6 3248.4 ± 16.4 3223.4 ± 63.6 100.836 46 0.9 3.9000 ±1.9 21.9521 ±2.2 0.6209 ±1.1 3113.6 ± 27.2 3224.8 ± 30.0 96.637 113 2.1 3.8838 ±1.8 21.9925 ±1.9 0.6195 ±0.7 3107.9 ± 18.0 3231.3 ± 28.2 96.253 155 1.6 3.8512 ±2.5 23.1483 ±2.7 0.6466 ±1.0 3214.8 ± 25.8 3244.6 ± 39.4 99.133 76 0.8 3.7209 ±4.3 24.4125 ±4.6 0.6588 ±1.7 3262.5 ± 43.3 3298.8 ± 67.7 98.940a, core 9 1.7 2.5632 ±2.6 44.1950 ±2.6 0.8216 ±0.5 3866.0 ± 14.5 3871.5 ± 38.6 99.940, shell 12 0.6 5.3507 ±2.8 13.0916 ±3.0 0.5080 ±1.0 2648.3 ± 21.5 2715.0 ± 46.4 97.5U*206Pb*-----------207Pb*C, %three Paleoarchean grains of 3407.1–3280.9 Ma age(U 51–101 ppm, U/Th = 1.6–2.3, C = 100.6–101.1%). Within the distribution of other grains, fourtime intervals are distinguished, conforming to thepeaks in the age histogram: 3.00–3.15 (5 grains),2.95–3.00 (10 grains), 2.75–2.85 (32 grains), and2.6–2.65 Ga (2 grains). Each of these intervalsincludes grains that are much different in the U content,U/Th ratio, and concordance value. Thus, in theage cluster of 2.75–2.85 Ga, the main peak in the PDdiagram (Fig. 4) presents two groups of grains: thenumerous low-uranium (22 grains; U 28–94 ppm,U/Th = 0.4–3.2, C = 70.4–101.2%) and the higheruranium (U 105–338 ppm, U/Th = 0.9–4.1, C =DOKLADY EARTH SCIENCES Vol. 431 Part 1 2010
322KOZHEVNIKOV et al.No.2015105Sample S-3976n = 910109 Sample 5883n = 438765432102000 2400 2800 3200 3600 4000MaFig. 4. Multimodal distribution of 207 Pb/ 206 Pb ages of zirconsfrom the samples S-3976 and 5883 in PD diagrams.72.1–102.6%). The other intervals contained aboutequal numbers of high- and low-uranium grains.The second sample 5883 of about 15 kg mass wastaken from the bottoms of the upper layer of finegrainedpebbled quartz conglomerates with quartz–feldspar cement, in the section of sedimentary andvolcanogenic rocks of Jatulium at the western boundaryof the Onega syncline. The detrital accessory mineralswere presented by zircon, tourmaline, garnet,titanite, titano-magnetite, and thorite.Zircons were divided into two large groups by theirmorphology, mineral inclusions, and transformationdegree. The first group was presented by light pinktransparent fine grains (75–200 µm) of various roundnessdegree or of retained faceting, with a weak zonalityin most of the cases (Fig. 2). The fine syngeneticmonophase mineral inclusions in these zircons werepresented by quartz, feldspars, and F-apatite. Notransformations were revealed in this group of zircons.The second group is formed by coarser zonal grains(125–350 µm), from rounded muddy brown–cherrycoloredto poorly rounded brown and pink ones ofhyacinth habit. In the grains of this group, syngeneticmono- and polyphase inclusions of thorite, monazite,anorthoclase, Cl-apatite, quartz, biotite, chlorite,pyrite, titanomagnetite, and iron hydroxide werefound. The intense transformations of zircon accompaniedwith the supply of Th (to 0.43 wt %), Mn (to0.63 wt %), Al (to 1.95 wt %), Fe (to 2.64 wt %), Ca (to1.61 wt %), Na (to 0.96 wt %), Hf (to 0.46 wt %), andlight REEs (to 0.72 wt %) were revealed zonally aspatches and dendritic systems. The microveinlets wereformed by quartz and apatite (Fig. 2). Outgrowths ofchlorite, biotite, caolinite, and globular zircon intergrowthswith thorite were observed, as well as those ofrutile with hematite.The study of the isotope system at 43 points (Fig. 3,table) showed wide variations in the U, Th, and Pbcontents, the U/Th ratios, and the concordance values(C), as well as the multimodal age distribution (Fig. 4).The most ancient Hadean age value of 3871.5 ±38.6 Ma is related to the core part of a grain of the leasturanium content (U 9 ppm, U/Th = 1.7, C = 99.9%).The age of the metamorphic shell around the core wasdetermined as 2715 ± 46.4 Ma (U 12 ppm, U/Th =0.6, C = 97.5%). The remaining set of the grains in thePD diagram is subdivided into four age clusters of pronouncedlydifferent uranium content. Cluster 1(3150–3247 Ma) included two low-uranium grains(U 46–57 ppm, U/Th = 0.9–1.3, C = 96.6–99.1%) andsix higher-uranium ones (U 113–266 ppm, U/Th =0.3–24.5, C = 96.2–100.8%). Cluster 2 (2903–2946 Ma)contained eight grains among which five were lowuraniumones (U 18–71 ppm, U/Th = 0.9–3.8, C =98.2–100.7%) and three were higher uranium (U 121–289 ppm, U/Th = 0.9–3.8, C = 96.0–100.6%). Incluster 3 (2819–2846 Ma), four grains were low-uranium(U 25–62 ppm, U/Th = 1.2–1.5, C = 96.2–98.4%) and one was higher uranium (U 216 ppm,U/Th = 1.2, C = 100.9%). Cluster 4 (2706–2743 Ma)was formed by six grains of low uranium content(U 12–89 ppm, U/Th = 0.6–1.8, C = 92.4–98.6%).Most probably, these grains were metamorphic andanalogous to zircon in the shell of a grain with aHadean-age core.The common characteristics of the treated detritalzircons from Jatulian terrigenous rocks located at differentsites of the Karelian craton are the multimodaldistribution of their ages within 2.61–3.87 Ga and theheterogeneity of the grain sampling in the morphology,mineral inclusions, and geochemistry of fresh andtransformed parts of grains testifying to the heterogeneityof their sources. Quite a synchronous growth ofzircon grains differing greatly in U and Th contentmight represent the evolution of fluidal conditionsunder the transformations of crystallized or metamorphosedacidic crust rocks.The isotope ages obtained for detrital zircons makea substantial contribution to the Early Precambrianscale of isotope age for the Fennoscandian shield andopen the prospects for discovering a detrital substancerelated to the Hadean–Mesoarchean impact events.The detection of the oldest zircons gives the precondi-DOKLADY EARTH SCIENCES Vol. 431 Part 1 2010
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