HadeanâArchean Detrital Zircons from Jatulian Quartzites ... - Springer

More documents

Recommendations

Info

HADEAN–ARCHEAN DETRITAL ZIRCONS 319LakeLakeLake12345678910111213141516WHITE SEALakeS-3976Lake5883LakeOnegaLakeLakeLadoga0 25 kmFig. 1. Geological scheme of Karelia and the location of testing sites: (1) Paleozoic cover; (2–7) Vendian (2), Riphean (3, includingrapakivi granites), Vepsian (4), Kalevian (5), Jatulian (6), and Sumian–Sariolian (7) Paleoproterozoic deposits; (8–10)Archean greenstone vulcanites and sediments (8), granite gneisses (9), highly metamorphosed and TTG complexes (10); (11, 12)Sumian laminated intrusions (11) and charnockites (12); (13) White Sea mobile belt (WSMB); (14, 15) WSMB borders: shear(14) and thrust (15); (16) sites of sampling for microprobe testing of zircons.andesibasalts, 2.5–2.4 Ga), and at Sariolian polymicticconglomerates (2.4–2.3 Ga).The monofraction of zircons includes several graintypes different in size, occurrence and composition ofmineral inclusions, character of variations, and thepresence and type of zonality, which was found whenthey were studied with a VEGA II LSH set (analystA.N. Safronov), with the following local U–Pb datingusing a SHRIMP II ion microprobe (analyst I.P. Paderin).All of the 46 zircon grains implanted into theplate were rounded. The grain size and elongationcoefficient varied within 150–350 µm and 1.1–3.3,DOKLADY EARTH SCIENCES Vol. 431 Part 1 2010
320KOZHEVNIKOV et al.Goyazite–florencite2871 ± 212681 ± 1550 µmQuartz5883-32698 ± 14 100 µm100 µm2867 ± 15S-3976ThoriteGematiteQuartz–OrthoclaseApatite80 µm5883-2QuZr15883-25Zr220 µmApFig. 2. CL and BSE images of zircons treated.206 Pb/ 238 U0.90.70.50.30.10.950.850.750.650.550.45Sample S-3976n = 9118002200Sample 5883n = 43220026002600300030003400340038003800400040000.350 10 20 30 40 50207 Pb/ 235 UFig. 3. Diagrams with the concordance for zircons fromJatulian terrigenous rocks.relatively. About a quarter of the grains were roundedfragments of large zircon crystals. Most of the grainsshowed oscillation zonality; three grains were characterizedby sectorial zonality, and one grain of 2681 ±15 Ma concordant age (U 128 ppm and U/Th = 2.5)was homogenous in CL (Fig. 2). Sixteen grains containedno mineral inclusions; the remaining 30 grainscontain inclusions of quartz, biotite, apatite, feldspar,thorite, and iron hydroxide. The inclusions and thoseof hydroxyl-containing minerals as well are oftenlocalized in the centers of zircon grains. The age determinedat five points of the grains as such was from2691 ± 22 to 2704 ± 43 Ma (U 36–358 ppm, U/Th =0.74–1.62). In eight grains, we found syngeneticinclusions of microcrystalline intergrowths of phosphatesfrom the group of florencite–goyazite–gorceixite(Fig. 2). The ages of zircons containing theseinclusions were 2867 ± 15 to 2871 ± 21 Ma (U 68–102 ppm, U/Th = 1.03–1.20), 2698 ± 14 Ma(U 130 ppm, U/Th = 1.2), and 2642 ± 21 Ma(U 532 ppm, U/Th = 2.18). The large fraction of zircongrains with phosphate inclusions (to 20%) showsthat low-depth granitic pegmatites characterized bythese minerals were probably present in a destroyedsource.In studying the isotope systems at 91 points of zircongrains by means of the LA–ICP–MS technique atthe Arizona Laserchron Center (Tucson, UnitedStates) by the procedure of [15] (Fig. 3, table), widerangevariations were found for the U, Th, and Pb content,U/Th ratio, and concordance value (C), as wellas the multimodal distribution of ages (Fig. 4). Thewhole set of grains included 2 Eoarchean (3.6–3.85Ga), 3 Paleoarchean (3.2–3.6 Ga), 57 Mesoarchean(2.8–3.2 Ga), and 29 Neoarchean (2.5–2.8 Ga)zircon grains. Two Eoarchean grains of 3837.2 ± 42.1and 3650.5 ± 21.7 Ma showed close characteristics (U116 and 123 ppm, U/Th = 2.1 and 1.5, C = 100.6 and100.2%). Somewhat less uranium was contained inDOKLADY EARTH SCIENCES Vol. 431 Part 1 2010
Page 1: ISSN 1028-334X, Doklady Earth Scien
Page 5 and 6: 322KOZHEVNIKOV et al.No.2015105Samp

HadeanâArchean Detrital Zircons from Jatulian Quartzites ... - Springer

Create successful ePaper yourself

Delete template?

Save as template?

HadeanâArchean Detrital Zircons from Jatulian Quartzites ... - Springer