ISSN 0016-7029, Geochemistry International, 2016, Vol. 54, No. 8, pp. 651–659. © Pleiades Publishing, Ltd., 2016. Original Russian Text © V.V. Akinin, I.L. Zhulanova, 2016, published in Geokhimiya, 2016, No. 8, pp. 675–684.
Age and Geochemistry of Zircon from the Oldest Metamorphic Rocks of the Omolon Massif (Northeast Russia) V. V. Akinin and I. L. Zhulanova N.A. Shilo North-East Interdisciplinary Scientific Research Institute, Far East Branch, Russian Academy of Sciences, ul. Portovaya 15, Magadan, 685000 Russia e-mail: а
[email protected],
[email protected] Received August 19, 2014; accepted March 12, 2015
Abstract—This study provides SHRIMP-RG data on zircons from garnet gedritites, the products of retrograde metamorphism of eclogite-like rocks constituting belonging to the basement of the Omolon Massif. The earliest episode recorded by oscillatory-zoned cores having high HREE and Ti contents occurred at 3.25–3.22 Ga (Paleoarchean) and is interpreted to represent an upper age limit of a metamorphic or magmatic protolith. One zircon core with a pronounced negative Eu anomaly yielded a concordant age of 2.6 Ga, which is interpreted to mark a Neoarchean episode of granite formation. The studied population of zircons provides the most distinct record of a Paleoproterozoic (1.9 Ga) event, which is marked by formation of garnet gedritites under amphibolite-facies conditions. This event is recorded by transparent recrystallization rims of preexisting large zircon grains and small newly-formed grains, which are characterized, compared with their cores, by lower crystallization temperatures and one order of magnitude lower concentrations of U, Th, and HREE, and the presence of garnet micro-inclusions. Keywords: eclogite-like rocks, Archean, zircon, SHRIMP, Omolon massif DOI: 10.1134/S0016702916060021
INTRODUCTION The Omolon Massif (OM) is one of the long-evolving tectonic elements in the continent–ocean transition zone of Northeast Asia. The massif is composed of ancient crystalline basement rocks and a weakly deformed Rhiphean–Mesozoic volcano–sedimentary cover. It has long been regarded as one of median massif, called inliers, lying within the Mesozoic epicratonic Verkhoyansk-Chukotka orogenic area and has recently been recognized as a cratonic terrane or a microcontinent and its relationships with the North Asian craton have been interpreted variably. Geologic and in part geochronological results point to a complex, multistage evolutionary history of the pre-Riphean crystalline basement of the OM (Gel’man, 1974; Bibikova, 1989; Zhulanova, 1990; Kotlyar et al., 2001; Shevchenko, 2006). These ancient rock complexes, however, remain poorly studied using modern isotope geochemical and geochronological techniques. Most exposures of pre-Riphean basement rocks are located in the southern part of the OM. Of particular interest is the Aulandzha Block, which is located in the extreme southeastern part of the massif and include an area of the deepest, well-exposed Archean continental crust of the Verkhoyansk–Chukotka region (Decisions …, 2009, pp. 9–29). Accessory zircons from gneisses of the Aulandzha Block first dated by conven-
tional ID TIMS yielded a U–Pb age greater than 3 Ga (Bibikova et al., 1978), which was later confirmed by ion microprobe SHRIMP I analyses conducted in Australia (Bibikova, 1989). Zircons from granulite and granite gneiss of the Paren’ and Avekova (Taigonos) Blocks, tectonically belonging to Mesozoic zones surrounding the OM on the south (Fig. 1), yielded a series of Archean and Early Proterozoic ages, interpreted as dating the granulite-facies metamorphism and granitization (Bibikova et al.,1978; Bibikova, 1989). However, all of the dates were found to be discordant with respect to the 207Pb/206Pb and 206Pb/238U ages, and the oldest Archean ages were calculated from a upper intercept of discordia and thus need to be reproduced. In the present study, we report results on zircons from garnet gedritite, a rare variety of melanocratic metamorphic rocks in the Aulandzha Blocks, which presumably formed during retrograde metamorphism of high-grade feldspar-free rocks. These rocks, commonly referred to as amphibole-bearing eclogites (Gel’man, 1974; Zhulanova, 1990) or garnet meta-ultramafites (Zhulanova et al., 2014) in the literature, are described here as “eclogite-like” rocks. The goal of this study was to obtain robust geochronological constraints on the evolution of the OM crystalline basement.
651
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161° E 64° N
N
VC M O A
KKCOA
S
Sea of Okhotsk
Siberian platform 329
1
6
2
7
3
8
4 5
329
4 km
9 10
11
12 161° E
Fig. 1. Schematic geologic map of the Aulandzha Block, compiled by I.L. Zhulanova. 1—Riphean and Pleozoic sedimentary rocks; 2–8—Archean: 2—biotite granite–gneiss with lenses of garnet meta-ultramafites, 3—garnet-biotite plagiogneiss, biotite–hypersthene schist, 4—diopside plagiogneiss, two-pyroxene–amphibole schist, garnet–biotite gneiss, 5—amphibolite, diopside–amphibole schist, 6— predominantly garnet–biotite gneiss, with minor cordierite, 7—diopside amphibolite, two-pyroxene–amphibole, garnet–diopside–amphibole schist, 8—charnockitoid, amphibole–two-pyroxene schist, leucocratic granulite; 9—garnet meta-ultramafites experienced retrograde metamorphism and sampling location; 10—geologic boundaries; 11— faults; 12—orientation of metamorphic foliation. Inset: the hatched areas show outcrops of pre-Riphean metamorphic rocks within the folded structures of Northastern Russia (from SW to NE: Okhotsk, Omolon-Taigonos, East Chukotka). VCMOA, Verkhoyanskо-Chukotka Mesozoic Orogenic Area. KKCOA, Koryak– Kamchatka Cenozoic Orogenic Area. The black dot denotes location of the Aulandzha Block. Coordinates of sample 329a: 63°42′20′′ N, 160°47′40′′ E.
GEOLOGIC AND PETROGRAPHIC CHARACTERIZATION OF DATED ROCKS The Aulandzha Block is interpreted as a horst of the basement of the OM with an area of 350 km2 that outcrops at the surface along a system of faults bordering the massif on the southeast. In the northwest, the metamorphic rocks are unconformably overlain by Riphean carbonate–terrigenous units (Fig. 1). The internal structure of this horst block differs from that of surrounding units. Recent data revealed a fragment (western half) of a structural dome with its core composed of charnockitoids clearly metasomatic in origin, but relics of the protolith generally remain, inlcuding amphibole–two-pyroxene schist, sometimes contain-
ing garnet, and locally leucocratic granulite. The dome flank dipping monoclinally underneath the sedimentary cover contains a thick stratified granulite unit, which crops out as a pre-metamorphic layering consisting of alternate bands of mesocratic hornblende gneisses, schists, and high-alumina rocks. The core and the flank are separated by a wide (4–5 km) band of biotite granite gneisses forming a blended unconformity (Zhulanova, 1990). The eclogite-like rocks form a distinct chain of 5– 50 m-thick lenses surrounded by granite gneiss and can be traced sporadically for 5 km. These lenses, parallel to each other and conformable with the overall dome-shaped structure of the horst, are interpreted as fragments (tectonic outliers) of the base of a charnockitoid antiform (Kotlyar et al., 2001) and represent typomorphic units of the Povarninsk complex, the lowermost division of the approved stratigraphic scheme for the Archean of the Verkhoyansk-Chukotka region (Decisions…, 2009). Most individual bodies differ in texture (massive or layered), proportions of major minerals and grain size (fine- or medium-grained), and the presence of minor minerals. The petrotype is represented by mediumgrained varieties with a granoblastic texture consisting of equal amounts of garnet, clinopyroxene, and brown hornblende. Common accessory minerals (0–10%) are orthopyroxene, spinel, rutile, ilmenite and others. Some samples of garnet-free rocks may contain up to 20% orthopyroxene. These rocks show clear evidence for their early formation. They also locally exhibit mingmatization and recrystallization textures in the vicinity of coarser leucosomes. Kelyphite textures around garnet grains range from narrow local rims containing orthopyroxene and plagioclase and broader gedrite–plagioclase rims to complete pseudomorphs of fine-grained plagioclase–hypersthene– green spinel symplectite, containing considerable proportions of ore mineral. Plagioclase often shows evidence of secondary muscovitization. The term “eclogite-like” is chosen to reflect the presence of the almandine–pyrope garnet coexisting with clinopyroxene, whose low Na2O content (0.54–0.68 wt %) does not allow these rocks to be classified as eclogites s. stricto (Classification …, 1992). The only lens-like body (8 × 20 m) is characterized by a coarse- to giant-grained and strongly heterogeneous texture and a lack of crystalline foliation. A large (2–80 mm) dark red crystal of garnet contrasts with the surrounding hornblende forming aggregates of black, lustrous, randomly oriented crystal, up to 15 mm across. The presence of small (up to 50 cm long) polymineralic lenses gives these rocks a vaguely banded structure. One of these lenses contains mediumgrained segregations (up to 7 cm across and 10 cm long) greenish grey in color, previously described as garnet gedritites (Zhulanova et al., 2014). The lower edge of this lens (6–8 cm) is rimmed by a symmetri-
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cally banded aggregated of gedrite, bytownite, and sapphirine. The least altered sample of eclogite-like rock from our collection with orthopyroxene-plagioclase rims constituting not more than 10 vol.% of the rock (sample А339) is distinguished chemically on the TAS diagram (The Petrographic …, 2009) as an ultramafic picrobasalt (Table 1). Unlike the lavas with a respective basicity, this rock has elevated Al2O3 and CaO contents and lower Mg-number while the range of other parameters is similar to that of hornblendite (The Petrographic …, 2009, Table 6, p. 117). Compared to sample А339, the dated garnet gedritite (sample 329a) has higher SiO2 and K2O contents (Table 1) and plots at the interface of four fields (ultramafic picrobasalts, moderately alkaline picrobasalts, trachybasalts, and basalts). The garnet gedritite has higher Al2O3and lower CaO contents than the eclogite-like rock. Based on major element oxides, it has no analogues in magmatic rocks. In addition, this garnet gedritite has high concentrations of Rb, Ba, Zr, and Hf (Table 1). In the dated sample 329a (10 × 7 × 3 cm, about 600 g), reaction rims (1–5 mm) are discernible with the naked eye around garnet porphyroblasts (10– 15 mm) that make up about 20 vol.% of the rock. Microscopic examination reveals differences in rim composition and internal structure. A detailed petrological examination was conducted by Avchenko O.V. (Zhulanova et al., 2014) on a segment where rounded garnet (up to 1 cm across) is surrounded by a symplectite of orthopyroxene, plagioclase and gedrite with an average grain size of 0.05 × 1 mm. With distance from the reaction rim, these minerals occur as relatively large grains (2–5 mm) forming monomineral granoblastic aggregates, with irregularly distributed biotite constituting up to 3 vol % of the rock. The texture and microprobe analysis of sample 329a indicate two generations of minerals: early minerals (high-Mg garnet, less basic plagioclase, and low-alumina orthopyroxene) and late minerals (garnet with high XFe and more basic plagioclase, high-alumina orthopyroxene and gedrite replacing this garnet). Mineral thermobarometry shows that the early assemblage formed under high-pressure granulite-facies conditions (t = 950– 900°C, P = 1.05–0.95 GPa). The retrograde assemblage gedrite–orthopyroxene–plagioclase indicates amphibolite-facies conditions during late-stage decompression accompanied by a significant decrease in temperature (t = 700–650°C, P = 0.65–0.55 GPa). The high Na2O content of gedrite (2.29–2.41 wt %) as well as high trace element concentrations and wholerock composition of sample 329a suggest that the eclogites-like rocks experienced allochemical retrograde metamorphism in the presence of fluid (Zhulanova et al.,2014). Thin sections of the garnet-bearing gedritite contain abundant zircon grains. They are brownish, transGEOCHEMISTRY INTERNATIONAL
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Table 1. Chemical composition of eclogite-like rocks (sample A339) and garnet gedritite (sample 329a) Component
А339
329а
SiO2
43.45
44.70
TiO2
0.94
1.09
Al2O3
15.57
18.37
Fe2O3 FeO MnO MgO CaO Na2O
4.49 11.00 0.26 10.35 12.59 1.21
2.87 10.25 0.22 14.21 5.32 1.20
K 2O
0.29
1.51
P2O5
0.06 0.26 100.48 n.d.
0.04 0.70 100.49
1.47 23.03 29.09 0.72
43.24 567.50 315.50 12.73
LOI Total H2O– Rb Zr Ba Hf
0.16
parent, 0.05–0.4 mm in size (Fig. 2). Smalls grains are isometric, while larger grains have a slightly elongated prismatic shape and smooth edges. Zircon is visible in all rock-forming minerals, including gedrite and sap-
opx + sp + pl
pl ged
pl
bt zr opx
opx zr 200 μm
Fig. 2. Photomicrographs of garnet gedritite. Zircon is confined to symplectite intergrowths of plagioclase and hypersthene. Thin section 329a” in parallel nicols. bi— biotite, ged—gedrite, opx—orthopyroxene, pl—plagioclase, sp—spinel, zr—zircon. 2016
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phirine. Growth twins and aggregates of grains of different size are described from zircon populations. A specific feature of zircon inclusions in garnet is a set of thin fractures that have the appearance of spider legs. Pleochroic halos in biotite are developed around zircon, which occurs as elongated grains oriented along cleavage cracks. In some cases, zircon grains can be traced as chain-like alignments of different length (up to 10 grains) across a symplectite (blind hidden cracks). Zircons were not found in thin sections of the eclogite-like rocks. Therefore, the petrographic observations indicate that growth of zircon in the garnet gedritite either post-dates or is contemporaneous with the formation of kelyphitic rims. Some small zircon grains may have resulted from the breakdown of the early mineral assemblages. METHODS Zircons were separated from the garnet gedritite (sample 329a) in which the Р–t conditions of formation of a retrograde gedrite-orthopyroxene-plagioclase assemblage were studied (Zhulanova et al., 2014). Zircon grains were extracted using a standard procedure, inlcuding crushing, heavy liquids and electromagnetic separation. To eliminate contamination, the sample was hand-crushed in an iron mortar. More than forty grains together with chips of zircon standards R33 and VP10 were mounted in a 25 mm diameter and 4 mm thick epoxy resin disc and sectioned in half, and the mount surface was then polished to expose the grain interiors. Examination at a range of magnifications (20 to 500×) in transmitted and reflected light allowed identification of areas, which are free of internal fractures and micro-inclusions and suitable for ion microprobe analysis. Polished grains were imaged with cathodoluminescence (CL) and backscattered electrons (BSE) on a JEOL JSM 5600 scanning electron microscope to identify internal structure and compositional zoning. U–Pb dating of individual zircons was performed on the USGS-Stanford sensitive high-resolution ion microprobe reverse geometry SHRIMP-RG at Stanford University Microanalytical Laboratory (California, USA). Analytical procedures for U–Pb dating followed methods described by Williams (1998). The SHRIMP-RG provides higher mass resolution than the standard forward geometry of the SHRIMP I and II (Clement and Compston, 1994). The SHRIMP-RG geometry minimizes the background noise, which, coupled with high-resolution, sample pretreatment with acid and rastering the primary beam for 90–120 s before the analysis allows a high-precision measurement of 204Pb in zircon, inlcuding the surface contamination (the primary ion spot has a diameter of 25 to 30 μm and a depth of 2 μm). Each analysis consisted of five cycles of measurements. The U–Pb zircon data were calibrated against the R33 zircon standard (419 Ma quartz diorite of the
Braintree complex, Vermont, Black et al., 2004), which was analyzed over the course of the analysis runs for reliable Pb/U calibration after every four unknown grains. The concentrations of U and Th were calibrated against the CZ3 standard (550 ppm U). The Pb/U ratio was calibrated using an empirical relationship between 206Pb+/U+ and UO+/U+ and then normalized to the 206Pb/U of the standard zircon. Data reduction was performed using Squid and Isoplot software (Ludwig, 2003) following the methods described elsewhere (Williams, 1998; Ireland and Williams, 2003). A comparison of ID TIMS and SHRIMP analyses of TEMORA1, TEMORA2, and R33 standards shows that the accuracy of SHRIMP 206Pb/ 238U dates can be no better than 1–1.5% (Black et al., 2004). Trace element abundances (15 trace elements, including REE, Hf, U, Th, Ti, and Fe) were measured on the same spots of zircon grains that were used for U–Pb age determinations using homogeneous zircon standards MAD-green (4196 ppm U) and CZ3 (550 ppm U) (Mazdab, Wooden, 2006). Estimated precision errors (2σ) for MAD-green were ±3% for Hf, ±5% for HREE (Dy, Er, Yb), ±10–15% for Ti, P, Sc, Y, and MREE (Ce, Nd, Sm, Eu, and Gd), and ±40% for La (all values). RESULTS A total of 47 zircon grains, 100–300 μm across, were extracted from sample 329a and mounted in epoxy resin, 16 of which were used for isotope geochemical studies (Table 2, Figs. 3, 4). All of them are slightly elongated, grey and pink crystals, oval in shape, with smooth edges. Most crystals display oscillatory-zoned CL-dark cores overgrown by non-luminescent rims with slightly discernible structural elements (Fig. 4). Geochronological data. The concordia curve (Fig. 3) shows a series of discordant analyses (7 spots) and three clusters of sub-concordant analyses (10 spots; < 6% discordance). The oldest cluster of sub-concordant analyses obtained on three zoned cores gave a weighted mean 207Pb/206Pb age of 3232 ± 50 Ma (Table 2, cluster 1; Fig. 4a). One core analysis that forms a separate cluster 2 gave a concordant age of 2.6 Ga (Fig. 3, spot 10). Finally, the rims of large zircon grains and a series of smaller grains define a tight cluster of concordant ages with a weighted mean age of 1907 ± 6 Ma calculated from six determinations (MSWD = 0.8; p = 0.4, N = 6; Table 2, cluster 3; Fig. 4b). Four out of seven discordant analyses plotting away from the concordia line (2, 3, 11.1, 13) lie on the isochron (discordia), whose upper and lower intercepts with concordia correspond to the ages of 3425 ± 28 and 911 ± 39 Ma, respectively (MSWD = 0.33 ± 1σ). Two extreme clusters (1 and 3) with similar statistical parameters may define another discordia because spots 8 and 15 will be plotted on the same line. In this
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89 179 152 200 132 59
2213
880 1726 875 834 1294 147
1225
2 3 15 11.1 8 13
16
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0.026 0.007 0.005 0.011 0.039 1.397
0.579 0.310 0.540 0.172 0.096 0.059
17 47 46 27 21 34
544 1041 726 1002 459 388
1805
4 9 12 11.2 14 7
2 3 15 11.1 8 13
16
8 10 14 11 6 12 9.5
0.6 0.4 1.2 0.7 0.1 0.3
0.1 0.2 0.1 0.2 0.4 1.2
0.2
0.4 2.5 0.8
Nd
437
167 366 306 362 578 73
14 11 65 16 15 20
713
415 222 175
12.2
0.7 1.1 1.8 1.1 0.4 0.6
0.3 0.5 0.3 0.4 0.3 1.0
0.7
1.1 3.1 1.5
Sm
1.87
0.10 0.11 0.18 0.25 0.11 0.41
0.58 0.79 0.22 0.52 0.46 0.85
0.09
0.45 0.33 0.38
8.53
0.52 0.55 1.03 1.14 0.09 0.08
0.18 0.35 0.21 0.31 0.20 0.64
0.06
0.76 3.06 0.81
Eu
2472 ± 12
1425 ± 56 1609 ± 17 2278 ± 11 2896 ± 37 2787 ± 37 3127 ± 55
1883 ± 21 1893 ± 28 1897 ± 14 1908 ± 24 1931 ± 29 1918 ± 21
2573 ± 17
3191 ± 45 3129 ± 25 3072 ± 90
103
7 17 14 12 4 8
1 3 2 3 2 3
9
13 19 8
Gd
3118 ± 3
2584 ± 12 2837 ± 18 2577 ± 4 3340 ± 3 2978 ± 7 3389 ± 17
1885 ± 32 1947 ± 36 1913 ± 12 1921 ± 24 1913 ± 25 1881 ± 41
2608 ± 3
3219 ± 3 3253 ± 4 3213 ± 11
236
40 97 63 78 31 37
3 6 6 4 3 5
46
64 52 22
Dy
+25
+50 +49 +14 +16 +8 +10
+0 +3 +1 +1 –1 –2
+2
+1 +5 +6
268
100 184 129 185 86 68
2 5 6 3 2 4
83
151 74 37
Er
2.14
4.04 3.53 2.36 1.76 1.85 1.60
2.93 2.93 2.92 2.90 2.86 2.84
2.04
1.56 1.60 1.64
Total (1) 238 U/ Pb/206Pb % disc. 206 age, Ma Pb
207
Pb
10773 12662 12052 12219 14199 10517
9713 11290 11842 10069 11227 9635
13976
11486 12160 11376
Hf
0.2400
0.1730 0.2018 0.1727 0.2762 0.2197 0.2851
0.1208 0.1207 0.1172 0.1181 0.1177 0.1299
0.1753
0.2557 0.2614 0.2548
206
Pb/
Total 207
399 10528
243 472 311 433 235 117
1 6 9 3 3 4
132
356 136 77
Yb
0.6
4.4 1.2 0.6 1.6 1.6 2.2
1.3 1.7 0.8 1.4 1.7 1.3
0.8
1.8 1.0 3.7
±%
1225
880 1726 875 834 1294 147
48 36 220 53 49 68
1701
761 431 348
U
0.2
0.7 1.1 0.2 0.2 0.4 1.1
1.2 1.8 0.7 1.3 1.3 1.1
0.2
0.2 0.2 0.7
±% Pb*
Fe
0.6
4.4 1.2 0.6 1.6 1.6 2.2
1.3 1.7 0.8 1.4 1.7 1.3
0.8
1.8 1.0 3.7
668 603 589 918 33 4
3 2 11 2 2 322
2213 1192
89 179 152 200 132 59
27 28 47 27 22 56
146 91
329 40 136 28 129 156
Th
2.14
4.04 3.53 2.36 1.76 1.85 1.60
2.95 2.93 2.92 2.90 2.86 2.89
2.04
1.56 1.60 1.64
206
57.3
19.6 29.7 17.6 34.9 28.8 14.0
11.8 14.2 12.7 14.5 19.9 1375.0
26.1
27.5 24.4 13.7
Ti
0.240
0.173 0.201 0.172 0.276 0.220 0.285
0.115 0.119 0.117 0.118 0.117 0.115
0.175
0.256 0.261 0.255
Pb*
206
(1) U/ ±% 207Pb*/
(1) 238
941
813 859 802 879 856 779
761 780 769 782 815 –
845
851 837 777
to C
0.2
0.7 1.1 0.2 0.2 0.4 1.1
1.8 2.0 0.7 1.3 1.4 2.3
0.2
0.2 0.2 0.7
±% U
0.73
0.73 0.40 0.62 0.97 0.22 0.11
0.90 0.85 0.78 0.94 0.79 1.08
0.08
0.60 1.20 0.69
Eu/Eu*
15.45
5.89 7.88 10.05 21.60 16.38 24.51
5.39 5.62 5.53 5.59 5.64 5.50
11.86
22.57 22.49 21.43
235
Pb*
(1) 207
0.6
4.4 1.6 0.6 1.6 1.7 2.5
2.2 2.6 1.1 2.0 2.2 2.6
0.8
1.8 1.0 3.7
0.467
0.247 0.284 0.424 0.567 0.541 0.624
0.339 0.341 0.342 0.345 0.349 0.347
0.491
0.640 0.625 0.610
0.6
4.4 1.2 0.6 1.6 1.6 2.2
1.3 1.7 0.8 1.4 1.7 1.3
0.8
1.8 1.0 3.7
0.9
1.0 0.7 0.9 1.0 1.0 0.9
0.6 0.7 0.8 0.7 0.8 0.5
1.0
1.0 1.0 1.0
(1) Corr.co ±% 206Pb*/ ± % eff. 238 U
Sub-concordant zircons form three clusters (1, 2, and 3) different in CL zoning patterns, age, and geochemical characteristics; the remaining zircons are discordant. For spots 7 and 16 (data shown in italics), we assume the capture of Ti- and Fe-rich inclusions. Errors of individual zircon analyses are 1σ. 206Pbc and 206Pb* are common and radiogenic lead, respectively. (1)—corrected for common lead using the measured 204Pb. R33 zircon standard calibration uncertainty is 0.27% (not included in errors of individual zircon analyses). Analyses were performed on the USGS-Stanford SHRIMP-RG microprobe at the Stanford University Microanalytical Laboratory (USA, analyst V.V. Akinin). Trace element concentrations in ppm. Temperature (t° С) was calculated based on the concentration of 48Ti in zircon (Ferry and Watson, 2007) provided that the activity of TiO2 and SiO2 in the system is unity.
3
4 3 3 5 7 11
5
30 23 36
Ce
11
10 13 4 11 4 8
1 1 0 0 0 1
1
1.370 231
0.036
500
10
2
0.096 0.845 0.173
5 1 6
1
869 484 225
Cluster Spot nos. nos.
3
La
27 28 47 27 22 56
48 36 220 53 49 68
Y
146
1701
4 9 12 11.2 14 7
2
10
5 1 6
1
1 4 4
(1) Th, 206Pbc, 206Pb* Th/U 206Pb/238U ppm % ppm age, Ma
329 136 129
U, ppm
761 431 348
Cluster Spot nos. nos.
Table 2. Geochronological and geochemical data for zircons from garnet gedritite, Aulandzha Block (sample 329a)
AGE AND GEOCHEMISTRY OF ZIRCON FROM THE OLDEST METAMORPHIC ROCKS 655
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AKININ, ZHULANOVA
0.7
3400
(а) 5 1
3000
13
6 8
2600
0.5
10
2200 1800
0.3
16
1200
15
(b)
206
Pb/238U
11.1
4 7 14 12 9 11.2
Fe, ppm
3
1400
2
0 0.1
0
4
8
12 207
16 20 Pb/235U
24
28
Fig. 3. U–Pb SHRIMP-RG data for zircon from garnet gedritite (sample 329a) plotted in a concordia diagram. Error ellipses are plotted at 1σ. the black and white scale bar shows iron concentrations in studied zircon grains (weak correlation with discordance). For details on two sub-concordant clusters (a) and (b) see Fig. 4.
case, the upper and lower intercept ages will be 3221 ± 40 and 1904 ± 25 Ma, respectively (MSWD = 0.97 ± 1σ). It is theoretically possible to obtain the other discordia line, but the available analytical data are insufficient to assess the validity of concordia intercept ages. Almost all discordant analyses were obtained from zircon cores.
206
0.68
(а) Weighted mean Pb/ Pb age = 3232 ± 50 Ma MSWD = 27 5
Geochemical data. The studied zircon domains form clusters with different concentrations of radioactive elements. For example, the oldest zoned cores (clusters 1 and 2) have several times (an order of magnitude) higher concentrations of U and Th than domains in cluster 3 (348–1701 vs. 36–220 ppm U and 129–329 vs. 22–56 ppm Th, respectively; Table 2). The discordant spots are similar to those in clusters 1 and 2 but show a considerably wider range of U concentrations (147–1726 ppm). The chondrite-normalized REE patterns of the analyzed zircons are characterized by a depletion of LREE relative to HREE (La/YbCH < 0.001), a strong positive Ce anomaly (Ce/Ce* = 7–76) and a weak negative Eu anomaly (Eu/Eu* = 0.6–1.0). The only exception is the core of grain 10 (cluster 2) having a more pronounced Eu anomaly (Eu/Eu* = 0.08, Table 2; Fig. 5а). Oscillatory zoned zircon cores (clusters 1 and 2) are more enriched in HREE than the rims (cluster 3): concentrations of Dy, Er, and Yb in the cores are an order of magnitude higher than in the rims (Table 2; Fig. 5a). In order to test the well-known hypothesis that the concentrations of HREE in zircon decrease due to preferential partitioning of HREE to simultaneously or subsequently crystalized garnet owing to its higher partition coefficient (http://earthref.org/KDD; Van Westrenen et al., 2001), we examined all zircon grains analyzed using the SHRIMP-RG to select inclusions (b) Concord. age = 1907 ± 6.2 Ma MSWD = 0.80 Probability of concordance = 0.37
207
14
3280
0.36
0.66 3200
Pb/238U
3160 3120
0.62
3080 1
0.60
206
206
Pb/238U
0.64
7
3240
0.35 12
1920
0.34
0.58
9
1840
0.33 6
0.56 0.54 19
20
21 207
1
1
22 Pb/235U 5 5
23
24
0.32 5.0
4
11.2
5.2
5.4 207
6 6
5.6 Pb/ U 4 4
6.0
77
11.1 11.1 11.2 11.2
100 μm
5.8
235
99 100 μm
Fig. 4. Paleoarchean sub-concordant (a) and Paleoproterozoic concordant (b) clusters of data points and CL images of zircon grains. GEOCHEMISTRY INTERNATIONAL
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Figure 5c shows a correlation between zircon crystallization temperature and Hf content, while both discordant and concordant analyses with different ages define a common trend. This relationship is generally consistent with the isomorphic substitution of Ti4+ (ionic radius 0.074 nm) to an eightfold coordinated site of Zr4+, also occupied by Hf4+ (ionic radius 0.083 nm) (Hoskin and Schaltegger, 2003). Five out of seven discordant grains have elevated Fe concentrations (from 668 to 1192 ppm, Table 2), which can be explained by zircon alteration and аn enrichment of the damaged metamic structure of zircon in non-formula elements (Geisler et al., 2007). DISCUSSION The oldest sub-concordant analyses of zircon cores yielded a weighted mean 207Pb/206Pb age of 3232 ± 50 Ma (cluster 1) suggest that the OM basement can be no younger than Paleo-or Mesoarchean, since this date corresponds to the Paleo-Mesoarchean boundary (3200 Ma) of the International Chronostratigraphic Chart (Cohen et al., 2012). However, the Paleoarchean age cannot be ruled out based on discordant results obtained on most zircon cores and an upper intercept age of 3.4 Ga. This can be supported by a similarity of our discordia in both intercepts to that of Bibikova (Bibikova et al., 1978). Because the studied zircon generations were produced during retrograde granulitefacies metamorphism of eclogite-like rocks, the Paleoarchean age may constrain the upper (ie minimum) limit for the granulite-facies metamorphism in the OM basement. GEOCHEMISTRY INTERNATIONAL
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Ti-in-zircon crystallization temperatures (Ferry and Watson, 2007) range from 760 to 880°C (Table 2). It was found that sub-concordant cores grew at higher temperatures than rims: 777–851°C for cluster 1 versus 761–815°C for cluster 3 (Table 2, Fig. 5c). The Ti concentrations measured within a single zircon grain ranges from 35 ppm in the core (spot 11.1) to 14 ppm in the rim (spot 11.2; Table 2, Fig. 4, see photomicrographs), which is consistent with a temperature difference of 100°C (880 vs. 780°C). The maximum temperature of 940°C was calculated for one discordant spot 16 having high Тi, Y, Ce, and Fe concentrations, which suggests the probability of capturing a REEbearing phase by ion microprobe. At the same time, this value is within the range calculated from petrological data (see above).
11.1 5 1 10 6
(а)
100
12 9 11.2 14 4
10 1 0.1
La Ce Pr Nd PmSmEu Gd Tb Dy Ho Er Tm Yb Lu
(b) CL
TL
14 SHRIMP pit
100 µm
Garnet: SiO2 = 39.3 Al2O3 = 22.3 MnO = 1.0 FeO = 21.3 MgО = 12.1 CaO = 2.8 (wt %)
(c)
14 000 13 000 Hf, ppm
suitable for microprobe analysis. Two garnet microinclusions (about 10 and 30 μm) were found in the core of a rounded zircon grain (spot 14, Fig. 5b). The analysis showed that the composition of these garnet inclusions (45% almandine, 45% pyrope, 8% grossular, and 2% spessartine) is almost identical to that of the garnet found in association with gedrite (Zhulanova et al., 2014).
657
12 000 11 000 10 000 9 000 700
750
800 850 Temperature, °С
900
Fig. 5. Trace element data for zircons from garnet gedritite (sample 329a): (a)—REE patterns for dated zircon domains; HREE-depleted Paleoproterozoic rims, (b)— photomicrographs of garnet inclusions in Paleoproterozoic zircon (cluster 3, grain 14), CL—cathodoluminescence, TL—transmitted light, to the right – garnet composition, wt %, (c)—relationship between Hf concentrations and zircon crystallization temperature calculated from the concentration of 48Ti in zircon (Ferry and Watson, 2007) provided that the activity of TiO2 and SiO2 in the system is unity. REE concentrations are normalized to chondrite values (McDonough and Sun, 1995). The composition of garnet was analyzed on a Camebax microprobe by V.V. Akinin (Shilo North-East Interdisciplinary Scientific Research Institute, Far East Branch, Russian Academy of Sciences). Data points on plots (a), (c): filled squares are cores of large zircon grains (cluster 1), open squares are rims of large zircon grains and small grains (cluster 3), star denotes a grain yielded a concordant age of 2.6 Ga (Fig. 3, spot 10).
At the same time, several features of Paleoarchean cores of large zircon grains, such as oscillatory zoning under cathodoluminescence and transmitted light and 2016
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elevated Th/U (0.33–0.45) are more commonly observed in igneous than in high-grade metamorphic zircons (Rubatto et al., 2001; Hoskin and Scaltegger, 2003), although the latter indicator, as such, cannot be used as a universal discriminant for the origin of zircon (Harley et al., 2007). The available data show that the protolith of the studied eclogite-like rocks was Al-rich ultramafic rocks (hornblendite group). Such rocks are interpreted as cumulates of ultramafic Zr-undersaturated magmas and it is highly unlikely that they would contain primary igneous zircons. Therefore, a metamorphic origin for Archean cores of these zircons cannot be ruled out. A concordant Neoarchean age of 2.6 Ga obtained from the core of one zircon grain (cluster 2) marks the final stage of the Archean evolution of the OM basement. This date is consistent with previous ages obtained on zircons from granite–gniesses of the Aulandzha Block inlier, which, according to geological evidence, obviously post-date the eclogite-like rocks and apobasitic charnockitoids (sample 328a, our unpublished data), as well as with an 207Pb/206Pb age of 2810 ± 50 Ma obtained by Bibikova (1989) on a zircon from granite–gneisses of the Paren’ Block. One zircon grain with a concordant age of 2.6 Ga has the highest U concentration compared with other Archean zircons (1701 and 348–761 ppm, respectively, Table 2), which is typical of the ancient granitoids and other granitization products (Bibikova, 1989). The presence of a strong Eu anomaly (Eu/Eu* = 0.08) in this zircon grain may theoretically reflect either plagioclase fractionation during melt crystallization or a reducing environment of zircon growth, which leads to the stability of two-valent cations Eu2+ incompatible with Zr4+ because of their larger ionic radii (0.125 nm vs. 0.084 nm). The second explanation for Eu anomaly seems to be more plausible given that the 2.6 Ma zircon was extracted from the garnet-bearing gedritite and that this age is close to the timing of formation of Neoarchean granite-gneisses in the region. In other words, the age of 2.6 Ma obtained in this study probably reflects the changes in the U–Pb isotope system of Paleoarchean zircons from garnet gedritites during metamorphism, which was accompanied the emplacement of terminal Archean granitoids to the OM basement. The rims of the studied zircons with a Paleoproterozoic age of 1.9 Ga (cluster 3) differ markedly in their features from the Archean cores. They are colorless and transparent in transmitted light and are virtually free of fluid and mineral inclusions. They also have an order of magnitude lower U and Th, generally lower REE and distinctly lower crystallization temperatures. These features are characteristic of rounded zircon grains in cluster 3. Based on these results, the age of 1.9 Ga can be interpreted to reflect the timing of solid-state recrystallization of garnet gedritites (Hoskin and Schaltegger, 2003). The difference in Ti-in-zircon temperatures
between core and rim of grain 11 (880 vs. 780°C) points to the retrograde character of Paleoproterozoic metamorphism. Petrographic observations support the potential presence of multiple generations of symplectites in the studied rocks (late generations can be represented by fine-grained dactylitic intergrowths of spinel and heavily muscovitized plagioclase). The presence of microscopic garnet inclusions in a small rounded zircon (grain 14), which are similar in composition to garnet found in association with gedrite, may provide the basis for interpreting a population of 1.9 Ga zircons with a similar morphology as a separate generation. At the same time, the presence of discordia (clusters 1 and 3, spots 8 and 15) may reflect the impact of a Paleoproterozoic thermal event on the U–Pb isotope system of early generation zircons. The established genetic link between the studied zircons and retrograde metamorphism of eclogite-like rocks suggests that the granulite-facies rocks predate the amphibolite-facies metamorphism in the OM basement. This conclusion has important implications for understanding the evolution of pre-Riphean basement of Northeast Asia, as well as for updating the serial legends of new-generation geological maps. CONCLUSIONS The results of the first isotope–geochemical study of zircons from garnet gedritites produced during retrograde metamorphism of eclogite-like rocks of the Aulandzha Block are consistent with a protracted, multistage history of pre-Riphean crystalline basement of the Omolon massif. The earliest episode manifested by the formation of a polymetamorphic complex occurred during the Paleoarchean (3.25–3.22 Ga, possibly to 3.4 Ga). This episode is recorded by oscillatory-zoned zircon cores having high HREE and Ti contents. The Neoarchean (ca. 2.6 Ga) metamorphic episode has left a less distinct imprint. Our results indicate that the Paleoproterozoic (1.9 Ga) metamorphic event that caused the formation of garnet gedritite under amphibolite-facies conditions is recorded by transparent recrystallization rims of preexisting large zircon grains and small newly-formed grains, which show distinctly lower crystallization temperatures and one order of magnitude lower concentrations of U, Th, and HREE compared to their cores. The age of 1.9 Ga is consistent with all previous Paleoproterozoic dates, whichat mark the final stages of basement evolution within the Omolon massif and the formation of the mature continental crust (Kotlyar et al., 2001; Shevchenko, 2006). ACKNOWLEDGMENTS The authors thank M. Coble (Stanford University) for technical assistance in performing SHRIMP-RG dating, and E.V. Bibikova for thoughtful and valuable
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suggestions for their thoughtful and valuable suggestions. Zircon studies were supported by the Russian Foundation for Basic Research (project nos. 12-05-00874 and 1605-00949) and NSF (EAR-0948673, PI—E. L. Miller). The preparation of the manuscript was supported by the Far East Branch of the Russian Academy of Sciences (project no. 15-I-1-008) and federal grants for government contracts on research project no. 2 of the Shilo NorthEast Interdisciplinary Scientific Research Institute, Far East Branch, Russian Academy of Sciences. REFERENCES E. V. Bibikova, V. A. Makarov, T. V. Gracheva, and K. B. Seslavinskii, “Age of the oldest rocks of the Omolon Massif,” Dokl. Akad. Nauk SSSR 241 (2), 434–438 (1978). E. V. Bibikova, U-Pb Geochronology of the Early Stages of the Evolution of Ancient Shields (Nauka, Moscow, 1989) [in Russian]. L. P. Black, S. L. Kamo, C. M. Allen, D. W. Davis, J. N. Aleinikoff, J. W. Valley, R. Mundil, I. H. Campbell, R. J. Korsch, I. S. Williams, and C. Foudoulis, “Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards,” Chem. Geol. 205, 115–140 (2004). Classification and Nomenclature of Metamorphic Rocks: A Manual, Ed. by N. L. Dobretsov, O. A. Bogatikov, and O. M. Rosen (OIGGM SO RAN, Novosibirsk, 1992) [in Russian]. S. W. J. Clement and W. Compston, “Ion probe parameters for very high resolution without loss of sensitivity,” US Geol. Surv. Circ. 1107, (1994). K. M. Cohen, S. Finney, and P. L. Gibbard, International Chronostratigraphic Chart: Chart Reproduced for the 34th International Geological Congress (International Commission on Stratigraphy, 2012). J. M. Ferry and E. B. Watson, “New thermodynamic models and revised calibrations for the Ti-in-zircon geothermometer,” Contrib. Mineral. Petrol. 154, 429–437 (2007). T. Geisler, U. Schaltegger, and F. Tomaschek, “Re-equilibration of zircon in aqueous fluids and melts,” Elements 3, 43–50 (2007). M. L. Gelman, “Geological problems of the oldest metamorphic complexes of the Northeast USSR,” in Main Problems of Biostratigraphy and Paleogeography of the Northeast USSR, Ed. by K.V. Simakov (DVNTS AN SSSR, Magadan, 1974), pp. 73–79 [in Russian]. S. L. Harley, N. M. Kelly, and A. Moller, “Zircon behaviour and the thermal histories of mountain chains,” Elements 3, 25–30 (2007).
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P. W. O. Hoskin and U. Schaltegger, “The composition of zircon and igneous and metamorphic petrogenesis,” in Zircon, Ed. by J.M. Hanchar and P.W.O. Hoskin, (Mineral. Soc. Am., Washington, 2003), Rev. Mineral. Geochem. 53, 27–62 (2003). T. R. Ireland and I. S. Williams, “Considerations in zircon geochronology by SIMS,” in Zircon, Ed. by J.M. Hanchar and P.W.O. Hoskin, (Mineral. Soc. Am., Washington, 2003), Rev. Mineral. Geochem. 53, 215-241 (2003). I. N. Kotlyar, I. L. Zhulanova, T. B. Rusakova, and A. M. Gagieva, Isotope Systems of the Magmatic and Metamorphic Complexes of Northeast Russia (SVKNII DVO RAN, Magadan, 2001) [in Russian]. K. R. Ludwig, “Isoplot 3.00, a geochronological toolkit for Excel,” Berkeley Geochronol. Center Spec. Publ., No. 4, (2003). F. M. Mazdab and J. L. Wooden, “Trace elements analysis in zircon by ion microprobe (SHRIMP-RG); technique and applications,” Geochim. Cosmochim. Acta 70. Suppl. 1, A40 (2006). W. F. McDonough and S.-S. Sun, “The composition of the Earth,” Chem. Geol. 120, 223–253 (1995). Petrographic Code of Russia. Magmatic, Metamorphic, Metasomatic, and Impact Rocks, Ed. by O. A. Bogatikov and O. V. Petrov (VSEGEI, St. Petersburg, 2009) [in Russian]. Resolution of 3rd Interdisciplinary Regional Stratigraphic Conference on the Precambrian, Paleozoic, and Mesozoic of Northeast Russia, Ed. by T.N. Koren’ and G.V. Kotlyar (VSEGEI, St. Petersburg, 2009) [in Russian]. D. Rubatto, I. S. Williams and I. S. Buck, “Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia,” Contrib. Mineral. Petrol. 140, 458–468 (2001). V. M. Shevchenko, Archean and Proterozoic of the Omolon Massif. Petrology and Isotope Age (SVNTs DVO RAN, Magadan, 2006) [in Russian]. W. Van Westrenen, B. J. Wood, and J. D. Blundy “A predictive thermodynamic model of garnet-melt trace element partitioning,” Contrib. Mineral. Petrol. 142, 219–234 (2001). I. S. Williams “U-Th-Pb geochronology by ion microprobe: applications of microanalytical techniques to understanding mineralizing processes,” Rev. Econ. Geol. 7, 1–35 (1998). I. L. Zhulanova, O. V. Avchenko, and O. I. Sharova, “Garnet metaultramafites and garnet gedritites of the Omolon microcontinent: deep diaphthoresis and its geological– tectonic interpretation (Northeast Russia),” Fundamental. Issled. 8 (6), 1393-1399 (2014). http://search.rae.ru I. L. Zhulanova, Earth’s Crust of Northeast Asia in the Precambrian and Phanerozoic (Nauka, Moscow, 1990) [in Russian].
Translated by N. Kravets
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