Origin of Zircon from a Lherzolite Lens Based on its U-Pb SHRIMP Age, Lu-Hf Isotopic System and Trace Elements: An Input to Revealing of 2.8Ga Magmatism in the Paleoarchean Bug Granulite Complex (The Ukrainian Shield)

The BGC is located in the southwestern Dniester-Bug Province of the Ukrainian shield (Figure 1A) and consists of felsic orthogneiss with minor mafic-ultramafic enclaves. The BGC units show imprints of several geological events: 3.7-3.66, 2.8, 2.3 and 2.0Ga [1-3]. Age of 3.3Ga has been recently obtained for magmatic zircon from the orthopyroxenite enclave (zircon, U-Pb SHRIMP, [3]). The broad peak around 2.8Ga on the histogram of U-Pb zircon ages reflects the age of high-temperature metamorphic event [1,2]. But simultaneous igneous mafic-ultramafic rocks were mot found within studied area. So, the purpose of our study is to fill this gap of magmatic activity.

The BGC (Figure 1A) consists of foliated and migmatized enderbites (now orthogneisses) with enclaves of mafic rocks and layers of met sediments [2,4]. The study area (Figure 1B) is a typical example of all these rocks. In addition, a few ultramafic enclaves occur in the area. Orthogneiss is 3.8-3.6Ga old [3,5-8]. Geological record in all these rocks demonstrates their complicate tectonic evolution. The deformation D1 led to a N-S-striking or NW-SEstriking steep foliation and sometimes was accompanied by weak migmatization. The next deformations (D2 and D3) resulted in W-E to WNW-ESE trending steep shear zones and foliations of the same orientation.

Enclave UR17/2 is a small lens-like body of a lherzolite mantled by a westernize, occurring in the orthogneiss (Figure 1C). The upper (in the picture) pinching part of the lens (17/2-4, Figure 1C) is represented by a pargasite-plagioclase ortho-pyroxenite. This lens and orientations of its structural elements (Figure 1B) formed during the second deformation when the W-E to WNW-ESE striking shear zone and steep lineation developed. The lherzolite consists mainly of olivine, orthopyroxene, clinopyroxene, phlogopite, minor serpentine, carbonate and accessory chrom spinel, pentlandite, apatite, magnetite (Figure 1D). A mineral assemblage of websterite is the same beside the absence of olivine and the higher abundance of phlogopite (Table 1). While the lherzolite hasusual mineral composition its geochemistry is atypical for mantle rocks: low # mg (0.86), high Ni (>3000ppm) and Ni/Cr>4, which makes the age of the lherzolite more interesting [9]. The pinched part of the lens is strongly recrystallized under high-grade conditions and consists completely of metamorphic minerals: Opx, Pl, Prg, Mgt (Figure 1B, UR17/2-4, Table 1).

Table 1:Major element composition of minerals from lherzolite, web sterite and inclusions in zircon from a peridotite lens UR17/2.

Figure 1:Figure 1A: Location of the Dniester-Bug Province in the tectonic structures of the Ukrainian Shield (grey, tectonic provinces; dotted, transitional zones separating provinces).

Figure 1B: Sketch geological map of the northern part of the Odessky quarry [3].

Figure 1C: Geological sketch of exposure UR-17/2.

Figure 1D: Stereo plot of planar and linear fabrics in exposure UR-17.

Zircon was separated from a websterite margin (sample UR 17/2-3; Figure 1C). Zircon population is characterized by the heterogeneous morphology. Using Back-Scattered Electron image (BSE) data we approximately may distinguish three zircon groups (Zircon I, II, III). Zircon I forms rounded grains ca. 200*250μm in size, some grains are faceted (Figure 2). Several small zircon grains hosted in orthopyroxene are interpreted as zircon I. In Cathodo- Luminescence (CL) images these grains are medium or weak and display a broad zoning. In BSE both small and large grains of zircon I are homogeneous and grey in color.

Zircon II represented by 3 grains, CL images reveal a two-phase internal texture: the dark grains contain CL-bright domains with a complex texture. BSE images show an irregular pattern of grey and light-grey domains corresponding to different Zr/Hf ratios (Figure 2).

Figure 2:Cathodoluminescence and back-scattered electron images of selective zircons from groups I-III. Numbers of grains correspond to the numbers in Table 1 & Table 3. The analytical craters (white circles) with numbers and the measured 207Pb/206 Pb ages are indicated.

Table 2:Analytical U-Pb SIMS SHRIMP data on zircons from web sterite UR 17/2-3.

Major and trace element analyses were produced using a JED-2200 (JEOL) microprobe coupled with an SEM JSM-6510LA spectrometer at the Institute of Precambrian Geology and Geochronology, RAS, St.-Petersburg. The microprobe analyses were carried out with current settings of 20kV and 1.5nA, and the ZAFcorrection method was used (Table 2).

The majority of zircon I and II shows a narrow range of REE concentrations and a typical magmatic chondrite-normalised REE profile (La to Lu slope, Eu and Ce anomalies; Figure 3; Table 3. Small negative Eu- and high positive Ce- anomalies, high amounts of Y and Hf up to 11000ppm suggest the magmatic origin of zircon [10,11]. Zircons I and II having the moderate Th and U concentrations and the moderate Th/U ratio (average 0.73) are typical for magmatic rocks (Table 3). Zircon grains 3.1, 5.1, 9.1 reveal a more flatter REE pattern (average Lun/Lan=306, Smn /Lan=3; Figure 3). These grains contain significant amounts of Ca, Ti, Li, Sr, Nb, Ba caused by secondary alteration produced by H2O-rich fluid [12].

Figure 3:Chondrite-normalized REE patterns for zircon from the websterite (sample UR 17/2-3).

Table 3:Trace element and REE concentrations in zircon from web sterite (UR17/2-3).

Table 4:Composition of zircon grains web sterite (UR-17/2-3).

Figure 4:Mineral inclusions in zircon. Numbers within the inclusions correspond to those in Table 1 & Table 4. The hydrothermal changes (“wormy convolute textures”) are seen in plot “E” (026, 027) and “J” (059). The composition of zircon: points 9, 26, 27, 35, 36, 48, 59 are in Table 4.

Orthopyroxene, amphibole and phlogopite inclusions in both zircon groups are similar in composition to those from the host websterite (Table 1). These petrographic and geochemical data suggest that zircon growth was essentially synchronous with orthopyroxene and phlogopite crystallization in the websterite matrix. Zircon III is represented by elongated grains composed of cores visible in BSE filled by numerous mineral inclusions, and rims with radial fractures. These grains are strongly altered as indicated by dapple texture and wormy convolute texture seen in BSE images (Figure 4). Zircon III strongly contrasts with zircon I and II in Lu, Li, U concentrations, and Lu/Hf, Th/U ratios (Table 3). Zircon III is characterized by a more flat REE distribution. Chemical composition of zircon III corresponds to that of zircon from ordinary mafic rocks [13]. But low Th/U ratio indicates the metamorphic alteration of zircon III (Hoskin and Schaltegger, 2003). High abundances of Ba, P, Ca, Nb, low Lun/Lan(average = 258), and Eu-, Ce-anomalies make them similar to the altered grains of zircons I and II. So, zircon III is magmatic in origin and was strongly altered by fluids. Distinguished zircon groups are not detectable in Zr/Hf ratio. Although some variations exist the average Zr/Hf value is about 40 (Table 4).

Analytical craters are of 30x2μm in size. Rho-correlation coefficient. The last hydrothermal process recovered in zircon III formed wormy convolute texture of zircon and altered mineral inclusions (Figure 4). The convoluted domains contain 1.5-1.6wt.% of Hf and significant amounts of non-formula elements such as Fe, Mn, Ca, Mg and Na (Table 4). Mineral inclusions are oval or rounded in shape. Their fine-grained matrix composed of intensively altered Opx and Cpx, Phl, minor Ap and Carb (Figure 4); (Table 1). Fractures cross-cutting all internal textural elements of zircons are filled by very fine opaque minerals.

14 isotope in-situ analyses [3] were performed for 10 zircon grains separated from the websterite (UR17/2-3, Figure 1C; Table 2). A regression line through the ten data points defines upper intercept at age of 2814±51 Ma and lower intercept at 1474±140 Ma (Figure 5). Value 2814 Ma is not entirely acceptable because

Figure 5:U-Pb concordia diagram for zircon from the websterite (sample UR 17/2-3). Inset shows U-Pb SIMS isotope ages histogram for the Bug Granulite Complex, n= 420; the UR17/2-3 data are shown in black. The age values of the grains with discordance more 10% are excluded.

1. Some points do not lie close to the regression line.

2. The lower intercept age is difficult to interpret.

Despite the existences of the experimental points near the lower intercept of the Discordia, it seems that the age of 1474±140 Ma has no geological meaning because all known U-Pb zircon ages vary from 3.75 to 1.9Ga (Figure 5). Furthermore, a Rb-Sr isochron (calculated for phlogopites, ultramafic samples UR17/2-2 and UR135) give the age of 1700±19Ma (unpublished our data). K-Ar ages from the studied Bug region show the peak values for biotite of 1775Ma (35% of all ages), and a range of 1975-2125Ma for amphibole (63% of all ages) [14]. These data represent cooling ages of the Bug granulite complex during its exhumation. Possibly, the age of 1475±140Ma may identify local hydrothermal alteration.

207Pb/206Pb ages determined for six sub concordant grains (discordancy less than 7%) of zircon I are within the range of 2.83-2.77Ga. Two sub concordant grains (4 points) of zircon II yield 207Pb/ 206Pb ages of 2.77 to 2.72Ga (Table 2). So, the age of magmatic crystallization of zircon I are assumed to be of 2.83-2.77Ga. Probably, the age of about 2.77-2.72Ga reflects a metamorphic process which changed composition of zircon and mineral inclusions.

Hafnium isotope analyses [3] was performed for 10 zircon grains from the websterite which were dated by the U-Pb method. These yielded similar initial 176 Hf /177Hf ratios (0.280690- 0.280749) and identical values of 176Lu/177Hf and 176Yb/177Hf except the grain 4.1 (zircon III) with very high U (6675 ppm; Table 2). In case of the high Lu/Hf ratio LA-MS ICP technique used here does not permit a correct evaluation of the 176Hf/177Hf isotopic ratio. Hafnium isotopic data for the grain 4.1 are not robus and were excluded from interpretation. Meanwhile, not only the grain 4.1 (zircon III) has higher Lu/Hf in comparison with zircon I and II. As seen in Table 3, other grains of zircon III have also high Lu amounts and high Lu/Hf ratio. It suggests that studied websterite crystallized from not completely homogeneous melt.

The Lu-Hf isotope measurements were performed using a 193- nm Ar-F COMPex-102 laser, a DUV-193 ablation system (Lambda Physic) and a Neptune MC-ICP-MS (Thermo Fisher) at the CIR. The Faraday cups’ configuration allowed the simultaneous registration of 172Yb, 174Yb, 175Lu, 176(Hf+Lu+Yb), 177Hf, 178Hf and 179Hf. The position of data-points of zircon grains on the diagram εHf(t)- εNd(t) within “terrestrial array” indicates their crystallization from the melt (Figure 6A) [15]. If we use Nd isotopic data on the lherzolite (Table 5), the data-points shifted to the area of xenogenic zircons contained inherited non-radiogenic hafnium. These data indicate the zircon crystallization from a hybrid melt.

Figure 6:Hf and Hf-Nd isotope plots for zircon from websterite (sample UR17/2-3).

Figure 6A: Isotopic Hf-Nd systematic. Solid lines define area for magmatic zircons. The location of zircon points (1) for websterite with εNd(t) values (Table 6) and (2) those assuming crystallization from the melt with Nd-isotopic characteristics of lherzolite UR 17/2-2.

Figure 6B: Hf evolution diagram. Evolution lines: depleted mantle - 176Lu/177Hf=0.0384 1 - ultramafic rock - 176Lu/177Hf=0.028, 2 - gneisses - 176Lu/177Hf=0.009.

Table 5:Hf isotopic compositions of zircon separated from web sterite (UR-17/2-3).

Table 6:Sm-Nd isotopic data for the rocks of the peridotite lens.

Calculated εHf (2.8Ga) varies from -8.0 to -10.7 (average value -9.0) (Figure 6B). Negative εHf(t) value shows that a pre-existing crust was assimilated by peridotite melt (Table 6).

Thus, the difference between the Nd isotope composition of lherzolite and websterite is consistent with a model of contamination of original lherzolite melt by the older rock. If the minimal value εHf (2.8Ga) corresponds to the maximal contribution of a gneiss contaminant with εHf(t) of 0.007-0.009 [16,17], a two-stage Hf model age of ca. 3.65Ga may be calculated for a contaminant.

A synthesis of combined Lu-Hf and U-Pb isotopic zircon data indicates hybrid origin of melt from which zircon crystallized. Practically all zircon grains show signs of alteration which increases from zircon I to zircon III. This alteration was accompanied by a local redistribution of Zr and Hf and indicates a significant fluid reworking that led to redistribution of these elements in zircons II and III. In general, all zircon grains maintain near chondritic Zr/Hf ratios. The last process during which the wormy convolute texture was formed caused a strong decrease in Zr/Hf and was probably synchronous with the alteration of pyroxene inclusions in zircon and with the origin of carbonate in zircon and in the rock matrix.

The mantle-derived lherzolite melt entrained the older crust. The marginal position of the websterite (Figure 1C) suggests the host orthogneiss as a contaminant. It is supposed that a peridotite magma was hot enough for TTG-gneiss melting. Melt intruded into orthogneiss formed dykes and geological setting of the investigated lenses does not contradict with this proposition. The geochemical and Lu-Hf isotope data show that zircon from lherzotite lens crystallized from a hybrid websterite melt which formed as a result of assimilation of original ultramafic melt by partially molten host orthogneiss.

The similarity of the mineral inclusions in zircon and minerals in host rock indicates that the age 2814±51 Ma corresponds to the intruding time of the lherzolite dyke. Thus, it evidences a pulse of ultramafic magmatic activity in the Dniester-Bug Province about 2.8Ga ago.

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