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 Bug granulite complex (BGC) comprises mainly Paleoarchean orthogneisses and represents the oldest portion of the Ukrainian shield. It has a very long (3.8-2.0Ga) and a complex evolutionary history and contains numerous mafic and ultramafic enclaves of different age and origin. This study is focused on a zoned peridotite lens within the orthogneisses. Zircon separated from a marginal websterite part of the lens yielded a U-Pb (SIMS SHRIMP II) age of 2814±51Ma. In-situ U-Pb and Lu-Hf isotopic analyses combined with BSE and CL imaging and trace element data provided evidence of magmatic origin of zircon. The εHf (2.8Ga) of zircon varies from -8.5 to -10.7 and indicates that the ultramafic melt contained an old component probably assimilated from the host orthogneiss. Field observations and obtained analytical data suggest that the lens is a fragment of a lherzolite dyke, and the age 2.81Ga is interpreted as the age of its crystallization.


Introduction
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][2][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 not found within studied area. So, the purpose of our study is to fill this gap of magmatic activity.

Geological Setting
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][6][7][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 websterite, 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 orthopyroxenite . 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. A mineral assemblage of websterite is the same beside the absence of olivine and the higher abundance of phlogopite (Table 1). While the lherzolite has usual 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).

Morphology and Composition of Zircon
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).       Table 1 & Table 3. The analytical craters (white circles) with numbers and the measured 207Pb/206 Pb ages are indicated. 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 Lu n /La n =306, Sm n /La n =3; Figure 3). These grains contain significant amounts of Ca, Ti, Li, Sr, Nb, Ba caused by secondary alteration produced by H 2 O-rich fluid [12].  Trace element concentrations were analyzed by ion microprobe Cameca IMS-4f (Jaroslavl branch of PhT1 RAS, Russia). Lu/Hf are the isotopic data (see Table 5), *-for the calculation of Lu/Hf in zircon grains R12, R14 and R15 were used elemental data (this Table). 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 4E). 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 "w" -zircons with wormy convolute texture of II or III group, nd -not detectable. The analytical method used is as in the Table 1.

Arch & Anthropol Open Acc
alteration of zircon III [11]. High abundances of Ba, P, Ca, Nb, low Lu n /La n (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).
The last hydrothermal process recovered in zircon III formed wormy convolute texture of zircon and altered mineral inclusions ( Figure 4E). 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 crosscutting all internal textural elements of zircons are filled by very fine opaque minerals.

Zircon U-Pb SIMS SHRIMP Age
14 isotope in-situ analyses for the used technique see, [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

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 subconcordant grains (discordancy less than 7%) of zircon I are within the range of 2.83-2.77Ga. Two subconcordant 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.

Lu-Hf Isotopic System in Zircon
Hafnium isotope analyses for the used technique see, [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 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 6), 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 6A: Isotopic Hf-Nd systematic. Solid lines define area for magmatic zircons [15]. 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.  The Lu-Hf isotope measurements were performed using a 193nm 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. 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 5).
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.

Discussion and Main Conclusion
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.