Genesis of positive Eu anomalies in acid rocks from the Eastern Baltic Shield

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<ul><li><p> 439</p><p>ISSN 0016-7029, Geochemistry International, 2006, Vol. 44, No. 5, pp. 439455. Pleiades Publishing, Inc., 2006.Original Russian Text E.N. Terekhov, T.F. Shcherbakova, 2006, published in Geokhimiya, 2006, No. 5, pp. 483500.</p><p>INTRODUCTIONLong-term studies of high-grade metamorphic rocks</p><p>in the eastern part of the Baltic Shield resulted in thefinding of their abundant acid varieties with positive Euanomalies. Rocks exposed at the Earths surface mostcommonly have negative Eu anomalies (if any) [1], andthus, studying rocks with positive anomalies is of sig-nificant interest and may hopefully provide insight intothe behavior of Eu in the Earths crust. Most of our sam-ples were collected at the LaplandBelomorian Belt(LBB) and the closest surroundings (Fig. 1), althoughanalogous or fairly similar rocks with positive Euanomalies were also found elsewhere at the BalticShield [2, 3].</p><p>The Eu anomaly is calculated by dividing the chon-drite-normalized Eu concentration of a rock by the half-sum of the normalized concentrations of Sm and Gd(the two neighboring elements) and is denoted Eu/Eu*.The deepest negative Eu anomalies, which likely deter-mine the composition of the upper crust, occur in gran-ites and metasomatic rocks, whose Eu/Eu* are some-times less than 0.01 [4], i.e., their Eu concentrations are100 or more times lower than they can be expectedjudging from the concentrations of other REE. No vol-canic rocks of mantle provenance show Eu depletion(i.e., Eu/Eu* = 0.65) typical of the average compositionof the upper crust [1], and MORB, within-plate volca-nics, and volcanic rocks of island arcs equally rarelydisplay positive and negative Eu anomalies. This putsforth the question as to where is Eu concentrated. Thewide occurrence of post-Archean rocks with negativeEu anomalies and, as a consequence, the typical REEpatterns of post-Archean sediments (the PAAS line</p><p>reflects the average composition of the eroded rocks)provides indirect evidence of the occurrence of deep-seated (lower and middle crustal) rocks with comple-mentary REE patters, i.e., with positive Eu anomalies.However, most rocks of granulite complexes that arenow exposed at the surface (and could previously belocated in the lower crust) only occasionally have posi-tive Eu anomalies. Persistently present positive Euanomalies are typical only of anorthosites, whichaccount only for insignificant volumes in granulitecomplexes. Anorthosites commonly consist of cumulusplagioclase. Plagioclase, a mineral thought to beresponsible for the development of Eu anomalies [1], isknown to be unstable at depths of &gt;40 km, and, hence,Eu anomalies should be related to some crustal pro-cesses. It is worth noting that rocks with positive Euanomalies are quite often found in Precambrian com-plexes and become progressively rarer in younger com-plexes, whereas the amount of rocks with negative Euanomalies increases. Hence, one of the mechanismsresponsible for Eu enrichment can be its temporaryaccumulation at a certain structuralgeochemicalboundary of the Earths crust. Similar to the apparenthorizon, some boundaries in the Earths crust seem toalways occur at a distance. For example, the boundarybetween ductile and brittle deformations alwaysoccurs at approximately the same depth (1015 kmfrom the surface), regardless of the processes that cantake place.</p><p>ANALYTICAL TECHNIQUESAll of our rock samples were analyzed for La, Ce,</p><p>Nd, Sm, Eu, Gd, Er, and Yb by plasma spectroscopy on</p><p>Genesis of Positive Eu Anomalies in Acid Rocks from the Eastern Baltic Shield</p><p>E. N. Terekhov and T. F. Shcherbakova</p><p>Institute of the Lithosphere of Marginal and Epicontinental Seas, Russian Academy of Sciences, Staromonetnyi per. 22, Moscow, 109180 Russia</p><p>e-mail: tereh@ilran.ru</p><p>Received May 17, 2005</p><p>Abstract</p><p>The eastern part of the Baltic Shield contains an abundance of acid rocks with positive Eu anoma-lies. These rocks are vein granites and blastomylonites of similar chemical composition but with variable K</p><p>2</p><p>Oconcentrations. The rocks are depleted in Ti, Fe, Mg, Ca, Rb, Zr, and REE, but are enriched in Ba and Sr, a factsuggesting a deep-seated nature of the fluids that participated in the genesis of these rocks. A zone favorable forthe derivation of these rocks was transitional from brittle to ductile deformations. The rocks were produced dur-ing the tectonic exhumation of lower and middle crustal material a horizontal extension. Shock decompressionfacilitated the inflow of reduced fluids, which, in turn, ensured the partial melting of the host rocks along openfractures and controlled REE fractionation with the development of Eu maxima.</p><p>DOI: </p><p>10.1134/S0016702906050028</p></li><li><p> 440</p><p>GEOCHEMISTRY INTERNATIONAL</p><p>Vol. 44</p><p>No. 5</p><p>2006</p><p> TEREKHOV, SHCHERBAKOVA</p><p>N41N41N41N42N42N42</p><p>N44N44N44</p><p>PRPRPR222</p><p>PRPRPR111</p><p>PRPRPR111N033N033N033</p><p>200220022002</p><p>KrKrKr</p><p>LBLBLB</p><p>ARARAR</p><p>PSPSPS</p><p>F33F33F33</p><p>LBLBLB</p><p>KpKpKp</p><p>PRPRPR111</p><p>BBBBBB</p><p>BBBBBBARARAR222</p><p>ARARAR222</p><p>ARARAR</p><p>ARARAR</p><p>MBMBMB</p><p>CKBCKBCKB</p><p>PZPZPZ</p><p>ARARAR222</p><p>PRPRPR111I-V 0I-V 0I-V 0</p><p>PRPRPR222</p><p>k24k24k24KKMKKMKKM</p><p>k17k17k17k84, 88, 89k84, 88, 89k84, 88, 89</p><p>941259412594125</p><p>778778778767767767</p><p>941039410394103</p><p>242424</p><p>k79k79k79940594059405</p><p>PRPRPR111</p><p>Topozero</p><p>Topozero</p><p>Topozero</p><p>PRPRPR111</p><p>PRPRPR111</p><p>ARARAR</p><p>ARARAR222</p><p>PRPRPR111</p><p>ARARAR</p><p>ARARAR222</p><p>KMKMKM</p><p>ARARAR222</p><p>k102k102k102k104k104k104</p><p>BBBBBB</p><p>PRPRPR111</p><p>ARARAR222</p><p>ARARAR</p><p>0 30 60 90 km</p><p>Barents Sea</p><p>WHITESEA</p><p>Kem</p><p>Belomorsk</p><p>Caledo</p><p>nides</p><p>1</p><p>2</p><p>3</p><p>4</p><p>5</p><p>6</p><p>7</p><p>8</p><p>9</p><p>10</p><p> b</p><p>b c</p><p>b</p><p>b b</p><p>b</p><p>b</p><p>AR</p></li><li><p> GEOCHEMISTRY INTERNATIONAL</p><p>Vol. 44</p><p>No. 5</p><p>2006</p><p>GENESIS OF POSITIVE Eu ANOMALIES IN ACID ROCKS 441</p><p>a Monospec 1000 analyzer [5] at the Institute of theLithosphere of Marginal and Epicontinental Seas, Rus-sian Academy of Sciences. Ni, Rb, Ba, Sr, Zr, and Ywere determined on a TEFa device at the same institute.Conventional silicate analyses were conducted at theVinogradov Institute of Geochemistry, Siberian Divi-sion, Russian Academy of Sciences. The quality of theanalyses was checked by using internationally certifiedstandards and by comparing the results obtained by dif-ferent techniques for the same samples.</p><p>BRIEF GEOLOGICALPETROGRAPHIC OVERVIEW</p><p>According to their morphological characteristics,acid rocks with positive Eu anomalies from the easternBaltic Shield (Fig. 1) are classed into two groups(Fig. 2). One of them occurs as relatively thin veins ofgranites and orthotectites, which usually cut across thehost rocks or, more rarely, are conformable with them.The other group comprises linearized rocks (blastomy-lonites).</p><p>The </p><p>granites</p><p> occur as veinlets a few centimetersthick and as veins up to 150 cm or, occasionally, up to1.5 m. The granites sometimes occur as orthotectitesthat compose irregularly shaped patches or lenses 2030 cm across. More rarely, orthotectite lenses arearranged along two perpendicular directions. Both thegranites and the orthotectites are very leucocratic rocksand contain no more than 25% biotite. A distinctivefeature of the orthotectites is their texture, which variesfrom coarse-grained to pegmatoid. Another of theirnoteworthy features is biotite rims along boundariesbetween the orthotectites and the host rocks, a featurethat reportedly [6] testifies that the rocks were formedby melting in situ. When the granites with positive Euanomalies occur among complicatedly folded graygneisses or dome structures of charnockites and ender-bites, they occur as randomly oriented veins (Figs. 2d, 2f).In the monoclinal sequences of the Lapland and Belo-morian complexes, the granites most commonlydevelop as veins with strikes parallel to those of thehost structures but with opposite dips (Figs. 2a2c, 2e).The granites usually compose of cutting veins, some ofwhich filled the fractures of brittle deformations. Someminerals of the host rocks, for example, hypersthene,bluish quartz, kyanite, and hornblende can be containedin the granite veins (although in smaller amounts and</p><p>recrystallized). We are not aware of any data on theradiological ages of these granites, and their host rockswere dated at 3.1 (gray gneisses and amphibolites) to1.75 Ga (subalkaline granites) [2]. In the southeasternpart of the Lapland granulite belt (in the Porya Bayarea), the granites are intruded by 1.72-Ga lamproites[7]. Geological evidence indicates that the latter rockswere emplaced into material within a zone of brittledeformations and inherit the structural style from thegranite veins.</p><p>The </p><p>vein rocks</p><p> that looked like granites in the expo-sures turned out to comprise of more than one petro-graphic variety (as can be seen in thin sections). Theseare biotite nebular plagioclase migmatites, leucocraticgranites, orthotectites, and muscovitequartz rocks.The rocks have granoblastic and, in places, hypidio-morphic-granular textures, and many of them exhibittraces of deformations, which are variably pronouncedin individual veins (samples 24/1, 9405/2, N41/2, andN033/3). Due to this, the textures of the rocks are cata-clastic: along with grains 12 mm, which are most typ-ical of these rocks, as they also contain smaller (0.10.2 mm) grains, which have uneven, ragged outlinesand occur between larger grains. Large plagioclasegrains are flattened and have a patchy extinction, andbiotite plates have stringy habits. All of the rock variet-ies have analogous compositions: they consists of pla-gioclase, quartz, microcline, biotite, and occasionalmuscovite and secondary aggregates of epidote-zoisite.The proportions of leucocratic minerals vary, particu-larly the amounts of microcline, which can be con-tained in the rocks in the form of antiperthitic inclu-sions, anhedral interstitial grains, or large porphyro-blasts. Some samples (sample 24/1) contain singleanhedral large (12 </p><p> 7 mm) microcline prisms. In thisrock, a large microcline grain contains small anhedraldomains of brown older plagioclase and myrmekiteswith small vermicular quartz grains that display simul-taneous extinction. The groundmass of this rock con-tains no microcline. Quartz is also unevenly distributedin the rock: the mineral can be contained in the form ofsmall equant or randomly and irregularly shaped grainsin the interstitial space or lens-shaped aggregates up to37-mm long. The quartz shows no traces of deforma-tions and was likely recrystallized after the cataclasisand mylonitization of the rock. Its amount varies from30% (of the total rock volume) in sample 24/1 to 50%in the samples 94103/4 and 9405/2. The strongly silic-</p><p>Fig. 1.</p><p> Schematic geological map of the LaplandBelomorian Belt and surrounding structures with our sampling sites of acid rockswith positive Eu anomalies. (</p><p>1</p><p>) Alkaline intrusions: (</p><p>a</p><p>) Paleozoic, (</p><p>b</p><p>) Late Paleoproterozoic; (</p><p>2</p><p>) Paleoproterozoic granitoids:(</p><p>a</p><p>) postkinematic (1.81.7 Ga), (</p><p>b</p><p>) synkinematic (1.851.8 Ga), (</p><p>c</p><p>) prekinematic (2.62.4 Ga); (</p><p>3</p><p>) Sumian intrusive magmaticrocks (2.52.45 Ga) layered intrusions, coronites (drusites), and gabbro-anorthosites of the LaplandBelomorian Belt); (</p><p>4</p><p>) volcano-sedimentary rocks of the Karelian Formation (2.61.8 Ga): (</p><p>a</p><p>) Ludicovian, Jatulian, (</p><p>b</p><p>) Sumian, Sariolian; (</p><p>5</p><p>) Late Archean(</p><p>a</p><p>) greenstone belts and (</p><p>b</p><p>) kyanite-bearing rocks of the Belomorian Belt; (</p><p>6)</p><p> Archean granite-gneisses of tonalitetrondhjemitegranodiorite composition; (</p><p>7</p><p>) Late ArcheanPaleoproterozoic (</p><p>a</p><p>) migmatites and (</p><p>b</p><p>) garnet amphibolites; (</p><p>8</p><p>) granulites: (</p><p>a</p><p>) LateArchean and (</p><p>b</p><p>) Paleoproterozoic; (</p><p>9</p><p>) major faults and the LaplandBelomorian detachment (heavy line), a normal fault that con-trolled the exhumation of the lower and middle crustal rocks to the surface; (</p><p>10</p><p>) sampling sites of acid rocks with positive Eu anom-alies (sample numbers correspond to those in (</p><p>a</p><p>) Tables 1, 2 and (</p><p>b</p><p>) in [2]). KMKarelian Massif, BBBelomorian Belt, LBLapland Belt, CKBCentral Kola Block, MBMurmansk Block, PSPechenga Structure, I-VImandraVarzuga Structure.</p></li><li><p> 442</p><p>GEOCHEMISTRY INTERNATIONAL</p><p>Vol. 44</p><p>No. 5</p><p>2006</p><p> TEREKHOV, SHCHERBAKOVA</p><p>Subalkaline granite</p><p>Maficgranulites</p><p>Acid granulite</p><p>Enderbite plagioclase migmatite developingGarnet amphibolite and plagioclase migmatite in the bottom part of the Lapland Belt</p><p>Granite-gneiss</p><p>charnockite</p><p>Subalkaline</p><p>granite</p><p> Granite-gneiss of the Archean basement</p><p>Migmatite</p><p> Plagioclase migmatite</p><p>Fine-grained</p><p>Vein granite</p><p> Pink and gray </p><p>100 m500</p><p>2 m10() (b)</p><p>(c) (d)</p><p>(e) (f)</p><p>Granite-gneiss</p><p>BlastomyloniteMafic granulite</p><p>kyanite-bearing rock</p><p>granite-gneiss</p><p> after mafic granulite</p><p>(g) (h)</p></li><li><p> GEOCHEMISTRY INTERNATIONAL</p><p>Vol. 44</p><p>No. 5</p><p>2006</p><p>GENESIS OF POSITIVE Eu ANOMALIES IN ACID ROCKS 443</p><p>ified rocks contain minor amounts of muscovite in theform of thin elongated (23 mm) laths. These granitesseem to be related to acid leaching zones and composetheir axial parts. In some instances, the granitesaffected by acid leaching are completely devoid of theiroriginal features and transformed into muscovitequartz rocks (sample 9435/1) or preserve their originalfeatures in relict grains, as in sample 767/21, which is amuscovitequartz rock with patches of old plagioclaseand skeletal kyanite prisms. The latter indicate that thepristine rock was kyanitegarnetbiotite gneissaffected by leaching. The mafic mineral of the veinrocks is dark olive biotite, whose contents vary fromsingle platelets to 25%. In places, the biotite isreplaced by zoisite or earthy aggregates of epidote.</p><p>It should be emphasized that the vein rocks some-times bear relics of hornblendebiotite (sample 24/1) orbiotite (sample N033/30) plagioclase migmatites, con-taining 1520% biotite. Relics of these pristine rocksare also distinguishable by the occurrence of older pla-gioclase, which is completely saussuritized. The rocksoften display traces of cataclasis, partial mylonitiza-tion, and blastesis, which suggest brittle deformationsin the vein rocks during their final evolutionary stages.The data presented above and the fairly uneven distri-</p><p>bution of microcline in the vein rocks indicate that theywere formed in situ.</p><p>The geological nature of the vein granites with pos-itive Eu anomalies is uncertain and was interpretedvariably. To some extent, this is explained by the factthat these rocks often occur among gray gneisses,whose genesis is also disputable. Most foreignresearchers believe that gray gneisses were producedvia magmatic crystallization, and veins of granites withpositive Eu anomalies are the final products of this pro-cess [8]. Most Soviet and Russian geologists considergray gneisses to be the products of metasomatic alter-ations in mafic rocks, and the granites with positive Euanomalies are thought to correspond to the final stage ofgranitization: the microcline stage of this process [9].The results of our research indicate that granites withpositive Eu anomalies are present not only among graygneisses, but also among granulites, anorthosites, mas-sive and gneissose charnockites and enderbites, subal-kaline granites, quartz monzonites, and kyanite-bearingrocks, and hence, studying these rocks may shed lightonto global events in the evolution of the Earths crust.</p><p>The </p><p>blastomylonites</p><p> compose tectonic zones rang-ing from a few to a...</p></li></ul>