oxygen isotopes in the cretaceous-paleogene granites of primorye and some problems of their genesis
TRANSCRIPT
150
ISSN 1819-7140, Russian Journal of Pacific Geology, 2008, Vol. 2, No. 2, pp. 150–157. © Pleiades Publishing, Ltd., 2008.Original Russian Text © G.A. Valui, E.Yu. Moskalenko, A.A. Strizhkova, G.R. Sayadyan, 2008, published in Tikhookeanskaya Geologiya, 2008, Vol. 2, No. 2, pp. 62–71.
INTRODUCTION
The genetic problem of granite magmas is of perma-nent interest and is widely debatable in the recent geo-logical literature. Isotopic and geochemical data arewidely applied in studying the complex processes ofgranite formation and, especially, the influence ofcrustal contamination on the composition of primarymelts [15, 20, 35–39, etc.]. However, the granitoidrocks of Primorye are isotopically poorly studied. Nosystematic isotopic studies have been done for themwith the exception of a few individual
87
Sr/
86
Sr determi-nations in the areas of the following ore deposits: Vos-tok-2 [7, 8, 23], Tigrinoe [6], Lermontovka [24],Arsen’evskoe [9], and Voznesenskoe [16, 33]. In thiswork, we report the first oxygen isotope data on thegranitoids of Primorye and their genetic interpretation.
BRIEF REVIEW OF OXYGEN ISOTOPE COMPOSITION OF GRANITOIDS
The study of the oxygen isotopic composition of themagmatic rocks (including granite batholiths) showedthat the crustal rocks are enriched in
18
O as compared tomantle magmas having
δ
18
O = 6‰. Sedimentary rockshave higher
δ
18
O varying from +12 to +20‰ [22, 35].
The oxygen isotope composition of magmatic rocksis determined by several factors: (1) the crystallizationtemperature, (2) the
δ
18
O in the initial magma, (3) the
fractionation, (4) the retrograde effect caused by reequili-bration at the subsolidus temperature, and (5) the interac-tion with aqueous solutions [22].
Magmatic rocks show an increase in
δ
18
O withincreasing SiO
2
. The oxygen isotope composition var-ies from +5.4 to +6‰ in ultrabasic rocks; from +5.5 to+7.4‰ in gabbro, basalts, anorthosites, andesites, tra-chytes, and syenites; and increases up to +13‰ in gran-ites and pegmatites [22].
According to Taylor [35], all granites, quartzmonzonites, granodiorites, and tonalites (and their vol-canic analogues), in terms of their oxygen isotopiccomposition, can be subdivided into three groups:
(a) low-
18
O with
δ
18
O less than +6‰;(b) moderate-
18
O with
δ
18
O between +6 and +10‰;(c) high-
18
O with
δ
18
O more than +10‰.The isotopic composition of the granite rocks
depends on the protolith composition. This was demon-strated for the first time by O’Neil and Chappel [22]using the example of Australian New England batholithconsisting of a great number of granite plutons groupedin four series. The mineralogy and geochemistry of thetwo former series indicate that they were formed bymelting of aluminous sedimentary rocks and corre-spond to S-type granites. Two other series were derivedfrom magmatic rocks and correspond to I-type granite.The first group is enriched in
18
O as compared to the lat-ter. The S-type granites have
δ
18
O from +10 to +12.5‰,
Oxygen Isotopes in the Cretaceous–Paleogene Granites of Primorye and Some Problems of Their Genesis
G. A. Valui, E. Yu. Moskalenko, A. A. Strizhkova, and G. R. Sayadyan
Far East Geological Institute, Far East Division, Russian Academy of Sciences, Vladivostok, Russia
Received January 16, 2007
Abstract
—The Cretaceous–Paleogene granites of the Eastern Sikhote Alin volcanic belt (ESAVB) and LateCretaceous granitoids of the Tatibin Series (Central Sikhote Alin) are subdivided into three groups according totheir oxygen isotope composition: group I with
δ
18
O from +5.5 to +6.5‰, group II with
δ
18
O from +7.6 to+10.2‰, and group III with less than +4.5‰. Group I rocks are similar in oxygen isotope composition to thatof oceanic basalts and can be derived by melting of basaltic crust. Group II (rocks of the Tatibin Series) havehigher
δ
18
O, which suggests that their parental melts were contaminated by sedimentary material. The low
18
Ocomposition of group III rocks can be explained by their derivation from
18
O-depleted rocks or by subsolidusisotopic exchange with low-
18
O fluid or meteoric waters. The relatively low
δ
18
O and
87
Sr/
86
Sr in the granitoidsof Primorye suggest their derivation from rocks with a short-lived crustal history and can result from the fol-lowing: (1) melting of sedimentary rocks enriched in young volcanic material that was accumulated in thetrench along the transform continental margin (granites of the Tatibin Series) and (2) melting of a mixture ofabyssal sediments, ocean floor basalts, and upper mantle in the lithospheric plate that subsided beneath the con-tinent in the subduction zone (granites of the ESAVB).
Key words:
granitoids, oxygen isotopes, Sr isotopic ratios, Eastern Sikhote Alin volcanic belt, Primorye.
DOI:
10.1134/S181971400802005X
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OXYGEN ISOTOPES IN THE CRETACEOUS–PALEOGENE GRANITES 151
whereas I-type granites are characterized by valueslower than +10 (from +7 to +9.9‰).
Later studies showed that granite rocks around theworld show significant variations in
δ
18
O [35, 37, 38].Some plutons have a practically constant oxygen isoto-pic composition, which is positively correlated with theinitial
87
Sr/
86
Sr. With increasing distance from conver-gent plate margins, the plutons show an increase in
18
O[35, 37].
Numerous oxygen and strontium isotopic determi-nations were made for the volcanic rocks of Japan. TheHonshu and Hokkaido islands have fairly mature conti-nental crust and consist of two island arc systems: eastand west ones. The East Japan arc through the Idzu arcgrades into the Mariana arc, which is a typical intraoce-anic island arc initiated on the relatively thin (17–20 km)Miocene oceanic crust. The oxygen isotopic composi-tion of the andesite-dominated volcanic rocks of theMariana arc shows no significant crustal contamina-tion. The oxygen isotopic composition varies from +5.5to +6.8‰, demonstrating weak enrichment in
18
O frombasalts to dacites, which can be explained by closed-system fractional crystallization [15, 27, and others].The initial Sr ratio (from 0.70332 to 0.70348) in thesouthern part of the Mariana arc is somewhat lowerthan that in the northern part (0.70365 to 0.70394).
The volcanic rocks of the Japanese islands ascendedthrough thick continental crust and differ from those ofthe Mariana arc in higher
δ
18
O = (+6.3)–(+9.9)‰ and
87
Sr/
86
Sr = 0.70357–0.70684, which indicates an intensecrustal contribution to their genesis [29, 30].
Differentiated series of individual Japanese volca-noes demonstrate two distinct trends: (1) simultaneousincrease of
δ
18
O and initial
87
Sr/
86
Sr from basic to felsicmagmas; (2) a significant increase of
δ
18
O at almostconstant or very insignificant growth of
87
Sr/
86
Sr. Thefirst trend is interpreted as contamination of primitiveisland arc magmas by granites and granodiorites widelyabundant in the central part of Japan, while the lattertrend is explained by contamination of the same mag-mas by metasedimentary rocks [15, 31].
Isihara and Matsuhisa [26] studied the oxygen iso-topic ratios in the Miocene granitoids of the Outer Zoneof southwestern Japan and found that S-type granitoidshave higher
δ
18
O than I-type with the boundary being at+10.5‰. They also consider that parent melts of S-typegranites contained greater amounts of sedimentaryrocks (from 45 to 64 wt %) than those of I-type granites(from 30 to 43 wt %).
CHARACTERISTICSOF THE STUDIED GRANITOIDS
The studied objects were Cretaceous–Paleogenegranites of the Eastern Sikhote Alin volcanic belt(ESAVB) and Late Cretaceous granitoids of the TatibinSeries (Central Sikhote Alin). According to reconstruc-
tions of Khanchuk [28], they were formed in the settingof a transform continental margin (Fig. 1).
Granitoids of the Eastern Sikhote Alin Volcanic Belt
The authors established that the intrusions (Oprich-nenskii, Vladimirskii, Valentinovskii, and Zapovednyi)of the eastern part of ESAVB (at the coast of the Sea ofJapan—I group) form large (tens of kilometers) mul-tiphase bodies consisting of equigranular diorite (88–72 Ma), granodiorite (69–65 Ma), and granite (64–60 Ma,according to K–Ar determinations) rocks [2, 4]. Theycrystallized at 650–750
°
C and belong to the magnetiteseries. The massifs of the western part of the volcanicbelt within the Dal’negorsk district (group II) and Kras-norechensk Rise (group III) are monophase and consistof sharply porphyritic rocks ascribed to ilmenite series.They crystallized at 750–850
°
C and 800–900
°
C, respec-tively. Based on the K–Ar determinations, the ages of theDal’negorsk gabbrodiorites, granodiorites, and granitesare, respectively, 83, 69–72, and 60–63 Ma; the Kras-norechensk monzogranodiorites define ages within 83–87 Ma [3]. They form small bodies (a few kilometers inthe Dal’negorsk district and tens of kilometers in theKrasnorechensk district) and are accompanied by boro-silicate and base metal deposits in the Dal’negorsk dis-trict and tin–base metal deposits in the Krasnorechenskdistrict, whereas intrusions of the coastal group hostonly insignificant magnetite–skarn and molybdenumoccurrences. The thickness of the Earth’s crust withinEastern Sikhote Alin based on seismic and gravimetricdata accounts for 25–30 km, and that of the granitelayer is 5–8 km, while the thickness of the granite mas-sifs is no more than 1.5–2 km [1, 13].
The petrological analysis of the obtained materialsshows that group I of the intrusions was formed fromlower temperature melts that contained less than 3 wt %H
2
O and were generated at lesser depths (15–20 km)than the group II and III massifs. The latter wereobtained from the higher temperature melts with an ini-tial water content more than 3 wt % and at a depth of20–25 km (Dal’negrosk volcanic structure) and 25–30 km(Krasnorechensk Rise). Such a difference was presum-ably caused by deepening of magmatic chambersinward the continent. The initial fluid content deter-mined the different crystallization dynamics and char-acter of the fluid separation. Respectively, in one case,this led to the formation of differentiated massifs ofequigranular rocks (group I), and in the other case, tothe formation of undifferentiated intrusions of sharplyporphyritic rocks and fluid release in the host rocks (thegroup II and III massifs) [3, 4].
The granitoid rocks of the Eastern Sikhote Alin vol-canic belt distinctly demonstrate the differentiation ofthe parental melts at different levels and stages of themelt evolution. Fractional differentiation in the meltgeneration zone led to the formation of a cotectic rockseries having a similar REE distribution [3, 4]; largemultiple diorite–granodiorite intrusions in the eastern
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VALUI et al.
part of the belt; and monophase bodies of gabbrodior-ites, granodiorites, or granites in the Dal’negorsk dis-trict, which were formed from individual portions ofdifferentiated magmas.
From east to west, the degree of melt differentiationdecreases from multiple coastal massifs through theDal’negorsk monophase massifs to monophase weaklydifferentiated monzodiorite–granodiorite bodies of theKrasnorechensk Rise simultaneously with increasingcrustal thickness (from 25 to 30 km).
Granitoids of the Sikhote Alin Plutonic Belt
The intrusions of the Tatibin Series are mainly con-fined to the Western and Central sutures of Primoryeand their subsidiary faults forming the Khungari–Tati-bin or Sikhote Alin [5, 17, 18] plutonic belt. The latterhas no volcanic analogues and consists of individualNE-trending linear plutons. The belt extends meridion-ally over more than 500 km (Tatibin Series) andincludes the Dal’nii Arminskii complex of the biotite–amphibole–granodiorite–adamellite–granite intrusions,as well as almost coeval gabbro–monzonite–syenitesmassifs (Berezovskii and Araratskii intrusions). Based
on K–Ar determinations, the series has an age of 105–85 Ma [5, 14].
The granodiorite–adamellite–granite complex of theTatibin Series is most abundant in the Dal’nii Arminskiiarea of Sikhote Alin and confined to the thickest part ofthe Earths’ crust, which reaches 35–40 km here. Thegravimetric fields above intrusions show local negativegravity anomalies, which indicates a significant thick-ness of the plutons (8–10 km) or the existence of deep-seated granite roots [5, 11, 12, 21].
The Tatibin granite massifs are multiple and occupyareas up to 400 km
2
. The complex consists of two rocktypes: (1) granodiorite–adamellite (86–72 Ma) of theDal’nii, Izluchinskii, Ust Arminskii, and Priiskovyimassifs; (2) adamellite–granite (81–72 Ma) massifs(Arminskii, Vodorazdel’nyi, Mechta, and others).Occasionally both types are juxtaposed in one massif,for instance, in the Priiskovyi Massif.
In the southern part of Primorye, the Tatibin Seriesincludes the Livadiiskii (98–102 Ma) and Krinichnyi(104 Ma) granodiorite massifs and the Uspenskii gran-odiorite–granite massif (106- to 124-Ma-old granodior-ites and 80- to 96-Ma-old granites) [5, 14].
123
100 km0
44°
46°
134° 136° 138°
Vladivostok
Grodekovskii
Krinichnyi
Livadiiskii
Nakhodka
TazgouUspenskii
Zapovednyi
Valentin
Valentinovskii
Evstaf’evskii
Ol’ginskii
Ol’ga
Vladimirskii
Brinerovskii
Rudnaya Pristan
Oprichnenskii
BerezovskiiAraratskii
Kavalerovo
Dal’negorsk
Krasnorechensk
Ternei
Dzhigit R.
Mal. Kema
Kem
a R.
Zimnii
Vodorazdel’nyi
Bol’shaya Ussurka R.
Dal’nerechensk
Ussuri R
.
LakeKhanka
SEA OF JA
PAN
Fig. 1.
Location scheme of the studied granitoid intrusions in Primorye: (
1
) granitoid intrusions, (
2
) eastern Sikhote Alin VolcanicBelt, (
3
) Central Sikhote Alin.
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OXYGEN ISOTOPES IN THE CRETACEOUS–PALEOGENE GRANITES 153
Detailed petrogeochemical characteristics of thestudied massifs are reported in [2–4, 11, 19, 21, andothers].
RESULTS
The oxygen isotope composition in the quartz–feld-spar fraction of the granitoids of Primorye was ana-lyzed at the Laboratory of Stable Isotopes of the FarEast Geological Institute using the standard technique.Oxygen from 2- to 5-mg-weight samples was extractedby BrF
5
fluorination in a nickel reactor at a temperatureof 650
°
C. The extracted oxygen was purified from reac-tion products and reagent remains using cryogenic
traps and a KBr absorber and was frozen in individualcontainers on silicagel at liquid nitrogen temperature.The
18
O/
16
O isotopic ratios were measured using a dou-ble-inlet Finnigan MAT-252 mass spectrometer. Thereproducibility of the
δ
18
O (1
σ
) of the samples was0.2‰ with
n
= 5. The results were calibrated using theinternal and international standards NBS-28 and NBS-30.The measurement results are presented in the table andFig. 2.
In terms of the oxygen isotope composition, thestudied granites can be subdivided into three groups:(1)
δ
18
O from +5.5 to +6.5‰; (2)
δ
18
O from +7.6 to+10.2‰, and (3) less than +5.5 and up to –0.2‰. Thefirst group includes the diorites of the Oprichenskii
Oxygen isotope composition (
δ
18
SMOW) of the quartz–feldspar fraction of the granitoids of some intrusions of Primorye
Intrusion Sample no. Rock SiO
2
, %
δ
18
O, ‰
87
Sr/
86
Sr
Granitoids of the Eastern Sikhote Alin volcanic belt
Magnetite
Oprichnenskii B-300 Diorite 60.84 +6.5
B-1212 Granite 72.26 +4.7
Vladimirskii B-496 Adamellite 69.46 +6.4
B-720 Granite 72.58 +0.2
Valentinovskii B-1007c Diorite 53.45 +3.5
B-901 Granodiorite 66.78 +5.6
B-915 Granite 71.64 +6.2
B-991 Granite 72.50 +3.2
Zapovednyi B-1154a Diorite 58.35 –0.2
B-1147c Granite 71.92 +2.0
Ilmenite
Lapshin Spring A-192c Monzonite 65.28 +8.5 0.7088*
Nikolaevskii B-1554a Gabbrodiorite 53.0 +2.3
Dal’negorsk B-1498i Adamellite 69.3 +4.7
Granitoids of the Tatibin Group of the Sikhote Alin plutonic belt
Ilmenite
Vodorazdel’nyi A-6 Granite 73.75 +9.2 0.7056 [7]
Zimnii C-1006 Adamellite 69.75 +6.3 0.7068 [7]
Uspenskii B-1341 Granodiorite 67.16 +9.6 0.7050 [34]
B-1352 Granite 75.52 +10.2 0.7070 [34]
Krinichnyi KC20/68 Granodiorite 68.05 Pl +9.0 0.7048 [9]
Qtz +10.6
Livadiiskii B-29 Granodiorite 62.62 +6.4 0.7048 [34]
Tazgou B-1356 Granite 70.50 +8.6
Grodekovskii Complex
Grodekovskii Gr-17 Granodiorite 68.38 +7.6 0.7074 [8]
Note: The oxygen isotope composition was analyzed at the Laboratory of Stable Isotopes of the Far East Geological Institute, Far EastDivision of the Russian Academy of Sciences in Vladivostok; the analysts were N.P. Konovalova and E.S. Ermolenko.
* The
87
Sr/
86
Sr ratios in the monzonites of Lapshin Spring were determined at the Laboratory of Isotope Geochemistry of the VinogradovInstitute of Geochemistry and Analytical Chemistry, Siberian Division of the Russian Academy of Sciences; the analyst was G.S. Plyus-nin. Other
87
Sr/
86
Sr ratios were taken from literature data.
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VALUI et al.
Massif and granodiorites of all the studied massifs. Thesecond group includes the granites of the Vodor-azdel’nyi and Uspenskii massifs, and monzonites of theLapshin spring. The third group includes the granites ofthe latest phases of the Vladimirskii and Valentinovskiiintrusions, as well as the diorites of the ValentinovskiiMassif and gabbrodiorite porphyry of the Nikolaevskiistock.
The obtained results can be interpreted as follows.The rocks of the first group with
δ
18
O = (+5.5) to(+6.5‰), which is similar to that of oceanic basalts,could be formed by partial melting of the basaltic crust.
The rocks of the second group are represented bygranites of the Tatibin Series of Central Sikhote Alin,which ascended through thicker continental crust(36 km as compared to the 25 km in the coastal zone ofPrimorye). They have higher
δ
18
O, which, according tostudies of Isihara and Matsuhisa [26] of the granitoidsof the External Zone of southwestern Japan, could indi-cate the participation of sedimentary material duringthe formation of the parent melts of these intrusions.
The rocks of groups I and II are ascribed to typicalgranitoids according to the Taylor classification [33].
The rocks of group III with low and negative
δ
18
Oare ascribed to
18
O-depleted granites. As was men-tioned by Taylor [35, 37], this can be explained eitherby melting of low-
18
O rocks or by later subsolidusexchange with low-isotopic hydrothermal fluids of
meteoric waters. Group III includes granites of theyoungest phases of multiple intrusions of the easternSikhote Alin as well as small gabbro and diorite stocks(hundreds of meters across) that were emplaced in thefissured rocks, which are favorable for oxygen isotopicexchange with meteoric waters. Among the studied grani-toids, the lowest
δ
18
O were found in the diorites (–0.2‰)and granites (+2.0‰) of the Zapovednyi intrusion,which was formed in the deep-seated Central Fault, thezone of intense circulation of meteoric waters andwater–rock isotopic exchange.
A sample taken from borehole 20 (depth 68 m) inthe granodiorites of the Krinichnyi Massif has δ18O =+9.0‰ in the plagioclase and δ18O = +1.6‰ in thequartz. The quartz–plagioclase 18O fractionation tem-perature was calculated using formula [32]:
Thus, the temperature of the quartz–plagioclaseoxygen isotope fractionation during crystallization ofthe granodiorites of the Krinichnyi Massif was 729°C.
GENETIC INTERPRETATIONOF THE DATA OBTAINED
The oxygen and initial strontium isotope ratios canbe used to solve the genesis of felsic magmas, i.e., thecomposition of protoliths, the mechanism of their gen-eration (fractionation of basaltic magmas or melting ofcrustal rocks), and the fraction of sedimentary rocksduring crustal contamination.
According to some researchers [25, 36], the Sr–Oisotope composition of igneous rocks could be used todiscriminate between mantle and crustal contamina-tion. The crustal contamination (assimilation of thehost rocks in the magma chamber) yields a negativecorrelation between the Sr content and the initial Sr iso-topic ratios with an upward convex mixing hyperbole inthe Sr–O isotope diagram. Contamination of sourcerocks (mantle metasomatism) must be accompanied byincreasing Sr and a strongly downward convex mixingcurve [15, 36].
The data points of the studied massifs analyzed fortheir Sr isotope composition were plotted in the δ18O–87Sr/86Sr diagram (Fig. 3). It was established that theisotopic characteristics of adamellites of the ZimniiMassif suggest source contamination with the ratio ofthe Sr content in the magma to that in the contaminantof 1 : 5, whereas the granites of the Vodorazdel’nyi
1000 αqtz–plln 1.59 106/T2×=
αqtz–pl 1000 δqtz+( )/ 1000 δpl+( )=
= 1010.6/1009 1.00158=
1000 1.00158ln 1.59 106/T2×=
T2 1.59 106/1.584×=
T 1000 K 729°C.= =
50 60 70 80SiO2, %
–2
0
16
14
12
10
8
6
4
2
δ18O, ‰
Basalts
Sedimentaryrocks
12345
Fig. 2. Diagram of δ18O–SiO2 in the granitoids of someintrusions of Primorye: (1) gabbro and diorites, (2) monzo-nites, (3) granodiorites, (4) granites, (5) tonalities of low-Ktonalities of the Tanzawa Series [26]. Lines connect therocks from the same intrusion: (1) Uspenskii, (2) Oprichn-enskii, (3) Valentinovskii, (4) Nikolaevskii–Dal’negorskii,(5) Vladimirskii, (6) Zapovednyi. Field of sedimentaryrocks, line of proportions of mixing of parent melts, anddashed line of the boundary between ilmenite and magnetitegranites of Japan after [26].
1
234 5
6
3
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OXYGEN ISOTOPES IN THE CRETACEOUS–PALEOGENE GRANITES 155
Massif of the same Tatibin Series define the curve ofcrustal contamination with Sr(M) : Sr(K) = 2 : 1. Thedata points of the granodiorites of the Livadiiskii Mas-sif are plotted in the contamination line of 1 : 2, whilethe granodiorites of the adjacent Krinichnyi Massifshow a crustal contamination curve with the proportionof 5 : 1. The parent melts of the monzonites of the Lap-shin spring were contaminated in the source in propor-tions of 1 : 5 (Fig. 3).
Thus, the parent melts of granodiorites were con-taminated in the source, while granite magmas werederived by crustal contamination.
The similar 87Sr/86Sr ratios in the rocks of the adja-cent Livadiiskii and Krinichnyi Massifs indicate thattheir parental melts were formed in a similar way buttheir subsequent evolution was different. The grano-diorite melts of the Livadiiskii Massif reached theircrystallization magmatic chamber without changes andformed a homogeneous monophase massif with preser-vation of the initial oxygen isotope composition δ18O =+ 6.4‰. The granodiorites of the Krinichnyi Massifwere subjected to differentiation and crustal contami-nation, which caused an increase of δ18O up to +10‰.The increase of δ18O in the granodiorites of the Krin-ichnyi Massif also can be explained by high-tempera-ture (250–450°C) isotopic exchange in the magmatic
chamber with meteoric waters having δ18O = 0 to+18‰ [22]. This evolution was presumably responsiblefor the formation of the Krinichnyi gold deposit,whereas the Livadiiskii granodiorite intrusion is barren,containing only scattered accessory gold, whose com-position was determined using a JXA-5A microprobeby Romanenko (Far East Geological Institute of the FarEast Division of the Russian Academy of Sciences) asAu94Ag5.
In the δ18O–87Sr/86Sr diagram showing isotopic vari-ations in the crustal rocks [25, 29, 36, 37], the datapoints of the studied granitoids are plotted in the fieldof altered oceanic basalts: granodiorites of the Livadi-iskii Massif and adamellites of the Zimnii Massif fall inthe lower part of the field on the boundary with mantlevalues, while the Lapshin monzonites and Grodek-ovskii granites are plotted in the central part of thisfield. The granodiorites of the Krinichnyi and Uspen-skii massifs fall in the field of ophiolite basalts, whilegranites of the Uspenskii Massif plot in the greywackefield (Fig. 4). It is believed [22, 35, and others] thatgranitoids preserve the isotopic characteristics of theirsource rocks. This indicates that the granodiorite mag-mas were derived from basaltic crust. The granites wereobtained from the same rocks with participation of sed-iments.
In the δ18O–87Sr/86Sr [10] diagram, most of the datapoints of the studied granites of Primorye are plottedalong model curve 3, which defines the mixing of amantle component with Sr = 500 with a hypotheticalcrustal reservoir with 87Sr/86Sr = 0.708. The exception
0.702 0.704 0.706 0.708 0.710 0.71287Sr/86Sr
4
6
8
10
12
14
δ18O, ‰
Crusta
l conta
min
atio
n
Source contamination
Sr (M):S
r (K) =
5:1
1:100
1:5
1:2
M
K
12
34567
Fig. 3. Model Sr–O isotopic plots of mixing of mantle (M)and crustal (C) matter at different Sr contents in magmasand contaminants [25, 36]. The numbers in lines denote theratios of the Sr contents in the mantle or magma to that inthe contaminant. Data points of the studied granitoids:(1) monzonite (massif of Lapshin Spring); (2) granite (Vodor-azdel’nyi Massif); (3) adamellite (Zimnii Massif); (4) gran-odiorite (Krinichnyi Massif); (5) granodiorite (LivadiiskiiMassif); (6) granite (Grodekovskii Massif); (7) granodioriteand granite (Uspenskii Massif).
Mantle
0.702 0.706 0.710 0.714 0.71887Sr/86Sr
1 2 3 4 5 6 7
6
8
10
12
14
16
18
20
δ18O, ‰
Ophio
lite
bas
alts
Bathyaloceanicsediments
Continentalsediments
Geosynclinalsediments, melange
Arkosesand quartzites
GranitoidsAlteredoceanicbasalts
Low-δ18O hydrothermally altered rocks
Fig. 4. Variations of the isotopic composition of the Earth’srocks with data points of the studied granitoids [29, 32, 36, 37].(1–7) as in Fig. 3.
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VALUI et al.
is the adamellite of the Zimnii Massif, whose isotopiccharacteristics do not follow the model curves of thisdiagram (Fig. 5). The isotopic characteristics of theGrodekovskii intrusion located in the Khanka medianmassif shown for comparison correspond to modelcurve 1, which differs in its higher Sr ratio, as themonzonites of the Lapshin spring as compared to thegranites of the Tatibin Series. It should be noted that thestudied granitoids of Primorye have a lower 87Sr/86Srisotopic ratio and Sr content than the Phanerozoic gran-itoids of Eastern Transbaikalia, which are considered[10] to be derived from the lower crust or Paleozoicmetasediments involved in magma generation owing tosubduction and collision.
CONCLUSIONS
Thus, the oxygen isotope study first carried out forthe granite rocks of Primorye revealed wide variationsin δ18O, which could have been caused by both differ-ent genesis of the initial magmas and the isotopicexchange in the magmatic and subsolidus conditions.
In our opinion, the low isotopic ratios (δ18O and87Sr/86Sr) in the studied granitoids presumably indicatethat they were formed from granitoid magmas withshort-lived crustal evolution [15, 22] and could haveresulted from the following: (1) melting of sedimentaryrocks with great amounts of young volcanic materialthat was accumulated in the trench along the transformcontinental margin (ilmenite granites of the TatibinSeries); and (2) melting of a mixture of deep-water sed-iments, ocean floor basalts, and upper mantle in thelithospheric plate subducted beneath the continent(magnetite granites of the ESAVB).
The obtained conclusions require additional studiesto refine the isotopic equilibration in the mineral–meltsystem during formation of the granitoids of the Pri-morye region.
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Mantle
0.704 0.708 0.712 0.716 87Sr/86Sr5
6
7
8
9
10
11
12
δ18O, ‰ SMOW123
1
2345678910
Fig. 5. Model Sr–O isotopic mixing curves of mantle andcrustal material at different Sr contents in the magma andcontaminant [10]. (1–7) As in Fig. 3. (8–9) Mixing modelsof the mantle and crustal source with different parameters(respectively): (8) 87Sr/86Sr = 0.705, Sr = 1000 ppm,δ18O = 6 and 87Sr/86Sr = 0.719, Sr = 350 ppm, δ18O = 12;(9) 87Sr/86Sr = 0.705, Sr = 500 ppm, δ18O = 6 and 87Sr/86Sr =0.719, Sr = 350 ppm, δ18O = 12. (10) Compositional fieldof the East Transbaikalian granitoids [10]. Trends (1, 2,and 3) are the mixing models of the mantle (Sr = 500 ppm)and hypothetical crustal reservoirs with 87Sr/86Sr = 0.716,0.712, and 0.708, respectively.
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