Isotopic and Trace-Element Constraints on Mantle and Crustal Contributions to Siberian Continental Flood Basalts, Noril’Sk Area, Siberia

0016.7037/93/$6.00 + .oo

Isotopic and trace-element constraints on mantle and crustal contributions to Siberian continental flood basalts, Noril’sk area, Siberia

JOSEPHL.WOODEN,'GERAI.DK.CZAMANSKE,'VALERI A.FEDORENKO,~NICHOLAST. ARNDT,~~ATHERINE~HAUVEL,~ ROBIN M. BOUSE.'BI-SHIA W. KING,’ ROY J. KNIGHT,’ and DAVID F. SIEM? ‘US Geological Survey, 345 MiddIefieid Rd.. Menlo Park. CA 94025, USA *Central Research Institute of Geological Prospecting for Base and Precious Metals, Varshavskoye Sh. 129B, Moscow I 13545. Russia ‘GPosciences Rennes, UniversitC de Rennes 1. 35042 Rennes cedex, France “US Geological Survey, Denver Federal Center, Denver, CO 80225, USA

Abstract-We present a tightly controlled and comprehensive set of analytical data for the 250~Ma Siberian flood-basalt province. Consideration of major- and trace-element compositions, along with strontium, lead, and neodymium isotopic compositions, strongly supports earlier Russian subdivision of this magmatism into three magmatic cycles, giving rise to three assemblages of eleven basalt suites in the ascending order lvakinsky-Gudchikhinsky, Khakanchansky-Nadezhdinsky, and Morongovsky-Samoedsky. Geochemical and isotopic discontinuities of varying magnitude characterize most of the boundaries between the eleven recognized basalt suites in the Noril’sk area. Although we conclude that the dominant volume of erupted magma originated from an asthenospheric mantle plume, none of the lavas is interpreted to directly represent asthenospheric melts, which would have been far more magnesian. On the basis of thermal considerations, we consider it unlikely that vast volumes of basaltic melt were produced directly from the continental lithospheric mantle beneath the Siberian craton. Moreover, there is little evidence from mantle xenoliths that the geochemical signatures of such melts would correspond to those of the Siberian flood basalts. Studies of melt migration lead us to conclude that transport of ~thenosphe~c melt through the lithosphe~c mantle would be rapid, by fracture propagation. Lavas from the Gudchikhinsky suite have negligible Ta-Nb anomalies and positive tNd values, and their parental magmas presumably interacted little with the continental lithospheric mantle or crust. All other lavas have negative Ta-Nb anomalies and lower fNd values that we attribute to interaction with continental crust. The model that we have developed requires discrete contributions from the plume and complex processing of all erupted magmas in the continental crust. The earliest magmas represent small percentages of melt formed in equiljbrium with garnet. Over time, the percentage of melting in the source region and the volume of magma produced increased, and garnet was no longer stable in the plume source. All of the plume-derived melts initially contained more than 20 wt% MgO and became less Mg rich by fractionation of olivine as they traversed the lithospheric mantle. We conclude, however, that the most significant control on the geochemical and isotopic compositions of all the erupted lavas was processing of mantle-derived magma in crustal reservoirs during periodic replenishment, periodic tapping, continuous crystal fractionation. and wallrock assimilation. Rapid eruption of an extremely large volume of processed magma that varied little in chemical and isotopic composition produced the sequence of relatively monotonous tholeiitic basatts that constitute the 2,300-m-thick third assemblage of the Siberian flood-basalt province near Noril’sk.

INTRODUCTION Becauseour investigation of the petrology, chemistry, and TNEOPPORTUNITYTOSTUDY theSiberianflood-basaltprov- origin of the SFBP was directly motivated by a desire to un- ince (SFBP) arose in 1990 through a joint memorandum of derstand the temporal and geochemical relations between understanding between the USGS and the Ministry of Geol- this basaltic volcanism and the economically important ore- ogy of the former USSR for study of the famed Noril’sk- bearing intrusions-an effort that could lead to discovery of Talnakh mining district of Siberian Russia (Fig. 1). These similar deposits associated with flood-basalt provinces else- continental flood basalts (CFBs) not only are of extraordinary where-our focus here is on characterization of the basaltic interest in their own right but also are related to the evolution system in terms of its evolution and endmembers. The pro- of the magmatic ore deposits of ~o~l’sk-Talnakh, which cesses that modified the source magmas are of interest insofar constitute one of the outstanding mineralized districts in the as they may also be responsible for the variety of magmas world. These ore deposits are unique in their association with recognized as having formed the ore-bearing intrusions and, a flood basalt province: in the inordinate amount of magmatic in addition, the concentrations of sulfide melt associated with sulfide ore found in association with mafic-ultramafic intru- the intrusions. We are less concerned herein, however, with sions that are 15-18 km long, 1-3 km wide, and less than the challenge of addressing the much broader problems rel- 350 m thick; and in the exceptionally high contents of Cu, evant to the origin of CFBs (e.g., CARLSON. 1991).None- Ni, and platinum-group elements (PGEs) in these ores theless, largely because it appears to represent the most viable (CZAMANSKE eta]., 1992b; NALDRETT eta]., 1992). means of generating large volumes of basaltic magma in a 3677 3678 J. L. Wooden et al

Upper Permian to lower Messlc flood basalts

m ~umginsky and Ssmoedsky suites F-_“7 Middle Ca~niferous to Upper Permian (Tungusska series) sedimentary rocks m Mokulaevsky and Kharaelakhsky suites a Ordovician to Lower Carboniferous m Morongovsky suite sedimentary rocks

m Ivakinsky, Syverminsk Gudchikhinsky, D Cambrian sedimentary rocks Khakanchansky, Tuk lonsky, and Nadezhdinsky suites 0 Upper Proterozoic sedimentary rocks 1-Talnakh ore junction 3~~okhtokhsky granodior~e 2-Noril’sk ore junction

FIG. L. Simplified geologic map of the Noril’sk area. showing major structural features. sampling locations, and subsurface outlines of the fully differentiated, ore-bearing intrusions (black, true scale).

nonrifting environment, we have chosen to embrace the the relation of the ore-bearing intrusions to the basdts on plume model as an explanation for SFBP volcanism. Thus, the basis of an equally extensive data set. part of this report is devoted to “testing” the plume model In this report, we sharply limit discussion of the field char- (e.g., CAMPBELL and GRIFFITH& 1990) against our data set acteristics of the basalts and previous work on their petro- and to developing a comprehensive and credible model for graphic and geochemical characteristics. The following re- the evolution of SFBP magmas. Future reports will consider ports, all in English, provide additional background ma- Petrogenesis of flood basalts in Siberia 3679 terial and analyses: FEDORENKO (1981, 1991, 1993), 2). This sequence is formed by alternation of lava flows ( from AL’MUKHAMEDOV and ZOLO?‘UKHIN ( 1988), ZOLOTUKHIN a few meters to 100 m thick, average 15 m thick) and tuff and AL’MUKHAMEDOV (1988, 1990, 1991), LIGHTFOOT et horizons (from several tens of centimeters to 50-100-m thick, al. (1990b, 1993a,b), SHARMA et al. (1991, 1992), and rarely 200-400 m thick). In the Noril’sk area, there are about BROGMANN et al. ( 1993). In many respects, the major- and 200 lava flows and thirty tuff horizons. Lava flows of similar trace-element characteristics discussed in the following text composition and texture are grouped into units tens to are comparable to those reported by LIGHTFOOT et al. ( 1990b, hundreds of meters thick. Single lava and tuff units typically I993a,b) for a comparable, but significantly less stratigraph- extend tens to hundreds of kilometers and have sharp con- ically complete, suite of samples, many of which are from tacts. borehole SG-9, collared approximately 2.9 km S. 2O”W of Lavas of the SFBP differ from the flood basalts in other borehole SC-32. provinces in their high proportion of amygdaloidal material. All of the lava flows can be divided into lower, massive and upper, amygdaloidal zones, which average 36% of the flow FIELD, PETROGRAPHIC, AND AGE RELATIONS thickness (from 49% in the lower suites to 3 1% in the upper The 250-Ma SFBP is one of the world’s largest CFB prov- suites of the sequence). The amygdaloidal basalts that char- inces, with relatively continuous exposures of basalt covering acterize the upper parts of the flows contain about 30% an area of at least 3.4 X IO5 km*. MILANOVSKIY ( 1976) amygdules by volume. estimated that rocks associated with this early Mesozoic flood- Volcanic rocks commonly rest on terrigenous sedimentary basalt magmatism once covered an area of 4 X lo6 km2 and rocks of the Tungusska series ( C2-P2) above a slight uncon- had a volume of 2-3 X lo6 km3. The Noril’sk area lies at formity: a few tens of meters to as much as 200-300 m of the extreme northwestern corner of both the Siberian platform sedimentary rocks was eroded. In some places, Tungusska- and the most extensive, relatively undisturbed exposures of series argillites conformably grade into SFBP tuffs, indicating the flood basalts. Impingement of a mantle plume beneath that no significant uplift preceded volcanism. Beneath the the margin of a Precambrian craton is most consistent with Tungusska series lies a 3,700-4,700-m-thick, Cambrian geologic and tectonic features of the SFBP, because the period through Upper Devonian sedimentary sequence containing 270-220 Ma was a time of continental collision and the Ural differing proportions of dolomite, marl, limestone, argillite. orogeny in northern Siberia (ZONENSHAIN et al., 1990). anhydrite, siltstone, variegated shale, sandstone. and coal (see MAKARENKO (1976) and ZONENSHAIN et al. (1990) noted ZEN’KO and CZAMANSKE, 1993, Fig. 1). These sedimentary that rifting took place west and north of Noril’sk during the rocks rest on a Precambrian basement of schists, granitic Middle to Late Triassic. We interpret the modestly exten- gneisses, and granites whose age is estimated at about 2 Ga sional features noted by KUZNETSOV and NAUMOV (1975) from common-lead isotopic analyses of xenoliths (J. L. and ZONENSHAIN et al. (1990) to be characteristic of the Wooden, unpubl. data). At present, little is known about Noril’sk area, as responses to stretching above a rising plume these crustal rocks but they are the subject of active inves- head (e.g., GRIFFI~HS and CAMPBELL, 1991, Fig. 13). tigation as a component of our research. In the Noril’sk area, the SFBP is represented by lavas, tuffs, The volcanic sequence of the Noril’sk area is divided into and intrusive rocks, with respective volume proportions of eleven suites on the basis of the chemical composition and this mafic magmatism estimated at 84: IO:6 ( FEDORENKO, texture of the lavas and correlation of tuff units: seven suites 1991). Because of the economic importance of the few, are subdivided into subsuites (Fig. 2). FEDORENKO ( 198 1) uniquely characterized ore-bearing intrusions ( NALDRETT et combined these suites into three basalt assemblages: ( 1) al., 1992: ZEN’KO and CZAMANSKE, 1993 ), the mafic intru- Ivakinsky-Gudchikhinsky, (2) Khakanchansky-Nadezhdin- sions have been much studied; most are sheetlike and now sky, and ( 3 ) Morongovsky-Samoedsky. Lavas of each assem- compose as much as I.5-20% of the sedimentary section for blage have specific signatures of major- and trace-element 2 km below the base of the basalts. About fifteen distinct chemistry. The geochemical boundary between the first two types, distinguished by petrographic and geochemical char- assemblages is sharp. The upper Nadezhdinsky (Nd,) and acteristics, and age in relation to the lava suites, are recognized lower Morongovsky (Mr, ) subsuites constitute a transition in the Noril’sk area ( FEDORENKO et al., 1984; NALDRETT et between the second and third assemblages. In the Noril’sk al., 1992 ). In contrast to most other CFB provinces, SFBP area, the Ivakinsky-Gudchikhinsky assemblage constitutes activity was characterized by initial and continuing explosive about 7%, the Khakanchansky-Nadezhdinsky assemblage volcanism; the 1ava:tuff volume ratio is estimated to be as about 14%, and the Morongovsky-Samoedsky assemblage the high as 4: 1 for the entire province. Basaltic tuffs of the first- remaining 79% of the total lava volume. For the SFBP as a and second-basalt assemblages typically contain 10-l 5 ~01% whole, a speculative estimate is that third-assemblage volcanic of fine-grained (0.1-I mm) minerals and lithic fragments rocks constitute 90-95% of the total volume, with the re- from the underlying sedimentary rocks. Exposed within the maining 5- 10% divided among first-assemblage volcanic thick basal tuff of the upper Morongovsky ( Mr2) subsuite is rocks ( -3O%), second-assemblage volcanic rocks (50-60%), a diatreme (less than 6 by 14 m in surface expression) that and Guli volcanic rocks ( lo-20%). Tholeiitic basalts are has carried up 5-30-cm-diameter fragments of the upper dominant among SFBP Iavas and are the only lava type in crystalline basement and overlying Riphean volcanic and many areas. Alkaline and subalkaline basalts (2.5 ~01%) and sedimentary rocks which lie at a depth of 9-10 km. picritic basalts ( 1 ~01%) are found in the Noril’sk area. Un- The thickest (>3,500 m thick) and most continuous se- usual lava types are characteristic ofthe Guli magmatic center, quence of volcanic rocks is present in the Noril’sk area (Fig. near the south boundary of the Yenisey-Khatanga trough, 3680 J. L. Wooden et al more than 500 km northeast of Noril’sk. There, trachybasalts, sion. On the basis of this and comparable ages for other basalt trachyandesitic basal& nepheline basanites, ankaramites, samples (see CAMPBELL et al., 1992), they concluded that high-Ti picrites, and meimechites, as well as rare dacites and the Siberian flood basalts dated by them have lost about 2% rhyodacites, are present, in addition to typical SFBP basalts. of their radiogenic argon. Subsequently, Dalrymple et al. The Kaltaminsky ankaramites (found in the upper part of (unpubl. data) obtained an 40Ar/39Ar age of 248.9 + 2.8 for the Morongovsky suite) and Ikonsky andesitic basalts (found the Lower Talnakh intrusion, and CAMPBELL et al. ( 1992) in the upper part of the Kharaelakhsky suite) present in the obtained a U-Pb age of 248 f 4 Ma for the Noril’sk I intrusion, volcanic sequence northeast of Noril’sk are related to the based on SHRIMP ion-microprobe dating of zircon extracted Guli magmatic center. from a sample of taxitic leucogabbro collected in the Med- We note that SHARMA et al. ( 1991) appeared to imply a vezhy Creek open-pit mine of the Noril’sk I intrusion. The geographic distinction between basalts of the Noril’sk and combined results of this and earlier studies (e.g., FEDORENKO Putorana areas, whereas SHARMA et al. ( 1992)more properly et al., 1984; LIGHTFOOT et al., 1990b, 1993a) suggest that addressed the temporal evolution of SFBP volcanism. In the Noril’sk I intrusion may be related in time to eruption general, first- and second-assemblage lavas are included in of the thick tuff unit at the base of the Mrz subsuite (Fig. 2), their Noril’sk-area samples, and third-assemblage lavas in a midpoint in the volcanic sequence. their Putorana-area samples. Our report is the first to char- Inasmuch as the ages obtained for the Noril’sk I intrusion acterize an essentially continuous, well-controlled strati- are not statistically different from the age of 25 1.1 f 3.6 Ma graphic section through the SFBP. recently determined by SHRIMP for the Permian-Triassic Unconformities in the volcanic sequence are rare, small boundary (CLAQU&LONG et al., 199 1 ), CAMPBELL et al. (a few meters of section missing), and local; traces of weath- ( 1992 ) argued that eruption of the Siberian flood basalts may ering are slight. Apparently, the topography was flat, and have contributed to the Permian-Triassic extinction event. eruptions quickly followed one another ( FEWRENKO, 1979). The magnetostratigraphy of the late Permian and early On the basis of paleontologic and geochemical data, FEDOR- Triassic (e.g., HAAG and HELLER, 199 1 ) and the fact that all ENKO ( 1991) concluded that tuffs of the lower third of the but the Ivakinsky suite are normally polarized (LIND and sequence formed in water-filled basins as shallow as a few SCHEKOTUROV, 199 I ) lead to the inference that the entire meters, whereas the upper tuffs were erupted subaerially. SFBP was formed in about 600,000 years (CAMPBELLet al., There is no evidence of subaqueous eruption of lava, even 1992: CZAMANSKE et al., I992a). for the lower suites. Volcanism took place in a lowland environment that was inherited from Late Permian (Tungusska SAMPLE SELECTION, PREPARATION, .4ND ANALYSIS series) time, when coal-forming detritus accumulated. Lava The fifty-three samples reported on here were chosen on the basis was predominately erupted in an environment of well-com- of trace-element chemistry from our larger set of seventy-nine samples pensated subsidence; some overcompensation took place of the SFBP in the Noril’sk area. Most of the samples are from bore- later. FEDORENKO ( 1979, 199 1) suggested that this subsidence hole SC-32, which penetrated 2.640 m of the lower suites of the of some 3.5 km was caused by the evacuation of magma SFBP (Figs. I and 2); for these samples, the final digits in the sample designation represent the depth in the borehole in meters. Supple- chambers. mentary samples were collected from surface outcrops at four localities The depressions evident in the present tectonic structure (Fig. 1) to complete stratigraphic coverage of all eleven basalt suites of the SFBP (Figs. 1 and 3 ) formed during Late Triassic to and their subsuites (Fig. 2). This stratigraphic coverage is from within Early Jurassic time; these include the Kharaelakh, Noril’sk, 44 m of the top of the basalt sequence at locality I6F to within 15 and Vologochan depressions (Fig. 3c) and the broad Tun- m of the SFBP-Tungusska series contact in borehole SC-32. Sample Mik-1 is clinopyroxene spinifex from a 45-86 m thick, differentiated gusska basin to the southeast. During the period of volcanism, flow exposed along the Mikchanda River ( DODIN and GOLUBKOV, the paleostructure was simpler, and the paleodepressions were 1971: RYABOV et al., 1977). broad and gentle. Paleotectonic reconstructions allow the in- Where present, weathered rinds were sawed or ground from all ference that the boundaries between the three basalt assem- outcrop samples. All sawn surfaces and the circumference of all core blages are related to two distinct periods of structural reor- samples were ground free of striations on a sintered-metal lap im- pregnated with loo-mesh diamonds. Samples were passed twice ganization (Fig. 3; FEDORENKO, 1979, 1991). Maximal through a successively constrained iron-jawed crusher and reduced thicknesses of first-assemblage lavas are found in the north- to powder in an alumina-lined shatterbox. Many of the samples are central part, of second-assemblage lavas in the east-central considerably altered (Table I; data for HzO’ and CO*). For the part, and of third-assemblage lavas northeast of the Noril’sk borehole samples, this alteration is unrelated to the present erosional surface and, for the first and second assemblages, may in part be due area. This reorganization of the magma-supply system is re- to emplacement of the subjacent ore-bearing intrusions. Rare sec- flected in geochemical distinctions between assemblages, ondary-alteration veinlets and evident amygdules were removed dur- suggesting access to different staging reservoirs and, possibly, ing sample preparation. We are acutely aware that alteration could to melts from different parts of the mantle plume. be responsible for some of the geochemical and isotopic variations Attempts to date the inception of flood-basalt magmatism that we document, but we do not believe that it compromises our principal conclusions. have led to numerous conflicting ages in the range 254-238 Major elements were determined by X-ray fluorescence; Ba, CU. Ma. In an attempt using the 40Ar/39Ar laser age-spectrum Cr. Ni. Rb. Sr, Zn, and Zr were determined by energy-dispersive X- technique, DALRYMPLE et al. ( 1991) obtained an age of 249 ray fluorescence; rare earth elements ( REEs) and the rest of the minor t 1.6 (20) Ma on biotite from a mineralized vein in the ore- elements were determined by instrumental neutron activation; and bearing Noril’sk I intrusion, in comparison with an apparent lead was determined by isotope-dilution mass spectrometry. The major- and trace-element data presented were produced by a team age of 244.9 * 1.8 Ma on plagioclase from basalt sample SG- effort and represent the best obtainable from the LJSGS laboratories. 32-25 15.4 of the Syverminsky suite that is cut by this intru- Analytical uncertainties in these techniques, as applied in the USGS, Petrogenesis of flood basalts in Siberia 3681

JJ3SUITE 0

Sm3

Aphlrtc basatts Subalkaline Tt-a@te -200 Es3 m basal& /xJ Polkllltic bar&s Alkallne trsehybasalts

ThakNtlc basans m Tutfs

m Terrlgcnout sedt- mentary rocks EXPOSURE 15-F

-600

BOREHOLE

-600

4ooa

-120C

-14OC

E d -16OC E ac 5 s: -18OC

-2000

-22OC

-240C EXPOSURE

-260C

-2800

-3000

-3200

FIG. 2. Composite stratigraphic section showing the petrography ofthe basalts in the Norit’sk area and their classification into suites and subsuites. Distribution of 78 samples shown by ticks to the right ofthe columns. Table 1. Major-element, trace-element and isotopic data for the SFBP basal& of the Noril’sk area. ..- ----16F’2.~.. 16F-19 ,6F-17 15F-36 SC-32 SO-32 SG-32 15F-25 SG-32 ;$;:: S:;;Z SG-32 SF&$2 SG-32 S,G&y SO-32 SG-32 SG-32 SG-32 56-32 SG-32 SG-32 SG-32 $,G-$ .%-32 --ST- .x* 87 1% 76"s 718 ,070 1255 1311 1415 1528 164.5 1701 1782 1838 1982 2069 Sm3 Sm3 Smj Km Km Hi4 H;; Hr4 Hr4 Hr3 tir, Mkq f&J tik; Mki Mkl w? MC2 Mr2 W Nda NO3 Ndz NO2 Ndr Ndl -44 -94 -565 -672 -754 -767 -896 -952 -961 -l225 -1370 -1419 -1499 -1770 -1903 -1955 -2011 -2115 -2228 -2345 -2401 -2482 -2538 -2670 -2682 -2769 ______--_____. -___- -___ 49.41 49 92 49.25 49.67 49.60 49.31 49.39 54 90 48.80 49.24 49.45 4776 49.74 49.41 49.90 49.79 49.71 49.64 49.67 49.99 5055 50.&r 52.87 52.96 51.44 53.67 1516 15.15 1535 15.42 1561 1530 1523 14.12 15.54 15.&i 15.43 1641 15.66 15.70 15.46 15.39 15.41 15.66 15.39 15.73 15.81 15.43 15.08 15.15 15.23 1529 1234 11.94 11.95 12.28 12.28 12.55 12.65 1167 12.30 1193 12.40 12.48 12.42 11.67 11.57 11.43 10.99 11.58 11.72 IO.58 10.57 IO.61 10.17 0.69 9.57 9.w 7.23 7.37 767 6.63 652 677 6.MI 3.92 7 80 7.59 740 7.36 6.83 6.63 6.60 7.10 7.65 7.41 7.49 7.59 6.62 6.93 643 6.79 7.32 6.38 1155 11.21 1164 11.49 ,140 11.27 1163 7.66 11.40 11.09 10.93 12.07 1101 12.67 12.35 12.60 12.22 11.74 11.49 12.39 12.17 11.8O 10.59 10.77 10.64 10.02 2.25 2.12 2.16 2.34 2.36 2.32 2.23 297 207 2.14 2.24 2.09 2.30 2.03 2.17 2.11 2.02 2.08 2.18 2.03 2.34 2.25 2.28 225 249 2.32 018 0.56 022 0.24 0.25 0.51 0 14 1.77 0.42 0.48 0.30 0.1 1 0.30 0.10 0.22 0.12 0.36 0.36 0.47 0.25 0.58 0.62 1.17 114 2.12 1.42 145 1.38 1.38 1.52 1.55 158 1.62 2.31 1.32 133 1.41 1.36 1.45 1.25 1.21 1.21 1.W 1.19 1.24 1.11 0.98 1.15 1.09 0.97 0.90 0.93 017 0.16 018 0.21 0.21 0.21 0.21 052 015 016 0.15 0.16 017 0.16 0.16 0.15 0.14 0.14 0.15 0.15 0.22 0.17 014 ” 13 0.11 0.14 021 020 0.21 0.21 0.21 019 022 0.16 0.21 0.20 0.20 021 0.20 0.19 O.%B 0.20 0.16 0.20 0.19 0.18 0.17 0.20 0.17 0.16 0.17 0.17 1.68 2.07 0.78 1.10 1.94 t 96 1.50 t.O1 1.71 1.47 1.01 1.41 1.19 1.65 1.26 1.13 1.x) 1.78 1.36 0.19 1.34 1.31 0.96 O.SJ 3.32 1.87 080 1.18 I62 192 0.42 0.75 3 53 1.52 1.57 16a 067 3.41 2.62 1.63 1.60 1.13 056 032 3.49 1.99 1.71 1.02 0.67 0.51 0.69 0.01 0.03 0.04 0.03 001 0.18 0.10 014 0.05 0.04 0.03 0.10 A.: 0.04 0.03 0.04 0.W 0.09 0.17 0.05 0.07 0.04 0.05 1.04 0.24 0.10 d).0t m.01 Co.01 UIOl CiIOl C0.01 COOI dJ.01 CU.01 CU.01 d.01 d.01 c0.01 0.01 Co.01 aI01 rS.Oi dOI 4.01 4.01 4.01 CO.01 0.01 0.03 co.01

120 149 124 130 140 163 a4 550 141 112 123 68 90 124 146 152 1.58 296 223 331 285 762 380 1.32 181 216 100 113 112 70 30 164 123 131 119 112 126 116 99 106 72 127 0.18 009 0.00 0.39 0.48 c.08 0.37 <,2 i.% 0.28 0.17 0.40 0.64 Ei 0% 0.18 2.59 0% 1.26 0.46 0.70 2.71 2.62 253 2.89 3.19 3.14 2.93 6.93 2.25 2.32 2.43 2.52 2.25 2.36 2.44 2.43 2.42 2.59 2.83 3.44 3.07 2.61 3.28 10 6 8 8 12 8 27 13 10 7 13 0 IO a 41; 2:3 12.3714 190 1.70 1.74 233 246 253 :;0 711 1.2s 1.54 1 .'sl 1.49 1.99 2507 IS5 2% 2.12 2751 3!:0 3!,4 515 10 46 IO 14 2 12 13 23 & 37 iit 4 &J 4 36 23 39 : 37 40 3"9 357 & A $ :: 32 zJ E El 201 181 195 205 Iso 186 394 176 180 176 215 z&l 20s 216 M", 2% 239 402 oE2 0271 0281 0.355 0.380 0377 0330 1.495 0%3 0.223 0.264 0280 0.269 0.268 oz4 0.258 0.301 0.291 028 0% 0.415 0% 0.501 0.427 0:8:7 1.021 1.033 1072 1.418 1.540 1.582 I.311 4.515 0.752 0.854 08&o 1.010 0.989 l.O69 1.064 0.694 I.228 1.252 1.729 2.111 2.143 3.504 3.182 2.807 3.838 0396 0.401 0.534 0.763 0.76, 0.610 0.666 1.142 0.326 0.436 0.303 0.355 0.407 0.426 0.487 0.419 0.4% 0.487 0.686 1.082 0.657 1.110 0.994 0.815 1.139 33 26 32 38 32 43 27 36 23 26 25 25 35 26 33 78 98 :: 87 75 2 77 93 2 96 75 i1 10.5 :: 101 106 106 :A :2" 8427 : LE 113 106 117 128 133 132 133 311 96 101 104 105 zi z 86 IO3 IO6 IO8 119 133 141 130 109 142 51 51 51 48 46 50 48 29 54 53 50 53 50 47 50 48 51 49 43 41 40 38 107 178 159 130 111 170 180 56 126 135 166 148 147 151 124 136 4$ 142 64 it 2 14365 126 145 146 106 103 122 116 30 164 176 137 155 137 Ii3 122 132 148 142 140 115 108 104 E :: 2410 2624 12.4 13.5 10.3 11.4 123 14.5 13.7 :.A 13.4 to.4 13.3 11.6 11.4 11.5 to.4 99 11.3 15.4 11.3 7.7 2.1 6.1 c5 c.5 c.5 c.5 9.9 9.3 9.7 11.4 17.5 145 16.9 10.4 13.5 17.4 10.6 10.4 10.4 11.4 11.4 9.6 13.4 10.0 3.2 2.0 8.5 c5 .x5 c.5 <.5

0.28 8.00 794 1001 9.59 10.34 9.32 34.5 5.01 6.62 7.02 777 848 7.23 7.91 7.45 6.51 5.38 5.90 10.73 14.46 13.15 18.8 17.1 16.7 19.9 20.8 19.8 18.5 23.4 25.0 25.2 21.4 79.8 ldl 15.7 17.4 G9.a 21.2 18.1 198 18.1 163 19.8 20.7 24.5 32.3 28.9 41.1 37.3 34.7 44.1 13.72 72.56 10.61 14.80 15.20 15.40 14.17 46.5 9.58 10.66 11.54 12.81 13.19 12.46 G-6 11.67 10.09 12.67 12.72 14.37 10.02 15.64 18.8 17.6 17.0 20.8 3.88 3.58 3.69 4.22 4.25 4.21 4.22 10.48 3.26 3.37 3.53 3.59 3.67 3.42 3.52 3.31 2.98 3.44 3.43 3.57 3.75 3.98 4.54 4.03 3.60 4.60 1.32 123 1.23 1.33 t 42 1.37 1.34 2.92 1.16 1.16 1.21 1.23 1.25 1.10 114 1.08 1.02 1.13 1.15 1.09 1.06 1.13 1.18 1.07 1.04 1.16 470 4.19 4.36 4.59 4.95 525 5.12 10.15 4.15 4.14 4.24 3.94 4.60 3.93 4:s 3.69 3.57 3.61 4.06 3.94 3.99 4.51 4.77 3% 3.62 4.63 0.797 0.729 0.734 0605 0.851 0.675 0.829 1.463 0.715 0.712 0.702 0.660 0.724 0.813 0.686 0.592 0.617 0.656 0.697 0.637 0.654 0.724 0.729 I.114 1034 1.072 1.180 1.191 I.241 1195 1.702 1.067 1.047 0.995 0.966 1025 0.862 0349 0.660 0.905 O.QSU I.036 0.6Sl 0.SS-I 0.969 1.121 0.9290.652 0.5S60.7S3 ;:g: 0.461 0.442 0455 0.523 0499 0.516 0.505 0.616 0453 0.440 0.438 0.431 0.437 0.4M1 0.411 0.361 0.374 0.417 0.432 0.359 0.474 0.431 0.417 0.370 . - 2.88 2.72 _..977 3.30 3.11 3.25 321 3.61 2.78 2.79 2.76 2.75 273 2.46 2.58 2.40 2.33 2.86 2.75 2.20 3.05 2.72 2.57 2.30 1.99 2.50 0.442 0.399 0.414 0.478 0.464 0.479 0.472 0.513 0.414 0.391 0.414 0.415 0.419 0.382 0.373 0.352 0340 0.363 0.404 0.331 0.459 0.384 0.378 0.352 0.297 0.355

35.05 39.45 40.37 39.95 4103 40.99 30.57 40.67 39.26 36.13 35.61 44.25 39.87 35.81 33.43 37.63 32.36 38.64 44.52 40.76 45.18 39.88 41.82 8878 20.08

1325 14.92 1960 21.53 1976 20.68 15.12 1008 16.71 17.93 11.96 15.15 14.06 15.73 13.73 15.05 17.28 12.78 14.66 16.76 x1.36 17.61 12.32 12.74 24.56 5.61

18.286 18.231 18.664 18.770 18.644 18640 16.568 17.630 18.570 18.453 18.468 18.459 18.347 18.623 16.588 18.625 18,765 18.759 16.677 18514 16.903 16.533 16.083 16.089 15.524 17.940

17.764 17.641 17.909 17.919 18.063 18.022 17970 17.231 17.909 17.74, 17.992 17.860 17.791 18.001 16.045 18.030 18.061 18.254 i8.0S7 17.661 16.099 17.836 17.596 17.565 17.552 17711

15.467 15.465 IS.514 15.516 15.505 15511 15.508 15440 15506 15.409 15502 15.509 15.462 15.525 15.522 15.529 15.527 15.561 15.516 15.511 15.541 15.508 15.491 15.489 15.521 IS.482

15.460 15.435 t5.474 15472 15.465 15469 15.477 15.419 15.474 15.463 15.478 15.478 15.463 15.483 15.494 15.499 15.492 15.%X? 15.466 15.477 15.500 15.472 15.466 15.463 15.471 15.470

36159 38096 36.249 38.197 38.148 38.136 38.077 37.965 38.171 38.180 38.199 38244 28.168 36.242 38.200 38.233 38.210 38.337 38.197 38.235 38.067 36.237 36.113 38.124 36.632 37996

37.723 37.805 37.746 37.700 37.637 37.626 37696 37.477 37.689 37.731 37.756 37.693 37.672 37.734 37.756 37.617 37.740 37.934 37.716 37.881 37.559 37.674 37.617. 37.803 37.552 37.747

070506 0.70525 0.70511 0.70516 0.70510 0.70522 0.70494 0.70698 0.70479 0.70512 070542 0.70521 070642 070547 070659 0.70565 0.70555 0.70652 0.70565 0.70643 0.7066O 0.70697 0.70446 0.71030 0.71088 0.71021

0.70462 0.70472 0.70468 070467 0.70469 0.70472 070459 0.70570 0.70414 070464 0.70460 0.70509 0.70516 0.70527 0.70540 0.7064, 0.70508 0.70518 0.70503 0.70624 0.70623 0.70641 0.70849 0.70672 0.70822 0.70872 Table 1. Major-element, trace-element and isotopic data for the SFBP basalts of the Noril’sk area.-(Continued) _~_. Sample no. SG-32 SG-32 SG32 IF-32 lF-30 1F-27 ,F-25 ,F-22 Mtk-l 66-22 X3-32 5632 SG-32 SG-32 SG32 SG-32 SG.32 SG-32 SC%32 SG-32 SG-32 SG32 SG-32 SG-32 SG-32 SG-32 sG-32 21443 2149.8 2212.5 2226.7 2245.5 2275.5 2301 2328 2332.7 2357.5 2375.2 2386.2 2464.8 2533.7 2543.9 2597.1 2605 2616 2622 2624.6 SlA8 Ndt W No, Tk Tk Tk Tk Sk Tk Hk Go2 G@ Go2 G@ Go2 Go1 Gdj Sv SV ‘H.4 IV3 IV3 Iv2 tv2 (VI iv1 W Biw!_. -2844 -2850 _,2913 -2922 -2932 -2972 -2982 -3056 -3097 -3116 -3146__=-&,71 -3198 -3203 -322% -3245 -3256 -3335 -3385 -3404 -3414 -3467 -3475 -3466 -3492 -3495

53 06 5233 53.40 47.65 47 79 49.86 49.68 49.94 51.92 5272 48.41 49.47 46.53 49.06 48.17 SO.73 49.74 49.61 53.15 5259 53.60 53.77 52.15 51.06 47.13 47.73 4720 15.05 1536 1525 12.15 12.73 15.67 15.78 15.71 9.23 1509 a.72 ll.ca 9.73 10.05 10.32 14.00 18.00 15.73 15.51 15.58 14.76 14.07 14.57 15.69 15.64 15.67 14.92 938 967 9.26 11 59 II IS 1019 961 10.10 6.37 9.34 13.75 12.86 13.06 12.67 13.51 12.6% 9.19 11 aa 10.06 9.98 11.77 13.78 12.72 1362 15.41 15.24 13.50 7.54 7.19 6.82 167 165 919 881 951 13.51 7.0% 16.2 14.0 13.2 17.1 16.1 7.06 6.a.l 6.28 7.09 5.95 5.19 2.87 3.81 4.42 4.62 5.36 4.03 a.95 10.75 941 915 941 1175 12.82 10.62 16.97 10.35 7.96 7.69 12.11 a.00 6.74 a.11 0.91 9.91 6.79 10.12 576 4.9% ?.a% 5.2s 7.13 451 9.45 2.80 231 275 1.20 1.10 1.89 1.80 2.42 086 2.26 1.10 2.95 1.44 1.10 1.24 3.76 3.41 3.52 3.9% 2.64 3.45 3.00 3.38 4.57 3.27 4.62 341 2.03 1.14 1.82 041 0.31 0.20 010 0.33 0 61 1.92 0.03 0.06 0.16 0.05 0.07 0.85 1.37 0.48 1.34 0.76 2.67 3.53 1.84 I.84 143 1.27 2.33 091 0.96 0.92 0.79 0.73 0.95 0.93 092 0.39 0.90 143 1.61 1.43 1.45 1.49 2.37 1.44 2.14 1.86 1.80 2.25 2.58 2.55 2.41 3.62 3.89 3.66 0.12 012 012 00% 0.09 0.11 0.10 0.0% <.05 0.13 0.12 0.14 0.14 012 0.14 0.22 0.17 0.32 023 0.25 0.36 1.13 0.88 0.78 135 1.37 1.26 0.17 0.1% 016 0.1% 0.18 0.17 0.16 0.17 0.14 0.2, 0.x) 0.17 0.1% 0.17 0.17 0.21 0.14 0.16 0.16 0.19 0.32 0.22 0.32 0.20 0.12 0.24 3.39 134 1.82 3.70 4.6% 1.62 1.88 2.98 0.97 4.2% 3.67 395 2.62 4.50 4.04 2.93 2.43 3.6% 2.92 i:::, 2.57 2.08 1.45 3.00 3.77 4.42 2.50 067 058 0.59 1.30 t m 3.07 3.12 0.41 0.47 1.23 2.70 2.02 2.72 2.70 3.23 0.61 0.45 081 0.90 2.32 1.09 1.23 2.62 0.77 0.82 0.95 0.76 050 020 0.22 0.1, 0.;; 0.07 0.19 0.05 0.17 6.82 0.67 047 2.64 0.22 0.46 0.57 0.50 0.10 0.17 0.16 0.08 0.09 0.25 0.49 2.10 1.i?j 160

353 376 127 116 206 14% 101 136 326 35 19 34 221 375 70 388 361 691 1470 929 937 870 1164 E 172 268 581 a31 387 316 395 2337 366 & 721 741 95 255 164 163 11 22 0.26 0.64 0.53 2 34 4.88 0.05 0.16 2.M) ,",'s 0.42 186 1.44 1.79 0.31 0.01 0.05 LZ 0.68 073 1.18 OiS Oli% o:i9 0.32 3!io 2.60 2.89 2.97 136 1.40 I.80 155 1.53 0.62 2.41 2.14 2.54 2.17 2.13 2.17 3.69 2.35 450 4.50 4.37 8.91 8.60 6.56 5.39 7.64 7.90 7.10 13 1P 7 6 10 11 fi 14 4 1% 19 m 34 34 45 44 2.01 5'00 3.83 0.L 01&L 1.43 1.76 1.26 Oil 6:i4 OliI I::4 1.44 1.06 OZ 1.89 G? 1.30 2.44 3!;1 S:8 14.26 10.63 3:;4 6Y2 5.15 7.88 Rl 12 13 3 57 13 6 11 40 IO a2 22 51 ii :: z 29 30 34 3"7 ii ii 23 A 283 25 2", ;i :z is 22 ;: zz :; zir 24 337 271 335 175 13% 270 275 276 EL 187 187 140 172 379 937 179 493 :A l&Y! 446 449 516 367 261 z 0.459 0.458 0.455 0.14% 0169 0199 0.177 0.123 0219p7 0.422 0!$4 0.477 0.404 0.391 0.438 0.745 O.AMI 0.779 0.941 0.864 1.528 1.806 I&?% 1.327 2.405 2.512 2.254 3.063 2.929 2.885 0.529 0.525 0683 0.603 0.64a 0.601 2.624 0.856 1.172 0.930 0.912 0.942 1.979 o.aS7 2.438 3.63a 3.147 El.842 7.204 4.867 3.643 6.230 5.788 0.746 0.66% 0.714 0.150 0.206 0.196 0.123 0.140 0.207 O.Q%Q 0.390 0.465 0.344 0.367 0.340 0.427 0.158 o.Kl4 0.338 0.809 1.S60 1.617 1.267 1.158 EE 2.272 2.317 21 12 16 17 29 24 19 25 15 24 35 23 36 27 48 57 Sa 41 59 ;: a4 73 51 :: z? :: 114 119 116 110 1t2 113 137 79 112 z 90 101 Ill 158 112 ii :; 167 131 131 124 :z 57 75 69 67 37 114 92 99 et an 10t 162 112 222 235 199 309 410 422 232 394 402 386 42 103‘tl a3 Ei 10152 10050 12752 40 40 93 70 a0 60 46 34 26 33 38 36 ;: 56 21 126 122 153 125 2 105 103 i(: i: :i ii+ 38 32 :; 37 46 54 56 135 1031 a34 a25 764 616 147 103 54 111 56 46 23 32 2 54 39 0”: c.5 :t! 3zS10.6 38312.9 1579.3 133 1588.9 cf! 6.5 14.1 II.8 9.6 14.0 12.0 6.1 CL5 <.5 <5 c.5 c.5 <.5 <5 4 c.5 <.5 c.5 c.5 3.0 6.6 6.3 9.3 13.5 c.5 7.9 a.7 9.9 7.5 9.6 7.3 2.6 <5 c.5 <5 K.5 c.5 <.5 c.5 c5 <.s <.5

166 169 16.28 4.80 5.13 7.06 6.37 5.06 4.66 16.9 6.74 6.o6 6.94 6.71 6.74 12.31 9.18 25.8 26.4 22.0 45.5 61.9 50.7 38.6 53.5 51.5 50.7 340 3i.i 37.3 11.41 Il.67 17.6 15.3% 12.07 10.97 36.0 18.3 20.0 16.8 16.0 la.3 30.7 20.5 49.0 52.9 48.0 92.8 111 92.1 121 119 1569 17.6 18.1 6.88 7.55 8.83 6.63 8.08 6.82 17.5 12.05 13.6% 12.21 11.30 13.08 19.1 13.11 24.8 26.2 24.5 43.1 AZ 58.5 47.6 57.7 56.6 :2"6 352 391 3.96 1.94 2.02 2.56 2.46 2.33 1.81 3.64 3.21 3.85 3.43 3.25 3.40 5.12 3.32 5.74 5.64 5.37 a.30 13.65 11.72 Il.85 11.53 10.39 106 i.i2 1.07 "71 0.74 0.93 1.01 0.90 054 t .oo 1.03 I .25 1.08 I.12 1.11 1.78 1.30 1.67 1.73 1.~1 2.28 3.88 3.52 :9G4 2.93 2.74 2.95 364 375 374-.. 2.37 2.43 2.92 2.85 2.77 1.99 3.98 3.61 4.10 3.82 3.44 3.75 5.58 3.25 5.73 5.55 5.71 7.69 t2.80 11.61 9.99 10.39 10.59 9.51 Ii613 0,625 0.597 0.388 0.392 0,477 0.466 0.493 0.292 0.605 0.545 0.619 0,556 0564 0.569 0.876 0.576 0.664 0.868 0.871 I.235 lB90 1.731 1.411 1.531 1.555 1.492 0.781 083O 0.523 0.518 0651 0.625 0.896 0.416 0.762 0.689 0.733 0882 0.696 0.627 I.044 0.675 1.161 1.090 1.140 1.581 2.363 2.185 I.685 2.038 1.979 1.9e9 0.313 E.!z 0.343 0.231 0.274 0.289 0.316 0.239 0.234 0.242 0.363 0.259 0.441 0.425 0.432 0.588 0.773 0.747 0.62% 0.768 0.808 0.746 1.99 2.25 2.12 141 1.42 1.69 1 75 I.63 1.12 z.ou 1.39 1.54 1.38 1.41 1.38 2.13 154 2.82 2.45 2.53 3.70 4.73 4.60 3.64 4.78 4.64 4.48 0.314 0.326 0.308 0m5 0.211 0.250 0.267 0.274 0.1% 0.300 0.189 0.208 0.184 0.197 0.19Q 0.299 0.219 0.375 0.345 0.349 0.518 0.670 0.646 0.536 O.%QQ 0.707 0.646

101.15 36.0% 48.59 41.46 41.97 3066 21 91 33.49 42.69 26.62 93.49 62.47 42.36 St.80 9353 68.85 22.67 124.38 55.39 72.13 32.82 29.60 63.27 65.21 7963 48.29

24.09 6.46 11.72 11.46 16.04 a.57 4.36 7.07 14.34 9.09 41.47 24.13 15.29 21.90 3292 14.49 5.32 30.02 25.46 13.88 16.77 6.07 7.51 19.61 32.60 26.38 la.64

18.742 18056 17796 17.462 17.449 17.627 17568 16.152 18.036 16.490 19.117 18.856 18.73% 18.865 18.951 18.545 16.010 16.675 18.468 18.244 18.235 18.017 17.865 18134 lQ.taa la.928 la914

1779 17.721 17.333 17009 16.815 17.288 17.415 17,672 17.466 16.130 17.477 17.902 18.133 l%.oal 17.649 17.972 17.793 17.488 17.461 17.695 17,493 17.698 17.568 17.35s 17.879 17.605 la.168

15.533 15.480 IS.443 15.419 15.425 15.432 15.414 15.502 15.478 15.525 15616 15.616 15.588 15.593 15.596 15.520 *5.462 15.527 15.547 15.515 i5.468 15.506 15.494 15.456 15.603 15.544 15.530

15.484 15.463 15.419 16396 15.392 15.414 15405 15.488 15.449 15.507 15.532 15.56a 15.557 15.549 15.530 15.491 15.451 15.466 15.496 15.467 15.431 15.492 15.479 15.415 15.637 15.487 15.492

39.050 36211 38.026 37450 37.515 37770 37573 36.157 36.150 38235 38.559 38.313 35.194 38.2Q5 38x34 38.731 37.9il 38.944 3aaoa 38.370 36.227 38.202 38.031 34.081 38.481 38.462 38.185

37.791 37.737 37.421 36943 36.993 37.389 37300 37.740 37.619 37.904 37.395 37.535 37.667 37.6m 37.220 37.874 37.629 37.396 37.596 37.681 37.330 37.794 37.662 37.293 37.669 37.469 37.584

0.71003 0.70652 0.70957 0.70636 0.70649 0.70612 070602 0.70576 0.70675 0.70993 0.70860 0.70697 070651 0.70647 070640 0.70714 0.70724 0.70651 0.70722 0.70571 0.70859 0.70865 0.70667 0.70841 0.70g84 0.70736 0.70710

0 70616 0.70743 0.70767 0.7056% 0.70553 0.70600 0.70590 0.70528 0.70832 0.70719 0.70571 0.70673 0.70621 0.70%4X, 0.70607 0.70640 0.70678 0.70590 0.70%3@ 0.70546 0.70725 0.70710 0.70809 0.70757 0.70587 0.70650 0.70575 3684 J. L. Wooden et al.

FIG. 3. Paleoisopach maps for the three lava assemblages in the Noril’sk area ( FEDORENKO,1979, 1991): (a) Ivakinsky-Gudchikhinsky: (b) Khakanchansky-Nad~zhdinsky: and (c) Morongovsky-Samoedsky. Limitsofbasalt outcrop slightly generalized from Fig. 1.Only the upper ~orongovsky lavas were contoured for panel (c) because younger lavas are strongly eroded. The Kharaelakh (K). Noril’sk (N), and Vologochan (V) depressions are indicated in (c). Contours in meters.

are discussed in detail by BAEDECKER( 1987), and typical results the samples with the highest U/Pb ratios. Absolute uncertainties in from replicate analyses of standard rocks are presented by BACON the age-corrected 206Pb/204Pbratios are generally less than 0.05 unit and DRUIT~ ( 1988). In this report. we have recalculated all major- but may reach 0.10 unit for a few samples. These uncertainties in and trace-element data to anhydrous compositions, based on sum- the age-correction calculations, however, are much less than the large mation of the ten major oxides to 100% following the assumption lead isotopic variations that characterize the basalts (measured zo6Pb/ of total Fe as FeO. ‘04Pb ratios range from 17.45 to 19. I?: Table I 1. Lead and strontium isotopic compositions were measured on a Neodymium isotopic compositions were measured on a Finnigan- Finnigan-MAT 262 mass spectrometer in static mode at the USGS MAT 262 mass spectrometer at G6osciences Rennes. Chemical tech- laboratory in Menlo Park, CA, Sample preparation for lead isotopic niques are slightly modified from WHITE and PATCHETT(1984). and concentration analyses followed the procedures of WOODENet Neodymium isotopic analyses were made in a static mode, 143Nd/ al. ( 199.2). For strontium isotopic analyses, the HBr wash from the ‘44Ndratios were normalized to ‘4hNd/‘44Nd = 0.7219, and the pro- lead isotopic separation was collected, dried, dissolved in 2 N HNOj, cedure gave a ‘43Nd/‘44Nd ratio of 0.5 I I823 ? 12 (20) for the La dried, and redissolved in 2 N HCl. Strontium was then separated, Jolla neodymium standard. using 2 N HCI on a cation-exchange column and run on single Ta filaments. Within-run precision is f0.00002 OF better. %/*%r ratios are normalized to ‘%r/%r = 8.3752. Standard NBS-987 gave an MAJOR-ELEMENT, TRACE-EI,~~~ENT, AND ISOTOPIC average ?Sr/*%r ratio of 0.7 1026 I 3 during the period of these C~i.~~A~ERISTI~S OF THE LAVA SUITES analyses. For lead isotopic measurements. unce~ainties in the ratios Analytical data are listed in Tables 1 and 2 and seiectively in individual runs are typically +0.05% (2~). Thermal-fractionation corrections are empirically determined from numerous measurements plotted in Figs. 4 through 12. When describing the chemistry on standards NBS-98 I and NBS-982, and average 0.1 Is/mass unit. of these rocks, our selection of elements has sometimes been Total uncertainties in isotopic ratios, which are dominated by vari- based on ( 1) better analytical data for our data set (e.g., Ta ations in the thermal-fractionation correction, are estimated conser- in preference to Nb) or (2) availability of comparable data vatively to be kO.08, 0.10. and 0.14% (2~). respectively, for 206Pb/ 2aaPb, 2a7Pb/204Pb,and ZosPb/204Pbratios. on the basis of numerous (e.g., as a measure of HREE depletion, we use ( Sm/Yb)N runs on rock and metal standards over several years. To monitor the rather than (Gd/Yb), because gadolinium data are absent entire analyticaf scheme. standard whole-rock samples BCR-1 and in many data sets). {The subscript “N” indicates that the BHVO- 1 are processed periodically for lead and strontium isotopic elemental concentrations have been normalized using the analyses by the same techniques, During this study, measured (un- corrected for fractionation) lead isotopic ratios for Fourteen samples primitive-mantle normahzation values of HOFMANN, 1988.) of BWVO-I ranged from 18.622-18.646 (206Pb/2”Pb). lS.497- In discussing trace-element ratios, the least compatible ele- 15.5 15 (2”7Pb/204Pb), and 38.104-38.160 (*osPb/204Pb), consistent ment during mantle magmatic processes is taken as the nu- with the fractionation correction factors determined from lead stan- merator (e.g., Rb/Sr, La/Sm, or Th/Ta). These ratios nor- dards NBS-98 1 and NBS-982. mally increase during fractionation or with decreasing per- Isotopic data for the basalts are corrected for an age of 250 Ma ( DALKYMPLEet al.. 199 I )_Correction of the lead isotopic data uses centage ofpartial melting. Measured and initial isotopic ratios measured lead (0.61-14.26 ppm). thorium (0.53-7.20 ppm), and and values are reported in Tables 1 and 2, but all discussion uranium (0.12-3.24 ppm) contents, and unce~ainties in the age- in this report is in terms ofinitial lead and strontium isotopic corrected data are greater than those associated with the measured ratios and tNd values. data because of the additional uncertainties in the lead, thorium. and uranium contents. Generous analytical uncertainties of +3 percent The elements barium, cesium. potassium, and rubidium for lead. 25 percent for thorium, and tl0 percent for uranium pro- show considerable scatter in mantle-normalized element dia- duce significant errors only in the age-corrected 2ohPb/204Pbratios of grams (Fig. 5 ). This group ofelements is known to be mobile Petrogenesis of flood basalts in Siberia 3685 during low-grade alteration, and their aberrant behavior, in Table2. Isotopiccompositions of Nd in selected samples addition to the petrographic evidence of alteration and the Sample Suite %m/‘MNd’ (‘43Nd/‘aNd)M (‘*3Nd/“‘Nd)t ENd(2MMa) relatively high HZ0 and occasionally high CO* contents mea- 16F-19 SW3 0.171 0.512618*8 0.512338 +0.4 sured in many samples, reflects the alteration undergone by 56-32-87 Hr4 0.165 0.512681*6 0.512411 +1.9 these rocks. High strontium contents are measured in some 56-32-670 HTl 0.185 0.512691i6 0.512388 +1.4 SC-32.1255 Mkl 0.171 0.51261ti5 0.512332 +0.3 samples (e.g., sample 36-32-2375.2 ofpicritic Gudchikhinsky SG-32-1311 Mr2 0.179 0.512688i6 0.512396 +1.6 basalt contains 900 ppm Sr). However, calculated initial “Sr/ ‘X-32-1645 Mrl 0.150 0.51240X6 0.512214 -2.0 ‘X-32-1701 Nd3 0.142 0.51240&S 0.512176 -2.7 “‘Sr [ ( R7Sr/86Sr)1] ratios vary systematically with respect to SG-32-1838 Nd2 0.146 0.512125+6 0.511887 -8.4 stratigraphic position (Fig. 6a), and there is generally a good LX-32.2069 Ndl 0 133 0.51206= 0.511844 -9.2 SG-32-2144.3 0.136 0.512063?6 0.511841 -9.3 correlation between ( 87Sr/8hSr), ratios and initial Q.,,,( tNd.7 ) Ndl SC-32-2212.5 Ndl 0.132 0.511967*5 0.511751 -11.0 values (“T” taken as 250 Ma). Although interpretation of lF-32 Tk 0.171 0.512458+9 0.512178 -2.7 lF-30 Tk 0161 0.5124226 0.512158 -3 1 the strontium isotopic data for individual samples may be lF-22 Tk 0.174 0.512605B 0.512320 +0.1 open to question, the overall trends are interpreted to reflect !X-32-2245.5 Gd2 0.161 0.512795%3 0.512531 +4.2 X-32-2328 Gd2 0.174 0.512773+5 0.512489 +3.4 those of the lavas at the time of their eruption. The REE X-32-2357.5 Gdl 0.162 0.512681+5 0.512415 +1.9 contents show coherent patterns in mantle-normalized ele- SG-32-2464.8 Sv 0.130 0.512371+4 0.512158 -3.1 X-32-2533.7 Iv3 0.116 0.512332+6 0.512142 -3.4 ment diagrams, with no obvious anomalies (Fig. 5). The 56-32-2624.6 tw 0.119 0.512501?4 0.512306 -0.2 neodymium isotopic data are least likely to have been affected ‘Calculated using Sm and Nd contents from Table 1. 143Nd/l44Nd normalized to 146Nd/l44Nd = 0.7219. by alteration. Estimated overall error, i O.OilOO20. In most respects, the major- and trace-element character- Measured value for La Jolla standard, 0.511834+8 (2s) istics discussed in the following text are comparable to those reported by LIGHTFOOT et al. ( 1990b, 1993a,b) for a comparable, but more stratigraphically limited, suite of samples ratios are low ( 17.5-17.7), and tNd,T = -3.1 for the single from the Noril’sk area. analyzed sample. The (87Sr/shSr)l ratios, however, range from 0.70.55-0.7064 and thus are comparable to those of the Iv, lvakinsky and Syverminsky subsuite; they increase with increasing MgO content.

The two lowermost suites, the Ivakinsky and Syverminsky, Gudchikhinsky comprise alkaline and subalkaline basalts with high contents of Na*O, K20, and most incompatible trace elements (Fig. Gudchikhinsky-suite lavas range from basalt to picrite 5a). and varying Si02 contents (Fig. 4~). The three subsuites (6.6-18.2 wt% MgO for the samples of Table 1, and 5-22 of the lowermost, Ivakinsky suite are alkaline and subalkaline wt% MgO if other data sets are included, e.g., FEDORENKO, basalts with overlapping Mg# but distinctive SiOZ contents: I98 1; LIGHTFOOT et al., 1990b, 1993a,b). The Gd, subsuite Iv,, 47.1-47.8: Iv*, 51.1-52.2; and Iv3, 53.6-53.8 wt% (Figs. is composed of a glomeroporphyritic flow, overlain by por- 4a,c). The Mg# for the Syverminsky samples are distinctly phyritic basalts. Both samples of Gd, basalt from borehole higher than those for the Ivakinsky samples and are com- SG-32 appear to be albitized, in comparison with the fresher parable to those of third-assemblage basalts (Fig. 4a); how- samples analyzed by LIGHTFOOT et al. (1990b, 1993a,b), ever, the REE characteristics of the Syverminsky suite are whose data show that the Gdl glomeroporphyritic basalt dif- quite different from those of third-assemblage rocks. For both fers from the Syverminsky basalts in having lower REE con- the Ivakinsky and Syverminsky suites, mantle-normalized tents, particularly LREEs. The Gd, porphyritic basalts are element diagrams show a steep slope defined by REEs and geochemically quite similar to the Syverminsky basalts, as thorium, with a maximum at barium and a sharp decrease stated by FEDORENKO ( 1981) on the basis of an extensive for rubidium and cesium (Fig. 5a). There are negative tan- set of major-element data. Picritic and pi&e-like basalts that talum anomalies in all samples (Fig. 4g); the smallest anom- form the Gd2 subsuite are as much as 150-200 m thick; in alies [(Ta/La)N - 0.81 are seen for alkali trachybasalts of restricted areas, picritic basalts interfinger with basalts of the the Iv, subsuite (-47 wt% SiO>), and larger anomalies [(Ta/ Gd, subsuite. The Gd2 picritic basalts of Table 1 contain La), - 0.51 characterize basalts of the Iv2 and Iv3 subsuites. I3.2- 18.2 wt% MgO, but some samples contain as much as The negative tantalum anomaly increases regularly with 22 wt% MgO (anhydrous). Average-weighted MgO contents stratigraphic height for the three samples of the Syverminsky for the Gd2 subsuite range from 9- 14.7 wt% in different parts suite. of the Noril’sk area ( NALDRETT et al., 1992. Fig. 5e), largely Isotopic variations in the Ivakinsky suite are related to as a function of modal olivine content. Using Mg-Fe distri- composition and stratigraphic position (Figs. 6a,c, 7c, 8, 9b). bution, only the least magnesium rich of these picrites can The relatively silica-poor flows of the Iv, subsuite have the be calculated to have been in equilibrium with FoBOolivine lowest ( R7Sr/86Sr), ratios (0.7058-0.7065 ), the least negative (the most magnesian olivine reported in the Siberian basalts tNd.Tvalue (-0.2), and the highest initial 206Pb/204Pb [ ( 206Pb/ by RYABOV et al., 1985). Thus, whereas it is permissive that 204Pb),] ratios ( 17.8- 18.2). The alkaline, more silica-rich magmatic liquids containing 18 wt% MgO were erupted, we basalts of the Iv2 and Iv3 subsuites have ( 87Sr/86Sr), ratios conclude that these picrites contained varying amounts of = 0.7061-0.7076 and (206Pb/204Pb), ratios = 17.4-17.7; a intratelluric olivine upon emplacement, much of which was single analysed sample has tNd.T = -3.4. Basalts of the Sy- entrained from an intermediate reservoir. Moreover, although verminsky suite have isotopic compositions that continue the basalts of the suite have major-element compositions the pattern of the Iv, and Iv3 subsuites. The (206Pb/204Pb), consistent with derivation by fractional crystallization of pa- 3686 J. I,. Wooden et al.

TVHr o Mk n Mr2 v * Mr,+Nd3 -1200 o Nd,+Nd, - A Tkp cc l Tkb BQ-160C E x Hk o Gd2 $-2ooc x Gd, (ss . sv -24oc

-280C

-32OCIi-

-36OC) L 20 30 40 50 60 70 -2 4 6 8 10 12 14 16 f8 47 48 43 50 51 52 53 54 550 10 20 30 40 50 60 Mg#, total Fe MgO, wt% SiO2, wt% La, PP~

-1600

-2800

-3600 t -.- _! 1.0 1.5 2.0 2.5 3.0 0.9 1.1 1.3 1.5 1.7 1.9 2.1 :2.3 5 1.0 1.5 -2.0 2.5 (La/Sm)N WWN (Thfla)N

FIG 4. Stratigraphic profiles of major- and trace-element chemistry throughout the basalt sequence for (a) Mg#, based on total Fe content; (b) MgO content: (c) Si02 content; (d) La content; (e) ( La/Sm)N ratio: (f) (Gd/Yb), ratio: (g) (Ta/La), ratio; and (h) (Th/Ta)N ratio. Primitive-mantle-normalization values from HOFMANN ( 1988). Dotted lines aid in distinguishing the stratigraphic progression through the first, second, and third basalt assemblages.

rental picritic liquids, such ratios as ( La/Sm)N and (Ta/Yb), distinguished isotopically from the picritic Gdz subsuite by change significantly within the suite and rule out a simple its lower cNd.r value (+ I .9 vs. +3.4 and i-4.2: Fig. 8) and c~stallization relation, as do their isotopic compositions. lower (~~7Pb/2~Pb)~ ratios ( 15.45-15.49 vs. 15.53-15.57; Fig. General characteristics of the rocks are moderate contents of 7a); (siSr/s6Sr)I ratios broadly overlap (0.7064-0.7067 vs. incompatible elements (for a given Mg#) and moderate (La/ 0.7057-0.7067; Fig. 6). Picrites of the Gudchikhinsky suite Sm)N but relatively high (Gd/Yb)N ratios (Fig. 4e, f ). With are characterized by uniformly high ( 207Pb/ro4Pb)I ratios (Fig. near-chondritic ( Ta/La)N ratios (Fig. 4g), the absence of a 7a), and the entire suite has high (R7Sr/ShSr)r ratios relative negative tantalum anomaly distinguishes rocks of the Gud- to [email protected] (Fig. 8a). Although it is presently uncertain chikhinsky suite from those ofall other Siberian basalt suites. whether these isotopic characteristics are magmatic or are The Gd, subsuite of marginally alkaline basalts can be related to postcrystallization alteration, coherency in the iso- Petrogenesis of flood basalts in Siberia 3687 topic data has led us to accept the measured data as mean- they have the highest (Th/Ta), ratios found in any SFBP ingful. basalt suite. Basalts of the Nd, and Nd2 subsuites have low MgO contents (6.3-7.3 wt%) but moderate Mg# that decrease Tuklonsky irregularly with increasing stratigraphic height (from -0.57- -0.53), and Si02 contents that range from 51.4-53.7 wt% The Kakanchansky suite, which consists predominately of (Fig. 4a,b,c). Among SFBP basalts, those of the Nd, and Nd2 basaltic tuffs, is represented by a single sample of basaltic tuff subsuites are notably depleted in copper, nickel, and PGEs; (86-32-2226.7, Table 1). In the eastern part of the Noril’sk yet the transition from Nd, to Nd2 is marked by a sharp area, minor flows occur within the Kakanchansky suite that increase in contents of copper ( lo-75 ppm), nickel (24-53 are similar to the Tuklonsky basalts, and in the western part ppm), and chromium (72-106 ppm). This marked change of the area there occur flows similar to those of the Ndi sub- in transition-metal contents at the Nd,-Nd2 boundary is not suite. The Tuklonsky basalts and pi&es of the second as- evidenced in the large-ion-lithophile (LIL)-element or iso- semblage show a wide range in major-element contents (e.g., topic data (Figs. 4d, 6a,c) and so is interpreted as due largely 8.8-16.7 wt% MgO) but show little variation in trace-element to a change in the efficiency of sulfide-melt extraction ( NAL- ratios. We might attempt to explain the picrite-basalt asso- DRETT et al., 1992; BROGMANN et al., 1993). ciation in this suite by fractional crystallization, but the neo- Basalts of the Nd, and Nd2 subsuites have uniformly high dymium, strontium, and lead isotopic data indicate a more ( 87Sr/86Sr), ratios (0.7074-0.7087 ) and correspondingly low complex relation. The Tuklonsky picrites vary in stratigraphic cNd,r values (-8.4 to - 11). Three of the four samples analyzed position and have a relatively small volume (flows less than anchor the bottom of the ( 87Sr/86Sr)i vs. tNd,T trend for the 35 m thick and a few kilometers long). The volume of the Noril’sk lava sequence; the fourth, the stratigraphically lowest Tuklonsky picrites in the Noril’sk area has been estimated Nd, sample. lies to the left of this trend (Fig. 8). ( 87Sr/86Sr)i at 180 km3, in comparison with 14,860 km3 for the Gud- ratios increase irregularly from the lowest sampled Ndi flow chikhinsky pi&es ( FEDORENKO, 198 1). The Tuklonsky to the uppermost Ndl and lowermost Nd2 flows, followed by pi&es probably also carried intratelluric olivine ( FoSO by a decline in (87Sr/86Sr)i ratios (Fig. 6a; better revealed by electron microprobe) from an intermediate reservoir and thus data of LIGHTFOOT et al., 1993a). Samples of the Nd, and do not represent eruption of high-Mg liquids. Nd2 subsuites have relatively low ( 206Pb/204Pb)i ratios ( 17.3- The overall trace-element characteristics of the Tuklonsky 17.8), a feature that characterizes most ofthe Si02-rich sam- picrites differ significantly from those of Gudchikhinsky pic- ples in the Noril’sk lava sequence (Fig. 7; see the Iv2, 1v3, rites. The Tuklonsky picrites contain slightly lower contents and Syverminsky discussions in the preceding text). In ad- of incompatible elements and Ti02, FeO, nickel, and chro- dition, these samples define a clear linear trend, parallel to, mium, but higher A&O3 and strontium contents at a given but above, the STACEY and KRAMERS ( 1975) mode1 growth Mg#. Their chondrite-normalized REE patterns are notably curve for average crust on the ( 206Pb/204Pb)i vs. ( 208Pb/204Pb)i flatter; the extent of enrichment in LREEs is similar in the diagram (Fig. 7b), and lie in the lead isotopic field defined Gudchikhinsky and Tuklonsky picrites (Fig. 4e), but the by the relatively Si02-rich basalts of the Noril’sk lava sequence conspicuous depletion of HREEs that characterizes the Gud- (Fig. 7). chikhinsky pi&es is absent in the Tuklonsky pi&es (Fig. 4f). Basalts and picrites of the Tuklonsky suite have relatively Nd3 and Mr, large, negative tantalum anomalies (Fig. 4g). Basalts of the Nd, and Mr, subsuites have chemical features The Tuklonsky picrites are distinctive from the Tuklonsky transitional between those of the Nd, and Ndz subsuites and tholeiites and all other lava suites because of their very low those of all the overlying basalts. This transitional, but seem- (2”6Pb/204Pb)i ratios (16.8-17.0 vs. 17.3-17.9 for the tholei- ingly coherent, group of rocks includes both glomeropor- ites; Fig. 7). With respect to other samples of the Noril’sk phyritic ( Nd3) and aphyric (Mr, ) basalts. With increasing lava sequence analyzed in this study, the Tuklonsky picrites stratigraphic height, there is a progressive decrease in Si02 have slightly retarded (87Sr/“6Sr)i ratios (0.7056 ? 1) for content and (La/Sm)N, and an increase in (Ta/La)N and their tNd,T values (-2.7 and -3.1; Fig. 8). In contrast, the tNd,J VahXS (Figs. 4, 8 ). single tholeiitic basalt sample analyzed for strontium and The isotopic systematics of the Nd3 subsuite show a sharp neodymium isotopic composition is on the main (87Sr/86Sr), change from those of the two lower subsuites but are quite vs. tNd,T trend for Noril’sk [ (s7Sr/8hSr)i = 0.7053 and tNd,T similar to those of the single sample analyzed from the lower = 0. I]. The other two tholeiitic basalt samples were analyzed Morongovsky (Mr, ) subsuite. The isotopic compositions of only for strontium isotopic composition and have higher Nd3 and Mr, samples ((8’Sr/86Sr), = 0.7062-0.7064, tNd,T (“‘Sr/*‘Sr), ratios (0.7059 and 0.7060). For the three basalt = -2.0 to -2.7, and (206Pb/204Pb)i = 17.8-18.0) clearly are samples from locality lF, (8’Sr/*6Sr)i ratios increase with intermediate to those obtained for samples from the Nd,- increasing stratigraphic position (Fig. 6a) and negatively Nd2 and upper Morongovsky ( Mr2) subsuites and all other correlate with ( 206Pb/204Pb)i ratios (Fig. 9a). These isotopic stratigraphically higher basalt suites on plots of elemental data indicate that the Tuklonsky picrites and tholeiites are contents vs. isotopic ratios, or isotopic ratios vs. stratigraphic not simply related by olivine addition or subtraction. height; in each plot the data generally lie along a linear trend Nd, and Nd2 (Figs. 6a. 7, 8, 9). Simple physical mixing of magmas from these two endmember groups is an acceptable, though not All the Nadezhdinsky basalts are strongly enriched in necessarily exclusive, process to produce the isotopic and LREEs but have relatively flat HREE patterns (Fig. 4e, f ); compositional characteristics of the Nd3 and Mri subsuites. 3688 J. L. Wooden et al.

1000 1000 0 1838 d 0 16F-19 Sm h 3 l 1970 > Nd2 . 16F-17 > t i CI 16F-2 Sm. I

n 15F-36 100 Km I- A 54 >

El I k 11 I I 66 k I1 I t I I I i lL”““l’I-tll’lJ CsRbBaTh U K TaLaCePbSrNdSmP ZrTi Y Yb Cs RbBaTh U K TaLaCePbSr NdSm P Zr Ti Y Yb

1000 r- 0 a7

g l 196 Hr4 a 15F-25 a IF-27 . 260.5 Tkb ; . 1 F-25 100 . 524.5 . iF-22 Hr3 Hrl

d / I, I I I I I I I, I, , I I I I 1- CsRbBaTh U K TaLaCePbSrNdSmP ZrTi Y Yb Cs RbBaTh U U TaLaCePbSr NdSm P Zr ii Y Yb

1000 0 719 o 2245.5 f l 799 b . 2275.6 0 1070 D 2301 Gd2 t ? . 1203 . 2328 . 1255 I= 100 I 2332.7 I 100 ~ A 1311 A 2357.5 Gd 1 t v 1415 v 2375.2

f 1 I i I 1 I I I I I I I I I I / I I 1 1 Eg ” ” ” ” ” I ’ ” I / ’ CsRbBaTh U K Ta LaCePb Sr NdSm P Zr Ti Y Yb CsRbBaTh U K TaLaCePbSrNdSmP Zr Ti Y Yb

o 2386.2 . 2464.8 Sv 0 2515.4 z 1000 . 2533.7 * 1000 r- A 2543.9 _? ‘“3 ~1 1645 a e Mr1 I A 2597.1 . 1701 Iv2 i v 2605 I= c q 1782 > Nd3 v 2616 100 IV

/ I / 8 / , 9 : / / 8 / , , I : I i 1 I CsRbEiaTh U K TaLaCePbSrNdSm P Zr Ti Y Yb Cs RbBaTh U K Ta La Ce Pb Sr NdSm P Zr Ti Y Yb Petrogenesis of flood basalts in Siberia 3689

A Sm + Km v Hr o Mk

-800 q Mr2 * Mr,+Nd3 o Nd2+Nd, -1200 . Tkp C . Tkb 2 a, -1600 x Hk

E E( Gd2 1 +j -2000 EI Gd, l sv C% a... 0 Iv3 * -2400 * 0 Iv2 o* . Iv, -2800 ,oo

-3200

-3600 L 0.704 0.705 0.706 0.707 0.708 0.709 1 2 4 5 15.38 15.42 15.46 15.5 15.54 15.58 87Sr/%r at 250 Ma *07Pb/204Pb at 250 Ma

FIG. 6. Stratigraphic profiles through the basalt sequence for (a) (87Sr/86Sr)I ratio: (b) Th/U ratio: and (c) (*“Pb/ ‘04Pb ), ratio

Upper Morongovsky ( Mr2) to Samoedsky permost basalt suites are much more limited than those found in the earlier lava suites. With the exception of two samples, Above the Nd3 and Mr, subsuites, five basalt suites con- (s7Sr/86Sr), = 0.7046-0.7054, tNd,T = 0.3-1.9, and (*“Pb/ stitute the third assemblage, which forms a 2,300-m-thick *04Pb), = 17.6-18.2 (Figs. 6a, 7, 8, 9). The exceptions are section that shows relatively little compositional variation. in the Hr, subsuite: ( 1) sample SG-32-260.5 is a composi- Almost all samples have MgO contents between 6.3 and 7.8 tionally normal tholeiite with the lowest ( 87Sr/86Sr), ratio wt%, Mg# between 0.48 and 0.56, and SiOz contents between (0.704 I ) determined in this study, and (2) sample 15F-25, 47.8 and 49.9 wt%. An exception is sample 15F-25, from a the high-silica Ikonsky andesitic basalt, is compositionally single lkonsky andesitic basalt flow, IO m thick and 0.5 km and isotopically similar to the Iv*, Iv3, and Sverminsky ba- long, in the Kharaelakhsky suite; it has chemical character- salts. istics similar in many ways to Iv2 and Iv3 alkaline and sub- Whereas limited isotopic variation within these third-as- alkaline basalts (Figs. 4, 5). In the third-assemblage basalts, semblage suites is their primary feature, systematic isotopic variations in trace-element abundances, trace-element ratios, variations are found in some of the suites and provide im- and isotopic compositions are moderate; several ratios show portant insights into the magmatic processes that produced systematic trends, with changes in these trends largely oc- the upper lava sequence. In a stratigraphic profile, the uni- curring within the Kharaelakhsky suite at the level where the formly higher ( “7Sr/86Sr)l ratios ofthe Mokulaevsky samples Ikonsky flow occurs [e.g., lanthanum, thorium, and uranium and the regular decrease in ( 87Sr/86Sr), ratios (0.7054-0.705 1) contents and ratios involving REEs (Fig. 4) and Th/U (Fig. upward in this suite are notable (Fig. 6a). Steps in the (“Sr/ 6b) 1. Basal@ of the third assemblage are moderately enriched “Sr), profile are found at the boundaries between the Mo- in LREEs and have relatively flat HREE patterns, quite sim- rongovsky and Mokulaevsky suites (0.705 I-0.7054) and be- ilar to those of the Tuklonsky picrites (Fig. 4e, f ). Tantatum tween the Mokulaevsky and Kharaelakhsky suites (0.705 I - anomalies, though ubiquitous, are smaller than in rocks of 0.7046). With the exceptions noted in the preceding text, the second assemblage (Fig. 4g). variation in (87Sr/86Sr)l ratio in the three uppermost suites Isotopic variations in the Mrz subsuite and the four up- is quite small (0.7046-0.7049). Neodymium isotopic data

FIG. 5. Mantle-normalized element diagrams ( WHEATLEY and ROCK, 1988) for the basalt suites (Table I ), normalized to HOFMANN’S ( 1988) primitive-mantle values. Lettered in the order ofsuperposition: (a) Ivakinsky (Iv) and Syverminsky (Sv); (b) Gudchikhinsky (Gd,, basalts; Cd*, picrites); (c) Tuklonsky (Tkb, basalts; Tkp, picrites); (d) Lower and Middle Nadezhdinsky (Nd, and NdZ); (e) Upper Nadezhdinsky ( Nd3) and Lower Morongovsky (Mr, ): (f) Upper Morongovsky (Mr2) and Mokulaevsky (Mk); (g) Kharaelakhsky (Hr); and (h) Kumginsky (Km) and Samoedsky (Sm). Leading “SC-32-” deleted for all borehole samples. 3690 J. L. Wooden et al.

15.58 for these five suites are limited; available data lie along the upper part of the main ( s7Sr/86Sr), vs. tNd,T correlation trend for the Noril’sk lava sequence (Fig. 8a). The lead isotopic 2 15.54 systematics are clearly the most variable isotopic parameter 5: for this group. Although the variation is somewhat random, cv 15.50 limiting bounds of the data trends with respect to stratigraphic 3 position indicate that ( 2aPb/2”Pb)1 and ( 2”7Pb/204Pb), ratios n decrease upward, whereas ( 20sPb/ ‘04Pb), ratios are relatively a 15.46 constant (Figs. 6c, 7a,b). The Mokulaevsky suite shows a iz stratigraphically upward decrease in all lead isotopic ratios B that roughly correlates with a decrease in ( S7Sr/s6Sr)l ratios a 15.42 (Fig. 9a). i 15.38rc ' ' ' 1 ' ' ' ' ' 16.8 17.0 17.2 17.4 17.6 17.8 18.0 18.2 18.4 GEOCHEMICAL AND ISOTOPIC VARIATIONS WITHIN THE SFBP

b The geochemical and isotopic data presented in the pre- cu38.0 - ceding text clearly indicate a varied and complex magmatic t system. These data are explicable, however. within a frame- 0 37.8 - work based on ( 1) the three basalt assemblages originally z z 37.6 -

g 37.4 $ y 37.2 E co 37.0 . z 36.8' "I A bo m 16.8 17.0 17.2 17.4 17.6 17.8 18.0 18.2 18.4 -2s: * 0.700 0.704 0.708 0.712 0.716 0.720 AA *o 0 “,I , , , ,.I_01 $*+ _.*._.... $_I_... 0.704 0.705 0.706 0.707 0.708 0.709 Nd2 + 87Srl86Sr at 250 Ma 61, , / , r. / ‘1 ’ Nd, . Tkp El 8 . Tkb vm •c Gd2 A, 0 R El q Gd, 48 q . n 0

16.8 17 17.2 17.4 17.6 17.8 18 18.2 18.4

206Pb/204Pb at 250 Ma 0 00 -10 c i 0 -121 / ’ 1 ’ 1 1 16.8 17.0 17.2 17.4 17.6 17.8 18.0 18.2 206Pb/204Pbat 250Ma

FIG. 8. (a) (87Sr/86Sr), ratio vs. c Nd,Tvalue: (b) ( *06Pb/204Pb)1ratio vs. Q.,~,~value (data set of Table 2). Dotted lines in the legend separate the three basalt assemblages (see text). MORB, midoceanic-ridge FIG. 7. (a) (2WPb/2MPb), vs. (207Pb/2WPb), ratios; (b) (2”Pb/2MPb), basalt; OIB, oceanic-island basalt ( HOFMANN et al.. 1986; K. P. Jo- vs. ( zosPb/2”Pb), ratios; and (c) (*&Pb/*“Pb), ratio vs. SiOz content. chum, unpubl data); CFB, continental flood basalt (CARLSON and All data corrected for 250 m.y. of radiogenic-lead growth, using mea- HART, 1988; CARLSON, 1991; L~GHTFOOT and HAWKESWORTH, sured lead, thorium, and uranium contents. STACEY and KRAMERS' 1988; LIGHTFOOT et al., 1990a; MAHONEY, 1988). Data for inset ( 1975 ) model growth curves for average crust shown for reference. presented for 250 Ma. Petrogenesis of flood basalts in Siberia 369 I

/ I LIGHTFOOT et al., 1990b, 1993a,b). Most of the lava flows in these suites are strongly fractionated, as indicated by low Mg# (Fig. 4a). These compositional characteristics find some parallels in the compositions of high-Ti CFBs, such as those from the Karoo or Parana basalt provinces f DUNCAN et al., 0 1984; PEATE et al.. 1992). or in the compositions of ocean- 0 - 3 0 island basalts (OIBs). They can be explained if a mantle 0°0 source with near-chondritic (Gd/Yb), ratios melted at rel- 3 b atively high pressure under conditions in which garnet was % 1 a residual phase: high contents of incompatible trace elements suggest that the percentage of melting in the mantle source region was relatively low. The tendency of the more silica- rich samples of these two suites to have high ( Th/Ta)N ratios, relatively high ( 87Sr/X”Sr), ratios, and low ( *~Pb/~“4Pb)~ ratios 168” i i ‘ i b.704 0.705 0.706 0.707 0.708 0.709 indicates that fractionation was accompanied by crustal contamination. The near-vertical trend of Syverminsky-suite samples on the plot of tantalum vs. thorium contents (Fig. b 10) indicates that mixing occurred between batches of magma 54 i- O3 0 that had diverse fractionation/contamination histories. As * discussed by LIWTFOOT et al. ( l993b) for a similar plot of 53 t x l 0 zirconium versus thorium contents, fractionation and partial 0 h melting can cause variations in the contents ofthese strongly 0 0 to moderately incompatible elements only along lines of nearly constant ratio. Contamination with crustal material, I characterized by high thorium contents and high Th/Ta ra- I tios. is needed to produce the Th/Ta ratios of about 4.5 in 4 the lower Syverminsky flows; Th/Ta ratios near 2.3 in the upper Syverminsky flows reflect eruption of magma with a 48 1 Th/Ta ratio near that of primitive mantle. The trend of the Syverminsky data suggests that magma with a composition comparably to that of the tholeiitic members of the overlying Gudchikhinsky suite was one ofthe mixing components. This mixing of early. crustally contaminated magma with less contaminated and distinct magma resembling an immediately overlying suite is repeated at least once again, as represented by the contrast between the compositions of the Ndi-Ndz and Nd3 basalts of the Nadezhdinsky suite. The Gudchikhinsky suite has some perplexing aspects: it is characterized by high ( Gd/Yb)N or ( Srn/YbtN ratios simitar Ftc. 9. (a) (*‘Sr/86Sr), vs. (z~Pb/*04Fb)~ ratios; fb) (8’Sr/s6Sr)i ratio vs. SiOZ content. to those of the Ivakinsky and Syverminsky suites, but lower ( La/Sm)N ratios comparable to those of thestratigraphically higher suites (Figs. 4e,f, 1 la,b). The combination of high (Gd/Yb)N or (Sm/Yb), ratios and high contents of incom- proposed for the volcanic sequence on the basis of petrology, patible elements also suggests the presence of residual garnet chemical composition, and paleotectonic reconstruction in its source and a relatively low percentage of melting. The ( FEDORENKO, I98 1) and now substantiated by more exten- geochemical characteristics of the Gudchikhinsky suite are sive trace-element and isotopic data, and (2) several major transitional in many respects between those typical of the controls on the geochemical and isotopic compositions of fn-st assemblage and those of the upper two assemblages. Its the individual volcanic suites. These controls are (a) differ- (Th/TafN ratios are nearer to unity than those for any other ences in parental-magma geochemistry caused by variations suite and therefore most closely resemble those of astheno- in source geochemistry, in source mineralogy as a result of spheric mantle: however, it has the highest ( 207Pb/204Pb), melting at different pressures and temperatures, and in the relative to (206Pb/204Pb), ratios and unusually high (*‘Sr/ percentage of melting; (b) fractionation of parental magmas, ‘%r)i ratios relative to tNd,Tvalues (Figs. 7, 8). These isotopic varying amounts of assimilation/partial melting of crustal features might be related to postcrystallization alteration, but rocks during fractionation, and variation in the composition the REE characteristics and (Th/Taf, ratios should not be of the contaminants; and (c) mixing of magmas. affected by alteration and thus indicate a unique petrogenesis The lvakinsky and Syverminsky suites of the first assem- for this suite relative to others in the Noril’sk lava sequence. blage have high incompatible-element contents and high (La/ The range of 6Nd.Tvalues (-3 to f4, Fig. 8) in all first-assem- Sm)N and (Gd/Yb), or (Sm/Yb)N ratios (Figs. 4e,f, 11: blage lavas is similar to that ofcertain other CFBs where even 3692 J. L. Wooden et al.

larger ranges are observed (e.g., Deccan, MAHONEY, 1988; 8 Columbia River, CARLSON, 1984) and coincides with the : : j lower tNd,T part of the OIB field. Our inte~retation of this ti i 71i (Th,8.8) / q ,1.’ i range of values is that the more negative 6Nd.Tvalues probably : Th/la=B.S\ ,f . ,...... represent the effects of crustal contamination, as indicated by the ( 87Sr/s6Sr)l ratios and compositional data. The mod- : /I : n n estly positive tNd,Tvalues that characterize the Gudchikhinsky 6- suite are consistent with a model of partial melting of an OIB-like mantle source. We have few data for the tuffaceous Kakanchansky suite that initiates the second assemblage. The second suite in this assemblage, the Tuklonsky, is more depleted in LIL elements than any other suite in the lava sequence (Figs. 4d, 12a). The relatively flat REE patterns and low REE contents that characterize this suite mark a major change to relatively high- percentage melting of mantle sources without garnet, presumably an indication of melting at lower pressures than those characteristic of first-assemblage magmas. The lowermost flow in the Tuklonsky suite has initial isotopic com- mantle, Th/Ta=2.3 positions similar to those found in third-assemblage basalts (Figs. 7,s. 9). Overlying Tuklonsky flows have higher (*‘Sr/ *‘Sr), ratios and lower initial lead (Fig. 9a) and neodymium ( LIGHTFOOT et al., 1993a) isotopic compositions. These isotopic trends seemingly are forerunners of similar but more extreme trends seen in the overlying Nadezhdinsky suite. LIGHTFOOT et al. ( 1993a,b) argue that the Nadezhdinsky b.0 0.4 0.8 1.2 1.6 2.0 2.4 suite represents Tuklonsky-like magmas modified by con- Ta, ppm tamination with crustal material-a conclusion with which we agree. LIGHT-FOOTet al. ( 199Ob) had originally suggested iA Sm * Mrl+Nd3 EC Gd;l n Iv1 that Mokulaevsky-like magmas were parental to the Na- o Km o Ndz+Ndl q Gdl 8 GdL dezhdinsky suite, a model that required assimilation of as v Hr A Tkp 0 sv ’ SVL much as 40% of tonatitic upper crust. However, the geo- o Mk b Tkb 111Iv3 l IVL chemical characteristics of the Tuklonsky suite are such that n Mr2 x Hk 0 Iv2 much less contamination ofa Tuklonsky-like parental magma - would be required. The isotopic characteristics of the Tuk- FIG. 10. Tantalum versus thorium contents. Large symbols rep- lonsky suite, especially the combined lead and strontium iso- resent our full seventy-nine sample data set; smaller dots, squares, topic data (Fig. 9a) that emphasize relatively unradiogenic and crosses represent data from LIGHTFOOTet al. ( 1990b, 1993a,b) lead isotopic compositions for the second assemblage, also for the designated basalt suites. Primitive-mantle ratio from HOF- MANN ( 1988). L, lower continental crust. based on TAYLOR and favor a connection between the Tuklonsky and the Nd, and MCLENNAN ( 1985). U. average upper continental crust (CONDIE, Ndz subsuites (it is impo~ant to continue to distinguish these 1993). two subsuites from the Nd3 subsuite, as discussed in the following text). Samples of the Nd, and Nd2 subsuites have thorium and tantalum contents quite different from those of most other lava suites (Fig. IO), and crustal contamination ing, produced by an influx of upper Morongovsky-like magma of Tuklonsky-like magmas is a reasonable way to produce into a magmatic system previously dominated by Nd, and the observed, relatively high Th/Ta ratios, as discussed for Ndr magmas. This stage of magma mixing ended second- the Syverminsky suite. Most of the Ndi and Ndz samples, assemblage magmatism, and very low LIL-element. Tuklon- however, have relatively constant Th/Ta ratios over a range sky-like magmas are not observed again in the Noril’sk lava of thorium and tantalum contents, supporting the fact that sequence (Fig. 12a). The change from second- to third-as- fractionation was an important process within these subsuites, semblage magmatism is associated with a notable shift in the once contamination had established the new Th/Ta ratio. principle locus of eruptive magmatic activity to the northeast The uppermost subsuite of the Nadezhdinsky, Ndll, and (Fig. 3) and an increase in the volumes of magma erupted the lowermost subsuite of the Morongovsky, MrZ , mark the (Fig. 2). break between the second and third assemblages. Both the Third-assemblage basalts are notable for their relatively geochemical and isotopic characteristics of these subsuites constant major- and trace-element compositions and trace- are transitional between the Ndl-Ndz and Mr2 subsuites. The element ratios; however, both isotopic compositions and some pattern of data on the plot of thorium vs. tantalum contents trace-element contents show significant variations within this (Fig. 10) and on most other compositional and isotopic dia- assemblage. Trace-element variations are most notable in ra- grams is most readily interpreted to result from magma mix- tios involving U: Th/U ratios show significant changes at Petrogenesis of flood basatts in Siberia 3693

0 Km v Hr 0 Mk a Mr2 Mrl + ,*.....N.d%;;

Nd2+ ’ Nd. A Tkp b Tkb

.*.*...*.,.....*.x Hk ts Gd2

s q Gdl Gondwana low- i7 CFE . sv u \ u Iv3 0 Iv2

n Iv,

l.ot,,‘,“‘,““,,“““‘,“’ 0.5 1.0 2.5 3.0

FIG. il. (a) (La/Srn)~ vs. (ThjTa), ratios; (b) (Sm./Yb)n vs. (Th/Ta)u ratios, compared with these ratios in other types of basalt (insets). Dotted lines in the legend separate the three basalt assemblages (see text). MORB, midoceanic- ridge basalt; OIB, oceanic-island basalt (HOFMANN et al., 1986, and K. P. Jochum, unpubl. data): CFB. continental flood basalt (CARLSON and HART, 1988; CARLSON, 1991: LIGHTFO~T and HAWKESWORTH, 1988; LIGHTFOOT et al.. 1990% MAHONEY, 1988). “Karoo picrites” (Nuanetsi picrites) from ELLAM and Cox ( 1989. 199 I ): “Gondwana low- Ti CFB” (includes Tasmanian dolerites) from HERGT et ai. ( 1989, 1991). U, ratios for average upper continental crust ( CONDIE, 1993); Band S, ratios for the Bolgokhtokhsky granodiorite and upper crystalline basement, respectively, of the Noril’sk area (G. K. Czamanske, unpubl. data). 3694 J. L. Wooden et al.

every major suite boundary and regular variation trends occur In Fig. 11, second- and third-assemblage suites define trends within suites (Fig. 6b). The possibility that postemplacement that are well displaced from those of first-assemblage lavas. uranium mobility is responsible for these variations is dis- the main distinction being the lower ( Sm/Yb)N ratios in most counted because of the systematic variation of Th/U ratios ofthe younger lavas, whose ( Sm/Yb)N ratios approach those with stratigraphic position. Incompatible trace-element con- of primitive mantle and suggest that little or no garnet was tents show an increase at the stratigraphic level at which the residual during mantle melting. In Fig. 1la-b. one end of the Ikonsky andesitic basalt flow was emplaced in the Kharae- trend is occupied by Tuklonsky pi&es and third-assemblage laksky suite, and only slowly return to earlier, lower values basalts with low ( La/Sm)N and ( Sm/Yb)N and moderate (Figs. 4e, 12a). This is essentially a third example of the ( Th/Ta)N ratios (-3.5, Fig. 10). and the other by basalts of effects of magma mixing. The Ikonsky flow has distinct iso- the Nd, and Ndz subsuites with intermediate ( La/Sm)N and topic compositions but, unlike the LIL-element contents, ( Sm/Yb)N and very high ( Th/Ta)N ratios. The increase in isotopic compositions of overlying flows return immediately (Th/Ta), ratios is accompanied by increases in SiOz content to values typical of the third assemblage (Fig. 6a,c). Strontium and (87Sr/86Sr)1 ratios and a decrease in tNd.1 values. The and lead isotopic compositions show significant shifts at sev- trend is toward the compositions of continental crust. esti- eral suite boundaries and commonly define distinct groups mates of which are shown in the diagrams. The average com- in the plot of ( R7Sr/86Sr)I vs. ( 206Pb/204Pb)1 ratios (Fig. 9a). positions of a varied suite of upper-crustal xenoliths (S, Fig. 11 insets) from the diatreme of Morongovsky-age 25 km south of the city of Noril’sk ( MASL~V and NESTEROVSKY, 196 1) suggest that local crust may have had a particularly favorable composition. The overall characteristics of second- and third-assemblage lavas find parallels in the compositions of many low-Ti CFBs (DUNCAN et al., 1984; PEATE et al., a 0.708 1992) and are well removed from the compositions of oceanic r basalts. The relative uniformity of composition of third-as- 5: a 0.707 semblage basalts and the contrast they provide with first-assemblage rocks are also similar to those of other CFBs. such 5 as the Karoo sequence, which opens with alkali volcanic rocks G 0.706 and picrites with varying compositions and terminates with % 2 a thick sequence of chemically monotonous basalts. 2 0.705

THE PLUME MODEL AND THE 0.704 PETROGENESIS OF CFBs 0 4 U ppm’ There has been considerable recent enthusiasm for the ar- 0.709, I I , I 1 , , gument that CFBs form when a starting mantle plume im- pinges on the base of the continental lithosphere (RICHARDS et al., 1989; CAMPBELL and GRIFFITHS, 1990; GRIFFITHSand CAMPBELL, 1990, 199 1). We have chosen to analyze our data within this framework, but in no way intend to imply that other models might not be valid. A particularly favorable feature of the plume model is its ability to provide a large volume of mafic magma over a very short interval of time. We believe that the Siberian flood basalts and the ore-bearing intrusions had a common source, and our most direct evidence that this source was an asthenospheric plume comes from lead and osmium isotopic studies of the intrusions. WOODEN et al. ( 1992) and WALKER et al. ( 1993) show that the lead, neodymium, and osmium isotopic compositions of the ores and ore-bearing intrusions correspond closely to those of some Hawaiian basalts. Their osmium isotopic compositions are distinctly different from those found to characterize the lithospheric mantle beneath Siberia (PEARSON et al., 1991). Limitations of space preclude a balanced review of the numerous models for CFB petrogenesis (we refer the reader to CARLSON, 1991). but we will address the considerable FIG. 12. (a) Uranium content vs. (*'Sr/%r),ratio: (b) ( La/Yb)N recent discussion concerning the possible significant contri- vs. (87Sr/86Sr)l ratios. bution of the continental lithospheric mantle to CFB vol- Petrogenesis of flood basal& in Siberia 3695

canism. Atthough numerous reports (see CARLSON, I991 f formed by melting close to the peridotite solidus, would be mention the possibility that the trace-element and isotopic highly magnesian. Parameterization of available, high-pres- characteristics of CFBs were acquired through assimilation sure experimental data (e.g., TAKAHASHI, 1986, 1992; of continental crust, many dismiss this possibility and con- HERZBERGet al., 1990) using a procedure similar to that of clude that such features of CFBs as high ( 87Sr/86Sr)1 ratios MCKENZIE and BICKLE (1988) yields an MgO content of and negative Ta-Nb anomalies are inherited directly or in- 22-24 wt8 for low-percentage partial melts at 50 kbars (M. directly from sources in the continental lithospheric mantle. J. Cheadle, pers. commun., 1993). We stress this fact because LIGHTFOOTet al. ( 1993a,b) came to this conclusion in their it is critical to the following interpretation of the genesis of studies of the SFBP. An argument frequently advanced for CFBs. During ascent through the lithospheric mantle, as- involvement of the lithospheric mantle is that certain char- thenospheric magmas may react with peridotitic wall rocks, acteristics of CFBs are not reproduced in simple models of the most notable modification of their composition being the crust-magma interaction. Key parameters include the com- crystallization of olivine and consequent decrease in MgO bination of high SiOz contents, high (8’Sr/84Sr)~ ratios, and content. If major-element equilibrium were reached between negative Ta-Nb anomalies with high strontium and titanium magma and watt rock, the MgO content of the magma when contents, a series of attributes inconsistent with simple mixing it arrives at the base of the crust will be similar to that of of basalt and granite. Moreover, the typical absence of cor- peridotite at ambient subcrustal pressures. At 12- 15 kbars. relation of major-element compositions with trace-element such melts will contain 12-15 wt’lt MgO according to the abundances and isotopic compositions is inconsistent with a MCKENZIE and BICKLE( 1988) parameterization. However, relatively simple model of assimilation accompanied by frac- we consider it unlikely that low-viscosity melts, rapidly as- tional crystallization (AFC). cending by fracture propagation (NICOLAS, 1986. 1989: Because third-assemblage basalts represent the vast bulk SPENCE and TURCOTTE. 1990), will fully equilibrate with of SFBP magmatism, these rocks must play a central role in the lithosphere (see discussion by WUPPERT and SPARKS, any model for the genesis of this province. A straightforward 1985) and suggest that these melts reached the Moho with explanation for their relatively uniform compositions might MgO contents of 15 wt% or more. seem to be that these magmas formed by moderate amounts These primary plume-derived melts would have been in of melting of a single uniform source, and in view of their equilibrium with garnet, as well as olivine, orthopyroxene, volume and composition, this source is conveniently placed and clinopyroxene, and this assemblage would leave a dis- in the mantle. On the basis of their moderate (87Sr/86Sr), tinctive trace-element imprint, expressed by a characteristic ratios and tNd,T values and, particularly, nearly ubiquitous depletion in HREEs. If the partial-melt fractions were low, negative tantalum anomalies, CFB sources in continental the melts would have high contents of incompatible elements: lithosphe~c mantle enriched in a “su~uction component” the act-assemblage lvakinsky and Syverminsky suites have have been advocated (cf. HAWKESWORTHet al., 1984a,b; all these characteristics. With increasing percentages of melt- HERGTet al.. 1989, 1991; CARLSON,199 1). Starting with ing, the contents of incompatible elements would decline, relatively cold lithospheric mantle, however, it is difficult to and MgO content would increase, though not dramatically. devise thermal schemes that can provide the required volumes Eventually, garnet would be exhausted, and the trace-element of magma (ARNDT and CHRISTENSEN, 1992). Moreover, pattern would lose its HREE depletion; these features char- numerous studies of xenoliths from the continental litho- acterize second- and third-assembIa~e magmas. The Gud- spheric mantle provide no evidence that significant parts of chikhinsky suite represents tholeiitic and picritic magmas with the lithospheric mantle have been enriched in this manner flat LREE patterns but depleted HREEs, i.e.. patterns that or have contributed significantly to flood volcanism. In con- basically combine the trace-element characteristics found in trast, SHARMAet al. ( 1992) conclude that the observed geo- the lower part of the first assemblage with those of the second- chemical and isotopic characteristics of the third-assemblage and third-assemblage lavas. Because the Gudchikhinsky suite basalts of the SFBP were inherited without major changes lies stratigraphically between the Ivakinsky and Syverminsky from an asthenospheric mantle plume. suites and second-assemblage lavas, a possible explanation Compositions of mantle melts are controlled by the com- for its geochemicat characteristics is that it represents tran- position and mineralogy of the source and the conditions sitional melting conditions. In contemplating this transition, under which melting took place. The conditions under which we note that there is no indication of a significant time break a subcontinental plume might melt differ greatly from those during SFBP volcanism. We offer two suggestions: ( I ) mag- associated with oceanic magmat~sm. The principal difference mas parental to the first assemblage may have been generated is the pressure, which profoundIy influences magma com- as the plume head was en route to its highest level, or (2) in position. If the lithosphere beneath the Siberian craton had response to the plume. the continental lithospheric mantle a normal thickness of - 150 km. an assumption supported may have been thinned by extension or thermal erosion, at- by available geophysical data (ZORIN and VLADIMIROV, lowing the plume to ascend to a shallower level by the time 1989). the pressure at its base would have been about 50 of Tuklonsky magmatism. kbars. Thermomechanical modelling by ARNDT and CHRIS- Some isotopic and trace-element constraints for the plume T136En: ( 1992) has shown that during plume-lithosphere in- source can be estimated from the compositions of OIBs and teraction. erosion of the lithosphere is generally small and from the studies of the ores and ore-bearing intrusions. The most melting takes place at sublithospheric depths and pres- lead, neodymium, and osmium isotopic compositions re- sures. Magmas formed under these conditions, even those ported by WOODENet al. ( 1992 ) and WALKER et al. ( 1993 ) 3696 J. I_. Wooden et at. indicate that, with respect to those systems, the plume source compositions are therefore not an argument for a source of region was similar to that for Hawaiian basalts. Although uniform composition, and the same is also true for those OlBs define large fields in traditional isotopic diagrams (e.g., trace elements influenced by the fractionation of olivine, cli- HART et al., 1992), they do not display the more extreme nopyroxene, or plagioclase. compositions-in particular, high ( 87Sr/86Sr)i ratios and low The second argument hinges on strontium abundances, tNd,i values-that characterize many CFBs and samples from which are moderate in third-assemblage basalts and high in the continental lithospheric mantle (Fig. 8a). CFBs are dis- the Nd, and NdZ subsuites. Unlike many CFBs, which have tinguished from OIBs and midoceanic-ridge basalts ( MORBs) negative strontium anomalies that are readily attributable to by a key trace-element parameter-negative Ta-Nb anomalies plagioclase fractionation, the Siberian flood basalts have neg- are present in most CFBs, but are absent, or extremely rare ligible or positive anomalies on mantle-normalized plots (Fig. and small. in oceanic basalts (Fig. 11; e.g., ARNDT and 5). The absence of negative strontium anomalies in rocks CHRISTENSEN, 1992, their Fig. 4 ). The SFBP is underlain by that have fractionated plagioclase indicates either an excess a thick sequence of Paleozoic carbonates, evaporites, and ar- of strontium in the parental magma or contamination with enaceous sedimental rocks, a gneissic basement of Precam- Sr-rich material. Although it might be argued that high stron- brian age, and a thick continental lithosphe~c mantle. Melts tium content is an expected feature of a lithosphe~c mantle generated in the asthenosphere would have traversed all of enriched with “subduction component,” such a source should these rocks during their ascent and may have interacted with also have high cesium, rubidium. and barium contents (e.g., them during periods of ponding; each is a possible contam- WHITE and PATCHETT, 1984; SUN and MCDONOUGH, 1989). inant. However, elevated (“‘Sr/%r)i ratios, low tNd.r values, This is not apparent in the Siberian flood basalts. The Bol- unradiogenic lead isotopic compositions, and especially neg- gokhtokhsky granodiorite, which crops out 50 km west of ative Ta-Nb anomalies point to old continental crust as the Noril’sk and presumably formed by partial melting of lower most significant contaminant. crustal rocks. contains 1,600 to 1,700 ppm Sr. Among others, The following argunlents indicate that derivation of the HJLDRETH and MOORBATH ( 1988) have shown the potential Siberian flood basalts directly from the mantle, whether as- for assimilation of lower-crustal materials to elevate the thenospheric or lithospheric, is not tenable. First, and most strontium contents of mantle-derived melts. For rocks of the powerful, is the uniform, yet highly evolved, composition of Nadezhdinsky suite, positive correlations between strontium most third-assemblage basalts. With MgO contents near 7 content and indices of crustal contamination, such as (Th/ wt% and Mg# of 50-55, these are not primary mantle melts- TaJN and ( 8’Sr/86Sr)I ratios and cNd,T’values. provide evidence their compositions are far removed from magmas formed by of contamination with Sr-rich material. Thus, the strontium the melting of mantle at sublithosphe~c depths. The crys- contents of the Siberian basalts, rather than indicating a tallization ofat least 30% olivine is required to derive a basalt mantle origin, appear to provide an indication of the geo- containing 7 wt % MgO from a primary magma containing chemical characteristics of their crustal contaminants and more than 20 wt% MgO, as would have formed by melting processing. of mantle peridotite at pressures prevailing at the base of the Finally, there is no evidence for the common existence of continental lithosphere (-50 kbars). Even if the effects of a subduction-enriched component in the continental litho- extensive interaction between the primary melts and lithospheric mantle, let alone beneath CFB provinces. Although spheric mantle are allowed, the magmas reaching the Moho material apparently is transported from subducted oceanic would contain at least 12-i 5 wt% MgO. Thus, additional crust into the mantle w-edge overlying subduction zones, there evolution must have taken place in the crust, an observation is no com~lling evidence that this mantle wedge is typically supported by consideration of the mineralogy and chemical transformed into lithospheric mantle. Although evidence for compositions of the erupted basalts. Phenocrystic plagioclase a “subduction component” has been found in analyses of in many flows and aspects of their major-element composi- xenoliths from the mantle beneath island arcs ( MAURY et tions, such as nearly constant A&O3 contents and decreasing al., 1992), complications of analyses of ultramafic nodules CaO with decreasing MgO contents, indicate that plagioclase from the continental lithospheric mantle show few of the and clinopyroxene have also fractionated. The minimal distinctive features of this component: e.g.. negative Ta-Nb change in ( Gd/Yb)N that accompanies relatively constant anomalies and enrichment in LIL elements are absent (e.g., Al203 contents in third-assemblage basalts indicates that gar- MCDONOU~H, 1990, 1992; MENZIES, 1992 ). The well-pub- net was not present in the fractionating assemblage. The licized, but relatively rare, metasomatized peridotite nodules, presence of plagioclase and an absence of garnet is evidence which are enriched in incompatible elements and have ra- that the fractionation took place at pressures less than 9 kbars diogenic isotopic compositions, have negligible or positive (depths less than about 30 km: W~NTHER and NEWTON, 1990; Ta-Nb anomalies. This type of metasomatism is best related RAPP and WATSON. 1993 ). to influx of low-percentage melts ( HAWKESWORTH et al., Whereas these arguments indicate that fractionation ofthe 198413; MENZIES et al., 1987). It is interesting to speculate mantle-derived melts took place within the continental crust, that if the Siberian continental lithospheric mantle had indeed this fractionation was not haphazard. During the evolution formed in a subduction environment and at one time had a of the third-assemblage basalts, the major-element abun- crustal geochemical signature, this signature has been over- dances were buffered so efficiently that only magmas with a whelmed in this Precambrian lithospheric mantle by a con- restricted range of compositions escaped the reservoir and tinuous, billion-year-long influx of low-melt fractions from reached the surface. The relatively uniform major-element the underlying asthenosphere. Petrogenesis of flood basalts in Siberia 3697

THE ROLE OF A MAGMA RESERVOIR THAT IS of the crust probably are locally produced by the influx of PERIODICALLY REPLENISHED, PERIODICALLY extremely large volumes of mafic magma. Such partial melts TAPPED, AND CONTINUOUSLY FRACTIONATING may constitute a significant proportion of the material that DURING ASSIMILATION OF CRUSTAL WALLROCK is assimilated and mixed into the fractionating magmas. If Our interpretation of the data for the Noril’sk lava sequence the main variables in the RTFA process are held approxi- requires four distinct magma types-one for each of the as- mately constant and a steady state is approached, the erupted semblages, with the Gudchikhinsky pi&es representing a lavas should display “. . ( 1 ) uniformity of composition fourth type which was transitional between that represented ; (2) large differences of composition between the lava by the two earlier suites of the first assemblage and that rep- and the parental magma with respect to major and trace ele- resented by the second assemblage. There is indication of ment contents and ratios; (3) strong control of liquid com- two major periods of mixing between strongly fractionated position by the low pressure phase equilibria . .” (O’HARA, + contaminated magma and more weakly contaminated 1977, p. 507 ). These are important features of the SFBP and + fractionated magma-one in the Syverminsky suite and other CFBs. another within the Nadezhdinsky suite, associated with the In Table 3, we demonstrate the feasibility of this process transition from second- to third-assemblage basalts. Frac- by modeling the generation of (a) typical basalt from the tionation affected all magmas significantly-none have MgO voluminous third assemblage and (b) an average basaltic contents close to possible primary mantle melts-and some composition for the heavily contaminated Ndi subsuite. The of the magmas to a high degree (e.g., the Iv*, 1~3, Ndi, and parental magma in both cases was based on the composition Ndz subsuites). Variations in isotopic compositions within of the Tuklonsky picrites. but with a relatively flat pattern suites and correlation of isotopic ratios with SiOz contents of mantle-normalized trace elements. Three contaminants indicate that contamination has obviously accompanied were considered. Contaminant 1 has a composition based fractionation in several specific cases and probably plays at on CONDIE’S ( 1993) average upper continental crust (for all least some role in all cases. elements except tantalum and thorium, this composition is We propose that the geochemical and petrologic variations comparable to that of TAYLOR and M~LENNAN’S (1985) observed within the SFBP can be explained in terms of a upper continental crust): it was used to model bulk assimi- model that combines ( 1) temporal evolution in the com- lation of felsic material (either granitoid or metasedimentary ) position of mantle-derived magma, a feature that is consistent or a partial melt of granitic composition by the parental with a mantle-plume source, and (2) processing in the crust, magma. The strontium content is relatively high ( 1.000 ppm), which involves modifying the compositions of these different though lower than the 1,600-1,700 ppm found in the Bol- magma types through regulation of magma throughput, gokhtokhsky granodiorite, a local pluton (see the following crystal fractionation, and assimilation/partial melting of text) that has previously been used in contamination models crustal rocks. We recognize the possibility that the magmas ( LIGHTFOOT et al., 1993a,b). Contaminant 2 has a mafic interacted with, or were contaminated with, material from composition based on RUDNICK and PRESPER’S ( 1990) av- the continental lithospheric mantle. Indeed, there is mod- erage composition of lower crustal xenoliths. but with lower erately convincing evidence of such a process in other regions Ta/La ratio. In this rather extreme case. we model bulk con- ( EL.L~M et al., 1992). However, at present these processes tamination with metabasalt that might constitute a mafic un- are not well understood and we prefer to attribute the der-plate of the lower continental crust. Contaminant 3 has “crustal” signature of CFBs to crustal contamination. We a tonalitic composition based on a series of 8-kbar partial- therefore propose that fractionation and contamination in a melting experiments on metabasalt ( RAPP et al., 199 1). The dynamic magma reservoir in the lower continental crust was major-element composition was based on the averages from the dominant control on the geochemical characteristics of their experiments, and the trace-element contents were es- third-assemblage basalts and the Nd, and Ndz subsuites. timated from the data of RAPP et al. ( 199 1, Fig. 6). In this A series of reports by O’HARA ( 1977. 1980; O’HARA and case, we model selective contamination with a low-melting MA 1’tsamarium and ytterbium contents) would be improved basalts. by adopting a lower ( Sm/Yb)N ratio in the parental magma These results were obtained with a choice of input param- of the third-assemblage basalts, a change consistent with gen- eters that we believe to be reasonable. For example, for Y eral evolution within the lava pile. There is also a mismatch (the fraction of liquid extracted as a lava flow at the end of for nickel in the Nadezhdinsky modelling, probably resulting each cycle), we used a value of 0.002 for the third-assemblage from sulfide-melt fractionation (NALDRETT et al., 1992; basalts and 0.0 1 for the Nadezhdinsky basalts. According to BRUCMANN et al., 1993), a process not considered in the these parameters, the average thickness of a Siberian flood- calculations. basalt flow, earlier estimated at 15 m, would relate to a magma The fit for the third-assemblage basalts is equally good for reservoir that was effectively 1.5 km thick during production all three contaminants, but the calculated amount of assim- of Nd, and Ndz basalts and to a reservoir that was effectively ilated mafic crust is improbably high, at 62%. For this reason, 7.5 km thick during production of third-assemblage basalts. a model of selective assimilation of partial melts of mafic The larger reservoir responsible for the third assemblage was crust is probably most reasonable ifthe reservoir was resident situated where the ascending mafic magmas reached a point

Table 3. Formation of flood basalts in RTFA magma chambers.

S’Sr 206Pb Th Ta La Pb Sr Nd Sm Yb NI Si02 Al203 Fe0 MgO CaO _ 143Nd _ S6sr la4Nd 204Pb Third-assemblage basalt

Pa”?“1 0.2 0.1 2.1 0.6 60 4.0 1.3 1.2 500 47.0 10.0 11.0 16.0 11.0 0.7030 0.51260 16.00

Contaminant 1 5.7 0.6 30 17 1000 26 4.5 2.2 20 66.0 15.0 4.5 2.2 3.5 0.7070 0.51250 17.40

Contaminant 2 1.6 0.6 13 2.0 422 15 3.5 1.9 100 50.5 16.5 9.1 7.7 10.0 0.7070 0.51250 17.40

Contaminant 3 4.0 0.5 24 12 600 19 5.0 3.0 10 66.0 17.0 4.0 1.0 3.0 0.7070 0.51250 17.40

Model basalt 1 1.4 0.37 9.0 4.0 194 12 3.2 2.1 130 49.9 15.6 12.2 7.8 11.0 0.7052 0.51271 17.77

Model basalt 2 1.2 0.46 9.6 2.1 169 14 3.7 2.4 129 49.2 15.9 12.6 7.7 10.6 0.7054 0.51266 17.49

Model basalt 3 1.2 0.30 6.2 3.3 190 11 32 2.2 127 50.6 15.9 11.6 7.4 11.4 0.7052 0.51271 17.05

Measured basaii 1.2 0.32 6.6 2.0 200 13 3.6 2.8 130 49.4 15.4 12.0 7.1 11.4 0.7051 0.51267 17.60

Contaminant 1 (granitoid) Contaminant 2 (mafic) Contaminant 3 (tonalitic) Input parameters Proportions of phases Input parameters Proportions of phases Input parameters Proportions of phases X 0.0014 01 0.50 X 0.0015 01 0.45 0.0012 01 0.55 Y 0.001 cpx 0.35 Y 0.001 CPX 0.35 : 0.001 CPX 0.30 q 0.10 pleg 0.15 P&Won 0.10 Plag 0.20 0.15 Plag 0.15 Proportion of contaminant 6% of contaminant 62% PrZportio” of contaminant 9% ~~ ___ _ ~___ Nadezhdinsky basalt

Parent 0.2 0.1 2.1 0.6 60 4.0 1.3 1.2 500 47.0 10.0 11.0 16.0 11.0 0.7030 0.51260 16.00

Contaminant 5.7 0.6 40 7.0 1200 35 6.6 2.9 100 66.0 15.0 4.5 2.2 3.5 0.7100 0.51155 17.50

Model basalt 2.5 0.56 17.6 4.0 341 19.1 4.5 2.6 114 52.8 15.6 10.2 7.1 9.6 0.7064 0.51210 17.60

Measured basalt 3.1 0.49 17.2 3.9 310 16 3.9 2.1 45 53.0 15.3 9.5 6.9 10.1 0.7065 0.51210 17.60

Input parameters Proportions of phases Distribution co;ffiiienya(D: all;Jculat~bs) X 0.014 01 0.55 Sr Nd Sm Yb Ni Y 0.010 0.25 01 ,001 ,001 ,001 ,001 ,001 ,001 ,001 ,010 10.0 0.25 z3 0.20 CPX .002 ,020 ,050 ,050 ,100 .200 ,300 1.0 1.0 PrZportio” of contaminant 15% Ptag ,001 ,001 .OOl .OOl 2.0 ,001 .OOl ,020 0.0

Explanation: 1. Compostion of steady-state liquid. calculated using the equation coCb = (X(t+S((Cg/Co)- I)]+ Yl(t- X?) from O’Hara (1g77) l-(l-X-Y)(l-xp1)

a. Input paranleters X - mess fraction of cumulate formed in each cycle. Y - mass fraction extracted es a lava flow. q -fraction of total cumulate precipitated to provide the energy to digest wall rocks b. Proporiions of phases Mass fraction of olivine. clinopyroxene. and plagioclase in fractionating assemblage c. Proportion of contaminant Represents proportion of contaminant in steady-state liquid, calculated from mass balance. Petrogenesis of flood basal& in Siberia 3699

of neutral buoyancy in the lower crust, perhaps near the dis- to have some specific compositional differences (e.g., LIL- continuity represented by the crust-mantle interface (FUR- element contents), their primary (La/Yb)N and (87Sr/86Sr)1 LONG and FOUNTAIN, 1986). The smaller reservoir, whence ratios would have been similar if both were produced by high- the Nadezhdinsky basalts were derived, may have been at percentage partial melting of a common mantle source and this level or, more likely, given the need for a more isotopically underwent a similar amount of RTFA processing. RTFA evolved contaminant, at a midcrustal level. There is little processing of Tuklonsky-like. second-assemblage magma& evidence as to whether the lower crust of this poorly known involving extensive assimilation, would increase (La/ Yb)N area was composed of underplated mafic rocks or relatively and ( 87Sr/8bSr)i ratios to the high values observed in the Ndi felsic granulites, but the materials contributed to the RTFA and Ndz subsuites. Magma input to the reservoir would have reservoir may have been comparable in either case. Low- been low at this stage. Magma mixing between the highly percentage partial melting of relatively mafic rocks has been modified Nd, and Ndz magmas and a less modified third- shown to produce tonalitic liquids ( RAPP et al., 199 I ; RAPP assemblage magma would scatter compositions back along and WATSON, 1993), whose incorporation could produce the RTFA trend, during a stage representing greatly increased effects comparable to those that might be expected from bulk magma input to the reservoir. If the contents of an incom- assimilation of more felsic crust. patible element, rather than an elemental ratio, are used in We emphasize that the particular model parameters listed comparable plots (e-g., uranium vs. (“Sr/%r)i, Fig. 12a), in Table 3 are by no means unique, and that similar agree- the strongly LIL-element-depleted, second-assemblage Tuk- ments can be obtained by using different input parameters. lonsky magmas are distinguishable from the third-assemblage For example, the absolute values of parameters X and Y do magmas, and separate RTFA and magma-mixing trends are not control the composition of the steady-state magma; only recognizable. Even the fact that there is no simple relation the ratio is important. We chose small values in the case of between the isotopic compositions of the picritic and basaltic third-assemblage basalts mainly because a large reservoir is lavas of either the Gudchikhinsky or Tuklonsky suites is most more likely to yield basalts of relatively uniform composition. unde~tandable if the magmas to these suites were undergoing We used an MgO content of 16 wt% for the mantle-derived complex processing in a reservoir from which cumulus olivine magmas fed into the RTFA reservoir. This represents a de- could be entrained. crease of 4-9 wt% from the MgO contents considered typical Because of the fundamental differences in melting con- of primary magmas derived from a plume impinging on the ditions discussed earlier, both elemental contents and ele- base of the continental lithosphere ( TAKAHASHI, 1986, 1992 ), mental ratios are distinct in ( 1) first-assemblage magmas and a decrease that is attributed primarily to olivine fractionation (2) second- and third-assemblage magmas. Almost any ele- as these magmas ascended through the pyroxene-bearing mentaf-variation diagram will distinguish these two groups. l~thosphe~c mantle. As noted earlier, it is possible that frac- Isotopic differences among the least modified members of all tionation and interaction with the lithospheric mantle during three assemblages, however, are not so distinct (e.g., Figs. 7, transit could result in even lower MgO contents for the input 8, 9), and isotopic-variation diagrams alone will not always magmas and notable modification of isotopic and trace-ele- identify their fundamental petrogenetic differences. This ment characteristics. If, however, the MgO contents were too characteristic of SFBP magmatism is consistent with impo- low (less than about 12 wt%), it becomes impossible to pro- sition of most of the isotopic variations on parental magmas duce the major-element, trace-element, and isotopic com- by RTFA processing. In view of the complexity of the system positions of the erupted basalts using realistic parameters. and the processing that we believe the magmas to have un- We believe the qualitative parameters used in this model to dergone, we consider it premature to speculate on the het- be plausible. and report these results to illustrate that processes erogeneity of the plume source with respect to the various in a crustal magma reservoir can produce the critical char- isotopic systems. We think that the parental melts represent acteristics of flood basalts, including their highly evolved yet a common plume source with minimal isotopic variation. relatively uniform major-element compositions, a decoupling On the basis of the combined lead and osmium isotopic stud- of the major-element compositions from trace-element and ies (WALKER et al., 1992, 1993), there is reason to believe isotopic compositions, and the imposition of crustal trace- that this source was characterized by subtle isotopic hetero- element and isotopic characteristics. On the basis of isotopic geneity. and incompatible-element data. PENG et al. ( 1993 ) came to a similar conclusion in their study of the lower six formations ISOTOPIC COMPOSITION AND AGE OF ?-HE CRUST of the western Deccan Traps. The Ivrea Zone of northern INVOLVED IN SFBP MAGMATISM Italy contains mafic intrusions that are postulated to represent magma reservoirs which have operated much as we propose There is evidence that the crustal components involved in (e.g., VOSHAGE et al., 1990). Noril’sk-area magmatism had a range of isotopic composi- One or more dynamic RTFA reservoirs in which magma tions and that the basalt sequence reveals a temporal pattern throughput varied substantially may explain some of the in- to isotopic variation in the contaminant. The silica-rich triguing elemental and isotopic correlations in the NOril’sk members of the Iv2 and Iv) subsuites and the Syverminsky lava sequence. An example may be the good positive corre- suite provide the first evidence for the isotopic compositions lation between (La/Ybfru and (87Sr/S6Sr)i ratios shown by of a crustal contaminant. At 250 Ma, its x7Sr/8”Sr ratio must all second- and third-assemblage samples (Fig. 12b). A]- have been 0.708 Or greater, its tNd Value -4 or lower, and its though the parental magmas for the two assemblages seem lead isotopic composition moderately unradiogenic (Table 3700 J. L. Wooden et al

4; Figs. 7, 8,9). If the isotopic compositions of the Tuklonsky Table 4. Limiting isotopic compositions of possible contaminants of tholeiites and picrites also reflect crustal contamination, then the SFBP at 250 Ma. a component with strongly unradiogenic Pb-isotopic com- %r ‘06Pb 207Pb 208Pb positions is required (Table 4); however, its *‘Sr/*‘%r ratio Lavas &Nd =G 204pb s 204pb (0.706 or higher) could be lower than that involved in the Iv2. kg, 20.708 s-4 517.3 ~~15.42 s37.3 evolution of the Ivakinsky subsuites, whereas its cNd value and Sv would have been about the same. Tk 20.706 s-4 $16.8 mineral as- text are generally applicable in the context of the mantle- semblage, and the percentage of melting in the source plume model for the origin of CFBs. Where the lithosphere region. is thick, melting within the plume will take place at relatively 3) The observed chemical and isotopic characte~stjcs of the high pressure in the presence of garnet, and the percentage lavas were acquired in RTFA magma reservoirs through of melting will be relatively low. Such conditions produce bulk assimilation and/or partial melting of crustal wall- the parental magmas of high-Ti-K basalts. Where and when rocks. Contamination was greatest for Nadezhdinsky-suite the lithosphere is thinner, the plume may ascend to shallower magmas, but processes in RTFA reservoirs also exerted levels, with the result that the percentage of melting will be a dominant control on the compositions of the chemically greater. Garnet may be exhausted, and the magmas will con- monotonous flood basalts that constitute the 2,300-m- tain relatively low contents of moderately to highly incom- thick third assemblage. patible trace elements. These magmas may be parental to 4) Earlier subdivision of Siberian flood-basalt volcanism into low-Ti-K basalts. Processing in RTFA magma reservoirs may three assemblages ( FEDORENKO, 198 I ) finds excellent well be the norm. This processing imposes a crustal signature support in our new geochemical and isotopic data and on the magmas, as expressed most strongly by low (Ta/La), fits well within the context of the mantle-plume model. ratios and evolved isotopic compositions: the overall char- 5) Whereas certain suites of lavas in other CFB provinces acteristics of the parental magma will be retained in the con- pose difficulties for any model and point to additional tents of moderately compatible elements (e.g., titanium). controls during ascent through the continental lithospheric We recognize that this model cannot explain the evolution mantle. we suggest that processing of CFB magmas in of all the lavas in CFB sequences. The Nuanetsi picrites of RTFA magma reservoirs may be the norm rather than the Karoo province in South Africa, for example, remain a the exception. problem. Certain features of those rocks, such as the presence of large negative Ta-Nb anomalies in samples with very high contents of incompatible elements and high MgO contents Ackno~~llrdRments-We gratefully acknowledge Igor Migachev, Al- exander Likhachev, and Alexey Volchkov of the Central Research ( ELLAM and Cox, 1989, 1991), or the positive correlation Institute of Geological Prospecting for Base and Precious Metals for between +,d,7 and yes values ( ELLAM et al., ! 992), appear their strong support of our joint study. The Noril’sk State Nickel to require some type of interaction between a picritic parental Concern and Noril’sk Expedition facilitated our collecting of samples magma and continental lithosphe~c mantle, as discussed by in September 1990. The counsel of Joe Arth, Charlie Bacon. Tom Bullen. Wes Hildreth, Howard Wilshire, Mike Zientek, and other ELLAM and Cox ( 199 1) and ARNDT and CHRISTENSEN USGS colleagues throughout the course of this study is much ap- ( 1992). For nearly all CFBs, however, the hypothesis that preciated. Richard Walker kept us informed of late-breaking devel- the crustal signature is largely imposed in an RTFA magma opments on the Re-0s front. Joe Arth. Tom Bullen, Richard Carlson, reservoir holds promise, as discussed in more depth by ARNDT Clark Johnson. John Mahoney, Stephen Moorbath, Bob Rapp, and et al. (1993). George Tilton provided challenging reviews that contributed to our presentation. Susan Garcia provided excellent assistance with com- puter graphics. We greatly appreciate the editorial assistance of George CONCLUSIONS Havach.

The geochemical and isotopic characte~zations of the SFBP in the Noril’sk area presented here and by LIGHTFOOT et al. ( 1990b, 1993a) are in excellent agreement. We concur with many of the detailed petrologic inferences that LIGHT- REFERENCES FOOT et al. ( 1990b, 1993~) made from their data, e.g., that extensive crustal contamination was involved in the forma- AL’MUKHAMEDOVA. 1. and ZOLOTUKHIN V. V. ( 1988) Geochem- tion of the Nadezhdinsky lavas. However, we disagree that istry of low-potassic basalts from Siberian and Deccan traps. In the iithospheric mantle was significantly involved in SFBP Deccan Flood Busah (ed. K. V. !%JBBhRAO), pp. 341-35 1. Geo- magmatism ( LIGHTFOOTet al., 1993a); instead we favor the logical Society of India. T. and U. ( 1992) The role of lithospheric processing of magmas derived from an asthenospheric mantle ARNDT N. CHRISTENSEN mantle in continental flood volcanism: Thermal and geochemical plume in crustal RTFA reservoirs. The formulation of our constraints. J. Geoph~:s. Res. 97, 10,967-lo,98 I. model was greatly influenced by the relatively monotonous ARNDTN.T..CZAMANSKE G. K..WOOIXN J.L..and FEDORENKO 3702 J. L. Wooden et al.

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