Magmatic and metamorphic evolution of the oceanic crust in the western flank of the MAR crest zone at 15°44′N: Investigation of cores from sites 1275B and 1275D, JOIDES resolution

353

ISSN 0869-5911, Petrology, 2008, Vol. 16, No. 4, pp. 353–375. © Pleiades Publishing, Ltd., 2008.Original Russian Text © S.A. Silantyev, Yu.A. Kostitsyn, D.V. Cherkashin, H.J.B. Dick, P.B. Kelemen, N.N. Kononkova, E.M. Kornienko, 2008, published in Petrologiya, 2008,Vol. 16, No. 4, pp. 376–400.

INTRODUCTIONThis study focused on a collection of samples from

the oceanic basement recovered by deep-sea drilling inthe crest zone of the Mid-Atlantic Ridge (MAR) northof the

15°20

'

Fracture Zone (Fig. 1). Drilling was car-ried out during Leg 209 of the drilling vessel

JOIDESResolution

within the framework of the Ocean DrillingProgram (ODP). The region was selected as a first-pri-ority site on the basis of an application, whose prepara-tion was contributed by one of the authors of this paper.The main goal of the deep-sea drilling carried out dur-ing Leg 209 was the investigation of the section of theoceanic crust composed of mantle peridotites and gab-bros (Hess-type crust) in a slow-spreading ridge (Caseyet al., 1996;

Shipboard Scientific Party

, 2003). Thedrilling was aimed at investigating various petrologicaland geochemical aspects of rock formation in the axialzone of a slow-spreading mid-ocean ridge (MOR): the

composition and structure of mantle materials, genera-tion and transport of magmatic melts, and intracrustalmetamorphic and hydrothermal evolution of the plu-tonic complex.

The region considered in this study is of specialinterest, because it is situated 10 miles from the axialzone of the MAR (Fig. 1), and drill cores obtained inthis region provide insight into the composition andstructure of the basement older than the magmaticproducts of the segment of the rift valley at the same lat-itude. The drill holes of Site 1275 are located along theroute of the

Shinkai

submersible atop a high, which wasinterpreted using the bathymetric data as an oceaniccore complex (Fujiwara et al., 2003). Currently, thisterm usually refers to gabbro–peridotite complexescomposing inside corner highs typical of the regions ofMAR intersection with offsets of transform faults (e.g.,Blackman et al., 2002). ODP sites 1275B and 1275D

Magmatic and Metamorphic Evolution of the Oceanic Crust in the Western Flank of the MAR Crest Zone at 15

°

44

'

N: Investigation of Cores from Sites 1275B and 1275D,

JOIDES Resolution Leg 209

S. A. Silantyev

a

, Yu. A. Kostitsyn

a

, D. V. Cherkashin

a

, H. J. B. Dick

b

, P. B. Kelemen

b

, N. N. Kononkova

a

, and E. M. Kornienko

a

a

Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia

e-mail: [email protected]

b

Woods Hole Oceanographic Institution, Woods Hole, MA 02543 USA

Received November 5, 2006

Abstract

—This paper reports the results of an investigation of a representative collection of samples recoveredby deep-sea drilling from the oceanic basement 10 miles west of the rift valley axis in the crest zone of the Mid-Atlantic Ridge at 15

°

44

'

N (sites 1275B and 1275D). The drilling operations were carried out during Leg 209of the drilling vessel

JOIDES Resolution

within the framework of the Ocean Drilling Program (ODP). The oce-anic crust was penetrated to a depth of 108.7 m at Site 1275B and 209 m at Site 1275D. We reconstructed thefollowing sequence of magmatic and metamorphic events resulting in the formation of a typical oceanic corecomplex of slow-spreading ridges: (1) formation of a strongly fractionated (enriched in iron and titanium)tholeiitic magmatic melt parental to the gabbroids under investigation in a large magma chamber located in ashallow mantle and operating for a long time under steady-state conditions; (2) transfer of the parental mag-matic melt of the gabbroids to the base of the oceanic crust, its interaction with the host mantle peridotites, andformation of troctolites and plagioclase peridotites; (3) intrusion of enriched trondhjemite melts as veins anddikes in the early formed plutonic complex, contact recrystallization of the gabbro, and development in the peri-dotite–gabbro complex of enriched geochemical signatures owing to the influence of the trondhjemite injec-tions; (4) emplacement of dolerite dikes (transformed to diabases); (5) metamorphism of upper epidote-amphibolite facies with the participation of marine fluids; and (6) rapid exhumation of the plutonic complex tothe seafloor accompanied by greenschist-facies metamorphism. The distribution patterns of Sr and Nd isotopesand strongly incompatible elements in the rocks suggest contributions from two melt sources to the magmaticevolution of the MAR crest at 15

°

44

'

N: a depleted reservoir responsible for the formation of the gabbros anddiabases and an enriched reservoir from which the trondhjemites (granophyres) were derived.

DOI:

10.1134/S0869591108040036

354

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SILANTYEV et al.

are located in the crest zone of the MAR, 10 miles westfrom the axis of the rift valley at

15°44

'

N. Site 1275Dis located 90 m south of Site 1275B. The section of theoceanic crust was penetrated to a depth of 108.7 m atSite 1275B and 209 m at Site 1275D. The results ofdrilling showed that the crustal section of this MARsegment is made up of the following rocks: troctolites(14%); gabbroids (74%), including gabbro, gab-bronorite, Fe–Ti gabbro, and

Ol

gabbro; diabases(10%); and granophyres (2%) (

Shipboard ScientificParty

, 2003).

The goal of this study was to reconstruct the mainstages of the magmatic and metamorphic evolution ofthe western flank of the MAR at

15°44

'

N and estimatemagmatic and metamorphic parameters within a morethan 100-m section penetrated by drilling of the plu-tonic complex of the slow-spreading ridge. In addition,the obtained new data on the magmatic and metamor-phic products of the western flank of the MAR at

15°44

'

N were compared with the petrological andgeochemical data obtained previously for the plutoniccomplexes of the MAR crest between

13°

and

15°30

'

N,at

30°

N, and those recovered at ODP Site 375B in theSouthwest Indian Ridge (Dick et al., 1991; Silantyev,1998;

Expedition 304/305 Scientists

, 2006).

OBJECTS AND METHODSThe collection studied included 32 samples of the

following rocks. Site 1275B: troctolite (1), gabbroids(15), granophyre (1), and diabase (1); Site 1275D: troc-tolite-like

Pl

peridotite (1), gabbroids (9), and grano-phyres (4) (Table 1).

The position of each of the samples in the recovereddrill core was carefully documented. It should be notedthat the collection available to us was sufficient to char-acterize the petrology and geochemistry of almost thewhole section penetrated at Site 1275D (11–205 m),whereas the material from Site 1275B represented onlytwo depth levels, 30 m and 87–100 m (Fig. 2).

The compositions of major minerals were deter-mined at the Vernadsky Institute of Geochemistry andAnalytical Chemistry, Russian Academy of Sciencesby the methods of local analysis using a Cameca SX100 electron microprobe with four vertical spectrome-ters operating at an accelerating voltage of 15 kV and abeam current of 30 nA. The following standard sampleswere used: SC olivine for olivine, chrome spinel forspinel, and augite for pyroxene.

The contents of major and some trace elements inthe rocks were determined by I.A. Roshchina by X-rayfluorescence analysis using a PANalytical AXIOS

46°45

′

W 46°30

′

W

15°45

′

N

15°30

′

N

Axis of MARrift valley

Ocean depth

~5000 m

Thalweg axis of

“15°20

′

”

Fracture Zone

Site

1275

Ocean depth

~1500 m

Fig. 1.

Location of ODP Site 1275 according to


(2003).

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MAGMATIC AND METAMORPHIC EVOLUTION OF THE OCEANIC CRUST 355

Advanced spectrometer with a scanning channel. Forthe analysis of major rock-forming elements, a samplepowder was blended with a flux (

Li

2

B

4

O

7

) in a propor-tion of 1 : 12 and fused at a temperature of

1050°ë

; theheated bead was flattened on a graphite substrate duringquenching. For the concentration ranges of the samplesstudied, the relative standard deviations were calcu-lated from independent repeated measurements as1.2% for SiO

2

; 3.5% for Al

2

O

3

; 6.2% for FeO; 8% forNa

2

O, MgO, P

2

O

5

, K

2

O, CaO, and TiO

2

; 10% for S;14% for Cr

2

O

3

; and 17% for MnO.Trace elements were analyzed by ICP-MS using an

Element-XR spectrometer. The samples were digestedwith HF +

HNO

3

in Teflon containers at a temperature

of

180°ë

. The method provided the complete decom-position of rock samples, including the relatively insol-uble minerals of the rocks (zircon, monazite, etc.). Thesensitivity of the instrument was calibrated within thewhole mass range using standard solutions, which con-tained all of the elements analyzed in the samples. Thequality of measurements and drift of instrument sensi-tivity were checked by alternated analyses of samplesand standards. The detection limits were from 0.01 ppbfor heavy and medium elements (U, Th, REE, etc.) to0.1 ppb for light elements (Be, Sc, etc.). The analyticalerror was 3–5% relative.

The analysis of Sr and Nd isotopes was carried outat the Vernadsky Institute of Geochemistry and Analyt-

Table 1.

Rock types and their distribution in drill holes and cores

Sample no. ODP number Position in section, m Rock

75B-1 6R.2 1 < W 30 Troctolite

75B-2 20R.4 1 < W 95.5 Gabbro

75B-3 20R.4 1 < W 95 Fe–Ti gabbro

75B-4 21R.1 1E < W 100 Gneissic gabbro

75B-5 20R.4 1 < W 95 Gabbro

75B-6 20R.4 2A < W 97 Fe–Ti gabbro

75B-7 20R.4 2A < W 97 Gabbro

75B-8 20R.3 1D < W 94.5 Coarse-grained gabbro

75B-9 20R.1 7B < W 94 Gabbro

75B-10 20R.4 2B < W 97.5 Coarse-grained gabbro

75B-11 18R.1 10B < W 87 Trondhjemite at a contact with gabbro

75B-12 20R.4 2B < W 97.5 Leucocratic gabbro

75B-13 20R.4 2C < W 98 Gabbro

75B-15 20R.3 3A < W 94 Diabase

75B-16 20R.3 3A < W 94 Gabbro

75B-17 20R.3 3D < W 94.7 Coarse-grained gabbro

75B-18 20R.3 4 < W 95 Gabbro

75D-1 5R.1 2 < W 23.5 Trondhjemite

75D-2 31R.3 1B < W 145 Trondhjemite

75D-2A 31R.3 1B < W 145 Recrystallized gabbro

75D-3 31R.1 2 < W 153.5 Gabbro-troctolite

75D-4 43R.4 4 < W 205 Leucocratic gabbro

75D-5 2R.2 2 < W 11 Gabbro-troctolite

75D-6 30R.2 3 < W 140 Gabbro

75D-7 28R.1 3 < W 129.5 Fe–Ti gabbro

75D-8 9R.2 4 < W 45 Plagioclase peridotite

75D-9 29R.2 5 < W 135 Gabbro

75D-10 42R.1 5 < W 196.5 Gabbro

75D-11 37R.2 3B < W 174 Gabbro

75D-12 35R.3 3A < W 164.5 Trondhjemite

75D-13 40R.1 8 < W 185.5 Trondhjemite

75D-14 43R.1 8B < W 200.5 Gabbro

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SILANTYEV et al.

ical Chemistry, Russian Academy of Sciences using aTRITON TI multi-channel mass spectrometer.

PETROGRAPHIC TYPES, MINERALOGY AND FORMATION CONDITIONS OF ROCKS

Ultrabasic Rocks

Two samples of ultrabasic rocks were recovered bydrilling. The petrographic differences between them areprobably limited to the degree of their alteration andmodal content of plagioclase. Typical troctolites (sam-ple 75B-1) showing an allotriomorphic texture charac-teristic of this rock type (major minerals show no idio-morphic outlines) were recovered at Site 1275B. The

troctolites are composed of olivine (50–60%,

Fo

90

)partly replaced by serpentine, prehnitized plagioclase,and chlorite (clinochlore) (Table 2). Large olivine relicsin the troctolites are rimmed by a fibrous talc–actinoliteaggregate. Peridotites from the drill core of Site 1275Dare macroscopically ordinary abyssal peridotitesalmost completely replaced by serpentine (sample75D-8). These peridotites also contain abundant brownspinel grains and poorly discernible completely chlori-tized plagioclase relics. It is reasonable to suppose that,similar to Site 1275B (sample 75B-1), the peridotites ofSite 1275D (sample 75D-8) are troctolites (althoughmore melanocratic) or plagioclase peridotites. Chromespinel from the plagioclase peridotite of Site 1275D is

210

200

190

180

170

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

Depth belowseafloor, m

Site 1275Ç Site 1275D

0% 50% 100% 0% 50% 100%

75D-5

75D-1

75D-8

75D-775D-975D-6

75D-275D-2A

75D-3

75D-12

75D-11

75D-1375D-1075D-1475D-4

75B-1

75B-2,75B-3,75B-4,75B-5,75B-6,75B-7,

75B-9,75B-8, 75B-10,75B-11,75B-12,75B-13,75B-15, 75B-16,75B-17, 75B-18

108.7 m

1

2

3

4

5

209 m

Fig. 2.

Distribution of various petrographic types of rocks drilled at sites 1275B and 1275D (


, 2003) andposition of samples considered in this paper in the section. (1) Gabbro, (2) olivine gabbro, (3) granophyre, (4) diabase, and (5) troc-tolite.

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similar in chromium content to chrome spinels from themost depleted residual peridotites of the MAR but ismuch richer in iron (Mg# = 0.25–0.45),

V

2

O

3

(0.30–0.50 wt %), and

TiO

2

(1.8–2.5 wt %) (Table 2).

In the preliminary report on the results of drillingduring Leg 209 of the

JOIDES Resolution

(

ShipboardScientific Party

, 2003), it was supposed that the miner-alogical features of troctolites from sites 1275B and1275D indicate their origin by the recrystallization ofresidual peridotites of the shallow mantle at the expenseof their magmatic interaction with the parental melts ofgabbroid intrusions. The results of our study are also

consistent with such a mechanism of the formation oftroctolites at

15°44

'

N on the MAR: these rocks containolivine relics with compositional parameters similar tothose of olivines from oceanic mantle peridotites andmantle nodules. Olivine from the troctolites of Site1275B differs from that of typical olivine gabbroids ofslow-spreading MOR (e.g., Site 735B, SouthwestIndian Ridge; Mayeda et al., 2002) in higher contents ofnickel and calcium and higher Mg# values.

ShipboardScientific Party

(2003) interpreted the troctolites ofsites 1275B and 1275D as impregnated dunites.

Table 2.

Compositions of olivine, spinel, titanomagnetite, and ilmenite, wt %

Sample no. Phase SiO

2

TiO

2

Al

2

O

3

Cr

2

O

3

FeO* MnO MgO CaO NiO V

2

O

3

ZnO Total

75B-1 Olivine 40.64 0.02 0.05 0.02 10.15 0.18 49.02 0.13 0.30 n.d. n.d. 100.51

75B-1 Olivine 40.42 0.03 0.01 0.01 10.19 0.20 49.11 0.10 0.33 n.d. n.d. 100.40

75B-1 Olivine 40.39 0.03 0.00 0.03 10.18 0.17 49.10 0.07 0.30 n.d. n.d. 100.28

75B-1 Olivine 40.05 0.03 0.03 0.04 10.31 0.17 49.34 0.05 0.34 n.d. n.d. 100.35

75B-1 Olivine 39.99 0.02 0.04 0.03 10.14 0.18 49.08 0.07 0.31 n.d. n.d. 99.86

75B-1 Olivine 40.42 0.00 0.00 0.05 10.09 0.16 50.45 0.09 0.27 n.d. n.d. 101.68

75D-3 Olivine 38.97 0.01 0.04 0.05 15.38 0.28 46.72 0.08 0.21 n.d. n.d. 101.88

75D-3 Olivine 38.53 0.01 0.04 0.03 15.54 0.29 46.64 0.09 0.21 n.d. n.d. 101.40

75D-5 Cr spinel 0.05 8.38 15.21 33.36 41.62 0.50 1.66 n.d. n.d. 0.47 0.69 101.95

75D-7 Cr spinel 0.07 2.09 20.67 43.00 23.30 0.35 11.51 n.d. n.d. 0.31 0.28 101.58

75D-8 Cr spinel 0.03 5.02 18.22 43.47 22.87 0.32 10.97 n.d. n.d. 0.27 0.00 101.17

75D-8 Cr spinel 0.07 6.39 17.24 43.01 24.06 0.33 9.73 n.d. n.d. 0.36 0.07 101.28

75D-8 Cr spinel 0.18 2.31 15.84 43.35 26.57 0.37 8.73 n.d. 0.40 0.35 0.23 98.34

75D-8 Cr spinel 0.14 1.83 15.25 43.69 30.24 0.44 6.15 n.d. 0.13 0.30 0.30 98.48

75D-8 Cr spinel 0.10 2.48 18.46 42.89 23.33 0.37 10.81 n.d. 0.12 0.49 0.22 99.27

75D-8 Cr spinel 0.13 2.17 18.44 42.59 23.08 0.36 10.51 n.d. 0.24 0.40 0.23 98.15

75B-2 Titanomagnetite 0.07 1.26 1.21 0.19 92.27 0.05 0.03 n.d. n.d. 2.04 0.11 97.23

75B-2 Ilmenite 0.06 31.03 4.64 0.17 60.86 0.70 0.25 n.d. n.d. 1.15 0.36 99.24

75B-9 Ilmenite 0.07 50.79 0.07 0.17 46.94 0.76 0.51 n.d. n.d. 0.69 0.00 100.00

75B-9 Ilmenite 0.04 51.15 0.11 0.02 47.43 0.71 0.21 n.d. n.d. 0.57 0.00 100.28

75B-15 Ilmenite 0.04 51.73 0.02 0.03 49.22 1.16 0.15 n.d. n.d. 0.36 0.05 102.75

75B-15 Titanomagnetite 0.04 3.54 1.24 0.09 91.70 0.09 0.08 n.d. n.d. 1.95 0.00 98.74

75D-6 Ilmenite 0.06 52.27 0.30 0.05 47.93 0.72 0.20 n.d. n.d. 0.33 0.09 101.96

75D-6 Titanomagnetite 0.60 6.65 1.71 0.09 86.83 0.14 0.13 n.d. n.d. 0.64 0.00 96.79

75D-8 Ilmenite 0.03 54.21 0.03 0.51 39.25 4.00 4.19 n.d. n.d. 0.17 0.00 102.40



75D-13 Ilmenite 0.04 52.20 0.08 0.01 47.71 1.46 0.11 n.d. n.d. 0.27 0.06 101.95

75D-13 Ilmenite 0.07 52.16 0.06 0.05 47.70 1.28 0.07 n.d. n.d. 0.23 0.34 101.96

75D-13 Ilmenite 0.05 52.26 0.02 45.97 1.20 0.11 n.d. n.d. 0.32 0.22 100.16

75D-13 Ilmenite 0.05 52.30 0.04 0.05 47.42 1.07 0.13 n.d. n.d. 0.27 0.00 101.34

Note: Here and in Tables 3 and 4, n.d. denotes not determined.

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Gabbroids

The sample collection is dominated by ophitic gab-bro composed of clinopyroxene, orthopyroxene, pla-gioclase, amphibole (usually, actinolite or hornblende,and occasionally cummingtonite), and chlorite. A char-acteristic feature of these rocks is the abundance ofopaque phases: magnetite, titanomagnetite, ilmenite,and subordinate pyrite, which leads to the developmentof a sideronitic texture in some samples. In addition totypical ophitic gabbros, the following minor petro-graphic types of gabbro were observed in the drill coresfrom

15°44

'

N: troctolitic gabbro [olivine + plagioclase +clinopyroxene + titanomagnetite + actinolite + chlorite(clinochlore)], gabbronorite, leucocratic gabbro (pla-gioclase + clinopyroxene + iron-rich chlorite + prehnite +titanomagnetite), and gneissic gabbro. Troctolitic andleucocratic gabbros are more common at Site 1275Dcompared with Site 1275B. Some of the aforemen-tioned gabbroid varieties show evidence for high-tem-perature recrystallization, which is most pronounced inthe gneissic gabbro at the contact with felsic veinlets.Such gabbros contain abundant oriented aggregates ofplagioclase and pyroxene neoblasts. Biotite wasdetected in some gabbro samples (leucocratic gabbrosfrom Site 1275B and gabbros enriched in opaquephases from Site 1275D). Apatite, titanite, and epidoteoccur in the leucocratic and variably recrystallized gab-broids of Site 1275D.

The composition of olivine from the most magne-sian (23.27 wt % MgO in the rock, see below) troctolitegabbroids of Site 1275D is transitional between oliv-ines from typical

Ol

gabbros of slow-spreading ridgesand troctolites from Site 1275B and corresponds to

Fo84 (Table 2).

A specific feature of pyroxenes from the gabbroidsof the ODP holes studied is high iron fractions: the FeO*content of clinopyroxene (augite, En36–38Fs19.7–22Wo40–43)ranges from 9 to 17 wt %, and that of orthopyroxene(hypersthene, En51–55Fs39–45–22W3–6) is from 24 to 30 wt %(Table 3). It was previously noted that clinopyroxenesfrom the gabbroids of the plutonic complex of MARsegments between 15°00' and 15°30'N show conspicu-ously high iron contents (up to En40Fs18Wo42) (Silan-tyev, 1998). This phenomenon was interpreted bySilantyev (1998) as a reflection of the high degree ofdifferentiation (iron content) of the host gabbro. Thedata of Table 3 show that pyroxene from the gabbroidsof the plutonic complex of the MAR crest at 15°44'Nshows even higher iron content than pyroxenes from thegabbroids investigated by Silantyev (1998). In the sam-ples described here, this phase (En43.9Fs21Wo35.1) ischemically similar to pyroxene from some olivine gab-bros enriched in opaque minerals from Site 735B at theSouthwest Indian Ridge (Niu et al., 2002). The highcontent of iron in pyroxenes from the gabbroids of1275B and 1275D is in agreement with the very highFeO* content in these rocks (up to 22.56 wt %, seebelow).

The variations in plagioclase composition in thegabbroids depend, on the one hand, on the petrographictype of the host rocks and, on the other hand, on theconditions of their metamorphic transformation. Thegabbro-troctolites contain both primary plagioclasecorresponding mainly to labradorite–bytownite (57–80% An) and secondary oligoclase (29% An) formedduring the metamorphic recrystallization of the rocks.In general, plagioclase from the gabbroids of the twosites ranges in composition from 80–50% An (ophiticgabbro, gabbronorites, giant-textured plagioclase gab-bro, and gneissic gabbro) to 40% An (leucocratic gab-bro). The metamorphic plagioclase of these rocks con-tains 26–13% An.

Phlogopite was detected in the leucocratic varietiesof gabbroids from Site 1275B and gabbro enriched inopaque minerals from Site 1275D. This mica is compo-sitionally similar to the phlogopite described previ-ously in the plutonic complex at the intersection of theMAR and the 15°20' Fracture Zone (Silantyev, 1998)but shows a higher iron content. The composition ofphlogopite from the gabbro recovered by drilling dur-ing Leg 209 of the JOIDES Resolution (samples 75B-12and 75D-7 from our collection) is transitional betweenthe fields of phlogopite from the gabbros and associat-ing trondhjemites from the MAR axis immediatelynorth of the 15°20' Fracture Zone, and phlogopite fromthe leucocratic gabbro of Site 1275B falls within thefield of phlogopite compositions from trondhjemites(Table 3, Fig. 3).

Ilmenite occurs in the gabbroids of the both drillholes. Its composition varies considerably in TiO2 con-tent, from 22–36 wt % in the ophitic gabbro with mod-erate contents of Fe–Ti oxides to 49–54 wt % in thegabbros enriched in opaque phases (Table 2). In thegabbroids of sites 1275B and 1275D, ilmenite closelyassociates with magnetite forming characteristic trellis-type textures of titanomagnetite exsolution (Budding-ton and Lindsley, 1964). Such relationships of Fe–Tioxides are characteristic of olivine-bearing gabbros(e.g., Halama et al., 2004). Indications for ilmenitereplacement by titanite were observed in some samplesof ophitic gabbros and gabbro-troctolites (for instance,samples 1275B-11a and 1275D-10c).

Calcic amphibole is one of the most abundantphases in the rocks of the plutonic complex penetratedduring Leg 209. According to the nomenclature ofLeake et al. (1997), the amphibole of the gabbroids isrepresented by the following solid solution series: acti-nolite–ferroactinolite, edenite–ferroedenite, and ferro-pargasite–hastingsite (Fig. 4).

The aluminous amphibole with compositionalparameters shown in Table 4 is often referred to asbrown amphibole and is considered as the highest tem-perature hydroxyl-bearing phase in gabbroids fromslow-spreading MOR (Meyer et al., 1989; Silantyev,1998; Shipboard Scientific Party, 2003; Expedition304/305 Scientists, 2006). As was noted by us previ-

PETROLOGY Vol. 16 No. 4 2008


Table 3. Compositions of clinopyroxene (Cpx), orthopyroxene (Opx), and phlogopite (Phl), wt %

Sample no. Phase SiO2 TiO2 Al2O3 FeO∗ MnO MgO CaO Na2O K2O Cr2O3 NiO Cl Total

75B-15 Cpx 52.90 0.15 0.85 11.71 0.41 13.48 21.47 0.27 0.00 0.02 0.00 n.d. 101.4175B-2 Cpx 51.00 0.48 1.23 12.35 0.37 13.02 21.11 0.27 0.00 0.04 0.00 n.d. 100.1775B-3 Cpx 51.29 0.44 1.62 12.48 0.42 13.01 20.66 0.32 0.00 0.00 0.00 n.d. 100.3375B-3 Cpx 50.63 0.72 1.90 13.57 0.38 13.21 19.07 0.34 0.00 0.02 0.00 n.d. 99.8475B-4 Cpx 51.40 0.65 1.73 12.71 0.39 12.93 20.55 0.33 0.00 0.01 0.07 n.d. 100.8375B-6 Cpx 51.77 0.61 1.63 13.27 0.35 13.43 19.76 0.35 0.00 0.01 0.00 n.d. 101.3675B-8 Cpx 51.58 0.49 1.75 13.32 0.34 13.29 17.77 0.64 0.07 0.04 0.01 n.d. 99.3275B-8 Cpx 50.86 0.67 1.84 12.83 0.31 12.74 20.44 0.43 0.01 0.00 0.00 n.d. 100.1575B-10 Cpx 50.54 0.57 1.88 13.76 0.41 13.59 17.69 0.33 0.00 0.02 0.04 n.d. 98.8175B-10 Cpx 50.57 0.56 1.53 12.61 0.45 13.22 19.81 0.37 0.00 0.01 0.00 n.d. 99.1775B-12 Cpx 52.02 0.20 0.47 13.35 0.40 12.81 21.25 0.32 0.03 0.04 0.00 n.d. 100.9175B-16 Cpx 51.02 0.43 1.53 12.92 0.32 12.57 18.65 0.29 0.02 0.16 0.03 n.d. 98.0775B-16 Cpx 51.05 0.65 1.58 15.05 0.39 12.99 18.33 0.40 0.03 0.00 0.01 n.d. 100.7275B-17 Cpx 51.09 0.52 1.66 13.69 0.41 12.51 20.17 0.36 0.02 0.00 0.06 n.d. 100.8075B-18 Cpx 52.60 0.03 0.10 12.46 0.39 12.58 23.24 0.15 0.01 0.03 0.00 n.d. 101.7975B-18 Cpx 51.66 0.14 0.62 14.17 0.32 12.20 21.55 0.26 0.01 0.01 0.02 n.d. 100.9675B-18 Cpx 51.62 0.19 0.82 15.02 0.35 12.06 20.84 0.29 0.00 0.01 0.08 n.d. 101.3475D-3 Cpx 50.09 1.16 3.29 4.12 0.16 17.83 21.28 0.38 0.00 0.67 0.04 n.d. 99.4375D-6 Cpx 50.59 0.25 0.90 15.05 0.48 11.37 20.43 0.32 0.02 0.02 0.00 n.d. 99.5475D-6 Cpx 50.66 0.33 1.09 16.59 0.41 11.76 18.96 0.36 0.02 0.00 0.02 n.d. 100.2175D-6 Cpx 51.14 0.01 0.34 16.07 0.38 10.92 19.51 0.19 0.01 0.00 0.00 n.d. 98.6775D-6 Cpx 50.42 0.50 1.11 15.88 0.45 11.63 19.93 0.35 0.01 0.03 0.05 n.d. 100.5875D-9 Cpx 51.36 0.02 0.17 19.29 0.57 8.01 20.81 0.29 0.00 0.01 0.00 n.d. 100.9175D-10 Cpx 51.76 0.64 1.98 12.02 0.38 12.84 21.18 0.35 0.01 0.03 0.00 n.d. 101.4775D-11 Cpx 52.18 0.54 1.45 16.60 0.41 14.80 15.25 0.38 0.00 0.02 0.00 n.d. 101.6475D-11 Cpx 52.08 0.71 1.66 13.37 0.36 13.53 19.58 0.45 0.00 0.01 0.04 n.d. 102.3875D-14 Cpx 51.43 0.74 2.38 12.03 0.33 13.33 20.41 0.26 0.01 0.02 0.00 n.d. 101.2775D-14 Cpx 51.38 0.20 1.43 16.68 0.47 11.32 17.87 0.29 0.01 0.01 0.01 n.d. 99.7575D-2 Cpx 52.37 0.07 0.22 14.84 0.50 10.88 22.63 0.21 0.00 0.00 0.00 n.d. 101.7175D-2 Cpx 50.51 0.52 2.02 13.93 0.36 12.33 20.26 0.36 0.00 0.01 0.00 n.d. 100.3275B-5 Opx 51.75 0.25 0.86 24.50 0.64 19.03 3.05 0.02 0.00 0.00 0.01 n.d. 100.1175B-5 Opx 52.20 0.34 0.84 25.59 0.64 19.59 1.41 0.00 0.02 0.05 0.03 n.d. 100.8075B-6 Opx 51.80 0.33 0.57 26.41 0.61 19.16 1.66 0.06 0.02 0.03 0.04 n.d. 100.8075B-6 Opx 52.16 0.35 0.72 26.34 0.70 19.37 1.77 0.00 0.00 0.03 0.00 n.d. 101.6375B-7 Opx 51.84 0.34 0.65 25.59 0.62 19.55 1.89 0.06 0.00 0.00 0.01 n.d. 100.5675B-7 Opx 51.55 0.29 0.31 26.47 0.65 18.82 1.83 0.04 0.01 0.03 0.00 n.d. 100.0275B-8 Opx 51.65 0.21 0.17 27.48 0.65 18.73 1.13 0.03 0.00 0.01 0.03 n.d. 100.1875B-8 Opx 50.64 0.24 0.13 27.44 0.66 18.41 1.65 0.03 0.00 0.06 0.02 n.d. 99.3275B-18 Opx 51.39 0.28 0.68 28.52 0.04 18.37 1.79 0.03 0.01 0.65 0.02 n.d. 101.8475D-7 Opx 50.91 0.10 0.32 36.56 1.25 8.56 0.98 0.10 0.01 0.03 0.01 n.d. 99.1475D-11 Opx 52.48 0.38 0.76 26.09 0.55 19.82 2.04 0.04 0.02 0.01 0.04 n.d. 102.2475D-11 Opx 51.35 0.38 0.64 30.05 0.62 16.85 1.62 0.04 0.01 0.05 0.03 n.d. 101.6475B-12 Phl 35.00 3.95 12.35 25.46 0.11 7.74 0.25 0.34 7.75 0.36 0.00 1.01 94.3375B-12 Phl 35.87 3.90 13.21 25.10 0.07 9.09 0.04 0.21 8.88 0.03 0.00 0.83 97.2475B-12 Phl 36.01 4.29 13.30 25.23 0.10 9.07 0.03 0.32 8.64 0.05 0.00 0.79 97.8375D-7 Phl 37.87 2.56 12.19 28.07 0.13 6.96 0.07 0.35 7.37 0.07 0.01 0.70 96.3675D-7 Phl 37.55 0.92 12.90 28.43 0.11 7.53 0.08 0.33 7.32 0.01 0.02 0.14 96.0675D-7 Phl 35.56 3.40 12.61 31.83 0.17 4.84 0.06 0.38 7.96 0.05 0.00 1.27 98.45

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ously (Silantyev, 1995), the content of chlorine in theanionic group of amphibole from oceanic basic rockscorrelates with its aluminum content and reaches themaximum values in pargasite–edenite hornblende. Inaccordance with this empirical relation, the highestchlorine contents in amphiboles from the gabbroids ofsites 1275B and 1275D were detected in hastingsite andferropargasite from the most Fe-rich gabbroids (bulk-rock FeO* is 17–22 wt %). Thus, the highest contentsof chlorine were detected in the most Fe-rich amphib-oles analyzed (Table 4). In all cases, calcic amphiboledevelops after pyroxene, and low-alumina amphiboles(actinolite) form also fibrous aggregates at the bound-ary of olivine and plagioclase in the troctolites and troc-tolitic gabbros.

Subcalcic amphibole of the cummingtonite–gruner-ite series was found in some gabbro-troctolite and gab-broid samples from the zones of igneous contact withgranophyres and diabases (at both sites) (Table 5,Fig. 4a). The origin of this mineral in the rocks of thedrilled section can be attributed to several factors. In theolivine-rich gabbro and gabbro-troctolite, the forma-tion of cummingtonite is evidently controlled by pla-gioclase–olivine interaction, which, according to Expe-dition 304/305 Scientists (2006), occurred in the troc-tolitic gabbro of the MAR under the mobile behavior ofCaO and SiO2 and temperatures higher than those ofamphibolite-facies metamorphism. During the high-temperature metamorphism of gabbronorites, cum-

mingtonite may form homoaxial pseudomorphs afterorthopyroxene, which are replaced during subsequentcooling by hornblende (Deer et al., 1963). Since cum-mingtonite was also identified in recrystallized gab-broid samples from the contact zones of granophyresand diabases, it is possible that this phase was formedduring gabbro recrystallization at the expense of thetemperature effect of contact magmatic interaction. Itshould also be noted that the appearance of cumming-tonite in the rocks can be related to the occurrence ofimmiscibility in the ternary tremolite–pargasite–cum-mingtonite system, which was observed in pargasitichornblende at an increase in pressure (Sharma and Jen-kins, 1999). It is evident that the composition of thehost rocks strongly affected variations in cummingto-nite composition: the most iron-rich varieties wereobserved in the gabbro with a high FeO* content(14−17 wt %), whereas the most magnesian cumming-tonites were found in the gabbro-troctolites. The chlo-rine content of cummingtonite from the rocks investi-gated here ranges from 0.01 to 0.33 wt % and shows nodistinct correlation with iron content (Table 5).

The compositional parameters of amphibole fromthe gabbroids of sites 1275B and 1275D, as well as itsrelationships with Fe–Mg silicates of igneous origin(clinopyroxene, orthopyroxene, and olivine) suggestthat the amphibole was formed by the metamorphismof the rocks of the plutonic complex penetrated by drill-ing.

The mineralogical features of the gabbroids of sites1275B and 1275D are similar to those of the rocks ofthe trondhjemite–gabbro association from the MARsegment between 15° and 15°30'N (Silantyev, 1998).However, the data presented above indicate that bothprimary (magmatic) and secondary (metamorphic)phases in the rocks of the plutonic complex at 15°44'NMAR show in general higher iron contents comparedwith the same phases from the rocks of the peridotite–gabbro–trondhjemite association of the MAR between15° and 15°30'N. The gabbroids from the two drillholes associating with granophyre (trondhjemite) dikesand veins contain phlogopite, which was evidentlyrelated to the contact magmatic interaction of the gab-bro and granophyre injections. This mechanism couldbe in part responsible for the albitization of plagioclasein the gabbro. It should be noted that the gneissic vari-eties of gabbroids are much less common in the MARcrest at 15°44'N compared with a more southern seg-ment of the rift valley between 15° and 15°30'N: twosamples from Site 1275B and one sample from Site1275D. The ultrabasic rocks recovered during Leg 209,similar to the associating gabbros, show indications ofhigh-temperature recrystallization: relics of olivine ofpresumably mantle origin in the troctolites of Site1275B and iron-rich spinel with high contents of tita-nium and vanadium in the plagioclase peridotites ofSite 1275D.

5 10 15 20 25 300

2

4

6

8

10

MAR trondhjemites

MAR gabbros

TiO

2, w

t %

MgO, wt %1 2 3 4 5 6

Fig. 3. Variations in MgO and TiO2 contents in phlogopitefrom the gabbroids of sites 1275B and 1275D. (1) Gabbro1275B-12 and (2) gabbro 1275D-7. Also shown are thecompositions of phlogopite from (3) trondhjemites and(4) gabbroids of the MAR between 15° and 15°30'N (Silan-tyev, 1998), (5) mantle residues from the MAR (Cannat andCasey, 1995; Silantyev, 1998), and (6) continental mantleperidotites (Ionov, 1988).



Ca‚ > 1.5;(Na + K)A < 0.5 Ca‚ > 1.5; (Na + K)A > 0.5; Ti < 0.5

1

2

3

0.5

4

5

67

89

0.5

7.5 6.5 5.5

Mg#

1—tremolite2—actinolite3—ferroactinolite

4—edenite5—ferroedenite6—pargasite

7—magnesiohastingsite8—ferropargasite9—hastingsite

75Ç-275Ç-275Ç-275Ç-2

75Ç-375Ç-375Ç-475Ç-9

75Ç-1775Ç-1875D-275D-7

75D-975D-1375D-14

Si, f.u.

(Ca + Nab) < 1

Cummingtonite

Grunerite

78

Si, f.u.

Mg#

75Ç-1175Ç-1275Ç-17

0.5

75D-475D-575D-975D-12

Fig. 4. Classification (Leake et al., 1997) of (a) calcic and (b) subcalcic amphiboles from the rocks recovered at sites 1275B and1275D. (a) Gabbro samples 75B-2, 75B-3, 75B-4, 75B-9, 75B-17, and 75B-18 from Site 1275B; gabbro samples 75D-7, 75D-9,and 75D-14 from Site 1275D; and trondhjemite samples 75D-2 and 75D-13 from Site 1275D. (b) Gabbro samples 75B-12 and 75B-17 from Site 1275B; trondhjemite sample 75B-11 from Site 1275B; gabbro samples 75D-4, 75D-5, and 75D-9 from Site 1275D;and trondhjemite sample 75D-12 from Site 1275D.

Granophyres

In the drill core samples studied, granitoid rocksform thin veins showing cross-cutting (magmatic) con-tacts with the gabbroids. These rocks have distinct gra-nitic (sample 1275B-11b) or doleritic (sample 1275D-12)textures and are made up of albite (11–18% An) or oli-goclase–albite (22% An), actinolite, cummingtonite,

and titanite; there are also plagioclase grains containingup to 45% anorthite component, which are probablyxenocrysts. The mineralogical and geochemical (seebelow) characteristics of vein granophyres from sites1275B and 1275D allow us to assign them to MARtrondhjemites, which were previously described bySilantyev (1998) and Silantyev et al. (2005). Because ofthis, the felsic rocks described by Shipboard Scientific

(a)

(b)

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Table 4. Compositions of calcic amphibole, wt %

Sample no. SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O Cl Total

75B-15 52.29 0.20 3.83 17.44 0.31 13.49 10.99 0.56 0.05 0.08 99.3775B-15 43.44 2.44 10.99 18.16 0.27 10.94 10.78 2.40 0.24 0.00 99.8475B-2 47.26 0.30 4.88 23.82 0.42 8.94 10.35 1.20 0.24 0.85 98.4475B-2 40.70 0.02 12.24 26.12 0.26 4.65 11.02 2.12 0.32 1.55 99.0075B-2 50.70 0.35 3.29 18.52 0.33 12.13 11.78 0.66 0.07 0.21 98.2275B-3 49.35 0.20 2.38 18.25 0.34 12.38 8.71 0.41 0.03 0.04 92.2475B-4 49.94 0.23 3.44 18.73 0.25 10.24 10.20 0.32 0.09 0.00 93.8475B-6 51.58 0.13 3.21 20.21 0.27 11.67 11.39 0.48 0.02 0.02 99.2175B-8 53.30 0.22 1.01 17.49 0.25 14.43 11.02 0.22 0.10 0.07 98.1975B-9 41.93 0.75 11.94 20.15 0.26 8.97 11.28 2.12 0.40 0.64 98.6475B-10 40.20 1.28 10.39 23.23 0.22 6.79 10.95 2.11 0.76 1.15 97.1975B-10 40.26 0.51 9.96 23.88 0.24 6.53 10.98 2.12 0.78 2.07 97.4175B-10 50.62 0.40 2.18 18.15 0.24 13.95 9.98 0.45 0.12 0.16 96.4275B-10 54.79 0.08 0.79 10.20 0.21 19.37 11.87 0.22 0.04 0.04 97.7975B-12 53.34 0.16 0.32 20.89 0.38 13.34 9.73 0.12 0.04 0.08 98.4475B-13 51.48 0.18 3.40 18.81 0.27 12.89 10.53 0.66 0.03 0.01 98.3975B-16 51.83 0.30 1.07 19.35 0.52 15.57 12.01 0.26 0.00 0.01 101.1475B-16 56.74 0.11 0.98 7.66 0.05 19.60 13.37 0.15 0.05 0.08 99.0475B-16 54.53 0.10 0.92 17.89 0.59 15.15 9.04 0.10 0.02 0.01 98.4675B-17 47.59 0.21 5.47 18.68 0.32 11.52 8.85 0.61 0.12 0.15 93.9575B-18 48.22 0.11 4.28 25.47 0.34 8.79 11.00 0.88 0.07 0.32 99.7675B-18 39.30 0.70 12.06 27.66 0.27 4.27 11.37 1.92 0.78 1.93 100.5475B-11 45.53 1.20 6.02 24.03 0.32 8.13 10.20 1.46 0.65 0.55 98.5275B-11 53.34 0.11 0.69 17.88 0.28 14.45 10.18 0.22 0.05 0.05 97.5075B-11 49.68 0.49 3.05 23.56 0.40 11.27 8.80 0.64 0.23 0.23 98.6575B-11 46.23 1.09 5.49 23.78 0.24 8.64 10.47 1.39 0.51 0.44 98.6475D-3 56.71 0.03 0.14 2.77 0.13 24.49 13.15 0.08 0.00 0.01 97.6075D-4 52.40 0.20 1.42 17.51 0.38 15.60 9.20 0.36 0.11 0.05 97.4175D-5 50.76 0.36 2.85 19.93 0.37 13.22 9.43 0.69 0.11 0.20 98.0675D-5 54.14 n.d. 3.31 9.95 0.14 18.23 12.64 0.54 0.01 0.03 99.0975D-5 54.51 n.d. 0.79 15.74 0.47 16.68 8.88 0.21 0.01 0.06 97.7375D-5 51.06 0.52 2.81 18.53 0.26 13.30 10.53 0.70 0.12 0.12 98.1975D-5 52.06 0.09 3.74 11.45 0.24 16.98 12.38 0.59 0.04 0.01 97.6575D-7 41.07 n.d. 12.66 29.04 0.34 3.58 10.90 2.11 0.30 0.36 100.4075D-7 44.79 0.26 5.56 24.44 0.29 5.94 9.11 1.09 0.73 0.63 93.4075D-7 48.34 0.62 3.78 28.77 0.50 6.62 9.32 0.82 0.26 0.32 99.4475D-9 51.31 0.16 2.03 24.89 0.29 8.92 10.30 0.35 0.05 0.04 98.3875D-9 50.08 0.14 2.76 27.23 0.29 6.41 11.59 0.50 0.07 0.05 99.2475D-9 45.24 0.75 5.26 30.62 0.31 4.42 9.75 1.31 0.77 0.75 99.5375D-9 45.75 1.32 5.59 26.50 0.28 7.20 9.34 2.13 0.46 0.19 99.5775D-14 48.81 0.22 6.14 18.88 0.25 12.16 10.58 1.11 0.04 0.11 98.2975D-14 43.06 2.25 10.63 18.85 0.21 10.19 10.97 2.38 0.27 0.03 98.9275D-2 52.31 0.15 1.50 18.75 0.32 12.94 11.95 0.31 0.02 0.04 98.3575D-2 51.41 0.38 1.35 25.13 0.50 9.22 10.66 0.35 0.05 0.03 99.4175D-12 49.95 0.41 2.38 24.71 0.29 7.58 11.37 0.48 0.07 0.05 97.3275D-12 51.69 0.31 1.44 21.87 0.26 10.79 10.70 0.54 0.06 0.11 97.9775D-13 49.29 0.88 4.54 18.58 0.28 12.53 10.43 1.39 0.26 0.03 98.4975D-13 50.25 0.54 3.94 17.86 0.25 14.29 10.19 1.25 0.22 0.02 99.0975D-13 47.34 1.13 6.31 16.84 0.21 13.59 10.96 1.90 0.35 0.05 98.8775D-13 48.98 0.78 4.10 20.95 0.55 11.37 10.13 1.43 0.33 0.08 98.9675D-13 44.93 1.00 5.77 23.63 0.46 8.55 10.56 1.79 0.56 0.21 97.4875D-13 46.78 1.41 5.96 19.90 0.35 11.41 10.27 1.91 0.35 0.13 99.19



Party (2003) as granophyres are referred to astrondhjemites in this paper.

Diabases

Diabases from the two drill holes are represented inour collection by a single sample (75B-15), whichshows a typical doleritic texture. Similar to the associ-ated gabbroids, the diabase is enriched in opaquephases. Diabase sample 75B-15 is composed of albi-tized plagioclase (14% An and 47–65% An), clinopy-roxene, hornblende (pargasite), actinolite with a vary-ing Al2O3 content (1.8–6.3 wt %), and iron-rich chlo-rite. Thus, this sample is mineralogically a uralitizeddolerite.

The relationships of various petrographic types ofrocks and their mineralogical characteristics suggestthat the latest events in the magmatic history of theMAR plutonic complex at 15°44'N were injections oftrondhjemite veins into the previously consolidatedgabbroid pluton and, probably somewhat later, theemplacement of diabase dikes. The P-T conditions ofgabbroid formation at sites 1275B and 1275D werequantitatively estimated for a series of samples inwhich coexisting orthopyroxene and clinopyroxeneswere analyzed. For this purpose, we used the TPF pro-gram (version 2 for MS Windows) designed byA.N. Konilov, V.I. Fonarev, D.M. Sultanov, andA.A. Grafchikov (Konilov, 1999). Four selected gabbro

samples (75B-5, 75B-6, 75B-8, and 75D-11) yieldedtemperatures of 985–1180°ë corresponding to pres-sures of 2–4 kbar (determined by the geothermobarom-eters of Kurepin, 1979; Kretz, 1982; and Nickel et al.,1985). In addition, we used the INFOREX database ofexperimental data on phase equilibria in igneous rocks(Ariskin and Barmina, 2000). In this case, thermometrywas also applied to orthopyroxene–clinopyroxenepairs, whose compositional variations allowed us toestimate the temperature range of gabbroid formationas 900–1000°ë. Silantyev (1998) reported tempera-tures of 900–1020°C (P ≤ 4 kbar) for the formation ofplutonic rocks from the MAR axis between 15°00' and15°30'N. He also demonstrated that the development ofbiotite in the gabbroids of the 15°30'N MAR segmentreflects the influence of magmatic interaction betweenthe host gabbro and invading trondhjemite veins, andthe latter crystallized at temperatures similar to those ofthe formation of the host gabbro.

The thermodynamic parameters of the crystalliza-tion of gabbroids collected south and north of the15°20' Fracture Zone were independently estimated byPlechova et al. (1998) by the computer simulation ofthe crystallization differentiation of basaltic magmas ofgiven compositions using the COMAGMAT programcomplex (Ariskin et al., 1993). Temperatures of 1120–1150°ë were obtained by this method for gabbronoritecrystallization at an MAR segment north of the 15°20'Fracture Zone.

Table 5. Compositions of cummingtonite, wt %

Sample no. SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O Cl Total

75B-12 51.60 0.12 0.49 31.67 0.64 12.03 1.31 0.13 0.04 0.07 98.12

75B-12 52.02 0.10 0.79 22.24 0.54 14.59 4.33 0.14 0.03 0.07 95.04

75B-17 51.32 0.27 0.64 26.26 0.60 17.93 4.19 0.06 0.01 0.01 101.29

75B-11 49.51 0.13 1.85 27.42 0.57 10.64 5.47 0.34 0.08 0.12 96.42

75D-3 53.85 0.56 2.04 9.12 0.34 28.75 5.05 0.16 0.03 0.00 100.33

75D-4 52.74 0.02 0.77 25.82 0.68 15.42 1.70 0.21 0.01 0.04 97.69

75D-4 51.53 0.12 0.81 29.57 0.84 12.61 1.44 0.14 0.00 0.07 97.37

75D-4 51.57 0.00 1.13 28.70 0.90 11.86 2.55 0.23 0.03 0.01 97.20

75D-4 49.13 0.27 2.44 30.89 0.72 8.18 5.29 0.36 0.13 0.21 97.63

75D-5 52.33 0.10 0.95 26.61 0.56 14.14 2.73 0.22 0.02 0.09 98.00

75D-5 55.00 0.06 0.56 15.33 0.68 18.87 5.14 0.10 0.01 0.06 96.00

75D-5 53.46 0.03 0.42 21.49 0.94 18.44 2.13 0.12 0.01 0.05 97.60

75D-9 50.91 0.40 1.82 29.67 0.61 9.61 5.51 0.40 0.10 0.10 99.53

75D-11 52.01 0.17 1.73 22.35 0.66 15.60 4.38 0.30 0.09 0.09 97.68

75D-11 53.39 0.18 1.87 21.99 0.64 15.30 5.28 0.42 0.03 0.08 99.33

75D-12 51.05 0.22 1.31 29.41 0.86 9.31 5.22 0.30 0.06 0.09 98.06

75D-13 50.99 0.24 1.53 25.92 0.43 12.20 6.37 0.40 0.06 0.07 98.21

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SILANTYEV et al.

Taking into account the data presented in this paper,their comparison with previous results, and the accu-racy of the geothermometers (for instance, the geother-mometer of Kretz, 1982 is accurate to ±60°ë), it can beconcluded that the rocks of the plutonic complex of theMAR crest at 15°44'N crystallized within the tempera-ture range 950–1100°ë and a pressure of no higher than4 kbar.

All of the petrographic types of rocks recovered bydrilling at the crest of the MAR bear extensive indica-tions of metamorphic transformations, which are typi-cal of plutonic complexes from slow-spreading MOR.Ultrabasic rocks are subjected to serpentinization,which is most extensive in the plagioclase peridotites ofSite 1275D. The mafic rocks show mineralogical evi-dence for metamorphism within a wide temperaturerange, which is primarily suggested by variations in thecompositional parameters of amphibole, the most abun-dant mineral in these rocks. The metamorphic condi-tions of the gabbro–trondhjemite association of the plu-tonic complex of the MAR at 15°44'N can be estimatedusing empirical relations between the compositionalcharacteristics of amphibole from oceanic metabasitesand the P-T parameters of their metamorphism (Silan-tyev, 1995, 1998). As can be seen in Fig. 5, the amphib-ole-bearing metamorphic mineral assemblages of theserocks were formed at temperatures from ≥600 to200°C. The pressure of metamorphic transformations

in the rocks of the plutonic complex was no higher than4 kbar (for assemblages with the most aluminous horn-blende). The assemblage of aluminous actinolite(4.9 wt % Al2O3) and plagioclase (36% An) in the gab-broids was formed at shallow depths of the crustal sec-tion (≥1 kbar). The pressure was estimated after Plyus-nina (1983) and Hammarstrom and Zen (1986). Low-aluminum actinolite (≤2 wt % Al2O3), albite, and chlo-rite represent the lowest temperature metamorphicassemblage from the rocks of sites 1275B and 1275D,which developed, according to Alt et al. (1986) andSilantyev (1995), within the uppermost few hundredmeters of the section of the oceanic crust. Under thesame conditions, prehnite replaced calcic plagioclase inthe troctolites in association with chlorite and serpen-tine developing after primary mafic minerals.

As was pointed out above, the most aluminous andiron-rich hornblendes from the gabbroids show highchlorine contents (up to 2.07 wt %), which can be usedas an independent indicator of metamorphic tempera-tures for oceanic plutonic rocks. The very high chlorinecontents in amphibole were observed only in samplesfrom Site 1275B, whereas aluminous amphibole fromall of the rock samples from Site 1275D never con-tained more than 0.75 wt % chlorine. This difference inamphibole composition may reflect the nonuniformityof the fluid regime and hydrothermal circulation in thetwo blocks of the oceanic crust spaced by 90 m.

The presented data indicate that the metamorphicparameters of the plutonic complex at 15°44'N MARcorrespond to those estimated previously for plutonicrocks from the slopes of the rift valley between 15° and15°30'N (Silantyev, 1998). However, the structural andtextural characteristics of metamorphic rocks recoveredfrom drill holes 1275B and 1275D suggest that themetamorphism responsible for their formationoccurred without significant influence of dislocationstresses, which lead to the formation of ultrabasicmylonites and amphibolites occurring in some insidecorner highs of the MAR, including those at the15°20'N Fracture Zone.

Thus, the petrological data considered above indi-cate uniform conditions for the formation of plutoniccomplexes in slow-spreading ridges, which is sup-ported by the consistent thermodynamic conditions ofthe magmatic and metamorphic stages of evolution ofthe peridotite–gabbro–trondhjemite association of thethird layer of the MAR over a considerable extent of itsaxis. On the other hand, the same data reveal regionalgeochemical differences between the plutonic com-plexes of the MAR manifested in their specific miner-alogical and geochemical features (e.g., high iron con-tent in the major minerals of the plutonic complex ofthe 15°44'N MAR region).

4 8 12 16 200

1

2

3

4

< 400°C 400−500°C 500−600°C > 600°CN

a 2O

, wt %

Al2O3, wt %

Fig. 5. Temperature dependence of variations in Al2O3 andNa2O contents in amphiboles from the plutonic complex ofthe MAR recovered at sites 1275B and 1275D. The filledsymbols are samples from Site 1275B, and the unfilled sym-bols are from Site 1275D. Squares are gabbros, circles aretrondhjemites, and triangle is diabase. For the sake of com-parison, thin crosses show the compositions of amphibolesfrom the plutonic rocks of the MAR between 13° and 17°N(Silantyev, 1998).



GEOCHEMICAL CHARACTERISTICS OF THE ROCKS OF THE PLUTONIC COMPLEX

OF THE MAR CREST AT 15°44'N

All of the petrographic types of gabbros recoveredat Site 1275B within two narrow depth intervals, 30 mand 87–100 m, are petrochemically strongly evolvedgabbroids of slow-spreading MOR or late-stage gab-bros after Miyashiro and Shido (1980) (Fig. 6, Table 6).In contrast, the gabbros of Site 1275D are similar to thegabbroids of Site 735B in the Southwest Indian Ridge(Meyer et al., 1989) in that they represent the wholecompositional spectrum of MOR gabbroids, i.e., theyinclude varieties of the early, middle, and late stagesaccording to the aforementioned classification. Thisdifference in the petrochemical characteristics of thegabbros is evidently related to the fact that the availablecollection of gabbroid samples from Site 1275Dgeochemically characterized practically the wholethickness of the section of this drill hole at depths from11 to 205 m below the seafloor. The enrichment ofilmenite in all petrographic types of gabbro from thewestern flank of the crest area of the MAR at 15°44'Nand the Mg# and TiO2 variations (Table 6) indicate thatilmenite was the liquidus phase in the magmatic systemresponsible for the formation of the plutonic complexof this MAR segment, and the parental melt of the gab-broids was rich in titanium. The diabases drilled at Site

1275B are geochemically related to the associated gab-broids (Fig. 7, Table 6). As can be seen from Table 6and Fig. 7, the gabbroids from sites 1275B and 1275Dare different from the MAR gabbroids composing theslopes of the rift valley between 15° and 15°30'N andthe section of the crest area of the Southwest IndianRidge (Site 735B) in higher iron contents. It is also evi-dent that some of the samples are recrystallized gabbrostransitional in composition between troctolitic gabbro,gabbronorite, and trondhjemite. It is noteworthy thatphlogopite occurs in the gabbroids of this type, and itscompositional variations (see above) may indicate theformation of phlogopite in these rocks at the expense ofinteraction between the vein injections of trondhjemitesand the host gabbros.

Figure 8 clearly illustrates geochemical variations inthe rocks of interest. There is a trend of trace elementenrichment from ultrabasic rocks (troctolites and pla-gioclase peridotites), through the field of basic rocks(gabbro, gabbro-troctolite, and gabbronorite), and totrondhjemites (granophyres). It can also be seen inFig. 8, that the crest zone of the MAR north of the equa-tor comprises mantle peridotites enriched in incompat-ible elements relative to the model mantle source ofN-MORB. It was previously noted (Silantyev, 2003)

10 10010.01

0.1

1

10

100

1

2

3

1 2 3

TiO

2, w

t %

FeO*, wt %

Fig. 6. Variations in FeO* and TiO2 contents in plutonicrocks from sites 1275D and 1275D. Fields 1, 2, and 3encompass the compositions of gabbros from the MAR ofthe early, middle, and late stages after Miyashiro and Shido(1980). Filled symbols are samples from Site 1275B, andunfilled symbols are samples from Site 1275D. Squares aregabbros, circles are trondhjemites, and triangle is diabase.Also shown are (1) trondhjemites and (2) gabbroids fromthe MAR between 13° and 17°N (Silantyev, 1998) and(3) gabbroids from Site 735B (Dick et al., 1991).

0.2 0.4 0.6 0.8 1.00

1.0

0.2

0.4

0.6

0.8

0.8

0.6

0.4

0.2

0

1.00FeO*

Na2O + K2O MgO

1 2 3

Fig. 7. Comparison in the AFM diagram of the composi-tions of rocks from the plutonic complex drilled in the crestzone of the MAR at 15°44'N and their petrographic analogsfrom the 13°–17°N MAR segment, Southwest IndianRidge, and Carlsberg Ridge (Indian Ocean). Filled symbolsare samples from Site 1275B, and unfilled symbols are sam-ples from Site 1275D. Squares are gabbros, circles aretrondhjemites, triangle is diabase, and diamonds are peri-dotites. Also shown are (1) gabbroids from the MAR seg-ment between 13° and 17° (Silantyev, 1998) and Site 735Bin the Southwest Indian Ridge (Dick et al., 1991),(2) trondhjemites from the MAR at 15°30'N, and (3) grani-toids from the Carlsberg Ridge.

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that the geochemically enriched and usually mostdepleted peridotites of the MAR show mineralogicalindicators (presence of phlogopite and magmatic horn-blende) of their interaction with enriched melts of theintraplate type. In addition, Fig. 8 shows that the troct-olites and plagioclase peridotites of sites 1275B and1275D are enriched in Zr and Nb relative to the DMcomposition. On the other hand, with respect to compo-sitional parameters (Al2O3/SiO2 and MgO/SiO2), theultrabasic rocks considered here are similar to thecumulate plagioclase peridotites of ophiolitic com-plexes (see, for example, the composition of plagio-clase peridotites from the Troodos complex reported byAllen, 1975).

The data of Table 7 and Fig. 9 characterize the REEdistribution patterns of rocks from the plutonic com-plex penetrated at ODP Site 1275. It can be seen that thechondrite-normalized REE contents of gabbroids fromsites 1275B and 1275D correspond to those of AtlanticMORB (Hofmann, 1988) and gabbroids of ophioliticlayered complexes (for instance, Semail ophiolite, Pal-lister and Knight, 1981). All of the silicic rock samplesshow strong LREE enrichment. It is also intriguing thatthe ultrabasic rocks are slightly enriched in LREE rela-tive to the chondrite composition. The chondrite-nor-malized (Sun and McDonough, 1989) La/Sm ratiosincrease in the following sequence of petrographic rocktypes: gabbroids [(La/Sm)cn = 0.51–0.91]–troctolites

Table 6. Major-element compositions of the rocks, wt %

Sample SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Cr2O3 L.O.I. Total

75B-1 40.58 0.12 5.00 7.60 0.13 33.01 4.03 0.01 0.03 0.35 9.35 100.20

75B-2 45.88 3.91 12.86 17.26 0.22 5.52 11.32 2.95 0.10 0.07 0.01 0.00 100.10

75B-3 42.62 5.53 10.32 19.92 0.25 6.04 12.47 2.38 0.06 0.04 0.01 0.00 99.64

75B-4 44.46 4.60 12.34 16.27 0.21 5.69 12.82 2.68 0.10 0.03 0.01 0.42 99.62

75B-5 42.94 5.17 11.42 18.20 0.23 5.84 12.88 2.36 0.06 0.02 0.01 0.54 99.67

75B-6 40.88 7.93 9.05 22.56 0.27 6.37 13.77 2.03 0.06 0.06 0.01 0.00 102.99

75B-7 45.60 3.94 11.65 17.70 0.25 6.28 12.18 2.61 0.08 0.04 0.01 0.00 100.33

75B-8 48.28 3.26 12.98 14.37 0.23 6.01 10.79 3.42 0.42 0.10 0.01 0.11 99.97

75B-9 43.30 6.34 10.57 17.54 0.23 5.36 12.33 2.63 0.20 0.13 0.01 0.21 98.85

75B-10 44.38 5.14 12.81 17.32 0.22 5.06 11.60 2.65 0.12 0.08 0.00 0.00 99.38

75B-11 51.64 3.66 9.20 13.89 0.20 5.50 9.25 4.50 0.08 0.34 0.00 1.10 99.36

75B-12 46.93 3.77 11.29 17.23 0.23 6.20 10.52 2.80 0.19 0.08 0.01 0.37 99.62

75B-13 48.04 4.44 13.71 13.20 0.20 5.62 11.37 3.28 0.17 0.10 0.01 0.14 100.28

75B-15 47.89 2.76 13.61 13.68 0.21 5.86 12.10 2.96 0.17 0.02 0.00 0.17 99.43

75B-16 45.83 3.72 11.26 17.54 0.24 7.40 10.72 2.61 0.11 0.07 0.02 0.87 100.39

75B-17 48.18 2.42 13.38 13.97 0.20 5.74 11.00 3.31 0.12 0.05 0.01 0.79 99.17

75B-18 46.18 3.51 11.65 17.40 0.24 5.96 11.30 2.70 0.14 0.14 0.01 0.07 99.30

75D-1 56.72 1.51 18.85 4.49 0.09 2.62 5.43 7.52 0.12 0.16 0.00 1.22 98.73

75D-2 65.35 0.93 16.85 3.62 0.06 1.09 4.05 6.41 0.31 0.08 0.00 0.12 98.86

75D-2A 53.29 0.86 15.50 9.03 0.16 5.20 10.46 4.09 0.12 0.05 0.01 0.17 98.94

75D-3 43.46 0.69 9.22 9.56 0.15 23.37 6.66 0.45 0.11 0.05 0.22 6.13 100.07

75D-4 48.46 3.72 13.47 15.04 0.26 3.26 9.13 4.32 0.30 1.56 0.00 0.26 99.78

75D-5 47.39 0.40 5.72 13.48 0.14 22.58 3.44 0.41 0.11 0.10 0.27 5.04 99.08

75D-6 45.71 3.64 13.67 18.41 0.23 2.45 8.36 4.46 0.40 1.13 0.00 0.84 99.30

75D-7 42.77 5.56 11.12 21.64 0.26 4.27 10.62 2.92 0.31 0.24 0.01 0.29 100.01

75D-8 39.45 0.15 2.18 8.62 0.13 35.78 0.59 n.d. 0.02 0.03 0.88 12.82 100.64

75D-9 47.90 3.44 13.88 15.35 0.22 2.85 9.43 4.56 0.20 1.39 0.00 0.64 99.86

75D-10 49.52 1.62 12.40 15.08 0.25 8.10 9.36 2.50 0.14 0.09 0.01 1.25 100.32

75D-11 48.12 2.32 12.63 15.46 0.21 7.00 10.27 3.00 0.07 0.09 0.01 0.09 99.27

75D-12 58.72 1.41 17.36 5.47 0.09 1.62 5.45 8.24 0.23 0.27 0.00 0.55 99.41

75D-13 66.58 0.56 15.44 4.22 0.07 1.37 2.26 7.85 0.16 0.11 0.00 1.02 99.64

75D-14 45.55 3.70 12.05 16.56 0.21 6.00 11.51 2.65 0.06 0.05 0.01 1.14 99.48



0.1 1 10 1000.010.1

0

10

100

1000

Zr,

ppm

Nb, ppm

MORBPeridotite 1275B

Peridotite 1275DGabbro 1275B

Gabbro 1275DTrondhjemite 1275B

Trondhjemite 1275D

MAR peridotite

Fig. 8. Distribution patterns of incompatible elements in the rocks. Compositions of mantle residuals of MAR areas north and southof the 15°20' Fracture Zone, Atlantis massif (MAR, 30°N), intersection of the MAR with the Oceanographer Fracture Zone (ourdata), and 43°N MAR (Shibata and Thompson, 1986). The composition of the depleted mantle (DM, asterisk) is after Hofmann(1988), and the compositions of MORB are after Kostitsyn (2004).

and plagioclase peridotites [(La/Sm)cn = 1.34]–trondhjemites [(La/Sm)cn = 1.48–2.85]. Similar to thegabbroids, the diabase shows geochemical signaturesof N-MORB derivatives with (La/Sm)cn = 0.51. Theabove-described REE distribution patterns in the rocksof the plutonic complex of the MAR at 15°44'N suggestthat variations in the trace-element composition of thegabbros (degree of enrichment) could be related to boththe heterogeneity of their parental melts and interactionwith trondhjemite injections. In addition, these dataallow us to make a tentative conclusion that the diabasedike complex was formed after the emplacement oftrondhjemites. The diabase investigated here has asharp magmatic contact with trondhjemites, and its pet-rographic analogs from sites 1275B and 1275Ddescribed by Shipboard Scientific Party (2003) showsimilar relations with all lithological rock varietiesfrom the drilled section. It should be noted that therecrystallized gabbroids and troctolites from the zoneof their magmatic contact with trondhjemites areenriched in trace elements, which was not observed inour diabase sample.

As was noted above, the mineralogical andgeochemical characteristics of felsic veins from theplutonic complex of the MAR at 15°44'N indicate thatthey are typical oceanic granitoids assigned to thetonalite–trondhjemite–granodiorite suite of slow-spreading ridges, which was distinguished by Silantyevet al. (2005). According to the existing geodynamicclassifications based on variations in trace element con-

tents in granitoids, the trondhjemites of sites 1275B and1275D are confined to the field of within-plate granites(WPG) (Pearce et al., 1984) (Fig. 10) or derivatives ofocean island magmas (Eby, 1992). The aforementionedspecific geochemical signatures of felsic rocks from theplutonic complex drilled at sites 1275B and 1275Dwere also observed in granitoid veins and dikes cuttingthe peridotites and gabbros of the crest zone of theMAR at 30°N (Expedition 304/305 Scientists, 2006),14°49'N (Bazylev et al., 2001), and 5°30'N (Savel’evaet al., 2003).

Despite the fact that almost all of the samplesexhibit signs of low- and medium-temperature meta-morphism, the rocks retain magmatic correlationsbetween the most inert elements (Zr, Nb, and Y) and themost mobile elements during metamorphism (Na2Oand CaO). It can be supposed that the contents of majorelements in the rocks of the plutonic complex of theMAR at 15°44'N were not significantly affected bymetamorphism and accompanying interaction with sea-water.

Variations in the Sr and Nd isotopic compositions ofgabbro, diabases, and peridotites from sites 1275B and1275D correspond to those of igneous products fromthe rift valley of the northern MAR (Table 7). The gra-nophyres associated with these rocks show elevated87Sr/86Sr values, and their 143Nd/144Nd ratios are signif-icantly lower than those of the diabases, gabbroids, andperidotites. The data of Table 7 suggest that the

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SILANTYEV et al.

Sm Eu Gd Td Dy Ho Er Tm Yb LuLa0

10

100

1000

(Pm)NdPrCe

Rock/Chondrite

Fig. 9. Chondrite-normalized (Evensen et al., 1978) REE distribution patterns in the rocks of the plutonic complex of the MAR at15°44'N. The hatched field is the composition of N-MORB (Hofmann, 1988). Symbols are the same as in Fig. 8.

trondhjemites of sites 1275B and 1275D were derivedowing to the magmatic evolution of an isotopicallyenriched source producing intraplate-type melts. The Srisotopic compositions of ultrabasic rocks (troctolitesample 75B-1 and plagioclase peridotite sample75D-8) reflect mainly the degree of serpentinization ofthe ultrabasic protolith and, according to calculations,may indicate a relatively low water/rock ratio (W/R =0.1–10, Fig. 11) during the hydration of rocks in theplutonic complex. The obtained W/R estimates are con-sistent with the suggested negligible chemical transfor-mations during the metamorphism of the protolith ofthe plutonic complex at 15°44'N MAR.

A comparison of variations in the Nd isotopic ratiosand trace element compositions of the rocks shows anagreement between the enriched composition of thesource (low Nd isotopic ratios) and the enriched char-acter of related rocks (high concentration of incompat-ible elements and low Sm/Nd ratios). As can be seen inFig. 12, there are systematic differences in trace-ele-ment characteristics between the trondhjemites and thehost gabbroids, which provides compelling evidencefor the derivation of these two rock groups from differ-ent magma sources.

A comparison of the obtained Sr and Nd isotopicdata for the rocks of the plutonic complex from15°44'N MAR with the available data on the isotopiccharacteristics of their petrographic analogs composingthe slopes of the rift valley and corner highs south of theregion considered (12°–15°30'N MAR segment)(Fig. 13) clearly indicates a contribution from an

enriched component represented by the veintrondhjemites to the magmatic systems of the MARcrest zone.

DISCUSSION AND CONCLUSIONS

The structure of the oceanic crustal section drilled atsites 1275B and 1275D and the relationships betweenits plutonic rocks allow us to infer the uniformity ofpetrogenetic processes responsible for the formation ofoceanic core complexes at slow-spreading ridges. Thisis suggested by the obvious similarity of sections andpetrographic rock types of the plutonic complex con-sidered here and many other complexes of the crestzone of the MAR (13°–15°30'N and 30°N) and thecomplex penetrated at Site 375B in the SouthwestIndian Ridge (Dick et al., 1991; Silantyev, 1998; Expe-dition 304/305 Scientists, 2006).

The obtained data on the petrology and geochemis-try of the rocks recovered by drilling at the MAR crestat 15°44'N can be used to attempt a reconstruction ofthe magmatic and metamorphic history of a large plu-tonic complex of the MAR situated 18 km west of therift valley axis.

A characteristic feature of practically all petro-graphic types of gabbro from sites 1275B and 1275D istheir high iron content. On the other hand, these rocksare usually enriched in ilmenite and have high titaniumcontents at minor variations in iron. Such peculiaritiesin the compositions of gabbroids from the plutoniccomplex of the 15°44'N MAR segment are indicative of



10 100 10001

10

100

1000

WPG

syn-COLG + VAG

ORG

Y, ppm

Nb,

ppm

1 2

Fig. 10. Variations in Y and Nb contents in the granitoids ofvein injections from the plutonic complex of the MAR at15°44'N. Compositional fields (Pearce et al., 1984): syn-COLG + VAG, syncollisional granites and island-arc gran-ites; ORG, mid-ocean ridge granites; and WPG, within-plate granites. (1) Trondhjemites from Site 1275D and (2)trondhjemites from Site 1275B.

their crystallization from a strongly fractionated tholei-itic melt. It should be noted that the distinct geochemi-cal indicators of the high degree of differentiation of theparental melt of the gabbroids are not consistent withthe trace element distribution patterns of these rocks. Itcan be supposed that this geochemical peculiarity isrelated to the fact that our collection of gabbroids isdominated by rocks strongly enriched in Fe–Ti oxidephases, whose abundance may results in the appearanceof sideronitic textures.

Given the considerable thickness of the section ofplutonic rocks with the above-described geochemicalsignatures, we can assume that the magmatic systemresponsible for the formation of the parental melt of thegabbroids evolved in a large magma chamber function-ing for a considerable time under steady-state condi-tions. The cotectic crystallization of melts producingN-MORB (spreading association) at MAR segmentsbetween 12° and 20°N occurs at depths of 36–42 kmcorresponding to pressures of 12–14 kbar (Dmitrievet al., 1999). On the other hand, the analysis of dataobtained during Leg 209 of the drilling vessel JOIDESResolution led to the conclusion (Shipboard ScientificParty, 2003) that the proportion of gabbroids and peri-dotites observed in the sections penetrated by drilling(20–40% gabbroids and 60–80% serpentinized peridot-ites) is also characteristic of MAR segments between14°39' and 15°44'N to depth levels of the oceanic litho-

0.703 0.704 0.705 0.706 0.707 0.708 0.709 0.7100.7020.5128

0.5129

0.5130

0.5131

0.5132

0.5133

MORBGabbro;TrondhjemitePeridotite

143Nd/144Nd

87Sr/86Sr

0.1

75Ç-11

75D-13

1 10

1001000

10000

SW

Fig. 11. Variations in strontium and neodymium isotopic ratios in the samples compared with MORB data (Kostitsyn, 2004). Alsoshown is the model line of compositions formed by the interaction of ultrabasic rocks (Nd = 1 ppm, 143Nd/144Nd = 0.51316, Sr =16 ppm, and 87Sr/86Sr = 0.7027) with seawater. Numbers along the curve denote the water/rock ratio. The concentrations of neody-mium and strontium and their isotopic compositions in the water of the Atlantic Ocean are after Piepgras and Wasserburg (1980)and Andersson and Wasserburg (1992).

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SILANTYEV et al.

0.25 0.3 0.35 0.4 0.450.20.5131

0.51312

0.51314

0.51316

0.51318

0.5132

10 10010.5131

0.51312

0.51314

0.51316

0.51318

0.5132

143 N

d/14

4 Nd

Sm/Nd

Nd, ppm

75D-2A

75D-214

3 Nd/

144 N

d 75D-2A

75D-2

Peridotite 1275BPeridotite 1275DGabbro 1275BGabbro 1275DTrondhjemite 1275BTrondhjemite 1275D

Fig. 12. Correlations of neodymium isotopic ratios with the characteristics of trace-element enrichment of the rocks. The solid lineconnects the compositions of trondhjemite (75D-2) and gabbro (75D-2A). The arrows indicate the direction from depleted toenriched compositions.

sphere below the Moho boundary. Numerous findingsof mantle residues containing gabbroid veins and dikesin the near-axial zone of the MAR between 14° and16°N (e.g., Cannat et al., 1992) support this suggestion.The possible size of magma chambers beneath slow-spreading ridges can be estimated from the data of seis-mic profiling, which was carried out in the 37°17'Nsegment of the rift valley. This investigation providedthe first evidence for the existence beneath the MAR ofa magma chamber with an area of 21–28 km2 at a depthof 3 km below the seafloor (Singh et al., 2006). Theseauthors also reported geophysical evidence for theexistence of faults bounding the rift valley and probablyextending to the magma chamber beneath the axialzone of the ridge. Thus, there are reasons to suggest thatthere are active magma chambers in the shallow mantlebeneath the MAR at 15°44'N responsible for the forma-tion of the gabbroids of oceanic core complexes.

Gabbroid intrusions related to such chambers wereemplaced in shallow mantle materials composed ofvariably depleted peridotites and caused their high-temperature recrystallization in the contact zones,

which resulted in the formation of troctolites and pla-gioclase peridotites. The above estimates suggest thatthis recrystallization occurred within the temperaturerange 950–1100°ë. In turn, the signs of gabbro recrys-tallization could be related to the injections oftrondhjemite veins, which were observed over theentire depth range of the section and had, therefore,played a considerable role in the development of theplutonic complex of 15°44'N MAR. As was notedabove, the observed distribution patterns of incompati-ble elements in the rocks suggest a possible intrinsicchemical heterogeneity of gabbroids at 15°44' N MAR.However, despite the obviously minor fraction (2%) oftrondhjemite veins and dikes in the drilled section ofthe plutonic complex (Shipboard Scientific Party,2003), it cannot be ruled out that these rocks affectedthe chemical compositions of the host gabbroids andperidotites.

The distribution of major and incompatible ele-ments and mineralogical characteristics of the gabbrosand granophyres of sites 1275B and 1275D suggest thatthe emplacement of felsic dikes and veins caused the



12 14 16 18100.702

0.7024

0.7028

0.7032

0.7036

0.704

12 14 16 18100.5129

0.513

0.5131

0.5132

0.5133

87Sr

/86Sr

of

rock

143 N

d/14

4 Nd

of r

ock

Northern latitude

(a)

(b)

Northern latitude

Fig. 13. Comparison of variations in the isotopic composi-tions of (a) strontium and (b) neodymium in the plutoniccomplex of the MAR at 15°44'N with the isotopic charac-teristics of MORB from the MAR rift valley between 10°and 17°N. Filled symbols are samples from Site 1275B, andunfilled symbols are samples from Site 1275D. Squares aregabbros, circles are trondhjemites, triangle is diabase, anddiamonds are peridotites. Crosses are data for basalts afterDoso et al. (1991). Unfilled triangles are the Sr and Nd iso-topic characteristics of the petrographic analogs of the gab-broids studied here from the axial zone of the MARbetween 15° and 15°30'N (Silantyev, 2003).

recrystallization of the host gabbroids and peridotites.The formation of such gabbroids was restricted to thelevels of the drilled sections where trondhjemite bodieswere observed.

As was noted above, phlogopite occurs in therecrystallized gabbroids at the contact withtrondhjemite injections. The formation of this high-temperature hydrous phase in the gabbros was evi-dently not related to the subsequent metamorphic trans-formation of the protolith. High-temperature phlogo-pite and hornblende are rather common in oceanic gab-broids and more silicic plutonic rocks (Cannat et al.,1992; Silantyev, 1998) and occur in residual abyssalperidotite, usually in the same plutonic complexes(Bazylev et al., 2001). Phlogopite was formed at 900–920°ë in the MAR gabbroids (Silantyev, 1998) and870–950°ë in the spinel harzburgites (Bazylev et al.,2001). In both cases, it was suggested that the main fac-tor of the formation of high-temperature phlogopite andhornblende in the gabbroids and mantle residues of theMAR was the interaction of these rocks with OIB-typeintraplate melts. Mayeda et al. (2002) supposed thatmicroscopic veinlets composed of Cpx + Opx + high-temperature Hbl + Pl from the olivine gabbro of ODPSite 735B at the Southwest Indian Ridge were gener-ated by the high-temperature interaction of magmaticminerals with a fluid phase at low W/R values. Theseauthors recognized the universally accepted concept onthe relation between the character of seawater percola-tion into the oceanic basement of slow-spreading ridgesand its plastic deformations and argued that the forma-tion of the aforementioned microscopic veins did notinvolve fluids of marine origin and predated the stage ofplastic deformations in the plutonic complex and, prob-ably, its complete solidification. Mayeda et al. (2002)supposed that fluid released from the parental mag-matic melt of the gabbro during late stages of its evolu-tion was most likely the agent responsible for the for-mation of the high-temperature amphibole-bearingvein association of the plutonic complex of slow-spreading MOR. The isotopic data presented hereshowed that the trondhjemite veins are systematicallydifferent from the gabbros in Nd isotopic characteris-tics. Thus, high-temperature hydrous phases wereformed in the plutonic rocks of oceanic core complexesowing to the interaction of rocks with enriched meltsunrelated to the parental magmas of the gabbroids.

The diabase sample from Site 1275B studied by usis the only petrographic type of rocks from our collec-tion showing no mineralogical and geochemical indica-tions of magmatic interaction with trondhjemite veins.Therefore, it can be supposed that the diabase dikeswere emplaced during the latest stage of the magmaticevolution of the plutonic complex of 15°44'N MAR.The obtained isotopic and geochemical data indicate agenetic link between the diabases drilled at Site 1275Band the associated gabbroids.

The reconstruction of the main stages of the meta-morphic history of the plutonic complex of 15°44'NMAR is the only efficient tool for deciphering the tec-tonic evolution of an oceanic core complex along itsexhumation path from the shallow mantle to the seaf-loor surface. Evidence for high-temperature metamor-phism (500–600°ë) was found in all of the petro-graphic types of rocks, which indicates that the sectionsshown in Fig. 2 were already formed by the time of thismetamorphic stage. The composition of high-tempera-ture metamorphic amphibole (high chlorine content)suggests that a fluid of marine origin participated in the

372


SILANTYEV et al.

Tab

le 7

. T

race

ele

men

t con

tent

s (p

pm)

and

Sr a

nd N

d is

otop

ic c

hara

cter

istic

s of

the

rock

s

Ele

men

t75

B-1

75B

-275

B-6

75B

-775

B-1

075

B-1

175

B-1

375

B-1

575

B-1

875

D-1

75D

-275

D-2

A75

D-8

75D

-11

75D

-12

75D

-13

Li

6.29

1.29

0.78

0.72

0.99

7.45

0.68

0.79

0.95

11.4

22.

620.

206.

932.

487.

09B

e0.

110.

310.

080.

200.

281.

320.

270.

230.

321.

972.

650.

920.

130.

302.

412.

55Sc

10.8

65.9

62.3

64.0

44.4

56.8

52.2

54.9

61.6

17.4

7.1

46.8

8.0

37.4

20.0

7.5

V45

1318

828

1128

1119

439

470

798

1073

3071

319

5760

651

79C

r27

1122

4.0

3.3

2.4

4.4

1.9

10.3

15.0

16.2

3.8

3343

0038

5.0

4.7

Co

9382

6169

5962

5953

7215

1135

126

5215

18N

i16

6323

1416

1435

1514

1747

8.0

3021

9948

7.9

8.2

Cu

3058

4447

4910

3538

521.

384.

329

1.31

4294

688

Ga

4.24

25.6

14.6

19.2

16.2

15.3

17.4

17.2

20.8

29.1

27.2

19.6

3.41

16.8

27.2

24.9

Rb

1.83

1.77

0.96

1.22

1.51

0.84

2.67

3.42

2.48

0.47

0.67

0.44

3.57

3.55

0.62

2.51

Sr10

821

089

.914

813

013

418

716

315

328

215

018

515

.211

516

911

2Y

4.89

28.4

21.6

22.7

15.6

102.

222

.716

.529

.337

.835

.139

.93.

6522

.274

.947

.5Z

r21

.157

.335

.939

.843

.089

.269

.824

.859

.156

.472

.170

.74.

8436

.347

928

2N

b0.

523.

113.

882.

344.

1023

.17.

341.

642.

3622

.820

.94.

890.

513.

7640

.720

.4B

a20

.950

.716

.933

.031

.745

.939

.342

.48.

3625

.274

.3L

a1.

143.

261.

661.

672.

1614

.33

3.33

1.36

54.

3911

.73

21.6

94.

890.

781

2.23

31.7

515

.09

Ce

2.78

8.87

3.85

4.98

5.71

36.7

98.

733.

9611

.24

27.5

144

.71

14.5

31.

831

6.45

73.1

834

.95

Pr0.

401

1.49

0.91

0.91

0.86

5.40

1.38

0.71

91.

783.

634.

932.

250.

277

1.12

9.91

4.81

Nd

1.94

8.04

5.32

5.31

4.26

26.1

97.

404.

289.

2316

.73

18.9

111

.30

1.29

66.

3341

.26

21.9

3Sm

0.54

82.

822.

062.

121.

778.

412.

461.

743

3.07

5.38

4.93

4.40

0.37

62.

2112

.10

5.93

Eu

0.20

81.

520.

851.

130.

911.

891.

311.

061.

263.

952.

421.

700.

163

1.01

4.28

1.34

Gd

0.75

14.

243.

163.

282.

2712

.29

3.62

2.45

4.51

6.08

5.38

5.50

0.45

93.

1313

.79

7.22

Dy

0.81

74.

963.

803.

993.

2614

.94

3.98

3.32

4.89

7.38

6.31

7.62

0.55

63.

8615

.50

7.65

Ho

0.17

91.

090.

831

0.87

10.

653

3.39

0.86

40.

707

1.04

51.

499

1.35

61.

580.

136

0.85

03.

191.

686

Er

0.54

13.

242.

472.

601.

8410

.64

2.63

2.03

3.14

4.16

3.79

4.47

0.40

32.

518.

855.

04Y

b0.

541

2.97

2.28

2.37

1.57

10.7

92.

481.

862.

793.

573.

313.

960.

451

2.42

7.79

4.74

Lu

0.08

40.

460.

350.

370.

251.

610.

390.

290.

430.

570.

520.

590.

074

0.38

1.20

0.71

Hf

0.52

71.

701.

001.

201.

222.

821.

910.

851.

631.

712.

161.

860.

155

1.10

11.2

77.

74T

a4.

814.

992.

142.

770.

603

1.70

0.56

0.21

00.

181

1.77

3.74

0.77

00.

087

0.27

22.

881.

99Pb

0.61

0.99

0.28

0.74

0.61

0.62

0.81

0.49

0.98

1.44

1.40

0.62

0.47

0.76

1.70

1.29

Th

0.45

0.35

0.16

0.24

0.11

1.65

0.40

0.05

0.67

1.47

2.21

0.18

0.09

0.14

2.37

6.16

U0.

389

0.05

10.

017

0.02

80.

038

1.22

0.06

40.

028

0.09

20.

321

0.26

10.

061

0.71

80.

035

0.86

00.

993

87Sr

/86Sr

0.70

8214

0.70

2619

0.70

2653

0.70

2690

0.70

2786

0.70

3477

0.70

2751

0.70

2712

0.70

2660

0.70

2931

0.70

2885

0.70

2733

0.70

7886

0.70

2621

0.70

3127

0.70

3662

±2σ

0.00

0033

0.00

0023

0.00

0026

0.00

0036

0.00

0023

0.00

0013

0.00

0020

0.00

0020

0.00

0025

0.00

0015

0.00

0023

0.00

0015

0.00

0029

0.00

0025

0.00

0022

0.00

0014

143 N

d/14

4 Nd

0.51

3173

0.51

3172

0.51

3157

0.51

3164

0.51

3165

0.51

3157

0.51

3169

0.51

3156

0.51

3185

0.51

3178

0.51

3137

0.51

3160

0.51

3150

0.51

3193

0.51

3119

0.51

3130



high-temperature metamorphism of the protolith. Therough estimation of the pressure conditions of high-temperature metamorphism presented above showedthat the whole complex probably occurred at a crustaldepth level of about 12 km (4 kbar) during this stage ofmetamorphic evolution. This estimate of metamorphicpressure is consistent with the results of isotopic inves-tigations indicating W/R values of 0.1–10, which ischaracteristic of the deepest levels of the oceanic crust.It is noteworthy that high-temperature metamorphicamphiboles with high chlorine contents were observedin the gabbroids recovered from those levels of the sec-tion where trondhjemite dikes and veins were detected.Perhaps, temperature perturbations in the zones oftrondhjemite emplacement resulted in the formation ofaluminous amphibole and also affected the salinity ofmetamorphic fluid (Fig. 14). Cummingtonite-bearinggabbros are confined to the same levels of the sections,which may be indicative of cummingtonite formationin the rocks owing to contact metamorphism. The sub-sequent metamorphic history of the rocks of the 15°44'NMAR plutonic complex recorded in the compositionsof coexisting amphibole and plagioclase included astage of greenschist metamorphism at pressures of≤1 kbar (≤3 km). The same conditions corresponded tothe serpentinization of ultrabasic rocks and develop-ment of prehnite in the troctolites. As was noted above,the W/R value was 0.1–10 both in the peridotites andgabbroids exposed now on the seafloor. This can beconsidered as evidence for the rapid exhumation ofrocks of the plutonic complex at 15°44'N MAR, and theamplitude of ascent can be roughly estimated as 12 km.

Summarizing the above consideration, the follow-ing sequence of magmatic and metamorphic events canbe deduced for the formation of the typical oceanic corecomplex of the slow-spreading ridge in the crest zoneof the 15°44'N MAR segment.

(1) Formation of a strongly fractionated (enriched iniron and titanium) tholeiitic magmatic melt, whichserved as a parental liquid for the gabbroids, in a large

magma chamber located in the shallow mantle andfunctioning for a considerable time under steady-stateconditions.

(2) Migration of the parental magmatic melt of thegabbroids into the base of the oceanic crust and itsinteraction with the enclosing mantle peridotites, whichresulted in the formation of troctolites and plagioclaseperidotites. Consolidation of the gabbroid massif.

(3) Formation of trondhjemite veins and dikes in theplutonic complex by derivatives of an enriched melt.Recrystallization of gabbros in the contact zones.Development of enriched isotopic and geochemicalsignatures in the peridotite–gabbroid complex owing tothe influence of the trondhjemite injections.

(4) Emplacement of dolerite dikes (transformedsubsequently into diabases).

(5) Metamorphism of the upper epidote-amphibolitefacies with the participation of fluid of marine origin.

(6) Rapid exhumation of the plutonic complex to theseafloor surface accompanied by greenschist-faciesmetamorphism.

The following conclusions can be drawn from theresults of our study.

(1) Extremely differentiated (Fe–Ti) gabbroidscompose a significant portion of the oceanic crustalsection in some MAR regions.

(2) Signs of magmatic interaction betweentrondhjemite injections and host gabbroids wererecorded in the plutonic complex of the 15°44'N MARregion.

(3) The distribution of Sr and Nd isotopic character-istics and strongly incompatible elements in the rockssuggests contributions of two main magma sources tothe magmatic evolution of the crest zone of the MAR at15°44'N: a depleted reservoir responsible for the for-mation of the gabbroids and diabases and an enrichedreservoir producing trondhjemite (granophyre) melts.

120

0.2 0.4 0.6 0.80

160

80

40

0

200

0.5 1 2 2.50

96

92

88

84

1001.5

+ Cum

PhlPo

sitio

n in

cor

ebe

low

sea

floor

, m

Cl, wt %

+ Cum

Phl

+ Cum

+ Cum

Cl, wt %

+ Cum

+ Cum

(a) (b)

Fig. 14. Variations in chlorine contents in amphibole from metagabbroids from various depth levels in the sections of (a) Site 1275Dand (b) Site 1275B. Squares are gabbros, circles are trondhjemites, and triangle is diabase.

374


SILANTYEV et al.

(4) It was established that igneous rocks inherit thelevel of trace-element enrichment (depletion) fromtheir magma source; in particular, the geochemicallyenriched trondhjemites bear isotopic signatures of a rel-atively more enriched material compared with that ofthe gabbroids, whose trace-element composition ismore similar to typical MORB.

We believe that some of these conclusions may bevalid for many other oceanic core complexes. It is obvi-ous that these essential tectonic units of the lithosphereof slow-spreading MOR (for instance, inside cornerhighs) provide important insight into the petrologicaland geochemical aspects of petrogenesis in MOR seg-ments with a low magmatic budget and geochemicaleffects related to the geologic evolution of the crust–mantle system.

ACKNOWLEDGMENTS

The authors are grateful to D.K. Blackman (ScrippsInstitution of Oceanography, USA) and A.A. Ariskin(Vernadsky Institute of Geochemistry and AnalyticalChemistry, Russian Academy of Sciences) for the fruit-ful discussion of our results. This study was financiallysupported by the Russian Foundation for BasicResearch, project nos. 06-05-64003 and 05-05-64699.

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Documents

Magmatic and metamorphic evolution of the oceanic crust in the western flank of the MAR crest zone at 15°44′N: Investigation of cores from sites 1275B and 1275D, JOIDES resolution