GEOTECTONIC SIGNIFICANCE of GNEISSIC AMPHIBOLITES from the VEMA FRACTURE ZONE, EQUATORIAL MID-ATLANTIC RIDGE José Honnorez, Catherine Mével, Raymond Montigny

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GEOTECTONIC SIGNIFICANCE OF GNEISSIC AMPHIBOLITES FROM THE VEMA FRACTURE ZONE, EQUATORIAL MID-ATLANTIC RIDGE José Honnorez, Catherine Mével, Raymond Montigny

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José Honnorez, Catherine Mével, Raymond Montigny. GEOTECTONIC SIGNIFICANCE OF GNEISSIC AMPHIBOLITES FROM THE VEMA FRACTURE ZONE, EQUATORIAL MID- ATLANTIC RIDGE. Journal of Geophysical Research, American Geophysical Union, 1984, 89 (B13), pp.379-390. insu-01894668

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GEOTECTONIC SIGNIFICANCE OF GNEISSIC AMPHIBOLITES FROM THE VEMA FRACTURE ZONE, EQUATORIAL MID-ATLANTIC RIDGE

Jose Honnorez

Rosenstiel School of Marine and Atmospheric Science, University of Miami, Florida

Catherine M4vel

Laboratoire de P4trologie, Universit4 Pierre et Marie Curie, Paris, France

Raymond Montigny

Laboratoire de Pa14omagn4tisme, Institut de Physique du Globe, Universit4 Louis Pasteur Strasbourg, France

Abstract. A large collection of gneissic fled by processes restricted to fracture zones. amphibolites was recovered by two close dredge Whether transform faults provide sections in hauls from the deepest part of the north facing "normal" oceanic crust is still debated [Thompson slope of the transverse ridge forming the south and Melson, 1972; Bonatti and Honnorez, 1976; wall of the Vema Fracture Zone. Serpentinites and Francheteau et al., 1976; Fox et al., 1976; various types of gabbroic rocks ranging from un- Bonatti et al., 1979]. Finally, comparisons with deformed slightly uralitized gabbros and norites the lower portions of ophiolite complexes are con- to flaser and mylonitic gabbros were associated troversial because of the postoceanic evolution with the gneissic amphibolites (in the shallowest and the questionable origin of the ophiolites. of the dredge hauls). Petrological studies in- Nevertheless, several models have been sug- dtcate that the amphibolites were derived from gested which attempt to explain the geophysical similar gabbroic rocks that reequilibrated under properties of the deeper crust in terms of rock stress in the conditions of the amphibolite types recovered in dredge hauls or observed in facies. On the other hand, the associated meta- ophiolite complexes. It should be emphasized that gabbros display sequences of secondary minerals, probably most of these models are, at least local- indicating complex cooling and cataclastic ly, correct. According to these models the lower histories without reaching metamorphic equili- oceanic crust, also called "layer 3," is quite brium. We tentatively suggest that the gneissic variable in thickness and nature. Three main rock amphibolites and associated metagabbros formed in types and their various combinations are most of- a vertical shear zone generated in oceanic layer 3 ten mentioned as major constituents of layer 3: by tectonism associated with the Vema Fracture partly serpentinized peridotites [Hess, 1959], Zone. Hydrothermal circulation of seawater along gabbros [Ewing and Ewing, 1959; Gutenberg, 1959; the highly permeable shear zone was activated by Raitt, 1963; Fox et al., 1973] and a variety of magmatic intrusions. K/Ar dating suggests, within hornblende-bearing basic and metabastc rocks. The relatively large analytical uncertainties, that latter include "dyke swarms mostly metamorphosed the amphibolite metamorphism took place 10 m.y. to the hornblende bearing facies" [Cann, 1970] and ago, i.e., at a time when the dredging sites were "amphibolites and hornblende gabbros" [Christen- located in the vicinity of the spreading center. sen, 1970, 1972; Christensen and Salisbury, 1975]. Most of the oceanic metamorphic rocks which Introduction have been dredged so far from the seafloor are characterized by preserved igneous textures and The nature of the lower oceanic crust is still nonequilibrium mineral assemblages. It seems that largely a matter of speculation mainly because the ocean floor metamorphtsm is related to circulation International Program of Ocean Drilling (IPOD) of hydrothermal solutions along open fractures in phase of the Deep Sea Drilling Project (DSDP) fell absence of stress [Cann and Funnell, 19•67; Melson short of reaching lower crustal depths. Therefore et al., 1968; Cann, 1969, •979; Miyashiro et al., our source of knowledge of the deep crust rests 1971; Kirst, 1976; Humphris and Thompson, 1978]. upon ocean floor geophysical properties, dredge However, oceanic metamorphic frocks have been re- hauls from the major fracture zones, and compar- covered from the seafloor in which preexisting tson with ophiolite complexes. However, all three igneous textures and mineralogy have been oblit- sources supply only indirect information. On the erated. These rocks display strong foliatton and one hand, the geophysical properties are inferred sometimes multiple brecciation, mylonitization, or from either large-scale surveys and experiments or folding. They were dredged mostly, but not exclu- small-scale laboratory measurementsof dredged or sively, from transform fault zones [Honnorez and cored samples. On the other hand, metamorphic Bonattt, 1975]. rocks dredged from the major transform faults have True amphibolites in which igneous textures always been suspected to represent crustal mater- have been completely obliter@ted were collected by tal which had been generated or, at least, modi- two dredge hauls from the Vema Fracture Zone (equatorial Atlantic) during two different cruis- Copyright 1984 by the American Geophysical Union. es: P7003 of the R/V Pillsbury (University of Miami [Honnorez et al., 1977] and CH78 of the Jean Paper number 4BO776. Charcot (Centre National pour l'Exploitstion des 0148-0227/84/004B-0776505.00 Oc4ans (CNEXO)). The purpose of this paper is to

11,379 11,380 Honnorez et al.: Significance of Gnetssic Amphibolites

amphibolites and associated metagabbros from the "Banded and granular amphibolites" from fault active portion of the Vema Fracture Zone and to scarps of the Mid-Atlantic Ridge, at 6øN, had been determine through petrological evidence and K/Ar mentioned by Bonattt et al. [1970, 1971], but when dating the conditions and age of the metamorphic studied in detail, the same rocks were called event. The implications suggested by their pres- "foltated or banded" and "granular amphibolitized ence In such a tectonic environment are also dis- or amphibolttic metagabbros" [Bonatti et al.,1975] cussed. The physical and magnetic properties of and were ascribed to the greenschist-amphibolite the same rock collection will be presented in transitional facies. Jehl [1975] extensively another paper discussing the feasibility for described and analyzed a "schistose amphibolite" gneisstc amphibolites to represent a major com- from the Gibbs Fracture Zone. It is made up of ponent of layer 3. green hornblende, "intermediate" plagioclase, and magnetite. Oceanic Amphibolites and Nomenclature A "foliated metagabbronorite" from hole 3•4, DSDP leg 37 to the Mid-Atlantic Ridge was studied Matthews et al. [1965] appear to be the first in great detail by Helmstaedt and Allen [1977]. ones to have briefly described "banded gabbros" The rock is made up of granulated relict plagto- from the crest of the Carlsberg Ridge, near a •lase, orthopyroxene, and clinopyroxene, the transcurrent fault. They were descrlbed in great latter being partly replaced by tremolite and detail by Cann and Vine [1966], who call them actinolttic hornblende. The rock was interpreted "banded hornblende gabbros." They are made up of as having been deformed at temperatures ranging alternating green hornblende and An 50 plagioclase from granulite to amphibolite facies during the lenticular bands and rarer cltnozoisite lenses. cooling of the gabbros even though it lacks the It is not known whether the plagtoclase is meta- amphiboltte facies mineral paragenesis. morphic or an igneous relict. Ito [1979] and Ito and Anderson [1983] studied Bodganovand P!oshko [1967] described in detail a stratigraphically controlled collection of plu- two fragments of "banded amphibolite" dredged with tonic rocks which were collected by submersible metagabbros from the RomancheFracture Zone. Both from the Mid-Cayman Rise along two vertical pro- rocks are formed by alternating bands of brown or files spanning 700 m water depth. The rocks range greenish-brown hornblende and acidic plagio- from gabbros to amphibolttes, and all are more or clase. Hornblende is partly retrograded to actin- less deformed, but the deformation is irregularly oltte. Ploshko et al. [1970] described the ac- distributed within a hand specimen or even a thin companying "gabbro-amphibolites" where igneous section. Most of the rocks have undergon• partial textures are well preserved and magmatic relic recrystallization and display amphiboltte mineral minerals are abundant. They contain commonbrown paragenests. An average of 15% of the total rock or green hornblende along with actinolite. collection was transformed to amphibolites, but Aumento et al. [1971] reported two distinct the samples which have completely reached equt- mineral assemblages in foliated "amphibolites" librium under the conditions of the amphibolite from the Mid-Atlantic Ridge, near 45øN. The first facies are exceptional. Metamorphismwould have type is made up of green hornblende and plagto- taken place during the cooling of the gabbros, and clase (oligoclase-andesine), with magnetite, hornblende would have formed from igneous plagio- sphene, epidote, biotite, and sertcttized ortho- clases and pyroxenes between 500 • and 750•C, clase. It is attributed to "the lower grade of whereas olivine would have been altered at tem- almandine amphibolite facies." The second type eratures ranging from 700 • to 200•C. Oxygen also contains dtopside and would correspond to isotope study of minerals separated from these "the highest grade of amphiboltte facies." The rocks [Ito and Clayton, 1983] showsthat plagto- orthoclasepresence of in quartz,the 45øN biotite, amphibolites and seriticized is most seawaterclaseand oramphiboles altered seawaterhave exchanged (6 0 oxyge•• 3 /•),with unusual for oceanic rocks even though Aumento et whereas pyroxene and oltvine relicts did nat. al. do not consider these samples as erratics. Moreover, the decrease in extent of the isotopic "Gneissic gabbros" and "amphibole schists" are exchange between plagtoclase and seawater and the mentioned but not described by Thayer [1969] as decrease in amphibole abundancewith crustal depth having been dredged from the Mid-Atlantic Ridge, indicate that the water circulation rapidly de- at 30•N. Miyashiro et al. [1971] briefly decreased with crustal depth: the water/rock ratio scribed "gnetssic and banded metagabbros" from the dropped from 1.7 to 0.2 over a 300-m crustal depth crestal region of the Mid-Atlantic Ridge adjacent interval. Hence, Ito and Clayton [1983] infer to the Atlantis Fracture Zone, at 30•N. Plagto- that the seawater circulation diminishes rapidly clase and pyroxene porphyroclasttc relicts coexist with depth and hence that the seawater circulation with intersttttal brown hornblende. Mtyashiro et was essentially restricted to the upper portion of al also mentioned, without describing it, a the plutontc layer and seawater-derived solutions "schistose amphibolite" from the fracture zone. barely reached the bottom of this layer. Unusual "amphibolttes" were dredged in the One can see from this brief review that a large Atlantic from the base of the Palmer Ridge, at variety of hornblende-bearing rocks recovered from 42øN [Cann and Funnell, 1967; Cann, 1971], which the seafloor have been called "amphibolites" and are made up of green hornblende and calcic plagio- that as pointed out by Honnorez and Ito [1979] and clase of unspecified origin (igneous or metamor- Cann [1979], the term amphibolite has often been phtc?). Igneous pyroxene relicts are abundant. used tndtscrtmtnantly and, sometimes, misused. The peculiarity of these amphibolites is that the Some of the rocks are gabbros containing horn- igneous textures of the parent rocks are well pre- blendes that are thought to be of igneous origin served and no foliation was observed. Cann and [e.g., Malcolm, 1979; Prichard and Mitchell, Funnell [1967] and Cann [1971] indicate that the 1979]. Such rocks are by no means amphibolites. amphibolites are derived from basalts and doler- Most of the so-called amphibolttes are partly ites. metamorphosed gabbros which have not reached full Honnorez et al.: Significance of Gneissic Amphibolites 11,381

equilibrium in the conditions of the amphibolite

facies. Such rocks often display relicts of a ß ...-.-.-::....,. :,•... gabbrotc mineralogy and texture [e.g., Cann and Funnell, 1967; Ploshko et al., 1970; Miyashiro et ....-•:.',}?i•;:..?X...... '.;.-::-.;::,;?,. '.-..':..:,,--..•,•..-r - "•"';'-.- *'"' •, "';-;•:',f i ß al., 1971; Bonatti et al., 1975; Helmstaedt and ..'- ?'c.':-:;-.:::'i'.,,.'...... ß,..;}:...,.;;;"':':;:::•:• •;!;:!;:;!";,:,'--•;. "::::::':• ..??"'•:'L.'* ;...:.,-> ..'..:':...... •::-;•;;':'•,..:•';;...... •'...o -. •-:.," •.". .... ,....:, ...... ?...... ':..•'-;•.o::-•...;•'._•.'•;•...... *"'*,,*''":"":.';.,,.,...:..:....:?•,.' .-<...... -' •...... Allen, 1977]. Oceanic rocks which completely - ß...•' ..,. ?--.-.'-:;'.ß ', :'•",-'; '"".."?':"--• :?'"'...•:,%': - .;. ' ?::':-•,'"'.,.:'...•"•i; '.---..:•;."•*:-'-"...... "i:?-;;-;i;•.?'"" -'"'" •':-:'c:-.":.•...•-'."-. ;',.:.:'-r;:½ •'•':""'.:..:-?':::•.:.?;...•'" ?•'..:??'.... recrystallized in the conditions of the amphi- ::, .....':..', *... ;.:' -' :-....* .•;:,-:.':;:"..;-:-? :.....:.;'"..'..•...::.,•-•;;*'i ':.,.•:.;. bolite facies are much less commonly reported. In - "' .,i. '.:;•'' "..-;': ',.':--:"- .,,'::';'{'.". '"':-:.:':-' .- ':':..." ,.'.•".'*....:..

re-suited from the intense deformation and ...•..- . .'*-.•-;:...;,.. '---.": ... ß -'•..•;.-,-":"':.:•;:..:.-,..-:,2• -'- - ,'-...... ,i•..'..% • '.•.:...;...... -...... •... -' ' . - ß::. ø' ' '..•:';.:.:;-..'i :-•"-,.•' "/":'-;':'..'.' . ' -- '" .. recrystallizatton of the parent igneous rocks ..'r*'-'.•..;. '.q•?"-?../.i'."•. '.;:':::.:•i !i::i:;;:-•"'•'•' '•-'-" ..:--,. . which can only be assumed [e.g., Cann and Vine, .....•.,.,•...... •...... * :'.;•.....;.:.:,i•.., ,I .?'•?.i!?'.;/•,..•..•."• 7'.,,*.....:'..,':.'.%.,-½•i•i.½ ...... 1966; Bodganov and Ploshko, 1967; Jehl, 1975]. ',-,,..: ....;:,:..,..<:...--..:!•":,-..'•, ..,...;•.. Gabbros completely reequtlibrated in the amphi- ,?""-'?;'i?.".,??"'"...?½ .... '?'%"':::'"':'?::X:?•'='"'=*'"?''..... bolite facies without losing their original ...... ::.-...-:..•,....-.....-.-:..? ...•...... ,..::,,-.,--:...•--..: ,,,,. .. ß,• -•.::'-'11;%. igneous texture are the rarest [Ito, 1979]. ....'ß ...."•. '"}' "i'•:•" .. ..::•ii•'.:"•"•:"•"•"•':'•'• ":!:"-;::..-'-:.:;•-"*'-:'"'•.,.:-.;-.': ...... ,'.,'•... •" •,-"•'•"i...... ß¾...:• .....;i:...... •...?.'.•' ...... :.-::':<. '?¾:•"--'•'•.i!•:i7;.';..?.'-:..' '*•";•-,':..:•"-•t?' .- ..:--.:. • It is therefore important to define accurately •'""'-" ...... "•'•*•' •:-:•,'•.....'•'<•C½"-"• •"•½,;:;C.•-.,. the terminology used in the context of this ?:::..:½•:'"'½...... ? --;:-:•!...<;*,s.'•..i,.,.,.;:: "'":; '"•"':". ' paper. We call "amphibolite" a metamorphic rock whose major mineral paragenesis, hornblende, and •s.::... calcic plagioclase indicate that it reequilibrated -2::/- under the conditions of the amphiboltte facies...... :;...... ;,,.. .. Such rocks may or may not exhibit foliation de- ¾.:. ..:.:. pending on the intensity of deformation and are accordingly qualified as "gneissic" or "with gabbroic texture," respectively. In the latter case, the name is followed by the textural name of the parental igneous rock. All of the other partly metamorphosed rocks displaying igneous relict minerals are called "hornblende-bearing flaser metagabbros" or "hornblende-bearing metagabbros," respectively. Table 1 presents the nomenclature used in the present paper to designate oceanic amphibolites and associated rocks.

Sample Location and Tectonic Setting Fig. 1. Hand specimen showing the gradation from The material studied in this paper consists of metagabbro to gneissic amphibolite. (left) unde- a set of gabbros, metagabbros, and gneissic amphi- formed uralitized gabbro showing a dark horn- bolites (Figure 1) collected in two dredge hauls blende "vein" (P7003-19AA). (right) (top to from the base of the north facing wall of the bottom) uralitized lineated gabbro (P7003-19DD), transverse ridge forming the south wall of the flaser gabbro (P7003-19G) and gneisstc amphi- Vema Fracture Zone at about 11øN in the Atlantic bolite (P7003-19E) in which magmatic texture is no Ocean (see Figure 2). One dredge haul by the Uni- more visible. versity of Miami's R/V Pillsbury in 1970 recovered mainly mylonitized serpentinites and cataclastic gabbros with very few amphibolites and only two site and about 2900 m below the ridge crest in- basalt fragments from depths ranging from 2050 to cluded numerous amphibolites and deformed serpen- 2850 m below the crest of the transverse ridge. tinites with very few gabbros, which includes a In 1978, a dredge haul that CNEXO's R/V Jean Char- Fe, Ti-rich gabbronortte and leucoferrodiorite cot recovered at about 15 km east of the preceding [Ohnenstetter and Ohnenstetter, 1980]. The rocks from the Vema Fracture Zone are compared with a single amphibolite specimen (sam-ple GS7309-51K) TABLE1. ProposedNomenclature of Metamorphic from the foot of the northern wall of the Romanche Hornblende-BearingMetabasites Fracture Zone. Table 2 summarizesthe geographic From the Oceanic Crust location, the bathymetric tnforma-tion, and the lithologic composition of the three dredge hauls where amphibolites were found. The Vema Fracture Zone offsets the Mid-Atlantic Complete Partial Ridge left laterally by about 320 km. It is Recrystallization Recrystallization markedby a narrow and almost straight east-west (Equilibrium (Metastable trough filled with about 1 km of sediments. The Paragenesis) Paragenesis) sediment/water interface lies 5 km below sea- ..... level. The topography of the Vema Fracture Zone has been described by Heezen et al. [1964] and Van Dynamic Gneissic Hornblende-bearing Andel [1969] and Van Andel et al. [1967, 1971]. hydrothermal amphibolites flaser gabbros The petrologic information about rocks dredged metamorphism from the fracture zone walls can be found in papers by Bonatti et al. [1971, 1974], Bonatti and Static Amphibolites Hornblende-bearingHonnorez [1976], Honnorezand Kirst [1975], Melson hydrothermal with gabbroic metagabbros and Thompson[1971], Prinz et al. [1976], and metamorphism textures Thompsonand Melson [1972]. The VemaFracture ...... Valley is bound by two steep walls with 30 - 50 • 11,382 Honnorez et al.: Significance of Gneissic Amphibolites

i , • 4•4 i 40 ø 30 ø 20 ø ,,=• VœMA VEMA FRACTURE ZONE

. -12 ø

I•NORTHERN-v-RIDGE CREST--•

I v t I v t _ .,o

Fig. 2. Structural sketch map of the Vema Fracture Zone and location of the dredge hauls. slopes, but the topography of the two walls is the Vema Fracture Zone was interpreted as a "stag- strikingly contrasted. The north wall does not nant" crustal block, i.e., a wedge of basement display any topographic anomaly and appears to be material which does not spread as fast as the a representative but probably disrupted section surrounding seafloor [Bonattt and Honnorez, through the oceanic crust [Bonatti and Honnorez, 1971]. A tentative Mesozoic age was suggested by 1976] whose surface progressively slopes away from Honnorez et al. [1975] for the shallow water lime- the Mid-Atlantic Ridge following the Sclater et stone capping the transverse ridge. al. [1971] relationship between distance from spreading axis and water depth. On the other hand, the south wall corresponds Sample Descriptions to a narrow (10-25 km wide) transverse ridge run- ning parallel to the valley and reaching up to 550 In hand specimen, the amphtbolites display ir- m beneath sealevel. Similar transverse ridges are regularly spaced discontinuous white streaks of common features of transform faults which displace plagioclase up to 5 mm thick in a dark brownish- slow-spreading mid-ocean ridges [Bonatti et al., black amphibole background. Both the amphibole 1979; Bonatti and Chermak, 1981]. Van Andel et al. [1971]have noted that this topograpicanomaly cleavagesand the layer alternation impart a is not a constructionalfeature built up by vol- strongfoliation to the rock. The grain size canic activity. It has been interpreted as re- varies considerably. Some samples actually look sulting from the uplift or "protusions"of an like fine-grainedmylonites, others like medium- upper mantle derived serpentinttebody into the grained gneisses (grains up to 1 mmin dia- fracture zone [Bonatti and Honnorez, 1971, 1976; meter). All of the specimens are coated with a Thompson and Melson, 1972; Honnorez et al., 1975; thin manganese crust. Bonatti, 1976, 1978]. Slabsof crustal material Microscopically, the mineral assemblageis riding "piggyback"on top of serpentinite pro- simpleand constant. Theobserved paragenesis is trusion were uplifted and eventually emergedCa-plagioclase + hornblende• clinopyroxene+ [Honnorezet al., 1975;J. Honnorez,unpublished, ilmenite, typical of the amphtbolitefacies. Only manuscript,1977]. the Romanchesample is devoidof ilmenite. All the minerals appear metamorphic except possibly On the basis of regular wide-beam precision apatite and very scarce zircon which are ir- depth recorder surveys it was initially assumed regularly distributed in the rocks and are con- [e.g. Bonatti et al., 1971; Bonatti and Honnorez, centrated in lenses aligned with the foliation. 1976; Bonatti, 1978] that these steep and smooth Both minerals may be interpreted as broken mag- walls corresponded to one or two faults with atic crystals or as recrystallized crystals after throws of a few kilometers. High-resolution magmatic grains. studies with multibeam side scan sonars ("sea- Two types of amphibolites can be distinguished beam") during leg 78 of the R/V Jean Charcot [H. on the basis of their texture: gneissic and mylo- D. Needham et al., unpublished data, 1979] indicate that the southern face of the south wall is nitic amphibolltes. The mineral paragenesis of the 15 studied amphibolite samples is summarized made up of numerous small fault scarps with throws in Table 3. of a few hundred meters and narrow ledges in ac- cord with the model proposed by Francheteau et al. [1976], whereas the northern face appears to be Gneissic Amphibolites formed by two or three fault scarps separated by Gneissic amphibolites are by far the most com- two terraces. mon type and are well illustrated by sample The transverse ridge forming the south wall of CH78DrlO-20. Plagioclase forming the white layers Honnorez et al.: Significance of Gneissic Amphibolites 11,383

(Figure 3) usually occurs as granular, slightly zoned crystals up to 0.5 mm in diameter. Larger crystals, up to 1 mm long, are occasionally present and are commonly characterized by an undulatory extinction. Hornblende is usually zoned from light brown cores to green rims, although an inverse zonation is observed in a few cases. Trains of small ilmenite crystals emphasize the foliation, and their proportion varies from sample to sample. Apatite and zircon may also occur but are always very rare. In some samples, thin layers of small rounded clinopyroxene crystals (50 - 200 •m in diameter) are observed lining up parallel to the foliatton. They never form elon- gated porphyroclasts as in flaser gabbros such as

Theythose aredescribed particularlyby Helmstaedt abundant inand sampleAllen CH78Dr10-[1977]. metamorphic77(Figure in4). origin. The clinopyroxenes appeartobe In samples P7003-19E and 19F, amphibole-rich layers consist of homogeneousbrown to olive brown hornblende crystals forming a decussate texture along with randomly scattered interstitial il- menitegrains (Figure 5). Rare roundedzircon crystals are observed between the hornblende grains. The plagioclase rich streaks are madeup

ofdiameter. trains ofPlagioclase equant, roundedcrystals plagioclase display strong0.2mm zo-in ningand patchy extinction in P7003-19Fbut are o o u• o homogeneousin P7003-19E. • u•

Myloniti c Amphibolites o o o o • • Four samples of very fine grained mylonitic amphibolite exhibit a dark green, almost black color in hand specimen. Rounded hornblende and o o plagioclase porphyroclasts up to 1 mmin diameter occur in a finer-grained (50 •m average diameter) matrix made up of green hornblende, plagioclase, • o o ilmenite, and, in CH78Dr10-81only, clinopyroxene o co • (Figure 6) Most of the gneissic and mylonitic amphibolite samplesretrogradeappear metamorphic to have event.been slightly Mineralsaffected typical byof a the greenschistfacies such as chlorite,actinolite, epidote, albite, prehnite, and sphene commonly occur either as lenticular zones parallel to the foliation or as parallel walled veins crossing the latter. Sphene commonlyrims the ilmenite crystals. Iron hydroxides (?) are abundant in sample CH78Dr10-75, where they form rusty patches. A zeolite tentatively identified as analcite forms veins in sample GS7309-51K. In the vicinity of the veins, hornblendemay grade into actinolite or be lighter brown in color with occasional green patches.assemblage Ais thirdobservedtype in of sample retrogressive P7003-19F, mineralwhere a poikiloblastic sodic plagioclase (An 20%) en- closes small hornblende crystals and "corroded" Ca-plagioclase grains. Clusters of prehnite microcrystals occur within or around the sodic plagioclase. It is important to notice that the retrograde event occurred after deformation, as demonstrated by the veins intersecting the foliation. On the whole, the retrograde metamorphic minerals do not account for much of the amphibolite except in sample CH78Dr10-126 where the plagioclase is largely replaced by albite, sericite, and prehnite. The mineralogical assemblage of the gneissic amphibolites are summarized in Table 3. 11,384 Honnorez et al.: Significance of Gneissic Amphibolites

TABLE 3. Mineralogical Composition of the Studied Samples of Amphibolites

Dredge Haul Sample Rock Type Typomorphic Paragenesis Retromorphic Minerals CH78DrlO DrlO-126 gnelsslc amphibolite horn•iende+plagioclase+actinolite+chlorite+epidote+ Verna without cpx ilmenite albite+white mica+sphene Fracture DrlO-14 gne•ss•c amphibolite hornblende+plagioclase+ chlorite+epidote+sphene+white Zone without cpx ilmenite mica DrlO-18 gne•ss•c amphibolite hornblende+plagioclase+ chlorite+epidote+sphene without cpx ilmenite DrlO-20 gne•ss•c amphibolite hornblende+plagioclase+ without cpx ilmenite DrlO-75 gne•sslc amphibolite hornblende+plagioclase+ iron hydroxides without cpx ilmenite DrlO-83 gne•ss•c amphibolite hornblende+plagioclase+ epidote without cpx ilmenite DrlO-86 gne•ss•c amphibolite hornblende+plagioclase+ epidote+white mica+sphene without cpx ilmenite DrlO-16 gne•ss•c amphibolite hornblende+plagioclase wl th cpx +cpx+ilmenite DrlO-23 gne•ss•c amphibolite hornblende+plagioclase actinolite+chlorite+sphene w• th cpx +cpx+i lmeni te DrlO-77 gnelss•c amphibolite hornblende+plagioclase white mica+sphene wl th cpx +cpx+i lmenite Drl 0-81 myl oni tic hornblende+plag i oc lase sphene amphi bol ite +cpx+ i lmeni te DrlO-82 mylonitic hornblende+plagioclase+ sphene+chlori te+epidote amphi bol ite i lmeni te

P7003 19E gneissic amphibolite hornblende+plagioclase actinolite+prehnite+epidote+ Verna without cpx +ilmeni te chlori te+sphene+albi te Fracture 19F gneissic amphibolite hornblende+plagioclase epidote+sphene+prehnite+ Zone without cpx +ilmenite traces chlorite+albite+actinolite+ quartz

GS7309 51K gneissic amphibolite hornblende+plagioclase epidote+chlorite Romanche without cpx Fracture Zone

Associated Gabbros and Metagabbros Amphtbole composition and distribution suggest a complex evolution. Three main types of occur- A complete transition exists between undeformed rences are observed: gabbros and gnetssic rocks, indicating that re- 1. Both clinopyroxenes and orthopyroxenes crysallizatton progressed with deformation of the contain blebs of reddish brown, strongly pleo- gabbrotc rocks up to the point where the foliated chrotc amphibole which are kaersutttlc and there- rocks do not contain any igneous relict mineral or fore thought to be of magmatic origin. It is re- texture. The numerous gabbrotc rocks found along markable that this amphibole is preserved even with the gnetssic amphibolites in dredge haul when pyroxene is completely replaced by actinolite P7003-19 display a continuous range of textures or green hornblende. from undeformed igneous fabrics of various types 2. Rarely, clinopyroxenes and orthopyroxenes to cataclastic, mylonittc, and gneisstc textures. (e.g., in sample P7003-19I) are directly rimmed by The main minerals of the least deformed gabbros discontinuous brown or olive-green hornblende. (e.g., P7009-19BB,X,V) are subhedral to euhedral, 3. More commonly, cltnopyroxene, generally calcium-rich plagtoclase (An 55 to 70), anhedral displaying orange to brown stains (iron hydroxides augitic clinopyroxene, tttanomagnetite, and il- and/or smectites), is partly or completely re- mentte as "exsolution" lamellae and/or discrete placed by multiple rims of amphibole: 1) an in- granules. Forsterttlc olivine and hypersthene are ner, finely fibrous tremolite-actinolite mosaic much less frequent except in two samples (P7009- clouded with secondary magnetite dust; this rim is 19I and Y) which are an augtte-beartng norite and almost colorless near the pyroxene core and pro- a troctoltttc metagabbro, respectively. The gresstvely becomes greener as it grades into 2) an amount of opaques varies from samples that are Fe, intermediate rim of green to olive-green fibrous Ti oxide-rich gabbros to others which contain actinolitic hornblende without secondary opaques; almost no opaques. this rim becomes outwardly more homogeneous, 3) a Plagtoclase displays kinked twin planes, un- well-crystallized (i.e., optically continuous) dulatory extinction, and incipient mortar struc- zoned brown hornblende whose terminations even- ture. The granulation of the igneous plagioclase tually extend into bluish-green chlorine-rich is accompanied by its replacement by more sodtc hornblende which also fills veins cutting the ad- phases. jacent plagtoclase crystals (Figure 7). A similar Honnorez et al.: Significance of Gneissic Amphibolites 11,385

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sequence of amphibole rims has been described by shear zones are marked by diffuse blackish horn- Bonattt et al. [1975] in metagabbros from the blende rich veins up to 2 cm thick which are par- crest of the Mid-&tlantlc Ridge, at 6øN. allel to the lineation. The "veins" are essen- The orthopyroxene is often altered into a mix- ttally made up of tdtoblasttc olive-green or ture of iron oxyhydroxides and/or smectites (id- bright to bluish-green hornblende with rare small dingsite?) starting from the crystal outlines and plagioclases and opaque granules. Close to the a crack network. The ultimate result is an id- veins the plagtoclase is more finely granulated dingsittc looking boxwork. The rare olivine crys- than in the remainder of the rock, and the clino- tals are generally almost completely replaced pyroxenes are completely replaced by various green either by an orange "tddlngsite" or by a mixture amphtboles ranging from actinolites to acttnolittc of talc(?) and secondary magnetite. hornblendes to hornblendes. Strongly pleiochrotc The primary titanomagnetite is extensively orange to yellow scales up to 0.25 mm in length of replaced by hematite, maghemite, goethite, and a micalike smecttte fill out the interstices be- lepidocrocite. It is often surrounded by sphene. tween other minerals and cracks or are scattered Large tlmentte grains are associated with numerous within the plagioclase. smaller (recrystallized?) ilmenite granules. I1- The flaser gabbros (e.g., samples P7003-19G, H, mentte is often partly replaced by sphene and, and S) display textures ranging from place to more rarely, rutile. Large tlmenite crystals ex- place, from that of the lineated metagabbros close hibit undulatory extinction. to the hornblende veins to that of mylontttc gab- Several samples of uralittzed gabbros are bros. They are characterized by a further granu- strongly ltneated and/or crossed by narrow shear zones (samples P7003-19AA, CC, DD, and KKK). The .U•/• .. ."..•...•-•.-• ..... •,. ,• ,•.-• ,•.•.....-•-,.. •. •--,,...... •,. '•,½-• *•,,•. • .•,.••--.• • .,,,,,•-.,•,,•, ..... •.-.

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' .%...:,.--•;...:.;,:.-:-.::.:;:?::...:--:•;-.?.':•.,.•...... •:--.:-..•.. --:::•...-•::•-:.'••.:...;..-?":.. .,:.: ...... ,,. :¾-•.. ß .,',--..-- ...... ,,.•;.•.'.•?'½.• • ...**•. '....?..:.,...... ?.•,.:,-'.':-:•..e .',:--•'• ...... •...,:'.• • ?•::.:½.,?. • •...:.",!%•*•-.•-:•...... •,•":',.;..:.:::•:,•:?:.:.•.•.'•:?..,.....•,•.•:,.....:.::.?'• ..;:•:..•.•.....;?::.::-.•...... ,.,•...?•' •' ..:?•;.'%-.. :...... :;...•::.'. '•?-'• •:,.,';'r .•*';•..,.,'",-.;" •-•'-;• :•: '•.,-'•:;'%?' • .•;::•, ' •.'---";• "•.',..•.;.•.r: ,:"..-.:; '-: :..•:'•-' •. ..,- ,,,,....:.-,. ½.,•:.•'....,•.•½::.;•:.:•:::,:...-.. ...-.;:..:....:..•:;•;,.•:.;•.:.•..,:...... :...... •...... ,:...?:?.•:,•.-•.;... •.•:.'<:.½•-•:• ..... •-•,..•.;;::;::::.'"•-;:'.:•:;;• ...... ,'•,;:½,:...%.•-:•'-..-•.•. ,•..-... :; .....• ,.::.. ..:½•:•:•,:.-::¾•.,-..: •...•'; ...... ';:... '•:, Fig.6. CH78Dr10-81. Mylonittc amphtboltte with ' .--.-•::•;-'-':':'•---•-* ::.: -• • .....'•'"'?'""•.•,.v-::.•-• '. ß "•---::'•':?"•:"'•:'•..•'•.:::;--:.;::•.•:: --...:':--'*';•ß ,-% ::':"•'. ?...... -•. :..,•- ß..---'--?'•.. <: :.½ ,.:"-,:::..:::-' ';;:•;::-.-•:•;.-....-::½.-•%,-½-::-:•:•:•-':•-,•,...... :...... • *-'::-:'.."• .., rounded porphyroclasts of hornblende (dark gray) •':-:.'•;•?•:;•:':"'•'•½ r•..•,.::;;...:...•%•...:`::....•,..:..•*.`•.•...:•::•:..•..:t..:...•;;•:•:;?;;•::•?;•..•?.•.`?;:.;•:..•..`.•:• -..... :--,,•::•:...?'•.;•-•:".,?*:-:;";'"•'."•::•-' ...... and plagioclase (white) in a ftne-gratned matrtx Fig. 4. CH78Dr10-77. Layer rich tn rounded conststtng of plagtoclase, hornblende, and il- crystals of clinopyroxene (light gray) in a mentte (black). Ilmentte ts particularly abun- gnetsstc amphiboltte made up of plagtoclase dant in this rock which has the bulk composition (whtte), hornblende (dark gray), and ilmenite of an iron, tttaniu•rich gabbro. (Photo- (black). (Photomicrograph with uncrossed nicols.) micrograph with uncrossed ntcols.) 11,386 Honnorez et al.: Significance of Gneissic Amphibolites

metagabbros, and they always appear to have formed the last. 2. Less frequent are olive to bluish-green •-'..+.•;;•:•:-• . •' •. ,•• . : •- .•. ',, * .• • . ' .. • ...... •-.. :•---'-::':••.'.'•-.' .... • •' • ' I ,•.•.• • • *'"• • • • hornblende veins crossing both plagioclase and ••: ...... ? •..:'•-.•.• ..., ....,•-.-::-.••: • •..• •ß _ •• •,.•,•..,•., • .ß •• ,. ½[•• •,• ..-. •.:z•-:••z•.. uralitized clinopyroxenes. -• -•c.'"•>'• •'•, , ' - • -' • • ?•'.* .•f' •' • •., .• '-•.... 3. Two of the flaser metagabbro samples (P7003-19H and S) contain up to 1 mm thick rims of ; "i'• '• •:• ' . •: •',' .... '- •'...... sutured quartz, with scattered epidote, and vermi- ;..,.....• q•' ,., . -• ,, m• ß • ß . .•.•- • •., •. •.::"• culated chlorite grading into yellow-orange smectite. ,. • .•,....•. . . ;..•:• •,. - _• •,. • . '..• -. ..•fi•? ..-i• .--,:.- .... Analytical Data

Mineral Chemistry ?:-•:-...... ß . . .. .• • :..:'?.• .:.::.•--•...-:...... ?•....%•...• :.:f•..•...... ?-¾•• •.-.-•-•:..,...... , .,•-.:.-•.....•..•..• , ..,....:.-;:•'•-.•..•-_-•.-• ...... •..... •••• •:..• .c-•..••... -.•,. • . • ß , :... Samples of gnetssic amphtboltte have been ..'A?•-•'•:•.:•-:.:.;.;';::""?":'•'-•'"":'';...... '•:•.;•;•-•.;•:..• ['i"•'•:•';: ":':• ••'. .•":"":•::¾" "-•-•' '• "'..... ß •:•-•+•-:-:•'- ::½ ;•'•"":'• .'•:-• •:••' .:..:•:•'* x •:',"-••.%7 ...... :,.•....77• • • •'.: selected for microprobe analyses: four without Fig. 7. P7003-19•. Replacement of magmatic clinopyroxene (DR10-126, Dr10-75, and P7003-19E cltnopyroxene in an undeformed uralitized gab- and F), three with clinopyroxene (Dr10-23, Dr10- bro. The center of the pyroxene pseudomorph is 77, and Dr10-16), and one mylonite (Dr10-81). replaced by actinolite, rimmed by bro• horn- They also represent the range of bulk rock com- blende then green chlorine-rich hornblende pro- positions. For comparison, minerals from the jecting into veins. The magmatic plagioclase Romanche Fracture Zone gneissic amphibolite sample (white) is preserved, (Photomicrograph with GS7309-51K have also been analyzed. urictossed nicols.) The analyses on CH78Dr10 samples have been carried out on a CAMECA MS46 microprobe, using natural minerals as standards. Elements con- lation of the plagioclase, an almost complete centrations have been calculated with the EMPADR replacement of the clinopyroxene (by various green VII program [Rucklidge and Gasparini, 1969]. The amphiboles), and a massive breakdown of the analyses on the P7003-19 samples were carried out opaques to leucoxene or sphene. The dominant on an ARL electron microprobe equipped with an amphibole is a tan to olive-green pleochroic horn- energy dispersive X ray spectrometer and using blende containing scattered small patches of kaer- natural minerals as standards. The Reed and Ware sutttic amphibole. Roundedapatite grains 0.5 mm [1975] correction procedure was used. in diameter occur sometimeswithin the hornblende Amphibol•s. The amphibole compositions are augen. Fractures in the plagioclase porphyro- quite homogeneous in a given amphibolite sample clasts are filled with chlorite and smectttes or but vary from sample to sample from magnesio- are crisscrossed by a network of narrow veinlets hornblende to ferro-hornblende according to of more sodic plagioclase. In sample P7003-19G, Leake's [1978] classification (Table 4). a.lbtte is the only plagioclase. Independently of the amphibolttes bulk com- The mylonttic metagabbros (e.g., P7003-19Q) are positions, amphiboles are quite similar with madeup of alternatingdark and thick and clear respectto their A1IV Tt and alkalis contents and thin bands. The dark bands are formed by a which have been demonstrated to depend mostly on blackish green almost opaque matrix composed of physical conditions [Shido and Mtyashiro, 1959; extremely fine (from 0.003 mm to submicroscopic) Liou et al., 1974; Raase, 1974; Spear, 1975]. In granules of green amphiboles and opaque miner- all but one of the amphibolites from the Vema als. The clear bands are mainly made up of plagioclase granules. Lenses of granulated apa- ttte also occur. Rounded Ca-rich plagioclase and tinction are observed. The amphiboleaugen are -..•.'•::.'.••'•"•'"•"• ...... •.•%;...•:.:•:.•. greenamphibole augendisplaying undulatoryex- •..-.:•.• •00•:.:.•...... '.." 7-.'¾'•-.' -'.:.... •. '..:.•f•Z•.:•%•,,-'•"?•"?:•:•":.•.•/"...... '...... "' rarelyzoned with a fibrousactinolitic rim sur- • ...... •'. ,, ':•'"'.. •- -?'?'•-..•½.:.•:.•.•.•.-..,...... •::•.?::..• ,.::•..,-.•.•::•.•.:..:.•::?•; ...... :.• rounding a monocrystallinehornblende core. The .... latter exceptionallycontains a small clino- pyroxenerelict with the characteristicyellow . .• •.•.•.?,.•..•....--••Z.>..• :--•: ..... • ...... -%:: stain (Figure8) ß Scaley,pleochroic (gold yellow - • • • ...... •:.--. •. -.... • - ..•.• .... - .•,• ..• ....•::: .... ;:.-•:: '%•.::...: ...... ".:::•:.'•.. :•. to orange)smectite occursthrough the rock either •:.-•?•:..-...... •-•:•..•r::•::f.•:.' ...... •:...... •:•::•..•:::•::.:•:...... associated with the green amphibole or, more rarely, forming lenses by itself. The undulatory extinction of the various amphiboles, the presence tn some augen of an acttnoltte rim surrounding hornblende cores, or the fact that scaley smectlte forms lenses indicate that the last shearing stage followed a retrograde metamorphism of (partly?) amphibolit•zed metagabbros. Several types of secondary mineral veinlets are observed tn various gabbroic rocks associated with Fig, 8. P7003-19H. Flaser gabbro, The rock the gneissic amphibolites: sheared, and a follatton is visible. However, l. •e most frequent veinlets are made up of magmatic clinopyroxene is still present as a pleochroic yellow-orange smectttes. These veins' porphyroclast. (Photomicrograph with uncrossed exist in all of the various types of gabbros and nicols,) Honnorez et al.: Significance of Gnetssic Amphtbolites 11,387

2.0 '

PTH(Km)

I0 km

-2: 2 • P7005.19 J.5 - XG5

, ß

0.2 04 06 0.8 I 6 Na+K Fig. 9. A1TM versus Na+K diagram for amphiboles.Dots, all amphtbolesin gneissic amphibolites; crosses, amphiboles in associated metagabbros (AA, P7003-19AA, uralttized gabbro; G, P7003-19G, flaser gabbro; H, P7003-19H, flaser gabbro).

FractureZone, the amphiboleTi contentvaries 0.30%) In zonedcrystals, A1 TM Ti andalkalis between 0.1 and 0.2 cattons per 23 oxygens, and is decrease from the light brown center to the green not correlated with the titanium content of the periphery (Figure 9). This can be interpreted as rocks. This is well explained by the presence of a progressive lowering of the temperature as the another titanium-bearing phase, ilmenite, in the crystal grows. The chlorine content is below rocks. Titanium-rich rocks contain abundant tl- detection. menite, while the titanium content of the amphi- The Mg/Fe ratio of amphiboles in the gneisstc bole essentially depends on physical conditions amphibolttes varies from sample to sample. Figure and is consistent with upper amphibolite facies 10a demonstrates that it is related to bulk rock [Raase, 1974]. This is not true in sample P7003- compositions. All the representative points 19E, which is extremely rich in titanium (8.8% scatter about a single line on the Fe-Mg diagrams TiO^) In this case, the TtO /FeO ratio of the except sample Dr10-75 This rock is characterized roc•;s toohigh for ilmentte to2 use up all ofthe bya highoxidation (Fe203/FeO' = 1.2)which may be Ti. Hence the amphiboles are more titantferous in related to retrograde alteration. Actually, when this sample than in any others. The titanium con- its oxidation ratio. is fixed at 0.4 (value for tent of amphiboles from the Romanche sample sample Dr10-81, rock having about the same iron (GS7309-51K) is much lower (0.07%) and in the content), the MgO/MgO + FeO of the rock de- absence of ilmenite in the rock (Table 3) may be creases: the representative point is shifted dependenton the bulk rock composition(TiO 2 = towardthe left and plots on the sameline as the others. There is no significant compositional difference between the various porphyroclasts and the Mg•(Mg•FeO) amp I small grains of the matrix of the Dr10-81 mylonite. This observation suggests that the metamorphic conditions did not vary during the mylonttization.

M• '(Mg,Fe) amp 81..319F 16

Fig. 10a. MgO/(MgO + FeO) of amphiboles versus bulk host, in amphibolite rocks.51K, GS7309-51K; 19E and 19F, P7003-19E and 19F, respectively; the others are CH78Dr10-16, 23, 75, 77, 81 and 126, Mg/(Mg+Fe) cpx respectively. The dashed line joins the representative plot of host rock with its present oxidation ratio (right) and with an oxidation Fig. 10b. Mg/(Mg + Fe) of amphtbole versus asso- ratio of 0.4 (left). See text for explanation. ciated pyroxene in amphibolites. 11,388 Honnorez et al.: Significance of Gneisstc Amphtbolttes

0 o•D ,..--• 0 I O0•Oh Oc•C'q •D o'•o'• c,q •D I 0,.--•1• • 0.,•'

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O• 00 -.• r'- O• 0 u'• 0 O0 .-• 0

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'"'• 00 C'•l 0 C'•l u'• ,..--• 000,1

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O• 0,-• -• O• 00 • O0 • 0-• ß ß ß ß ß ß ß ß ß ß ß ß

0 0 "•' 00 I• oO O• oO .,•' O• ,..•

ß ß ß ß ß ß ß ß ß ß ß ß •o .-• o o .-• o • u'• ,-• o o o,!

ß ß ß ß ß ß ß ß ß ß

c• 0 • O0 •000 • • Honnorez et al.: Significance of Gneissic Amphibolites 11,389

For comparison,microprobe analyses of selected plagioclaseand the Na20content of the bulk rock amphiboles of the associated metagabbros are pre- is not simple. Calcic plagtoclase is stable at sented in Table 5. P7003-19AA is a ltneated ural- the temperature under which metamorphtsm oc- ttized gabbro (host) containing a green hornblende curred. The An content of secondary plagtoclase vein. The amphtboles in the host metagabbro is a may thus be dependent on the composition of the ferro (to ferroan) pargastttc hornblende, whereas primary plagtoclase it recrystalltzed from. it varies from magnesio- to ferro-hornblende and The poiktloblastic plagioclase porphyroclasts ferro-edentttc hornblende in the veins. In the of sample P7009-19F contain less than 20% An. flaser gabbro sample P7003-19H, two amphiboles Albtte (An 6) has been analyzed in retromorphic were identified as magnesio-hornblende and mag- veins from Dr10-126 in equilibrium with epidote. nesian hastingsttic hornblende. In the mylonittc No intermediate composition between calctc metagabbro P7003-19G the amphtboles range from plagtoclase and retrograde albite was found, actinolite to ferro-edinttic hornblende to mag- indicating a gap between the high-temperature nestan hastingstttc hornblende to ferro-horn- metamorphism and the low-temperature retromorphic blende. event. Compared with the hornblende of the gneisstc amphtbolite, the amphtboles of associated meta- Bulk Rock Chemistry gabbros are characterized by a much wider field of composition, suggesting a lack of reequiltbratton Table 8 presents the bulk rock chemical analy- during metamorphism. It is remarkable that in an ses of 19 gnetssic amphtbolites from the Vema A1 versus alkalis diagram (Figure. 9), repre- Fracture Zone and the single amphibolite from the sentattve points are systematically shifted toward Romanche Fracture Zone. The bulk rock composi- more alkaline compositions (i.e., sodic, for the tions have to be discussed with caution in terms potassium content is always low). Moreover, the of comparison with magmatic equivalents. It is ferro-hornblendes and ferro-edenitic hornblendes clear from an AFM diagram (see Figure 11) that in the veins contain abundant chlorine (e.g., the range of chemical compositions of amphibolites Table 5• •n•!y•4•. 6). The significance of this is similar to those of oceanic gabbros and meta- observation is discussed in the following chapter. gabbros [Bonattt et al., 1971; Miyashiro and Cltnopyroxene. Clinopyroxene from the gneisstc Shido, 1980; Prinz et al., 1976; J Honnorez, un- amphibolttes is apparently metamorphic judging published. data 1977]. Table 8 shows that the from textural relationships. The clinopyroxene rocks are rather heterogeneous in composition. composition is very homogeneous in a given sample The most variable elements are titanium, iron, (Table 6) but varies somewhat among various speci- silica, and aluminum. Five rocks (CH78Dr10-81, mens. All the pyroxenes belong to the diopstde- Dr10-75, P7003-19E, Q, and AA) are titanium- and hedenbergiteseries They are always A1TM iron-rich and correlattvely aluminum-and silica- alkali-, and Ti-poor, as expected from metamorphic poor. Their compositions are consistent with pyroxenes [Girardeau and M•vel, 1982]. Like the those of iron- and titanium-rich gabbros. On the amphiboles, the Mg/Fe ratio of the metamorphic other hand, three other amphibolite samples pyroxenes is related to the bulk rock composition (GS7309-51K and CH78Dr10-13 and 129) are very (see Figure. 10b). titanium poor (<1%Ti02) and relatively iron poor •gi___o_•_se•.The rock texture indicates that (<8.6%FeoT). Suchrocks probably correspond to the plagioclase coexisting with hornblende in the adcumulate gabbros almost completely devoid of gnetssic amphibolites is also secondary and has opaques. The potassium content of the gneisstc recrystalltzed under stress. Only in samples amphibolttes is always very low, except in CH78- Dr10-23 and Dr10-81 are there large crystals Dr10-126 in which calcic plagioclase is partly scattered in a finer-grained matrix of plagioclase altered to secondary white mica: its higher and displaying undulatory extinction which could potassium concentration is thus explained by a be interpreted as magmatic porphyroclasts in a relatively low-temperature secondary alteration. recrystalltzed matrix. Plagtoclases from all rocks are calcic, but their composition varies Age of Metamorphism from one specimen to another, generally ranging from An 48 to An 66. Their iron and potassium Dating oceanic metamorphic rocks is a difficult contents are always low, even in the large crys- process because none of the minerals they contain tals, again typical of secondary plagioclase (0.4 are favorable to isotopic dating methods. K/Ar and 0.5%, respectively). dates have nevertheless been attempted on isolated In sample P7003-19E the plagioclase grains are amphiboles and plagioclases and on bulk rock sam- homogeneous, as indicated by the lack of composi- ples. The analytical precision of the results is ttonal difference between the core and the rim of rather poor because potassium concentrations are the simple grains (Table 7). Unzoned highly cal- very low in both amphiboles and plagtoclases and ctc plagtoclases also occur in specimens Dr10-77 thus in whole rocks. and GS7309-51K. Both amphtbolites are charac- Plagioclases and hornblendes were separated terized by a very high CaO/Na20content. As will from the 80-160 •m fractions by heavyliquids and be discussed later, these two rocks are inter- magnetic separation. Different sieve sizes, how- preted either as metamorphosed rodingites or ad- ever, were used for samples Dr10-20 and Dr10-77. cumulate gabbros devoid of or containing little They are mentioned in the table of results. An 8- Fe-Ti oxides. In the other rocks, plagtoclases mm-diameter core was directly fused for Ar analy- are commonly zoned from a calcic core to a more sis of the whole rock Dr10-23. On the other hand, sodic rim (Table 7). In the mylonite Dr10-81 the the Ar analysis of the whole rock Dr10-83 was per- small plagioclase grains of the matrix are similar formed on the 250-350 •m fraction after ultrasonic in composition to the more sodic rims of zoned cleaning. porphyroclasts. The analytical procedure, more extensively de- The relationship between the An content of the scribed elsewhere [Westphal et al.,1979], was the 11,390 Honnorez et al.: Significance of Gneis$ic Amphibolites

ß ß ß ß

..... j•j ß Honnorezet al.: Significanceof GneissicAmphibolites 11,391

TABLE 6. Representative Chemical Analyses of Clinopyroxenes From the Amphibolites.

CH78Dr 10-77 CH78Dr 10-23 CH78Dr 10-81 CH78Dr 10-16

SiO2 51.67 51.63 50.64 52.54 TiO2 0.24 0.11 0.21 0.25 FAle•03 7.021.94 0.709.76 14.451.01 8.031.32 MnO 0.21 0.45 0.72 0.23 MgO 14.87 13.65 11.49 14.49 CaO 23.42 23.01 21.10 23.69 Na2 0.10 0.32 0.25 0.33 Total 99.47 99 ß 63 99 ß87 100.88

Cations based on 6 Oxygens Si 1.931 1.954 1.947 1.946 AIIV 0.069 0.031 0.046 0.054 A1VI 0.016 ...... 0.004 Ti 0.007 0.003 0.006 0.007 Fe2+ 0.219 0. 309 0.465 0.249 Mn 0.007 0.014 0.023 0.007 Mg 0.828 0.770 0.659 0.800 Ca 0.938 0.933 0.869 0.940 Na 0.007 0.024 0.019 0.023 Total 2.022 2.053 2.041 2.030

following: argon was analyzed by isotopic dilu- spar. The analytical uncertainties, however, tion mass spectrometry. Potassium was measuredby preclude any definite conclusion. flame photometry with a lithium internal stan- In spite of high analytical errors the bulk dard. Ages were computed with the K decay con- rock sample Dr10-83 yields a calculated age that stants proposed by Steiger and Jaeger [1977]. is compatible with that of the amphibole sep- Plus-minus figures were estimates of analytical arate. On the other hand, bulk rock sample Dr10- precision at one standard deviation. They were 23 indicates an age of 17.8 ñ 1.6 m.y. The lack calculated following the procedure given by Cox of data on the mineral phases, however, prevents and Dalrymple [1967]. giving any sound and definite interpretation of The results of K-Ar age analyses are given in that age. Table 9. The following section will be devoted to Two observations lead us to conclude that a assessing the geological meaning of the dates metamorphic event has occurred about 10 m.y. yielded by the three systems investigated in this ago: the minerals from sample DrlO-16 yield study, hornblendes, plagioclases, and whole rocks. almost concordant ages close to this value, and The hornblendes DrlO-81 and DrlO-20 display the age determination of five hornblendes fall in evidence of alteration (chloritization?) that may this range. A 10 ñ 2 m.y. age has been determined explain both their high contamination by atmos- by U-He method on sample P7003-19E originating pheric argon and their relatively low age values, from the same area (D.E. Fisher, personal com- 7.8 • 3.7 and 7.1 • 2.3 m.y., respectively. The munication 1983). It has to be noted that two other hornblendes can be regarded as fresh. Two independent methods give similar answers for the of them (i.e., DrlO-77 and DrlO-16) yield ages of same type of rock. the same order of magnitude, 8.6 • 1.4 and 9.6 ñ 1.5 m.y., respectively. The amphibole,DrlO-83, Discussion on the other hand, displays a higher value of 13.1 Parental Rocks ñ 1.8 m.y. Is there any real difference between the ages of DrlO-20 and DrlO-837 Dalrymple and The gneissic amphibolites are completely devoid Lanphere's [1969] critical value test, at 95 % of relict igneous textures and minerals; hence the confidence (according to which there is a real nature of their parental rocks must be inferred. difference between the calculated ages of two However, several observations suggest that these minerals), is not quite conclusive. If one as- rocks were originally gabbros. sumes then that DrlO-16, DrlO-20, DrlO-77, DrlO- Bulk rock chemical composition. We have 81, and DrlO-83 have been metamorphosed contem- demonstrated that the range of chemical composi- poraneously, the event would have probably tions of the 19 studied gneissic amphibolites occurred between 8 and 12 Ms. overlaps that of the oceanic gabbros and meta- One plagioclase, DrlO-16, gives a K-Ar age, gabbros from the same region of the Mid-Atlantic 10.9 ñ 2.5 m.y., close to that of the coexisting Ridge. hornblende which gave 9.6 ñ 1.5 m.y. On the other Dredge haul composition. Dredge haul CH78DrlO hand, the other plagioclase, DrlO-83, yields an collected abundant serpentinites and gneissic age of 19.6 • 4.1 m.y. This discrepancy might be amphibolites, a few gabbros, and no basalt. In due to an occurrence of excess argon in the feld- dredge haul P7003-19, basalts were scarce, while 11,392 Honnorez et al.: Significance of Gneisstc Amphtbolites

o ,-.-• • I r-.o

u•

o Honnorezet al.: Significance of Gneissic Amphibolites 11,393

TABLE 8. Bulk Rock Analyses of Amphibolites

CH78 CH78 CH78 CH78 CH78 CH78 CH78 CH78 CH78 CH78 CH78 Dr ! 0 Dr 10 Dr 10 Dr 10 Dr 10 Dr 10 Dr 10 Dr 10 Dr 10 Dr 10 Dr 10 126 13 14 16 18 20 23 75 77 81 82

Si 2 49.08 50.90 50.01 46.96 51.45 49.71 50.63 42.81 45.07 43.37 48.76 TiO• 0.81 0.78 1.09 2.07 0.64 1.02 1.14 4.88 0.86 5.00 1.77 A1263 15.76 15.50 14.57 13.46 14.89 15.89 14.06 11.40 14.19 12.16 14.01 Fe203 2.67 1.86 2.86 3.29 2.71 3.09 2.58 10.54 4.36 5.46 3.81 FeO 6.92 7.20 7.20 8.48 6.76 6.37 7.38 8.87 6.83 12.27 8.09 MnO 0.09 0.18 0.18 0.20 0.15 0.34 0.15 0.28 0.16 0,20 0.16 MgO 7.63 7.39 7.43 8.66 6.79 7.13 7.18 6.80 9.23 6.83 7.24 CaO 10.92 11.33 11.32 12.23 10.87 11.18 11.86 10.09 15.03 9.98 11.34 2.78 2.68 3.02 2.08 3.40 3,02 2.95 2.32 1.80 2.30 2.75 0.49 0.17 0.16 0.14 0.15 0.14 0.12 0.17 0.15 0.13 0.10 2.70 1.42 1.71 2.01 1.65 1.74 1.47 1.60 2.25 2.24 1.61 Total 99.76 99.41 99.55 99.58 99.46 99.63 99.52 99.73 99.96 99.94 99.64

CH78 CH78 CH78 CH78 CH78 P7003 P7003 GS P7003 P7003 Dr10 Dr10 Dr10 Dr10 Dr10 19E 19F 7309 19AA 19AA 83 87 127 128 129 51K host Vein in Meta- gabbro

50.81 48.80 49.43 49.70 50.93 38.06 45.92 46.00 42.14 40.93 1.21 1.85 1.58 2.00 0.86 8.79 1.30 0.30 6.66 5.22 13.97 15.33 14.50 14.83 16.05 11.78 10.00 17.38 11.06 10.98 4.12 2.50 2.19 1.57 1.65 3.64 2.36 1.06 5.79 3.47 FeO 6.75 '7.76 8.44 8.08 6.72 13.22 9.60 4.48 12.29 15.11 MnO 0.45 0.15 0.16 0.15 0.16 0.23 0.20 0.08 0.30 0.25 MgO 6.27 7.73 8.33 8.92 7.93 9.82 12.58 11.47 6.16 7.76 CaO 9.41 10.66 10.54 10.46 10.89 8.80 11.85 16.07 10.47 9.67 Na20 3.96 2.78 2.50 2.50 2.85 2.21 1.75 1.42 2.69 2.77 K20 0.18 0.09 0.06 0.09 0.13 0.11 0.23 0.04 0.14 0.24 HO+ 2.33* 2.49 2.12 2.23 1.78 2.13 3.03 2.70 2.65 3.23 P•O5 n.d. 0.06 0.04 0.03 0.05 0.01 0.04 0.03 0.02 0.04 Total 99.46 100.20 99.89 100.56 100.00 98.79 98.86 101.03 100.37 99.67

serpentinites,jabbros, and metagabbros were very causeimportant vertical uplifts tookplace along abundant. We have demonstrated that the complete the southern wall of the Vema Fracture Zone [Hon- transition exists between undeformed gabbros to norez et al., 1975]. gneissic amphibolites from the latter dredge haul. Both dredge hauls are part of a sampling Temperature Conditions sequence of eight dredge hauls along both the north and sou•h face of the transverse ridge of The mineral assemblage observed in the gneissic the VemaFracture Zone, at about 43øW. amphibolites is typical of the amphibolite Occurrenceo• apa•ite• In several samplesof facies: hornblende+ Ca-plagioclaseñ ilmenite ñ gneissic amphibolite, apatite occurs as small clinopyroxene. That the rocks have reached equi- grains lining up along the foliation. This min- librium in most cases is demonstrated by the eral is abundant in differentiated Fe, Ti-rich absence of igneous relics, the homogeneous com- gabbros from the higher levels of the magmacham- position of secondary minerals, and the parti- ber in the oceanic crust [Bonatti et al., 1971; tioning of Fe and Mg between hornblende and clino- Prinz et al., 1976; Ohnenstetter and Ohnenstetter, pyroxene (Figure. 10b). According to Liou et al. 1980]. These grains may be relict broken magmatic [1974] the amphibo!ite facies sensu stricto (co- crystals preserved through amphibolite facies existing calcic plagioclase and hornblende) begins metamorphism. approximately at 550øC at low pressure. From the Depthof.,dredging. The gneissic amphibolites titanium contentof the hornblendes(Raase, 1974) have been dredged from 2050 to 2850 m below the it is reasonable to estimate equilibrium temper- crest of the transverse ridge. This observation ature to be between 550• and 650•. In the mylo- is consistent with the abundance of differentiated nitic amphibolites the minerals of the matrix are gabbroic rocks such as numerousFe, Tt-rich gabbro similar in composition to the porphyroclasts. norites and leuco-ferrodiorite in the upper part Therefore it can be inferred that the myloniti- of magmachambers in the oceanic crust. However, zation occurred within the same temperature range this argumentshould be regardedwith caution be- as the amphibolite facies recrystallization, the 11,394 Honnorezet al.: Significance of Gneissic Amphibolttes

o z4

A M Fig. 11. AFM diagram of the gneissic amphibolites from the Vema and Romanchefracture zones compared to gabbros and metagabbros from the Mid-Atlantic Ridge. Solid line surrounds the field of the metagabbros [Honnorez and Kirst, 1975; Bonatti et al., 1975;J Honnorez, unpublished data, 19XX]. Dashed line surrounds the field of the gabbros [J.Honnorez, unpublisheddata, 19XX; Priaz et al., 1976; Miyashiro and Shido,1980]. 1, uralitized gabbro P7003-19AA; 2, vein in gabbro P7003-19AA; 3, gneissic amphibolite CH78Dr10-81; 4, gneissic amphibolite CH78Dr10-75; 5, gneissic amphibolite P7003-19E; 6, gneissic amphibolite CH78Dr10-82; 7, gneissic amphibolite CH78Dr10-127; 8, gneissic amphibolite CH78Dr10-16; 9, gneissic amphibolite CH78Dr10-14; 10, gneissic amphibolite CH78Dr10-13; 11, gneissic amphiboli[e CH78Dr10-23;.13, gnetssi• amphibolite CH78Dr10- 128; •4, gneissic amphibOliteCH78Dr10-83; 15, gneissic amphiboliteCH78Dr10-18; 16, gneissic amphibolite CH78Dr10-20; 17, gneissic amphibolite CH78Dr10-126; 18, gneissic amphi-boliteCH78Dr10-129; 19, gneissic amphiboliteCH78Dr10-77; 20, gneissic amphibolite P7003•19F; 21, gneissic amphibolite GS7309-51K; 22, rodingite P7003-23B; 23, rodingite P7003-17R; 24, alkali gabbro GS7309-; 24A; 25, alkali gabbro P6707-25F; 26, anorthosite GS7309-58II.

only difference being the intensity and/or dura- the greenschist facies before starting to recrys- tion of shearing. tallize into amphibolites. The low-temperature metamorphic minerals (smectite, actinolite, albite, prehnite, chlorite, quartz, epidote, sphene) observed in late veins Heat Source correspond to greenschist facies conditions and lower temperatures formed during cooling of the Let us consider now the possible heat sources lithosphere after shearing of these rocks stopped. present in the oceanic crust. The presence in the associated metagabbros of 1. Magmatic activity at (or close to) spread- igneous relict minerals and textures and the large ing centers is the most logical and most often variety of secondary amphiboles (pargasitic horn- cited heat source. The hydrothermal circulation blende, hornblende, actinolite) occurring in a resulting from the warming up of seawater by the single specimen demonstrate a lack of equilibrium tholelitic magma would be responsible for the heat related to a complex cooling story. The frequence necessary to form the oceanic amphibolites. of an inner actinolite rim separating the igneous 2. Off axis basaltic rocks with alkaline ten- pyroxene from the hornblende and the scarcity of dencies(Bonatti et al., 1979) are really emplaced metamorphic hornblende rims dlrectly surrounding along fracture zones.. Such rocks were recovered pyroxene relicts suggest that the gabbros had al- from the St. Paul Fracture Zone [Melson et al., ready cooled down to temperatures corresponding to 1967] and the Romanche Fracture Zone (J. Honnorez Honnorezet al.: Significance of GneissicAmphibolites 11,395

TABLE 9. K/At Ages

K20 40Attad 40Attad , Sample Mineral x 100 Age ñ l o, Ma (wt %) (10-11mole/g) 40Attotal

Dr10-81 amphibole 0.089 0ß 09986 2.27 7.8ñ 3.7 (80-160 •m) Dr10-83 amphibole 0.095 0.1796 6.52 13.1 + 1.8 (80-160 plagioclase 0.184 0.5229 2.01 19.6 +_ 4.1 (80-160 •m) whole rock 0.126 0.2703 5.71 14.8 +_ 1.5 (250-350 •m) Dr10-16 amphibole 0.080 0.1111 6.78 9.6 ñ 1.5 (80-160 plagioclase O,157 O,2470 4,92 10.9 +_ 2.5 (80-160 Dr10-20 amphibole 0.091 0.09281 2.19 7.1+ 2.3 (50-71 •m) Dr 10-77 amphibole 0.093 0.1148 5.17 8.6 +_ 1.4 (90-112 •m) Dr10-23 whole rock 0,080 0,2056 7,58 17.8 +_ 1.6 (not crushed)

Analysts_R.Montigny and R Thuizat (I..P.G. Strasbourg), I e = 0.581 x 10-10 yr-1; 18 = 4.962 x 10-10 yr-1; 40K/Ktotal = '1.167 x 10-4mol/mol. andE. Bonatti, unpublisheddata, 1978)but none the oceanicmetamorphic processes. In ophiolite were foundfrom the VemaFracture Zone. complexes,oceanic metamorphism is mainly recorded 3. The heat for metamorphismcould be deve- as static, having occurredin the absenceof loped mechanicallyduring shearingalong the stress[Spooner and Fyfe, 1973;Gass and Smewing, transformfault zone. Reitan [fromHyndman 1972] 1973;Mgvel, 1975; Elthon and Stern, 1978;COish, calculatedthat friction distributedthrough 3 km 1977; Stern and Elthon, 1979;Liou and Ernst, of crust•uring • relativelyshort period of time 1979]. However,some examples of foliated meta- (i.e., 10J to 10v years)may raise the temperaturemorphic rocks have been described, usually in the throughseveral kilometers of crustby about30 ø- gabbrosequence. From field relationshipsthey 80oC. seemto result from two main types of processes: 4. We must also consider the possibility of Type 1. Horizontal localized shear zonesmay contactmetamorphism by a high-temperatureultra- inducethe formationof flaser gabbrosand gneis- mafic intrusion becausethe gneissicamphibolites sic amphibolites,with a foliation subparallelto andrelated metagabbros are associated with set- the layeringin the gabbrosequence- They pro- pentinites in both dredgehauls from the south bably form close to the ridge axis by ductile wall of the VemaFracture Zone. This wall belongs deformationof the still hot crust and are there- to a transverseridge whichis consideredby some fore related to the spreading. Suchrocks occur, authors as an ultramafic diapiric protrusion for instance, in the Apennines[Corte$ogno et al., [Bonattiand Honnorez, 1971, 1976; Bonatti, 1976, 1975], in the westernAlps [Mdvelet al., 1978; 1978]. Steenet al., 1980], at CanyonMountain [Thayer, The combinationof shearing-inducedheat with 1980and personalcommunication, 1982], in the Dun magmaticintrusion driven hydrothermalheat at Mountainbelt, NewZealand [Kidd, 1981], and in spreadingcenter was already advocated by Bonatti Newfoundland[Girardeau, 1979; Casey et al., 1981; et al. [1975] to explain the formationof meta- Girardeauand Mdvel, 1982]. In NewfOundland, gabbrosfrom the crest of the Mid-AtlanticRidge, minorvertical shear zones,subparallel to the at 6øN. The metamorphicprocess which generated meandike trend, are interpretedas resulting from them was called "contact-hydrothermal-dynamicthe formationof the rift valley walls [Caseyet metamorphism." al., 1981]. Type 2. Larger amphibolite bodies may be Shearin•"' relatedfaults. to In majorthis case,accidents the shearingsuch as shouldtransform be Thegneissic rocks are also characterizedby vertical. Suchrocks have been described in the crystallizationunder stress. Thelack of field CoastalComplex, Newfoundland [Karson and Dewey, relationshipsfor rocks dredgedfrom the ocean 1978;Casey et al., 1981]and in Oman[Smewing, floor makes the interpretation of their foliated 1980]. textures difficult. Therefore, although many In situ observationsfrom submersiblesdescribe ophiolitesprobably did not format typical mid- planar structuresin oceaniclayer 3. Theyare oceanridges, the studyof metamorphicrocks in interpretedas shearzones because gneissic rocks ophiolite sequencesis helpful in understandinghave actually beensampled from these zones. But 11,396 Honnorez et al.: Significance of Gnetssic Amphibolites

TABLE 10. Water Contents of Gneissic Amphtbolites, Gabbros, Metagabbros, and Rodingites From the Mid-Atlantic Ridge Fracture Zones

Average H20+ Number H• O+, Range, of References %z % Samples

Unaltered and slightly 1.5 0.84-1.71 8 Prinz et al. [1976] and uralitized gabbros J. Honnorez (unpublished data, 1979) Gneissic amphibolites 2.1 1.21-3.03 19 this paper Metagabbros 3.7 1.51-8.79 40 Bonattt et al. [1975] and J. Honnorez (unpublished data, 1979) Rodingites 9.3 7.10-11.55 2 Honnorez and Kirst [1975]

their orientation is variable: it has a wide seated layered gabbros appears to be fresh. range in the Cayman trough [Stroup and Fox, 1981], The fact that all of the metagabbros and amphi- is mostly subhorizontal in the Gorringe bank bolites dredged from the seafloor contain about 10 [CYAGORGroup, 1984], and is vertical andparallel times moreatmospheric argon than_•res• basalt to the ridge axis in the Kane fracture zone (H. J. glass (0,35 x 10-u versus 0.035 x 10 v cm• of VAr •. Dick, personal communication,1983). All these (D.E. Fisher• personal communication, 1983) indi- shear zones seem to be related to type 1. cate that the plutonic rocks and their metamorphic In the absence of direct observation it is im- equivalent reacted with seawater. possible to conclude anything about the origin of It has been pointed out that the hornblendes the shearing observed in the Vema Fracture Zone from gnetssic amphtbolites contain no chlorine and rocks. However, the abundance and the large size generally less sodium (see Figure 9) than those of the gneissic amphtboltte samples (one dredge from associated metagabbros which may contain an haul contained half amphtbolites, half serpen- appreciable amount of chlorine. Similarly, in the ttnttes, and almost no gabbro) as well as the fact partly amphtbolttized metagabbros from the Mid- that these rocks occur in two dredged hauls taken CaymanRise [Ito and Anderson, 1983] the most C1- 15 km away from each other seem to preclude narrow rich hornblendes with up to 0.66% C1 occur just shear zones in mostly undeformed gabbros. We below the crustal level where the most altered think rather that the gnetsstc amphibolites were gabbros were collected. This may indicate, ac- formed in a large vertical shear zone related to cording to Ito and Anderson, the formation of a the transform fault activity as in type 2. dense brine seal in the crust during metamorphtsm. As suggested by Ito [1979] and Ito and Origin of Water Claytoncalculated[1983], fromo• 80/160 the basis the of composition water/rockofratios the The gnetsstc amphtboiites are richer in water fluid phase evolves toward water deficient brines than gabbros and poorer in water than metagabbros as it reacts with the crust. This suggests that from the Mid-Atlantic Ridge and its fracture zones the fluid phase responsible for the crystal- {see Table 10). This intermediate water content 1tzation of chlorine-rich amphiboles in the is related to the presence of hornblende which metagabbro was more evolved (i.e., had lost more contains less water than lower-grade metamorphic water) than the solution reacting to form the phases. gneisstc amphtbolites. According to Lister [1982], seawater percolates The lack of magnetite and the presence of through the oceanic crust along a "cracking front" tlmenite in the gnet$sic amphtbolites indicate during the "active stage" of the crust cooling. that relatively low oxygen fugactty prevailed The depth and temperature of this front are re- during metamorphism. Most of the iron of the lated to the temperature at which the crustal parental gabbros entered the hornblende (and rocksare sufficiently stressedto crack. This eventuallythe _cltnopyroxene)as Fe2+. Minor cracking temperatureis still unknownbecause of amountsof Fe2+ also entered the accessory poor knowledge of the mechanical behavior of ilmentte whose abundance was determined by the oceanic rocks. However, one can assume that the titanium content of the rock. This is the reason cracking front and, consequently, the hydrothermal why the hornblendes from the gneissic amphibolites metamorphtsm, reach down to a zone of the oceanic are relatively rich in iron (1.014 to 2,874 mol crust directly overlying the magmachamber and, FeT per half unit cell). later, farther down through the now solid magma chamber itself. At such time, the hydrothermal A•e of Met•morphism circulation and alteration have decreased in intensity ("passive stage") because most of the Dredge hauls CH78Dr10 and P7003-19 recovered heat has already been lost by the now solid gab- the gnetsslc amphtboltte at about 196 and 207 km, bros. Lister's theory seems to be confirmed by respectively, from the spreading axis of the Mid- the observation that in several ophiolite com- Atlantic Ridge. Ages of crustal emplacement of plexes, massive gabbros and even part of t•e 12.3 or 14 m.y., and 13 or 14.8 m.y., respective- underlying cumulate gabbros are hydrothermally ly, are calculated using the Minster and Jordan altered, whereas the major part of the more deeply [1978] 1,6 or 1.4 cm/yr half spreading rate values Honnorez et al.: Significance of Gneissic Amphibolites 11,397

between South America and Africa or North America gnetssic amphibolites are likely to be a common, and Africa, respectively. Within the limit of if not abundant, constituent of oceanic layer 3. analytical errors, K/At datings of gneissic amphi- The near absence of amphibolite facies metabasalts bolite bulk rocks and isolated phases are, on the is probably due to the difficulty of oceanic layer whole, consistent with the calculated ages of the 2 rocks to reach the high-temperature regime re- crust. This means that the amphibolites were gen- qutred for their ductile deformation and recrys- erated in the vicinity of the ridge axis. How- tallization. ever, this inference does not help us to determine whether the metamorphic event affected newly em- Acknowledgments. We extend our appreciation to placed gabbroic rocks or older gabbros forming a the officers and the crews of the R/V Pillsbury stagnant block which would have been rejuvenated and Jean Cha•cot for their collaboration during as it moved past the spreading center. legs P7003 and CH78, respectively, to the Vema Fracture Zone. We are grateful to E. Bonatti and Conclusion: Formation of the Gneisstc D. Needhamfor allowing us to study the samples of Amphibolttes and Associated Metagabbros in the gneiss!c amphibolites dredged during these two VemaFracture Zone legs. We also wish to thank A. Anderson, H. J. B. Dick, W. G. Ernst, D. E. Fisher, E. Ito, J. We have demonstrated that the gnetssic am- G. Liou, R. Kidd, J. R. Kienast and P. W. Kirst phibolttes from the Vema Fracture Zone were for stimulating discussions and reviews of this derived from various types of oceanic gabbroic paper. This research was supported by grants from rocks. Most of them were dredged from about 3 km the National Science Foundation (OCE 79-09318) and subbottom, while metagabbros and flaser gabbros the Centre National de la Recherche Scientifique were dredged from shallower depths (i.e., between (ATP-IPOD). 2 and 3 km). Their mineral assemblage shows that References the rocks equilibrated in the amphibolite facies conditions which require a thermal gradient of 200 Aumento, F., B. C. Loncarevic, and D. I. Ross, to 320ø/km. The depths at which the gnetssic Hudson geotraverse: Geology of the Mid- amphibolites were dredged have obviously no sig- Atlantic Ridge at 45øN, Philos ß Trans ß R. Socß nificance for calculating thermal gradient in the LondonSet. A__,268, 623-650, 1971ß ocean crust during the amphibolite facies meta- Bodganov, Y. A., and V. V. Ploshko, Magmatic and morphic event. Their strong foliation implies metamorphic rocks of the deep-water Romanche stress during recrystallization. Transformzones trench, Dokl. Ak. Nauk SSSR, 177, 173-176, are characterized by intense shearing along vet- 1967. tical planes subparallel to the direction of Bonattt, E., Serpentintte protrusions in the spreading. The friction and mostly somemagmatic oceanic crust, Earth Planet. Sct. L•tt., 32___, intrusions are the most likely heat sources in 107-113, 1976. these zones. We suggest that the fault zone may Bonatti, E., Vertical tectonism in oceanic have acted as the conduit of the ascendantlimb of fracture zones, Earth P•anet. Sci. Lett., 37• a hydrothermal convective cell. At depths of sev- 369-379, 1978. eral kilometers subbasementthe stress combined Bonatti, E., and A. P. Chermak, Formerly emerging with the hydrothermal circulation induces the crustal blocks in the equatorial Atlantic, ductile deformation and equilibrium recrystal- •ectonoph•sics, 72___,165-180, 1981. ltzation of the gabbros in amphtbolite facies Bonatti, E.,and J. Honnorez, Non-spreading crustal

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