26. "BASEMENT" ROCKS OF GORRINGE BANK

PREFACE

Igneous rocks have been recovered from the GorringeBank in the eastern North Atlantic, through deep drilling(see Chapter 2), dredging and piston coring. The results arepresented here. Two interesting and contrasting suites ofrock have been discovered. One is an "ophiolite" suitethought to represent the original oceanic crustal layergenerated by accretion at the axis of a Mid-Oceanic Ridge.The other, a silica deficient alkaline suite, more closelyresembles igneous bodies located behind subducting zonesand may be associated with younger intrusions accompany-ing the development of a compressive plate boundary along

the Azores-Gibraltar seismic belt (Le Pichon et al., 1970;and McKenzie, 1970). This chapter gives petrographicdescriptions and a single chemical analysis of selectedsamples and offers brief discussions of their geologicalsignificance.

REFERENCES

Le Pichon, X., Bonnin, J. and Pautot, G., 1970. TheGibraltar end of the Azores-Gibraltar Plate boundary: anexample of compressive tectonics (Abstract). UpperMantle Committee Symposium. Flagstaff, Arizona. July,1970.

McKenzie, D. P., 1970. Plate tectonics of the Mediterraneanregion. Nature. 226, 239.

26.1. PETROGRAPHY OF THE GORRINGE BANK "BASEMENT"

Jose Honnorez, Rosenstiel Institute of Marine and Atmospheric Sciences, University of Miami, Miami, Floridaand

Paul J. Fox, Department of Geological Sciences, State University of New York at Albany, Albany, New York

INTRODUCTION

Igneous rocks were obtained at Site 120 on the northernflank of Gorringe Bank (water depth 1711 meters, seeChapter 2). Core 8 recovered approximately 0.9 meter ofbasement rocks below Lower Cretaceous, partly silicifiedpelagic sediment. The basement was encountered at 246meters below the sea floor and was penetrated to 253.4meters before the hole was abandoned.

RESULTS

Most of the recovered igneous material is a holocrystal-line metagabbro composed of large crystals (ranging from0.5-1 cm in length) of gray feldspar and tan pyroxene withinterstitial patches of dark minerals. The texture is panallo-triomorphic granular (Figure 1). The Ca-plagioclases aregenerally twinned according to albite and/or pericline,and/or Carlsbad laws. They are typically slightly zoned witha core An content of about 55-60 per cent. Largest crystalsof Ca-plagioclase are often broken resulting in twisted andoffset twin planes. The partial alteration of the pyroxenehas produced a dense network of dark parallel lines whichhave clouded the pyroxenes and thereby obscured theinterference colors. As a result, the pyroxene species couldnot be identified with certainty. The outer portion of thepyroxenes grades into an amphibole, generally actinolite,which surrounds the pyroxene by a felty growth of fibers.

In some instances, however, the actinolite is in opticalcontinuity with pyroxene. Upon occasion the mantlingamphibole is a zoned pleochroic hornblende with pleo-chroism ranging from either bright orange to dark redbrown, or from light green to dark blue green. Thehornblende often contains small euhedral prisms of apatite.The finer grained material filling the interstices between thepyroxene and Plagioclase crystals is made up of fine fibrousgreen chlorite, an unidentified mineral (similar to chlorite),and longer blades of a colorless amphibole (tremolite?).Rare patches of quartz are associated with the chlorite. Theaccessory opaques are small round bodies which have beenpartly replaced by a submicroscopic colorless material. Theresults of a major element chemical analysis and normativemineral computation for a metagabbro sample (120-8-7) aregiven in Table 1 (analysis by J. Honnorez).

DISCUSSION

The basement sample recovered at Site 120 is a gabbrooriginally composed of Ca-plagioclase, pyroxene, andopaque minerals which have subsequently reequilibratedto changing physiochemical conditions. Occurrence of thesecondary amphiboles (actinolite and hornblende), chlorite,and quartz suggest that the gabbro recrystallized underamphibolite greenschist metamorphic conditions.

A few small chips of light green gray, glomeroporphy-ritic metabasalt were found accompanying the core of

747

A. J. GAVASCI, P. J. FOX, W. B. F . RYAN

. . . » • . - ' - •

Figure 1. Reverse print of a thin section enlargement of the ho lo crystalline meta-gabbro of Core 8, Site 120, Gorringe Bank,illustrating the Cataclastic texture developed under amphibolite/greenschist metamorphic conditions.

TABLE 1Chemical Analysis and CIPW Normative Minerals:

Sample 120-8-7

ChemicalAnalysis

(Per Cent)

SiO2

AI2O3Fe2O3

FeOMnOMgOCaONa2OK2OTiO2

P2O5H2OH2O+

Total

48.6117.181.195.610.149.948.653.050.200.510.030.973.62

99.70

CIPW NormativeMinerals(Per Cent)

ORABANDIHYOLMTILAPH2O+H2O-

Total

1.1825.8032.60

8.1112.0212.65

1.730.970.073.620.97

99.70

metagabbro at Site 120 (Figure 2). The 0.1-1.0 mmphenocrysts are predominantly twinned plagioclases whichhave been partially albitized. There are lesser amounts ofchlorite-pseudomorphed olivine. The phenocrysts are set ina fine grained matrix of acicular Plagioclase, microlites,chlorite, and other alteration products.

The basaltic samples suggest that a basalt originallycomposed of calcic Plagioclase phenocrysts and fine-grainedmatrix underwent metamorphism which partially albitizedthe calcic Plagioclase and produced the chlorite. Thealbitization of the Plagioclase and the occurrence ofchlorite indicate a greenschist facies metamorphism.

Metabasalt in the greenschist facies and metagabbro inthe greenschist amphibolite facies are often recovered bydredging from the tectonic escarpments of the Mid-OceanicRidge System and fracture zones (for example, see discus-sion in Melson et al., 1968; Cann, 1969; and Miyashiro etal., 1971). The occurrence and abundance of these "rocktypes have led investigators to conclude that metabasalt inthe greenschist facies and metagabbro in the greenschist andamphibolite facies are representative of a portion of oceanicbasement (Layer 2) and oceanic crust (Layer 3). Themetamorphic gabbros and basalts recovered at Site 120 aresimilar to some of the metagabbros and metabasaltsdredged from the escarpments of the Mid-Oceanic Ridgeand fracture zones, (see Miyashiro et al., 1971 for adescription of oceanic metamorphic rocks.) The occurrenceof metagabbros and lesser amounts of metabasalts at thebase of Hole 120 on the north flank of Gorringe Banksupports the suggestion of Le Pichon et al. (1970) thatGorringe Bank is a slab of uplifted oceanic crust.

REFERENCESCann, J. R., 1969. Spilites from the Carlsberg Ridge, Indian

Ocean./. Petrol 10, 1.

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26.2. PETROGRAPHY OF ROCKS FROM CRESTAL AREA OF THE GORRINGE BANK

Le Pichon, X., Bonnin, J. and Pautot, G., 1970. TheGibraltar end of the Azores-Gibraltar Plate boundary: anexample of compressional tectonics (Abstract). UpperMantle Committee Symposium. Flagstaff, Arizona, July,1970.

Melson, W. G., Thompson, G. and van Andel, Tj. M., 1968.Volcanism and metamorphism in the Mid-AtlanticRidge, 22°N latitude./. Geophys. Res. 73, 5925.

Miyashiro, A., Shido, F. and Ewing, M., 1971. Metamor-phism in the Mid-Atlantic Ridge near 24° and 30°N. InA discussion on the petrology of igneous and metamor-phic rocks from the ocean floor. Bullard, E., Cann, J. R.and Matthews, D. H., (Eds.). Phil. Trans. Roy. Soc.London A, 268, 589.

Figure 2. Reverse print of a metabasalt (spilite) from adrill bit sample. The dark patches are porphyritic andglomeroporphyritic clusters of Plagioclase accompaniedby chlorite pseudomorphs set in a very fine-grainedmatrix of acicular feldspar, chlorite, and alterationproducts. Relic crystal outlines suggest that olivine,now replaced by chlorite, was originally present.

V

>•'»*• Λ* 1 m π> - *

v " . * ' , . v , . " ' *

26.2. PETROGRAPHY OF ROCKS FROM THE CRESTAL AREA OF THE GORRINGE BANK

Anna T. Gavasci, Department of Geology and Geography, Hunter College of the City University of New York, New YorkPaul J. Fox, Department of Geological Sciences, State University of New York at Albany, Albany, New York

andWilliam B. F. Ryan, Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York

INTRODUCTION

Samples of igneous rock were recovered from the crestalarea of Gorringe Bank in 1965 by coring and dredging fromthe R/V Robert D. Conrad of the Lamont-Doherty Geologi-cal Observatory. The sampling objective was to obtainmaterials which would shed light on the composition andgeological history of this uplifted part of the Azores-Gibraltar seismic zone. Numerous boulders and rock frag-ments were obtained in one dredge, RC 9-7 (36°44.6!N,ll°05.8'W; water depth = 104-130 m), and two piston

cores, RC 9-206 (36°40.5'N, 1 l°04.4'W; water depth = 736m) and RC 9-208 (36°42;N, 11 °07.3'W; water depth = 104m).

Much of the recovered material was badly weathered,and most of the exposed rock faces were encrusted withcarbonate crusts including colonies of byrozoa and coral.Smaller pebbles were often rounded, probably indicatingabrasion near an ancient surf-zone on the bank.

Twenty samples were studied in thin section with apetrographic microscope. The analyses are presented inTables 1 and 2 according to the two igneous rock suitesrecognized.

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A. J. GAVASCI, P. J. FOX, W. B. F. RYAN

TABLE 1Oceanic Suite

TABLE 2Alkaline Suite3

Sample Rock Type

MajorMineral Metamorphic

Composition Facies

Oceanic Ciustal RocksRC9-206-2 Metagabbro

RC9-206-3

RC9-206-5

RC9-206-6

RC9-206-11

RC9-206-13

RC9-208-1

Cataclasticmetagabbro

Cataclasticmetagabbro

Cataclasticmetagabbro

Cataclasticmetagabbro

Cataclasticmetaanorthosite(gabbrofragment)Cataclasticmetagabbro(anorthosite)

Labradorite,hornblendeLabradorite,diallage,iron oxidesLabradorite,diallage,apatite, ironoxidesLabradorite,diallage,apatite,iron oxidesLabradorite,diallage,apatite,iron oxidesLabradorite,diallage

Labradorite

Amphibolite

Greenschistamphibolite

Greenschistamphibolite

Greenschistamphibolite

Greenschistamphibolite

Greenschistamphibolite

Amphibolite

Amphibolite

Amphibolite

Ultramafic (Mantle?) Rocks

RC9-206-12 Amphibole- Augite,biotite pyroxene aegerine-augite,

brown horn-blende, biotite,apatite, intersti-tial zeolites

RC9-208-2502 Pyroxene bear- Brown horn-ing hornblendite blende, augite,

interstitialapatite, sphene,zeolite(?)

OCEANIC SUITE

The first group of rocks (7 of 20 specimens) ischaracterized by Cataclastic metagabbro which is composedof primary labradorite, diallage, and iron oxides withvarying amounts of secondary amphibole (blue greenhornblende, and/or actinolite), chlorite, and in somesamples, quartz and epidote. Most of the metagabbrosamples exhibit Cataclastic textures. These metagabbrosamples are similar to the metagabbro sample cored at Site120 on the north flank of Gorringe Bank (described inChapter 26.1) as well as the metagabbros recovered fromthe bounding walls of transform faults (see Miyashiro et al.,1971), and they are thought to compose the oceanic crust(Layer 3).

Two of the twenty samples are ultramafic specimens.One is an amphibole-biotite pyroxenite characterized byaugite, aegirene-augite, brown hornblende, biotite, apatite,and interstitial zeolites. The other is a pyroxene-bearinghornblendite (Figure 1). The occurrence of actinolite andblue green hornblende, which mantle the primary brown

Sample Rock Type

MajorMineral Metamorphic

Composition Facies

Alkaline Intermediate Series

RC9-208-2

RC9-208-4

RC9-208-5

RC9-208-6

RC9-208-8

Aegirinephonolite

Aegirinephonolite

Sanidinephonolite

Noseanphonolite

Sanidinephonolite

RC9-208-2504 Sanidine-noseanphonolite

Alkaline Mafic Series

RC9-208-3 Maficphonolite

RC9-206-1 Maficphonolite

RC9-208-250i Maficphonolite

RC9-208-2503 Welded tuff(or devitri-fied lava)

RC9-206-7 Alkalicmetabasalt

Aegirine, sani-dine, nepheline,analcite, ironoxidesAegirine, sani-dine, nepheline,zeolitesSanidine, nephe-line, aegirine,nosean, ironoxides, zeolitesNosean(?), sani-dine, nepheline,aegirine, zeo-lites, andesiteSanidine,nepheline,aegerine, anal-cite, zeolitesSanidine,nosean, nephe-line, aegirine,analcite(?),zeolites

Sanidine, anal-cite, nosean(?),aegirine-augite,aegirine, zeolitesAegirine, sani-dine, nepheline,nosean, zeolite,kao UniteNosean, aegir-ine, nepheline,sanidine, biotite,anorthoclaseNosean, horn-blende, biotite,pyroxene, leu-cite(?), aegirine-augite, aegirine,sanidine, anorth-clase, glassymatrixAltered olivine,augite, Plagio-clase, inter-stitial fe Id spa-to id (nosean:),melilite(?)

Zeolite

Zeolite

Zeolite

Zeolite

Zeolite

Zeolite

Zeolite

Zeolite

Zeolite

Zeolite

Zeolite

a>10 per cent feldspathoids.

hornblende and pyroxene, indicates amphibolite faciesmetamorphism; the occurrence of zeolite in veins andinterstitial zeolite suggests later stage retrogressive zeolitemetamorphism. Ultramafic rocks of this type have beenrecovered in dredge hauls from the base of transform faultescarpments (Dmitriev et al., 1971; Bonatti et al., 1971).The ultramafic rocks recovered from transform faults areinterpreted as representing the lower part of oceanic crustor upper mantle(?).

750

26.2. PETROGRAPHY OF ROCKS FROM CREST AL AREA OF THE GORRINGE BANK

Figure 1. Reverse print enlargement of a thin section ofpyroxene bearing hornblendite (RC 9-208-2502);revealing a medium-grained fabric and brecciatedappearance. The larger crystals (arrows) are brownamphibole of an edenite composition. Scale bar repre-sents 1 millimeter.

ALKALINE SUITE

The second group of samples (11 of 20 specimens) is asuite of silica deficient, alkaline rocks (Figure 2) and ischaracterized by the occurrence of pyroxene (aegirine),feldspar (sanidine), and feldspathoids (nepheline and/ornosean) as the principal minerals and, depending on thesample, lesser amounts of analcite, hornblende, pyroxene(augite, or aegirine-augite), and iron oxides. The occurrenceof zeolite in the matrix and in veins in all the samplessuggests zeolite facies metamorphism. The occurrence ofsuch undersaturated alkaline rocks in the cores and a dredgelocated on the crest of Gorringe Bank is unusual becausethis igneous rock type (phonolite) is not characteristic ofthe rock types typically recovered from the ocean floor(dredge hauls invariably recover one or a combination ofthe following rock types: tholeiitic basalt, alkaline basalt,metabasalt (zeolite and greenschist facies), gabbro, meta-gabbro (greenschist and amphibolite facies), and serpen-

Figure 2. Reverse print enlargement of a thin section ofmafic phonolite (RC 9-208-250j), containing largecrystals of nosean (arrow) and numerous thin laths ofsanidine. Many of the more mafic minerals are replacedby dark green pyroxene and iron oxides. Biotite is alsoreplaced and has inclusions of zircon without apleocroic halo. Scale bar represents 1 millimeter.

tinized periodotite. Although phonolite rocks occur inminor amounts in a late stage differentiate in associationwith alkaline basalts on some volcanic oceanic islands (e.g.,Ascension and St. Helena), phonolites are more generallyfound in association with igneous bodies located behindsubducting zones (e.g., Andes).

The occurrence of metagabbros in bottom samples fromthe crest of Gorringe Bank, a part of the Azores-GibraltarRidge, is not surprising and in fact these rocks suggest thatthe oceanic crust (Layer 3) is exposed along a portion ofGorringe Bank. The occurrence of a suite of intermediateand mafic alkaline rocks composed of feldspathoids, how-ever, is not similar to rocks previously sampled from theocean basin, and this suite is certainly not compatible withthe rock types thought to be generated along accretingplate margins of the Mid-Oceanic Ridge System. Rather,

751

A. J. GAVASCI, P. J. FOX, W. B. F. RYAN

these feldspathoid rich rocks are characteristic of igneousrocks found behind some subducting margins. Although thefeldspathoidal phonolites could represent later stage differ-entiates associated with an alkaline basalt seamount on theAzores-Gibraltar Ridge, we interpret the occurrence of thefeldspathoidal phonolites as reflecting a short and rapidsubducting phase which accompanied the deformation ofthe Horseshoe Abyssal Plain sediments and uplift of theGorringe Bank. As to the age of the intrusions, we havenoted welded tuffs containing xenoliths of mafic phonolitein direct association with baked skeletal debris of formerneritic fauna. This evidence would place the intrusion ofthe phonolite at a time when the bank was already shallow.According to the facies sequence of pelagic sedimentrecovered at Site 120, the transition there from a deepoceanic sea bed commenced during or after the EarlyMiocene, and was essentially completed by the EarlyPliocene.

REFERENCES

Bonatti, E., Honnorez, J. and Ferrera, G., 1971. Peridotite-gabbro-basalt complex from the equatorial Mid-AtlanticRidge. In A discussion on the petrology of igneous andmetamorphic rocks from the ocean floor. Bullard, E.,Cann, J. R., and Matthews, D. H., (Eds.). Phil. Trans.Roy. Soc. London A. 268, 385.

Dmitriev, L. V., Vinogradov, A. P. and Udintsev, G. B.,1971. Petrology of ultrabasic rocks from rift zones ofthe Mid-Indian Ocean Ridge. In A discussion on thepetrology of igneous and metamorphic rocks from theocean floor. Bullard, E., Cann, J. R. and Matthews, D. H.(Eds.). Phil. Trans. Roy. Soc. London A. 268, 403.

Miyashiro, A., Shido,F. and Ewing, M., 1971. Metamor-phism in the Mid-Atlantic Ridge near 24° and 30°N. InA discussion on the petrology of igneous and metamor-phic rocks from the ocean floor. Bullard, E., Cann, J. R.,and Matthews, D. H., (Eds.). Phil. Trans. Roy. Soc.London, A. 268, 589.

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