Abstract
A diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the Southwest Indian Ridge (54°S, 7°16′ E) between the Islas Orcadas and Shaka Fracture Zones. The cumulus nature of the gabbroic rocks in the suite is indicated by phase, modal and cryptic layering, igneous lamination, and low incompatible element abundances. We present a mass-balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk-rock major element compositions. The model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid. Calculated proportions of trapped liquid range from 3%–15%; values that are characteristic of adcumulates to mesocumulates. Models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids, thus explaining the high TiO2 contents observed in magnesian clinopyroxenes. Cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers, but disequilibrium plagioclase compositions suggest some crystallization at greater depth. Furthermore, basalt compositions projected onto the olivine-clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures (from less than 10 to greater than 20 kb) as well as polybaric fractional crystallization. We suggest that the Southwest Indian Ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros. The adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large, but ephemeral magma chambers below segments of the Southwest Indian Ridge.
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References
Barnes SJ (1986) The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions. Contrib Mineral Petrol 93:524–531
Batiza R, Vanko DA (1985) Petrologic evolution of large failed rifts in the eastern Pacific: petrology of volcanic and plutonic rocks from the Mathematician Ridge area and the Guadalupe Trough. J Petrol 26:564–602
Bender JF, Langmuir CH, Hanson GN (1984) Petrogenesis of basalt glasses from the Tamayo Region, East Pacific Rise. J Petrol 25:213–254
Bryan WB (1983) Systematics of modal phenocryst assemblages in submarine basalts: petrologic implications. Contrib Mineral Petrol 83:62–74
Bryan WB (1986) Linked evolutionary arrays: a logical structure for petrologic modeling of multisource, multiprocess magmatic systems. J Geophys Res 91:5891–5900
Bryan WB, Dick HJB (1982) Contrasted abyssal basalt liquidus trends: evidence for mantle major element heterogeneity. Earth Planet Sci Lett 58:15–26
Bryan WB, Moore JG (1977) Compositional variations of young basalts in the mid-Atlantic Ridge rift valley near lat 36°49′ N. Geol Soc Am Bull 88:556–570
Campbell IH (1978) Some problems with the cumulus theory. Lithos 11:311–323
Campbell IH (1987) Distribution of orthocumulate textures in the Jimberlana Intrusion. J Geol 95:35–54
Cann JR (1974) A model for oceanic crustal structure developed. Geophys J R Astr Soc 39:169–187
Cann JR (1981) Basalts from the ocean floor. In: Emiliani CE (ed) The oceanic lithosphere (The sea: v7). Wiley, New York, pp 363–390
Crane K (1985) The spacing of rift axis highs: dependence upon diapiric processes in the underlying asthenosphere. Earth and Planet Sci Lett 72:405–414
Detrick RS, Purdy GM (1980) The crustal structure of the Kane Fracture Zone from seismic refraction studies. J Geophys Res 85:3759–3777
Dick HJB (1989) Abyssal peridotites, very-slow spreading ridges and ocean ridge magmatism. In: Saunders AD, Norry MJ (eds) Magmatism in the ocean basins. Geol Soc Lond Spec Publ No 42, pp 71–105
Dick HJB, Fisher RL, Bryan WB (1984) Mineralogic variability of the upper most mantle along mid-ocean ridges. Earth Planet Sci Lett 69:88–106
Dickey JS, Frey FA, Hart SR, Watson EB, Thompson G (1977) Geochemistry and petrology of dredged basalts from the Bouvet triple junction, South Atlantic. Geochim Cosmochim Acta 41:1105–1118
Einarsson P, Bandsdottir B (1980) Seismological evidence for lateral magma intrusion during the July 1978 deflation of the Krafla volcano in NE Iceland. J Geophys 47:160–165
Elthon D (1987) Petrology of gabbroic rocks from the Mid-Cayman Rise spreading center. J Geophys Res 92:658–682
Elthon D, Casey JF, Komor S (1982) Mineral chemistry of ultramafic cumulates from the North Arm Mountain Massif of the Bay of Islands ophiolite: Evidence for high-pressure crystal fractionation of oceanic basalts. J Geophys Res 87:8717–8734
Engel CG, Fisher RL (1975) Granitic to ultramafic rock complexes of the Indian Ocean ridge system, western Indian Ocean. Geol Soc Am Bull 86:1553–1578
Fisher RL, Dick HJB, Natland JH, Meyer PS (1986) Mafic/ultramafic suites of the slowly spreading Southwest Indian Ridge: PROTEA exploration of the Antarctic plate boundary, 24° E–47° E. Ofioliti 11:147–178
Fisk MR (1978) Melting relations and mineral chemistry of Iceland and Reykjanes ridge basalts. PhD thesis, University of Rhode Island
Fisk MR, Schilling JG, Sigurdsson H (1980) An experimental investigation of Iceland and Reykjanes Ridge tholeiites. I. Phase relations. Contrib Mineral Petrol 74:361–374
Fox PJ, Gallo DG (1984) A tectonic model for ridge-transform ridge plate boundaries: implications for the structure of oceanic lithosphere. Tectonophysics 104:205–242
Fox PJ, Stroup JB (1981) The plutonic foundation of the oceanic crust. In: Emiliani C (ed) The oceanic lithosphere (The Sea: v7). Wiley, New York, pp 119–218
Francheteau J, Ballard RD (1983) The East Pacific Rise near 21° N, 13° N and 20° S: inferences for along-strike variability of axial processes of the mid-ocean ridge. Earth Planet Sci Lett 64:93–116
Grove TL, Gerlach DC, Sando TW (1982) Origin of calc-alkaline series lavas at Medicine Lake Volcano by fractionation, assimilation and mixing. Contrib Mineral Petrol 80:160–182
Grove TL, Bryan WB (1983) Fractionation of pyroxene-phyric MORB at low pressure: an experimental study. Contrib Mineral Petrol 84:293–309
Grove TL, Baker MB, Kinzler RJ (1984) Coupled CaAl-NaSi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry. Geochim Cosmochim Acta 48:2113–2121
Kuo L-C, Kirkpatrick RJ (1982) Pre-erilptive history of phyric basalts from DSDP Legs 45 and 46: evidence from morphology and zoning patterns in plagioclase. Contrib Mineral Petrol 79:13–27
Hamelin B, Allegre CJ (1985) Large scale regional units within the depleted upper mantle revealed by an isotope study of the Southwest Indian Ridge. Nature 315:196–199
Hebert R, Bideau D, Hekinian R (1983) Ultramafic and mafic rocks from the Garret Transform Fault near 13°30°S on the East Pacific Rise: igneous petrology. Earth and Planet Sci Lett 65:107–125
Hekinian R, Thompson G (1976) Comparative geochemistry of volcanics from rift valleys, transforms and aseismic ridges. Contrib Mineral Petrol 57:145–162
Henderson P (1968) The distribution of phosphorus in the early and middle stages of fractionation of some basic layered intrusions. Geochim Cosmochim Acta 32:897–911
Hodges FN, Papike JJ (1976) DSDP Site 334: Magmatic cumulates from oceanic layer 3. J Geophys Res 23:4135–4151
Irvine TN (1980) Magmatic infiltration metasomatism, double-diffusive fractional crystallization, and adcumulus growth in the Muskox and other layered intrusions. In: Hargraves AB (ed) Physics of magmatic processes. Princeton University Press, Princeton, pp 325–383
Irvine TN (1982) Terminology for layered intrusions. J Petrol 23:127–162
Kerr RC, Tait SR (1986) Crystallization and compositional convection in a porous medium with application to layered igneous intrusions. J Geophys Res 91:3591–3608
Kimball KL, Spear FS, Dick HJB (1985) High temperature alteration of abyssal ultramafics from the Islas Orcadas Fracture Zone, South Atlantic. Contrib Mineral Petrol 91:307–320
Klein E, Langmuir CH (1987) Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness. J Geophys Res 92:8089–8115
Kusznir NJ (1980) Thermal evolution of the oceanic crust: Its dependence on spreading rate and effect on crustal structure. Geophys J R Astr Soc 61:167–181
Langmuir CH, Bender JF (1984) The geochemistry of oceanic basalts in the vicinity of transform faults: observations and implications. Earth and Planet Sci Lett 69:107–127
Lawver LA, Dick HJB (1983) The American Antarctic Ridge. J Geophys Res 88:8193–8202
le Roex AP, Dick HJB (1981) Petrography and geochemistry of basaltic rocks from the Conrad fracture zone on the America-Antarctic Ridge. Earth Planet Sci Lett 54:117–138
le Roex AP, Dick HJB, Erlank AJ, Reid AM, Frey FA, Hart SR (1983) Geochemistry, mineralogy and petrogenesis of lavas erupted along the Southwest Indian Ridge between the Bouvet Triple Junction and 11 degrees East. J Petrol 24:267–318
le Roex AP, Dick HJB, Reid AM, Frey FA, Erlank AJ, Hart SR (1985) Petrology and geochemistry of basalts from the American-Antarctic Ridge, southern ocean: implications for the westward influence of the Bouvet mantle plume. Contrib Mineral Petrol 90:367–380
Liou JG, Ernst WG (1979) Oceanic ridge metamorphism of the East Taiwan ophiolite. Contrib Mineral Petrol 68:335–348
Lindsley DH, Anderson DJ (1983) A two-pyroxene thermometer Proc Lunar Planet Sci Conf 13th, Part 2. J Geophys Res 88 Suppl A887-A906
McBirney AR, Noyes RM (1979) Crystallization and layering of the Skaergaard intrusion. J Petrol 20:591–624
Melson WG, O'Hearn TO (1979) Basaltic glass erupted along the Mid-Atlantic Ridge between 0–37° N: Relationships between composition and latitude. In: Taiwani M (ed) Deep sea drilling results in the Atlantic Ocean: ocean crust. Maurice Ewing Series Vol 2, AGU, Washington DC, pp 249–261
Melson WG, Thompson G (1970) Layered basic complex in oceanic crust, Romanche fracture, equatorial Atlantic Ocean. Science 168:817–820
Melson WG, Thompson G (1971) Petrology of a transform fault zone and adjacent ridge segments. Phil Trans R Soc Lond A 268:423–441
Morse SA (1979) Kiglapait geochemistry II. Petrography. J Petrol 20:591–624
Miyashiro A, Shido F (1980) Differentiation of gabbros in the Mid-Atlantic Ridge near 24° N. Geochem J 14:145–154
Muir ID, Tilley CE (1964) Basalts from the northern part of the rift zone of the mid Atlantic Ridge. J Petrol 5:409–434
Mutter JC (1985) Multichannel seismic images of the oceanic crust's internal structure: evidence for a magma chamber beneath the Mesozoic Mid-Atlantic Ridge. Geology 13:629–632
Pallister JS, Hopson CA (1981) Semail ophiolite plutonic suite: field relations, phase variation, cryptic variation and layering and a model of a spreading ridge magma chamber. J Geophys Res 86:2593–2644
Papike JJ, Cameron KL, Baldwin K (1974) Amphiboles and pyroxenes: characterization of other than quadrilateral components and estimates of ferric iron from microprobe data. Geol Soc Am Abstr Progr 6:1053–1054
Perfit MR (1977) Petrology and geochemistry of mafic rocks from the Cayman Trench: evidence for spreading. Geology 5:105–110
Rhodes JM (1983) Homogeneity of lava flows: chemical data for historic Mauna Loan eruptions. Proc 13th Lunar Planet Sci Conf, Part 2, Geophys Res 88:A864
Ryan MP (1987) Neutral bouyancy and the mechanical evolution of magmatic systems. In: Magmatic processes: physicochemical principles. Geochem Soc Spec Publ 1:259–287
Schilling JG (1985) Upper mantle heterogeneities and dynamics. Nature 314:62–67
Schilling JG, Bergeron MB, Evans R (1980) Halogens in the mantle beneath the North Atlantic. Phil Trans R Soc Lond, Ser A 297:147–178
Schouten H, Klitgord KD (1982) The memory of the accreting boundary and the continuity of fracture zones. Earth Planet Sci Lett 59:255–266
Schroeder BG, Thompson G, Sulanowska M, Ludden JN (1980) Analysis of geologic materials using an automated x-ray flourescence system. X Ray Spectrom 4:198–205
Sclater JG, Bowin C, Hey R, Haskins R, Peirce H, Phillips J, Tapscott C (1976) The Bouvet Triple Junction. J Geophys Res 81:1857–1869
Sclater JG, Fisher RL, Patriat P, Tapscott C, Parsons B (1981) Eocene to Recent development of the Southwest Indian Ridge, a consequence of the evolution of the Indian Ocean triple junction. Geophys JR Astr Soc 64:587–604
Shibata T (1976) Phenocryst-bulk rock composition relations of abyssal tholeiites and their petrogenetic significance. Geochim Cosmochim Acta 40:1407–1417
Shin CY, Gast PW (1971) Rare earths in abyssal tholeiites, gabbros and their mineral separates from the Mid-Atlantic Ridge near 24° N (abs). Am Geophys Union Trans 52:376
Shido F, Miyashiro A, Ewing M (1971) Crystallization of abyssal tholeiites. Contrib Mineral Petrol 31:251–266
Sigurdsson H, Sparks RSJ (1978) Lateral magma flow within rifted Icelandic crust. Nature 274:126–130
Sparks RSJ, Huppert HE, Turner JS (1984) The fluid dynamics of evolving magma chambers. Phil Trans R Soc Lond 310:511–534
Sparks RSJ, Huppert HE, Kerr RC, McKenzie DP, Tait SR (1985) Postcumulus processes in layered intrusions. Geol Mag 122:555–568
Stolper E (1980) A phase diagram for mid-ocean ridge basalts: preliminary results and implications for petrogenesis. Contrib Mineral Petrol 74:13–27
Streckeisen A (1976) To each plutonic rock its proper name. EarthSci Reviews 12:1–33
Tait SR, Huppert HE, Sparks RSJ (1984) The role of compositional convection in the formation of adcumulate rocks. Lithos 17:139–146
Tapscott CR, Patriat P, Fisher RL, Sclater JG, Hoskins H, Parsons B (1980) The Indian Ocean Triple Junction. J Geophys Res 85:4723–4739
Thompson G (1973) Trace-element distributions in fractionated oceanic rocks 2. Gabbros and related rocks. Chemical Geology 12:99–111
Tiezzi LJ, Scott RB (1980) Crystal fractionation in a cumulate gabbro, Mid-Atlantic Ridge, 26° N. J Geophys Res 85:5438–5454
Wager LR (1963) The mechanism of adcumulus growth in the layered series of the Skaergaard intrusion. Min Soc Am Spec Pap 1:1–9
Walker D, DeLong SE (1984) A small Soret effect in spreading center gabbros. Contrib Mineral Petrol 85:203–208
Walker D, Shibata T, DeLong SE (1979) Abyssal tholeiites from the Oceanographer Fracture Zone. Contrib Mineral Petrol 70:111–125
Whitehead WM, Dick HJB, Schouten A (1984) A mechanism for magmatic accretion under spreading centres. Nature 312:146–148
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Meyer, P.S., Dick, H.J.B. & Thompson, G. Cumulate gabbros from the Southwest Indian Ridge, 54°S-7° 16′ E: implications for magmatic processes at a slow spreading ridge. Contr. Mineral. and Petrol. 103, 44–63 (1989). https://doi.org/10.1007/BF00371364
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DOI: https://doi.org/10.1007/BF00371364