The plutonic foundation of a slow-spreading ridge

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doi: 10.1029/GM070p0001
Author(s): Dick, Henry J. B.; Robinson, Paul T.; Meyer, Peter S.
Author Affiliation(s): Primary:
Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA, United States
University of Michigan, United States
University of Wales College of Cardiff, United Kingdom
Bundesanstalt für Geowissenschaften und Rohstoffe, Federal Republic of Germany
Lamont-Doherty Geological Observatory of Columbia University, United States
Dalhousie University, Canada
Volume Title: Synthesis of results from scientific drilling in the Indian Ocean
Volume Author(s): Duncan, Robert A., editor; Rea, David K.; Kidd, Robert B.; von Rad, Ulrich; Weissel, Jeffrey K.
Source: Synthesis of results from scientific drilling in the Indian Ocean, edited by Robert A. Duncan, David K. Rea, Robert B. Kidd, Ulrich von Rad and Jeffrey K. Weissel. Geophysical Monograph, Vol.70, p.1-39. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0065-8448. ISBN: 978-1-118-66803-0 CODEN: GPMGAD
Note: In English with English summary. 73 refs.; illus., incl. strat. cols., 1 table, sketch map
Summary: Hole 735B drilled 500 m of gabbroic layer 3 at the SW Indian Ridge. The section consists of small intrusions with no evidence for a large, steady-state magma chamber. The complex stratigraphy represents multiple phases of magmatism, alteration, and ongoing deformation in the zone of lithospheric necking and crustal accretion beneath a slow-spreading ocean ridge. Magma evolution was by fractional crystallization of intercumulus melt in crystal mush, by melt-rock reaction and wall rock assimilation as batches of melt migrated upward through the crust. The major form of igneous layering, undeformed olivine gabbro cut by numerous layers of sheared ferrogabbro, formed by synkinematic differentiation in deforming, partially molten gabbro. This process drove late intercumulus melt into shear zones, where reaction, crystallization, oxide precipitation and melt trapping transformed the rock to ferrogabbro. The section underwent extensive brittle-ductile deformation, with shear zones forming while the section was still partially molten, under anhydrous granulite conditions, and at higher water-rock ratios in the amphibolite facies. The shear zone is extensively controlled both magmatic and subsolidus fluid flow and alteration. Alteration and circulation of seawater was tectonically enhanced, with extension and lithospheric necking superimposed on the dilational thermal stress available for cracking, resulting in high permeabilities. Alteration decreased abruptly in mid-amphibolite facies with the end of brittle-ductile deformation as the section was transferred into the rift valley wall and the zone of block uplift. Alteration conditions then closely resembled those of the statically cooled Skaergaard intrusion, with diopside replacing amphibole as the principal mafic hydrothermal vein mineral. This is attributed to low permeability and more reacted fluid with cracking driven only by thermal dilation during static cooling. Late trondhjemite veins formed by fractional crystallization and by wall rock anatexis of amphibolites during reintrusion of the section. This provides direct evidence that the section underwent multiple alteration and magmatic events as magmatism waxed and waned.
Year of Publication: 1992
Research Program: DSDP Deep Sea Drilling Project
ODP Ocean Drilling Program
Key Words: 05 Petrology, Igneous and Metamorphic; 18 Geophysics, Solid-Earth; Amphibolite facies; Anatexis; Atlantis II fracture zone; Body waves; Brittle deformation; Deep Sea Drilling Project; Deformation; Diorites; Ductile deformation; Elastic waves; Facies; Fractional crystallization; Gabbroic composition; Igneous activity; Igneous rocks; Indian Ocean; Intrusions; Magmas; Melts; Metamorphic rocks; Mid-ocean ridges; ODP Site 735; Ocean Drilling Program; Ocean floors; P-waves; Plutonic rocks; Plutons; S-waves; Seismic waves; Shear zones; Southwest Indian Ridge; Trondhjemite; Velocity structure
Coordinates: S324327 S324318 E0571618 E0571557
Record ID: 2007093153
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute.