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Journal of Petrology | Volume 43 | Number 5 | Pages 885-905 | 2002
© Oxford University Press 2002
Gabbroic Pegmatite Intrusions, Iberia Abyssal Plain, ODP Leg 173, Site 1070: Magmatism during a Transition from Non-volcanic Rifting to Sea-floor Spreading
1VIRGINIA MUSEUM OF NATURAL HISTORY, 1001 DOUGLAS AVENUE, MARTINSVILLE, VA 24112, USA
2DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF NORTH CAROLINA, CHAPEL HILL, NC 27599, USA
3DEPARTMENT OF GEOLOGICAL SCIENCES, VIRGINIA TECH, BLACKSBURG, VA 24061, USA
On the Iberia Abyssal Plain (Ocean Drilling Program Site 1070), gabbroic pegmatites and related rocks (127 ± 4 Ma, U–Pb zircon) intrude upper mantle that was subsequently exposed and serpentinized during Early Cretaceous non-volcanic rifting. The pegmatites include a 3–4 m dike or sill (the ‘main’ pegmatite), numerous dikelets of 1–5 cm thickness, and clasts within the overlying ophicalcite breccia. Exclusive of rodingitization, the main pegmatite contains 40–70% calcic andesine, 25–35% kaersutitic amphibole (Mg# 60–70), 5–25% augite (Mg# 70–80) and 1–2% ilmenite. The dikelets are more magnesian (Mg# up to 82 in kaersutite and 88 in augite). Most indications are that the high Mg#s in the dikelets reflect igneous compositions. Isotopic and elemental chemistry indicate that the pegmatite-forming melt was enriched in incompatible elements relative to normal mid-ocean ridge basalt, but not as enriched as Azores basalts. The amphibole-bearing plagioclase peridotites of the Iberia Abyssal Plain are an appropriate source for the pegmatite melts. A combination of decompression accompanying unroofing and heating from the upwelling asthenosphere beneath the developing rift caused P–T conditions in the amphibole-bearing lithosphere to exceed the dehydration-melting solidus (
1050°C), producing small-volume, enriched, hydrous melts. Pegmatite intrusion pre-dates unroofing at Site 1070 and post-dates syn-rift sedimentation and faulting of serpentinite seen inboard on the Iberia Abyssal Plain. Thus, serpentinization and unroofing were time-transgressive and the age of the non-volcanic sea floor formed by the unroofed mantle grows younger outboard, just as is the case for normal, volcanic sea floor.
KEY WORDS: abyssal plain; gabbro; Iberia; pegmatites; rifting
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