Igneous petrology of Hole U1309D, IODP Expeditions 304/305 at the Atlantis Massif, MAR 30°N

IODP Expeditions 304 and 305 at the Atlantis Massif (western rift flank of the Mid-Atlantic Ridge, 30°N) intended to investigate the processes that control formation of oceanic core complexes as well as the exposure of ultramafic rocks in very young oceanic lithosphere. The domal, corrugated surface of the 0.5-2 Myr old massif is interpreted as a detachment fault exposed at the seafloor. The domal core forms the footwall to the detachment fault system, and was the main focus of drilling at Hole U1309D (bottom: 1416 mbsf, recovery: 75%). Over 96% of Hole U1309D is made up of gabbroic lithologies, which comprise amongst the most primitive as well as freshest plutonic rocks known from the ocean floor. Impregnated mantle peridotites, now heavily serpentinized, constitute only 0.3% of recovered sequence and are restricted to four thin lenses in the uppermost 200 m. Subhorizontal diabase sheets of variable thickness (0.01 - 10 m) with chilled margins mainly occur in the uppermost 130 m of the hole (2.9%), but isolated thin dikes are also found at greater depths. Medium-grained to pegmatitic gabbros and gabbronorites form the most abundant rock types (56%). Both grain size and modal composition can vary strongly on a dm-scale, and commonly include minor quantities of olivine (rarely exceeding 10%), Fe-Ti-oxides, and/or orthopyroxene. Intercalated with these gabbros are medium-grained olivine gabbros (28%), which have an olivine mode that nearly always exceeds 20%. Their modes are highly variable on a submeter scale as well and they frequently grade into troctolitic gabbro and troctolite. Olivine-rich troctolite with more than 70% olivine is very rare at mid-ocean ridges, but comprises 5.4% of the recovered section. In contrast to troctolite, it has a cumulate-like texture with subhedral to rounded olivine and interstitial to poikilitic plagioclase and clinopyroxene in variable proportions. Another special but more common lithology is oxide gabbro (7.7% of the rocks recovered), defined by the occurrence of more than 2% modal Fe-Ti-oxide minerals. The oxide-bearing gabbros are most common within the gabbro sequence, and the Fe-Ti-oxides occur as heterogeneously distributed patches or seams. The oxide gabbros can occur as discrete dikelets crosscutting other lithologies, or display strong enrichment in ductile deformation zones. These lithologies appear to repeat in two separate and distinct intrusive packages of several hundred meters thickness each, with similar lithologic sequences comprising each. Where visible, the contact relationships suggest that gabbro is generally intrusive into olivine-rich lithologies (olivine gabbros and troctolites) and is itself intruded by zircon- and apatite-bearing felsic (``leucocratic'') dikes and oxide gabbros. We note also that the upper ~60 m of gabbros in Hole 1309B, located 20 m away from 1309D and cored to only 101.8 mbsf, do not correlate with this interval in Hole 1309D, although later stage diabase units do correlate between Holes.