ICDP Oman Drilling Project: varitextured gabbros from the dike–gabbro transition within drill core GT3A

Abstract. The Oman ophiolite (Samail massif, Sultanate of Oman) is the
largest sub-aerial exposure of oceanic lithosphere on Earth and provides the
opportunity to study the accretion and alteration of oceanic lithosphere
formed under fast-spreading conditions. Drill hole GT3A (23∘06′50.7′′ N, 58∘12′42.2′′ E) of the ICDP (International Continental
Scientific Drilling Program) Oman Drilling Project with a length of 400 m
aimed at penetrating the dike–gabbro transition of the Samail ophiolite
paleocrust in order to shed light on the role of the axial melt lens (AML)
during accretion of the lower plutonic crust. AMLs beneath fast-spreading
mid-ocean ridges are sandwiched between the sheeted dike complex and the
uppermost gabbros and are believed to feed the upper crust and, at least
partially, the underlying crystal mush. Typical gabbroic rocks from dike–gabbro transitions of fast-spreading
systems are the so-called “varitextured gabbros”, often showing
considerable variations in mineral mode, texture and grain size, which are
regarded as the frozen fillings of axial melt lenses. Here, we present a
detailed petrographic, microanalytical and bulk-chemical investigation of 36
mafic rocks from the drill hole GT3A, which represent mostly varitextured
gabbros, revealing a complex formation with several evolution stages.
Poikilitic domains formed first, corresponding to an early crystallization
stage, where only plagioclase and clinopyroxene of more primitive
composition crystallized. Later, domains of granular textures containing
also interstitial amphibole and Fe–Ti oxide were formed. This stage is
characterized by a magma evolution that underwent crystal fractionation
established by lower temperatures due to more efficient hydrothermal
cooling at the margin of the AML. A last stage is characterized by pervasive
hydrothermal alteration, where all primary minerals have been altered under
temperature conditions, varying from the magmatic regime down to greenschist
facies. A highlight of this stage is amphiboles showing noticeable
compositional zoning. The observation of peculiar microgranular domains,
representing relics of stoped exogenic material from the sheeted dike
complex, documents the upward migration of an AML in a replenishment event,
forcing the AML to burn through previously altered sheeted dikes. This
process is responsible for significant assimilation of hydrothermally
altered components, indicated by a marked Cl enrichment in the outer zones
of magmatic amphiboles. Petrological modeling involving gabbros and basalts
revealed that the GT3A rock suite followed a fractional crystallization
evolution trend, with a primitive MORB as parental melt with an estimated
water content of 0.2 wt % to 0.8 wt %. The modeled liquid lines of descent
suggest a magmatic evolution via fractional crystallization, where the
basalts correspond to frozen liquids, while the gabbros, especially the more
primitive ones, show a significant cumulate component.
