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

Abstract

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.