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doi:10.2204/iodp.proc.335.204.2016

Introduction

Ocean Drilling Program (ODP) Hole 1256D is the first complete penetration of the upper oceanic crust reaching the uppermost part of the gabbroic lower crust. Hole 1256D was drilled between ODP Leg 206 (Shipboard Scientific Party, 2003) and Integrated Ocean Drilling Program Expeditions 309 and 312 (Expedition 309/312 Scientists, 2006) and 335 (Expedition 335 Scientists, 2012). Hole 1256D is located in 3635 m of water in the Guatemala Basin (6°44.2′N, 91°56.1′W) on 15 Ma oceanic crust of the Cocos plate, which formed along the East Pacific Rise at a full spreading rate of >200 mm/y (Wilson, 1996). Below a 250 m thick section of sedimentary cover, 810 m of lavas and 346 m of sheeted dikes were penetrated with recovery between 5% and 30%. The first gabbroic rocks were encountered at 1407 meters below seafloor (mbsf) during Expedition 312. The recovered crustal section is characterized by a 115 m thick dike–gabbro transition zone that includes two ~52 and ~12 m thick gabbro intervals intruded into diabase dikes with granoblastic textures (Fig. F1) (Wilson et al., 2006; Expedition 335 Scientists, 2012; Ildefonse et al., 2014).

The gabbroic rocks from the two gabbro intervals (Gabbros 1 and 2; Fig. F1) are isotropic, referred to as “varitextured,” and are similar to those described at the same level of the crustal section exposed in the Oman ophiolite (e.g., MacLeod and Yaouancq, 2000; France et al., 2009). These gabbros potentially represent the top of the crystallized axial melt lens (Wilson et al., 2006; Koepke et al., 2008, 2011; France et al., 2009; Alt et al., 2010). The gabbroic bodies have been interpreted as relatively small intrusive bodies rooting into a larger melt lens below (Koepke et al., 2008; Alt et al., 2010). Another interpretation is the gabbroic bodies represent the main melt lens in which the granoblastic diabases between Gabbros 1 and 2 and those below Gabbro 2 represent large stoped enclaves of sheeted dikes (France et al., 2009).

We present here an analysis of the crystallographic-preferred orientations (CPO) of the most abundant primary silicate phases (plagioclase and clinopyroxene) in 9 samples from the Gabbro 1 and 2 intervals to document the variability of their magmatic fabrics. Seven of the nine measured samples (Table T1) were also used by Trela et al. (2015) for shape-preferred orientation (SPO) measurements of plagioclase. The samples used in this study are from 1412.99–1494.36 mbsf in Cores 312-U1256D-214R through 232R. All samples have textures that are essentially isotropic, with tabular plagioclase, subophitic clinopyroxene, minor orthopyroxene, Fe-Ti oxides, and olivines. Secondary amphibole is abundant. Some samples (e.g., 312-U1256D-231R-1, 92–94 cm) have anorthositic domains characterized by lower proportions of clinopyroxene and smaller grain size than the surrounding rock. Other samples contain veins filled with chlorite (e.g., 220R-1, 14–17 cm) and/or amphibole (e.g., 223R-2, 7–9 cm).

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