Previous   |    Next

doi:10.2204/iodp.proc.345.108.2014

Igneous petrology

Coring in Hole U1415H recovered seven different igneous lithologies from 10 intervals from a surficial “rubble” zone, which were collectively defined as lithologic Unit I (Fig. F1). Below we give a brief lithologic description for each rock type recovered in the different intervals based on macroscopic and, where available, microscopic observations.

Olivine gabbro

Olivine gabbro defines two intervals (Intervals 9 and 10) in Unit I in Hole U1415H (Thin Section 10; Sample 345-U1415H-1R-1, 51–53 cm [Piece 10]). The olivine gabbro is a medium-grained equigranular granular rock (Fig. F2) that consists of olivine (10%–18%), plagioclase (60%–65%), and clinopyroxene (17%–30%), with trace amounts of oxide. Olivine is fine grained and subhedral to anhedral with an irregular amoeboid habit. Plagioclase is medium grained and subhedral to euhedral with a tabular habit. Clinopyroxene is medium to coarse grained and anhedral with an interstitial to poikilitic habit. In one sample (345-U1415H-1R-1, 51–53 cm [Piece 10]), olivine also forms poikilitic crystals (Fig. F3), a feature that becomes characteristic in gabbro in Hole U1415J.

Olivine-bearing gabbro

Olivine-bearing gabbro defines one lithologic interval (Interval 1). A thin section was cut (Thin Section 7; Sample 345-U1415H-1R-1, 0–4 cm [Piece 1]) for microscopic observations. The olivine-bearing gabbro is medium-grained equigranular granular rock that is moderately to highly altered. Modally, the olivine-bearing gabbro consists of olivine (2%), plagioclase (60%), and clinopyroxene (38%). Olivine is fine grained and subhedral to euhedral with a subequant habit. Plagioclase is medium grained and subhedral to euhedral with a tabular habit. Clinopyroxene is medium grained and anhedral with an interstitial habit.

Orthopyroxene-bearing olivine gabbro

Orthopyroxene-bearing olivine gabbro defines two intervals (Intervals 3 and 8). In general, orthopyroxene-bearing olivine gabbro is medium-grained equigranular granular rock. Interval 3 is foliated. Modally, orthopyroxene-bearing olivine gabbro consists of olivine (6%–7%), plagioclase (70%), clinopyroxene (20%), and orthopyroxene (4%), with trace amounts of oxide. Olivine is fine grained and euhedral to anhedral with a subequant habit. Plagioclase is medium grained and euhedral to subhedral with a tabular habit. Clinopyroxene is medium to coarse grained and subhedral to anhedral with an interstitial to oikocrystic habit. Orthopyroxene is euhedral to subhedral with a prismatic habit.

Gabbro

Gabbro defines two intervals (Intervals 5 and 6) and is a medium-grained equigranular (Interval 5) or seriate poikilitic (Interval 6) granular rock. Modally, gabbro consists of plagioclase (50%–70%) and clinopyroxene (30%–50%), with trace amounts of olivine and oxide. Plagioclase is fine grained and subhedral to euhedral with a tabular habit. Clinopyroxene is medium to coarse grained and anhedral with an interstitial to oikocrystic habit.

Orthopyroxene-bearing gabbro

Orthopyroxene-bearing gabbro defines one interval (Interval 7). A thin section was cut (Thin Section 9; Sample 345-U1415H-1R-1, 33–36 cm [Piece 7]) for microscopic observations. Orthopyroxene-bearing gabbro is medium-grained inequigranular ophitic rock. Modally, orthopyroxene-bearing gabbro consists of plagioclase (50%), clinopyroxene (44%), orthopyroxene (4%), and olivine (2%). Plagioclase is medium grained and subhedral with a tabular habit. Clinopyroxene is anhedral with an irregular to subophitic habit. Orthopyroxene is subhedral to subhedral with a prismatic habit.

Olivine-bearing gabbronorite

Olivine-bearing gabbronorite defines one interval (Interval 4). A thin section was cut (Thin Section 8; Sample 345-U1415H-1R-1, 18–22 cm [Piece 4]) for microscopic observations. Olivine-bearing gabbronorite is medium-grained equigranular granular rock (Fig. F4). Modally, the olivine-bearing gabbronorite consists of olivine (3%), plagioclase (50%), clinopyroxene (35%), and orthopyroxene (12%), with trace amounts of oxide. Olivine is fine grained and anhedral with an irregular amoeboid habit. Plagioclase is medium grained and subhedral to euhedral with a tabular habit, showing a marked zoning in part exhibiting ghostly indistinct zonation (Fig. F5). Clinopyroxene is medium grained and euhedral to subhedral with a tabular to oikocrystic habit. Orthopyroxene is subhedral to euhedral with a subequant habit.

Anorthosite

Anorthosite comprised one interval (Interval 2). The rock is coarse grained and granular with a seriate grain size distribution. Plagioclase comprises 100% of the rock and is euhedral to subhedral with a tabular habit. Trace amounts of orthopyroxene and oxide are present.

Top of page   |    Previous   |    Next