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doi:10.2204/iodp.proc.345.106.2014

Igneous petrology

Coring in Hole U1415E recovered 16 different igneous lithologic intervals from a surficial “rubble” zone, which was 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. Two different lithologic domains were defined one thin section from Hole U1415E. Therefore, a table is provided listing the corresponding thin section, the number and nature of the individual domains, the characteristics of the contact between the domains, as well as a link for the corresponding image of the thin section with the domain boundaries marked (Table T2).

Gabbro

Gabbro defines five intervals (Intervals 6, 9, 10, 12, and 13) from Unit I. In general, the gabbros are medium grained with equigranular to subophitic textures. Modally, gabbro consists of plagioclase (35%–65%) and clinopyroxene (35%–65%), with trace amounts of oxide. Plagioclase is medium grained and subhedral to euhedral with a lath-shaped habit. Clinopyroxene is medium grained and anhedral with an interstitial habit.

Olivine-bearing gabbro

Olivine-bearing gabbro defines three intervals (Intervals 1, 7, and 15) from Unit I. In general, the olivine-bearing gabbro is a medium-grained, equigranular to subophitic granular rock. Modally, olivine-bearing gabbro consists of olivine (2%–3%), plagioclase (60%–72%), and clinopyroxene (25%–40%), with trace amounts of oxide and occasionally trace orthopyroxene. Olivine is fine grained and subhedral to euhedral with a subequant habit. Plagioclase is fine grained and subhedral to euhedral with a tabular habit. Zoning in plagioclase is scarce. Clinopyroxene is fine to medium grained and anhedral with an interstitial habit.

Olivine gabbro

Olivine gabbro defines two intervals (Intervals 14 and 16) in Unit I. In general, olivine gabbro is medium grained equigranular granular rock. Modally, olivine gabbro consists of olivine (15%–20%), plagioclase (50%–70%), and clinopyroxene (10%–35%), with trace amounts of oxide. Olivine is fine grained and euhedral to subhedral with a subequant habit. Plagioclase is medium grained and subhedral to euhedral with a tabular habit. Clinopyroxene is medium to coarse grained with an anhedral interstitial habit.

Gabbronorite

Gabbronorite defines two intervals (Intervals 3 and 5) in Unit I. In general, gabbronorite is medium grained and inequigranular with seriate to subophitic textures (Fig. F2). Modally, gabbronorite consists of plagioclase (35%–60%), clinopyroxene (18%–20%), and orthopyroxene (20%–35%), with trace (≤2%) oxide. Plagioclase is medium grained and subhedral to euhedral with a tabular habit. Clinopyroxene is medium grained and anhedral with an irregular interstitial to poikilitic habit. Orthopyroxene is anhedral with an irregular poikilitic habit (Fig. F3).

One gabbronorite sample (345-U1415E-1R-1, 21–23 cm) shows a patchy appearance resulting from the presence of two lithologically different domains consisting of orthopyroxene-bearing gabbro and disseminated oxide gabbronorite (Fig. F4; lithologies of the two domains are based on thin section descriptions). This feature was not observed in any other gabbro in Hole U1415E. A characteristic feature is the presence of orthopyroxene, oxide minerals and probably primary amphibole (now nearly completely altered to secondary amphibole), and oxide inclusions in plagioclase of the disseminated oxide gabbronorite, implying that this lithology is rather evolved compared to most other gabbroic lithologies recovered at Site U1415.

Troctolite

Troctolite occurs as a highly altered single piece in Hole U1415E (Interval 4; Fig. F5). The troctolite is a medium-grained equigranular rock. Modally, the troctolite consists of olivine (40%), plagioclase (45%), and clinopyroxene (15%), with trace amounts of oxide. Olivine is anhedral and completely altered. Plagioclase is anhedral to subhedral with a tabular habit. Clinopyroxene occurs as subhedral to euhedral prismatic grains.

Orthopyroxene-bearing olivine gabbro

Orthopyroxene-bearing olivine gabbro defines one interval (Interval 2) in Unit I. The highly altered orthopyroxene-bearing olivine gabbro is a medium-grained equigranular subophitic rock. Modally, orthopyroxene-bearing olivine gabbro consists of olivine (5%–7%), plagioclase (50%), clinopyroxene (40%), and orthopyroxene (3%), with trace amounts of oxide. Olivine is fine grained and euhedral to subhedral with subequant habit. Plagioclase is medium grained and euhedral to subhedral with a tabular habit. Clinopyroxene is medium grained and anhedral with a subequant habit. Orthopyroxene is euhedral to subhedral with an equant habit.

Anorthositic gabbro

Anorthositic gabbro defines a single interval (Interval 8) in Unit I (Thin Section 4; Sample 345-U1415E-2R-1, 0–3 cm). The highly altered anorthositic gabbro is a medium-grained equigranular granular rock. Modally, anorthositic gabbro consists of plagioclase (90%) and clinopyroxene (10%). Plagioclase is medium grained and subhedral with a tabular habit. Clinopyroxene appears to be medium grained and anhedral with an interstitial habit.

Doleritic gabbro

Doleritic gabbro defines a single interval (Interval 11) in Unit I. Doleritic gabbro is a fine-grained equigranular ophitic rock. Modally, doleritic gabbro consists of plagioclase (50%) and clinopyroxene (50%), with trace amounts of oxide. Plagioclase is fine grained and euhedral to subhedral with a lath-like habit. Clinopyroxene is fine grained and anhedral with an interstitial to ophitic habit.

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