Appendix C: Petrographic Descriptions of Sampled Nelchina Area Map Units

Peninsular Terrane

Gabbro, undifferentiated (Jgu) – The bulk of the outcrop is fractured, massive, light- to dark-

grey coarse-grained gabbro with a foliated appearance (Figure 3A). In thin section, gabbro

shows blocky subhedral to anhedral grains of hornblende, with fine-grained quartz in small

patches between larger grains. There is no mineral flattening or stretching, but feldspar is

twinned, and bent, so we interpret the “foliation” to be igneous flow banding as demonstrated by

Burns (1996) for mafic layered gabbronorites at the southern end of the Nelchina River

gabbronorite complex. The gabbro is intermingled with black, finer-grained bands that have no

fabric alignment in outcrop or thin section, but are more densely fractured and cut by calcite

veins (Figure 3A, 3B). The blacker, finer-grained zone shows ~80% amphibole in thin section

(Figure 3C). The amphibole is anhedral and nearly pristine, with minor chlorite alteration along

cleavage planes and grain fractures (Figure 3C). Its composition was determined by XRD to be

ferro-pargasite. Plagioclase, making up ~10% of the rock, is pervasively altered to zoisite and

white mica. Apatite and zircon are accessory phases, along with ilmenite mantled by titanite

(Figure 3C). Pyrite occurs as small disseminated grains. Zircon U-Pb age of the amphibolite is

189 Ma, and 40Ar/39Ar ages of amphibole from both the amphibolite and gabbro are 183 Ma.

Ages and field relations place both with the Border Ranges Ultramafic Mafic Complex

(BRUMC) of the Jurassic Talkeetna arc (Burns, 1985).

Tonalite dikes

Massive, blocky, pale grey to white dikes, up to 5m across, cut the gabbro in the northern part of

the field area. Margins are sharp and chilled. They appear as described by Burns (1996) as quartz

diorite. Thin sections reveal the dikes are made up primarily of anhedral, twinned plagioclase

and quartz with undulatory extinction. Plagioclase (An25-35) is partly altered to sericite, and all

mafic minerals have altered to chlorite. Based on mineral content, we classify our single sample

as tonalite.

Chugach Terrane

Metasedimentary rocks (Mzms) – Black very fine-grained metasediments, including white-gray

chert, green tuff, and rare limestone with a generally east–west striking foliation that parallels the

local trace of the Border Ranges fault. In thin section, black mudstone contains very fine- to fine-

grained quartz; very fine-grained calcite, mica, and graphite; and minor albite and chlorite.

Blocks include white-gray chert with preserved ribbons, including one block ~10m wide that is

folded. Bedding is preserved in the chert (Supp. Figure 1), and is discordant to the general

foliation. Green tuff blobs from 2cm up to 1m wide are present as wisps and thin layers swirled

in with the chert and black mudstone (Figure 3D). Limestone is rare in our field site. Joints and

fractures throughout Mzms are filled with quartz and calcite. Disseminated sulfides include

pyrite and chalcopyrite, which are visible in outcrop and hand sample, and cause orange staining.

Supplementary Figure 1. Chert block in Mzms (Mesozoic metasediment) showing disrupted primary bedding. (flower for scale)

Metavolcanic rocks (Mzmv) – Tracts of metavolcanics rock are non-uniformly distributed

throughout the McHugh Complex in this location, and are likely related genetically (see

geochemistry section). They are distinguished from Mzms by their bright green color (Supp.

Figure 2, top). Blocks of Mzmv contain cm-scale volcanic clasts. In places, the meta-basalt

preserves metamorphic fabric of chlorite and secondary albite defining foliation (Supp. Figure

2, bottom).

An ~100 m wide block of pillow lava is preserved within this assemblage (Mzpb of Figure 2) as

a less-altered “core” within an ~250 m-wide metavolcanic block. In some spots, individual

pillows are up to 1 m wide (Figure 3G). The rinds of the pillows are green, while the interiors

are black and less altered. Outcrop evidence of alteration throughout these rocks (and the entire

Supplementary Figure 2. Photos of Mzmv (Mesozoic metavolcanics). TOP: outcrop is green, typically with orange staining, with cm-scale clasts of volcanic material. BOTTOM: chlorite (pale blue-green swaths) and secondary albite (white needle-like grains) define foliation in a cross-polarized thin section photo of a sample of meta-basalt.

McHugh Complex) includes disseminated pyrite and calcite stringers, and quartz and calcite

veins cross-cutting foliation.

In the undeformed pillow lava, thin sections (Supp. Figure 3) reveal glass altered to chlorite,

and plagioclase laths up to 1mm long that are preserved in random orientations. Most plagioclase

is secondary albite, but some grains have been replaced by variably sized sericite and quartz,

which are sometimes intergrown with chlorite. Amygdaloidal voids are filled by calcite, chlorite,

and epidote (Supp. Figure 3).

Away from the core of the block, pillow structures become progressively more obscure until they

are entirely absent, however, geochemical data (Table 1) indicate that these all come from a

within-plate setting.

Mesomélange (Mzm) – This unit is made of complexly intermingled Mzms and Mzmv, with

blocks generally smaller than 10m. The boundaries between the blocks are commonly finely

laminated mudstone, but some blocks are in direct contact. The lamination in the mudstone

Supplementary Figure 3.

indicates mineral compaction and alignment and possible flow. In thin section, the tuffaceous

material is made up of rolled, sometimes broken grains of albite and quartz, with chlorite and

white mica growing around the porphyroclasts and defining foliation (Supp. Figure 4).

Sheared mélange (Mzs) – In outcrop, this assemblage is differentiated by its strong alignment

(Figure 3J) subparallel to the trace of the BRF. Lithologically it is most similar to Mzms, as it

contains both mudstone and green tuff. In Mzs, however, relict quartz clasts have flattened grains

with undulatory extinction (Supp. Figure 5), there is pervasive mineral fracturing, and black

anhedral material is present along foliation planes.

Supplementary Figure 4. Thin-section photos of tuff layer from Mzm (Mesozoic mesomélange). TOP: View in plane-polarized light, showing rounded, white to light-pink feldspar (albite) grains mantled by pale green chlorite that is aligned to define the foliation. BOTTOM: Same thin section view as above, shown in cross-polarized light. White mica (higher interference colors) is visible with the chlorite, and both are aligned. The albite grains appear to be porphyroclasts that rolled and in some cases broke during the metamorphism/deformation.

Cross-Cutting Dikes of Sanak–Baranof Belt

Hypabyssal (“felsite”) dikes (Pgi) – Three felsic hypabyssal dikes crosscut the mélange in the

map area. They range from ~25 m to ~150 m wide, are undeformed (lack mineral alignment) but

altered, and weather to orange-pink (Supp. Figure 6). Phenocrysts of albite, which make up

~10% of the rock, are 3–4 mm long and almost fully altered to sericite and calcite. The gray,

aphanitic groundmass consists primarily of quartz, feldspar, and white mica. Clusters of

randomly oriented chlorite, white mica, and oxides are likely alteration products of previous

mafic minerals. Although compositionally these dikes fall into the dacite field (Figure 6, sample

16Ba02), we adopt the classification of “felsite” by Burns et al. (1991) because the altered state

Supplementary Figure 5. Photomicrograph in cross-polarized light of a quartz clast from Mzs (Mesozoic sheared McHugh) showing flattened , aligned, and undulose quartz grains indicative of low-grade ductile deformation. Black anhedral material is aligned parallel to flattened grains from the lower left through the center of the slide.

of plagioclase makes their original compositions impossible to determine. One dike yielded a 53

Ma zircon U-Pb age.

Supplementary Figure 6. Photos of felsite dikes that cross-cut the McHugh mélange. (Left) Field photo showing pink-orange weathering of an ~5-m-wide dike cutting unit Mzm (Mesozoic mesomélange). (Right) Photomicrograph in cross-polarized light showing alteration of the undeformed dike material.