amphibolite

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Jonna Reamer May 6th, 2010

Amphibolite Report Gel 323

INTRODUCTION-

Off of route 9N in the northeastern Adirondacks near Jay, NY there is an outcrop exposed

containing an amphibolites dike within a hybrid gneiss, a sill and another gabbroic

anorthosite gneiss section.

FIELD SITE DESCRIPTION- route 9N near Jay, NY

This is a large outcrop consisting of 3 major rock groups, separated from top to bottom.

The middle layer is clearly a sill separating the other layers. There is very little obvious

foliation and the rock is black grey and white. There are some small blebs of an amphibolite

dike, which contains garnet, trailing though the lower calc-silicate gneiss with hornblende

amphibole layers in it (see figure 1). The sill is a metagabbro consisting of hornblende,

biotite, orthopyroxene, clinopyroxene, garnet, and plagioclase. The upper layer is gabbroic

anorthosite gneiss containg mostly clinopyroxene and plagioclase. The sill has a strike and

dip of 191/30. There are some slickenslides present, indicating faulting activity. There

are large areas of prehnite and calcite found in the lowest of the three sections and some

smaller areas where pyrite and garnet can be found (see figure 2).


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Figure 1: The outcrop as a whole. All three layers are present. The amphobolite dike is

clearly present.

Figure 2: Calcite.


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Thin section displays 30% mostly brown subidioblastic to xenoblastic hornblende grains.

Occasionally the hornblende may be found intergrown with opaques. The hornblende

grains tend to be smaller than some of the other minerals present averaging about o.5mm

to 1mm in diameter. Also present is 20% clinoproxene (augite) displaying 2blue/indigo

interference colors (see figure 4). These grains are larger than the hornblende averaging

2mm in diameter. The grains are xenoblastic, though a few of subidioblastic quality may be

found. Large quartz porphyroblasts make up 15% and are all xenoblastic. The

porphyroblasts can be up to 5mm in diameter but average about 4mm (see figure 5). Also

making up 15% of the slide is plagioclase with An 35, making it andesine. These are also

found in porphyroblasts (see figure 6) and are also xenoblastic. These porphyroblasts

average 2mm in diameter. Of a slightly lesser content at 10% is sphene. The grains are

xenoblastic and scattered throughout the slide. A platy stongly pleochroic mineral

identified as biotite makes up 7%. The grains are xenoblastic and small, averaging 1mm in

diameter. Lastly, 3% of the thin section is represented by opaques. They are xenoblastic

and unidentifiable in think section.

A representative field of view is included in figure 7.


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Figure 4: Clinopyroxene grain at 5x magnification.

Figure 5: large quartz porphyroblast at 5x magnification.


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Figure 6: large andesine plagioclase porphyroblast at 5x magnification.

Figure 7: 5x magnification

DISCUSSION/COMPARISON-

Though the hand specimen and thin section show no evidence of foliation, the outcrop in

certain areas does and thus identifies this as an amphibolites. The influence of the sill can

be seen in the unit due to the presence of sphene and biotite in thin section. Jaffe identified

this as a Sphene-Augite-Andesine Calc-Silicate Gneiss which does concur with what was


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found through investigation of the thin section, though less sphene than was indicated by

the literature was actually found in the thin section.

This being an amphibolites, the large presence of both pyroxene and plagioclase were

expected and indeed found at 20% and 15%, respectively. The An content of the plagioclase

at 35 (Andesine) agrees with what is normally seen in the Adirondack region, and therefore

comes as no surprise.

REFERENCES CITED-

Jaffe, E. B., Jaffe, H. W. Grenville Calc-silicate, anorthosite, gabbro,a nd iron-rich syenitic

rocks from the Northeastern Adirondacks. (p. 107-129) within Field Trip Guidebook. New

York State Geological Association. Editor Olmsted, J. F.

Shelley, D. Igneous and Metamorphic Rocks Under the Microscope: Classification, Textures,

Microstructures and mineral preferred orientations. Chapman and Hall (New York: 1993).

Whiteny, P.R., and Olmsted, J. F. (1993) Bedrock Geology of the Ausable Forks Quadrangle,

Northeastern Adirondack Mountains, New York.

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