PETROLOGY OF THE MARBLE CAKE OUTCROP, LONG LAKE, NEW YORK

The “Marble Cake” outcrop, which derives its name from the swirled appearance of alternating dark mafic-rich and white feldspar-rich layers, is located on Route 30, between Tupper Lake and Long Lake, NY. The outcrop strikes SSE, dipping steeply WSW.

Three rock units comprise the “Marble Cake” outcrop. These are a diopside-rich marble to the north, a “swirled” banded hornblende-feldspar gneiss in the middle and a potassium feldspar-garnet gneiss to the south. The marble is typical of marble found throughout the Adirondacks, consisting of coarse calcite with abundant green diopside and brown sphene. Somewhat unusual is the size of some of the clumps of diopside grains, up to 4 cm in diameter. The marble occurs as zenoliths in the banded gneiss. The swirled, banded gneiss contains cms thick dark bands of hornblende-plagioclase gneiss interlayered with light bands of perthitic potassium feldspar gneiss. Less common are bands of diopside-hornblende-garnet-plagioclase gneiss. Some potassium and plagioclase feldspar grains are up to 10 cm in diameter and are relict igneous phases. A few hornblendes have altered cores that we interpret to have been orthopyroxene. The southern part of the outcrop consists of less foliated pink potassium feldspar-garnet gneiss with significantly lesser amounts of biotite, hornblende, plagioclase, diopside and quartz. Garnets are commonly 1 cm in diameter, with some up to 4 cm. 120^ograin boundaries in both gneiss units are indicative of recrystallization during metamorphism and indicate that most igneous textures have not been preserved. Locally, myrmekitic textures of inter-grown plagioclase and quartz are found at K-spar/plagioclase contacts.

The marble zenoliths are a clear indication that the gneisses are intrusive in origin, and that the marble was picked up during ascent of the magma. The banded gneiss, with its low quartz content, was most likely a mangerite, part of McLelland’s Adirondack AMGC suite. The pink potassium feldspar-garnet gneiss has the composition of a syenite. The “Marble Cake” thus is characteristic of other intrusive igneous rocks in the Adirondack region. What is distinctive about this site is the well exposed zenoliths of marble, the large size of the diopside clots, large grains of both plagioclase and potassium feldspar, and dime to silver dollar size garnets.