Needle Mountains, Colorado

Proterozoic magmatism and tectonics

The Great Unconformity exposed at 12,000’ in the Needle Mountains, where the Cambrian Ignacio Quartzite unconformably overlies the ~1.43 Ga Eolus Granite at Overlook Point.

Eolus granite exposure around Mount Eolus, as seen from Lime Mesa near Overlook Point.

The involute comagmatic contact between the ~1.43 Ga Eolus Granite and central Trimble Granite on East Silver Mesa, mapped at 1:2200 scale. full size

Mapping in progress

Biotite-rich mafic schlieren at the Trimble-Eolus contact, East Silver Mesa. Undulose lower contact and gradational upper contact indicates comagmatic intrusion

Thick, sharp biotite schlieren in mixed or transitional Eolus-Trimble facies

Potassium feldspar megacryst in coarse, inequigranular two-feldspar granite from the eastern massif of the Eolus Granite

Modal layering defined largely by biotite abundance in the Trimble granite, scanned from a polished hand sample. Longest dimension is 17 cm. full size

Twilight trondhjemite gneiss in the foreground of Twilight Peak, as seen from the north. Eolus granite can be seen exposed on the far left across the Animas River canyon.

Cross-bedded quartzite of the Uncompahgre Formation near Snowdon Peak.

Basal conglomerate of the Uncompahgre Formation containing cobbles of banded hematite and jasper likely derived from BIF, as reported to occur in the eastern unit of the ~1.8 Ga Irving formation.

Syntectonic pegmatite dike (associated with the ~1.72 Ga Tenmile Granite) cutting the Twilight Trondhjemite Gneiss near between Snowdon and Twilight Peaks

Strongly banded gneiss at the contact between the Irving Formation and the Tenmile Granite near Twilight Peak

Large euhedral garnet in chlorite matrix, Irving Formation near Highland Mary Lakes.

Calc-silicate boudin in foliated amphibolite of the Irving Formation near Highland Mary Lakes.

Intrusive breccia with clasts of Irving formation amphibolite in a trondhjemitic matrix.

Highly deformed felsic orthogneiss associated with the Irving formation in Animas River canyon.

Wave ripples in the Ignacio Quartzite on Stag Mesa, a few meters above the Great Unconformity.