Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Tnfg North Fork leucogranite (early Oligocene) – The North Fork leucogranite
(Tnfg) forms the interior phase of the North Fork intrusion. The North Fork leucogranite
(Tnfg) is a white to pinkish-white, equigranular, medium-grained biotite leucogranite
(fig. 21) that is most similar in appearance to the California leucogranite (Tcm). A modal
analysis (2,070 points on slab stained for alkali feldspar) on a sample of North Fork
leucogranite from the western part of the intrusion in the Garfield quadrangle (1,600 feet
west of the Maysville quadrangle boundary) indicates 26.8 percent quartz, 40.0 percent
alkali feldspar, 31.3 percent plagioclase, 1.8 percent biotite-muscovite, and 0.1 percent
magnetite (Shannon, unpublished data, 1988). The granite is classified as granite b (IUGS
classification) or alkali-feldspar granite (Ramsay and others, 1985), depending on the
composition of the plagioclase. The North Fork leucogranite has mineralogical
characteristics that are transitional between the Mount Antero leucogranite and the
California leucogranite. The North Fork leucogranite contains only minor garnet that is
generally localized in zones close to the fine-grained granite border phase and the
margins of the intrusion. In addition, the North Fork leucogranite has accessory apatite
similar to that present in the Mount Antero leucogranite but lacking in the California
leucogranite (Shannon, 1988). It also contains accessory magnetite, ilmenite, monazite,
fluorite, and zircon.
A whole-rock chemical analysis (sample 05-575A, table 1) of the North Fork
leucogranite (Tnfg) from the Maysville quadrangle was obtained for this study. The
sample was collected from the eastern part of the intrusion, immediately interior to the
border zone. The sample is chemically classified as alkali granite (De la Roche and
others, 1980) and has elevated SiO 2 , Na 2 O, and K 2 O, and has low Fe 2 O 3 , CaO and MgO.
Shannon (1988), in a compilation of whole-rock chemical analyses of the Mount Antero
leucogranites, provided an analysis of the North Fork granite (from London, 1987,
written communication). The analysis is very similar to the above sample and included
trace element analyses for Rb (335 ppm), Nb (45 ppm), and Sr (75 ppm). The major- and
trace-element character of the North Fork leucogranite is similar to the whole group of
chemically evolved granites and rhyolites associated with the Mount Antero leucogranite
system.
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