St. Francois Mountains

The St. Francois Mountains in southeast Missouri are a mountain range of Precambrian igneous mountains rising over the Ozark Plateau. This range is one of the oldest exposures of igneous rock in North America.The name of the range is spelled out as Saint Francois Mountains in official GNIS sources, but it is sometimes misspelled in use as St. Francis Mountains to match the anglicized pronunciation of both the range and St. Francois County.See also: U.S. Interior Highlands The St. Francois Mountains in southeast Missouri are a mountain range of Precambrian igneous mountains rising over the Ozark Plateau. This range is one of the oldest exposures of igneous rock in North America.The name of the range is spelled out as Saint Francois Mountains in official GNIS sources, but it is sometimes misspelled in use as St. Francis Mountains to match the anglicized pronunciation of both the range and St. Francois County. The name of the range derives from the St. Francis River, which originates in the St. Francois Mountains. The origin of the river's name, which also was originally spelled 'Francois' in the French manner, is unclear. The area, as part of the Louisiana district of New France, is near some of the earliest French settlements in Missouri, where many French place names survive. Some sources conjecture that the name honors St. Francis of Assisi (1181/1182–1226), the patron saint of the Franciscan order, but none of the region's early explorers were Franciscans. Others propose that Jacques Marquette, a French Jesuit, named the river when he explored its mouth in present-day Arkansas in 1673. Before his voyage down the Mississippi River, Marquette had spent some time at the mission of St. Francois Xavier, named for the Jesuit missionary Francis Xavier (1506–1552). The spelling of the river's name shifted from 'Francois' to 'Francis' in the early 20th century. The St. Francois Mountains were formed by volcanic and intrusive activity 1.485 billion (1.485 x 109) years ago. By comparison, the Appalachians started forming about 460 million years ago, and the Rockies a mere 140 million years ago. When the Appalachians started forming, the St. Francois range was already twice as old as the Appalachians are today. The intrusive rocks of the area are composed of three types: subvolcanic massifs, ring intrusions and central plutons. The subvolcanic intrusives are similar in geochemistry to the associated rhyolite volcanics, which they intrude. They are granite with granophyric quartz, perthitic potassium feldspar, biotite and magnetite. They are intrusive into the rhyolites with development of fine grained granophyre at the contact. At depth they exhibit a coarse-grained rapakivi texture. The subvolcanic granites are the most widespread igneous rocks and were thought to have been covered with extensive volcanics that have been removed by erosion. The ring intrusives are high silica bodies which were intruded along ring faults associated with caldera collapse. Rock types include trachyandesite, trachyte, syenite and amphibole - biotite granite. They are commonly porphyritic. The central plutons are highly evolved two mica (contain both biotite and muscovite) granites. Distinctive accessory minerals include: fluorite, topaz, apatite, spinel, allanite, sphene and cassiterite. They are enriched in tin, lithium, beryllium, rubidium, barium, yttrium, niobium, uranium, thorium and fluorine and are referred to as 'tin granites'. Their circular to oval shape in plan view is consistent with emplacement within resurgent calderas. The exposed igneous rocks of the St. Francois are surrounded at depth by the slightly younger (~100 my) widely distributed igneous Spavinaw terrane. The Spavinaw rocks are intersected in drill core across southern Missouri, southern Illinois, northern Arkansas, southern Kansas and northeastern Oklahoma. The Spavinaw rocks occur in outcrop only near Spavinaw, Oklahoma. The rhyolites and ash flow tuffs of the Spavinaw are essentially identical to the volcanics of the St. Francois mountains (the Washington County volcanics). The Saint Francois Mountains were formed by igneous activity, whereas most of the surrounding Ozarks are developed on Paleozoic sedimentary rocks as a dissected plateau. The localized vertical relief was caused by erosion following uplift during the Pennsylvanian and Permian periods produced by the Ouachita orogeny to the south. Elevations and strata dips in the Ozark structural dome generally radiate downward and outward away from the Saint Francois mountains.