BH250-89
Title
Subject
Description
This Rhyolite is mapped as early Proterozoic, post-penokean in the Observatory Hill and at Marquette (Baxter Hollow suite).
BH250-89 represents rhyolitic volcanic rocks exposed near the Baxter Hollow Granite in the Baraboo Range and helps complete the broader magmatic story that preceded deposition of the Baraboo Quartzite. These rhyolites are Paleoproterozoic in age and are generally associated with igneous activity that occurred approximately 1,760 – 1,830 Ma, broadly overlapping with emplacement of the Baxter Hollow Granite. Together, they represent felsic magmatism that affected southern Wisconsin during early Proterozoic crustal evolution along the margin of the Superior Craton.
The rhyolite and the Baxter Hollow Granite are likely related as volcanic-plutonic equivalents formed from similar felsic magmatic systems. The Baxter Hollow Granite crystallized slowly at depth, producing its coarse-grained phaneritic texture, while BH250-89 formed when chemically similar silica-rich magma erupted at or near Earth’s surface and cooled much more rapidly. In many Proterozoic terranes, rhyolites and granites occur together as part of large felsic magmatic provinces associated with continental arc magmatism, extensional magmatism, or post-orogenic magmatic activity. The Baraboo area preserves evidence of this broader igneous history in both intrusive and extrusive forms.
Chemically, BH250-89 is silica-rich and dominated by quartz and feldspar components, with elevated potassium and sodium similar to many rhyolites. These rocks typically contain volcanic quartz, potassium feldspar, plagioclase, and minor biotite or iron oxides. In hand samples, rhyolite may appear pink, reddish, gray, or light tan depending on mineral content and oxidation history. Fine-grained groundmass dominates because the lava cooled rapidly, though phenocrysts of quartz or feldspar may be present if cooling occurred in multiple stages.
Texturally, BH250-89 commonly exhibits an aphanitic to porphyritic texture, with fine-grained groundmass surrounding larger phenocrysts of quartz or feldspar. Some rhyolites in the region may also preserve flow banding, devitrification textures, or welded volcanic textures depending on eruption style. In the thin section, quartz phenocrysts may show embayed margins, while feldspars may exhibit alteration or deformation features related to later tectonic events. Similar to the Baxter Hollow Granite, the rhyolite may not appear strongly metamorphosed in outcrops, but microscopic examination can reveal subtle deformation effects such as recrystallization and mild undulose extinction in quartz grains related to later regional tectonism.
These rhyolites likely contributed sediment to the developing Baraboo basin alongside the Baxter Hollow Granite. Once uplifted and exposed, both rock types underwent physical and chemical weathering. Feldspars altered to clays, unstable minerals broke down, and resistant quartz grains survived transport. Streams carried these sediments into coastal and shallow marine environments where repeated wave reworking concentrated exceptionally mature quartz sands that eventually became the Baraboo Quartzite.
The presence of both BH250-89 rhyolite and BH250-89b Baxter Hollow Granite tells an important regional story: before the Baraboo Quartzite became one of North America’s classic mature quartzites, an older volcanic-plutonic terrane had to form, uplift, weather, and erode. These rocks preserve the igneous roots of that story and provide a direct link between Proterozoic magmatism and the sedimentary evolution of the Baraboo basin.
Coverage
GPS Location: near 43°34'21.12"N, 89°15'59.68"W