Chinati Mountains caldera volcanic rocks, including Chinati Mountains Group, Mitchell Mesa Ignimbrite, and type area of Petan Basalt (TXOGcm;0)

State	Texas
Name	Chinati Mountains caldera volcanic rocks, including Chinati Mountains Group, Mitchell Mesa Ignimbrite, and type area of Petan Basalt
Geologic age	Oligocene
Lithologic constituents	Major Igneous > Volcanic > Felsic-volcanic > Rhyolite (Flow) Minor Igneous > Volcanic > Felsic-volcanic > Trachyte (Flow) Igneous > Volcanic > Mafic-volcanic > Andesite (Flow) Igneous > Volcanic > Felsic-volcanic > Dacite (Flow) Incidental Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
Comments	Chinati Mountain caldera volcanic rocks are about 1 km in thickness and include collapse breccias, rhyolitic to basaltic lavas, and a rhyolitic ash-flow tuff. Silicic to intermediate rocks are the largest volume of rocks. A large quartz monzonite intrusion domes the west side of the caldera; many dikes of peralkaline rhyolite intrude caldera-fill volcanic rocks at the north side of the caldera. According to Cepeda and Henry (1983) eight units make up the Group, from bottom to top: collapse agglomerate; lower trachyte, middle trachyte, nonporphyritic domes and flows, Cieneguita dome and flows, lower rhyolite, upper trachyte, upper rhyolite. On the basis of major and trace element chemistry, Cameron and Cameron (1986) recognized 6 groups within the Chinati Mountains caldera rocks--basalts, andesites, main series dacites to low-SiO2 rhyolites, low-high-field-strength (low HFS) cation group, high-SiO2 metaluminous rhyolites and high-SiO2 peralkaline rhyolites. Mitchell Mesa Ignimbrite (or Rhyolite) is the most voluminous and widespread ash-flow tuff of Trans-Pecos Texas and its eruption led to collapse of the Chinati Mountains caldera; it is a multiflow, single-cooling-unit, ash-flow tuff of high-silica rhyolite (77% SiO2); Mitchell Mesa unit is as much as 255 ft thick, averages about 45 ft. Petan Basalt (Trachyte) (also Jones Formation of southern Davis Mountains) is a sequence of porphyritic trachyte lavas that overlie the Mitchell Mesa Ignimbrite north and west of the Infiernito caldera. It contains 25-30 percent phenocrysts and glomerocrysts of plagioclase, clinopyroxene, orthopyroxene, magnetite, ilmenite, zircon, and apatite.
Stratigraphic units	Brite Ignimbrite of Rim Rock Country (same as Mitchell Mesa Rhyolite, or Ignimbrite), Petan Basalt (same as Bell Valley Andesite of Wiley Mountains area and the Jones Formation of southern Davis Mountains)
References	Cepeda, J.C., and Henry, C.D., 1983, Oligocene volcanism and multiple caldera formation in the Chinati Mountains, Presidio County, Texas: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations 135, 32 p. Henry, C.D., Price, J.C., Duex, T.W., and James, E.W., 1992, Geology of the Infiernito caldera and magmatic evolution of the Chinati Mountains, Trans-Pecos Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations no. 206, 56 p. Henry, C.D. and McDowell, F.W., 1986, Geochronology of magmatism in the Tertiary volcanic field, Trans-Pecos, Texas, in Price, J.G. and others, eds., Igneous geology of Trans-Pecos, Texas: The University of Texas at Austin, Bureau of Economic Geology Guidebook 23, p. 99-122. Cameron, K.L. and Cameron, Maryellen, 1986, Geochemistry of quartz-normative igneous rocks from the Chinati Mountains and Terlinga areas, west Texas--a comparison with Cenozoic volcanic rocks from Chihuahua and Baja Claifornia Sur, Mexico in Price, J.G., Henry, C.D., Parker, D.F., and Barker, D.S., eds., Igneous geology of Trans-Pecos Texas, field trip guide and research articles: The University of Texas at Austin, Bureau of Economic Geology, Guidebook 23, p. 143-163. Bureau of Economic Geology, 1992, Geologic Map of Texas: University of Texas at Austin, Virgil E. Barnes, project supervisor, Hartmann, B.M. and Scranton, D.F., cartography, scale 1:500,000.
NGMDB product	NGMDB product page for 68390
Counties	Brewster - Presidio

State

Name

Chinati Mountains caldera volcanic rocks, including Chinati Mountains Group, Mitchell Mesa Ignimbrite, and type area of Petan Basalt

Geologic age

Oligocene

Lithologic constituents

Major

Igneous > Volcanic > Felsic-volcanic > Rhyolite (Flow)

Minor

Igneous > Volcanic > Felsic-volcanic > Trachyte (Flow)

Igneous > Volcanic > Mafic-volcanic > Andesite (Flow)

Igneous > Volcanic > Felsic-volcanic > Dacite (Flow)

Incidental

Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)

Comments

Chinati Mountain caldera volcanic rocks are about 1 km in thickness and include collapse breccias, rhyolitic to basaltic lavas, and a rhyolitic ash-flow tuff. Silicic to intermediate rocks are the largest volume of rocks. A large quartz monzonite intrusion domes the west side of the caldera; many dikes of peralkaline rhyolite intrude caldera-fill volcanic rocks at the north side of the caldera. According to Cepeda and Henry (1983) eight units make up the Group, from bottom to top: collapse agglomerate; lower trachyte, middle trachyte, nonporphyritic domes and flows, Cieneguita dome and flows, lower rhyolite, upper trachyte, upper rhyolite. On the basis of major and trace element chemistry, Cameron and Cameron (1986) recognized 6 groups within the Chinati Mountains caldera rocks--basalts, andesites, main series dacites to low-SiO2 rhyolites, low-high-field-strength (low HFS) cation group, high-SiO2 metaluminous rhyolites and high-SiO2 peralkaline rhyolites. Mitchell Mesa Ignimbrite (or Rhyolite) is the most voluminous and widespread ash-flow tuff of Trans-Pecos Texas and its eruption led to collapse of the Chinati Mountains caldera; it is a multiflow, single-cooling-unit, ash-flow tuff of high-silica rhyolite (77% SiO2); Mitchell Mesa unit is as much as 255 ft thick, averages about 45 ft. Petan Basalt (Trachyte) (also Jones Formation of southern Davis Mountains) is a sequence of porphyritic trachyte lavas that overlie the Mitchell Mesa Ignimbrite north and west of the Infiernito caldera. It contains 25-30 percent phenocrysts and glomerocrysts of plagioclase, clinopyroxene, orthopyroxene, magnetite, ilmenite, zircon, and apatite.

Stratigraphic units

Brite Ignimbrite of Rim Rock Country (same as Mitchell Mesa Rhyolite, or Ignimbrite), Petan Basalt (same as Bell Valley Andesite of Wiley Mountains area and the Jones Formation of southern Davis Mountains)

References

Cepeda, J.C., and Henry, C.D., 1983, Oligocene volcanism and multiple caldera formation in the Chinati Mountains, Presidio County, Texas: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations 135, 32 p.

Henry, C.D., Price, J.C., Duex, T.W., and James, E.W., 1992, Geology of the Infiernito caldera and magmatic evolution of the Chinati Mountains, Trans-Pecos Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations no. 206, 56 p.

Henry, C.D. and McDowell, F.W., 1986, Geochronology of magmatism in the Tertiary volcanic field, Trans-Pecos, Texas, in Price, J.G. and others, eds., Igneous geology of Trans-Pecos, Texas: The University of Texas at Austin, Bureau of Economic Geology Guidebook 23, p. 99-122.

Cameron, K.L. and Cameron, Maryellen, 1986, Geochemistry of quartz-normative igneous rocks from the Chinati Mountains and Terlinga areas, west Texas--a comparison with Cenozoic volcanic rocks from Chihuahua and Baja Claifornia Sur, Mexico in Price, J.G., Henry, C.D., Parker, D.F., and Barker, D.S., eds., Igneous geology of Trans-Pecos Texas, field trip guide and research articles: The University of Texas at Austin, Bureau of Economic Geology, Guidebook 23, p. 143-163.

Bureau of Economic Geology, 1992, Geologic Map of Texas: University of Texas at Austin, Virgil E. Barnes, project supervisor, Hartmann, B.M. and Scranton, D.F., cartography, scale 1:500,000.

NGMDB product

NGMDB product page for 68390

Counties

Brewster - Presidio