Avon Volcanic District

The Avon Volcanic District is an approximately 275 square kilometer magmatic province in the southeast of the US state Missouri . It consists of over 80 courses and diatrema alkaline, ultramafic and carbonatitic composition. The rather unusual rock association is sometimes associated with the impact craters at the 38th parallel due to its geographical location .

Introduction and location

Location of the Avon Volcanic District within the lineament at the 38th parallel

The Avon Volcanic District, abbreviated AVD , sometimes also Avon Magmatic District , is located about 90 kilometers south of St. Louis in Missouri. It belongs to the Middle Mississippi Alkaline Province , an alkali rock province on the central Mississippi River , which also includes the Dent Branch-Bee Creek District and the Wauboukigou Alnöite District . The presence of Melilith is characteristic . Alkali magmatism in southern Arkansas and the Gulf of Mexico states is younger ( chalk ) and does not contain melilite either.

The district is located at the southeast end of the Farmington Anticline , a southeast-trending anticline structure , and is enclosed by two fault zones , the approximately 190-kilometer east-southeast trending Ste. Genevieve Fault Zone in the northeast and the parallel Simms Mountain Fault Zone in the southwest. The fault zone separates the Ozark Dome ( St. Francois Mountains ) in the southwest from the Illinois Basin in the northeast, with the Illinois Basin sinking up to 900 meters at the fault zone. The Ste Genevieve Fault Zone terminates at its southeast end in Northeast trending Reelfoot Rift , a grave Zone , which is part of the former Lower Midcontinent Rift System was. The Farmington Anticline is known for its Mississippi Valley-type mineralization .

geology

The igneous rocks of the Avon Volcanic District have intruded the Mesoproterozoic basement of the Eastern Granite Rhyolite Province , which is covered by Cambrian sediments about one hundred meters thick, ranging from the Lamotte Sandstone in the lying (northwest) to the Potosi Formation in the hanging wall (southeast). The Bonneterre Dolomite is stored on the Lamotte Sandstone , followed by the Davis Formation and the Derby Doerun Formation .

The lithosphere is around 200 kilometers thick here.

Petrology

The intrusives can be addressed as Melilithite , Alnöite , Calciocarbonatite and Kimberlite .

In the diatrema there are rocks that impress with their wealth of accretionary lapilli up to 4 centimeters in size . The lapilli are made up of a core and a shell area. Megacrystals of olivine are usually found in the core , but magnetite and clinopyroxene also occur. Cores made from xenolites of the lower crust are rare . The mantle area of the lapilli usually consists of a fine-grained matrix of olivine melilithite and contains regulated, slat-shaped melilith , relictual microcrystals of olivine and unchanged perovskite and other oxides. Lapilli that contain both melilithite and carbonatite at the same time are very rare. These may indicate a segregation process of silicate and carbonate magma or an incomplete mixing process.

The duct facies mainly consist of olivine melilithite and contain olivine and titanium-containing augite in a matrix of melilite and perovskite. In this case, clinopyroxene is usually replaced by calcite and the base material by a mixture of chlorite and carbonates.

Age

Zartman and colleagues (1967) found cooling ages for biotites from the Lower Devonian ( Emsium ) of 402 ± 19 million years BP . Zartman (1977) later reduced this age to 388 to 377 million years BP (Middle to Upper Devonian, Givetium to Frasnium ).

Individual evidence

↑ Nelson, WJ et al.: Ste. Genevieve Fault Zone . Office of Nuclear Regulatory Research, US Nuclear Regulatory Commission, Division of Radiation Programs and Earth Sciences, Missouri and Illinois, 1985.
^ Kidwell, AL: Post-Devonian igneous activity in southeastern Missouri . In: Missouri Geological Survey and Water Resources Report of Investigations . tape 4 , 1947.
↑ Callicoat, Jeff S. et al .: Pelletal lapilli in ultramafic diatremes, Avon Volcanic District, Missouri . In: Geological Society of America Abstracts with Programs . Vol. 40, No. 5 , 2008, p. 27 .
↑ Bridges, David L. and Hogan, John P .: Characterization of the ultramafic diatremes and dikes of the Avon Magmatic province, southeastern Missouri, USA . In: Geological Society of America Abstracts with Programs . Vol. 40, No. 3 , 2008, p. 12 .
↑ Zartman, RE et al .: K-Ar and Rb-Sr ages of some alkalic intrusive rocks from central and eastern United States . In: American Journal of Science . tape 265 , 1967, pp. 848-970 .

[1] Nelson, WJ et al.: Ste. Genevieve Fault Zone . Office of Nuclear Regulatory Research, US Nuclear Regulatory Commission, Division of Radiation Programs and Earth Sciences, Missouri and Illinois, 1985.

[2] Kidwell, AL: Post-Devonian igneous activity in southeastern Missouri . In: Missouri Geological Survey and Water Resources Report of Investigations . tape 4 , 1947.

[3] Callicoat, Jeff S. et al .: Pelletal lapilli in ultramafic diatremes, Avon Volcanic District, Missouri . In: Geological Society of America Abstracts with Programs . Vol. 40, No. 5 , 2008, p. 27 .

[4] Bridges, David L. and Hogan, John P .: Characterization of the ultramafic diatremes and dikes of the Avon Magmatic province, southeastern Missouri, USA . In: Geological Society of America Abstracts with Programs . Vol. 40, No. 3 , 2008, p. 12 .

[5] Zartman, RE et al .: K-Ar and Rb-Sr ages of some alkalic intrusive rocks from central and eastern United States . In: American Journal of Science . tape 265 , 1967, pp. 848-970 .