Leucite Hills

Leucite Hills
	View from the west flank of Steamboat Mountain over the Killpecker Dunes to the south to the Leucite Hills i. e. S., with Black Rock (far left) and Spring Butte (middle left) as the next bumps
part of	Rocky mountains
Coordinates	41 ° 52 ′ N , 108 ° 57 ′ W Coordinates: 41 ° 52 ′ N , 108 ° 57 ′ W
Type	fossil volcanic field
rock	Relief-forming: Lamproit; also: Siliziklastika
Age of the rock	Lamproit: predominantly Old Pleistocene; Silicoplastics: Cretaceous and Old Tertiary

The Leucite Hills are a hilly landscape in southwest Wyoming . Geologically, it is a fossil volcanic area ( Leucite Hills Volcanic Field ) whose relief-defining, comparatively rare alkaline volcanic and sub-volcanic rocks originated in the Pliocene and especially in the Pleistocene and are therefore extremely young in geological terms . The name Leucite Hills means something like Leucite Mountains in German and refers to the foid mineral leucite , which is an important component of the alkaline volcanic rocks.

Geography and geomorphology

Location of the Leucite Hills in Wyoming

The approximately 2,600 square kilometer area of the Leucite Hills in the narrower sense is located in southwest Wyoming east of the Green River in Sweetwater County , approximately 35 kilometers northeast of the small town of Rock Springs . It extends south of the dune landscape of the Killpecker Dunes on the northeastern edge of the Rock Springs Uplift . Geologically, however, two isolated elevations, one north of the Killpecker Dunes (Steamboat Mountain) and one approx. 15 kilometers northwest of Rock Springs (Pilot Butte) have to be added.

In the Leucite Hills region, 14 larger volcanic structures emerged over the last three million years , of which ash and lava cones as well as 50 lava flows and chimney fillings have been preserved, which are in the form of larger table mountains (mesas) and small table mountains and peaks (buttes). shape the landscape. Furthermore, 9 subvolcanic total courses and sills have been identified. However, the total volume of the volcanic rocks is relatively small at 0.58 cubic kilometers. The intrusion of the dikes and the effusion of lavas, ashes and ignimbrites took place in or over the relatively flat layers of the rock springs anticline structure. The secondary rocks come from the Upper Cretaceous ( Lance Formation , Lewis Formation , Almond Formation , Ericson Formation , Rock Springs Formation , Blair Formation and Baxter Formation ), from the Paleocene ( Fort Union Formation ) and from the Eocene ( Wasatch Formation and Green River Formation ). They consist of sandstones , siltstones , shale , coal layers and claystones .

The Leucite Hills (i. W. S.) include the following more or less closely spaced volcanic full forms:

The Madupit on Pilot Butte from the west

Badger's Teeth (formerly Twin Rock). Five solidified, east-west-oriented eruptions of lava from a chimney (with auto-recessed agglomerate), which sits on a conveyor passage. Petrographically it is an orendite, but chemically it is a madupite.

Black Rock - 800,000 years of BP. Easternmost occurrence. Consists of a pyroclastic olivine orendite at the base, which is covered by a lava flow. Contains tubers made of dunite with olivine crystals, some of which are gem quality (peridot).

Boar's Tusk - 2.56 million years BP. Striking volcanic vent from Wyomingite, which towers over its surroundings by almost a hundred meters. The agglomerate rock contains xenolites of the tertiary formations (clay and sandstones), granite fragments and car-related lamproite fragments .

Cabin Butte (formerly Osborn Mesa). Orenditic lava flow with a dome-like volcanic vent at the north end.

Deer Butte (formerly Cross Mesa). Remnants of a composite volcano with two lava flows and a production center in the south wall. The scoria consists of Wyomingite, the lava mainly of orendite.

Emmons Mesa - 940,000 years of BP. Orenditic lava flows and a volcanic cone very rich in xenolites ( amphibolite , anorthosite , granulite , granite and sedimentary rocks). Xenoliths (granites and sediments) are also found in the lavas.

Hill at last. Lava flows and pyroclastics from olivine orendite. Contain multiple populations of upper mantle olivine xenocrystals.

Hague Hill. This small hill south of Endlich Hill consists of Orenditic lavas.
Hallock Butte. Wyomingite apophysis of the N 320 trending Wortman vein.
Hatcher Mesa - 970,000 years of BP. With very rich xenolites from the basement such as granites, norite and mica slate . Remnants of a pent-up lava flow that fell back into the chimney. At the base Wyomingit followed by the predominant Orendit.
Iddings butte. Apophysis of the N 320 trending Wortman duct. Dense Wyomingitlava inside, volcanic agglomerates on the edge.
Mathews Hill. Small volcanic vent.
Middle Table Mountain - 2.05 million years BP. Smaller chimney made up of alternating layers of green Madupite and brown Wyomingite. The Madupitic lavas containing mudstone xenolites are low in silicon compared to the other lamproites of the Leucite Hills, but are enriched in MgO.
North Table Mountain - 1.24 million years BP. Consists of a single lava flow (with beautiful flow structures) of light gray orendite and dark gray wyomingite.
Pilot Butte - 3.00 million years BP. Is isolated northwest of Rock Springs in the Eocene of White Mountain. Type locality of the Madupit.
South Table Mountain - 1.81 million years BP. Consists of lava flows of olivine orendite and subordinate wyomingite. Quite rich in magnesium because of the numerous pheno- and xenocrystals of olivine. Small bulbs of dunite are also present.
Spring Butte - 810,000 years BP (formerly Orenda Butte or Orenda Mesa). Type locality of Orendit. Composed volcanic center of six cones, at least six, up to 20 meters thick lava flows and three corridors. The cones are made up of welded pyroclastic flows and contain breadcrust bombs. Sandstones of the Fort Union Formation and rare anorthosites are found as xenolites.
Steamboat Mountain - 1.70 million years BP. Is isolated north of the Killpecker Dunes and represents the world's most potassium-rich volcanite with more than 12 percent by weight in a glassy Wyomingite. Predominantly aphanitic, bubble-rich lava flows of orenditic composition, which can have sediment inclusions.
Weed butte. Wyomingite apophysis of the N 320 trending Wortman vein.
Circle Mesa - 950,000 years of BP. Most important lamproite outcrop in the Leucite Hills. Consists of a basal orendite lava flow up to 15 meters thick, which is covered by six interlocking volcanic cones. The current, which started with a layer of rubble, baked the underlying sediment to about 50 centimeters in red.

geology

Regional geological framework

The volcanic rocks of the Leucite Hills are part of the strata of the Greater Green River Basin . They are located in the central part of this basin on the northeastern edge of the Rock Springs Uplift (also Rock Springs Anticline ), an anticline structure formed during the Laramian mountain formation . Since its elevation, this structure has been subject to very strong erosion along a hidden displacement, so that today layers from the Upper Cretaceous bordered by layers of the Paleogene are exposed in its core. The underlying basement is part of the Wyoming Craton , the rocks of which have crystallization ages from 3200 to 2600 million years ( Archean ). It consists of an amalgam of backarc basins, island arches and fragments of microcontinents, which has been tectonically largely stable for around 2700 to 2600 million years, which was intruded by granites of the Archean.

The volcanic field of the Leucite Hills is about 100 kilometers north of the Cheyenne Belt , which separates the Wyoming Craton from the Colorado Plateau and is interpreted as a Proterozoic suture that was created along a former Wadati-Benioff zone .

An important tectonic lineament runs parallel to the axis of the Rock Springs Anticline in the N320 direction - the Farson lineament . Many extraction centers and corridors in the Leucite Hills follow its orientation, including the line of volcanic cones on the Zirkel Mesa and the Emmons Mesa. Approximately perpendicular to the anticline axis, numerous faults (smaller displacements) cross in the east-northeast direction that were created during the Maastrichtian .

Petrology of the volcanic rocks

The Lamproitlaves of the Leucite Hills were discovered in 1871 by Samuel Franklin Emmons on the occasion of a new survey of the 40th parallel in the western United States. The first rock analyzes were carried out for the first time in 1897 by both James Furman Kemp and Charles Whitman Cross , who were able to determine an extremely high K ₂ O content of almost 13 percent by weight.

All of the volcanic rocks found in the Leucite Hills are ultrapotassic (extremely potassium-rich) lamproites. These volcanic rocks are also peralkaline and have high magnesium numbers . They show high concentrations of compatible elements such as nickel and chromium . The LREE and incompatible elements potassium , barium , titanium , rubidium , strontium , fluorine and zirconium are also enriched . It is characterized by a depletion of the main elements aluminum , calcium and sodium .

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Circle Mesa in the southeast of the Leucite Hills (i. E. S.): 6 eroded ash cones from Wyomingite tuff

Petrographically, five lamproite varieties are distinguished from one another in the Leucite Hills, which in turn can be assigned to two distinct groups - the phlogopite lamproites and the Madupitic lamproites . The phlogopite lamproites include the Wyomingite , the orendite and the olivine orendite, whereas the madupite and the transitional madupite form the group of the Madupitic lamproites. The two groups differ fundamentally in their geochemical parameters, whereby the differences cannot be explained by fractionation or crust contamination. Rather, they are actually chemically distinct source regions.

Mineral inventory

The lamproites of the Leucite Hills hold a mineral inventory that reflects their peralkaline and ultrapotassic nature. As a rock group, they can be characterized by the presence of the following minerals:

Phlogopite - containing titanium
Tetraferriphlogopite - containing titanium
Richterite - contains titanium and potassium
Leucite - low in sodium and aluminum
Sanidin - rich in iron
Diopside
Apatite
Priderit or Jeppeit
Wadeit
Shcherbakovit
Perovskite

Phlogopite, diopside and apatite form phenocrystals , but can also occur in the matrix. The remaining minerals are only represented in the basic mass. In no case are the minerals nepheline , melilite , kalsilite , sodium-rich alkali feldspar , plagioclase , monticellite , garnets (with Ti and Zr) and aluminum-rich augite to be found in the Leucite Hills .

Chemical composition

Main elements

Quantitative analyzes of the chemical composition of samples from different locations in the Leucite Hills showed the following values:

Oxide wt.%	General bandwidth	Orendit Steamboat Mountain	Orendit Fifteen-Mile Spring	Wyomingit Circle Mesa	Madupit Pilot Butte
SiO ₂	41.02-55.82	55.43	54.08	55.01	45.30
TiO ₂	2.05-2.73	2.64	2.08	2.73	2.05
Al ₂ O ₃	7.20-9.99	9.73	9.49	9.57	7.64
Fe ₂ O ₃	2.05-2.73	2.12	3.19	4.28	5.89
FeO		1.48	1.03
MnO		0.08	0.05	0.06	0.12
MgO		6.11	6.74	6.28	10.60
CaO	3.30-12.72	2.69	3.55	4.11	11.00
Na ₂ O	0.54-1.85	0.94	1.39	0.98	1.67
K ₂ O	4.74-11.54	12.66	11.76	11.50	8.03
P ₂ O ₅	1.30-2.99	1.52	1.35	1.29	2.05
LOI		2.53	2.71	3.23	4.48
Mg #	0.72-0.79			0.73	0.76

The lamproites of the Leucite Hills are ultramafic , mafic, and intermediate rocks , with the madupites at the ultramafic to mafic and the phlogopite lamproites at the intermediate end of the spectrum. Underlining their ultramafic character, the Madupitic lamproites can achieve very high levels of MgO and CaO, but are generally lower in potassium than the extremely potassium-rich phlogopite lamproites.

Trace elements

Trace elements ppm	Orendit North Table Mountain	Wyomingit Circle Mesa	Madupit Pilot Butte
Cr	310	343	444
Ni	180	226	104
Zn	62	67	99
Rb	296	288	212
Sr	2020	1830	5719
Zr	265	159	529
Ba	6670	6240	7212
Ce	297	236	712
Nd	124	96.8	300
Sm	15.6		36.0
Hf	2.5		4.7
Th	16.7	13.7	44.8

The trace elements show an extreme enrichment of the incompatible elements , in particular barium and strontium reach very high values, especially in the Madupitic lamproites. Niobium and samarium are depleted within the trace elements (negative spike) . In the case of the rare earths , a very clear enrichment of the LREE and a depletion of the HREE can be seen, with the Madupitic lamproites also generally achieving higher concentrations here.

Isotope ratios

Neodymium-strontium isotope diagram with the position of Madupite and Orendite in comparison to other magma provinces

The following initial ratios were determined for the radioisotopes of Sr, Nd and Pb; the δ ¹⁸ O value is also given :

Isotopes	Orendit North Table Mountain	Wyomingit Circle Mesa	Madupit Pilot Butte
⁸⁷ Sr / ⁸⁶ Sr	0.70591	0.705741	0.70563
¹⁴³ Nd / ¹⁴⁴ Nd	0.51188	0.511872	0.51208
²⁰⁶ Pb / ²⁰⁴ Pb	17,273	17.227	17,583
²⁰⁷ Pb / ²⁰⁴ Pb	15,482	15.464	15.504
²⁰⁸ Pb / ²⁰⁴ Pb	37.280	37.318	37.523
δ ¹⁸ O	8.38	8.65	8.93

In the diagram ¹⁴³ Nd / ¹⁴⁴ Nd versus ⁸⁷ Sr / ⁸⁶ Sr, the lamproites of the Leucite Hills are all in the crustal enriched quadrants, whereby the phlogopite lamproites are again clearly separated from the Madupitic lamproites. In addition to consistently higher lead isotope ratios, the latter also have a higher ¹⁴³ Nd / ¹⁴⁴ Nd value. Compared to other magma provinces such as the Tuscan magma province , southeast Spain, West Kimberley or Gaussberg in Antarctica, which have almost identical ¹⁴³ Nd / ¹⁴⁴ Nd values, the Leucite Hills are characterized by their very low ⁸⁷ Sr / ⁸⁶ Sr values and stand out clearly from them. In addition, their ²⁰⁷ Pb / ²⁰⁴ Pb and ²⁰⁸ Pb / ²⁰⁴ Pb values are significantly lower. Together with the lamproites of Smoky Butte in Montana , they form a steep trend that is only slightly contaminated by crustal strontium and lies directly below bulk earth (average composition of the earth). All other magma provinces mentioned are on a flat trend and have been more or less contaminated.

Petrogenesis of the volcanic rocks

The upper mantle is generally regarded as the source region of the Lamproite in the Leucite Hills . Its petrological composition is likely to be either lherzolite or refractory Harzburgite . It is also assumed that this archaic mantle area is depleted in the basalt fraction , as basaltic magmas had previously been secreted. Only later did he experience metasomatic enrichment via vein networks carrying phlogopite. The age of this metasomatosis in the underlying mantle can be estimated from the neodymium isotope ratio of the lavas - the model ages obtained in the Mesoproterozoic are 1200 million years BP. The metasomatosis was possibly caused by a subduction at the edge of the Wyoming craton. This is in line with the general spatial distribution of lamproites, which, in contrast to kimberlites of the plate interior, usually occur at plate edges in mobile zones, which girdle archaic cratons provided with deep lithospheric roots.

Mirnejad and Bell (2006) also identify a second, much more recent (<100 million years BP) enrichment event that came to fruition from the Upper Cretaceous. Potassium-rich solutions were brought into the source region by mantle bulges and partial melting was induced. Either the Yellowstone hotspot 300 kilometers further north or the Colorado Plateau, which is rising up due to a mantle anomaly, 200 kilometers further south, are considered to be the energy suppliers . Usually the Farallon Plate , which is subducted to the east, is also considered to be the cause.

Age of the volcanic rocks

The volcanic rocks of the Leucite Hills formed from the Piacenzian 3.0 million years ago. The main activity was limited to the end of the Old Pleistocene , ie to the period 940,000 to 890,000 years BP.

Individual evidence

^ Carmichael, ISE: The mineralogy and petrology of the volcanic rocks from the Leucite Hills, Wyoming . In: Contrib. Miner. Petrol. tape 15 , 1967, p. 24-66 .
↑ Ogden, PR, Jr .: The geology, major element geochemistry, and petrogenesis of the Leucite Hills volcanic rocks, Wyoming: Phd. Dissertation . University of Wyoming, Laramie 1979, pp. 137 p .
Jump up ↑ Speer, JT: Xenoliths of the Leucite Hills volcanic rocks, Sweetwater County, Wyoming: MS thesis . University of Wyoming, Laramie 1985, pp. 57 p .
^ Smithson, SB: The geology of the southeastern Leucite Hills, Sweetwater County, Wyoming: MA thesis . University of Wyoming, Laramie 1959, pp. 92 p .
^ Ernst, WG: Metamorphic Terrains, Isotopic Provinces, and Implications for Crustal Growth of the Western United States . In: Journal of Geophysical Research . v. 93, 1988, pp. 7634-7642 .
↑ Zirkel, F .: Microscopical Petrography . In: US Geol.Expl. 40th Parallel, Report 6 . 1876, p. 259-261 .
↑ Kemp, JF: The Leucite Hills of Wyoming . In: Geol. Soc. At the. Bull. Band 8 , 1897, p. 169-182 .
^ Cross, W .: Igneous rocks of the Leucite Hills and Pilot Butte, Wyoming . In: Am. J. Sci . tape 4 , 1897, p. 115-141 .
↑ Mirnejad, H. and Bell, K .: Origin and Source Evolution of the Leucite Hills Lamproites: Evidence from Sr-Nd-Pb-O Isotopic Compositions . In: Journal of Petrology . tape 47 , 2006, p. 2463-2489 , doi : 10.1093 / petrology / eg1051 .
↑ Mitchell, RH and Bergman, SC: Petrology of Lamproites . Plenum, New York 1991, pp. 477 .
↑ Vollmer, R., Ogden, P., Schilling, J.-G., Kingsley, RH and Wagoner, DG: Nd and Sr isotopes in ultrapotassic volcanic rocks from the Leucite Hills, Wyoming . In: Contrib. Mineral. Petrol. tape 87 , 1984, pp. 359-368 .
↑ Lange, RA, Carmichael, ISE and Hall, CM: 40Ar / 39Ar chronology of the Leucite Hills, Wyoming: eruption rates, erosion rates, and an evolving temperature structure of the underlying mantle . In: Earth and Planetary Science Letters . tape 174 , 2000, pp. 329-340 .

[1] Carmichael, ISE: The mineralogy and petrology of the volcanic rocks from the Leucite Hills, Wyoming . In: Contrib. Miner. Petrol. tape 15 , 1967, p. 24-66 .

[2] Ogden, PR, Jr .: The geology, major element geochemistry, and petrogenesis of the Leucite Hills volcanic rocks, Wyoming: Phd. Dissertation . University of Wyoming, Laramie 1979, pp. 137 p .

[3] Jump up ↑ Speer, JT: Xenoliths of the Leucite Hills volcanic rocks, Sweetwater County, Wyoming: MS thesis . University of Wyoming, Laramie 1985, pp. 57 p .

[4] Smithson, SB: The geology of the southeastern Leucite Hills, Sweetwater County, Wyoming: MA thesis . University of Wyoming, Laramie 1959, pp. 92 p .

[5] Ernst, WG: Metamorphic Terrains, Isotopic Provinces, and Implications for Crustal Growth of the Western United States . In: Journal of Geophysical Research . v. 93, 1988, pp. 7634-7642 .

[6] Zirkel, F .: Microscopical Petrography . In: US Geol.Expl. 40th Parallel, Report 6 . 1876, p. 259-261 .

[7] Kemp, JF: The Leucite Hills of Wyoming . In: Geol. Soc. At the. Bull. Band 8 , 1897, p. 169-182 .

[8] Cross, W .: Igneous rocks of the Leucite Hills and Pilot Butte, Wyoming . In: Am. J. Sci . tape 4 , 1897, p. 115-141 .

[9] Mirnejad, H. and Bell, K .: Origin and Source Evolution of the Leucite Hills Lamproites: Evidence from Sr-Nd-Pb-O Isotopic Compositions . In: Journal of Petrology . tape 47 , 2006, p. 2463-2489 , doi : 10.1093 / petrology / eg1051 .

[10] Mitchell, RH and Bergman, SC: Petrology of Lamproites . Plenum, New York 1991, pp. 477 .

[11] Vollmer, R., Ogden, P., Schilling, J.-G., Kingsley, RH and Wagoner, DG: Nd and Sr isotopes in ultrapotassic volcanic rocks from the Leucite Hills, Wyoming . In: Contrib. Mineral. Petrol. tape 87 , 1984, pp. 359-368 .

[12] Lange, RA, Carmichael, ISE and Hall, CM: 40Ar / 39Ar chronology of the Leucite Hills, Wyoming: eruption rates, erosion rates, and an evolving temperature structure of the underlying mantle . In: Earth and Planetary Science Letters . tape 174 , 2000, pp. 329-340 .