Smoky butte

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Smoky butte
part of Rocky Mountains (foreland)
Smoky Butte (USA)
Smoky butte
Coordinates 47 ° 18 ′  N , 107 ° 0 ′  W Coordinates: 47 ° 18 ′  N , 107 ° 0 ′  W
Type fossil volcanic field
rock relief-forming: Lamproit
also: Silicoplastics of the Fort Union formation
Age of the rock Lamproit: predominantly Oligocene
siliciclastics: Paleocene and Old Tertiary
f1
p1
p4
p3
p5

The Smoky Butte is an Oligocene lamproit intrusion in Garfield County of Montana . The sub-volcanic rock contains quite rare minerals such as armalcolite and davanite .

Geography and geomorphology

The Smoky Butte to German Smoky Mountain , one is Härtling ( Engl. Butte ), the approximately 13 kilometers west-southwest of Jordan in the field of Missouri Breaks pending. Along with other Lamproitic elevations such as the Radial Dike Butte , the Bull Snake Knob and the Half Sediment Butte in the northeast as well as the Instrument Butte , the Ship Rock (not to be confused with the Shiprock in Arizona) and the Wall Rock in the southwest, it follows you N030-painting lineament.

geology

The Smoky Butte was described in detail by RE Matson in his doctoral thesis in 1960. It consists of passages and several small chimneys that penetrated the Paleocene sediment layers of the Tullock member of the Fort Union formation . Its sandstones , which lie directly above the Cretaceous-Tertiary border , were changed in contact metamorphosis. During the ascent, the Lamproit used the aforementioned lineament for a good 3 kilometers - a crustal weak zone that is now designed as a corridor that is up to 40 meters wide . The unstable corridor is split up into staggered, staggered areas. The edges of the aisle are mostly deterred, but can also show pahoehoe- like knitted structures.

The lamproite rocks belong to the phlogopite-lamproites, more precisely they are sanidine-diopside-Richterite-phlogopite-lamproites. The hypabyssal rocks show various forms and can appear vesicular, massive, glassy and brecciated, tuffs and other pyroclastics also occur to a lesser extent. Effusiva was not promoted.

Lamproite breccias and heterolithic breccias (mixing of vesicular, pumice-like lamproot fragments with secondary rocks of the weak contact metamorphic Fort Union Formation) are very common and build up some of the localities mentioned above. In blocks of heterolithic breccia, alternating layers of welded agglomerations and layered tuffs are often found.

Petrology

In terms of petrography, three different rock facies can be distinguished:

mineralogy

Overall, the lamproites from Smoky Butte carry the following minerals in addition to glass:

In contrast to the lamproites from the Leucite Hills in Wyoming, there is no leucite .

composition

The following chemical compositions and their CIPW standards could be determined for the three facies :

Oxide
wt.%		 Indoor Transition zone Outdoor area CIPW standard Indoor Transition zone Outdoor area
SiO 2 51.89 53.53 52.19 Q 0.00 0.00 0.22
TiO 2 5.17 5.23 4.96 Or 45.09 46.39 446.39
Al 2 O 3 9.05 9.08 9.04 From 4.07 3.00 2.79
Fe 2 O 3 3.86 4.39 5.02 Ac 11.10 11.74 6.04
FeO 1.31 1.02 3.50 Tuesday 6.18 5.24 15.26
MnO 0.08 0.08 0.08 En 16.81 16.95 12.80
MgO 7.90 7.78 7.98 No 0.02 0.33 2.93
CaO 4.44 4.43 4.90 Il 2.94 2.33 7.56
Na 2 O 1.97 1.93 1.14 Tn 2.04 7.38 2.40
K 2 O 7.63 7.85 7.85 Pf 4.75 1.70 0.00
P 2 O 5 0.25 0.22 0.22 Ap 0.55 0.48 0.48
LOI 5.32 4.41 1.49

The potassic lamproites from Smoky Butte , some of which are slightly oversaturated with silicon (since they are quartz normative), are located in the transition area from mafic to intermediate rocks . They are peralkaline, recognizable by the normative Akmit . They have also expressed high TiO 2 - and increased K 2 O contents . Their MgO values are 8 percent by weight. In contrast, they are depleted in Al 2 O 3 and P 2 O 5 .

Trace elements

Trace elements
ppm		 Smoky butte
Sc 14.0 17.2 14.6 13.0 13.3 15.4
Co 36.4 37.7 27.0 30.2 31.2 20.3
La 364 410 366 332 337 437
Rb 50 140 122 69 85 112
Sr 3150 2998 2332 2686 2915 2226
Pb 12.45 14.94 12.54 10.50 17.22 14.53
Ba 9550 11030 9020 9340 8709 20000
Ce 803 874 821 716 746 930
Nd 329 361 328 292 300 379
Sm 36.7 40.8 37.0 32.6 33.5 43.1
Hf 44.9 50.9 46.1 41.6 42.5 53.8
Th 6.66 7.76 6.41 5.29 5.26 7.33

Compared to other Lamproit provinces such as West Kimberley in Western Australia, the rocks of the Smoky Butte are depleted in rubidium , lead and thorium . What is striking, however, are the extremely high levels of barium (up to 2 percent by weight). Also hafnium shows elevated concentrations.

Isotope ratios

Neodymium-strontium isotope diagram with the position of the lamproites of Smoky Butte (blue) compared to other magma provinces

The following initial ratios were determined for the radioisotopes of Sr, Nd and Pb:

Isotopes Smoky butte
87 Sr / 86 Sr 0.70592 0.70587 0.70603 0.70633 0.70628 0.70630
143 Nd / 144 Nd 0.51137 0.51137 0.51143 0.51137 0.51128 0.51151
206 Pb / 204 Pb 16.145 16,469 16,146 16,037 16.025 16.643
207 Pb / 204 Pb 15.193 15,527 15.218 15.209 15.190 15.277
208 Pb / 204 Pb 36.27 36.58 36.37 36.25 36.195 36.68

The lamproites from Smoky Butte are among the least radiogenic rocks in the world with very low lead isotope ratios and very low ϵ Nd. Like the lamproites of the Leucite Hills, they follow a steep trend in the isotope diagram 143 Nd / 144 Nd compared to 87 Sr / 86 Sr with simultaneously low Sm / Nd and Rb / Sr ratios. They differ very clearly from rocks in other magma provinces.

Age

Using the potassium-argon method, Marvin and colleagues determined an age of 27 ± 3 million years BP for the lamproite on phlogopite, which corresponds to the Chattium . This is the youngest magmatic age known in eastern Montana to date.

Individual evidence

  1. ^ Wagner, C. and Velde, D .: Davanite, K 2 TiSi 6 O 15 , in the Smoky Butte (Montana) lamproites . In: American Mineralogist . tape 71 , 1986, pp. 1473-1475 .
  2. ^ Matson, RE: Petrography and Petrology of Smoky Butte Intrusives, Garfield County, Montana . In: Master of Science Thesis . Montana State University, Missoula 1960.
  3. ↑ Velde, D .: Armalcolite-Ti-phlogopite-diopside-analcite bearing lamproites from Smoky Butte, Garfield County, Montana . In: American Mineralogist . tape 60 , 1975, pp. 566-573 .
  4. ↑ Mitchell, RH: A review of the mineralogy of lamproites . In: Spec. Publ. Geol. Soc. S. Afr. 1985.
  5. ↑ Fraser, KJ et al .: Sr, Nd and Pb isotope and minor element geochemistry of lamproites and kimberlites . In: Earth and Planetary Science Letters . tape 76 , 1985, pp. 57-70 .
  6. ^ Marvin, RF, Hearn, BC, Jr., Mehnert, HH, Naeser, C. W., Zartman, RE and Lindsey, DA: Late Cretaceous-Paleocene-Eocene igneous activity in North-Central Montana . In: Isochron / West . tape 29 , 1980, pp. 5-25 .