Rare Earth Element Evidence for the Petrogenesis of the Banded Series of the Stillwater Complex, Montana, and its Anorthosites

Loferski, Patricia J.; Arculus, R. J.; Czamanske, Gerald K.

doi:10.1093/petrology/35.6.1623

Cited by 11 publications

(3 citation statements)

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“…Evidence for the migration of evolved interstitial liquid in the Upper Anorthosite zone is seen in high bulk-rock REE concentrations (La = ∼10 × chondrite) in some rocks that have flat to slightly negative Eu anomalies. These high REE concentrations occur in anorthosites for which the plagioclase separates from the same rocks have strong positive Eu anomalies and compose 85% + of the rock mode (Boudreau, 1986;Loferski et al, 1994) (Fig. 30).…”

Section: The Anorthosite Zonesmentioning

confidence: 99%

The Stillwater Complex, Montana – Overview and the significance of volatiles

Boudreau

2016

Mineral. mag.

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The geology of the 2.7 Ga Stillwater Complex in South-Central Montana is reviewed with a focus on the role of volatiles in locally modifying both the crystallization sequence of the evolving parent magma and the initially precipitated solid assemblages to favour olivine ± chromite. A secondary origin for these two minerals is particularly probable for the olivine-bearing rocks of the Banded series and, at a minimum, also increasing their modal abundance in the Peridotite zone of the Ultramafic series. Direct evidence for volatiles includes the presence of high-temperature fluid inclusions in pegmatoids and hydrous melt inclusions (now crystallized) in chromite and olivine from both the Ultramafic and the Banded series rocks. Indirect evidence includes the boninitic character of the parent magma, the presence of volatile-bearing minerals including high-temperature carbonates, rock textures, and Cl / F variations in apatite. Mechanisms which favour the formation of olivine (± chromite) over pyroxene include volatile phase boundary shifts induced by added H2O, incongruent melting of pyroxene by hydration of a partly-molten mush, and the near- to sub-solidus replacement of pyroxene by olivine and chromite by silica-undersaturated fluids. These mechanisms cast doubt that magmas with different liquid lines of descent were involved in the crystallization of the Stillwater Complex. A dry Stillwater magma would have been mineralogically and modally much less varied and lacking in high-grade platinum-group element and chromium deposits.

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Section: The Anorthosite Zonesmentioning

confidence: 99%

The Stillwater Complex, Montana – Overview and the significance of volatiles

Boudreau

2016

Mineral. mag.

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“…An Archean fractionated magma pool of possible impact origin is exposed as the stratiform Stillwater complex, 2.7 Ga, in southwest Montana, which was little deformed until it was tilted and faulted in early Tertiary time. It crystallized from diverse melts of mixed crustal and mantle sources (Loferski et al, 1994), and many of its rocks have Nd and Pb isotopic features indistinguishable from those of nearby older granites and gneisses (Czamanske and Bohlen, 1990). The complex consists of a basal 2 km of cumulate harzburgite and orthopyroxenite, another 2 km mostly of norite, and an upper 3 km of anorthosite (typically ~An 80 ), anorthositic troctolite, norite, and gabbro.…”

Section: Anorthosite and Allied Fractionates May Record Impact-melt Lmentioning

confidence: 99%

Earth's first two billion years—The era of internally mobile crust

Hamilton¹

2007

Geological Society of America Memoirs

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The magmatic and tectonic processes of the pre-2.5 Ga hot, young Earth differed profoundly from those of the modern planet. The ancient rocks differ strikingly in individual and collective composition, occurrence, association, and structure from modern rocks. Widespread forcing of Archean geology into plate-tectonic frameworks refl ects unwarranted faith in uniformitarianism and in inappropriate chemical discriminants, and disregard for the lack of features that characterize plate interactions. Archean crust records extreme and prolonged internal mobility and was far too weak and mobile to behave as rigid plates, required, by defi nition, for plate tectonics. None of the geologic indicators of subduction, arc magmatism, and continental sundering, separation, and convergence have been documented. No Archean oceanic crust or mantle has been recognized, and the only known basement to supracrustal rocks, including the thick basalts, high-Mg basalts, and ultramafi c lavas that typify greenstone successions, consists of tonalite-trondhjemite-granodiorite (TTG) migmatites and gneisses. A thick global melabasaltic protocrust likely formed by ca. 4.45 Ga, and from it TTG suites were extracted by partial melting over the next 2 b.y. Delamination of the increasingly dense restitic protocrust enabled rise of lighter and hotter depleted mantle and hence more melting. The oldest known crustal materials are zircons, which scatter in age back to 4.4 Ga and are recycled in migmatites whose fi nal crystallization was after 3.8 Ga, and in ancient sediments. Earth may have had a dense greenhouse atmosphere, not a hydrosphere, before 3.6 Ga, for the oldest proved supracrustal rocks are of that age, and older felsic crust may have been too hot to permit rise of dense melts. Rigid plates of lithosphere did not stabilize until a billion years after that and then were mostly small and local. Dense lavas erupted atop mobile felsic crust after 3.6 Ga produced a density inversion that was partly righted by sinking of the volcanic rocks and rising of the subjacent TTG. In some places, the early dense rocks retained cohesion and sank as synclinal keels between rising domiform diapiric batholiths. In others, the early dense rocks sank deep into mobile TTG crust, and only later in Archean time was the felsic substrate strong enough to enable dome-and-keel style. The TTG substrate rose slowly, with variable amounts of partial melting to generate more-fractionated melts

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“…For this reason and considering that the Basal Gabbro Unit is not per se part of the main layered sequence of the Uitkomst Complex, we have largely ignored this unit for the purpose of the present paper. The composition of the parental magma can also be calculated from cumulus minerals, using suitable partition coefficients (Lambert & Simmons 1987, Loferski et al 1994, Mathez 1995, or from a putative trapped melt component (Bédard 1994). However, these methods have to correct for re-equilibration of the cumulus phases with trapped melt and zoning in some minerals, such as plagioclase and orthopyroxene.…”

Section: Age Datingmentioning

confidence: 99%

Parental Magma and Emplacement of the Stratiform Uitkomst Complex, South Africa

Waal¹,

Maier²,

Armstrong³

et al. 2001

The Canadian Mineralogist

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The Uitkomst Complex, a Ni-Cu-PGE-Cr-mineralized layered basic intrusion situated approximately 200 km due east of Pretoria, South Africa, hosts commercially viable magmatic base-metal sulfide deposits. The Complex has a concordant 207 Pb/ 206 Pb zircon age of 2044 ± 8 Ma, suggesting that it is coeval with the Rustenburg Layered Suite (RLS) of the Bushveld Complex. Chemical modeling, using summation and least-squares optimization techniques, provides evidence that the boninitic Bushveld B1 magma is parental to both the lower ultrabasic and upper basic layered series of the Uitkomst Complex. The layered series crystallized in two stages, i.e., a lower conduit and an upper closed-system stage. The tubular shape of the Uitkomst Complex is the result of the intersection of a near-horizontal bedding plane fault with an existing vertical fracture zone under tensional conditions. During the conduit stage, a combination of magma mixing, contamination and flow dynamics may have facilitated sulfide formation and segregation. The identification of Bushveld B1 magma as the major parental magma of the Uitkomst Complex has significance in the exploration for similarly mineralized sub-RLS intrusive bodies. Mots-clés: massif intrusif basique stratiforme, complexe de Uitkomst, minéralisation en Ni-Cu-PGE-Cr, mélange de magmas, ségrégation de sulfures, Afrique du Sud. §

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Rare Earth Element Evidence for the Petrogenesis of the Banded Series of the Stillwater Complex, Montana, and its Anorthosites

Cited by 11 publications

References 0 publications

The Stillwater Complex, Montana – Overview and the significance of volatiles

The Stillwater Complex, Montana – Overview and the significance of volatiles

Earth's first two billion years—The era of internally mobile crust

Parental Magma and Emplacement of the Stratiform Uitkomst Complex, South Africa

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