Some critical differences between orogenic-plutonic and gravity-stratified anorthosites

Berrangé, Jevan P.

doi:10.1007/bf02029646

Cited by 17 publications

(4 citation statements)

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“…Anorthosites are widely distributed in terranes of Grenville age throughout the world, but there is still no general agreement on their genesis or even their classification. (See, for example, Middlemost, 1970;Berrange, 1965. ) Since the Roseland type is so restricted in occurrence, it has not been discussed in recent review articles on anorthosite, with only one exception (Isachsen, 1969).…”

Section: Roseland Anorthositementioning

confidence: 99%

Geology and mineral deposits of the Roseland District of central Virginia

Herz¹,

Force²

1987

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The Roseland district of Nelson and Amherst Counties, Va., is centered on an intermontane basin along the southeastern margin of the Blue Ridge. The Grenville-age igneous rocks in this area are less altered than elsewhere in the Blue Ridge, and exposure is sufficient to map their relations. The district was formerly an important producer of titanium minerals and still contains over 5 percent of U.S. identified resources of those minerals. The valuable minerals, rutile and ilmenite, formed in two stages which largely correlate with anorthosite and ferrodiorite, respectively.The oldest rocks (apparently pre-Grenville) are garnet-graphitepyroxene-blue quartz granulites and quartz mangerites. Platytextured leucocratic charnockites are associated with these rocks but are probably a younger neosome.The Roseland Anorthosite (about 1,050 Ma) intrudes these older rocks as a largely concordant basal sheet. Crosscutting relations with country rock are abundant, however. The anorthosite consists of andesine-antiperthite megacrysts in a granulated oligoclase-potassium feldspar matrix. Its margins contain pyroxene and blue quartz megacrysts, probably as a result of high-temperature reaction between anorthosite melt and country rocks. The unusually alkalic nature of the anorthosite has led to unusual rutile mineralization in this contaminated marginal facies and its immediate country rocks, especially in swarms of anorthosite sills in mangeritic rocks forming the anorthosite roof along the eastern side of the body.The Shaeffer Hollow Granite (about 990 Ma) is a coarse leucocratic blue quartz granite, porphyritic with tabular feldspars. It cuts granulite but is present largely as roof pendants and screens in the younger ferrodioritic plutons, except in a wide deformation zone where it appears to be the dominant protolith of mylonitic rocks.The Roses Mill and Turkey Mountain ferrodiorite-charnockite plutons (about 970 Ma) together form a largely concordant upper intrusive sheet. Intrusive relations with older units, however, are abundant. These rocks have high ilmenite-apatite-(zircon) contents that reflect distinctively high values of Ti, P, Zr, Fe/Mg, and K/Si. Blue quartz is present only as abundant xenocrysts. Much of these plutons is isochemically altered to biotitic augen gneiss. Layered concordant impure nelsonite (ilmenite-apatite rock) is locally found along the bases of these plutons, and nelsonite forms discordant bodies in the immediately underlying country rock. These lithologies probably formed by liquid immiscibility. Nelsonites and ilmenite-rich ferrodiorite constitute the main ilmenite resources of the district.The anorthosite and ferrodiorite-charnockite plutons are apparently not comagmatic but probably cogenetic. The anorthosite formed in a deeper crustal environment than ferrodiorite and probably was emplaced as a diapir into the higher crustal level.In the northwestern side of the district are ferrodioritic charnockitic rocks of the Pedlar massif. They are about 1,040 m.y. old but are separated from all othe...

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Section: Roseland Anorthositementioning

confidence: 99%

Geology and mineral deposits of the Roseland District of central Virginia

Herz¹,

Force²

1987

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Section: Introductionmentioning

confidence: 99%

“…An attempt to classify known anorthosites was proposed by Ashwal (1993) and modified by Ashwal and Bybee (2017) and Ashwal (2020). This classification includes the works of Buddington (1939, 1975); Berrange (1966); Romey (1968); Middlemost (1970); Moshkin and Dagelaiskaja (1972); Scoates (2000); and Namur et al (2011). Ashwal (2020) subdivided anorthosites into six basic types: (a) Primordial Anorthosites of the Moon , which are monomineralic coarse‐grained rocks forming the lunar highlands, with very calcic plagioclase (An 93–98 ) that crystallized not later than 4.3 Ga; (b) Archean Megacrystic Anorthosites which enclose calcic plagioclase (An61‐94, Ashwal & Bybee, 2017); (c) Proterozoic Anorthosites (massif‐type) formed between ~0.5 and ~2.6 Ga as small plutons (1–10 km 2 ) to large composite batholiths (up to 18,000 km 2 ) with their plagioclases crystals intermediate in composition (An30‐70, Ashwal & Bybee, 2017); (d) Anorthosites in layered mafic complexes in which variably thick anorthosites layers overlay mafic layers; (e) Anorthosites of oceanic settings , which by extension include those associated with ophiolites, likely resulting as cumulates from basaltic magmas (Samail‐Oman, Late Cretaceous, Pallister & Hopson, 1981; Marum‐Papua New Guinea, late Eocene, Jacques, Chappell, & Taylor, 1983); and (f) Anorthosites inclusions in other igneous rocks , originating as cognate plagioclase cumulates from the same magma as their host (cognate inclusions), as gradational differentiation products in the suite of their cognate igneous rocks, as an independent magma batch intruding the igneous host or as fragments of older anorthosites (xenolithic inclusions).…”

Section: Introductionmentioning

confidence: 99%

Petrology and geochemistry of the Batchingou anorthositic suite rocks (Bana volcano‐plutonic complex, Cameroon Volcanic Line): Inference on their origin and relation with host granites

et al. 2022

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This study focusses on field mapping, petrographic description, and geochemical analyses of the Batchingou anorthositic suite rocks (Bana Plutono-volcanic Anorogenic Complex, Cameroon Volcanic Line). This suite consists of gabbroic (gabbro, gabbronorite, norite, and anorthosite) and charnockitic (mangerite, jotunite, enderbite, and monzogranite) rocks and intrude Ypresian granites (51 ± 1 Ma), within an elliptically shaped corridor of 1200 Â 600 m 2 . Gabbroic rocks evolved by crystal fractionation of olivine, hypersthene, augite, plagioclase, and ilmenite following the path: gabbro ! gabbronorite ! norite ! anorthosite. There is little or no crustal assimilation in this suite, contrarily to the charnockitic suite which likely evolved from a jotunite melt by assimilation of the Pan-African granito-gneissic basement rocks. Anorthosite has high Eu/Eu* (2.04-2.66), typical of anorthosites formed as cumulates. The La/Ta ratios (9.75-20.1 in gabbro, 21-45 in anorthosite and 23-76 charnockitic rocks), La/Yb N (4.2-19.5), Nb/U (22-90), and Nb/Th (5-23) suggest that the parental magma of the Batchingou anorthositic suite rocks is an alkaline basaltic melt deriving from a garnet-bearing asthenospheric mantle, but was variably modified during ascent by lithospheric spinel lherzolites and crustal rocks. The Batchingou gabbro has characteristics of evolved tholeiites and match the composition of the Laramie high-Al gabbro, whereas jotunite shows compositional affinity to ferrodiorites from Proterozoic anorthosite complexes. The gabbro Ni (19-39 ppm) and Cu (44-118 ppm) contents are low, indicating that their parental melt is not primitive. The Batchingou anorthositic suite differentiated from a different magma batch postdating the host granitoids and is similar to the anorthosite dykes of the Mboutou ring complex (Cameroon) and most of the ring complexes of the Aïr Province (Niger).

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“…Massif-type anorthosites have certain characteristics (1): 1) They form plutons of batholithic proportions that are largely unlayered but appear to be concordant to country rock in Precambrian terranes.…”

mentioning

confidence: 99%