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Occurrence and Petrological Significance of Loveringite in Thewestern Laouni Layered Complex, Southern Hoggar

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Occurrence and Petrological Significance of Loveringite in Thewestern Laouni Layered Complex, Southern Hoggar Canadian Mineralogist Vol. 25, pp. 683-693 (1987) OCCURRENCE AND PETROLOGICAL SIGNIFICANCE OF LOVERINGITE IN THEWESTERN LAOUNI LAYERED COMPLEX, SOUTHERN HOGGAR. ALGERIA JEAN-PIERRE LORAND Laboratoire deMiniralogie, Museum national d'Histoire naturelle deParis. 61. rue deBuffon. 75005 Paris Cedex, France* JEAN-YVES COTTIN Dipartement de Giologie, 1ST, University d'Oran Es Senia, Oran, Atgirie GIAN CARLO PARODI Dipartmento di Scienze delta Terra, Universita di Roma «La Sapienzan, piazzale A-Moros, 00185 Roma, Italy ABSTRACT textes petrologiques et geochimiqucs diffcrents, les ortho- cumulats a olivine et chromite et les adcumulats d'anor- Loveringite(Ca. REE)(Fe, Ti, Cr)2l038 is an accessory thosites. Dans les orthocumulats inferieurs, la loveringite but not a rare mineral in layered complexes. It has been est ilroiiemcnt associee au spinellc chromifere dans les espa- identified in the lower banded zone ofthe Western Laouni ces imercumulus, coexistant avec pargasite litanifere, phlo- intrusive complex, southern Hoggar, Algeria. It occurs gopite sodico-potassique, ilmenite magnesienne, rutile et mainly in two distinct petrographic and geochemical set pseudobrookite magnesienne. Elle est plus richc en Ti et tings, in olivine - chromite orthocumulates and in anor- plus pauvre en Zr et en terrcs rarcsque dans les composi thosite adcumulates. In the lowermost orthocumulates, tions publiees. Dans les adcumulats d'anorthosite au som- loveringiteis closely associated with chrome spinel in the mct de la zone inferieure litec, la loveringite peut cristalli- imercumulus spaces, where it coexists with Ti-rich parga scr loin du spindle chromifere et cocxiste avec zirconolite site, (Na, K)-phlogopitc, magncsian ilmenite, rutile and CaZrTi207, zircon et de petites quantites d'ilmenite et magnesianpseudobrookite. It is richer in Ti and poorerin rutile. En consequence, elle est fortcment appauvrie en Ti REE than previously known examples. In the anorthositc et enrichie en Zr et terres rarcs. Certains grains de loverin adcumulates, at the top of the lower banded zone, the gite sont totalcment inclus dans le plagioclasccumulus et loveringite crystallized away from the chromespinel; it coex dissocies des mineraux magmatiqucs tardifs. La distribu ists with zirconolite CaZrTi207, zircon, and a minor tion et les variations de composition de la loveringite de amount of ilmenite and rutile. As a result, it is strongly la base au sommet de la zone inferieure litec sont expliquees depleted in Ti and enriched in Zr and the REE. It may be par la percolation de liquides fortement evolues, expulses fully enclosed in the cumulus plagioclase and is freeof latc- lors de la compaction de l'cmpilemem des cumulats. Apres magmaticminerals. Its distribution and compositional var ('accumulation d'oxydes riches en Ti dans les cspaccs imer iations from the base of the lower banded zone to the top cumulus des orthocumulats inferieurs, ces liquides ont etc are accounted for by percolationof strongly evolved liquids appauvris en Ti mais enrichis en Zr et terrcs rares. Us ont expelled from the pileof cumulates by filter pressing. After ete arretes pendant leur migration vers le haut par les adcu the accumulation ofthe Ti-rich oxides in the imercumulus mulats d'anorthosites formant un lit stagnant dans la cham- spaces of the lowermost orthocumulates, these liquids brc magmatique et se sont infiltres le long des limites dc became depleted in Ti but enrichedin Zr and the REE.They grain du plagioclase alors que eclui-ci emit encore en cours were arrested during upward migration by the anorthosite dc croissance adcumulus. L'cffct combine dc ces deux pro adcumulates that formed a stagnant layer in :he magma cessus est probablement responsable des inclusions de love chamber; also, they infiltrated along grain boundaries of ringite dans le plagioclase. the plagioclase as the latter was still undergoing adcumu- lus growth. The two processesarc likely responsible for the Mots-cles: loveringite, Laouni-ouest, complexe intrusifbasi- inclusions of loveringite in plagioclase. que stratiforme, anorthosites, infiltration, metasoma- tisme, Hoggar, Algeric. Keywords: loveringite, Western Laouni, basic layered com plex, orthocumulates, anorthosites, infiltration Introduction metasomatism, Hoggar, Algeria. Loveringite (Ca, REE) (Ti.Fe.Cr) 210J8 is the Ca SOMMA1RE end-member ofthe crichtonitc group, ofgeneral for mula AM2]0K. It was first described as a mineral La loveringite (Ca, REE) (Fe.Ti.Cr) 210Jg est un mine species by Gatehouse et at. (1978) and Campbell & ral accessoire mais non rare dans les complexes basiqucs Kelly (1978) from bronzitites and olivine - plagioclase stratifies. Elle a ete identified dans la zone inferieure litce orthocumulates of the Lower Layered Series in the du complexe intrusif de Laouni-ouest, Hoggar meridional, Jimberlana igneous intrusive complex, Australia. Algcrie. Elle est principalement presente dans deux con- Cameron (1978) reported a complex Ca - REE - Ti •Unite associee au CNRS n°286. oxide in anorthosites and bronzitite from the Criti- 683 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/3446347/683.pdf by guest on 25 September 2021 684 THE CANADIAN MINERALOGIST cal Zone ofthe eastern Bushveld Complex, the major tallization of loveringite arc discussed by reference and minor element chemistry of which is similar to to the percolation of intercumulus liquids that take that of loveringite. This mineral is closely associated place in magma chambers (Irvine 1980). As pointed with chrome spinel, ilmenite and rutile. Because of out by Campbell & Kelly (1978) and Haggerty (1983), the absence of X-ray data, Cameron could not decide the members ofthe crichtonite group have the struc whether his oxide mineral is loveringite or armalco- tural flexibility to accept all the large-radius incom lite. Another locality in the Bushveld Complex was patible elements (e.g., Zr, Ti, REE, Th and Nb) that recently revisited by Johan & Watkinson (1986), who are concentrated in residual liquids or fluids ofcumu confirmed the presence of loveringite, but their iden lates. Indeed, they are powerful petrogenetic indi tification relies exclusively on microprobe data. cators to trace the circulation of these liquids through In this paper, we describe the third occurrence of the cumulus pile of layered basic complexes. loveringite in an igneous layered complex. This mineral has been identified by X-ray diffraction and Geological Setting microprobe investigation in the Laouni layered com plex, southern Hoggar, Algeria. It coexists either The Pan-African Laouni igneous complex (600 - with Nb - Zr-rich rutile, Mg-rich pseudobrookite and 540 Ma), 250 km south of Tamanrasset, Algeria, magnesian ilmenite, or with zirconolite (Lorand & crops out as four small intrusive massifs within gra Cottin 1987a, b). Such a diversity of oxide minerals nitic and metamorphic formations ofthe Polycyclic in basic rocks has only been described in the KREEP Central Hoggar area (Caby & Bertrand 1977, Cot- lunar basalts (Haggerty 1973). The textural relation tin 1985). The samples studied come from the mas ships, compositional range and conditions of crys sif described as the "Western Laouni" by Donville Sft »pl« Lsvfti-larlt* -o-o • o-o- -o-o •Mas 81 ta ao • • ** SH40 aft »» 3S>WC • • 8M0I 80 ee-T5 »• «MSO ai 7» • aa M-eo • • • • •• • • FIG. 1. Schematic lithology ofthe lower banded zone ofthe Western Laouni massif, showing the distribution ofloveringiteand associatedopaque minerals (Ru rutile, Ps Mg-rich pseudobrookite, II ilmenite, Zir zirconolite). The number of stars is proportional to the relativeabundance of each mineral. Key to zones: 1 urali- tized gabbros, 2 olivine - chromite cumulates, 3 bronzite orthocumulates, 4 plagioclase- olivine cumulates, and 5 anorthosites. The shaded areascorrespond to intensely serpentinized zones. Fo, An, En arc the forsterite, anorthite and ensta- tite contents ofolivine, plagioclaseand orthopyroxene, respectively. Scale bar 20 metres. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/3446347/683.pdf by guest on 25 September 2021 LOVERINGITE IN THE WESTERN LOUNI COMPLEX, ALGERIA 685 (1962). The southern part of this massifis a concen Petrographic Features trically zoned body, about 20 km2 in size, sur of the Samples Investigated rounded by uralitized gabbros. Apart from the con siderable difference in size, the western Laouni body Fifteen samples from the lower banded zone and exhibits close similarities with the Jimberlana and the massive zone have been investigated in reflected Bushveld complexes in terms of both petrography light. The number of polished thin sections studied and order of crystallization of the cumulus phases for each sample ranged from 2 to 7 except for sam (Cottin & Lorand 1986). ple 2H85, which was studied using 12 polished thin In spite of local serpentinization, the lower part sections in order to obtain a better estimate of the ofthe massif (the lower banded zone) displays layer abundance of loveringite. ing well defined by the cyclic repetition of olivine According to the descriptive classification of Irvine - chromite cumulates, bronzite cumulates and (1982), bronzite - and olivine - chromite cumulates plagioclase - olivine cumulates over a thickness of range from meso- to orthocumulates. Olivine - chro a few hundred metres (Fig. 1). The lower banded mite cumulates show more than 25% by volume of zone is overlain by massive olivine gabbro and troc- intercumulus material composed oforthopyroxene, tolite
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