Na- Ti- Zr- H2O-rich mineral inclusions indicating postcumulus chrome-spinel dissolution and recrystallization in the Western Laouni mafic intrusion, Algeria

Abstract

Disseminated cumulus chrome spinel in the lower-most olivine orthocumulates from the Western Laouni intrusion (Southern Hoggar, Algeria) contains inclusions of silicates enriched in Mg-Ti-Na and H2O, and Fe-Ti (Zr) rich oxides, occurring either as numerous micro-inclusions or as large solitary cavities. Regardless of the inclusion type, titanian pargasite (or kaersutite) and the sodium analog of phlogopite predominate over orthopyroxene (En86 to En92) and Na-rich plagioclase (An50Ab50 to An7Ab93). Oxide inclusions are magnesian ilmenite (up to 37 mol% MgTiO3), rutile, loveringite (Ca,REE-(Ti,Fe,Cr,Mg,Zr)21O38) and accessory magnesian pseudo-brookite (“kennedyite”); apatite, Fe-Ni-Cu sulfides and alteration products (saussurite, bastite) are also present. Apart from kennedyite, all minerals included in chrome spinel form larger intercumulus crystals in the host rock as well. The micro-inclusions were trapped as the consequence of chrome spinel dissolving against the intercumulus liquid, 150 to 300° C below its liquidus temperature. The solitary cavities are attributed to fluid-assisted solid-state recrystallization of chains of spinel crystals; the role of capillary fluids is demonstrated by both the hydrous nature of solid inclusions and a strong positive correlation between the amphibole content and the abundance of solid inclusions in the rock. Phlogopite locally showing similar optical orientation within and outside the inclusions indicates that spinel was still recrystallizing down to 950° C. In the temperature range 1100-900° C determined for the formation of inclusions, chrome spinel probably trapped various combinations of crystals, liquids and fluids, the respective quantities of which varied greatly over distances of few tens of micrometres in a single spinel. The volume ratio of solids to liquids or to fluids must have been low enough to permit magmatic or hydrothermal reactions, which are otherwise never possible for the larger intercumulus crystals. Hence, the lack of olivine and clinopyroxene as inclusions in spinel while abundant in the host rock suggests that, as trapped, they completely reacted with the residual liquid to form phlogopite and pargasite respectively. Likewise, plagioclase and phlogopite have been enriched in Na against hydrothermal fluids to form albite and Na-phlogopite in the mineral inclusions.