Showing papers on "Subsolvus published in 1989"

Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks*

Abstract: Homogeneous and heterogeneous phase relationships in the alkali feldspars are reviewed, and behaviour diagrams developed. A1,Si ordering is almost certainly continuous and higher order in both albite and potassium feldspar and has been established reversibly or nearly so down to below 500 ~ in albite and possibly to ~ 200 ~ in potassium feldspar. The degree of order in intermediate albite changes strongly over a range of ~ 75-150 ~ depending on pressure, low albite being stable up to about 620650 ~ and high albite above about 725 ~ at low pressure. Symmetry is broken at ~ 980 ~ mainly by a cooperative shearing of the whole framework and not by A1,Si ordering alone; there is a thermal crossover near 700~ shearing being dominant above (high albite) and ordering dominant below (intermediate albite). In potassium feldspar symmetry is broken by A1,Si ordering at a temperature of about 500 ~ The change in degree of order with respect to temperature has been followed easily and reversibly in sanidine from ~ 1075 to ~ 550 ~ and to a lesser extent in microcline from 450 to 200 ~ Ordering rates in sanidine down to 500 ~ and ordering rates in microcline between 450 and 200 ~ are almost as fast as in albite. Ordering in sanidine at 500 ~ and below slows and then stops with the development of the tweed orthoclase domain texture. The tweed texture acts as a barrier to further order because the strain energy associated with the (incipient) twin domain texture balances or nearly balances the free energy decrease resulting from ordering. Ordering stops not because of the kinetics of A1,Si diffusion, but because the total driving force is very small or nil. Ordering can readily proceed to completion, with the formation of low microcline, only if the domain-texture barrier is overcome by processes involving fluids or strong external stresses. There is no barrier in albite. The symmetry-breaking process in alkali feldspar changes with composition from mainly shearing in albite to ordering in potassium feldspar. Symmetry is broken equally at a compositional crossover (metastable with respect to exsolution) near Abso 75 at low pressure and progressively displaced towards Or at higher pressures. Ordering in pure albite occurs by a (nearly) one-step path which progressively becomes two-step with substitution of Or. Diagrams showing the near-equilibrium variation of the order parameters at low pressure with composition and T are given, as well as two extreme phase and behaviour diagrams for complete coherent and complete incoherent (strain-free) relationships, These diagrams can be used to understand feldspar relationships and microtextures in hypersolvus and subsolvus rocks, the occurrence of orthoclase, and of intermediate and low microcline. K~O,VORDS: alkali feldspars, ordering rates, phase transformations, coherency.

An Anorthosite Suite in a Ring-Complex: Crystallization and Emplacement of an Anorogenic Type from Abontorok, Aïr, Niger

Abstract: Laminated anorthosite grading outwards into leucogabbro, gabbro, and monzogabbro occurs in a 2.6-km-diameter funnel-shaped intrusion, cut by a quartz alkali syenite plug and concentric syenite and granite ring-dykes. The anorthosite-gabbro series is laminated but not modally or otherwise texturally layered. The lamination, defined by large tabular plagioclase crystals, forms a set of inwarddipping cones, the dips of which decrease from 60-45° in the central anorthosite to 80% of the series, generally has homogeneous labradorite cores (An 62-58 in the whole series) and thin strongly zoned rims, which follow progressively longer solidus paths from the anorthosites to the gabbros. All rocks contain a late-magmatic alkali feldspar. Plagioclase is the main or only cumulus phase, the anorthosites being ad- to mesocumulates and the gabbros orthocumulates. Olivine (FO 49-41) is more abundant than clinopyroxene in most of the series. Depending on quartz content, the syenites and granites are hypersolvus or subsolvus and the depth of crystallization was calculated to be 5 ± 2 km.A Rb/Sr isochron for the syenites and granites gave an age of 399 ± 10 Ma with an initial strontium isotopic ratio of 0.7084 ± 0.0005. Ten samples from the anorthosite-gabbro scries have an average calculated initial ratio of 0.70582 ± 0-00004 at - 400 Ma, showing that the two series are not comagmatic. The anorthosite-gabbro series has parallel REE trends (La N/Yb N 7-10) with decreasing positive Eu anomalies and increasing total REE contents from anorthosite to gabbro; two monzogabbros have almost no Eu anomaly. The liquid calculated to be in equilibrium with the lowest anorthosite has almost no Eu anomaly and its normalized REE pattern lies just above those for the monzogabbros. The syenites and granites have complementary REE patterns with negative Eu anomalies.The inferred parental magma was alkalic and leucotroctolitic with high TiO 2 P 2O 5, Sr and K/Rb and with low MgO, very similar to parental magmas in the Gardar province, South Greenland. It was probably produced at depth by settling of olivine and clinopyroxene but not of plagioclase, which accumulated by flotation. It is suggested that plagioclase crystals from this lower chamber were progressively entrained (from 0% in the gabbros to 30-40% in the anorthosites), giving rise to the flow lamination in the upper chamber. The magma in the lower chamber may have been layered, because the plagioclase cores in the anorthosite are considerably richer in Or than those in the leucogabbros or gabbros. Overall convection did not occur in the upper chamber, whereas compositional convection occurred in the more slowly cooled central anorthositic adcumulates. © 1989 Oxford University Press.