Magmatic evolution of the Gaussberg lamproite (Antarctica): volatile content and glass composition

The lamproite of Gaussberg is an ultrapotassic rock where leucite, olivine and clinopyroxene microphenocrysts occur in a glass-rich groundmass, containing microliths of leucite, clinopyroxene, apatite, phlogopite and rare K-richterite.Abundant silicate melt inclusions occur in olivine, leucite and, rarely, in clinopyroxene microphenocrysts. Raman investigations on melt inclusions showed the presence of pure CO2 in the shrinkage bubbles. On the other hand, the glass of the groundmass is CO2-poor and contains up to 0.70 wt.% of dissolved H2O, as estimated by infrared spectra. It is inferred that CO2 was released at every stage of evolution of the lamproite magma (CO2-rich shrinkage bubbles), whereas H2O was retained for longer in the liquid. At Gaussberg, CO2 seems to have a major role at relatively high pressure where it favoured the crystallization of H2O-poor microphenocrysts; the uprise of the magma to the surface decreased the solubility of CO2 and caused a relative increase in water activity. As a consequence, phlogopite and K-richterite appeared in the groundmass.The glass composition of both the groundmass and melt inclusions suggests different evolutions for the residual liquids of the investigated samples. Sample G886 shows the typical evolution of a lamproite magma, where the residual liquid evolves toward peralkaline and Na-rich composition and crystallizes K-richterite in the latest stage. Sample G895 derives from mixing/mingling of different batches of magma; effectively glasses from melt inclusions in leucite and clinopyroxene are more alkaline than those found in early crystallized olivine. Leucite and clinopyroxene crystallized early from a relatively more alkaline batch of lamproite magma and, successively, a less alkaline, olivinebearing magma batch assimilated them during its rise to the surface.

Caloplaca Cancarixiticola, A New Species from South-East Spain Growing on Ultrapotassic Rocks

Caloplaca cancarixiticola Nav. -Ros., Egea & Llimona (Teloschistales, Lichenes) is described as new. It is characterized by a lobate thallus weakly attached to the substratum, and narrowly ellipsoid or ellipsoid-fusiform ascospores, (14.5–) 16–22(–25) × (4–)4–5–6um3 with a narrow equatorial wall-thickening, only (1–) 2.5–3.5(–4) um wide. This new species, related to the Caloplaca aurea-group of the subgenus Gasparrinia, was found on cancarixite, an ultrapotassic rock, in a volcanic region of Albacete (SE Spain).

Khewra Trap: An Unusual Ultrapotassic Rock in the Salt Range of Pakistan

The classical stratigraphic sequence of the Salt Range contains thin flows of an ultrapotassic rock at its base. Commonly known as Khewra trap, it occurs at the top of the very late Proterozoic to Early Cambrian rocks consisting of marly anhydrite/gypsum, and oil shales overlying evaporites. The trap is an unusual rock consisting of euhedral to skeletal, spinifex, stellate phenocrysts in a very fine-grained to cryptocrystalline, locally glassy, matrix. The phenocrysts (up to 3 cm long) are considered to be Mg-rich enstatite now completely pseudomorphed by a mineral aggregate principally made up of talc with subordinate amounts of Mg-rich clays and, locally, quartz. The matrix is unaltered and almost entirely made up of Na-Ca-poor and Mg-Fe-rich K-Feldspar (sanidine-orthoclase), with granules, specks and dendroids of Fe-Oxide. Talc, Mg-rich clays, quartz, dolomite, and Fe-oxide constitute the amygdules. Chemical analyses of the rock samples from the trap are remarkably similar in composition except for some variation in iron oxide due, probably, to leaching during alteration. The rocks consist approximately of 60 wt% SiO2, 0.7% TiO2, 11% Al2O3, 2-5% Fe2O3, (total), 0.02% MnO, 10% MgO, 0.4% CaO, 0.5% Na2O, 9% K1O, and 0.04% P2O5. Normatively the rocks are essentially made up of orthoclase and orthopyroxene. The volcanism may be related to Late Proterozoic-Early Paleozoic rifting which also resulted in deposition of the evaporites, however, the major element chemistry casts doubts on such an interpretation. Detailed trace- and rare earth element geochemistry is in progress to throw light on the petrogenesis of these highly unusual rocks.

Conditions of phlogopite crystallization in ultrapotassic volcanic rocks

Phlogopite occurs as an early crystallizing mineral in many ultrapotassic lavas of basaltic affinities. Based on high-pressure experiments in lavas of these compositions, the early crystallization of phlogopite is controlled in large part by the bulk compositions of the liquids from which it crystallizes but also by the total pressure and by the aH2O, with early phlogopite forming under a narrow range of aH2O, less than that represented by H2O-saturated conditions. Variations in fO2 do not appreciably affect phlogopite crystallization but high aCO2 suppresses its crystallization. In ultrapotassic magmas, phlogopite will preferentially incorporate K2O, TiO2, MgO, and Al2O3 relative to the coexisting early silicate minerals, olivine and clinopyroxene, and thus, on fractionation of these minerals, phlogopite will be more effective in reducing these oxides in residual liquids. Phenocrysts and microphenocrysts of phlogopite in ultrapotassic lavas are directly related with respect to their K/Ti, K/Al, K/(K + Na), and Mg/(Mg + Fe) ratios. Textural relations suggest phlogopite may form by reaction relationships involving liquid with olivine, and/or clinopyroxene. Such relationships are supported by the experimental studies on ultrapotassic rock compositions.