No abstract

Abstract-We report on a new angrite, Northwest Africa (NWA) 1296, a fine-grained rock with a magmatic texture of rapid cooling. Dendritic olivine (≈Fo 50 ) crystallized first in association with anorthite microcrysts (An 98-100 ) forming composite chains separated from one another by intergrown Al-Fe diopside-hedenbergite pyroxenes. In addition, some olivines with lower Mg# and increased CaO (up to 12%) are found between the chains as equant microphenocrysts. Pyroxenes and olivines are both normally zoned from Mg# = 0.52 to less than 0.01 in the rims. Ca-rich olivines are surrounded by, intergrown with, or replaced by subcalcic kirschsteinite. They appear after plagioclase crystallization stopped, at the end of the crystallization sequence. Minor phases are pyrrhotite, Fapatite, and titanomagnetite. Pyroxene is the last silicate phase to grow, interstitial to idiomorphic olivine-kirschsteinite. Numerous small vesicles and some channels are filled with microcristalline carbonate. The mode (vol%) is about 28% olivine, 3% kirschsteinite, 32% anorthite, 34% pyroxene, and 3% of the minor phases-close to that reported previously for D'Orbigny and Sahara (SAH) 99555. The bulk chemical composition of NWA 1296 is similar to D'Orbigny and SAH 99555; NWA 1296 differs by its texture and mineralogy, which are interpreted as resulting from rapid crystallization-an evidence of impact melting. Angrites cannot be produced by partial melting of a CV source because segregation of a "planetary" core is necessary to explain the low FeO/MgO ratio of magnesian olivines. Neither the odd Ca/Al ratio nor the very low SiO 2 content can be explained by conventional partial melting scenarios. We suggest that carbonate is the key to angrite genesis. This is supported by the striking similarities with terrestrial melilitites (low SiO 2 , superchondritic Ca/Al ratio, presence of carbonate). The lack of alkalies could be the result of either loss after impact melting or absence of alkalies in the source.

The bulk chemical composition confirms that NWA 1670 corresponds to a normal angrite melt that incorporated olivine. High Mg olivine xenocrysts and the associated mineralogy are typical of angrites. We suggest that it is an impact melt with relict phenocrysts. The strong silica undersaturation, the presence of Fo90 olivine xenocrysts and carbonate support their derivation as melilite-like melts in the presence of carbonate.

The melilite-bearing skarns of Cornet Hill (CH) and Upper Cerboaia Valley (UCV), in the Apuseni Mountains of Romania, occur at the contact between monzodiorite bodies of Ypresian age (Paleocene) and Neojurassic calcitic marbles. Typical wollastonite -grossular -diopside endoskarns are separated from exoskarns (tilleyite and spurrite or wollastonite at CH, wollastonite only at UCV), at most places, by a melilite-rich rock, in which veins and vein-like zones of recrystallization are composed only of idiomorphic melilite crystals reaching 15 cm across. Titanian garnet and wollastonite are the principal minerals associated with melilite (and also monticellite, perovskite, vesuvianite, cuspidine, spurrite, tilleyite, calcite, hydroxylellestadite, hydrogrossular and other minor alteration-induced minerals). A different association that includes aluminian diopside and grossular occurs (1) as veinlets in the marble close to the skarns and (2) as relict inclusions in endoskarns. From the geometrical relationships of the zone sequences and the veins, the textural features of the mineral associations and the inferred conditions of fluid-mineral equilibrium, these mineralogical peculiarities are interpreted as resulting from the superposition of two main stages. Firstly, there was circulation of a comparatively CO 2 -rich fluid formed the early aluminian diopside -grossular endoskarns, with depletion in Si (and Fe, Na, K) and inert behavior of Mg, Al, Ti. Then, a high-temperature (750°C) fluid circulated on both sides of the contact between marble and endoskarns, and developed the melilite-rich, titanian-garnet-bearing rocks partly at the expense of previously formed endoskarns, and spurrite or wollastonite (CH) or wollastonite (UCV) exoskarns at the expense of marble. The pressure of CO 2 was very low, less than 26 bars at UCV and 16 bars at CH, with a H 2 O pressure less than 750 bars. Not only Si and Ca were mobilized, but also Mg, Al and Ti, leached from the endoskarns and deposited in the veins and the nearby part of exoskarns. This stage, which occurred in the temperature range corresponding to the end of the crystallization of plagioclase in the monzodiorite, has pegmatitic chemical and textural features. The main flow of fluid ended with the development of tilleyite partly at the expense of spurrite and wollastonite at CH, and local high-temperature (about 710°C) recrystallization of the zonation, mostly in veins, especially in the endoskarn-exoskarn boundary, but also within the endoskarns. A monticellite -gehlenite association appeared in the melilite-rich rocks, later followed by vesuvianite, whereas in the endoskarn, vesuvianite developed together with coarse-grained wollastonite and grossular.