The typochemical features of pyroxenes from Paleozoic picrite dikes within Spitsbergen Archipelago

Burnaeva M.Yu.*, Antonov A.V.**, Sirotkin A.N. ***

*FGUP VNIIOkeangeologiya, St.Petersburg, Russia;** FGUP VSEGEI, St.Petersburg, Russia;***FGUNPP PMGRE, Lomonosov, Russia.

Picrite dikes are located within the West Spitsbergen Island, in the east and the south of an Andre Land in the area of a Krjusspjunten Cape, a Peterman Cape (western coast of Vejde-Fiord), a Purpurdallen Valley, and also around a glacier Rubinbreen - Haakon VII Land in the Devonian graben filled with redrocks. Geologists of FGUNPP PMGRE discern two types of the dikes: those saturated with xenoliths (type 1) and with mica (type 2). The dikes have been described earlier (Evdokimov, etc., 2006).

A pyroxene is one of the main rock-forming minerals for dikes of the both types. Discerned are three generations of the mineral: phenocrysts, groundmass microlites and xenocrysts. The pyroxene makes 20 to 60 % in the total, consisting mainly of microlites; phenocrysts and xenocrysts being under 1 %. Considered below is the composition of precisely the pyroxenes from dikes without regard for those from xenoliths..

Phenocrysts represent short-prism grains 0.4 to 1.2 mm large, occasionally up to 3 mm long, often colored in tints of brown, being rarely slightly greenish or achromatic. The grains are zonal with the largest showing up to 3 zones.

The groundmass pyroxene is present as long-prism grains colored brown, 0.05 to 1.0 mm long. The grains vary in dike section from small (0.05 to 0.2 mm) at the rim up to elongated (0.2 to 1.0 mm) in the center. Twinned grains and rosettes of needle-shaped units are discerned among microlites.

Microprobe analysis of the grains has been performed to reveal the features of composition of the pyroxenes (analysts: A.V.Antonov (FGUP “VSEGEI”), Ju.L.Kretser (LLC “PC +”)). The results are shown in Table 1.

Started with phenocryst crystallization the process of mineragenesis had been completed by forming groundmass microlites. Along the same direction, the composition of pyroxenes shows decrease in SiO₂, MgO, Na₂O and increase in TiO₂, CaO, Cr₂O₃ in dikes of the both types; contents of Al2O3 being slightly lower in dikes of type 1 and much higher in dikes of type 2. FeOt shows nearly no variations in quantity, but samples from dikes of type 1 contain for the most part ferrous oxide. A share of ferric oxide increases in pyroxenes from picrites of type 2 (table 1). It should be noted from the latter that oxidation of the environment for pyroxene generation in dikes of type 1 was of lower potential.

As a whole, the elements in pyroxenes of dikes of the both types demonstrate a similar nature. The difference is observed but in the content of Al₂O₃ that may be a result of its high content in the melt and undersaturation of the melt in SiO₂ at final stages of its crystallization.

All the pyroxenes studied concern Ca-Mg-Fe-clinopyroxenes. Three mineral varieties (diopside, augite, fassaite) are discerned by crystallochemical formulae (Mineraly, 1981). Among minals, dominating are diopside (20-77 %), tschermakite (3-25 %), hedenbergite (0-43 %), enstatite (0-27 %). The highest average contents of the first two and the latter two components have been revealed in microlites and in megacrysts of dikes of the both types, respectively.

Large phenocrysts of pyroxenes are characterized by zoning resulted from rapid change in crystallization conditions when the mineral composition has not managed to come to equillibrium with the surrounding melt. To study the zoning, 6 grains from 3 samples (table 2) were analyzed. Three zones are pronounced as a whole. Distribution of the elements within the zones shows a similar pattern in all the samples. The content of SiO₂ and MgO is established to increase toward the second zone then decreasing toward the rim. Al₂O₃ and FeOt in most part of the grains decrease toward the second zone then increasing toward the rim (exception is one grain from dike of type 1 showing subsequent core-to-rim increase in aluminia). TiO₂ and CaO increase from core to rim. The composition of the rim zone and in part of the second zone of grains is similar to that of microlites that indicates their syngenetic pattern.

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