Atoll Garnets in "Achtarandite" Serpentinites: Morphology, Composition and Mode of Origin
Atoll garnets in aposkarn serpentinite from the Wiluy River, Republic of Sakha-Yakutia, Russia, have the classic form comprising a garnet core, an intermediate zone filled with chlorite-group minerals and an outer garnet atoll. The core of an illustrated example is complexly zoned from schorlomite to grossular-andradite. Morphologically, the core is a rhombic dodecahedral crystal. The atoll crystallized as a tetragon-trisoctahedron with minor rhombic dodecahedron faces and is composed of hibschite and "hydroandradite". The atoll garnet formed as the result of selective dissolution and substitution by chlorite of an internal hibschite zone with columnar structure that became unstable under new conditions of crystallization. The pattern of dissolution traces defects in the garnet crystal. The growth of the atoll garnets reflects the main stages in the evolution of the Wiluy deposit itself and is associated with the development of the Siberian traps.
The main products of volcanic activity in the teschenite-picrite association (TPA) are shallow, sub-volcanic intrusions, which predominate over extrusive volcanic rocks. They comprise a wide range of intrusive rocks which fall into two main groups: alkaline (teschenite, picrite, syenite, lamprophyre) and subalkaline (dolerite). Previous 40Ar/39Ar and 40K/40Ar dating of these rocks in the Polish Outer Western Carpathians, performed on kaersutite, sub-silicic diopside, phlogopite/biotite as well as on whole rock samples has yielded Early Cretaceous ages. Fluorapatite crystals were dated by the U-Pb LA-ICP-MS method to obtain the age of selected magmatic rocks (teschenite, lamprophyre) from the Cieszyn igneous province. Apatite-bearing samples from Boguszowice, Puńców and Lipowa yield U-Pb ages of 103± 20 Ma, 119.6 ± 3.2 Ma and 126.5 ± 8.8 Ma, respectively. The weighted average age for all three samples is 117.8 ± 7.3 Ma (MSWD = 2.7). The considerably smaller dispersion in the apatite ages compared to the published amphibole and biotite ages is probably caused by the U-Pb system in apatite being less susceptible to the effects of hydrothermal alternation than the 40Ar/39Ar or 40K/40Ar system in amphibole and/or biotite. Available data suggest that volcanic activity in the Silesian Basin took place from 128 to 103 Ma with the the main magmatic phase constrained to 128-120 Ma.