Structure of Synnyr alkalic rock massif |
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| Abstract: | There are five intrusive complexes in the Synnyr range, one of which is the complex of alkalic igneous rocks discussed. It consists of several phases, some of which have been more or less completely rnetasomatized. These metasomatic rocks are considered by the author as a separate “complex.” K-A determinations on biotite from the syenites of these two complexes indicate an age of between 300 and 350 m.y. The alkalic rocks and their metasomatic equivalents form the equant Synnyr massif, situated at the intersection of two regional faults of northeast and northwest strike. The former is considered to have been the “magma channel.” The periphery of the massif is coarse alkalic syenites that underlie a ring-like erosional depression within which occur finer (trachytoid) Ne-bearing syenites and pulaskites. Contact zones consist of skarn, hornfels, and fenite. Within the intrusive the overall mineral composition of the metasomatized facies is not greatly different from that of the original rocks, but the texture has been changed by neocrystallization and recrystallization and the porosity is distinctly higher. Plagioclase has been replaced by almost pure albite and some of the potassium feldspar by intergrowths with kaliophylite. Fluorite and analcite occur as new minerals. In the late, low-temperature stage scapolite, zeolites, epidote-zoisite, and sparse quartz are found in some places. The other three complexes of the range are described very briefly: subalkalic syenite complex, a complex of silicic intrusive rocks, and a series of late lamprophyric dikes. – E. Ingerson. |
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