MESOZOIC ALKALINE MAGMATIC FORMATIONS OF THE KHOKHOI ORE FIELD (UPPER AMGA AREA)

The article provides the first description of the petrographic compositions of Mesozoic alkaline igneous rocks of the Khokhoy ore field. The Khokhoy field forms part of the Upper Amga area and is localized in the northwestern part of the Aldan-Stanovoy shield, at the junction of two major structural units – Olyokma granite-greenstone area and the Aldan granulite-gneiss area, in the zone of the meridional deep-seated Amga fault. Numerous of alkaline and sub alkaline igneous rocks of the Lebedinsky monzonite-syenite-granite complex present in this territory are related to the most productive stage – Mesozoic tectonic-magmatic activation of the Aldan shield. Within the Khokhoy ore fields, there are exposed the Northern Boskho, Western Boskho and Upper Khokhoy alkaline massifs of Mesozoic age, as well as rare mafic dikes whose mineral compositions have been poorly studied until recently. The purpose of this article was to obtain high-quality data on the quantitative mineral composition of all igneous rocks of the Khokhoy ore field with the use of crystallooptical methods. As a result of investigations carried out at the DPMGI SB RAS it was found that rocks of the Northern Boskho massif are largely leucocratic syenite porphyries with prevalent hypidiomorphic granular and porphyritic textures. The rocks of the Western Boskho massif differ in their quantitative mineral composition, primarily in structural characteristics, from the Northern Boskho syenites, and correspond to monzonites. They have a monzonitic texture with elements of hypidiomorphic and porphyritic textures. The rocks of the Upper Khokhoy massif mineralogically differ noticeably from those of the Northern Boskho massif; however, structurally, they are close to the Western Boskho monzonites. They show a well-defined monzonitic texture (with clear plagioclase idiomophism in contrast to K-feldspar). The crystallooptical studies of rare fragments of mafic igneous rocks of unclear genesis found during the field works showed they are tephrites. Also, the time sequence of intrusion of igneous rocks in the Khokhoy ore field is established. Generally, the data obtained may shed light on some problems concerning the evolution of Mesozoic tectonic-magmatic activation of the Aldan-Stanovoy shield.

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