Plagioclase of Intrusive Rocks from the Pervomai–Ayudag Island-Arc Complex (Mountainous Crimea): from Plagiolherzolite to Plagiogranite

• Published in: Moscow University geology bulletin Vol. 79; no. 3; pp. 431 - 443
• Main Authors: Ovsyannikov, G. N., Spiridonov, E. M., Filimonov, S. V.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01.06.2024; Springer Nature B.V
• Subjects: Abundance; Crystals; Earth and Environmental Science; Earth Sciences; Geology; Island arcs; Jurassic; Mesozoic; Mountains; Olivine; Plagioclase; Potassium; Quartz; Rock; Rocks; Silica; Crimea; andesine; island-arc Pervomai–Ayudag intrusive complex; Mesozoic rocks of the Мountain Crimea; labradorite; oligoclase; anorthite; bytownite
• ISSN: 0145-8752; 1934-8436
• DOI: 10.3103/S0145875224700418

The Middle Jurassic hypabyssal island-arc Pervomai–Ayudag intrusive complex in Mesozoic rocks of the Mountainous Crimea includes plagiolherzolite, olivine gabbronorite-dolerite, quartz and quartz-free gabbronorite-dolerite, quartz and quartz-free gabbronorite-diorite, quartz diorite, and plagiogranite. These rocks, except for quartz diorite and plagiogranite, have an abundance of anorthite (An 92–90 ) and bytownite (An 89–73 ). Such a composition of plagioclase is typical of island-arc mafic rocks. Due to the abundance of anorthite and bytownite depleted in silica, many types of gabbroids in the complex are quartz rich. Anorthite and bytownite are enriched in FeO (up to 0.9–1.0 wt %). The K 2 O content increases exponentially from anorthite to albite–oligoclase (from 0.0 n to 1.4 wt %). The potassium content of plagioclase decreases noticeably even at a small degree of epigenetic metamorphic alterations. The elongation of plagioclase crystals in gabbro of the middle parts of the intrusions (from anorthite to andesine) is from 2 to 5.5, usually 3–4. Effusive-like porphyritic microgabbro-dolerite of the quenching zone contains plagioclase phenocrysts with an elongation of up to 10; split laths of plagioclase with an elongation of up to 10 or more occur in their fine-grained matrix.