2.8-MA ASH-FLOW CALDERA AT CHEGEM RIVER IN THE NORTHERN CAUCASUS MOUNTAINS (RUSSIA), CONTEMPORANEOUS GRANITES, AND ASSOCIATED ORE-DEPOSITS

Diverse latest Pliocene volcanic and plutonic rocks in the north-centr al Caucasus Mountains of southern Russia are newly interpreted as comp onents of a large caldera system that erupted a compositionally zoned rhyolite-dacite ash-flow sheet at 2.83 +/- 0.02 Ma (sanidine and bioti te Ar-40/Ar-39). Despite its location within a cratonic collision zone , the Chegem system is structurally and petrologically similar to typi cal calderas of continental-margin volcanic arcs. Erosional remnants o f the outflow Chegem Tuff sheet extend at least 50 km north from the s ource caldera in the upper Chegem River. These outflow remnants were p reviously interpreted by others as erupted from several local vents, b ut petrologic similarities indicate a common origin ana correlation wi th thick intracaldera Chegem Tuff. The 11 x 15 km caldera and associat ed intrusions are superbly exposed over a vertical range of 2,300 m in deep canyons above treeline (elev. to 3,800 m). Densely welded intrac aldera Chegem Tuff, previously described by others as a rhyolite lava plateau, forms a single cooling unit, is > 2 km thick, and contains la rge slide blocks from the caldera walls. Caldera subsidence was accomm odated along several concentric ring fractures. No prevolcanic floor i s exposed within the central core of the caldera. The caldera-filling tuff is overlain by andesitic lavas and cut by a 2.84 +/- 0.03-Ma porp hyritic granodiorite intrusion that has a cooling age analytically ind istinguishable from that of the tuffs. The Eldjurta Granite, a pluton exposed low in the next large canyon (Baksan River) 10 km to the north west of the caldera, yields variable K-feldspar and biotite ages (2.8 to 1.0 Ma) through a 5-km vertical range in surface and drill-hole sam ples. These variable dates appear to record a prolonged complex coolin g history within upper parts of another caldera-related pluton. Major W-Mo ore deposits at the Tirniauz mine are hosted in skarns and hornfe ls along the roof of the Eldjurta Granite, and associated aplitic phas es have textural features of Climax-type molybdenite porphyries in the western USA. Similar Ar-40/Ar-39 ages, mineral chemistry, and bulk-ro ck compositions indicate that the Chegem Tuff, intracaldera intrusion, and Eldjurta Granite are all parts of a large magmatic system that br oadly resembles the middle Tertiary Questa caldera system and associat ed Mo deposits in northern New Mexico, USA. Because of their young age and superb three-dimensional exposures, rocks of the Chegem-Tirniauz region offer exceptional opportunities for detailed study of caldera s tructures, compositional gradients in volcanic rocks relative to cogen etic granites, and the thermal and fluid-flow history of a large young upper-crustal magmatic system.