Age of lamprophyres of the Middle Timan: First Rb-Sr data

Abstract

Dating of lamprophyres from magmatic complexes of the Middle Timan that are probably genetically linked to Timan diamonds is a problem of considerable importance. Up to now, the age of magmatism of this region was mainly defined by K‐Ar determinations obtained in the 1970s and 1980s. In this work, we attempted for the first time to study the Rb‐Sr systematics of the rocks of Middle Timan. Subalkaline basic rocks of the dike series have a wide areal distribution and belong to the lamprophyre group. These rocks are confined to deep-seated, longlived faults of NE-strike that is transverse to the general Timan northwestern extension of plicative and disjunctive structures. Host rocks are sandstones and clay shales of the Chetlas and Bystrin groups. In the development area of lamprophyres, the sediments of the Chetlas Group form a gently SW-dipping monocline complicated by small gentle folds. Beds commonly dip 10 ° ‐20 ° , becoming steeper near the Central Timan fault zone. The rocks of the Bystrin Group are deformed into simple folds whose limbs dip up to 40 ° [2, 5]. The lamprophyres compose hundreds of bodies (predominant dikes and a few stocks) that show no spatial relation to intrusive magmatism. They form several fields with the largest Kos’yu (Fig. 1) and Bobrovskoye fields with the area of approximately 1000 km 2 in the southeastern part of the Chetlas Kamen [2, 5].