Kinematic analysis of ultrahigh‐pressure–high‐pressure metamorphic rocks in the Chaglinka–Kulet area of the Kokchetav Massif, Kazakhstan

Abstract

The central part of the Kokchetav Massif is exposed in the Chaglinka–Kulet area, northern Kazakhstan. The ultrahigh‐pressure–high‐pressure (UHP–HP) metamorphic belt in this area is composed of four subhorizontal lithological units (Unit I–IV) metamorphosed under different pressure–temperature (P–T) conditions. The coesite‐ and diamond‐bearing Unit II, which consists mainly of whiteschist and eclogite blocks, is tectonically sandwiched between the amphibolite‐dominant Unit I on the bottom and the orthogneiss‐dominant Unit III on the top. Total combined thickness of these units is less than 2 km. The rocks of the UHP–HP metamorphic belt are affected by at least four deformational events post‐dating peak metamorphism: (i) The earliest penetrative deformation is characterized by non‐coaxial ductile flow in a NW–SE direction. The shear sense indicators in oriented samples from Unit I provide consistent top‐to‐the‐northwest motions and those from Unit III provide top‐to‐the‐southeast, south or south‐west motions; (ii) Upright folds with subhorizontal enveloping surface refold earlier foliations including shear‐indicators throughout the metamorphic belt; (iii) The third stage of deformation is denoted by large‐scale bending around a subvertical axis; and (iv) Late localized fault (or shear) zones cut all earlier structures. The fault zones have subvertical shear planes and their displacements are essentially strike‐slip in manner. The subhorizontal structure and opposite shear directions between Unit I and Unit III during the earlier deformation stage suggest north‐westward extrusion of UHP Unit II.

Thermobaric structure of the Kokchetav ultrahigh‐pressure–high‐pressure massif deduced from a north–south transect in the Kulet and Saldat–Kol regions, northern Kazakhstan

“…In order to form such a geologic and thermobaric structure, the highest grade rocks would have to be selectively transported from the mantle depths more rapidly than the lower grade rocks in this massif during exhumation ( Maruyama et al 1994 ). Integrating the structural analysis ( Ishikawa et al 2000 ; Yamamoto et al 1999 ) and the geochronology ( Jagoutz et al 1990 ; Claoué‐Long et al 1991 ; Troesch & Jagoutz 1993) of the Kokchetav UHP–HP massif, protoliths of the massif that were buried to mantle depths attained their metamorphic peak at 530–533 Ma (garnet–clinopyroxene, Sm–Nd, isochron and zircon SHRIMP ages), and were exhumed northwards to mid‐crustal levels by about 517 Ma (muscovite 40 Ar/ 39 Ar age). On the basis of estimated conditions of the AMP zone ( Fig.…”

Geology of the Kokchetav UHP‐HP metamorphic belt, Northern Kazakhstan

“…In this unit, sigmoidal polycrystalline aggregates, asymmetric pressure shadows, drag folds and shear bands clearly indicate a top‐to‐the‐south sense of motion ( Fig. 9; Yamamoto et al 2000 ).…”

Overview of the geology, petrology and tectonic framework of the high‐pressure–ultrahigh‐pressure metamorphic belt of the Kokchetav Massif, Kazakhstan

“…As with Unit I, dominant foliations are axial planar to outcrop‐scale WNW–ESE‐trending upright folds, and the earliest foliation, which is associated with intrafolial isoclinal folds, is subhorizontal ( Yamamoto et al 2000 ; Kaneko et al 2000 ).…”

“…Sigmoidal polycrystalline aggregates, asymmetric pressure shadows, drag folds and shear bands in rocks of Unit III, indicate top‐to‐the‐south sense of motion ( Yamamoto et al 2000 ). Asymmetric augen and S–C fabrics in the orthogneiss are related to late, left‐lateral strike‐slip deformation.…”