Timing of collision of the Kokchetav massif with Stepnyak Island arc on the basis of conodonts and radiolarians from siliceous rocks of juxtaposed terranes of different geodynamic seetings

The siliceous rocks from structures of the junction zone of the Kokchetav massif and Stepnyak trough that occur in turbidites of the accretionary wedge, silica-volcanogenic sequence of the Stepnyak trough and syntectonic olistostrome were dated on the basis of findings of conodonts and radiolarians within the Middle-Upper Arenigian (conodont zones O. evae, B. navis—lower Par. originalis). This range of time is marked by a powerful tectonic rearrangement, involved with the rearrangement of the accretionary wedge and overriding of the Kokchetav massif upon the Stepnyak fore-arc trough.

In northern Kazakhstan, siliceous rocks are widespread, often associated with volcanics, which are traditionally referred to as the Nikolskoe-Burluk Group recognized by Shlygin [1]. Structurally, this group belongs to the framing of the Kokchetav massif, in particular, to the zone of junction with the Stepnyak trough [2, 3]. The lower part of the group includes metamorphosed diabases, basalts, andesite-basalts, spilites, liparites and their tuffs, and siliceous siltstones. The upper part is made up of jaspers, jasper-quartzites, sandstones, conglomerates, siliceous mudstones. The group is exposed best of all near the town of Kokchetav and at the Chaglinka River. In the late 1980s, Ivanov [2] was first to find conodonts at the Chaglinka River. In the early 1990s, large-scale surveys were carried out with the purpose to date the siliceous and volcanogenic-siliceous formations of the Kokchetav massif, to outline their distribution in area, to locate them stratigraphically, and to correlate them with the regional stratigraphic scale of Kazakhstan. In 1990–92, the studies were carried out by a working group headed by Tsay [3]. They found about 50 new occurrences of the Ordovician conodonts in the Nikolskoe-Burluk Group of the northern Kokchetav massif and in the Zhanasui Formation of the southern Shat massif. These findings corroborated the Early Ordovician (Arenigian) age of the Nikolskoe-Burluk Group and its analogs rather than their Precambrian or Cambrian age as was supposed earlier.

This paper reports new data on findings of conodonts in the siliceous-volcanogenic formations of the zone of junction of the Kokchetav massif and Stepnyak trough. Siliceous rocks were sampled in the process of detailed geological mapping, which permitted them to be referred to different structures: turbidite olistostrome-bearing sections of accretionary wedge, syntectonic olistostrome, and association of basalts of the Stepnyak fore-arc trough (Fig. 1). More than 40 samples of siliceous rocks were collected, and some of them contain radiolarians and conodonts of different degrees of preservation. <...>