EARLY CAMBRIAN GRANITOIDS OF BOSHCHEKUL AND DZHALAIR-NAYMAN ZONES

(KFZAKHSTAN): STRUCTURAL POSITION, AGE AND SETTINGS OF FORMATION

Ryazantsev A.V.*, Degtyarev K.E.*, Kotov A.B.**, Salnikova E.B.**

*Geological institute RAS, Moscow, Russia, degtkir@ginras.ru

**Institute of Geology and Geochronology of Precambrian RAS, Saint Petersburg, Russia

akotov@peterlink.ru

Lower Paleozoic ophiolite and island arc complexes are widespread in the eastern part of Kazakhstan. Basalt-rhyolite series and related granitoids presence is their characteristic feature. Such formations are known and widespread in Boshchekul zone at the north-east of Central Kazakhstan (Khromykh, 1984; Ryazantsev, 2005; Degtyarev, Ryazantsev, 2007). They are recently revealed in the Dzhalair-Nayman zone of Southwestern Kazakhstan (Ryazantsev et al., 2008).

Ophiolites, basalt-rhyolite series and granitoids are exposed in the central (Temirastau Stream region, Azhe and Zhilandy Mountains environs) and southern (Tiyes and Semizbugu Mountains region) parts of Boshchekul zone. In the Zhilandy Mountains area ophiolites are represented by serpentinitized harzburgites and dolerites from sill in sill complex. Basalt-rhyolite sequence is structurally higher; basalts are rare at this location. Dolerites from sill in sill complex and basalts are cut by large granitoid massif with complex structure. Interrelations between granitoids and felsic effusives are tectonic. Coarse-grained amphibole and amphibole-biotite tonalites comprise the earlier phase in this massif. They are intruded by coarse-medium-grained leucocratic plagiogranites. Dikes and small bodies of plagiogranites with granophire texture are the latest derivates. Tonalites and some dikes of plagiogranites with granophire texture contain xenoliths of gabbro and dolerites.

Tonalite sample (D-7130 – 51°36’08,6” n.l., 73°52’57,5” e.l.) was picked out for U-Pb zircon datings. Accessory zircon was separated and the age of its crystallization 524±4 m.a. was obtained.

In Dzhalair-Nayman zone ophiolites compose extensive (200 km) belts and separate massifs. In the central part of zone the most full ophiolite sections are described in the country between Andassay and Karshigaly streams (Andassaysky massif) (Chu-Iliyskiy..., 1980). Among mantle restites the harzburgites prevail, lherzolites are rare. Layered complex occupies the largest square. Taxite gabbros occupy higher structural position, turning in dolerite dike complex. Plagiogranites form thin (up to 1 mm) injections in gabbros and dikes or large bodies (up to 200 m in Baygara Mountains area), in dolerites of dike complex. Plagiogranites are tectonically overlapped by basalt-plagiorhyolite sequence (up to 100 m). At one of locations in the country between Andassay and Karshgaly streams the fragment of ophiolite section includes: 1) massive microgabbros (65 m), 2) white and greenish medium-grained plagiogranites (170 m). Plagiogranite sample (Р-6234 – 44°46’35,4” n.l., 72°44’38,6” e.l.) was picked out for U-Pb zircon datings. Accessory zircon was separated and the age of its crystallization 519±4 m.a. was obtained.

At the south-east of Dzhalair zone, in Dulankara Mountains (Dulankara massif) dunites and clinopyroxenites of layered complex (50-100 m) comprise visible base of the ophiolite complex, turning upwards in melanocratic gabbros (200 m). Gabbros are tectonically overlapped by quartz diorites (300 m), gradually turning in leucocratic diorites and granodiorites (300 m) and then in leucocratic plagiogranites (400 m). Along the contact between gabbros and granitoids the lenses of cataclastic gabbros, blastomylonites, amphibolites after gabbros and gneissosed granites (up to 50 m) are traced. Nearby the base of diorites the veins of plagiogranites with granophire texture are observed. They are oriented perpendicularly or obliquely to the mylonite zone. Thickness of veins is 0.1-1.0 m, the extent, up to 50 m. Leucocratic plagiogranites are tectonically overlapped by volcanic series, represented by basalts, rhyolite, siliceous tuffites, and also by sills of sulfur-bearing felsites.

Plagiogranites with granophire texture (sample Р-695/2 – 43°42’04,6” n.l., 75°19’42,8” e.l.) from small (0.2 m) vein, cutting quartz diorites, were selected for U-Pb zircon datings. Accessory zircon was separated and the age of its crystallization 521±2 m.a. was obtained.

All obtained datings of granitoids from Boshchekul and Dzhalir-Naiman zones are similar and correspond to the termination of Early Cambrian (A geological time scale 2004).

Characteristic features of Lower Cambrian complexes of Boshchekul and Dzhalair-Naiman zones are uniformity of stratified sections structure and the same succession of granitoids intrusion. The latter begins from tonalites and quartz diorites, followed by leucocratic plagiogranites and completed by veins and dikes of plagiogranites with granophire texture.

These data confirm suggestions, that Lower Cambrian complexes of these zones belong to one paleostructure. The fragments of the latter are revealed in a number of zones of Eastern, Northern, Central and Southwestern Kazakhstan and are traced in modern structure, forming large shoe, more than 2000 km in length (Degtyarev, Ryazantsev, 2007).

Formation of basalt-rhyolite series and granitoids, tightly related to ophiolites, is typical for areas of back-arc rifting of ensimatic island arcs. Further these complexes comprised the basement, on which the formation of Middle Cambrian island arc differentiated volcanic series, widespread in Boshchekul zone, took place.

Work is supported by RFBR (project 06-05-65311), Programme of Earth Sciences Department of Russian Academy of Sciences №10.

References

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Ryazantsev A.V., Tolmacheva T.Yu., Degtyarev K.E., Kotov A.B., Salnikova E.B., Anisimova I.V., Yakovleva S.Z. Geochronological and biostratigraphical foundation of age of Dzhalair-Naiman zone ophiolites in Kazakhstan // General and regional problems of tectonics and geodynamics. Materials of XLI Tectonic conference. Vol. 2. M.: GEOS, 2008. P.214-219. (in Russian)