Mainitskiy terrane 

(island arc) 

(Northeastern part of Russian Northeast)

Commonly contains an older and a younger sequence. The older sequence consists of: (1) a lower unit of 
serpentinite and serpentinite melange that contain fragments of late Paleozoic and early Mesozoic 
ophiolites, and limestone with spilite and bedded jasper which contain Middle and Upper Jurassic 
radiolarians; and (2) an upper unit of graywacke, siltstone, tuff, bedded chert that contain rare Berriasian 
and Valanginian Buchia. Local olistoliths are common and are composed of ophiolite, limestone with 
Permian and Triassic foraminifera, plagiogranite, andesite, and rhyolite that are all metamorphosed to 
pumpellyite facies. One of largest olistoliths of tuff and flysch contains chert with Upper Triassic 
radiolarians. The younger sequence consists of a thick assemblage of Upper Jurassic and Lower Cretaceous 
island arc volcanic and sedimentary rocks composed of tholeiitic basalt, andesitic basalt, rhyolite, tuff, 
breccia, chert, siltstone, and sandstone that contain Callovian to Kimmeridgian, and Tithonian to 
Hauterivian radiolarians. The younger sequence is interpreted as a late Paleozoic oceanic crust sequence 
that is overlain by a Mesozoic primitive island arc sequence.

The Mainitskiy terrane also contains a complex series large tectonically imbricated nappes. (1) In the 
central part of the terrane is a tectonically disrupted ophiolite (Yagel melange) of Late Jurassic to Early 
Cretaceous (Callovian to Hauterivian) age that consists of large blocks of dunite, peridotite, gabbro, and 
diabase dikes, and pillow-basalt with hyaloclastite and chert in a serpentinite matrix. Also occurring in the 
melange are dikes and lava flows similar to oceanic tholeiite, and younger island arc volcanic rocks. (2) In 
the northern part of terrane are several tectonic sheets of dunite, harzburgite, gabbro, and ultramafic rocks 
that are structurally overlain by island arc volcanic rocks. (3) In the southern part of terrane is an 
assemblage of Upper Jurassic and Lower Cretaceous (Neocomian) tuff and graywacke and exotic blocks of 
Paleozoic and early Mesozoic sedimentary and igneous rocks.

For paleomagnetic determinations, sedimentary and volcanic rocks of Upper Jurassic to Lower Cretaceous 
age yield grade A data indicating a southerly displacement with respect to the Siberian platform of 
50o+16o. The Mainitskiy terrane is interpreted as a fragment of a Late Jurassic up to Middle Cretaceous 
island arc that is underlain by a basement of oceanic crust and primitive island arc rocks of Triassic to 
Early Jurassic age. The various sequences forming the terrane were amalgamated in the mid-Cretaceous. 
The terrane is unconformably overlapped by the Cretaceous sedimentary rocks of the Kuibiveem 
sedimentary assemblage (unit kb Kuibiveem sedimentary assemblage). REFERENCES: Aleksandrov, 
1978; Zinkevich, 1981; Pushcharovskiy and Til'man, 1982; Peyve, 1984, 1985; Byalobzhesky and others, 
1986; Grigor'gev and others, 1987; Gel’man and others, 1988; Stavsky and others, 1989; Shmakin, 1991; 
Vishnevskaya and others, 1991; Sokolov, 1992; Didenko and others, 1993.