PROBLEM OF HYPABYSSAL GRANITOID SPACE ON EXAMPLE OF SIZINDZHI MASSIF

(OKHOTSK-CHUKOTKA VOLCANOGENIC BELT)

Mishin L.F.

Yu.A.Kosygin Institute of Tectonics and Geophysics FEB RAS, Khabarovsk, Russia, lfmishin@mail.ru

The problem of space is the key one in magmatic geology. Its solution is in the sphere of field geology, in detailed structural studies. The Sizindzhi granitoid massif is located at joint of Kuidusun and Ulyinskaya volcanic zones. The massif stretches for 20 km to the south-east, and it is 5-10 km wide. Within it, all peculiarities of composition and structural interrelations that are characteristic of the I type hypabyssal granitoids are combined in the concentrated form. In addition, it is splendidly outcropped. Due to these circumstances, it appears fine polygon for solution of many problems that are related to granitoid magmatism. The host rocks are volcanites of the Chukotka belt in the south-western flank and aleurolites of the Verkhoyansk complex – in the north-western one. The host rock strike is conformable to the massif boundaries, on the whole, though in detail, the intrusive boundaries are of the cutting character.

Microdiorites and microgabbros are the earliest rocks within the Sizindzhi massif. These rocks occur as xenolyths and separate blocks through the whole massif and in all rocks, in fact. The size of individual bodies achieves 100 m. The nature of these rocks is insufficiently clear. The data that indicate the formation of microdiorites are available as a result of main and middle composition volcanic rock dioritization.

Middle-grained diorites, stratiform amphibole gabbros and quartz diorites are mostly spread in the south-western part of the massif, but some of them are often rather large, and outcrops of these rocks being isolated among adamellites occur through the whole massif. Diorites and quartz diorites ubiquitously contain assimilated xenolyths of microdiorites to various extent, and more often their relics like accumulations of restite minerals that impart a spotted appearance to the rocks. Xenolyths lack in stratiform diorite-gabbro series.

Adamellites and granodiorites are dominant rocks within the massif. Their absolute age on zircons by U-Pb method is 91 Ma. The term “granitization” is mostly suitable to the present stage of pluton formation. Adamellites of various density saturate the whole massif. The granitoid bodies form continium from drops less than 1 cm to large homogenous masses to 10 km² by area. Contacts of adamellite with country rocks are both sharp and diffusional. In the latter case, they have shadow schlieren texture and are saturated by xenoliths. The xenoliths are mostly represented by microdiorites. The form of granite extracts is various, i.e. the by-layer migmatization prevails in aleurolites, and rounded and sometimes rod-like extracts of granite material with sharp borders that remind xenoliths are typical for diorites. In the periphery, such pseudoxenolithic migmatites are followed by smaller (first centimeters) extracts of granite material, and porphyroblasts of oligoclase with quartz ichthyoglipts. In fissure rocks, granitization develops as veins and veinlets of complex morphology with formation of agmatite structures.

Hybrid granite-porphyries and fine-grained granites are hardly spread in the Sizindzhi masif. They compose thin dykes of body and separate stocks to 0.2 km² by area. They are saturated with hornfelsed xenoliths. The extent of primary structure preservation in xenoliths is directly depended on volume of intrusives. Small bodies are represented by magmatic breccia that consists of basalt and andesibasalt fragments cemented by fine-grained aplite of eutectoidal composition. In larger bodies, melanocratic spots filled in amphibole and biotite that preserve angular form of xenoliths remain. Away from the contacts, further assimilation of xenoliths and recrystallization of initially microaplite groundmass and rock homogenization take place. Hybrid granites formed by partial melting, the low-temperature granite eutectics, and further transformations of rocks occurred in solid state due to recrystallization, perhaps with addition of alkali and silica.

In final stage, numerous dykes of porphyry rhyolites and rhyodacites up to 1 km long and complexly branching systems of veins and dyke-like bodies of aphyric rhyolites and their glasses filling in wide zone of extension form in the south of the massif.

Magmatism of various ages in the Sizindzhi massif took place within the united, clearly formed boundaries without any mechanical effect on host rocks and those of early phases. As a result, an original structure that can be called xenolith in xenolith formed. This structure can be observed at the level of the whole massif and separate outcrop and even in individual sample. In view of injective tectonics, it is very difficult to explain the origin of such rock interrelations.

Spatial distribution of host rock xenoliths and their structural data are of peculiar significance for understanding the conditions of the Sizindhi massif formation. Xenoliths and septa of aleurolites occur exclusively in the northern and western flank zones of the massif, where granitoids contact rocks of the Verkhoyansk complex. Xenoliths of volcanic rocks are spread in the opposite zone of the massif that contacts volcanic rocks. The border of spread of terrigenous rock xenoliths and volcanites lies nearly along the axial part of the massif on continuation of volcanite contacts with terrigenous rocks in frame of the massif, its position practically does not depend on composition of magmatites, and relative quantity of xenoliths only changes. The most saturation of the massif with xenoliths of aleurolites is observed in the right bort of the Atarbai river and on the right bank of the Sizindzhi river. The mass measurements of the layering in xenoliths and detached masses showed their good concordance to aleurolite bedding in frame of the massif. Due to oriental circus diagrams, the concordance is established both in strike of layering and character of folding. The structural data indicate that xenoliths are minisepta of substrate in its primary bedding.

Distribution of xenoliths, skialites, shadow structures and character of contacts indicate the formation of diorites and adamellites by assimilation of microdiorites, aleurolites and volcanites in situ. Injective nature can be only recognized in small bodies of pluton formation in the regressive stage.