BATHOLITH FORMATION AS REFLECTION OF PALINGENESIS PROCESSES

AT STRUCTURAL STAGE BOUNDARIES IN OROGENIC AREAS

Rostovsky F.I.

Far East Geological Institute FEB RAS, Vladivostok, Russia, sakhno@fegi.ru

The origin of the batholith granitoid formations (Kuznetsov, 1964) remains the object of discussions. Such formations, originated at significant depths and cropped out by deep erosion in orogenic (mobile) belts, are considered to be formed during the main folding or immediately after it. the SikhoteAlin is one of such belts, in which the varied mineralization is related to the granitoid apical protrusions. The SikhoteAlin orogenic system, incorporating the Late Paleozoic midland WestSikhoteAlin and Late CretaceousPaleogene marginalcontinental EastSikhoteAlin volcanoplutonic belts, is in places overlapped by the postorogenic flat–lying plateaubasalts of the MiocenePliocene time. Within the orogen, at close hypsometric levels, both the sedimentary, volcanogenicsedimentary, and magmatic (volcanoplutonic) rocks of various age (Early Paleozoic to Neogene) and different formations, and also the “exhumed” fragments of its “crystalline basement” are cropped out. The latter are represented by gneisses and crystalline schists pressed in the form of wedges in the overthrust zones. A complicated wedge–mosaic picture of the SikhoteAlin geological map shows its many-stage fold-thrust structure. Within the orogenic system, several structural stages are distinguished – OS, DS, P, TJ₂, J₃K₁, K₂Pg, and N (postorogenic basalts) separated by regional structural unconformities complicated (with the exception of Ng) with different-scale thrusts. These structural stages, locally including the structural layers separated by angular unconformities, differ not only by a set of formations, but different deformational style as well. The various age structuralformational zones of the SikhoteAlin are in effect the shears of its different structural stages. Each of them began to develop with the origination of the ophiolite volcanoplutonic formations, and volcanoplutonic belts – with the origination of the andesite formations. The development of all of them was completed with granitoid (rhyolitic) magmatism. Vast massifs, the Early Ordovician (Shmakovka) along the western Sikhote-Alin boundary to the Late CretaceousPaleogene (coastal) on the Japanese Sea coast, are not fixed in the gravitational belt. It is explained by the fact that such massifs, which are grouped as broken echelon-like-oriented chains of N–E trend, form “platy” bodies 12 km thick with abundant dome-like protrusions of the roof. The massif morphology is governed by their formation in zones of being successively formed regional basal shears (decollements) at the boundaries of the different-age structural stages. In the SikhoteAlin dismembered relief, all of the basal thrusts trending N–E are mapped by a flexuous “bottom” of the largest massifs with indistinct boundaries (granitization of enclosing rocks). Deep parts of the massifs are composed of the “hybrid” formations with gradual transitions from quartz diorites and monzodiorites to granodiorites with “shade” textures. Apical parts of the massifs are dominated by differentcomposition granites. The latter are rarely overlapped with washout by sedimentary formations of the overlying structural layers (stages) (Rostovsky, 2003). The deep parts of most of the massifs are characterized by a gentle (10–30^o) orientation of the current lines with dip of W and NW points. The latter is shown by parallel location of xenoliths and schlieren. The near–surface parts of the granitoid massifs are characterized by analogous orientation of flat joints. The aplite dikes are often localized in the flat prototectonic fractures as in the perpendicular to them transverse fractures of N – W strike. The aplite gentle dikes grade up the dip directly into quartz veins with wolframite or cassiterite impregnation. In the transverse steep prototectonic fractures, greisens with wolframitecassiterite mineralization are localized.

The basal shears (decollements) formation was accompanied by dilatancy (Brace et al., 1966) of thrust zones. Relics of zones of dilatancy deconsolidation, saturated with highly mineralized hydrothermal solutions, have been recognized in the Kol’skaya superdeep hole beginning from the depth of 4.5 km (Kol’skaya…, 1984). Deviator stresses originated through the thrust formation are an order and more magnitude higher than the lithostatic pressure at the same depth. Together with the tribological effect they were responsible for the palingenesis of the dilatancy zones with the origination of magmatic melts. Crystallization mechanism of the latter is well seen in the shallow–water (“coastal”) granitoid massifs that crop out on the Japanese Sea coast. In the bottom of most of the massifs, the palingenemetasomatic hybrid rocks were crystallized in situ, and they do not show clear contacts with the underlying crystalline schists. Such hybrid rocks with gradual transitions from granodiorites of various composition to granites sometimes with garnet or with parallel bands of fine–crystalline biotites contain migmatite xenoliths, lens of biotite hornfels, and xenoliths of breccias. Composition of fragments in breccias depends on the underlying rock composition. Thermal convection in the process of crystallization of such shallow–water magmatic chambers was responsible for gradual vertical change of hybrid rocks by coarse and mediumgrained, sometimes banded biotite and biotitehornblende (grey) granites and then by finecrystalline leucocratic (pink) granites. In the latter ones, miarol cavities with quartz and orthoclase crystals are sometimes found. The grey and especially pink granites are characterized by apophyses broken into the enclosing rocks. The author observed gradual transitions from aplite granite to rhyolites at the highest hypsometric levels of the Vladimirsky, Ol’ginsky, and Evstafyevsky massifs. The rhyolites contain thin (12 mm) “interbeds” of fresh volcanic glass in places with plicated jointing. Origination of such rock association is explained by its formation from the anchieutectic melts under the near-surface conditions. KAr dating of the Vladimirsky (56 samples) and Ol’ginsky (23 samples) massifs shows that their deep zones began to develop in the Maastrichtian (7368 m.y.) and completed (apical parts) in the Danian (6558 m.y.) time. Thus, such “plateau–like” massifs 1.21.5 km thick were crystallized within the 810 m.y. age interval.

References

Kol’skaya superdeep hole: investigation of abyssal structure of the continental crust with drilling the Kol’skaya superdeep hole. Moscow: Nedra. 1984. 490p.

Kuznetsov Yu.A. Main types of magmatic formations. Moscow: Nedra. 1964. 388p.

Rostovsky F.I. Evolution of the volcanoplutonic formations in geotectonic development of the Sikhote-Alin // Volcanism and geodynamics. Ekaterinburg. 2003. P.802804.

Brace W., Paulding B.W., Scholz C.H. Dilatancy in fracture of crystalline rocks // Journ. Geophys. Res.1966. Vol. 71, N 16. P.3939–3953.

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