GRANITOIDS OF THE KOMSOMOLSK ORE DISTRICT (KOD) AS AN EXAMPLE OF

STRIKE-SLIP RELATED CRUSTAL GRANITIZATION

Mitrokhin A.N., Utkin V.P., Nevolin P.L.

Far East Geological Institute FEB RAS, Vladivostok, Russia, stakhor@yandex.ru

The research of the Sikhote-Alin's Cretaceous tectogenesis (e.g. Utkin, 1989) fixes the key role of strike-slip faulting in localization of granitoid magmatism occurrences that took place during the Cretaceous tectonomagmatic event. At the same time, in contrast to disjunctive (shear and destructive) forms of strike-slip faulting, the influence of the strike-slip related folding (as a plicative form of strike-slip faulting) on the character of localization and morphology of the Sikhote-Alin's Cretaceous granitoid intrusions is not known enough, although the attention of the regional researchers has been already paid to the fact of concordance of some large granitoid plutons with strike-slip related fold structures (e.g. Utkin, 1977). That the strike-slip related folding had actually an essential influence on localization of the specified granitoids is shown by our studies of morphology and infrastructure of the Aptian-Campanian granitoid intrusions of the Myaochan series in the KOD. The Myaochan series includes (e.g. Ognyanov, 1988; Gonevchuk, 2002) three intrusive assemblages in succession, as followed: the Puril granodiorite, Silinka monzonitoid, and Chalba granite.

1. We have shown (e.g. Mitrokhin, 1998) that the regional strike-slip related folding, which took place during the Aptian-Campanian time, occurred in the form of the single system of gentle linear uniform synclinal depressions and anticlinal uplifts superposing with the pre-Aptian terrigenous-flysch basement with about 18 km wave length as a buckle folding. Among them the Tzentralny anticline uplift together with the Zapadny and Vostochny depressions being conjugate with the former, are the most known (e.g. Ognyanov, 1986; Mitrokhin, 1998). The depressions are compensated by effusive-sedimentary strata of the Kholdami and Amut suites, the part of which is comagmatic with rocks of the Puril and Silinka intrusive assemblages, accordingly: Aptian-Albian rhyolites of the lower division of the Kholdami Suite and the Turonian-Campanian andesites of the Amut Suite (e.g. Ognyanov, 1986; Mitrokhin, 1998). The Zapadny depression is westerly conjugate with the also well-known Chalba uplift (e.g. Ognyanov, 1986; Mitrokhin, 1998) that hosts the massif of the same name, whose batholithic-like granites belong to the Chalba intrusive assemblage. In addition, there is figured out (e.g. Mitrokhin, 1998) the Ognensky depression being westerly conjugate with the Chalba uplift as well as the Eliberdan uplift being easterly conjugate with the Vostochny depression. The specified fold structures taken together are N040-050E-trending, being at naturally oblique angle to the N010-030E-trending Komsomolsk sinistral-fault zone, in which the Chalba, Myaochah, and Kholdami faults are the largest. Being under N340-350W lateral compression, their activation is well-known (e.g. Utkin, 1989) to predetermine, just as analogous faults everywhere in Sikhote-Alin, a structural style and geodynamics of the Aptian-Campanian regional folding and faulting as well as geodynamic conditions for localization of synchronous ore-magmatic formations.

2. The mentioned-above largest regional uplifts and depressions together with the Chalba, Myaochan, and Kholdami sinistral faults, control spatial position of the low-angle lens-like 0-5-km-thick Komsomolsk cryptopluton (KC) separated out by geophysicists (e.g. Lishnevsky, 1969) that includes practically all granitoid occurrences of the Puril and Chalba assemblages. At that, the KC roof relief is completely concordant with the geometry of the 18-km-wave-length fold system, which is also fixed by morphology of the pre-Aptian basement surface. This is especially indicated in the conjugation zone between the Chalba fault and Chalba uplift, which hosts the Chalba granite massif being considered to be (e.g. Lishnevsky, 1969) an eroded part of the KC.

3. The same situation is figured out for localization features of the Silinsky monzonitoids. The well-known Silinka monzonitoid massif lying over the KC (e.g. Lishnevsky, 1969) proves to be spatially controlled by the Tsentralny uplift that is underscored by its laccolithic form with thickening to the uplift root (hinge). Thereby, the fact is once more fixed that the regional NE-trending folding is similar that has been figured out by us (e.g. Mitrokhin, 1998) on the regional depression controlling the Aptian-Campanian effusive-sedimentary basins. At that, there takes place a rock composition change from basic (gabbro) through dioritic right up to acidic (granitic) varieties downward apical parts of the massif. It is necessary to stress that the change is smooth, but is not phase, i.e. is facial via transitional rock varieties without sharp contacts between them. There are direct structural evidences that the geometry of the massif’s facial zonality coincides with its morphology and, therefore, is concordant with the Tsentralny uplift. At the same time, the specified zonality corresponds completely to the succession of intrusive phases of the Silinka monzonitoids from gabbro to granite that is vividly observed in both an exocontact part of the Silinka massif and away from it, in particular, within Zapadny and Vostochny depressions. It stands for the crystallization of the massif rocks to be under a stable extension within the fold-related decompression zone being in the Tsentralny uplift hinge during the entire period of the Silinka assemblage’s formation that has been already said to come off synchronously with folding and sinistral faulting. At that, inflow of the monzonitoid magma into the decompression zone took place through the NNE-trending sinistral faults that is fixed by the availability of neck-like bodies in the massif bottom, for example, within the Solnechny fault. By the way, the same body is fixed geophysically for the KC by its thickening up to 7-7.5 km within Myaochan sinistral fault. The analysis of the structural style for the regional Aptian-Campanian folding (e.g. Mitrokhin, 1998) gives the development of fold-related decompression zones (being in fold hinges) to be quasi-ductile by means of crust shortening through the combination of two processes: both similar folding accompanied by thrusts and fan-cleavaging (fold-related reverse faulting). The foregoing is vividly observed for the Chalba granitic massif, whose morphology obeys the geometry here of fold, cleavage, and fault structures taken together.

4. Local granitoid and monzonitoid bodies attaching “cupola-like” morphology of the Komsomolsk cryptopluton are in the decompression zones being controlled by higher-rank folds (laccoliths, lappoliths, sills, etc.) and, more often, sinistral and dextral faults and their combinations (stocks and dikes within pull-apart structures, which are superimposed with the 18-km-wave-lengh folding during ripe stages of the regional strike-slip faulting.

References

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