GRANITE BATHOLITHS IN SPACE AND TIME ON THE ASIAN NORTH-EAST

Gelman M.L.

Northeastern Interdisciplinary Research Institute, Magadan, Russia, gelman@neisri.ru

Northeastern Siberia is a representative area of Phanerozoic granites. Some magmatic epochs are considered because of some changes of the pattern of igneous provinces were here during the geological history. Batholiths were formed in the PZ₁, PZ₂, MZ₂, N₁, and N₂ always and only in the continental terranes or closely to them. What is more, several bodies of sodic granites adjacent to the south margin of the Arctic Chukotka continental terrane and belonging to the PZ₃-MZ₁ ophiolitic association, can apparently be considered as an extension of the belt of MZ₁ granites in the Cordillera. Basic rocks are a feature of each magmatic epoch. Formation of batholiths during the PZ magmatic epochs, was related to the Pangea-II, whereas the same during the both MZ and CZ epochs separated from PZ ones by MZ₁ epoch of abundant and diverse basic magmatism, were related to its break-up.

A few well studied PZ batholiths occur in elevations of the basement of Omolon median mass. Where such elevations contain batholiths, there Archean basic granulites and anorthosites underwent a bulky granitization in the PRZ₁. From paleogeographic materials, there is inferred that a granite- metamorphic dome began to grow in the latest PRZ in the room of granitized rocks. Minor syenite intrusions and perhaps also acid eruptions happend in middle of the O. Mesozonal granite and nordmarkite batholiths formed here during the latest S to the D beginning. Then a mylonite dome superimposed then on the granite batholith. The lowermost sequences of the sedimentary cover of the Omolon median mass are affected by high-grade metamorphism, where they compose the roof of the batholiths. At whole this cover in whole was intruded in PZ₁ by epyzonal porphyritic syenites and granites. Where AR metamorphic rocks have bee not granitized, there the latest PRZ₃ to early PZ time was marked by midalkaline basic magmatism. The growth of the granite-metamorphic dome turned in PZ2 to voluminous ignimbritic eruptions and high level acid intrusions. PZ magmatic association on the Omolon median mass is compared in both age and composition with the H. Read’s granite series.

MZ₂ batholiths compose some huge belts. Petrographic and petrochemistric features of MZ₂ granites as well as CZ ones depend strongly on their position regarding ancient Pacific boundary. This phenomenon may manifest in a scale of an individual batholith and in a scale of batholith belts. Distribution of gabbro besides granite in a composition of any batholith, occurence such rocks as tonalite or quartz monzonite, uncommon andesine granite, a presence of garnet and cordierite, hypersthene, magnetite or ilmenite in granite mineral paragenesis, trend - variations in SiO₂, K₂O, K₂O+Na₂O, CaO/Al₂O₃ values a. o., etc. - all that applies to above variable features. Manners of emplacement of MZ₂ batholiths are diverse, and each of them is more characteristic for either batholiths belt. And emplacement of batholiths was accompanied everywhere by preceding or / and simultaneous or / and subsequent acid volcanism, diapiric granite intrusions. General cases of that manner include following. (a) The Kolymian batholith belt, slow (during J₂bt to J₃t, i.e., ca. 15-20 my) uplift of the vast volume of homogeneous viscous «dry» adamellite magma brought about anticlinal deformation and contact metamorphism of the roof rocks including deposits of J₂ submarine acid pyroclastic flow. (b) The Chukchee batholith belt, multistory recumbent bodies of granitic rocks and migmatites originated during the K₁v to K_1-2 (30-40 my) in the cores of granite - metamorphic domes; more basic magmas worked on the progressive stage of growth of these domes and more acid were intruded on the regressive stage of dome building. © The Okhotsk - Chukchee volcanic belt (OCVB), intrusions of basic, intermediate, and acid magmas stepped up in K_1-2 during ca. 20-30 my a whole body of each batholith, sometimes jointly but usually in complex succession of events including also episodes of volcanism and regional metamorphism. The manner (a) was realized in causal connexion with preceding rapid dipping of the earth crust along a fault and by affect of juvenile reducton fluid. On the contrary, the manner (b) is shown, where granite magmatism epoch has come after a stage of continent raising. And the manner © is typical of geologic history right on the continent margin.

Geology of the MZ granites as well as PZ ones implies that its formation was need of continental earth crust. There is very spectacular a gap in the chain of OCVB batholiths, where volcanic sequences of OCVB overlie the structures of South Anyui suture and Oloy zone. All over continental side of OCVB, these are only structures, where we infer that «granitic» layer in upper crust is strongly reduced or absent. Another evidence may be found near to borders between ocean related ophiolitic provinces and continent related granitic ones in Ranges Peculney and Zolotoy in Northeastern Siberia, also in the Kitakami Mountains in Northeast Japan. There are rifts, which are traced from ophiolitic provinces into continents, and mafic associations of plutonic and volcanic rocks are replaced incidentally by gabbro-to- granite batholiths. But how many acidic rocks is kept in continental crust, it is of no importance for MZ₂ batholith formation unlike PZ one. The thickness of ancient «granitic» layer is probably the greatest in the Kolymian batholith belt. It decreases toward OCVB and comes to nothing straight under batholiths on OCVB oceanward side. The MZ₂ Chukchee batholith belt is situated in a province of Tr₁ basic magmatism, and we may consider that ancient continental crust has been reduced here in Tr₁ time like this took place in the Siberian trappean province. As N₂ magmatic epochs, dependence of acid magmatism on the type of continental crust is very slight pronounced. An interesting example is shown in Southwest Japan. Border of Pacific forms the sharp bent here by coupling of the Japan and Bonin trenches, and pattern of the petrographic provinces yields to that bend ignoring the Median tectonic line, which marks the southern limit of preN felsic igneous rocks in Honshu. On the southern side of the Median line, there such rocks as picrobasalts and pyroxenites, gabbros, tonalites, potassic granites including garnet bearing varieties, change one after the another in the direction from Pacific border along the N₂ Shimanto granitic belt, that arose within the limits of preN ophiolitic terrane where «granitic» layer is absent according to Japanese seismological data. We know just such rock changes but connected with rise of thickness of Earth crust, regarding to MZ₂ petrographic provinces in Northeastern Siberia and in Japan too. And as N₂ magmatism in Southwest Japan, if we will go to its Inner Zone with typical continental crust from Pacific border, so we will see the same rock changes. Also «granitic» layer did not influence on distribution of areas of N₂ acid magmatism on the Kurile, Kamchatka, Koryak highland.

Formation of batholiths in OCVB continues basic magmatism and is accompanied by its manifestations. There lateral changes in petrochemistry of both gabbros and granites are coherent. The volum of gabbro diminish toward continent, and basic rocks, expect lamprophyre, are absent in compositions of Chukchian batholiths. It is like this in batholiths of garnet - cordierite granite of J₃ age typical in the Kolymian batholith belt. However more later K₁ batholiths also known in this belt, which are mostly peralkaline granite but embody substantial units of monzonites and biotite pyroxenites.

Acid magmas may be generated in the Earth’s crust or even in upper mantle, if there are minerals regarding to granite or phonolite eutectics. Chemical and thermal influence of juvenile fluid cause melts, which compositions belong to the thermal minima in the system Q-Ne-Ks. Composition of eutectics depend on activities K, Na, H₂O, CO₂, CH₄, F, Cl in fluid. Diversity of granite compositions reflects these circumstances. Formation of plutons of batholith dimensions is a phenomenon of self-organization.