GRANITE batholitHS OF the JAPANESE SEA BOTTOM

Lelikov E.P.

V.I. Il`ichev Pacific Oceanological Institute FEB RAS, Vladivostok, Russia, lelikov@poi.dvo.ru

Granites play an important role in the geological structure of the Pacific Ocean marginal seas. Numerous outcrops of the Pre-Cambrian, Paleozoic, Mesozoic, and Cenozoic granitoids are revealed on continental slope, underwater rises and ridges of the Sea of Japanese, Okhotsk, East China and Philippine seas. These seas are differed by type and earth crust composition, structure and time of folded basement formation that determine the features of each granitoid magmatism of each of them. On the basis of petrogeochemical classification (Tauson, 1977), of structural and tectonic positions of granitoid massifs, they are subdivided into three genetic classes: ultrametamorphogenic, palingenic and differentiates of andesitic and tholeiitic magmas, among which we distinguish granitoids of several formational and geochemical types (Lelikov, Malyarenko, 1994).

Large granitoid massifs that can be referred to batholiths in size and form are revealed on the underwater structures of the Japanese Sea – on the Korean Plateau and Yamato Rise. They differ by origin: the first ones are referred to the formations of palingenic-anatektoidal class, and the second ones represent derivates of andesite magma that predetermines peculiarities of their structure and geochemical specialization.

In the western part of the Japanese Sea, within the Korean Plateau, a large batholith, with its granites having been dredged from various ledges of the rise from the depth of 1700 to 950 m, has been found. The massif is extended within its studied part for approximately 200 km from north to south, and it is 40 to 70 km wide. It is largely composed by irregular-, coarse- and middle-grained pinkish-grey granites, often porphyry-like with phenocrysts of microcline, where biotite and leucocratic varieties are developed. Biotite-hornblende granodiorites, quartz syenites, and also quartz-feldspar pegmatites are more rarely observed.

These Middle Paleozoic granites form the large massif, developed in the process of palingenic melting the metamorphogenic formations of Pre-Cambrian, with relics not fully replaced in its structure. By iron and calcium contents and total alkalinity, these granites are similar to the gneiss-granites of Pre-Cambrian that testifies to the influence of Pre-Cambrian formations on the structure of described rocks. The massive is overlapped by the strata that are similar to the Devonian deposits of Korea, with the pebble of granites being observed in basal layers.

By chemical composition, subalkaline varieties and rocks of normal alkalinity are distinguished among granites. Rocks of the complex are enriched in Rb, Ba, and differentiation trends of these rocks in the Rb-Sr-Ba diagram are directed along Rb-Sr side of triangle. High concentrations of light rare-earth elements (LREE), low contents of heavy (HREE) ones are peculiar for them, that is expressed by high La/Sm (4,85-6,04), La/Yb (28,85-50,88) ratios and sharply fractionated spectrum of the rare-earth elements (REE) distribution. They are characterized by distinctly expressed negative europium anomaly related to high content of feldspars.

By content of some trace elements (Table 1) and chemical specialization, rocks of the complex correspond to palingenic granites of calc-alkaline series (Tauson, 1977).

Petrogeochemical peculiarities of rocks and structure of minerals allow us to refer them to the group of palingenic granites of granite batholith formation, the latter having been crystallized from magma with high content of water phase at rather low temperature 550-660⁰С in the abyssal facies of depth (Lelikov, Malyarenko, 1994).

Large massif of the Late Paleozoic complex (332,0-258,0 Ma) granitoids that extends for 200 km, at width of some outcrops 15-30 km has been studied on the underwater Yamato Rise, located in central part of the sea. Similarity of rocks from various sites of the rise, allows us to assume that the Yamato basis is composed by granitoids, which were exposed onto the surface in Mesozoic, then eroded and overlapped by the Lower Cretaceous and Paleogene deposits, with the pebble and boulders of these granites being in basal layers.

Its rocks are subdivided into those of calc-alkaline and subalkaline series. The first one includes quartz diorites, granodiorites, biotitic granites and leucogranites, and the second one - quartz monzonites, and subalkaline granites that is distinctly shown in the alkali-earth silicon diagram in the form of independent trends (Borodin, 1987). It is also expressed in the geochemical specialization of rocks, i.e. increase in the content of rare alkalis and decrease in siderophile elements from calc-alkaline to subalkaline differences (Table). However, in Rb - Sr - Ba diagram, the points of all rock structures are located along Sr-Ba axis of triangle in the field of derivatives of andesite magma. Sharply fractionated spectrum of distribution of the rare-earth elements (REE) is observed in rocks of both types. But it is expressed more distinctly in alkaline varieties, that is reflected in higher values La/Sm (4,14-6,64), La/Yb (14,44-40,32) ratios in subalkaline ones than in calc-alkaline La/Sm (1,77-3,22), La/Yb (3,64-7,70) ones. Granitoids are characterized by the negative europium anomaly.

Table 1. Average contents of trace elements (ppm) in granitoids.