Origin of the Earth's Crust and its Evolution

The granitic rocks including migmatite, gneiss and palingenetic plutonics found in the axial core of the reprdsentative orogenic belts such as the Abean and Hidaka belts in Japan are now believed to have been originally geosynclinal deposits refbrmed by syn-orogenic igneous activities in a viride sense. From the authors' own observation 'in the course of mapping in the respective fields above mentioned in Japan, and based on laboratory works, the authors now stand to accept a theory of granitization in making up granitic crust through orogenic process. As a matter of fact, not only in cratonic, sub-cratonic or older orogenic belts in the continental mainland of Asia, but also even in such younger mobile belts as the Japanese islands, the foundation is chiefiy composed of granitic rocks of the Precambrian age. Thus, the ear! iest stage of the geologic time became a problem to be discussed in the present paper as to the formation of granitic crust, since the source area for geosynclinal deposits must have eventually existed even in such an ancient day, and the depositional sea should have been also present, so far as the origin of the oldest granitic rocks are concerned. Thus, the earliest ocean and continent prior to the geologic time came to be a problem. Based on the recent progress of the Precambrian geology, the ur-continent might be existed in the present Pacific, as will be stated later on, while the present continent may have been under the sea water, viz. the ur-ocean. This assumption seems to be well consistent with the well known model for the structure, and interior movement of the early stage of the earth, presented by F.A. VEN! NG MEiNEsz in 1964. Namely, one hemisphere was a continent, while the other side was an ocean, caused by a single cell convective movement of the interior of the earth, when the core was not yet differentiated from the mantle. This may be called as the lunar stage of the earth's history. The third stage is the geological age in which ur-ocean became continent composed of terrestrial crust step by step, while ur-continent became ocean, the ancestral basin of the present Pacific, The principal course of development of the 516 M. MiNATo and M. HuNAHAsHi crust since the Late Precambrian may be assumed to have been a refbrmation of the terrestrial crust caused by new orogenic movements, and also by oceanization in a wide sense. The ocean might be also repeatedly reformed by the appearance of rises, ridges and troughs in accordance with orogenies in the continent. The principal cause of these movements may have been and is still in the movement of the interior of the earth. The convective movement itself might be however complex more and more through ages by the growth of the core. Eventually, the observable features of the earth of the present day are merely momentarily recognized. These features, as a matter of fact, have been brought from a long complicated history of the past. The crust itself is the historical being as well as the core and the mantle of the earth. The mutual relationship is now well established as having been ever intimate between the crustal movement and the interior movement of the earth. From various sources of geological information, the authors now stand to accept the existence of horizontal movement of the crust and repeated reformation o' the crust in various ways in the geologic past, even though the details of the authors' view may not be entirely as similar as the hypotheses of the continental drifting and ocean fioor spreading. Foundation of the young orogenic beXts Certaln geologically young mobile belt like an island arc is actually a product of polycyclic orogenic movements ranging from the Variscan to multiple phases of the Alpine movements. The Japanese islands are one of such examples, where the Abean (I Variscan), Honshu (ti Indosinian), Hiroshima (i Yenshan) and Hidaka (# Himalayan) movements occurred in succession. The complex structure brought by these repeated orogenic movements above mentioned in the Japanese islands was finally reformed by the youngest orogenic movement in the Neogene and Quaternary time to bring forth the so-called island arc structure (M. MiNATo, M. GoRAi 8i M. HuNAHAsHi (editors), 196S). Now, the Paleozoic deposits in Japan are now believed to have been evidently deposited on the basement complex of the Precambrian. At least this may be the case for the Honshu, Shikoku and Kyushu islands through various geologic evidences. Although the gneissic rocks supposed to have been originally belonging to the Precambrian complex in Japan do show rather young absolute age such as 420-320 × 106ys (K-Ar age) old. However, this may have been resulted from either repeated reactivation, progressive orretrogressive metamorphism and taphrogeny during the later orogenic movements. As a matter of fact, such imaginary Precambrian rocks are only locally cropping out in Japan, To be sure, these rocks have been brought to the surface by faulting, thrusting and igneous activity from the depth. It may be however eventually almost indisputable on the former wide development ORIGIN OF THE EARTH'S CRUST AND ITS EVOLUTION 517 of the Precambrian rocks in Japan as a basement complex, prior to the deposition of the Paleozoic formations. (M. MiNATo, 1966, 1968) Large island Borneo may be counted as another such example, and the large peninsula Kamchatka may be also belonging to a similar case like Japan in concern to the problem on basement complexes. In New Zealand, the Precambrian rocks are evidently developed in wide area being covered by the fossiliferous Cambrian formation. In the progress of geologic reconnaissance in future, existence of the basement complex of undoubted Precambrian age will be further proved in many islands belonging to island arcs. Such being the case, it is a quite dogmatic view to assume distinction on the foundation between the young mobile belts and the Caledonian, Variscan and older Alpine belts in the continent. Tlius, there may be originally no difference in foundation between cratonic, sub-cratonic and orogenic belts, since the foundation is chiefly composed of granitic rocks of Precambrian age, in both stable and mobile belts. ' There may be however perhaps young island arcs among the islands in the intra Pacific, whlch may apparently lack the granitic foundation, as a result of reformation of the terrestrial crust, as vLTill be stated later on. I "-i,M-"'"ti;5';-N -""" -"-" -x" ・V=:5.5 "--" """" -""" -mp"" ""-e vv vvv V=6.4 vv v V=8.1 zi "e"" ・V =,5.5 "--vv vv V=6.4 vv vv v