【摘要】 ~~更多还原

【Abstract】 Ⅰ. General SKETCH The Cenozoic basalt lavas are widely distributed along the coastal regionand its adjacent districts in eastern China. They form either widespread plateausor scattered mesa-like hills; and, in some places, the volcanic craters are stilladmirably preserved to-day. These basalt sheets were erupted extensively in lateTertiary and early Quaternary times. They belong to the multiple-vent basalts,but not the plateau basalt in the type of eruption. In this paper are given the results of a petrochemical study of the Ceno-zoic basalt in eastern China. Geological and petrographical studies of the basaltsare not described in detail. Judging from the chemical compositions, the mostoutstanding characteristic feature of the most basaltic rocks is the richness inalkalis, especially in K₂O, also the undersaturation in silica, and, therfore,the most of the basalts distinctly belong to the alkaline rock suite. Such chemicalcharacters are closely related to modal compositions. The general petrogra-phical characters of the basalt may be now as well described. In North Chinaand South Manchuria most basalts are trachy basalt and its allied types, andoccasional nepheline basalt. The constituent minerals of these basalt are cha-racterized by the presence of anorthoclase, potash-adesine, plagioclase, titanaugite,as well as by the abundance of olivine. In South China the main basalt is olivinebasalt, consisting of olivine, augite, and plagioclase as usual. At some placesthe basalt may be slightly silica saturated that the occasional presence of hyper-thene and quartz （xenocryst） is noteworthy. In North Manchuria the dominant basalt is leucite basalt, containing leucite and olivine as phenocryst and groundmass constituent. Xenocryst of quartz and feldspar grains are sometimes present.In Taiwan Province, basalt of Kuanyinshan volcano is of the tholeiite type,but, in some other districts, the alkaline basalt with feldspathoidal constituentalso occurs. In view of the above-mentioned facts, the petrographical difference seemsto be matched by the differences of the major tectonic forms of China. Thesefacts may be tabulated below: Geographical region Tectonic forms Basalt type 1. North Manchuria Granitized region in Varisclde Leucite basalt 2. North China & South Precambrian Sino-Korean Trachybasalt Manchuria massive 3. South China Huanan platform in Yenshanide Olivine basalt 4. Taiwan Himalayide Tholeiite basalt Ⅱ. Regional INVESTIGATION In Table 1 the chemical analyses of the Cenozoic volcanic rocks in easternChina published up to 1951 were collected, of which 74 analyses are basalticrocks. The available analyses of every basalt sheet are taken an average to re-present the regional composition of the basaltic rocks. The localities of the basaltsheets where the analyzed chemical data are referable are shown in Figure 1.In all, 21 localities of basalt are investigated. The average chemical compositionsof each basalt locality, together with the normative minerals, Niggli’s valuesand Zavarickij’s numbers are given in Tables 2 A and 2 B respectively （localnos. in Table 2 correspond to the numerous in circle in Figure 1）. For theconvenience of comparison, the average chemical composition of some standardbasalts （after S. R. Nockolds, 1954）, some alkaline basalt in New Zealand （afterW. N. Benson, 1944） and Uganda （after A. Holmes, 1937）, and the world’s ba-salts, together with the normative minerals, Niggli’s values and Zavarickij’snumbers of our calculation, are selected and given in Tables 3A, 3B, 7A, 7Brespectively. As mentioned above, the chemical composition of eastern China basalt ischaracterized by the high content of alkalies, and especially the K₂O. This cha-racter is definitely revealed by the normative feldspar, which is given in Table2 A and graphically shown in Figure 2. From this graphical representation,it is recognized that the normative feldspar becomes increasingly enriched ormolecular from South China to North Manchuria. Although the projected points appear to be scattered, a trend of clustering of the normative feldspars in accor-dance with its geographical distribution is apparent upon close inspection. Inthe normative pyroxene diagram, Figure 3, it is shown that most basalts of easternChina are rich in Wo molecular, falling near the Di-Hd line; On the otherhand, those of Kuanyinshan basalts of Taiwan Province are poor in Wo andcluster in the central part of the Di-Hd-Fs-En field. Therefore, that the easternChina basalt, excluding that of Kuanyinshan, belong to the alkaline rock suiteis justified. The comparison diagrams of qz, al-alk, C-（al-alk） and K-mg ofNiggli’s values, based on the data given in Tables 2 B, 3 B, and 7 B, are gra-phically shown in Figs. 4, 5, 6 and 7 respectively. As will be seen from thesefigures, in general, the qz and al-alk of Niggli’s value are lower, but C-（al-alk） and K-mg are higher than those of some common basalts. Fig. 8 isZavorickij’s petrochemical vector diagram, from which the petrochemical cha-racters of high alkaline property in eastern China basalts are clearly justified. Ⅲ. Serial COMPARISON For ascertaining and understanding the general chemical characters, theCenozoic eastern China basalt and its differentiated associations are divided,based on the SiO₂ content, into 5 chemical types, the average composition ofwhich is given in Table 5A. As the chemical composition of Kuanyinshanvolcanic rocks in Taiwan Province shows a distinct calc-alkaline affinity, it is,therefore, dealt with the same procedure as well. The average chemical compo-sition of the volcanic rocks is given in Table 5 B. Since the Cenozoic basalt ineastern China covers the region adjacent to the Circum-Japan Sea and the Ja-panese Archipelagoes which are well known by the occurrence of alkaline andcalc-alkaline rock suite respectively. The serial variation in the chemical com-position of eastern China volcanic rocks is examinated in comparing them withthose of the Circum-Japan Sea and the Japanese volcanic rocks. The data usedfor the Circum-Japan Sea and Japanese rocks, which are given in Tables 6 A and6 B, are based on Tomita’s and Yamada’s calculated results respectively. Zavaric-kij’s number of the same rocks, based on Zavarickij’s data, is given in Table6 C. Following various petrochemical methods, some graphic diagrams are cons-tructed in order to express the chemical characters of the rock series of easternChina more definitely. The difference in chemical composition as comparedwith the Circum-Japan Sea alkaline rock suite is shown in the normal varia-tion diagram Figure 9. Other petrochemical indices （proposed by Peacock, Hol- mes and Tomita respectively） obtained are summarized below: Ⅰ Ⅱ Ⅲ Ⅳ alkali-lime index （Fig. 10） 65.5 59.0 63.0 52.0 alkali-alumina index （Fig. 11） -- -- 67.8 63.3 Ol-Q index （Fig. 12） 20.0 43.0 72.5 76.5 Ne-Hy index （Fig. 13） -- -- 55.0 59.9 Di-C index （Fig. 14） 46.5 46.5 -- -- Ⅰ. Japanese volcanic rocks; Ⅱ. Taiwan volcanic rocks; Ⅲ. Circum-Japan Sea alkaline suite; Ⅳ. eastern China volcanic rocks.From these graphical solutions in Figs. 10-14, it may be justified that thevolcanic rocks in eastern Asia have increasingly enriched the alkali contentfrom the Japanese Archipelagoes to the mainland of China. Figure 15 is avariation diagram of the normative feldspar of the eastern China series, incomparison with that of the Circum-Japan Sea region, the Japanese Archipela-goes and New Zealand. As will be seen from the diagram, the variation of thenormative feldspar of eastern China rock series is quite discriminative. Notonly is it richer in or molecular, but the positions of point N and O （corres-pond to the respective Ol-Q and Ne-Hy indices as shown in Figs. 12 and 13）of the eastern China series are also in a later course than the others. Figure 16is a paragenetic diagram of the normative plagioclase and pyroxlene of theeastern China series. In the variation diagram of the normative pyroxene, Fig.17, the distinct difference in the course of variation between the alkaline andcalc-alkaline series is clearly revealed. In this connection, the directon of varia-tion in the normative pyroxene of the alkaline series differs from that of thecalc-alkaline series. It is also notable that the range of variation in the easternChina series is smaller, due to the presence of higher Wo molecular in bothbasic and acidic types, as compared with that of the Circum-Japan Sea series.Figure 18 is Zavarickij’s petrochemical vector diagram of the eastern Chinaseries compared with that of Circum-Japan Sea and Japanese series. It may be acomplemental diagram of the serial comparison diagram of eastern Asia volcanicrock series, prepared by Zavarickij （cf. A. N. Zavarickij’s Introduction to Petro-chemistry of Igneous Rocks, 1950, Fig. 61, p. 205, in Russian）. The distinctdifference of these rock series may be clearly observed in the graphical solution.In Table 7 A is given the average chemical composition of eastern China basalt,together with the composition of basaltic rocks of the world （including olvinebasalt and Toleiite, Ocean basalt and Continent basalt） for comparison. Theeastern China average is very similar to the average composition of the olivine- basalt of the Circum-Japan Sea region （based on Tomita, 1935, and recalculatedby the writer）, while it differed more or less from that of any of the world’sbasalts by the high content of alkali and especially K₂0. This character isdefinitely revealed by the normative feldspar and pyroxene, which are graphi-cally shown in Figs. 19 and 21 respectively. Figure 20 is Zavarickij’s petroche-mical diagram of the world’s basalt. As shown in these diagrams, it suggests atransitional relation rather than a sharp cleft in the petrochemical charactersof the world’s basalts. There are no distinct types of basalt magma but rathera continuous series from silica saturated （tholeiitic） to silica-undersaturated（olivine-basalt） rock, although the chemical distinction may be too subtle to beread directly from chemical analyses. Ⅳ. ORIGINAL Basalt Magma The original magma of eastern China basalt belongs to the olivine basaltmagma type. In this discussion it is assumed that the primary olivine basaltmagma is melted from a crust of peridotitic crystalline rock in the upper partof the sima layer. Olivine basalt magma may be modified by the gain of alimited amount of silica and some alkalies to produce a derived melt, whichappears to coincide with the tholeiitic magma.更多还原