MESOZOIC VOLCANIC ROCKS IN THE EASTERN PART OF CHINA
Wu Liren Qi Jinying Wang Tingdu Zhang Xiuqi Xu Yongsheng (Institute of Geology, Academia Sinica)
The Mesozoic volcanic rocks in the eastern part of China are well developed and cover an area of 1,200,000 km~2. Geological and petrological studies demonstrate that the formation and activity of magma of these volcanic rocks were closely related to plate tectonics.A lot of geologists consider that the Japanese Island Arc began to separate from the Eurasian continent after 120m.y. BP. The Mesozoic high-pressure and low-temperature metamorphic zone. (Dananao glaucophane greenschist series) in the eastern part of Taiwan Island, corresponding to the Sanbagawa Formation in the south-western part of Japan, .indicates the trace of Mesozoic plate movement. Therefore, the position of the whole trench in the Early Jurassic Epoch was near the Eurasian continent. In the Early Yanshanian (180—145m.y.) the Kula plate moving towards NNW subducted under the Eurasian plate to cause a strong folding and uplift of the earth crust. Intrusion was the main form of magmatic activity, And then, in the Late Yenshanian (140—70 m.y.), it gave rise to fracturing and depression. At the same time, some old deep faults were revived. These faults are usually characterized by anticlockwise displacement. This might be the early stage of the development of a multiple rift system. Volcanic extrusion became the predominant magmatic activity.Based on the association of volcanic rocks there can be divided three petrological regions. The main boundaries between these three petrological regions are controlled respectively by the Baodi-Changli-Jinzhou fault in the north, and the Changzhou-Yueyang deep fault in the south, in a roughly NEE direction. They are not parallel with but cross the Mesozoic trench line at an acute angle. These boundary lines are nearly perpendicular to the direction of Kula plate's movement and reflect the variation in depth of magma generation from south to north. The depth of magma formation is the greatest in the northern region, intermediate in the central region, and much shallower in the southern region. We call such kind of petrological zones "chemical zonation", and the bimetamorphic zones in the southwestern part of Japan, as well as in Taiwan Island and the mainland of China, "physical zonation".In the southern region the andesite-dacite-rhyolite series is the main volcanic rock association, in which rhyolite is the predominant member. Occasionally, there occur basalts (tholeiite, trachybasalt). Apart from basic volcanic rocks, all of them belong to calcalkali rock series. In the central region potash-rich volcanic rock associations are well developed, such as (trachybasalt)-trachyandesite-latite-trachyte.(phonolite). They are referred to alkali rock series, straddle-A or-B type of differentiation trend. While in the northern region the main volcanic rock association is basalt-andesite-dacite-rhyolite and belongs to calc-alkali rock series, in Yanshan district there often occur quartztrachybasalt, quartz-trachyandesite, quartz-latite, and rhyolite. In this case they are referred to a transitional type from alkali to calcalkali rock series.The composition of all the above-mentioned volcanic rocks is characterized by high alkali, especially potassium, and with K_2O/Na_2O≥1, but low titanium (TiO_3 usually1%).The results of the petrological, isotopic (O,S,Sr) and rare earth research lead us to the conclusions that volcanic rocks in this province are of multiple origin. The genera tion of rhyolitic magma in the southern region was essentially by anatexis of the lower part of the continental crust, and andesitic magmas of the northern and central regions might be generated by fusion of upper mantle mixed with some materials of oceanic crust and pelagic sediments in the subduetion zone under great compressive force. The presence of a large amount of biotites and amphiboles in these rocks indicates that, water has played an important role in the process of magma generation. Assimilation and contamination of the continental crust, even of sedimentary rocks, such as carbonates, gypsums and salts by the magma have also taken place. The potash-enriched trachybasalt-trachyandesite-latite-traehyte-(phonolite) rock series is quite similar to the so-called "shoshonitic magma type". These lavas were erupted at the time of fracturing' (rift) instead of uplifting during the Yenshanian movement, probably in a relatively more consolidated and stable area. The depth of magma formation cannot be very great, because we have never found anatectic inclusions, not even xenocrysts from them, and there are a large amount of watercontaining minerals in the andesitic rocks. The coexistence of olivine tholeiite together with them indicates that they have similar origin at a depth of 40—60km. Part of potassium has come from the mantle by partial melting, but an important part of it is from the continental crust.