Cenozoic Alkali Basalts Tuyon Basin And Its Tectonic Significance

With the interpretation of remote sensing image and the comprehensive field researches ofCenozoic volcanic rocks in the Tuyon Basin, we will discuss in detail about seven volcanoes andtessellated strike-slip faults in this paper. Of them, we proposed the term of the Toyon volcanogroup in the first time. All the volcanoes can be subdivided into two types: central-eruption-typeand flood-basalt-type, building a series of volcanic mechanism, including calderas, volcanicnecks and cone sheets. Tuyon volcanic rocks are classified as alkali basalts, pyroxene megacrystsand deep-seated mantle xenoliths, such as augite and peridotite, which have been observed in somebasaltic layer. We consider that the Cenozoic volcanic rocks in the Tuyon basin were resultedfrom primary mantle magma, which ascended along the faulted-channel and erupted from the severalvolcanoes almost at the same time, rather than the products of eruption during two periods(110~Ma and 67~6Ma) suggested by previous geologists. Tuyon volcanic rocks are classified as alkali series, which are low in silicon, enrichedin alkali, especially enriched in kalium. The types of the volcanic rocks include trachybasalt,basanite, basalt,and phonolitic basalt. A mass of pyrolite xenolith (such as peridotite andpyroxenide) and high-pressure mineral megacrysts (such as anorthoclase, augite and kaersutite)are found in cone sheets and BS volcanic rocks. Pyroxene megacrysts formed under the temperatureof 1200.85-1288.85℃ and the pressure between 0.93 and 1.67GPa, amphibole megacrysts crystallizedunder the pressure of 0.83-0,89GPa, temperature about 1000℃, and anorthoclase maybe at thetemperature of 900℃ around. The Tuyon Basin contains pyrolite inclusions. The mineralogical character of high-pressuremegacrysts, and the Mg*', NiO content and petrographic characteristic of volcanic rocks showthat the Tuyon Basin has the characteristics of primitive mantle magmas. The proof of isotopessuggests that Tuyon volcanic rocks were originated from the same depleted mantle region, and theinitial depth of melting, Zo, was 135km and the final depth of melting, Zf, was 120km. Partialmelting, instead of fractional crystallization, controlled the evolvement of Cenozoic volcanicrocks in the Tuyon Basin and the degree of which was lower than 5%. During the magmas ascendedand intruded, the high-energy, quick ascending primary mantle magmas with xenolith andhigh-pressure megacrysts didn't react with the crustal matter, which formed products, such ascone sheets. After then, the fall of magma energy and ascending velocity made the primary magmasassimilate with country rocks slightly, and their products, such as volcanic lava and volcanicnecks. With the increasing overlying pressure and the loss of magma chamber, collapse calderaformed. Cenozoic volcanic rocks in the Tuyon Basin belong to continental-intraplate basalt, and theformation environment is also continental-intraplate. Their origin was related to the large scalestrike-slip tectonic regime derived from the collision between India and Asia. It seems thatIndian-Asian collision and long-distance transfer were already completed at the beginning ofcollision. '...