Source of volcanic ash in the sediments of Sihailongwan maar lake, NE China, and its significance
四海龙湾玛珥湖沉积物中碱流质火山灰的来源及其意义

Sihailongwan maar lake is located in the Cenozoic Longgang volcanic field, northeastern China. Sediments of the Sihailongwan maar lake have been sampled by drilling under Chinese-German cooperative project. The sediments have not disturbed during the drilling. The varve was also well preserved in the sediment core of the Sihailongwan maar lake. Volcanic ash was recognized in the sediments of the Sihailongwan maar lake in the Longgang volcanic field of NE China. Volcanic ash studied is located in 70 cm below bottom of the Sihailongwan maar lake. This volcanic ash cannot comprise a continuous tephra layer, which consists of sparse pumice and tephra in the same depth of the sediment core. The particular shape and sharp rim of the volcanic glass shard suggest that the volcanic ash is the result of direct sinking to the bottom of the Sihalongwan maar lake from volcanic eruption rather than one of secondary transportation. Chemical composition of this volcanic ash by electron microprobe analyses shows that SiO2 varies from 74.72% to 76.64% , Al2O3 from 10. 22% to 11. 53% . TFeO from 3. 72% -4. 35% , Na2O from 4. 20% -4. 77% and K2O from 4. 18% -4. 43%. This characteristic suggests that the volcanic ash was derived from rhyolitic lavas rather than basaltic lavas. However, the volcanic rocks distributed in the Longgang volcanic filed consist of basaltic lavas and basaltic pyroclastic rocks; there is no rhyolite outcropped around the Sihailongwan maar lake. The concentrations of the major element oxides of the volcanic ashes are consistent with those of pumice from Tianchi volcano erupted in 1199 - 1200 AD, which is located to the northeast 200 km of the Sihailongwan, and show considerable differences from those of the volcanic rocks located in the Longgang volcanic field. This feature rules out the possibility that the volcanic ashes interbedded within the sediments of Sihailongwan maar lake were derived from the eruptions of the Longgang volcanic field. In addition, the sparse distribution of the ash in the sediment is consistent with this inference. The volcanic ash was considered to be the product of eruption of Tianchi volcano erupted in 1199-1200 AD based on a geochemical comparison between chemical composition of the volcanic ash and those of the Tianchi volcanic rocks erupted in 1199 - 1200 AD. Moreover, the age of the volcanic ash is around 1630 AD to 1085 AD on the basis of the previous 14C dating of the sediments of the Sihailongwan maar lake, which is consistent with the age (1199 - 1200 AD) of the Tianchi volcano erupted in historical times. This result probably shows that the products erupted from Tianchi volcano occurred in 1199 - 1200 AD reached the Longgang volcanic field. The Sihailongwan maar lake likely is Northwestern boundary of the product from the Tianchi volcano erupted in 1199 - 1200 AD because the volcanic ashes show fine-grained features and low contents in the sediments. The study implies that the magnitude of the Tianchi volc