关键词: 火山喷发物, 埃洛石, 风化, 琼北

Abstract: It is well known that weathering process initiates as soon as the volcanic products reach the earth surface. As a result, various kinds of secondary clay minerals and their polytypes are formed in different periods of weathering process. Based on this fact, the time sequence and succession of volcanic eruption can be inferred. It may provide, therefore, a convenient dating method applicable to the prediction and assessment of volcanic hazards. In this paper, the weathering features of clay minerals from 3 profiles of volcanic ejecta in Leihuling-Ma'anshan area, northern Hainan Island are studied in detail by using various techniques including scanning electron microscopy, energy spectroscopy and x-ray diffraction analysis. The results of this study have led to some new insights about the clay minerals and their weathering features, as well as the time sequence of volcanic eruption in this area. (1) The spheroidal halloysite is predominant in the secondary clay minerals formed by the weathering of volcanic products in this area. The interlayer water within halloysite decreases with the development of weathering, and as a result halloysite changes its form in a sequence of 10Å halloysite→(7Å+10Å)halloysite→7Å halloysite. (2) The Fe³⁺ content in halloysite is within the range of 9.9%～28.8%, and the average SiO₂/Al₂O₃ ratio is 1.62, belonging to iron rich halloysite. (3) Based on final products of weathering, it is postulated that the succession of volcanic eruption in this area was southeast Leihuling→north Ruhongcun village→east Ruhongcun village. (4) Taken the study of weathering products of volcanic ejecta in Japan as a reference, the eruption time of volcano in this area can be inferred to be 6～12ka BP.

Key words: volcanic ejecta, halloysite, weathering feature, north Hainan Island