| 其他摘要 | Anisian basalts have been identified in the Kafang-Laochang, Qilinshan (Gejiu area) and the Donglian village (Kaiyuan areas), Yunnan province, SW China. They are geologically located in the central zone of the Emeishan large igneous provinces. They are intruding in the Gejiu Formation (T2g). The Anisian basalts in Gejiu area are widely distributed and it covers an area of >125 km2. The Anisian basalts in Kaiyuan area are less exposed. Thanks to the close spatial relationship with the stratiform copper orebodies of the world-class Gejiu tin poly-metallic ore district, the Kafang-Laochang basalts have been well studied. The Kafang-Laochang basalts have suffered intense alteration. Moreover, due to the lack of Sr-Nd isotopic data and geochronology data, the petrogenesis and the tectonic setting of Gejiu Anisian basalts are still vague. The unclear petrogenesis of the basalts will largely hold back the understanding of the layered copper in the basalts. For the sake of those problems, the unaltered Qilinshan basalts and the Kaiyuan basalts are included in this study. Besides the systematic sampling, microscopic identification and major and trace elements analysis, the TIMS for the Sr-Nd isotopic ratios, the LA-ICP-MS for the U-Pb zircon and the low contents of PGE analysis are also carried out in our study. Base on the comprehensive study of mineralogy, petrology, geochemistry, Sr-Nd isotope ratios and chronology. The results are as following: The zircons dating result from Kaiyuan basalts show age of 248.2 ± 6.1 Ma (N = 9; MSWD = 0.25), and the zircons dating result of Kaiyuan andesite show age of 247.7 ± 1.4 Ma (N = 16; MSWD = 0.015). These results are very consistent. The Kaiyuan basalts are formed at 246～249 Ma, 10 Ma younger than the termination age (257 Ma) of Emeishan mantle plume event. The high K and Mg character of Kafang-Laochang basalts could not represent the source, but the result of granite-related hydrous fluid alteration. The K was primarily derived from the granite-related hydrous fluid and the Mg was come from the dolomite country rock. The olivine and the clinopyroxene have alter to the phlogopite (K and Mg rich mineral), causing the increase of K and Mg in the Kafang-Laochang basalts. The primitive mantle–normalized PGE patterns of the Gejiu-Kafang basalts are very uniform and show similar patterns with the Emeishan high-Ti basalts. During low-degree partial melting, the Pt-Pd rich sulfides in mantle control the PGE contents in melt. The Gejiu-Kaiyuan basalts have less PPGE contents than the Emeishan basalts owing to the early extraction of PPGE from the same mantle. The PGE features imply the Gejiu-Kaiyuan basalts are the remelted product of Emeishan plume. The Gejiu-Kaiyuan basalts are alkaline basalts with high TiO2 contents. These basalts have similar trace elements, REE, PGE patterns and Sr-Nd isotope ratios. The patterns and ratios are similar with the Emeishan high Ti basalts, indicating they are forming by a same event and correlation with the Emeishan plume event. Due to the Permian Emeishan plume event and the The Simao terrane collided with the South China/Indochina block in Middle Triassic time, the Yangtze continental lithosphere was suffered a relaxation, which triggered a rift event at 248 Ma. The alkaline magmatism occurred in this rift system. The alkaline basalts it is the low degree of partial melting (4～12 %) product of Emeishan plume. The magma has suffered clinopyroxene fractional crystallization and weak crustal contamination, erupted in the Gejiu-Kaiyuan rift at 248 Ma. During 83 Ma, the Kafang-Laochang basalts have suffered intense Yanshanian granite-related hydrous fluid alteration and the dolomite country rock contamination, the mineralcompositions and the major elements compositions have change dramatically. |
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