滇东南个旧白云山碱性岩年代学和地球化学及成因意义滇东南个旧白云山碱性岩年代学和地球化学及成因意义

报道了滇东南个旧超大型锡多金属矿区西区北部白云山碱性岩新的锆石U-Pb年龄、全岩地球化学和Sr-Nd同位素数据。LA-ICP-MS锆石U-Pb定年结果表明,白云山碱性正长岩形成于晚白垩世（80.0±0.6 Ma）,与个旧地区的中基性岩及花岗岩均为同一次构造岩浆事件的产物;碱性正长岩与霞石正长岩具有相似的主微量元素地球化学特征及Sr-Nd同位素组成,暗示二者很可能是源于同一富集地幔源区并经历了不同程度演化的产物。结合已有的元素和同位素组成结果,认为碱性岩、中基性岩和成矿花岗岩很可能分别源自富集的岩石圈地幔、正常的岩石圈地幔和地壳源区。在晚白垩世伸展构造背景控制下,源于不均一岩石圈地幔的碱性和中基性的岩浆底侵,促使中下地壳岩石部分熔融形成花岗质熔体,在上升至近地表过程中引起构造活动带成矿物质的富集,从而形成个旧超大型锡多金属矿床的矿化格局。可以说,源于富集地幔的碱性岩浆在含矿花岗质岩浆的成岩成矿过程中,应不只是提供热量的贡献。

Geochronology, geochemistry and genesis of the Baiyunshan alkalic rocks in Gejiu area of southeastern Yunnan Province

In this paper, the authors report new zircon U-Pb ages, whole-rock major and trace elements composition and Sr-Nd isotopic data for the Baiyunshan alkalic rocks in the northern part of the western area of the Gejiu tin-polymetallic ore district. The Late Cretaceous age (80.0±0.6 Ma) using the zircon LA ICP-MS U-Pb dating for the alkalic syenites indicates that they occurred with the contemporaneous gabbrodiorites and granites. This suggests that these rocks were all formed during the same magmatic-tectonic event. The geochemical characteristics and Sr-Nd isotopic compositions indicate that the alkaline syenite and the nepheline syenite were derived from an enriched mantle, and subsequently underwent different evolution processes. Combined with the previous studies, the authors hold that the alkali rocks, intermediate-mafic rocks and ore-related granites were derived from the enriched lithospheric mantle, the normal lithospheric mantle and the crust source, respectively. In the Late Cretaceous, the parental melts of the alkali rocks and gabbrodiorite magmas were formed by partial melting of the heterogeneity of the lithosphere mantle triggered by an extensional tectonic setting. Uprising of the above mantle melts promoted the partial melting of middle-lower crust and formed the granitic melts. These Late Cretaceous magmatism caused the activity and enrichment of the ore-forming material within a mutual tectonic belt, and then formed the superlarge tin-polymetallic deposit in the Gejiu ore district. It can be said that the enriched mantle-derived alkaline magmatism not only offered energy during the formation and mineralization of the ore-bearing granitic magma but also played some other roles.