コロンビア共和国エル・ロブレ火山成塊状銅硫化物鉱床母岩の地球化学と鉱床のテクトニックセッティング
1997 年 47 巻 3 号 p. 131-144
詳細
1997 年 47 巻 3 号 p. 131-144
Major and minor elements and mineral chemistry are presented for 19 green rock samples from the El Roble mining area. These rocks are compositionally bimodal ranging from 47.4 to 52.5% SiO2 and from 56.2 to 62.7% SiO2. The basic rocks are divided into three types according to TiO2 Content. The type 1 rocks are richest in TiO2 (2.4-2.8%), and their multi-element spider diagrams normalized against N-MORB and other geochemical discrimination diagrams indicate that they are similar to T-MORB or OIB. The type 2 rocks are lower in TiO2 (1.3-1.6%) than those of the type 1, and their multi-element spider diagrams resemble N-MORB, but merge the island arc characteristics together in rather high content of LIL elements. The type 3 rocks are lowest in TiO2 (1.1-1.3%) among the basic rocks and are transitional between MORB and island arc basalt in their multi-element spider diagrams and minor element (Ti, Zr, V and Sr) chemistry. The intermediate rocks, the type 4, are lower in TiO2 (0.2-0.8%) than the type 3 rocks and include boninite. They have the typical characteristics of island arc and/or fore arc in their multi-element spider diagrams, with distinct minus anomaly of Nb, and in Ti, Zr, V and Sr contents. The El Roble ore deposit occurs closely related with the type 2 and 3 rocks which have suffered the ocean-floor alteration ranging from greenschist facies to a transition state from greenschist to amphibolite facies. In contrast, the mineral assemblage of alteration minarals in the type 1 and 4 rocks indicate the prehnite-pumpellyite facies. Comparing with the geochemical data for the Recent volcanic rocks at variable tectonic settings, it is estimated that the type 1, 2 and 3 rocks were formed at the back-arc rift and the type 4 rocks at the island arc and/or fore arc. In the process of spreading and closing of the Colombian back-arc basin of Cretaceous age, the El Roble ore deposit probably formed at the spreading axis when it approached to the subduction zone and the back-arc magma was generated by mixing of T-MORB and island arc magma components.
