Element Geochemistry and Petrogenesis of High-K Potassic Dike Rocks in Two Types of Gold Ore Fields in Northwest Jiaodong, Shandong, China

摘要

This paper deals with the high|K, potassic dike rocks in two types of gold ore fields at Linglong and Dayigezhuang, Northwest Jiaodong. The rocks can be divided into three types, i.e., (1) lamprophyre, (2) andesite porphyrite, and (3) dacite porphyrite, based on their geological occurrence and space|time relationship with gold mineralization. These rocks were the products of early, synchronous and late mineralization, respectively. Element geochemistry shows that variations in chemical composition of major oxides follow the general rules of magmatic fractional crystallization. The fractional crystallization of mineral phases of augite in the early stage (namely in the lamprophyre stage) and hornblende and plagioclase in the late stage (namely from the andesite|porphyrite to dacite porphyrite stage) controlled the magma evolution. The rocks are enriched in alkali and have higher K-2O and lower TiO-2 contents, as well as strongly enriched in large ion lithophile elements such as Ba, Sr and Rb, and LREE but strongly depleted in transition elements such as Cr and Ni. Rb is depleted relative to Sr and Ba, and Rb/Sr ratios are low. Volatile constituents are abundant. These characteristics indicate that the initial magma originated from the metamorphic subduction ocean|crust that had been intensively contaminated by crustal materials, and retrogressive metamorphism is characterized by low|degree partial melting during back|arc spreading. Varying degrees of partial melting and different emplacement environments may be the main causes for the evolution of the rocks and mineralization in different degrees in the two gold ore fields at Linglong and Dayigezhuang, Shandong.