GEOCHEMICAL EVOLUTION CHARACTERISTICS OF TRACE ELEMENTS AND REE IN GEJIU GRANITES
Lu Jie (Institute of Geochemistry, Academia Sinica)
The evolution characteristics of Gejiu granites, Yunan Province are described in terms of their petrology, especially their trace element and REE geochemistry. The three major types of Gejiu granites: the porphyritic biotite monzonitic granite (stage Ⅰ), the medium-grained biotite-K-feldspar granite (stage Ⅱ), and the two-mica alkali-feldspar granite (stage Ⅲ) are thought to have been formed successively from the same magmatic source through fractional crystallization (Rayliegh fractionation), because linear correlations are observed in log(Rb/Sr)-logSn, log(Rb/Ba)-logSn, logLa-logSr, logCe-logSr, logEu-logSr, etc. In addition, the evolution of REE distribution patterns in these three major types of granites also reflects the magmatic differentiation features of Gejiu granites. Of granites of the three major stages, the two-mica alkali-feldspar granite of stage Ⅲ, underwent strong differentiation, and thus has the closest genetic relationship with Gejiu Sn-polymetallic ore deposits. Such Sn-polymetal mineralized granites are characterized by high Rb/Sr and Rb/Ba ratios, low K/Rb and ΣCe/ΣY ratios, and a remarkable Eu depletion.