Petrogenesis of Early Cretaceous riebeckite granophyres in southern Da Hinggan Mts.

This paper reports lithologic features，K--Ar age and geochemical data of riebeckite granophyres from Aliwula area in the southern Da Hinggan Mts．，aiming to reveal the petrogenesis of riebeckite granophyres． K-- Ar age of riebeckite granophyres is 126 ± 2 Ma，implying that the riebeckite granophyres formed in the Early Cretaceous． The granophyres are rich in riebeckites and with a lot of melt-fluid inclusion in its quartz phenocrysts． The granophyres are characterized by extensive enrichment in SiO2 ，FeO，and ( Na2O + K2O) and depletion in MgO and CaO，strong negative Eu anomalies and strong positive Ce anomalies． Additionally，the riebeckite granophyres not only have high total REE contents and display enrichment of HFSEs ( for example Zr， Hf，Nb，Ta) ，but also are strong in depletion of LILEs ( e． g． Ba，Sr) as well as high Ga /Al ratios． Primitive mantle-normalized REE pattern significantly displays REE M--W tetrad effect． REEs fractionate evidently and highly enrich in LREE，but are uneven distribution in the rocks． Taken together，we conclude that the riebeckite granophyres are similar to typical A-type granite，which could be derived from stretching environments in the Early Cretaceous． The granophyres originated from residual melt which underwent highly differentiation process， and were formed in magmatic-hydrothemal transition stage at last．