| Popis: |
The geochemistry of the Late Paleozoic–Early Mesozoic volcano-plutonic rocks of northern Mongolia is a key factor to understand the subduction-related magmatism at the southern margin of “Siberian continent (Siberian craton + accreted geologic units).” Many studies have been carried out in the volcano-plutonic rocks; however, fundamental questions on its detail petrogenesis still remain unanswered. This paper describes the geochemistry of the Permian–Triassic volcanoplutonic rocks at the Oyut-uul area, northeast of Darkhan City, Mongolia. The mid-ocean ridge basalt (MORB)-normalized multi-element pattern (spidergram) for the examined rocks shows an enrichment of large-ion lithophile elements (LILE) against high-field-strength elements (HFSE) and rare earth elements (REE), with clear negative Nb anomaly to suggest an arc-related igneous activity. The most data are plotted in the calc-alkaline field in the SiO2 vs. FeO*/MgO diagram. It is, thus, concluded that the examined rocks are of calc-alkaline series formed at an arc environment. The present samples, 53– 78 wt.% SiO2, high Al2O3 and Sr concentrations, high La/Yb ratio, and low K2O/Na2O ratio, have adakitic nature derived from oceanic slab-melt. They are quite similar to the marginal facies of the Kitakami adakitic granite of Japan in Sr/Y-Y relationship, MgO concentration, spidergram, and REE chondrite-normalized pattern. It is considered that the marginal facies of the Kitakami adakitic granite was resulted from interaction between the slab-melt and mantle peridotite/lower crustal amphibolite during their ascent. The Oyut-uul rocks might be also derived from oceanic slab-melt interacted with mantle peridotite/lower crustal amphibolite. |
