ХАРГАНАТЫН АМ ОРЧМЫН ВУЛКАНОГЕН ЧУЛУУЛГИЙН БОДИСЫН НАЙРЛАГА, ГАРАЛ ҮҮСЭЛ

Abstract

In this study, we report on U-Pb dating, major and trace elements with aim constraining
the tectonic setting of early carboniferous volcanic rocks near the Harganaty Am valley of
Southern Mongolia. These volcanic rocks, which are mainly composed of basalt, basaltic
andesite, andesite, rhyodacite and rhyolite, located in the eastern segment of Gobi-Tianshan
between the Gurvansaikhan terrane and the Tsagaan Uul block. The result of LA-ICPMS zircon
dating for two felsic rocks indicate that formed in the early carboniferous (ca. 325-333 Ma). The
mafic and intermediate rocks have SiO2 = 41.3-57.7 wt.%, K2O = 0.5-2.54 wt.%, Mg#=0.41-
0.57, Cr = 11-122 ppm, Ni = 4.82-188 ppm, whereas the felsic rocks have SiO2 = 41.3-57.7
wt. %, K2O = 2.1-4.86 wt. % Mg#=0.41-0.57, Cr = 10-91 ppm, Ni = 0.8-17.88 ppm. These
data are typically calc-alkaline suite. The mafic to intermediate rocks are characterized by
strong enrichment in large ion lithophile elements (LILE) such as Rb, Ba, Sr and Pb, depletion
in high field strength elements (HFSEs) such as Nb, Ta and Ti, depletion in heavy rare-earth
elements (HREEs), and positive Eu anomalies (Eu/Eu* = 1.72). On the other hand, the felsic
rocks show a strong depletion in Nb, Ta, Sr, P, and Ti, enrichment in Th, Rb, and, Ba, and
relatively negative Eu anomalies (Eu/Eu* = 0.56-0.74). Taken together, these data suggest that
the mafic to felsic volcanic rocks derived from the partial melting of newly accreted crust. The
early carboniferous volcanic rocks indicate an extensional environment, similar to a subduction
related rocks from orogenic belts.