Late Cambrian Syn‐Intrusive Thickening of the Ikh MongolArc Domains in the Mongolian Collage

Abstract

Structural analysis, U–Pb monazite and xenotime dating, Ar–Ar dating of biotite and amphiboleand thermodynamic modeling of peak metamorphic assemblages allow constraining the tectono‐thermalevolution of the migmatite‐magmatite domain on the southern periphery of Precambrian Zavkhan Block. Themain subvertical metamorphic fabric resulted from upright folding and almost complete transposition of earlysub‐horizontal foliation of uncertain age. P–T conditions of 760–790°C at 0.7–0.8 GPa and in situ U–Pbmetamorphic monazite 505–495 Ma ages characterize this tectono‐metamorphic event. It was associated withsyn‐deformational partial melting and intrusion of axial planar syntectonic leucogranite veins. Monazite andxenotime from these veins give U–Pb age of c. 500 Ma, confirming that the migmatization and formation ofsubvertical fabric were coeval. Such time constraints are contemporaneous with ages of gneissified Ikh‐MongolArc granite sheets intruding horizontally shortened partially molten crust thereby confirming syn‐compressionalnature of arc emplacement. Thermal modeling of cooling history of the whole migmatite‐magmatite domainconstrained by Ar–Ar ages is compatible with the shortening of the hot system followed by moderate erosion.The P–T and geochronological data of the Zavkhan block margin are almost identical to those of short lived lateCambrian magmatism and metamorphism described in the Khondalite belt in Far East China and in other placesof Mongolian Collage suggesting that these two domains may have formed a continuous belt.