地理情報学と地球統計学: 概要

Evidence of Elliptical Intrusions and Sr, Hf, Pb and O-H Isotopes of the Khanbogd Alkali Granite Pluton: Construction of Underplating Magma Associated with The Duplex Thrust

Dash Batulzii , Serjlkhumbe Amaramgalan*, Holk Gregory, Davaa Khishigsuren, Jargal Lkhamsuren and Otgonhuu Javkhlan

The Khanbogd peralkaline granite pluton, occupying more than 1000 km², is located in southern Mongolia. Previously, two magmatic bodies called Western and Eastern and the flattened shape of this pluton were discovered. Several elliptical intrusions within the pluton have been detected in Landsat satellite images. These are at least four elliptical intrusions in the Western and two intrusions in the Eastern body. The granites of the inferred intrusions differed in Alumina Saturation Index (ASI) and Cross, Iddings, Pirrson and Washington (CIPW), ⁸⁷Sr/⁸⁶Sri and δ¹⁸O values. Fe-Mg minerals in studied granites are garnet, aegirine and arfvedsonites. The low dD values (-155% to -170%) of arfvedsonite support the argument for meteoric fluid involvement in the crystallization source. Two stages of aegirine crystallization were determined before and later arfvedsonite, which formed from a phase induced by meteoric water. The domain structure of arfvedsonite indicates its crystallization from a new lagging phase, which is similar to a peritectic phase. However, the peritectoid-like system is suggested for garnet and aegirine-I. Stage II aegirine, which appeared after arfvedsonite, shows its crystallization caused by a change in magnetization when the water is exhausted in the solidus. The diffusion effect, represented by the droplets of arfvedsonite and II stage aegirine in K-feldspar followed the processes. Further, meteoric fluids resulted in disequilibrium δ¹⁸O values between quartz and feldspar, which occurred in the postsolidus state. Accordingly, the K-feldspar surface has effects, such as fractured and stippled. Two adjacent intrusive bodies and undeformed host rocks at the front part of the pluton are consistent with underplating magmatic factors during duplex-type thrusting. Another factor is the multiple sources for the melt: mantle, reworked seafloor and arc rocks, which are identified by the ratio of ⁸⁷Sr/⁸⁶Sri and δ¹⁸Oqz values. An underplate magma source is a suitable interpretation explaining the principles of low non-stable isotopes for alkali granites.