T1: Comprehensive understanding of the crustal evolution and resource exploration in Asia (Symposium)

[T1-P-02] Amphibole-bearing granitic rocks in the Ereendavaa block, NE Mongolia: Insights into multi-stage magmatic sources and crystallization conditions

*Munkhdelger Bold¹, Tatsuki Tsujimori¹, Yasuhito Osanai², Nobuhiko Nakano², Tatsuro Adachi², Otgonbayar Dandar¹, Fransiska Ayuni Catur Wahyuandari ² (1. Tohoku Univ., 2. Kyushu Univ)

Keywords:CAOB, geothermobarometer, EPMA

The Ereendavaa block (EDB) is located along the southern part of the Mongol-Okhotsk Belt within the eastern Central Asian Orogenic Belt. The EDB has experienced multi-stage magmatism through tectonic cycles, including northward subduction of the Paleo-Asian Ocean during the early Paleozoic and southward subduction of the Mongol-Okhotsk Ocean during the early Mesozoic. However, melt evolution and magma crystallization conditions during magmatic pulses at 540, 470, and 220 Ma are poorly constrained. This study aims to elucidate magmatic crystallization conditions through mineral chemistry (EPMA) analysis of amphibole-bearing granitic rocks from these periods. The granitic rocks in the EDB typically comprise quartz, feldspar, amphibole, and mica, with accessory minerals like opaque. Compositional data for feldspar and hydrous mafic minerals (amphibole and biotite) reveal the following compositions: (1)~540 Ma granodiorite: orthoclase, oligoclase, pargasite, and Mg-rich biotite; (2)~470 Ma granite: orthoclase, oligoclase, ferro-pargasite, and Fe-rich biotite; (3)~220 Ma granodiorite: orthoclase, andesine-oligoclase, edenite, and Mg-rich biotite. The mineral chemistry indicates a sub-alkaline nature for the 540 and 220 Ma granodiorites, while the 470 Ma granite is alkaline. Temperature estimates, based on Ti-in-Ca amphibole geothermometer, range from 957–869°C (540 Ma) and 918–858°C (470 Ma), to 825–782°C (220 Ma). Based on Al-in-Ca amphibole geobarometer, pressure estimates suggest these rocks formed at ~8–6 kbar, ~9–7 kbar, and ~4–3 kbar, respectively. Average oxygen fugacity values are -14.11 (470 Ma granite), -11.82 (540 Ma granodiorite), and -13.04 (220 Ma granodiorite). In summary, the parental magmas for the 540 Ma and 220 Ma granodiorites evolved from orogenic, calc-alkaline to sub-alkaline magmas, emplaced at middle (~26 km) and upper (~12.5 km) crustal levels. They crystallized at high (up to 957°C) and lower temperatures (up to 825°C) under oxidizing conditions. The 470 Ma granitoid likely evolved from more alkaline magmas, emplaced at deep crustal levels (~29 km) approaching the lower crust, and crystallized at high temperatures (up to 918°C) under more reducing conditions than the 540 Ma and 220 Ma granodiorites.