Petrogenesis of the early Carboniferous Xilinhot gabbro–diorite pluton in central Inner Mongolia: Magma evolution and tectonic significance

Abstract

The Xilinhot gabbro–diorite pluton in central Inner Mongolia comprises a microgabbro–hornblende gabbro–quartz diorite suite and diorite–microdiorite–quartz microdiorite suite, which intrude the Erdaojing complex in the northern orogenic belt of the southeastern Central Asian Orogenic Belt (CAOB). Zircons separated from hornblende gabbro and quartz diorite yielded sensitive high-resolution ion microprobe U–Pb ages of 326.8 ± 3.4 and 324.0 ± 1.9 Ma, respectively. There are two types of hornblende in the hornblende gabbro: isolated hornblende and hornblende clusters surrounding orthopyroxene. Only hornblende clusters occur in the quartz diorite. The amphibole is edenite in the diorite, whereas the microdiorite and quartz microdiorite contain hornblende. The isolated hornblende formed at higher P–T–H2Omelt conditions (3.3–5.0 kbar; 749–814 °C; 7.4–8.3 wt% H2O) than the hornblende clusters in the hornblende gabbro (0.9–2.2 kbar; 670–731 °C; 3.8–5.5 wt% H2O). Edenite in the diorite crystallized at higher P–T and lower H2Omelt conditions (1.8–4.8 kbar; 777–979 °C; 3.2–5.2 wt% H2O) than hornblende in the microdiorite (1.3–1.5 kbar; 697–711 °C; 5.0–5.6 wt% H2O). These thermodynamic parameters reveal a multi-level magmatic storage system. Hornblende gabbro, quartz diorite, diorite, microdiorite, and quartz microdiorite are generally characterized by flat middle to heavy rare earth element patterns and no Eu anomalies. Geochemical modeling of heavy rare earth element concentrations in hornblende gabbro suggest that the magma was generated by >17.5% partial melting of spinel lherzolite. Slab-derived fluid input can account for the enrichments in Sr, Th, and light rare earth elements, and depletions in Nb and Ta. The multi-level magmatic storage system and slab-derived fluid input suggest that the early Carboniferous magmatism was generated in an arc-related tectonic setting. The northern orogenic and Hegenshan ophiolite belts constitute an early Carboniferous fore-arc basin–arc–back-arc basin system.