Petrogenesis and magmatic-hydrothermal evolution time limitation of Kelumute No. 112 pegmatite in Altay, Northwestern China: Evidence from zircon U-Pb and Hf isotopes

摘要

Granitic pegmatites and associated rare-metal deposits, unique features in the Altay orogen, are located in a key tectonic position of the southern Central Asian Orogenic Belt (CAOB) and document the tectonic evolution of the Paleo-Asian Ocean. The Kelumute No. 112 pegmatite, which intruded into the Jideke two-mica granites, hosts abundant rare-metal (e.g., Li, Be, Nb, Ta) ore deposits and ranks only second to the Koktokay No. 3 pegmatite in both size and reserves. To explore the evolution time limitation of magmatic, magmatic-hydrothermal and hydrothermal stages in the pegmatite magma system, the tectonic setting and the genetic relationship between pegmatites and granites, this study presents zircon U-Pb and Hf isotopic compositions of the Kelumute No. 112 pegmatite and the wall rocks (two-mica granite and biotite granite) as determined by LA-ICPMS and LA-MC-ICPMS. The weighted mean ~(206)b/~(238)U ages of the internal textural zones of the pegmatite, including zones I, II, III, V and VI, are 238.3 ± 2.0 Ma, 233.5 ± 3.7 Ma, 188.3 ± 1.7 Ma, 218.8 ± 1.9 Ma and 210.7 ± 1.6 Ma, respectively. Wall rocks of two-mica granite and biotite granite are dated at 445.6 ± 4.3 Ma and 455.6 ± 5.4 Ma, respectively. Zircons from the Kelumute No. 112 pegmatite have lower positive ε_(Hf)(t) values ( + 0.03 to +2.35), with T_(DM) model ages of 1112-1225 Ma. Wall rocks show similar zirconε_(Hf)(t) values ( - 1.41 to +4.13) and T_(DM) model ages (1172 to 1515 Ma). However, three xenocrystic zircons from the granites are characterized by larger negative 6Hf(t) values ( - 5.85 to -9.83) and older T_(DM) model ages (1839 to 2090 Ma). Thus, the following conclusions can be drawn: 1) the Kelumute No. 112 pegmatite and its wall rocks (Jideke two-mica granite and biotite granite) have no genetic relationship, as indicated by the large gaps between their formation ages. However, they did originate from a common source composed of ancient crust and mantle-derived materials under two different tectonic settings; 2) the magmatic, magmatic-hydrothermal transition and hydrothermal stages of the No. 112 pegmatite magma lasted for ~5 Ma, ~23 Ma and ~22 Ma, respectively; and 3) the No. 112 pegmatite magma was most likely formed in a post-collision tectonic setting, indicating that block amalgamation and collisional orogeny of the CAOB continued into the Triassic.