A new model for the granite–pegmatite genetic relationships in the Kaluan–Azubai–Qiongkuer pegmatite-related ore fields, the Chinese Altay

Abstract

Pegmatites commonly form in the waning stage of magma evolution by fractional crystallization of volatile-rich magmas and may be important host rocks of strategic metals (e.g., Li, Be, Cs, Ta, and Nb) and high-quality gem minerals. This study reports new zircon U–Pb dating results and Hf isotopic compositions of the KLA803 pegmatite, the AZB-01 pegmatite, the JMK-09 pegmatite (abbreviated as the K–A–J pegmatites) and the Halong granite from the Chinese Altay to determine the potential petrogenetic relationships between them. The geochronological data document that the K–A–J pegmatites were emplaced at 224.6±2.3Ma, 191.6±2.0Ma and 192.0±2.3Ma, respectively, and they are characterized by negative to low positive εHf(t) values (from −1.0 to +6.3) and old model ages (TDM) (with the TDM1 from 874 to 597Ma and TDM2 from 1298 to 833Ma). In contrast, the Halong granite has an emplacement age of 398.3±2.4Ma and is characterized by higher positive εHf(t) values (from +9.9 to +15.2) and younger model ages (TDM) (with the TDM1 from 626 to 414Ma and TDM2 from 760 to 423Ma). They all have intruded into the Kulumuti group stratum, which has negative initial εNd(t) values (from −4.3 to −0.2) and old TDM model ages (between 1.22 and 1.56Ga). Based on the calculated results of the mixing ratios (f) of the initial magmas and the prevailing Paleozoic tectonic framework of the Chinese Altay, we establish two petrogenetic models for the K–A–J pegmatites: Model 1 refers to that these pegmatites originated from a mixed magma that was composed of 72–91wt.% depleted mantle components and 9–28wt.% lower crust components; and Model 2 refers to that they were derived from the partial melting of 38–83wt.% Halong granite and 17–62wt.% sedimentary rocks from the Kulumuti group. We also suggest that the initial magma of the Halong granite was significantly contributed by juvenile materials with a slight involvement of crustal materials. In Model 1, because LCT-type pegmatites (classified as Li–Cs–Ta enriched pegmatites associated with S-type granite that was produced by the partial melting of preexisting sedimentary rocks) have close geochemical affinities with crustal materials, the excessively high weight percentages (72–91wt.%) of the depleted mantle components in the initial magma of the K–A–J pegmatites indicate that this model is unrealistic. Therefore, we consider that Model 2 is more reasonable at present for interpreting the petrogenesis of the K–A–J pegmatites, and it needs to be verified in other pegmatite fields of the Chinese Altay.