HIDDEN CRUST-DERIVED MAGMATISM REVEALED BY THE ~23 MA POTASSIC MAGMATISM IN THE WESTERN LHASA TERRANE, TIBET: INSIGHT FROM ZIRCON U-PB DATING AND HF ISOTOPES

The postcollisional potassic and ultrapotassic magmatism in western Lhasa terrane has been well-studied in the past decade. The magmatic pulse during 21-25 Ma has been particularly well recognized by Ar-Ar dating in Shiquanhe, Xiongba, Bangba and Wenbu areas (Miller et al., 1999; Ding et al., 2003; Williams et al., 2004). Here, for the first time, we present our new zircon LA-ICPMS U-Pb dating and Hf isotope data on new potassic extrusive rock occurrences in the Xiongba and Bangba areas of the western Lhasa terrane, to further constrain their age and nature of their melt source regions. These rocks are high-K calc-alkaline dacite and shoshonitic trachyte, with their SiO₂ content ranging from 63 to 69 %, MgO from 0.6 to 2.5 %, and K₂O /Na₂O from 1.5 to 6.2.

One hundred zircon grains from seven potassic samples in Xiongba and Bangba give concordant LA-ICPMS U-Pb ages of 23-24 Ma. These zircon separates yield ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.282273 to 0.282531, corresponding to ε_Hf (t) values of -17.1 to –8.0. Their Hf depleted-mantle modal ages (T_DM) and crustal model ages (T_DM^C) are in the range of 1.0–1.4 Ga, and 1.6-2.2 Ga, respectively. These data collectively suggest that magmas of the alkaline rocks were derived mainly from a source region with significant contributions from mature crustal materials.

We also found two major clusters of inherited zircons with concordant U-Pb ages in the potassic rocks. The first (39 zircon grains) yields a peak age of ~90 Ma (MSWD = 2.0), with positive ε_Hf (t) values ranging from ~0 to 6. Such zircon ε_Hf (t) values are significantly lower than those of the contemporaneous granite (86.4 Ma) near Quxu in central Gangdese batholith (ε_Hf = 10–13, Ji et al., 2009), implying that the ~90 Ma magmatism with contributions from depleted mantle materials also exists in the western Lhasa terrane without outcrops yet recognized. The second cluster (10 zircon grains) yields a peak age of ~152 Ma, with negative ε_Hf (t) values of -11 to -5.2, suggesting the presence of Late Jurassic magmatism in the same area. Contemporaneous granitic magmatism (152 and 159 Ma) with different ε_Hf (t) values (10-15) was recently reported in Dazhuqu, central Gangdese batholith by Ji et al. (2009).

Our preliminary results reveal that the ~90 Ma and ~152 Ma magmatic events with different source regions to the central Gangdese batholiths probably volumetrically significant in the western Lhasa terrane. We interpret the hidden ~90 Ma magmatism as having generated by partial melting of the lower continental crust, whereas the ~152 Ma magmatism was derived largely from anatexis or remelting of the middle-upper crust with mature continental materials beneath the western Lhasa terrane.