Nd–Hf Isotopic Characteristics, Evolution Trend and Tectonic Setting of Triassic Magmatic Rocks in the Eastern Segment of East Kunlun Orogeny

As a major component of the western segment of the Central Orogenic System, the East Kunlun Orogeny is characterized by the largely exposed of Triassic magmatic rocks. Based on the collected zircon U–Pb geochronological data of 96 Triassic magmatic rocks in the eastern segment of the East Kunlun orogeny, the Triassic magmatic activity is limited to 212～252 Ma, and can be further divided into three stages: early– (238～252 Ma), middle– (226～238 Ma) and late–stage (212～226 Ma). Among them, the peak magmatic period is the early stage (238～252 Ma). The statistical results of 106 Nd isotopes of Triassic magmatite in the eastern segment of the East Kunlun Orogeny show that ε_Nd(t) values range from –9.4 to –1.7, mainly concentrated between –6.5 and –3.0, and the Nd model ages (T_DM(Nd)) range from 0.72 to 1.88 Ga, mainly concentrated between 1.00 and 1.80 Ga. The statistical results of 41 Hf isotopes (whole rock, zircon) of Triassic magmatite show that ε_Hf(t) values vary greatly (–8.4 to +12.4), mainly concentrated between –4.5 and +2.0, and the crustal model ages (T_DMC(Hf)) range from 0.49 to 1.80 Ga, mainly concentrated between 1.15 and 1.55 Ga. Overall, the Triassic magmatic rocks are mainly derived from the reworking of Mesoproterozoic crustal materials, with minor involvement of juvenile crust (＜ 1.0 Ga) and Paleoproterozoic crustal materials. From the early stage (237～250 Ma) to middle stage (226～238 Ma) and then to late stage (212～226 Ma), the Nd–Hf isotopic parameters seems exhibit a certain evolutionary trend. In the early stage especially in the early Triassic, the ε_Nd(t) values are higher, and positive ε_Hf(t) values occupy a large proportion, indicating the presence of more juvenile material in the source. In the middle stage, lower ε_Nd(t) values and negative ε_Hf(t) values dominate the major proportion, and Hf model ages (T_DMC(Hf)) reveals the presence of Paleoproterozoic crustal material. In the late stage, the Nd–Hf model ages reveal an increase in older crustal source components. This magmatic source evolutionary trend is consistent with the tectonic evolution setting of subduction to collision and then to post–collision in the eastern segment of the East Kunlun orogeny in Triassic period.