Zircon shrimp U-Pb chronology and geochemistry of Cenozoic orthoclase porphyry in Gongmaorima area, Northern Tibet plateau

Gongmaorima Cenozoic orthoclase porphyry is located at the eastern Hoh Xil, northern Tibetan plateau, and formed in oligocene, Paleogene period with SHRIMP U-Pb age of zircon is 26.51 Ma ± 0.79 Ma. It is characterized by high w(SiO₂) (58.62%-61.86%), enriched in ALK (Na₂O + K₂O 7.71%-10.09%), high in w (K₂O) (5.72%-7.75%), high in K₂O/Na₂O ratios (2.1-5.3), high in MgO (3.09%-4.61%) and Mg^# values (0.59-0.69), in dicating that it belongs to shoshonite series. Their ∑REE contents are very high (262.88 × 10^-6-371.65 × 10^-6), especially enriched in LREE (247.76 × 10^-6-352.29 × 10^-6), but depleted in HREE (15.12 × 10^-6-18.68 × 10^-6). The contents of w(Y) and w(Yb) are generally lower, which is from 17.43 × 10^-6-21.53 × 10^-6 (average 18.71 × 10^-6) and 1.30 × 10^-6-1.67 × 10^-6 (average 1.52 × 10^-6), respectively. REE patterns are strongly oblique to the HREE side with intensive enriched in LREE, and (La/Yb)_N values are from 24.49-33.85. Eu anomaly is invisible, which Eu/Eu^* from 0.74-0.92. The contents of Sr and Sr/Y value are also higher, which from 675 × 10^-6 ∼1 949 × 10^-6 and 38.73-90.52, respectively. In the primitive mantel-normalized spidergrams of trace elements, obvious negative anomalies showed in Nb, Ta, P, Ti and Y. These geochemical characteristics showed that Cenozoic orthoclase porphyry in Gongmaorima area belong to a suite of high-Mg and high-K C-adakites, and their potential source was equivalent to an eclogitic phase rock. They are generated by partial melting of lower crust section, implying that the crust had been thickened and an eclogitic lower had been formed during oligocene, Paleogene period in northern Tibetan plateau area.