Study Of Magmatism In Tengchong-baoshan Block, Western Yunnan And Its Tectonic Implications

The Tengchong Block and the Baoshan Block are the important branches of the Tethyan orogen of western Yunnan. The Early Paleozoic granitoids, located in the Baoshan, were dated at 480 ~ 486 Ma and 447 ~ 472 Ma by zircon U-Pb method. The Mesozoic-Cenozoic granitoids were dated at 83 ~ 85 Ma and 60 ~ 66 Ma. The Mesozoic-Cenozoic magmation in the Tengchong was three stages: 115 ~ 118 Ma, 67 ~ 74 Ma and 51 ~ 55 Ma. The granitoids in the Baoshan are mainly monzogranite, high-K calc-alkaline and peraluminous, with K2O/Na2 O > 1 and A/CNK > 1.1. These rocks are enriched in large-ion lithophile elements and Pb, and depleted in high field-strength elements. The Pinghe granitoids show similar chondrite-normalized REE patterns, with negative Eu anomalies(δEu = 0.25 ~ 0.48). Both the Pinghe granitoids and Nansa stock are enriched in light rare-earth elements, however, the other stocks show gentle chondrite-normalized REE patterns with strongly negative Eu anomalies. All analyses from ten samples have a wide range zircon ?Hf(t) values(-2.1 ~-12.4) and varying Hf-isotope crustal model ages(1.6 ~ 2.2 Ga). Such geochemical signatures indicate that the Early Paleozoic granitoids are S-type granite in character and were derived mainly from anatexis of ancient crustal materials with varying contributions from mantle-derived components. In the Tengchong Block, all samples dated at different ages have a wide range zircon ?Hf(t) values(-13.1 ~ 8.8) and varying Hf-isotope model ages(0.41 ~ 1.76) and were derived from anatexis of crustal materials with varying contributions from mantle-derived components.The comparability of geochronology, geochemistry and tectonics in the Tengchong Block, the Baoshan Block and the Lhasa Terrane suggest that the Tengchong Block and the Baoshan Block may represent part of an Early Paleozoic magmatic arc in the Gondwana continental margin facing the proto-Tethyan Ocean.