Petrogenesis of the Langdu High–K Calc–Alkaline Intrusions in Yunnan Province: Constraints from Geochemistry and Sr‐Nd Isotopes

Abstract:The Langdu high–K calc–alkaline intrusions are located in the Zhongdian area, which is the southern part of the Yidun island arc. These intrusive rocks consist mainly of monzonite porphyry, granodiorite, and diorite porphyry. The K2O content of majority of these rocks is greater than 3%, and, in the K2O‐SiO2 diagram, all the samples fall into the high‐K calc‐alkaline to shoshonitic fields. They are enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs; LaN/YbN= 14.3–21.2), and show slightly negative Eu anomalies (δEu = 0.77–1.00). These rocks have high K, Rb, Sr, and Ba contents; moderate to high enrichment of compatible elements (Cr = 36.7–79.9 ppm, Co = 9.6–16.4 ppm, and MgO = 2.2%‐3.4%); low Nb, Ta, and Ti contents, and characteristic of low high field strength elements(HFSEs) versus incompatible elements ratios (Nb/Th = 0.75, Nb/La = 0.34) and incompatible elements ratios (Nb/U = 3.0 and Ce/Pb = 5.1, Ba/Rb = 12.0). These rocks exhibit restricted Sr and Nd isotopic compositions, with (87Sr/86Sr)i values ranging from 0.7044 to 0.7069 and εNd(t) values from −2.8 to −2.2. The Sr‐Nd isotope systematic and specific trace element ratios suggest that Langdu high‐K calc‐alkaline intrusive rocks derived from a metasomatized mantle source. The unique geochemical feature of intrusive rocks can be modeled successfully using different members of a slightly enriched mantle, a slab–derived fluid, and terrigenous sediments. It can be inferred that the degree of partial melting and the presence of specific components are temporally related to the tectonic evolution of the Zhongdian island arc. Formation of these rocks can be explained by the various degrees of melting within an ascending region of the slightly enriched mantle, triggered by the subduction of the Garzê–Litang ocean, and an interaction between the slab–derived fluid and the terrigenous sediments.