U-Pb dating of the detrital zircons in the dark diorite enclaves from the Lincang granites in Mengku, Shuangjiang, western Yunnan and its geological significance

The Lincang granites in Mengku, Shuangjiang, western Yunnan consist, lithologically, of biotite monzogranite, biotite granodiorite and alkali feldspar granite. The dark mafic microgranular diorite enclaves (MME) were discovered in biotite monzogranite near Reshuitang, Mengku during our field geological survey. The present paper focuses on petrographic observation, LA-ICP-MS zircon U-Pb dating and whole-rock major elemental analysis of the dark diorite enclaves. The LA-ICP-MS zircon U-Pb age of 230.9 ±1.2 Ma for the dark diorite enclaves is roughly consistent with that of 229.2 ±0.8 Ma for the medium-to fine-grained biotite monzogranite. Geochemically, the biotite monzogranite is characterized by the enrichment of K₂O and Na₂O contents (Na₂O > K₂O) and Al₂O₃, and an average A/CNK ratio of 1.17, while the dark diorite has moderate contents of K₂O and Na₂O (Na₂O > K₂O), enrichment of Al₂O₃ and MgO, and an average A/CNK ratio of 0.77. Both the biotite monzogranite and dark diorite display the right-leaning LREE distribution patterns. They are all enriched in large ion lithophile elements (LILE) and depleted in the high field strength elements (HFSE) Na and Ta, and show relatively higher Mg^# values (30.50 to 61.41 for biotite monzogranite and 58.58 to 67.34 for dark diorite, respectively). The Cr and Ni contents in biotite monzogranite range in 46.96×10^-6 and 11.21×10^-6, respectively, which are lower than those in dark diorite (an average ranging in 197.62×10^-6 and 75.68×10^-6, respectively). The formation of the granitic magmas in the study area may well be closely related with the partial crustal melting caused by mantle fluid action and crust-mantle magmatic mixing in the continental-continental collision to post-collisional tectonic settings, implying that the Baoshan and Simao blocks were in the continental-continental collision to post-collisional periods at the age of 230 Ma.