The origin of the mafic microgranular enclaves from Jiama porphyry Cu polymetallic deposit, Tibet: Implications for magma mixing/mingling and mineralization

The research on magma mixing/mingling is instructive to unravel the interaction of crust-mantle, and discuss the geodynamic setting of magma and ore-forming process. The Jiama deposit, located in eastern part of the well-known Gangdese Metallogenic Belt on the Tibetan Plateau, is the largest porphyry-skarn Cu polymetallic system in this region. We studied the dioritic mafic microgranular enclaves (MMEs) in the host felsic porphyries from Jiama, at the aim of ascertaining the origin of rocks, improving the magmatic rock diagenetic model, and providing implication for magmatic mixing and mineralization. Petrographic observation shows that there are many typical textures in the dioritic MMEs and the host porphyries whose origin can be explained in terms of magmatic mixing and mingling, such as resorption of feldspar-quartz, quartz edging texture, feldspar sieve structure, feldspar reverse zoned, and acicular apatite morphology. The results of zircon LA-ICP-MS U-Pb isotopic dating show that the age (15. 3 +/- 0. 3Ma) of MMEs is consistent with those of the felsic host porphyries within the error range, further indicating the existence of magma mixing. The dioritic MMEs are similar in chemical composition to high-Mg diorite. These MMEs are characterized by low content of SiO2 (52. 44% similar to 59. 45%), high contents of K2O (3. 19% similar to 5. 62%), Mg0 (3. 53% similar to 6. 62%) and compatible trace elements (e. g., Ni : 86 x 10(-6) similar to 146 x 10(-6); Cr: 102 x 10(-6) similar to 228 x 10(-6)), as well as by high Sr/Y and La/Yb ratios. As for the characteristics of REE and other trace elements, the MMEs have higher Sigma REE values than those in host felsic porphyries with ratios of (LREE/HREE)(N) = 21 similar to 23, and they are enriched in LILE (Rb = 189 x 10(-6) similar to 284 x 10(-6), Sr = 498 x 10(-6) similar to 658 x 10(-6), Ba = 1247 x 10(-6) similar to 1378 x 10(-6)), while relatively depleted in HFSE (Nb, Ta and Ti). On chondrite-normalized REE and primitive mantle -normalized multielement plots, data for the dioritic MMEs fall in between domains for the ultrapotassic mafic rocks and the felsic host porphyries, which are generated by partial melting of metasomatized continental lithospheric mantle and subduction-modified juvenile lower crust, respectively. The Hf isotope data (epsilon(Hf)(t) = -0. 9 similar to 4. 6) also fall in between the fields for the ultrapotassic mafic rocks and the granodiorite porphyries (representing the host felsic porphyries). These features demonstrate that the dioritic MMEs were formed by the mixing between ultrapotassic and adakite-like melts, derived from metasomatized Tibetan lithospheric mantle and juvenile lower crust, respectively, and also indicate that the removal of the lower part of the thickened lithosphere also took place in the eastern Gandese belt. This also allows us to propose the varied contribution of ultrapotassic mafic melts derived from metasomatized Tibetan lithospheric mantle in the generation of high -potassium adakitic rocks which are widely distributed in the southern Lhasa terranes. In addition, the incorporation of ultrapotassic mafic rocks will add a large amount of water and metal materials to the felsic magmatic system, and this is the fundamental factor which controlled the formation of the Jiama superlarge porphyry-skarn type deposit.