Origin of the Miocene porphyries and their mafic microgranular enclaves from Dabu porphyry Cu-Mo deposit, southern Tibet: implications for magma mixing/mingling and mineralization

• Published in: International geology review Vol. 56; no. 5; pp. 571 - 595
• Main Authors: Wu, Song, Zheng, You-Ye, Sun, Xiang, Liu, Sheng-Ao, Geng, Rui-Rui, You, Zhi-Ming, Ouyang, Hai-Tao, Lei, Dong, Zhao, Zhong-Ying
• Format: Journal Article
• Language: English
• Published: Philadelphia Taylor & Francis 04.04.2014; Taylor & Francis Ltd
• Subjects: Dabu; lower crustal arc cumulates; mafic microgranular enclaves (MMEs); magma mixing/mingling; porphyry deposit; Tibet
• ISSN: 0020-6814; 1938-2839
• DOI: 10.1080/00206814.2014.880074

The origin of magmas that are linked to economic mineralization in porphyry deposits formed in continental collisional belts is controversial. In this paper, we studied the mafic microgranular enclaves (MMEs) and their host monzogranite porphyries (HMPs) from the Dabu porphyry Cu-Mo deposit in southern Tibet. Zircon SHRIMP U-Pb ages indicate coeval formation for the MMEs and HMPs in middle Miocene time (~15 Ma). The MMEs have high Mg# (50.7-60.8), low SiO 2 (53.2-62.5 wt.%), and high Cr (95-175 ppm) contents, with positive εHf(t) values ranging from +3.4 to +9.4. These results, along with the presence of phlogopite, suggest that the MMEs were most likely generated by partial melting of a metasomatic lithospheric mantle source region. The HMPs have high Sr/Y (88.2-135.7), La/Yb (25.0-31.9) ratios, and moderate Mg# (46.2-49.3) values. They have the same εHf(t) values (+3.3 to +7.7) with arc-like Palaeogene rocks. The HMPs also show typical arc magma characteristics such as enrichment in LILEs (e.g. Rb, Ba, Sr, and K) and depletion in HFSEs (e.g. Nb, Ta, Ti, Zr, and P). These results suggest a possible origin involving high-pressure remelting of thickened lower crustal arc cumulates related to earlier Neo-Tethyan subduction. The lower crustal arc cumulates dominated by garnet-bearing amphibolite facies could be the potential copper sources of the Dabu porphyry Cu-Mo deposit. Underplating of the mantle-derived mafic magmas could have provided heat input for melting of the hydrous lower crust. Reaction between the mafic and felsic magmas might have further increased Cu concentrations and contributed to subsequent mineralization.