矿床地球化学国家重点实验室知识库

The latest Devonian–early Carboniferous granitic intrusions in the Central Tianshan block are composed mainly of monzogranites and granodiorites. Here we present the petrology, geochemistry, and in situ zircon U–Pb ages and Hf isotopes of these intrusions. Bulk geochemistry suggests that the monzogranites and granodiorites are high-K, calc-alkaline, I-type granites. LA-ICP-MS zircon dating shows that the monzogranites and granodiorites formed at ca. 362 Ma and ca. 354 Ma, respectively. They are characterized by relatively high initial 176Hf/177Hf ratios (0.282571–0.282764) and positive εHf(t) values (+2.1 to +7.2). We interpret them to have been derived from partial melting of the Mesoproterozoic metamorphic basement of the Tianshan block and a significant addition of juvenile material. Compared to the monzogranites, the granodiorites are characterized by higher Sr (373–599 ppm), low Y (12.5–20.5 ppm), and Yb (1.21–2.04 ppm) contents, with relatively higher Sr/Y (26–32) ratios, analogous to those of modern adakitic rocks. The differences in geochemical characteristics between the monzogranites and granodiorites may reflect differences in the P–T conditions experienced by the two lithologies during partial melting. We propose that the latest Devonian monzogranites were possibly generated by partial melting of the Tianshan Mesoproterozoic basement rock with an influx of juvenile material in an arc setting. However, the granodiorites were likely related to the slab roll-back of subducted north Tianshan Ocean during the early Carboniferous.