矿床地球化学国家重点实验室知识库

The Paleoproterozoic Hekou Group in the western Yangtze Block is a volcano-sedimentary succession that is intruded by gabbroic plutons. Sedimentary rocks in the group include slates, marble and meta-siltstones interlayered with felsic metavolcanic rocks, metabasalts and metatuffs. Both the volcanic rocks of the Hekou Group and gabbros have undergone upper greenschist to lower amphibolite facies metamorphism. Metatuff samples from different layers have average zircon U-Pb ages of ∼1697 Ma, slightly older than the intruding gabbroic plutons with zircon U-Pb ages of ∼1684 Ma. Both metabasalts and metagabbros have similar elemental and isotopic compositions, indicative of a co-magmatic origin. They are rich in TiO₂ (mostly >2.5 wt.%), Zr (94.5–347 ppm), Ta (0.48–3.00 ppm) and Th (1.05–7.61 ppm) with high Nb/Y ratios (mostly >0.6) and LREE-enriched chondrite-normalized REE patterns, resembling within-plate mafic rocks. Their positive whole-rock ɛNd(t) (0.2 to +3.4) and zircon ɛHf(t) values (−3.3 to +8.4) suggest contributions from a depleted mantle source. The large range of ɛNd(t) and ɛHf(t) values, and variable degrees of Nb-Ta anomalies (Ta/La_PN = 0.17–1.75) are indicative of crust contamination during magma ascending. Both the metabasalts and metagabbros are considered to have formed in a continental rift setting.

Detrital zircon grains from meta-siltstones in the Hekou Group have U-Pb age populations mainly at 2070–1880 Ma, 2330–2250 Ma and 2900–2700 Ma. These age populations are comparable to those of the North Australian and North China Cratons in the Columbia supercontinent. However, in terms of geochemical features, the ∼1.7 Ga within-plate mafic rocks at Hekou are similar to those from the ∼1.7 Ga Leichhardt and Calvert Superbasins of the North Australian Craton, but different from those from the 1.7–1.2 Ga Zhaertai-Bayan Obo rift zone of the North China Craton. It is thus suggested that the Yangtze Block was more likely linked with the North Australian Craton in Columbia during the Late Paleoproterozoic. These rifting basins and mafic rocks may record the initial break-up of the Columbia supercontinent.