Search

The reduced nature of the subducting slab has been used to explain the relatively poor endowment of economic porphyry copper deposits in Paleo-Tethyan arc systems. The presence of numerous porphyry Cu deposits in the southern Yidun terrane potentially challenges this model, because these deposits are thought to be the product of Paleo-Tethyan subduction. To investigate this, two representative deposits (i.e., Pulang and Songnuo porphyry Cu deposits) from the southern Yidun terrane have been studied. Zircon U-Pb ages of pre- and syn-mineral porphyries in Pulang are 221 ± 2 Ma (MSWD = 0.89) and 215 ± 2 Ma (MSWD = 1.50), respectively. Zircon and garnet U-Pb ages of the Songnuo syn-mineral porphyry are 217 ± 2 Ma (MSWD = 1.15) and 223 ± 5 Ma (MSWD = 0.97), respectively. The Pulang and Songnuo porphyries both have high whole-rock Sr/Y ratios with minor or no negative Eu anomalies, consistent with derivation from hydrous magmas that underwent minor early plagioclase crystallization but abundant amphibole fractionation. The two porphyry intrusions are characterized by high large-ion-lithophile elements (LILE: Th, U, Ba, Rb) abundances, and low concentrations of high-field-strength elements (HFSE: Nb, Ta, Ti, Zr, Hf). They have similar initial Sr isotope ratios from 0.7055 to 0.7073, ƐNd (t) values from − 4.3 to − 1.6 and zircon ƐHf (t) values of − 1.81 ± 1.34 (n = 47) that are distinct from published results for the Permian–Early Triassic Paleo-Tethyan arc granitoids in the Sanjiang region but similar to the Neoproterozoic arc intrusions in the nearby western Yangtze craton. The Pulang and Songnuo porphyries have relatively high zircon EuN/EuN* ratios (0.59 ± 0.04, n = 52) and calculated Δ log fO2 (FMQ) values (1.61 ± 0.23, n = 52), consistent with relatively oxidized magmas. This feature is similar to the published data for the Neoproterozoic arc intrusions (EuN/EuN* = 0.55 ± 0.08; Δ log fO2 (FMQ) = 2.14 ± 0.60), but distinct from the published data for the reduced normal Paleo-Tethyan arc magmas (EuN/EuN* = 0.29 ± 0.10; Δ log fO2 (FMQ) = − 0.68 ± 0.88). Given that the most recent tectonic reconstruction models suggest that the Paleo-Tethys Ocean closed before ~ 220 Ma, we propose that the Pulang and Songnuo porphyries Cu deposits were formed in a postsubduction setting from oxidized melts generated by reactivating the Neoproterozoic arc root, unrelated to Paleo-Tethyan subduction. [ABSTRACT FROM AUTHOR]

Magmatic sources of porphyry deposits in postcollisional settings remain controversial. We have used new and published petrological and geochemical data for the Eocene–Oligocene porphyry Cu ± Mo ± Au deposits in the Sanjiang region, SW China, to address this outstanding issue. New data for three deposits (Machangqing, Tongchang, and Beiya) in the Ailaoshan–Red River porphyry Au-Cu-Mo belt (southern part of the Sanjiang region) suggest that ore-forming porphyries were emplaced at ~ 35 Ma, have high (87Sr/86Sr)i (0.7068–0.7071) and negative εNd(t) (− 6.9 to − 5.0), low zircon εHf(t) (− 5.3 to 4.5), and relatively high δ18O (5.9–9.0‰). Magmatic amphibole phenocryst compositions indicate that the parental magmas are all relatively oxidized (ΔFMQ = 1.7 ± 0.6), and H2O-rich (3.8 ± 0.3 wt% H2O). These results are consistent with those estimated from zircon compositions (ΔFMQ = 1.8 ± 0.8) and high whole-rock Sr/Y ratios (75 ± 31), respectively. Based on the new and published data, we suggest that the parental magmas for the Ailaoshan–Red River porphyry Au-Cu-Mo belt were derived from a preserved juvenile arc lower-crust and the underlying metasomatized subcontinental lithospheric mantle (SCLM) attributed to a Neoproterozoic subduction event, whereas the parental magmas for the Yulong porphyry Cu-Mo belt (northern part of the Sanjiang region) originated from the Permian–Triassic juvenile arc lower-crust and metasomatized SCLM. Additionally, parental magmas for these porphyry deposits are all oxidized and H2O-rich, and we attribute such characteristics to inheritance from mixed mantle-crust sources that were modified by previous oceanic slab subduction. [ABSTRACT FROM AUTHOR]