Cenozoic tectono-magmatic and metallogenic processes in the Sanjiang region, southwestern China

Porphyry deposits and oxidized magmas

The Lower Yangtze river belt is one of the most important metallogenic belts in China. The mechanisms responsible for ore genesis and the formation of related Cretaceous igneous rocks, such as adakite, A-type granitoid, and Nb-enriched basalt, remain controversial. Mesozoic granitoids in the Lower Yangtze river belt were mostly formed in the Early Cretaceous (140–125 Ma), and three granitoid belts—the inner, the south, and the north—have been defined according to petrological and geochemical characteristics. Previously, based mainly on negative eNd and high initial Sr isotope values, the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust, both of which require crustal thickening. Mesozoic crustal thickening, however, is not supported by the development of extensional basins in the region. From the Late Jurassic to Cretaceous, eastern China was closely associated with subduction of the Pacific plate in the south and the Izanagi plate in the north. The midocean ridge (MOR) between these two plates was drifting toward and likely subducting under the Lower Yangtze river belt. A ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt. Partial melting of subducting young, hot oceanic slabs close to the ridge formed adakitic rocks. The negative eNd values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle, and/or crustal materials through AFC process. A slab window opened during ridge subduction as indicated by A-type granitoids in the center of the inner belt. Nb-enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths.

Cretaceous ridge subduction along the lower yangtze river belt, eastern china

Determination of fluid/melt partition coefficients by LA-ICPMS analysis of co-existing fluid and silicate melt inclusions: Controls on element partitioning

The link between reduced porphyry copper deposits and oxidized magmas

Yulong ore-bearing porphyries, along the northwestern extension of the Red River–Ailao Shan fault system in eastern Tibet, consist of five porphyry deposits, containing a total of more than 8 million tons of copper resources. U–Th–Pb laser inductively coupled plasma mass spectrometry dating of zircon shows that the porphyries were emplaced in Early Tertiary (41.2–36.9 Ma), covering a period of ∼4.3 Ma, with formation ages decreasing systematically from northwest to southeast. The start of porphyry magmatism coincided with the onset of transpressional movement along the Red River–Ailao Shan fault system, implying a close link between these two events. Age sequence in intrusions can be plausibly explained by assuming that a region of melting in the lower northwestern plate moved southeasternward along the Tuoba–Mangkang fault relative to the upper plate. Zircon grains from the Yulong ore-bearing porphyries have higher Ce4+/Ce3+ than those from barren porphyries in the region. This suggests that the ore-bearing porphyries crystallized from a relatively oxidized magma, which has important implications for future ore exploration in the region and other Cu deposits in convergent margin environments in general.

Zircon Ce4+/Ce3+ ratios and ages for Yulong ore-bearing porphyries in eastern Tibet

The potassic alkaline igneous rocks along the Ailao Shan–Red River (ASRR) shear zone range in composition from ultramafic to felsic and are characterized by similar trace element patterns and radiogenic isotope ratios. Their distribution, and in some cases their structure and outcrop outline, suggest a genetic relationship to the ASRR shear zone. Zircons separated from 12 potassic alkaline bodies along the ASRR zone have been dated by an in situ U‐Pb method to temporally constrain the left‐lateral movements along the shear zone. The ages of the alkaline bodies range from 34.0 to 36.3 Ma. These new zircon ages, together with previous data, suggest that the onset age of the left‐lateral movements along the ASRR shear zone began at or slightly before 36 Ma and that the movements lasted from ≥36 to 17 Ma. When uncertainties are taken into account, the duration of motion on the ASRR shear zone coincides with the opening of the South China Sea, implying a genetic relationship between these two events.

https://www.jstor.org/stable/10.1086/527459

The age of the potassic alkaline igneous rocks along the Ailao Shan-Red River shear zone: Implications for the onset age of left-lateral shearing

The Yulong copper belt, along part of the Red River-Ailao Shan fault system and its northwestern extension in eastern Tibet, consists of five porphyry pipes that contain a total copper resource of over 8 million tons. The porphyries are characterized by high alkali content (K2O+Na2O > 6%), K2O/Na2O > 1, and marked negative Ti, Ta and Nb anomalies on mantle-normalized incompatible element diagrams. U-Th-Pb laser ICP-MS dating of zircons from the Yulong porphyries showed that they were emplaced over a 4.3 Ma period and that they Young systematically from northwest to southeast as follows: Yulong, 41.2 +/- 0.2Ma; Zalaga, 38.5 +/- 0.2Ma; Mangzong, 37.6 +/- 0.2 Ma; Duoxiasongduo, 37.5 +/- 0.2Ma; and Malasongduo, 36.9 +/- 0.4 Ma. We suggest that the source of the shoshonites was lower crust that was pushed into the mantle by the compressive component of transpressional movement on the adjacent Tuoba-Mangkang fault and that the compositional variation in the porphyries is due to mixing between magmas of different composition, generated by different degrees of partial melting of a heterogeneous source region.

Heng-Xiang Yu

Papers

Zircon Ce4+/Ce3+ ratios and ages for Yulong ore-bearing porphyries in eastern Tibet

The age of the potassic alkaline igneous rocks along the Ailao Shan-Red River shear zone: Implications for the onset age of left-lateral shearing

Geochronological and geochemical study on the Yulong porphyry copper ore belt in eastern Tibet, China