Search

The tectono-magmatic history of the Lüchun-Jinping microblock and its possible affinity with the Yangtze Craton are important elements for the reconstruction of Cenozoic plate tectonics in southeastern Tibet. In order to constrain the affinity and decipher the pre-Cenozoic paleopositon of the Lüchun-Jinping microblock, we focused on the petrogenesis of Eocene-Oligocene lamprophyres in the Lüchun-Jinping microblock. The lamprophyres yield zircon U Pb ages of 34.7–33.3 Ma and exhibit potassic-ultrapotassic features with elevated K 2 O/Na 2 O (1.4–4.0) ratios. They are characterized by high concentrations of compatible elements (e.g., Cr = 187–692 ppm, Ni = 31–218 ppm), large-ion-lithophile elements and light rare-earth elements enrichment, high-field-strength elements depletion, and high radiogenic isotopic values, i.e. (87Sr/86Sr) i = 0.7063–0.7078 and εNd(t) = −3.9 to −2.4. Combined with the low Nb/U ratios, these features suggest that the lithospheric mantle source was metasomatized by subduction-related fluids beneath the Lüchun-Jinping microblock. The relatively high Rb/Sr ratios and high heavy rare-earth element contents indicate that these lamprophyres were derived from partial melting of a phlogopite-bearing lherzolite within the spinel stability field. The parental magmas have experienced fractional crystallization of olivine and clinopyroxene during emplacement. Comprehensive comparisons between the lamprophyres from the Lüchun-Jinping microblock and the potassic-ultrapotassic mafic rocks from the western Yangtze Craton indicate that the Lüchun-Jinping microblock can be regarded as a dismembered part of the western Yangtze Craton due to continental extrusion and Cenozoic sinistral displacement. The compositional trends of the potassic-ultrapotassic mafic rocks suggest that the palaeogeographic position of the Lüchun-Jinping microblock was near the Dali area (west of the Binchuan) and close to the Jinshajiang suture zone before the Cenozoic. [Display omitted] • Mafic lamprophyre dykes from the Lüchun-Jinping microblock were emplaced at 35–33 Ma. • Geochemical compositions of mafic lamprophyres reflect an enriched SCLM domain. • The enriched SCLM domain was metasomatized by Proterozoic subduction-related fluids. • The Lüchun-Jinping microblock is a displaced part of western Yangtze Craton. • The pre-Late Eocene position of this microblock was near Dali and closer to the JSSZ. [ABSTRACT FROM AUTHOR]

A suite of lamprophyres, spatially associated with mafic lavas and potassic felsic intrusive rocks, was emplaced between 36.5 ± 0.2 and 33.7 ± 0.5 Ma (based on phlogopite 40 Ar/ 39 Ar dating) on the eastern side of the Ailao Shan-Red River shear zone in the western Yangtze Craton. These shoshonitic and ultrapotassic intrusive rocks post-date the ~ 60–55 Ma collisional event between the Indian and the Asian continents. They are characterized by: (1) enrichment in large-ion lithophile elements and light rare-earth elements with (La/Sm) n = 3.15–7.15; (2) strong positive Pb spikes; (3) depletion in high-field-strength elements (e.g. Nb/La = 0.08–0.98); (4) high initial 87 Sr/ 86 Sr (0.706–0.709) with negative εNd(t) values of − 10.5 to − 0.9; (5) old Nd model ages of 1542–945 Ma; and (6) radiogenic ( 207 Pb/ 204 Pb) i of 15.57–15.70 and ( 208 Pb/ 204 Pb) i (38.70–39.06). These features suggest that the mantle source was metasomatized by Proterozoic subduction beneath the Yangtze Craton. The lamprophyres have similar trace element patterns, and Sr–Nd–Pb isotope compositions, as coeval mafic lava, indicating a common source of metasomatized veined continental lithospheric mantle (CLM). Lower degree partial melting of metasomatic veins likely generated the lamprophyres, whereas the coeval mafic lava was likely derived from melting of phlogopite harzburgite. The lamprophyres and mafic lava have similar Sr–Nd isotope systematics as CLM-derived Neoproterozoic mafic rocks and Late Permian Emeishan low-Ti basalt in the region, indicating that they share the same Proterozoic source. We envisage that mantle plumes thermally eroded the Proterozoic metasomatized CLM beneath the western part of the Yangtze Craton during 825–750 Ma and 260–250 Ma, although residual metasomatized domains remained before being tapped by delamination after the India–Asia continental collision during the Paleogene period. [ABSTRACT FROM AUTHOR]