Potassic magmatism in western Sichuan and Yunnan Provinces, SE Tibet, China: Petrological and geochemical constraints on petrogenesis

Abstract

Potassic volcanism in the western Sichuan and Yunnan Provinces, SE Tibet, forms part of an extensive magmatic province in the eastern Indo-Asian collision zone during the Paleogene (40-24 Ma). The dominant rock types are phlogopite-, clinopyroxene- and olivine-phyric calc-alkaline (shoshonitic) lamprophyres. They are relatively depleted in Na2O, Fe2O3, and Al2O 3 compared with the late Permian-early Triassic Emeishan continental flood basalts in the western part of the Yangtze craton, and have very high and variable abundances of incompatible trace elements. Primitive mantle-normalized incompatible element patterns have marked negative Nb, Ta and Ti anomalies similar to those of K-rich subduction-related magmas, although the geodynamic setting is clearly post-collisnal. Spatially, some incompatible trace element abundances, together with inferred depths of melt segregation based on the Mg-15 normalized compositions of the samples, display progressive zonation trends from SW to NE with increasing distance from the western boundary of the Yangtze craton. Systematic variations in major and trace element abundances and Sr-Nd-Pb isotope compositions appear to have petrogenetic significance. The systematic increases in incompatible trace element abundances from the western margin to the interior of the Yangtze craton can be explained by progressively decreasing extents of partial melting, whereas steady changes in some incompatible trace element ratios can be attributed to changes in the amount of subduction-derived fluid added to the lithospheric mantle of the Yangtze craton. The mantle source region of the lamprophyres is considered to be a relatively refractory phlogopite-bearing spinel peridotite, heterogeneously enriched by fluids derived from earlier phases of late Proterozoic and Palaeozoic subduction beneath the western part of the Yangtze craton. Calculations based on a non-modal batch melting model show that the degree of partial melting ranges from 0·6% to 15% and the proportion of subduction-derived fluid added from ∼0·1% to ∼0·7% (higherr-Ba fluid) or from 5% to 25% (lower-Ba fluid) from the interior to the western margin of the Yangtze craton. Some pre-existing lithospheric faults might have been reactivated in the area neighbouring the Ailao Shan-Red River (ASRR) strike-slip belt, accompanying collision-induced extrusion of the Indo-China block and left-lateral strike-slip along the ASRR shear zone. This, in turn, could have trigered decompression melting of the previously enriched mantle lithosphere, resulting in calc-alkaline lamprophyric magmatism in the western part of the Yangtze craton. © Oxford University Press, 2004; all rights reserved.