Cenozoic mantle composition evolution of southern Tibet indicated by Paleocene (~ 64 Ma) pseudoleucite phonolitic rocks in central Lhasa terrane

Abstract

The question of whether continental subduction processes in collisional orogenic belts can trigger wide-spread mantle metesomatism and crustal material recycling remains unresolved. Miocene (25–8Ma) ultrapotassic rocks in southern Tibet are the only mantle-derived magmatic rocks emplaced after the collision between India and Asia and they have been linked to the onset of east-west extensional stresses as the surface uplift of the Tibetan Plateau reached near-maximum elevation. However, their petrogenesis remains highly controversial, particularly the issue of whether their extremely enriched Sr-Nd isotopic characteristics were related to metasomatism derived from subducted Indian continental materials during the Cenozoic. Here we report on a Paleocene silicate-unsaturated, pseudoleucite phonolitic dike, in the Rongniduo area of central Lhasa terrane. In-situ SIMS (secondary ion mass spectrometry) apatite U-Pb age indicates the dike was generated at 64.1±4.2Ma, which slightly predates the age of initial India and Asia collision (about 55–50Ma). This is the oldest age yet reported for ultrapotassic rocks in southern Tibet. Samples from this dike have distinctly more depleted Sr-Nd (whole rock: (87Sr/86Sr)i=0.7064 to 0.7062, εNd(t)=−1.5 to 0.4; in situ apitite: (87Sr/86Sr)i=0.7059 to 0.7060, εNd(t)=−2.0 to 0.4) isotopic compositions, than those of Miocene (25–8Ma) ultrapotassic rocks in the central Lhasa terrane ((87Sr/86Sr)i=0.7106 to 0.7399, εNd(t)=−10.6 to −18.5). Our new data provides important constraints on pre-collisional mantle characteristics beneath the Lhasa terrane. We suggest that these ~64Ma pseudoleucite phonolitic rocks were derived from the enriched lithospheric mantle metasomatized by subducted Tethyan oceanic materials in response to Neo-Tethyan slab roll-back. As a consequence, the younger Miocene ultrapotassic rocks, which display different geochemical compositions from the pre-collisional ultrapotassic rocks, were most probably derived from a mantle source metasomatized by subducted Indian continental materials after ~64Ma. Our results indicate that the addition of subducted continental components plays an important role in changing mantle constituents beneath collisional orogenic belts.