Zircon U-Pb age and geochemical characteristics of the Azha intrusion in the southern Lhasa Block, Tibet and their indications for the evolutionary history of the Neo-Tethys

A large number of magmatic rocks are exposed in the southern margin of the Lhasa Terrane, which provides great objects for studying the evolution history of the northward subduction of the Neo-Tethys along the southern margin of the Lhasa Terrane.In order to study mechanism of subduction evolution of Neo-Tethys, this paper conducts systematic petrological, geochronological and geochemical analyses of the Azha intrusions in the Shannan area of Tibet.Results show that the Azha intrusive rocks are mainly composed of quartz monzodiorite, formed during Late Cretaceous(95.0±1.4 Ma, 100.2±1.4 Ma).The samples are characterized by high silicon and sodium(Na₂O>K₂O), belonging to high-K calc-alkaline rocks.They have high Sr(588.47×10^-6, 649.65×10^-6), low Y(8.66×10^-6, 9.03×10^-6)and Yb(0.87×10^-6, 0.92×10^-6)and high Sr/Y ratio(65.17, 75.02), showing the geochemical characteristics of typical adakitic rocks.Their large-ion lithophile elements such as Rb and Sr are enriched, and their high field strength elements such as Nb, Ta and Ti are depleted, along with the enrichment of LREE and the positive anomaly of Eu.The samples show low(⁸⁷Sr/⁸⁶Sr)_i(0.703825, 0.703836), high ¹⁴³Nd/¹⁴⁴Nd(0.512790, 0.512798)and positive values of ε_Nd(t)(4.08 and 4.25).Combined with previous results, it is suggested that the Azha instrusion is the product of partial melting of oceanic crust due to ridge subduction of the Neo-Tethys.