Geochemical characteristics and petrogenesis of Miocene high Sr/Y rocks in Xigatze fore-arc basin, southern Tibet

The Xigatze fore-arc basin, one of the well-preserved fore-arc basins worldwide, consists of interesting nearly E-W trending dike swarms of basaltic to granitic compositions. Mid-Miocene more felsic dikes to the responses of fore-arc lithosphere provide important records to the post-collisional processes. Miocene (~15–11 Ma, zircon UPb ages) high Sr/Y magmatic rocks from the Xigatze area in this study are characterized by (1) rich in Na as compared with K contents (Na2O/K2O = 1.75–5.62) and high Al2O3 content (> 16 wt%); (2) high Sr but low Y contents, thus high Sr/Y (>81.0) ratios, resembling those of adakites; (3) enrichment in LREE and depletion in HREE with weakly negative or no Eu anomalies (Eu/Eu⁎ = 0.80–1.02); (4) small range in 87Sr/86Sr(t) (0.705080–0.706031) and εNd(t) (−3.1 − +0.4) values, indicating that the Xigatze Miocene magmatic rocks are derived from partial melting of juvenile lower crust with minor contributions from the ancient crust in the Lhasa terrane; (5) thickened crust with a crustal thickness of ~52–69 km estimated from (La/Yb)N values. As compared with those in the Gangdese high Sr/Y suite, the Xigatze ones have lower SiO2 and K2O contents, but higher Na2O/K2O, Sr/Y, and K/Rb ratios, the higher Sr/Y and K/Rb ratios implying a weaker effect by amphibole either as a residual in the source or as a fractionation phase during the magma ascent than that in the Gangdese suite. •The dioritic to granitic dikes in Xigatze fore-arc basin are formed at ~15–11 Ma.•These Miocene dikes show high Sr/Y and La/Yb ratios with adakitic affinity.•They are derived from partial melting of juvenile mafic lower crust.•The southern Tibet experienced crustal thinning from ~19–17 Ma to ~15–11 Ma.