Age and geochemistry of the Cuona leucogranite in southern Tibet and its geological implications

The Cuona leucogranite pluton is situated in the east of Himalayan orogen. LA-MC-ICP-MS zircon U-Pb dating re-veals that leucogranites were crystallized at 17.7±0.3Ma, representing the Miocene crustal anataxis. Geochemical studies show that the samples are characterized by high SiO₂(74.46%~75.57%), Al₂O₃(14.07%~14.64%), K₂O(4.19%~4.85%), K₂O/Na₂O ratios(1.09~1.31) and A/CNK values(1.15~1.25), enrichment of Rb, Th, U and depletion of Ba, Nb, Zr, Sr, and high ratios of Rb/Sr(17.75~29.50) with strong negative Eu anomalies(δEu=0.18~0.26). These features suggest that they are crust-derived high potassium calcalkaline and peraluminous S-type granite. The relatively high I_sr(0.78982~0.79276) and low ε_Nd(t)(-19.5~-18.2) are well comparable with data of the metapelite from Greater Himalayan Crystalline complex(GHC), indicating that the leucogranites were generated from their partial melting. The features of high I_sr and low Sr concentration as well as the decreasing Rb/Sr values with increasing Ba concentration demonstrate that the Cuona leucogranites were derived from muscovite dehydration melting under the water-absent condition, possibly triggered by structural decompression responding to the activity of South Tibetan Detachment system(STDS). It is held that the Cuona leucogranites reflect dynamics of structural decompression, dehydration melting and emplacement of the melt along STDS under the background of crustal extension and thinning.