Cretaceous Magmatism In Southern Qiangtang Block, Central Tibet:Tectonic Implications

Tectonic-magmatic evolution patterns in central Tibet play important roles in understanding geodynamic evolution of the Tibetan Plateau.In this study,we focused on the Cretaceous magmatism of the southern Qiangtang block with relatively low degree of research in the central Tibet.Combining with detailed fieldwork,petrography,Zircon U-Pb geochronology,whole rock geochemistry and Sr-Nd-Pb isotope,we attempt to reveal the relationship between the Cretaceous magmatism and the evolution Bangong-Nujiang Tethys.The main achievements of this paper are as follows:Base on the isotopo geochronology of magmatism in southern Qiangtang block,combined with the existing data,we established a time series of Cretaceous magmatism and divided the Cretaceous magmatism of the southern Qiangtang block into four period:145-125 Ma,125-100 Ma,100-87 Ma,87-73 Ma.The first period is characterized by a striking 145-125 Ma magmatic lull in southern Qiangtang block.The disappearance of the asthenospheric wedge as a result of flat slab subduction of the Bangong-Nujiang Tethys,leading to magmatic lull in the southern Qiangtang block.The second period is represented by Early Cretaceous?113.9±3.8 Ma?volcanic rocks from Qushenla Formation in the Duoma village,composed of andesite,dacite and rhyolite,and are charaterized by subalkaline and medium-K calc-alkaline,enriched with ligh rare earth elements,and(⁸⁷Sr/⁸⁶Sr)_i=0.70466-0.70537,?Nd?t?of 1.71-2.89.The original magma of the Qushenla Formation volcanic rocks is derived from the mixing of the mantle melt and the crust melt,which may be the result of the slab roll-back of the northern subducted of the Bangong-Nujiang Tethys.The thrid period is represented by Early Late Cretaceous?95.4±0.9 Ma and 94.4±0.8 Ma?andesites in the Biluoco area,characterized by high MgO contents and positive?Nd?t?and low(⁸⁷Sr/⁸⁶Sr)_i.The magma source iscomposed of the subduction plate melt,sediment and mantle wedge peridotite,which is related to the slab break-off of the northern subducted of the Bangong-Nujiang Tethys.The fourth period is represented by Late Cretaceous volcanic rocks from Abushan Formation?74.4±0.9 Ma and 75.7±0.9 Ma?in the Duoma village and Chuburi pluton?72.9±1.4 Ma and 73.7±1.5 Ma?in the Amdo area.Abushan Formation are highly fractionated I-type magmatic rocks,with high potassium calc-alkaline,and obvious negative Eu anomalies,(⁸⁷Sr/⁸⁶Sr)_i=0.70453-0.70491,?Nd?t?=1.63-2.35.Chuburi pluton are highly fractionated I-type granite with obvious negative Eu anomaly.(⁸⁷Sr/⁸⁶Sr)_i ranging from 0.7064 to 0.7089,with?Nd?t?from-1.94 to-5.59.The geochemical characteristics of the Abushan Formation volcanic rocks and Chuburi pluton are similar.The original magma was generated through various degrees of mixing between mantle-derived magma and lower crustal melts.The generation of the volcanic rocks was associated with lithospheric delamination beneath the Lhasa-Qiangtang collision zone.According to the study of multi-stage magmatism in the southern Qiangtang block,combined with sedimentary stratigraphy,paleomagnetic and tectonic deformation,we suggest that 145-125 Ma magmatism lulls have been attributed to flat slab subduction of the Bangong-Nujiang Tethys;125-100 Ma magmatism was generated by slab rollback of the Bangong-Nujiang Tethys;100-87 Ma magmatism was a result of slab break-off;and87-73 Ma magmatism was related to the lithospheric delamation in Lhasa-Qiangtang collision zone.