Eocene Post-collisional Magmatism Research In The Southern Margin Of The Gangdese Belt

Post-collisional alkaline magmatism is widespread in the global orogenic belts,and which is usually formed in an extension setting.It provides insights into the tectonic evolution and dynamic mechanism of tectonic transition from compression to extension setting in the orogenic belts.The Tibetan-Himalayan orogenic belt is the youngest and most spectacular continent-continent collisional orogenic belt in the world,formed by the subduction of the Neo-Tethyan oceanic slab and subsequent collision of the Indian Plate and the Eurasian Plate.It is the most important accretion-type orogenic event in the geological history of the earth since the Cambrian.The Lhasa Terrane is the southernmost block of the Eurasian Plate,it is the ideal area to study the tectonic evolution of The Tibetan-Himalayan orogenic belt.Mesozoic-Cenozoic magmatic rocks are widespread in the southern Lhasa subterrane,these rocks are related to the northward subduction of the Neo-Tethyan oceanic slab and subsequent the Indian continental lithosphere beneath the southern Lhasa subterrane.As a magmatic response,slab breakoff generally opens a slab window and consequently triggers partial melting of differing magma source regions to produce post-collisional magmatism（intense magmatism with compositional diversity,including basaltic magmatism showing within-plate basalt geochemistry,bimodal magmatism,and alkaline magmatism,etc.）.However,few alkaline magmatism related to the Neo-Tethyan oceanic slab breakoff have been identified.In this article,we present a combined study of field investigation,petrology,mineral chemistry,whole-rock geochemistry,zircon U–Pb ages and Sr–Nd–Hf–Mg isotopes for the Mayum alkaline complex,lamprophyre dykes and fine-grained granite veins in the southern Lhasa subterrane,in order to constrain the timing and dynamic mechanism of the Neo-Tethyan oceanic slab breakoff.The Mayun alkaline complex is composed of amphibole syenites,quartz syenite,alkali feldspar granites and mafic microgranular enclaves（MMEs）in the host quartz syenites.Their crystallization ages are 52.2±0.8 Ma,52.2±1.0 Ma,49.6±0.5 Ma and53.1±1.6 Ma,respectively,indicating that these rocks were simultaneously generated in the Early Eocene.These rocks are shoshonic with high Si O2（64.32–70.49 wt.%）,Na2O+K2O（6.63–9.43 wt.%）contents and K2O/Na2O ratios（0.89-2.14）.They are enrichment in LILEs（Rb,K）,LREEs,Th and U,and depletion in HFSEs（Nb,Ta,Ti）,HREEs,exhibiting arc-type characteristics.These rocks have homogeneous whole-rock(⁸⁷Sr/⁸⁶Sr)_iratios（0.7051–0.7056）,negativeε_Nd（t）values（–1.7 to–0.9）,and positive zirconεHf（t）values（+1.69 to+7.72）.Thus,the felsic magma was derived from the juvenile lower crust under the southern Lhasa subterrane,and went through fractional crystallization during the magma ascending emplacement.Compared with the host alkaline complex,the MMEs have lower Si O2 contents（55.16–63.67 wt.%）,very higher Fe2O3^T（4.08–6.58 wt.%）contents with variable Cr（3.45–226.00 ppm）,V（46–107 ppm）and Ni（2.66–6.16 ppm）contents.Meanwhile,the MMEs have similar patterns in trace and rare earth elements（such as obviously Eu anomalies,Eu/Eu*=0.24–0.38）and similar whole-rock Sr-Nd isotopes((⁸⁷Sr/⁸⁶Sr)_i=0.7051–0.7056,ε_Nd（t）=–2.1 to–1.4)with the host alkaline rocks.The MMEs might be derived from a mantle source,and magma mixing occurred between the MMEs and the host alkaline rocks.The lamprophyre dykes and fine-grained granite veins extended in the east-west trending in the Mayum region.The lamprophyre dykes were emplaced in the early Eocene（51.6±0.7 Ma）,they show low Si O2（52.21–53.40 wt.%）and K2O/Na2O ratios（0.21-0.32）,which are consistent to that of the MMEs in the quartz syenites.They derived from the same rich-Na mantle sources.The lamprophyre dykes have whole-rock(⁸⁷Sr/⁸⁶Sr)i ratios（0.7059）,negativeεNd（t）values（–1.2 to–1.3）,and zirconεHf（t）values（–1.64 to+1.65）,indicating that the mafic magma was derived from an lithospheric mantle source under the southern Lhasa subterrane.The fine-grained granite veins have crystallization ages of 50.0±0.8 Ma,they have high Si O2（74.63–77.36 wt.%）contents and K2O/Na2O ratios（0.97-2.50）,exhibiting similar geochemical characteristics with that of the Mayum complex,which is the high evolved products of the felsic magma.The Mg isotopic compositions of the Mayum alkaline rocks show a range of–0.22±0.02‰to–0.11±0.02‰,which is slightly heavier than the mantle(δ²⁶Mg=–0.25±0.07‰).Theirδ²⁶Mg values show correlations with Si O₂and Mg O contents.Theδ²⁶Mg values of the lamprophyre dykes and fine-grained granite veins are from–0.14±0.07‰to–0.06±0.02‰and–0.13±0.00‰to–0.11±0.02‰,respectively.The limited Mg isotope fractionation occurred during the segregation of biotite and the residue of K-feldspar.The Mayum alkaline,lamprophyre dykes and fine-grained granite veins are alkaline with high Ga/Al and Y/Nb ratios,their whole-rock geochemical characteristics show that they are A2-type granitoids,indicating that the tectonic translation from compression to extension setting already occurred in the southern margin of the Gangdese belt.The tectonic translation was related to the breakoff of the Neo-Tethyan oceanic slab in Eocene.Combined with the contemporary post-collisional alkaline magmatism,the onset timing of the Neo-Tethyan oceanic slab breakoff was constrained before～53 Ma.After the slab breakoff,the upwelling of the hot asthenosphere triggers the partial melting of both the metasomatized mantle and juvenile lower crust with the involvement of the Indian continental crust materials under the southern Lhasa subterrane and generated mafic and felsic magmas.The Mayum alkaline rocks was formed by magma mixing and fractional crystallization during ascending emplacement process.