Cenozoic Magmatism In The Ailaoshan Tectonic Belt,Sanjiang Region,SW China

The Ailaoshan tectonic belt situated along the southeastern margin of Tibet plateau,has experienced complicated tectonic evolution.The Eocene lamprophyres are characterized by enrichment in large ion lithophile elements and light rare earth elements,depletion in high field strength elements,high(87Sr/86Sr)i and negative ?Nd(t).These arc-like geochemical signatures point to an enriched lithospheric mantle source The low range of Nb/U,elevated ratios of Ba/La,and limited values of Th/Yb suggest this enriched mantle source was metasomatized by subducted slab-derived fluids.Moreover,the lamprophyres exhibit light Mg isotopic compositions,low Ti/Ti*and Fe/Mn,and high Na2O+K2O,further indicating the mantle was metasomatized by carbonate-rich melts.The Mg-Sr isotope binary mixing modeling highlights the metasomatism of clastic sediments.Combined with previous studies,the diverse metasomatic agents may be related to the Neoproterozoic and Permian oceanic subduction.The Early Oligocene granitoids with zircon U-Pb ages of 32?31 Ma have similar Sr-Nd-Hf isotopic compositions,falling along the same evolution lines with the Neoproterozoic meta-volcanic rock and the Neoproterozoic OIB-like amphibolites,which indicates the existence of OIB-like igneous rocks in the source.Furthermore,there are a lot of inherited zircons with different ages and sources,including:(1)the 800?700 Ma inherited zircons with a provenance of Neoproterozoic sedimentary rocks;(2)300?200 Ma ones with a derivation from the Paleo-Tethyan arc and the Triassic granites;(3)100?40 Ma ones with a source of Cretaceous intrusions.These inherited zircons indicate that a melange was developed above the Neoproterozoic OIB-like igneous rocks in the crust.The formation of such a complicated crustal structure is related to regional thrusting.The wall-rocks of the Early Oligocene granitoids show biotite 40Ar/39Ar total age of 46 Ma,which could represent the time of the regional thrusting.The sheared granitoid show muscovite and biotite 40Ar/39Ar ages of 27 Ma,and the undeformed granitoid have biotite 40Ar/39Ar age of 24 Ma.These ages are similar to the zircon U-Pb ages of 26?23 Ma obtained from the Late Oligocene leucosomes,representing the time of the anataxis,metamorphism and crustal uplift related to regional shearing.Based on the discussion above,the tectonic evolution of the Ailaoshan tectonic belt is investigated:(1)An OIB-like igneous rock was attached between the South China block and Indochina block,as a result of the closure of Paleo-Tethyan;(2)At 46 Ma,the OIB-like igneous rock and rocks with different ages were thrusted underneath the Ailaoshan belt during the continental collision,forming the complicated crustal structure,later at ca.32?31 Ma,it was melt to generate the granitoids;(3)At 31?23 Ma,the belt has sufferred crustal shearing,leading to a series of crustal extension,metamorphism,uplift and anataxis,which is favorable to the formation of the orogenic gold deposits.