Petrological And Geochemical Evidence For Early Permian Intra-oceanic Subduction In The Palaeo-Tethys Ocean, Sanjiang Area, SW China

The suthern Langcangjiang tectonomagmatic belt is located in the central part ofthe Nujiang-Lancangjiang-Jinshajiang tectonomagmatic belt, which is the one of themost important tectonic-magmatic-metallogenic belt in China. The mafic-ultramaficintrusions preserved in the area play key roles to understand the tectonic processesinvolved in the assembling of SW China. We took drill core and surface samples fromthe Banpo and Nanlinshan mafic-ultramafic intrusions in the southern Langcangjiangarea, and carried out studies including ID-TIMS zircon U-Pb dating, petrology andmineral chemistry, whole rock compositions and trace elements in clinopyroxene, andSm-Nd isotopes of selected samples. New data allow us to propose a newtectonomagmatic model that is different from any of the previous ones.Precise ID-TIMS zircon U-Pb dating indicates that the Banpo and Nanlinshanintrusions have similar ages of~295Ma and298Ma, respectively, confirming thatthey are the oldest igneous rocks in the Lincang-Jinhong igneous belt. The rocks fromthe Banpo and Nanlinshan mafic-ultramafic instrusition are all characterized by slightto moderate enrichments of light rare-earth elements relative to heavy rare-earthelements and depletions in some high field strength elements such as Nb, Zr, Hf.Whole rock εNd values for the Banpo and Nanlinshan intrusions vary between+3.4and+6.6. These characteristics are similar to those of lavas from modern island arcs.The geochemistry of the Banpo and Nanlinshan mafic-ultramafic intrusions revealsthat:(1) the ultramafic rocks of these intrusions are crystal cumulates of basalticmagma;(2) the parental magmas of these rocks are consistent with the derivatives ofarc magma derived from the depleted mantle wedge with <3%of a partial meltderived from subducted sediment, plus slab fluids from altered oceanic crust; and (3)the combination of positive εNd values with negative Nb and Zr-Hf anomaliessuggests that these two intrusions are the products of basaltic magmatism in an islandarc setting instead of a back-arc rifted basin. Based on the spatial and temporal distribution, lithological assemblage，mineralcompositions, whole-rock and mineral major and trace elements compositions, andSm-Nd istopes compositions of the mafic-ultramafic intrusitions, we propose that theLincang-Jinghong igneous belt represents an ancient composite arc system whichevolved from an oceanic arc in the Early Permian(298-295Ma) to a continental arcafter~285Ma. The transition occurred as a result of arc accretion which in turn tookplace due to persistent subduction of the Palaeo-Tethys Ocean along the westernmargin of the Simao Block.