Zircon U-Pb age, geochemistry and Hf isotope of two types of Late Ordovician super-aluminum granites in the Caojian area of western Yunnan, and its indication to Pan-African movement

The Caojian complex granite located in Yunlong County of western Yunnan is exposed on the eastern edge of the northern end of the Baoshan block, which is composed of four period granites of Early Ordovician, Late Ordovician, Late Cretaceous and Neogene. The main Late Ordovician intrusive is lithologically made of medium grained biotite monzogranite and the porphyritic medium-coarse grained biotite monzogranite, in which gneissic structure is well developed. Zircon LA-ICP-MS U-Pb ages are 453.9±3.3 Ma and 456.8±3.5 Ma, respectively. Both belong to post-orogenic strongly-peraluminous high-K calcalkaline peraluminous granite with enrichment of K、Rb and other large ions lithophile element, but depletion of the high field strength element such as Zr, Ta, Nb, Th etc. According to the Sr-Yb diagram, the former belongs to the Himalayan granite, which is a partial melt under higher pressure in lower crust environment. The latter belongs to the Nanling granite, which indicates partial melting under the condition of lower pressure in the middle-upper crust environment. In the light of A-B diagram, the former shows the trend to muscovite peraluminous granite, and its magma is mainly related to uplift and decompression. The latter shows the characteristics of cordierite-bearing biotite-rich peraluminous granite, and its magma is mainly related to the warming and heating caused by upwelling of the deep mantle materials. Zircon Hf isotope analysis shows that the former yields ε_Hf(t) value of 11.65~0.28, averaging -6.36. The latter gives ε_Hf(t) value of 11.61~-1.57, averaging -6.78. Both share the roughly equivalent source regions, and are ancient crustal materials, with a small amount of mantle material added to the medium grained biotite monzogranite. It shows that the process of deep crust-mantle interaction may be firstly uplift and decompression leading to lower crust materials low-degree partial melting, not far behind, because to upwelling of mantle materials, warmed and heated medium-upper crust materials happened to higher-degree partial melting, in the background of the post-collision phase of thePan-African movement. The two types of the Late Ordovician granites are the magmatism responses of mutual extrusion and collision between small blocks on the northern edge of Gondwana continent, it shows that there is the Pan-African movement in Baoshan block.