Geochronologv and geochemistry of hudong gneissic composite pluton in the junction of guangdong and guangxi provinces: Implications for early paleozoic tectono-magmatism along the northern margin of yunkai massif

The Hudong gneissic composite pluton is located in the junction of Guangdong and Guangxi provinces and is composed mainly of gneissic tonalite, granodiorite and monzonitic granite. LA-ICFMS U-Pb dating result of zircon from a gnessic granodiorite sample yields an emplacement age of 443. 1 ± 2. OMa. The gneissic tonalites and granodiorites are metaluminous ( A/CNK =0. 83 ∼ 0. 93 ) and belong to medium-K amphilx)le calc-alkaline granitoids (ACG) with relatively low SiO2 (62. 92% ∼67. 54% ) and K2O+ Na20 (3. 98% ∼5. 17% ). In contrast, the gneissic monzonitic granites are metaluminous to weakly peraluminous ( A/CNK =0. 92 ∼ 1.05) and belong to high K calc-alkaline granites ( KCG) with higher SiO 2(71.55% ∼ 72. 78% ) and K20 + Na20 ( 6.65% ∼ 7. 57% ). Rocks from the Hudong gneissic composite pluton are characterized by relative enrichment in LILEs (e. g. , U, Ba, Kb and Th) and LKEEs and pronounced depletion in HESEs (e.g. , Nb, Ta and Ti) and HKEEs, highly radiogenic Sr (87 Sr/86 Sr(i)= 0. 71268 ∼0.71482) and Pb (e. g. , 206Pb/204Pb (i)=18.17 ∼18.36), and nonradiogenic Nd (εNd(t) = - 9. 4 ∼2.6) isotopic compositions. Such geochemical features are typical of subduction-related arc magmatic rocks. In combination with the regional tectonic evolution, we consider that the Hudong gneissic composite pluton was most likely produced related to the southward subduction of the ocean floor and collision between the Yangtze and Cathaysian blocks. Asthenospheric upwelling followed by the oceanic subduction provided enough heat supply to melt both the overriding lithospheric mantle and the overlying recycled fertile crust of the Yunkai massif and ultimately to form the hybrid magma. This hybrid magma experienced different degrees of fractional crystallization to produce the wide compositional range of the Hudong gneissic composite pluton. We therefore conclude that the Hudong gneissic composite pluton was an important record of the oceanic subduction and subsequent continental collision in the northern margin of Yunkai massif during Early Paleozoic time.