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Geochemical and geochronological characteristics of the Fengmu mafic rocks in Hainan and its tectonic implications
Pages: 146-157
Year: Issue:  4
Journal: Acta Scientiarum Naturalium Universitatis Sunyatseni

Keyword:  Triassic mafic rocksLA-ICP-MS zircon U-Pb datingarc affinitypost-collisional settingFengmuHainan Island;
Abstract: Hainan Island,tectonically located between the Indochina and the South China Blocks,has undergone multiphased structural overprinting and complex tectonic evolution.Mafic rocks,newly discov-ered in the Fengmu area,Central Hainan,are coeval with Triassic porphyritic granite and mainly com-posed of plagioclase,clinopyroxene and hornblende.LA-ICP-MS zircon U-Pb dating results for the repre-sentative sample yield the weighted mean 206 Pb /238 U age of (240.3 ±2.6)Ma,interpreted as the crystal-lization age of the mafic rocks.They have SiO2 ranging from 48.99% to 56.00%,K2 O between 0.63%~3.94% and Al2 O3 between 14.56% ~17.77%,and have a K2 O /Na2 O ratio of 0.58 ~1.32.These rocks show high REE concentrations with ∑REE =214.7 ~522.7 μg/g,and (La /Yb)N =11.17 ~31.10,(Gd /Yb)N =2.00 ~4.50,δEu =0.74 ~1.14.They are characterized by subparallel right-slop-ing REE pattern,similar to those of island arc volcanic rocks,and by enrichment in LREEs and LILEs and depletion in HFSEs with significant Nb-Ta and Ti negative anomalies.The εNd (t)values ranges from -1.05 to -2.53.Our data indicate that these rocks originated from the enriched mantle source modified by subduction-related fluid /melt.In combination with other available data,it is proposed that the Feng-mu mafic rocks at 240 Ma generated at the post-collision extensional setting of the convergence of the South China with Indochina Block in response to the subduction of the Paleotethyan Ocean.
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