Hainan Island

tectonically located between the Indochina and the South China Blocks

has undergone multiphased structural overprinting and complex tectonic evolution. Mafic rocks

newly discovered in the Fengmu area

Central Hainan

are coeval with Triassic porphyritic granite and mainly composed of plagioclase

clinopyroxene and hornblende. LA-ICP-MS zircon U-Pb dating results for the representative sample yield the weighted mean

206

Pb/

238

U age of （240.3±2.6） Ma

interpreted as the crystallization age of the mafic rocks. They have SiO

2

ranging from 48.99% to 56.00%

K

2

O between 0.63%~3.94% and Al

2

O

3

between 14.56%~17.77%

and have a K

2

O/Na

2

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.. They are characterized by subparallel right-sloping 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 ε

Nd

(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 Fengmu 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.