Mesozoic Magmatism In Hainan Island (SE China) And Its Tectonic Significance

The Qiongzhong and Danxian batholiths, as the largest early Mesozoic syntectonic granite intrusions in Hainan Island, are mainly composed of porphyritic biotite two-feldspars granitoids, in which alkali-feldspar phenocrysts and dark minerals are foliated mostly in WE-direction. The granites are of metaluminous I-type. Major and trace elements show co-variations with SiO2, suggesting the fractional crystallization of plagioclase, hornblende, apatite and Ti-Fe oxides in the evolution of the granites. With systematic depletions of Ba, Nb, Sr, P and Ti, the granites display geochemical similarities to most granitoid rocks formed in the subduction-related environments. Both the Qiongzhong and Danxian granites are relatively high in initial 87Sr/86Sr ratios (0.7079 ~ 0.71 12 and 0.71 19 - 0.7194, respectively) and low in Nd(t) (- 8.9 ~ - 5.8 and -10.6 ~ -8.1, respectively). Their two-stage depleted mantle model ages (T2DM) range from 1.6 Ga to 1.8 Ga, implying a derivation from crustal sources with average crustal residence ages of early Mesoproterozoic. The zircon SHRIMP U-Pb dating results suggest that the Qiongzhong and Danxian granites were emplaced at 237 3 Ma and 186 3 Ma, respectively, which argued against the previous thoughts that all the foliated granites with orientated structure in Hainan Island formed duing the Hercynian - Indosinian episode.40Ar-39Ar and K-Ar age data indicate that the mafic dykes in Hainan Island were emplaced in three major episodes during -135 Ma, 105 ~ 117 Ma and 81-96 Ma. Mafic dykes from the southern Hainan are rich in Alkali and K and belong to shoshonitic, while those from the northern Hainan belong to high-K calc-alkaline series with lower K2O and Na2O. The mafic dykes are characterized by arc-like trace elements, high initial 87Sr/86Sr ratios and low eNd(t) values, and show somefamiliarities with the EMII-mantle source. The southern Hainan mafic dykes arestrongly enriched in LILE with high Nb/Ta ratios, indicating a magma source reconstructed by silicic-melt through partial melting of ancient subducted slides. However, the northern Hainan mafic dykes with chondritic Nb/Ta ratio should be derived from the fiuid-metasomatized mantle related to ancient subduction zone.The Qiongzhong and Danxian granites are belong to the KCG-type (K-rich calc-alkaline) granitoids according to the tectonic nomenclature by Barbarin (1999). These foliated KCG-type granites with original orientated structure are the results generated in a changing geodynamic orogenic environment with alternating compressional to extensional stress field. This suggests that Hainan island has been a part of South China since latest Permian, and been major in extrusion environment of the Tethys tectonic system. The Qiongzhong granite should have been formed at orogenic relaxing stage after the peak of Indosinian orogeny, and the Danxian granite were formed at the latest orogenic phase. In addition, the mafic dykes of Hainan island were formed in an intraplate extensional environment, similar to the mafic dykes in North Guangdong and Fujian coastal areas, all suggesting an extension regime tectonic in southeastern China during Cretaceous.