Mesozoic Magmatism In Yacheng-Sanya Area(South Of Hainan Island) And Its Tectonic Significance

The early Mesozoic magmatic plutons of Yacheng-Sanya area are located at the connection place of Huanan、India、South China Sea and Pacific plates. This area consists of monzonite granite、syenogranite、granite porphyry、 granodiorite、gabbro-diabase and bojite.The high-precision LA-ICP-MS zircon U-Pb dating results suggest that the magmatic plutons of Yacheng-Sanya area were mainly formed in Early Triassic Epoch, the syenogranite in the east of Sanya was formed in 246.6Ma-251.6Ma, the monzonite granite in the north of Sanya was formed in 245Ma, and the bojite between Yacheng and Sanya was formed in 241.3Ma.Chemically, the contents of SiO2 in the granites(monzonite granite、 syenogranite、granite porphyry) are between 71.83%-76.34%, and most of samples belong to metaluminous to weakly peraluminous rocks. The contents of K2O+Na2O are between 8.31%-14.05%, while the contents of ∑REE are between 137.9ppm -325.8ppm. Those samples have moderate to strong negative Eu anomalies(δEu=0.026-0.43). The granites are mainly depletion in LILE(Cs、K、Ba、Sr、Eu), HFSE (Nb、Zr、Ti) and have different degrees of enrichment in magmatopile (incompatible) trace elements (Th and Ta), LILE(Rb), HFSE(U), Nd, Sm, Gd. The content of the SiO2 in the Granodiorite is between 63.38%-68.35%. They are metaluminous and calc-alkaline rocks, and the contents of K2O+Na2O are between 8.71%-10.34%, while the contents of ∑REE are between 179.4ppm-253.9ppm. They show strong fractionation between LREE and HREE(∑Ce/∑Y=4.57-5.63 (La/Lu)N=14.50-19.69). They have weakly negative Eu anomalies (δEu =0.56-0.63). The Granodiorite is mainly depletion in LILE(K、Ba、Sr、 Eu), HFSE(Nb、P、Zr、Ti) and has different degrees of enrichment in LILE(Cs、 Rb), magmatopile (incompatible) trace elements(Th and La), HFSE(U), Nd, Sm, Gd. The SiO2 contents of the gabbro-diabase and bojite are between 47.13% -59.20%. They are calc-alkaline rocks, the K2O+Na2O contents are between 3.89%-6.97%, and the ∑ REE contents are between 121.2ppm-392.9ppm. They show moderate degree of fractionation between LREE and HREE(∑Ce/∑Y= 2.60-4.02, (La/Lu)x=7.99-12.24). The gabbro-diabase and bojite are mainly depletion in LILE(Sr、Ba、Ti), HFSE(U、Nb、Zr), magmatopile (incompatible) trace elements (Th、Ta), P and have different degrees of enrichment in LILE(Rb), HFSE(Nb、Ce), magmatopile (incompatible) trace elements (La), Nd, Sm, Gd.The Early Triassic Epoch granites in Yacheng-Sanya area are A-type granites and belong to the A2 - type according to the theories of Whalen(1987) and Eby et al(1992). Study area is in the stretch tectonic environment during 241.3-251.6Ma.Due to the reduced pressure, tensile and thinning crust, the upwelling asthenosphere mantle material caused the underplating, and heating the middle and lower crust, partial melting in the original rock, to produce the A-type granites. In addition, mixing between high temperature basic magma of the mantle and the low-crustal felsic acidic magma formed granodiorite. The not fully mixing basic magma formed dioritic inclusion and mafic dikes. In general, Yacheng-Sanya area was in the stretch tectonic environment during Early Triassic Epoch.