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RESEARCH ON THE GEOCHEMISTRY AND GENESIS OF MESOPROTEROZOIC GRANITES ON HAINAN ISLAND

XU De-ru, LIANG Xing-quan, CHENG Guang-hao, HUANG Zhi-long, HU Hui-dong (Changsha Institute of Geotectonics, Academica Sinica, Changsha 410013, China; 2. institute of Geochemistry, Academica Sinica, Guiyang 550002, China)  
Mesoproterozoic granites on Hainan island are composed of granite, granodiorite with the same origination, and show an evolution series evidently from granite to granodiorite and quartz-diorite with the evolution trend to calc-alkalic. The suits are in situ-semisitu prealuminous granites during the upheaval after collision of plate, have the genesis characteristics of mantle-crust two-component mixing, and are the result of situ-remelting of mid-lower crust bia-basic material(metasedimentary rocks and plao-amphibole-geniss within Baoban Group) and participating of mantle material by heating caused by crust thickinning and following underplating of tholeiitic magma because of the collision of plate, at last, the melting magma ascends, concretes and posits along the Gezhen ductile shear zone because of the mechanism of strike-slip dilatancy pumping. The granites are characterized of high SiO2, K2O, Rb, Ba, Ta, Ce and low P, Ti, Zr, Sr, Fe2O3+FeO, MgO, CaO contents, the specific value of elements of Zr/Nb, La/Nb, Ba/Nb, Rb/Nb, K/Nb, Ba/La and Cr, Go, Ni, V are closed to that in mid-lower crust, the content of Rb, Sr, Ba, Ta, Zr and ratio of K/Sr, Rb/Sr, Sr/Ba have relatively less variable boundary, show consistence of the composition of originative rocks or melting mode. The granites have strong contrasts between LREE and HREE, an important negative Eu abnormality, and consistency REF models with right-inclining type, are identical to that of metasedimentary rocks within the Baoban Group. The higher value of positive εNd than that of rocks within Baoban Group and the variant of(87Sr/86~ Sr), suggest the formative information of mantel-crust mixing. Considering the feature of space distribution of metabasic volcanic rocks, We think that the transformation of tectonic system during Mesoproterozoic era might go through the whole course from feeble subduction, collision to underplating.
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