GEOCHEMISTRY OF GNEISSIC ALKALINE GRANITES IN DONGHAI COUNTY, JIANGSU PROVINCE, AND ITS TECTONIC SIGNIFICANCES

Abstract

The Donghai region is tectonically located in the southwestern margin of the Su-Lu Orogenic Belt. Several Neoproterozoic gneissic alkaline granite plutons, with typical mineral assemblages of quartz + K-feldspar + oligoclase-albite + aegirine (aegirine-augite)/arfvedsonite, were outcropped in this area. Chemically, these granites are characterized by rich in silicon (SiO2 = 73.43% - 77.38%), alkaline (K2O+Na2O = 8.19% - 9.53%), poor in calcium and magnesium, and high FeO*/(FeO* +MgO) ratios (=0.83-0-94). They also show enrichments of Ga, Y, Zr, Hf, and depletions of Sr, P, Ti, and display high 104XGa/Al ratios (=2.76 -5-15). Chondrite-normalized REE distribution patterns of them are right-inclined, with (La/Yb)N rarios of 2.08-17-89, and display moderate-strong europium depletions ((Eu=0.12-0.66). On the discrimination diagrams proposed by Whalen et al (1987) and Eby (1990), they are all plotted in the A-type granite field. Considering that the rocks have high values of agpacity index (=0.99-1.22) and low A/NKC ratios (=0.78 -0.93), thus they should be grouped into peralkaline A-type granites. These granites show lower Y/Nb and Y/Ta ratios, with values of 1.85-9.72 and 4.71-30.14 respectively, and display clearly depletions of Nb, Ta, Ti on the primitive mantle-normalized spidergrams, implying that they preserve some geochemical fingerprints of the foregoing arc magmatism. On the diagrams for subdivision of A-type granites proposed by Eby (1992), they fall in the A2 field, which was interpreted as a characteristic of post-collisional granites. Integration of mineral chemistry and whole-rock geochemistry with geodynamic considerations suggests that the Donghai gneissic alkaline granites were emplaced during the earliest extensional stage of orogenic collapse, rather than produced in syn-rift period. This understanding has important significances both in further revealing the Neoproterozoic tectonic evolutions of the Su-Lu Orogenic Belt and in profoundly probing the breakup mechanism of the Rodinian supercontinent.