Geochemistry, age and tectonic significance of granitic rocks in north Altun, northwest China

Abstract

Granitic rocks in the Altun terrane, NW China crop out in a 50-km-long, E–W-trending belt along the southern margin of the Bashikaogong Basin, north Altun. They intrude the north Altun subduction complex and are hosted in Precambrian sandstones, schists, mudstones and tuffs. Four lithologic types are recognized; (I) gray, very coarsely phyric granite, (II) red, fine- to medium-grained, aphyric granite, (III) gray, medium- to coarse-grained, moderately phyric granite and (IV) pink, medium- to coarse-grained, moderately phyric granite. All of the granites are peraluminous with normative corundum ranging from 0.8 to 3.9 wt.%. Silica contents vary between 65.1% and 75.7%, and total alkalis (Na2O+K2O) range from 7.5 to 9.0 wt.%, giving K2O/Na2O ratios of 1.1 to 2.7. Rittman indices vary from 1.8 to 2.8, with an average of 2.3. Total rare earth elements (∑REE) are between 89 and 335 ppm, and all samples show significant but variable negative Eu anomalies on chondrite-normalized REE patterns. On mantle-normalized trace element patterns all the granites have pronounced negative Ba, Nb, Sr, P and Ti anomalies. Zircons from the granites show slightly different SHRIMP U–Pb ages for the different lithologies; 474.3±6.8 Ma for type I granites, 446.6±5.2 Ma for type II, 434.5±3.8 Ma for type III, and 431.1±3.8 Ma for type IV. The geochemical and geological data indicate that these are S-type granites formed by partial melting of crustal materials during or shortly after collision of the Altun and Tarim continental blocks in the Paleozoic. The age and geochemistry of the north Altun granites suggest that they can be correlated with similar rocks in north Qilian on the opposite side of the Altun strike-slip fault.