Geochronology and geochemistry of the Yilan greenschists and amphibolites in the Heilongjiang complex, northeastern China and tectonic implications

The Heilongjiang complex, extending along a suture zone between the Jiamusi and Songliao blocks in Northeast China, is composed mainly of blueschists, greenschists, meta-ultramafic rocks, quartzites, muscovite–albite schists and two-mica schists. Controversy has long surrounded the ages and tectonic settings of mafic rocks from the complex, which are crucial part of the complex. The lithological associations and their major and trace element compositions indicate that the mafic protoliths of the Yilan greenschists can be subdivided into alkali and tholeiitic basalts, which were derived from partial melting of a garnet-bearing and spinel-bearing mixed source, whereas the protoliths of the amphibolites are tholeiitic and were generated from the partial melting of spinel peridotite. Magmatic zircons from a tholeiitic amphibolite sample yielded a 206Pb/238U age of 256 ± 2 Ma, interpreted as its protolithic age. The sample also contains small amounts of older inherited zircons up to 344 Ma, which, together with its origin from shallow lithospheric mantle, indicate that the tholeiitic rocks were generated in a continental rift. The geochemical data suggest that further rifting led to the formation of an ocean between the Jiamusi and Songliao blocks, in which some oceanic islands developed, represented by the alkali basaltic protoliths of the Yilan greenschists. Magmatic zircons from an alkaline greenschist sample yielded a 206Pb/238U age of 162 ± 3.9 Ma, which, together with protolithic age of 141.8 ± 1 Ma previously obtained for the Yilian blueschist, support the model that the ocean between the Jiamusi and Songliao blocks closed at some time after ~ 141 Ma, not earlier at 210–180 Ma as previously considered.