GEMOC ARC National Key Centre

NATURE OF THE LITHOSPHERIC MANTLE BENEATH THE EASTERN PART OF THE XING'AN - MONGOLIA FOLDBELT: EVIDENCE FROM BOTH POTASSIC VOLCANIC ROCKS AND MANTLE XENOLITHS

Zhang, M.¹, O'Reilly, S.Y.¹, Griffin, W.L.^1,2

1 GEMOC Macquarie
2 CSIRO Exploration and Mining

The Phanerozoic Mongolia­Xing'an foldbelt is a major composite foldbelt ca 1500 km wide in Central-East Asia formed by amalgamation of several minor allochthonous terranes during subduction and collision between the Siberian Craton to the north and the Sino­Korean Craton to the south.  These terranes such as the Songliao Basin and the Jiamusi blocks are separated by Paleozoic orogenic belts such as the Greater and Lesser Xing'an Mts and Zhangguangcai Mts.  Geological evidence indicates that subduction against both craton margins could have occurred as early as early Cambrian, whereas paleomagnetic data suggest that the final suturing of the two continents probably was not complete until Jurassic.  However, the age and geochemical nature of the lower crust and lithospheric mantle are poorly known though various Proterozoic ages (eg 600-700 Ma or 1.7-1.8 Ga) have been proposed for the basement of the Songliao Basin and Paleoproterozoic (*2.3 Ga) crustal rocks occur on the eastern margin of the Xing'an Mountains.

The late-Tertiary to Recent mafic potassic volcanic rocks erupted at the northwestern boundary between the Greater Xing'an Mts and the Songliao Basin in the eastern part of the Mongolia-Xing'an foldbelt and covering an area of * 1400 km2.  These potassic rocks and their mantle ultramafic xenoliths (spinel peridotite and pyroxenite) provide constraints on the nature of the lithospheric mantle in the region.

The distinct geochemical signatures of the potassic rocks require the presence of an EM1-type lower lithospheric mantle in the garnet-stability field.  This inferred lower lithospheric mantle is refractory in major element terms but enriched in LILE and exhibits noticeable fractionation between HFSE and LILE (eg U+Th vs Rb+K) and among various HFSE (eg Nb+Ta vs Zr+Hf) and has very unradiogenic 143Nd/144Nd (_Nd ¾-12) and 206Pb/204Pb (¾ 16.30) ratios.  The evolution for such as an enriched mantle source indicates old (late-Archaean to early-Proterozoic) metasomatic event(s) and long-term preservation of such an enriched lithospheric mantle block beneath the Phanerozoic Xing'an Fold Belt.

Spinel peridotite xenoliths, fragments of the uppermost lithospheric mantle in the region, have moderately low concentrations of basaltic components, Y, Sc, and HREE.  However, clinopyroxenes from these xenoliths display heterogeneously enriched incompatible element and Sr-Nd isotopic signatures.  These geochemical signatures are transitional between peridotites from typical Archaean terranes and those from Phanerozoic terranes, implying a possible Proterozoic origin.

Therefore, geochemical data for the Chinese potassic rocks and entrained mantle xenoliths suggest that the lithospheric mantle beneath the Songliao Block is a Precambrian (Archaean (?) to early Proterozoic) mantle terrane that extends well beyond the present boundary of the Songliao Basin into the eastern flank of the Great Xing'an Mountains.  Further studies of mantle xenoliths and their basalt hosts along the Xing'an-Mongolia Foldbelt (eg Sikhote-Alin, Wangqing, Mudanjiang, Shuangliao, Dariganga and Central Mongolia) will provide further constraints on the nature and evolutionary history of lithospheric mantle beneath the Phanerozoic East-Central Asian fold belts.