Neng Jiang, of the Key Laboratory of Mineral Resources, Institute of Geology and Geophysics of the Chinese Academy of Sciences, Beijing, arrived this month to spend a year working with Rick Carlson on isotopic studies of  rocks from the ancient continental crust of China. Jiang's work focuses on studies of the age and origin of mantle and lower crustal xenoliths from the North China Craton in an attempt to distinguish whether the wide range in ages recorded in rocks from this area reflects several episodes of crustal growth or instead simply reworking of the 2.7-billion-year old basement.

The samples Jiang will be studying at DTM are accidental inclusions of lower crustal and upper mantle rocks picked up by the young Hannuoba basalts that erupted about 100 km west of Beijing. The Hannuoba basalts erupted through the North China Craton where rocks as old as 3.8 billion years have been found, but in an area known as the Trans-North China Orogen where the basement is 2500-2700 million years old.  This area of the North China Craton was subjected to intense metamorphism around 1800-1900 million years ago that is interpreted to reflect the time when the eastern and western blocks of the craton collided and welded together.  The area also was the site of extensive granitic magmatism approximately 140 million years ago.

Jiang’s previous work has shown that the 1800-million-year-old rocks, as well the younger granites, likely were derived by remelting of the older crustal basement rather than from new crustal addition events.  At DTM, he will be analyzing a variety of isotope systems in samples from the lower crust and upper mantle to investigate the causes and consequences of this major crustal reworking and the processes that led to internal chemical differentiation of the continental crust.