�Ժ�������������������������������ɽ�ҽ������ѧ�͵���ѧ�о����������: ������ײ�������������Ϊ 120 ±1. 3 Ma; �ϲ��İ�ɽ������Ϊ 110. 7 ± 1. 2 Ma����: �������������ڻ�ɽ�¼������ɣ��¼����Լ 10 Ma�������ڻ�ɽ�¼��붫������������������ɽ�¼���ȫ���ԶԱȡ��������ɽ����ʯ���Ϊ������ �� ��ɽ�ң����ڸƼ��� �� �߼ظƼ���ϵ�С�ϡ��Ԫ��������ʾ��ϡ��Ԫ�ظ�������ϡ��Ԫ�ؿ��𣬳ʸ����쳣��΢��Ԫ��������ʾ��Ը�����������ʯԪ�� (LILE) K����b��U��Th������ Nb��Ta��Ti �ȸ߳�ǿԪ�� (HFSE)����ϵ���ѧ������ǰ���о����ϣ�������Ϊ�����������������������������ɽ����Դ���µؿǵIJ������ڣ����̫ƽ������ŷ�ǰ��֮�¸��嵼�¼Ӻ���ʯȦ��������չ�����йء�

Chronological and geochemical data on the Mesozoic Longjiang stage volcanic rocks indicate that the rhyolite from the lower part of the Longjiang Formation formed in 120 ±1. 3 Ma and andesite from the upper part of the Longjiang Formation formed in 110. 7 ±1. 2 Ma�� It implies that the Longjiang Formation formed from two vol- canic events during an interval of about 10 Ma��and these two volcanic events could be compared with other related Early Cretaceous volcanic events of Northeast China�� The rock assemblages of Longjiang Formation are composed mainly of rhyolites and andesites��belonging to chemically calc- alkaline to high- K calc- alkaline series�� The ��EE patterns demonstrated L��EE enrichment and H��EE depletion��with weak negative Eu anomalies�� The volcanic rocks of Longjiang Formation were strongly enriched in large ion lithophile elements (LILE) such as K����b��U and Th��with depletion in high field strength elements ( HFSE) such as Nb��Ta and Ti�� Combined with geochem- ical characteristics and previous researches��the authors propose that the Early Cretaceous volcanic rocks in Xinsh- eng area��northern Heilongjiang were derived from partial melting of lower crust and were related to a post- orogenic extensional environment induced by the subduction of paleo- Pacific Plate to Eurasian Plate��