Abstract:

The Tianheyong basalts are basanites, which at least can be subdivided into three types of mineral assemblage. In general, the Tianheyong basalts are characterized by low silicon (w(SiO2)=4397％4568％), enriched alkali (w(K2O+Na2O)=591％765％), enriched potassium (w(K2O)=204％289％), high contents of titanium (w(TiO2)=218％237％), high magnesium value (Mg＃=6876), and high contents of REE (∑REE=(2466232982)×10-6). The chondrite normalized pattern of REE shows a right inclined smooth line, extreme enrichment of LREE, significant fractionation of LREE and HREE, without obvious anomaly of Eu(δEu=090102) and Ce(δCe=096100); extreme enrichment of incompatible elements, especially with peak value in high field strength elements (HFSE) of Nb, Ta and Th, and OIBlike distribution patterns of multielements; the Tianheyong basalts also have high content of compatible elements: Co((391489)×10-6), Ni ((130257)×10-6) and Cr((138320)×10-6). All of the above characteristics as well as poorcrystallized, abundant peridotite inclusions, few xenocrysts, and elements covariation trend indicate that the Tianheyong basalts were solidified from primitive basaltic magma. Trace elements ratio of Ba/Rb (1235) and variation of alkali metals might indicate the metasomatism of fluids. Petrogenesis simulation indicates that the Tianheyong primitive basaltic magma may have been the product of polybaric melting column with nonmodal partial melting processes of enriched lherzolite, which straddle the boundary of garnetand spinelfacies lherzolite. Magma may have been originated from low partial melting of source rocks(~<5％), which have ~1％ partial melting of garnet lherzolite and 2％5％ partial melting of spinel lherzolite. The comprehensive analysis indicates that the induced mechanism of source rock partial melting may have been the edgedriven convection, which resulted in the Tianheyong basalts having the deepest origin depth compared with that of eastern Jining and Hannuoba basalts.

Key words: petrogenesis; Tianheyong; basalt; primitive magma; polybaric melting column; edgedriven convection