| Summary: | 碩士 === 國立成功大學 === 地球科學研究所 === 87 === The Alashan Terrane located between the Qilian fold belt and the Inner Mongolia terrane. Proterozoic strata exposed along the southern margin of the Alashan block including metasediment and metabasalt assemblages, were intruded by at least two phases of granitoids. Two suites of basalt represent probable mid Proterozoic oceanic assemblages respectively. MORB-like metabasalts with La/Ybn=0.8 to 3.0 and Sm-Nd TDM model ages ariund 2.5 Ga are intercalated with metasediments having Sm-Nd TDM model ages of 2.1 to 3.0 Ga. Major and trace element discriminants (notably Th-Sc) indicate the sediments were deposited in a continental margin setting. A second suite of basalt with intraplate characteristics including La/Ybn ratios of 4.3 to 6.6, occurs as lenses in massive carbonates found along with similar metasediments to the first suite, and is interpreted as melange resultig from the accretion of a suite of ocean islands to the continental margin of North China. The granitoids can be divided into I-type and S-type, although both are products of variable mixing between basaltic material and partial melting of Precambrian basement. Two phases of granitoids have been observed. The earlier suit with age of 1.5Ga （or older）are syn-collisional granitoids with isotopic similarities (age ca.1063 Ma, Sm-Nd TDM=1.1Ga) to those observed in the Central Qilian terrane. A later suite of granitoids have an age of 255Ma with □Nd(255Ma)+0.6 to -25.4 and overlap the fields for volcanic arc, syn-collisional and intraplate granitioids in trace element discrimination diagrams. Geochronology studies suggest that the basement is older than 2 Ga and shows little relationship to the Caledonian.
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