Two contrasting paleozoic magmatic belts in northern Inner Mongolia, China: petrogenesis and tectonic implications

The Solonker zone, extending from Solonker to Sonidzuoqi and Xilinhot in northern Inner Mongolia, China, has been considered as the sature between the Manchurides and Altaids. Two magmatic belts have been recognized along the suture zone: an older, variably deformed are-related magmatic belt represented by the Baolidao magma suite, and a younger, collisional granite brit, represented by the Halatu plutons. The Baolidao suite is composed of gabbroic diorite, quartz diorite, tonalite and granodiorite. Hornblende is present in all rock types. The Halatu plutons consist mainly of two-mica adamellite and subordinate granodiorite and leucogranite. Hornblende is absent in all rock types. The bulk of Baolidao rocks were emplaced at ca. 310 Ma (zircon Pb-206/U-238 age). They are metaluminous to weakly peraluminous and possess high Na2O/K2O ratios from 2 to 20. They have no Eu anomalies in the REE patterns but show strong positive Sr anomalies in the primitive-mantle-normalized spidergrams. In contrast, the Halatu granites were formed at approximate to 230 Ma based on a whole-rock Rb-Sr isochron. They are weakly metaluminous to peraluminous and have rather uniform chemical compositions. In comparison with the Baolidao magma suite, they have much lower contents of FeO, MgO, CaO, TiO2, Cr, V and Sr, but are richer in SiO2, K2O and Rb. They also exhibit distinct negative Sr, P, Ti and Nb anomalies in thr spidergrams and significant negative Eu anomalies in the REE patterns, a feature typical of crustal melts. The most primitive rock type of the Baolidao suite (gabbroic diorite) has I-Sr = 0.7052 and epsilon (Nd)(T) = +2.4, whereas the other rock types show small variations in both I-Sr (0.7056-0.7059) and epsilon (Nd)(T) (+0 7 to -0.1). Geochemical modelling suggests that the gabbroic diorite was derived fi om a metasomatized upper. mantle and the other rock types were generated from the gabbroic diorite magma by fractional crystallization coupled with assimilation of older crustal material (AFC), and followed by hornblende-dominated fractionation in a closed system. The Halatu granites exhibit a range of Is, (0.7048-0.7063) and epsilon (Nd)(T) (+1.0 to -2.2), They were derived from sources dominated by young mantle-derived rocks with variable amounts of ancient continental crust. The largely juvenile isotopic signature for both the Baolidao suite and Halatu granites indicates a significant addition of new continental crust in northern inner Mongolia. (C) 2000 Elsevier Science B.V. All rights reserved.