Magma chamber evolution: implication for the generation of continental crust: A case study in Kekeli batholith, North Qilian Orogenic Belt

Traditionally, zircon in situ Hf (Zir-Hf) isotope composition is considered to record faithfully the Hf isotope composition of the magma at the time of crystallization and because zircon is the major host of Hf in granitoids, zircon Hf isotope composition is regarded as representing the whole rock Hf (WR-Hf) isotope composition (Schmitz et al., 2004). However, in Kekeli Batholith in the North Qilian Orogenic Belt, the zircon ɛHf(t) is dominantly < 0, whereas the whole-rock ɛHf(t) is mostly > 0. Such significant WR-Hf and Zir-Hf isotope decoupling demonstrates that (1) Zir-Hf isotope composition does not necessarily represent WR Hf composition, and (2) zircon is not the sole major host of Hf. Besides, WR-Hf isotope is largely decoupled from WR-Nd isotope but Zir-Hf is not. The significant positive correlation between WR-Hf isotope and TiO₂ indicates that the TiO₂-rich minerals may be important Hf host apart from zircons. This correlation combined with the broad negative correlations between both ɛNd and ɛHf and silica further constrain that Kekeli batholith suites are the hybrids of magma mixing. The discrepancy between WR-Hf and Zir-Hf isotope composition cannot be explained by the 'analytical bias' (i.e., the inherited zircons are analysed in the whole rock but not analysed in the in situ zircons). The possibility is that during homogenising process in the course of magma mixing, Ti-rich minerals such as amphibole (Amp), Fe-Ti oxides, biotite (Bi) etc. crystallize earlier in the more mafic composition and preserve the isotope signature closer to the mafic endmember. Zircons do not crystallize until the later stage when the hybrid magma is saturated in Zr. Therefore, zircons generally record isotope compositions closer to the felsic endmember. The decoupling between WR-Hf and Nd isotopes should be inherited from the mafic endmember. The Kekeli Batholith dated at 500 Ma in this study contains abundant cumulate assemblages dominated by amphibole and plagioclase plus minor other felsic minerals (Kfs, Qtz). The charnockite sample QL10-26 has the highest TiO₂, pyroxene with Amp reaction rims and the most radiogenic WR-Hf (ɛHf(t) = 26) and therefore represent the most primitive mafic endmember. The leucogranite samples have the lowest ɛHf(t) and ɛNd(t) and thus represent the crustal endmember. Other samples with intermediate composition between these two endmembers also have the intermediate isotopic compositions in between. The various zircon types with complex internal structures present in most samples reflect complex source materials. All these observations are consistent with the scenario that the mantle-derived basaltic magmas interacted with granitic magmas derived from the lower crust. Considering the petrotectonic associations, it is probable that the Kekeli Batholith may represent a fossil magma chamber that crystallized at the base of a continental arc crust associated with a subduction zone. Schmitz, M.D., Vervoort, J.D., Bowring, S.A., Patchett, P.J., 2004. Decoupling of the Lu-Hf and Sm-Nd isotope systems during the evolution of granulitic lower crust beneath southern Africa. Geology 32, 405.