| To better understand the origin of voluminous silicic rocks in a convergent continental margin,we have carried out an integrated study in which we have obtained geochronological,mineralogical,and isotopic?including whole-rock Sr–Nd–Pb,in situ zircon Hf?data,of the Heri?HR?pluton in West Qinling on the NE Tibetan Plateau.The pluton?235–233 Ma?is mainly composed of metaluminous to weakly peraluminous granodiorites with an I-type affinity,along with minor felsic porphyries.Detailed elemental data demonstrate that these granodiorites underwent fractional crystallization of hornblende and apatite,with plagioclase?i.e.sieve-textured plagioclase cores?accumulation to some extent.Except for porphyritic granodiorites,Pb isotopes for other analyzed samples are characterized by high radiogenic and uniform Pb isotopes((206Pb/204Pb)t:18.16–18.43,(207Pb/204Pb)t:15.57–15.59,and(208Pb/204Pb)t:38.16–38.36),together with limited variations in initial Sr((87Sr/86Sr)t:0.7078–0.7081)and Nd(?Nd?t?=-7.0–-6.4)isotopes with two-stage model ages(TDM2)of 1.57–1.51 Ga,collectively pointing to a derivation from the Mesoproterozoic basement rocks at the lower crustal level,or a comprehensive mixing of different-age components that generated an average crustal residence age.The Sr–Nd isotopic compositions of the porphyritic granodiorites are strikingly similar to those of the host granodiorites,implying their isotopic equilibration.Nonetheless,based on their Pb isotopic compositions,we speculate that the porphyritic granodiorites have included enriched lithospheric mantle source-derived components.Compared with experimental melt compositions of amphibolites,the HR granodiorites are probably derived from an amphibolitic source under hydrous conditions due to incongruent breakdown of amphibole and biotite.A K-rich protolith is not required to produce high-K I-type granitoids.Based on the temporal–spatial distribution of granitic intrusions in West Qinling and the regional tectonic evolution,our interpretation is that the HR batholith was formed due to the northward subduction of the Paleotethyan ocean.Together with previously published data and our new data,we propose that the HR granitoids were generated by partial melting of lower crustal amphibolites,with minor mantle-derived materials.By contrast,the coeval K-rich Sangke leucogranites in West Qinling were generated from an aluminous source?e.g.metapelite?under fluid-absent conditions during the subduction stage,and the Na-rich Xiaogouli leucogranites were derived from partial melting of metabasaltic protoliths under thickened conditions during the collision stage. |
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