The Sanchahe quartz monzonite intrusion is situated in the middle segment of the North Qinling tectonic belt, Central China mainland, and consists chiefly of sanukitoid-like and granodioritic-monzogranitic rocks. The sanukitoid-like rocks are characterized by quartz monzonites, which display higher Mg-# (55.0-59.0), and enrichments in Na2O+K2O (7.28-8.94%), Ni (21-2312 ppm), Cr (56-4167 ppm), Sr (553-923 ppm), Ba (912-1355 ppm) and LREE ((La/Yb)(N)=9.47-15.3), from negative to slightly positive Eu anomalies (Eu=+0.61 to +1.10), but also depletion in Nb, Ta and Ti. The granodioritic-monzogranitic rocks diaplay various Mg-# of 6.00-53.0, high Na2O+K2O (7.20-8.30%), Sr (455-1081 ppm) and (La/Yb)(N) (27.6-47.8), with positive Eu anomalies (Eu=1.03-1.57) and depleted Nb, Ta and TL Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) zircon U-Pb isotopic dating reveals that the sanukitoid-like rocks were emplaced at two episodes of magmatism at 457 +/- 3 Ma and 431 +/- 2 Ma, respectively. The monzogranites were emplaced at 445 +/- 7Ma. Sanukitoid-like rocks have their Hf(r) values ranging from +0.3 to +15.1 with Hf-depleted mantle model ages of 445 to 1056 Ma, and the monzogranite shows its Hf(t) values ranging from -21.6 to +10.8 with Hf-depleted mantle model ages of 635 to 3183 Ma. Petrological, geochemical and zircon Lu-Hf isotopic features indicate that the magmatic precursor of sanukitoid-like rocks was derived from partial melting of the depleted mantle wedge materials that were metasomatized by fluids and melts related to subduction of oceanic slab, subsequently the sanukitoid magma ascended to crust level. This emplaced mantle magma caused partial melting of crustally metamorphosed sedimentary rocks, and mixing with the crustal magma, and suffered fractional crystallization, which lead to formations of quartz monzonites. However, the magmatic precursor of the granodioritic-monzogranitic rocks were derived from partial melting of subducted oceanic slab basalts. Integrated previous investigation for the adackitic rocks in the south of the intrusion, the Sanchahe intrusion signed that the North Qinling tectonic zone was developed in an early Paleozoic transitionally tectonic background from an island arc to back-arc.
