| The Qinling orogenic belt has experienced a long time and complex tectonic evolution, with many multi-phase and multi-stage intrusives being developed. The studies on the petrogenesis of the intrusives including source rocks involve the formation of the Qinling orogenic belt, in particular the geodynamic processes. The Xiaomaoling composite intrusives dealt with in the dissertation are exposed at the western end of the Xiaomaoling-Doling uplift belt located in the northern most part of South Qinling. Based on the field work, the petrography, zircon U-Pb geochronology, systematic geochemistry and Hf isotope analysis of the composite intrusives are discussed in this paper.1. The Xiaomaoling composite intrusives are composed of early Songjiawuchang diabase (grabbro) and late Mihunzhen, Mogouxia and Yejiawan diorites. The diorites contain diabase (grabrro) xenoliths. The Xiaomaoling intrusives bear xenoliths of the Pre-Sinian Mogouxia Complex. Synchronous granitic dikes intrude into the Mogouxia Complex, which is covered unconformably by the Sinian Doushantuo group clastic rocks. It follows that the Xiaomaoling composite intrusives were formed after the Pre-Sinian Mogouxia Complex and before the Sinian Doushantuo group.2. The Songjiawuchang intrusive, Mihunzhen intrusive, Mogouxia intrusive and Yejiawan intrusive from the Xiaomaoling intrusives yield mean 206Pb/238U ages of 864.4±1.7 Ma,846.7±2.7 Ma,859.4±1.7 Ma and 861.1±1.8 Ma respectively by LA-ICP-MS zircon U-Pb dating, suggesting that these composite intrusives were formed during the Neoproterozoic (870～840 Ma). Accordingly, the Xiaomaoling composite intrusives are classified into three parts:early Songjiawuchang diabase (grabbro), middle Mogouxia quartz diorite-tonalite and Yejiawan quartz monzonite-monzodiorite and late Mihunzhen diorite-quartz diorite, that is to say, they are products of different evolutionary sequence of synchronous magmas.3. The early Songjiawuchang intrusive and middle to late Mogouxia, Yejiawan and Mihunzhen intrusives belong to two different series on their geochemical analysis. The early Songjiawuchang intrusive has high FeO, Fe2O3, MgO and TiO2 contents, low K2O and Na2O contents, with A/CNK=0.68～0.83; enriched LREE, negligible Eu anomalies or weakly positive Eu anomalies; low Rb, Ba, Th and U contents, slightly negative Nb, Ta, Ti and P anomalies, (Th/Nb)N ratios slightly greater than one, hight Zr content and Zr/Y ratios, showing a feature of the continental tholeiite. The middle to late Mogouxia, Yejiawan and Mihunzhen diorites have high K and Na contents, A/CNK less than 1.1; similar REE and trace elements feature, enriched LREE; LREE and HREE fractionation and negative Eu anomalies becoming stronger with the increasing acidity from Mihunzhen diorite-quartz diorite to Mogouxia quartz diorite-tonalite to Yejiawan quartz monzonite-monzodiorite; enriched Rb, Ba, Th, U and K and depleted Nb, Ta^ Ti and P for the Mogouxia, Yejiawan and Mihunzhen intrusives, indicating a geochemical feature of island arc volcanic rocks.4. The Lu-Hf isotopic analysis shows the Xiaomaoling composite intrusives of different stages have different sources. The early Songjiawuchang amphibolic diabase (gabbro) withεHf(t) value of 7.9～9.9 and TDM1 of 1107～1021 Ma, might be derived from depleted mantle contaminated by crustal material. The middle Mogouxia intrusive and Yejiawan intrusive have similar Hf isotopic feature, the former characterized by eHf(t) value of 4.1～5.1 and TDM2 of 1476～1427 Ma, and the latter byεHf(t) value of 3.2～5.9 and TDM2 of 1548～1371 Ma, indicating that they were derived from partial melting of juvenile crust formed in the middle-late Mesoproterozoic. The late Mihunzhen intrusive has mostly negativeεHf(t) value and TDM2 of 1970～1709 Ma, indicating that it resulted from the partial melting of a late Paleoproterozoic old crust mixed with some mantle material. 5. In combination with regional geological background and tectonic evolution characteristics of the Qinling orogen, it can be seen that the Xiaomaoling composite intrusives were formed in an early Neoproterozoic post-orogenic extensional environment.
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