| A large number of Yanshanian granite (porphyry) bodies, Indosinian graniteand dioritic porphyrite veins are widely exposed in the Shanyang-Zhashui area, middle Qinling Orogenic Belt. Most of those Yanshanian granite (porphyry) bodies are surrounded by porphyry-skarn Cu-Mo-Fe and Cu-Au-Fe mineralization. Nine typical granite (porphyry) bodies and three dioritic porphyrite veins in the Shanyang-Zhashui area were selected to be systematic studied in this thesis. Basic on the field geologic investigation, petrographic examination, electron microprobe analysis, LA-ICP-MS zircon U-Pb ages determination, geochemical analysis and Lu-Hf isotopic analysis, the source natures, petrogenesis and petrogenic tectonic setting for these granite (porphyry) bodies were studied in this paper. Finally, metallogenic geological setting for porphyry-skarn deposits was discussed. In order to study the relationship between the tectonic activity and magmatism and mineralization,40Ar-39Ar ages of the Fengzhen-Shanyang fault was determined in the thesis. According to previous works, this thesis yielded following understandings and results.The granitoid magmatism can be divided into three periods (Neoproterozoic age, Indosinian and Yanshanian, respectively) in the researched region. The first period (Neoproterozoic magmatism:864.4-680Ma) was represented by the Banbanshan magmatic complex, Shiligou monzonitic granite, Lengshuigou magmatic complex and Mihunzhen magmatic complex, etc., all of them are small rock blocks interruptedly distributed along the southern side of Fengzhen-Shanyang fault. These rock blocks, consisted of gabbro, diabase, quartz diorite, moyite, and monzogranite, fromed the Fengzheng-Shanyang accretionary complex-melange in Middle Qinling Orogenic Belt during the process of subduction-accretionary. The second period (Indosinian magmatism:227-199Ma) was represented by the granitic batholiths (including Dongjiangkou, Zhashui, Caoping and Shahewan). These granitic batholiths located in the northern part of Shanyang-Zhashui area, and consisted of monzogranite, biotite monzogranite, granodiorite, rapakivi granite, and so on. Mafic microgranular enclaves are common in these batholiths. The last period (Yanshanian magmatism:150.2-141.5Ma):Showing as the granite (porphyry) apophysis or stock, widely exposed in the researched region. Most of them consisted of diorite, quartz diorite, granodiorite and monzonitic granit; contain plagioclase, potassium feldspar, quartz and other major mineral; also contain dark minerals such as hornblende and biotite; and include accessory minerals like zircon, apatite, magnetite and titanite.Geochemical analysis indicated that plutons (porphyry) in Shanyang-Zhashui area belong to high-K calc-alkaline series (less samples belong to shoshonite series) with the metaluminous (most samples) to peraluminous (less samples) feature; geological and mineralogical characteristics testify their Ⅰ-type granite nature. Mg#ranging from19to68(most between45～60), correlation of biotite between FeO/(FeO+MgO) ratio and MgO, correlation of hornblende between TiO2and Al2O3and microgranular enclaves in the Chigou pluton proved the crust mantle migmatization in their magma source. Chondrite-normalized REE patterns showed ’shovel’ feature, no manifest Eu anomaly, enriched in large ion lithophile elements (such as K, Rb and Ba), and depleted in high field strength elements(e.g. Nb, Ta, Ti and P), indicated these plutons possess the common features belong to adakite and arc magmatic rocks; but the northern small plutons (porphyry) belong primarily to Adakite. Zircon εHf(t) range from-4.5to2.17, indicating that the magma may sourcing from high degree of magma mixing by crustal material and mantle material, and crustal melting are main ingredients. Hf isotopes two-stage model ages (TDM2),09～1.6Ga (most1.2～1.4Ga), indicated the magma formed by the melting of southern Qinling Orogenic Belt mafic lower crustal which formed in the period of Neoproterozoic to Mesoproterozoic can be the source of plutons (porphyry) from Shanyang-Zhashui area.Both mineralogical and geological characteristics show that these small plutons occurred under the tectonic setting from post-collision to extension, and belong to post-collision granitoid. After the north-south direction compressive stress disappeared in the period of late Jurassic to early Cretaceous, Qinling Orogenic Belt turned into the strong extension; large-scale magma activity were triggered; formed huge amount of crust-mantle mixed magma; finally ascended and emplaced to produce these plutons (porphyry) in Shanyang-Zhashui area.Potassic alteration, silicification, phyllic, chlorite and other porphyry alteration were widely developed in Shanyang-Zhashui area. Furthermore, Chigou porphyry deposits, Lengshuigou porphyry deposits and Xiaohekou porphyry deposits were discovered after the recent porphyry deposits studying and exploring in this area. Geological characteristics, lithogeochemical compostion and tectonic setting of these granite (porphyry) bodies are consistent with their counterpart of porphyry deposits in continental settings; but more deep depth of emplacement(8～14km), slightly enriched in Rb and Ba, no significant content of Sr, overmuch content of Mn, Y and Yb and relatively lower ratio of Sr/Y and La/Yb of these granite (porphyry) bodies differ from those characteristics of typical porphyry deposits at home and abroad. These differences are likely to be constraints on the mineralization intensity of porphyry-skarn deposits. As a result, there would be small to medium-sized porphyry-skarn deposits output in Shanyang-Zhashui area. |
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