Geochemical Characteristics And The Relationship For Cu、Mo Ore Forming Of Cretaceous Granitoids In Gangdese-xiaChayu Volcanic-magmatic Arc

Cretaceous Granitoids located between Indus-Yarlung suture and Bangong Nujiang Suture Zone in Gangdese-xiaChayu Volcanic-magmatic Arc, make up an important part of Gangdise metallogenic belt in Tibet. Cretaceous Granitoids distribute mainly along the south edge of Gangdese-xiaChayu from the east to west there, with an exposed area of over12000km2.Through fieldwork and data sorting combined with a systematic analysis and study on petrography, petrochemistry, petrogeochemistry, mineral chemistry, Sr-Nd isotope and isotope chronology, this thesis tries to achieve a basic understanding of such things as the causes, the magma sources, the temperature and the pressure condition, the tectonic setting of Cretaceous granitoids in this area. The relationship between the causes of Cretaceous granitoids and the evolutionary of gangdise collisional belt has also been studied in the thesis. With an analysis by isotope geochronology, it is drew out that the isotopic age of Early Cretaceous Granitoids is between104Ma and161.1Ma while Late Cretaceous Granitoids between65Ma and78.2Ma.Combined with previous materials, data and achievements during the fieldwork time of this thesis, by mapping units emplacement process they were divided into the following kinds:middle size quartz monzobiorite、middle size biotite monzogranite、middle porphyritic biotite hornblende monzonitic granite、fine biotite monzogranite、middle size grained porphyritic biotite monzogranite、middle-coarse grained porphyritic biotite monzogranite Each of them has different formations and geological characteristics.The common rock forming minerals in Cretaceous granitoids are quartz, andesine, oligoclase, orthoclase,common hornblende, oxybiotite. The common accessory minerals are magnetite and sphene in Early Cretaceous Granitoids.The common rock forming minerals in Granitoids Late Cretaceous Granitoids.are lots of quartz, plagioclase and so on. The common accessory minerals are magnetite, sphene, zircon and monazite geochronology.Invariable element geochemistry of Cretaceous Granitoids in this area has a characteristic as follows:the average quantity of SiO2contained in Early Cretaceous Granitoids is68.43。Al2O3>(Na2O+K2O+Ca0), with a overall high quantity of Silicon and Al2O3, Na20and K2O while a low quantity of MgO andMg#, meaning that they belongs to Calcium-Calcium Alkaline Granite. On the other hand, the average quantity of SiO2contained in Late Cretaceous Granitoids is71.15, Al2O3>(Na2O+K2O+CaO), meaning that they belongs to High K calc alkaline granitoids. The rock type developed from acid to alkaline from the Early Cretaceous to the Late Cretaceous. The magma demonstrated a characteristic of evolution from Type I to Type S. As for the components of the rare earth and trace elements, the situation is very obvious that light rare earth elements are of large amount while heavy rare earth elements are of small amount. The quantity of Lithophile elements such as B, Li are gradually increasing while that of ore forming elements such as Cu、Zn、Pb、Au and compatible elements such as Cr、Ni、Rb is reducing; Rb/Sr increasing while rates such as K/Rb、Rb/Cs、 Rb/Li are reducing from Early Cretaceous to Late Cretaceous. Intermediate phase is common there with an prevalent emplacement depth of2.5-10km. Early Cretaceous Granitoids form in the places where the temperature is from700℃to800℃and the pressure of more than3108P while Late Cretaceous Granitoids form in the similar places where the temperature is from680℃to750℃and the pressure of less than3×108P.With a comprehensive analysis and judgment, Cretaceous Granitoids here are formed by marginal volcanic arc associated with subduction. During the subduction between Indian Plate and Eurasian platein the later stage of Yanhanian, Cretaceous Granitoids in Gangdese-xiaChayu became an active continental edge and formed a volcano magmatic arc. Then its dramatic subduction went on and the anatecticmagma intruded from different directions. Magma in its way was fused and evoluted under the action of stress got an active emplacement like a hot air balloon exploration. A concentric ring distribution pattern was formed.This area has a favorable condition for ores and bears a suggestion of ore-forming geological condition. Meanwhile, geochemical prospecting, geology and synthetic images get result of a large-scale. Elements get on well with a distinguished concentration and high abnormal value as well as a favorable ore formation situation. All those ensure this area a bright prospect place for ore exploration.