Geochronology And Geochemistry Of The Yongzhu Granitoids In Middle-north Gangdese, Tibet

There are many Mesozoic magmatic rocks in middle-north Gangdese block, theirpetrogenesis and geological setting has long been a debate up to now. In this paper wereport Yongzhu granitoids in middle-north Gangdese and detailedly study petrology,geochemistry and zircon U–Pb ages of them, attempting to offer informations for solvingthese questions.According to characteristics of outcrop and zircon U–Pb ages, Yongzhu granitoidscan be divided into three stages, including Early Jurassic(176.4Ma), EarlyCretaceous(120~110Ma) and Late Cretaceous(~75Ma). Early Jurassic granite includesgranodiorite with gneissic structure. Geochemically, Early Jurassic granite belongs toperaluminous and medium-K-calc-alkaline granite. All these samples are enriched inLREE and depleted in HREE with significant negative Eu anomalies; LILE are enrichedand HFSE are depleted. Early Cretaceous granite includes granodiorite, porphyriticmonzogranite with abundant dioritic enclaves. The skarnized and mabled zone occures atzone granite and carbonatite contact. Geochemically, Early Cretaceous granite and dioriticenclave belongs to aluminous and medium-high-K-calc-alkaline granite. All these samplesare enriched in LREE and depleted in HREE; LILE are enriched and HFSE are depleted.Late Cretaceous includes monzogranite and porphyritic monzogranite with abundantdioritic enclaves. The contact zone between granite and carbonatite is skarnized zone.Geochemically, Late Cretaceous granite and dioritic enclave belongs to aluminous andmedium-high-K-calc-alkaline granite. All these samples are enriched in LREE and depleted in HREE; LILE are enriched and HFSE are depleted. Early Jurassic granite isundifferentiated S-type granite, while Early and Late Cretaceous granites areundifferentiated I-type granite. They are all characterized by high TZr, Mg#, Cr and Ni,which demonstrate the mantle-derived magma most likely involved in the petrogenesis ofYongzhu granitoids. The Cretaceous granite is likely related to magma mixing. The sourcerocks of Jurassic granite is sedimentary or metasedimentary rocks from middle-upper crust,while the source rocks of Cretaceous granites are metamorphic magmatic rock frommiddle-upper crust, with the hornblende and plagioclase as restite.Early Jurassic peraluminous, undifferentiated S-type granite maybe formed in thesetting of double subducton of Bangong-Nujiang Tethyan Ocean and Yarlung ZangboTethyan Ocean which caused the colliding middle-Gangdese block and meltingsedimentary or metasedimentary rocks from middle-upper crust(It may be affected bymantle-derived magma). Early Cretaceous undifferentiated I-tpye granite maybe formed inthe setting of south subducton of Bangong-Nujiang Tethyan Ocean and subduction slabbreak-off which made asthenosphere materials rise and lithosphere mantle and crust melt.In the result, basic magma and acidic magma formed and then they mixed. LateCretaceous undifferentiated I-tpye granite maybe formed in the setting of post-collisiontectonic setting which was late than Qiangtang-Lhasa block collision. The partialdelamination of thickened lithosphere should also be accompanied by rising asthenospherematerials and underplated mantle-derived magma which made middle-upper crust meltforming acidic magma. And then mantle-derived magma and acidic magma mixed. Thedynamic setting shows subducton of Bangong-Nujiang Tethyan Ocean, Qiangtang-Lhasablock collision and collapse after collision in middle-north Gangdese, in Mesozoic.Mineralization is related to Early and Late Cretaceous granites. The mineralizationtype is mainly skarned copper mineralization.