西藏甲玛埃达克质斑岩的地球化学特征及意义

分析了西藏甲玛石英闪长玢岩-二长花岗斑岩-花岗闪长斑岩岩相学、年代学及地球化学特征,研究其形成演化及构造环境,并建立岩浆成因模式。甲玛石英闪长玢岩-二长花岗斑岩-花岗闪长斑岩是具弧岩浆地球化学特征的准铝质高钾钙碱性岩岩石系列,同时具有O型(向C型过渡)埃达克质岩和I型(或磁铁矿型)花岗岩的特征。其成因与新特提斯洋板片脱水熔融形成的流体与地幔楔发生地幔交代作用形成的EMⅡ型富集地幔有关。中新世兰格期(15 Ma左右),由于拉萨地体东西向的"崩塌",导致拉萨地体开始伸展减薄,从而使交代地幔(EMⅡ型富集地幔)减压熔融,形成了富含Cu、Au等成矿物质的具I型(或磁铁矿型)弧岩浆地球化学特征的准铝质富钾O型埃达克质铁镁质岩浆,并在沿该时期的正断层及裂谷系统侵位过程中,不断与下地壳高钾及中—高Fe/Mg的花岗质岩浆发生不同程度的岩浆混合作用,伴随着相对较弱的岩浆结晶分异过程不断侵位浅地表,进而形成了独特的石英闪长玢岩-二长花岗斑岩-花岗闪长斑岩岩石组合。

Geochemical Characteristics and Significance of the Jiama Adakitic Porphyry,Tibet

Based on the petrography,chronology and geochemical dates,as well as the studies on the formation,evolution and tectonic settings of the quartz diorite porphyrite,adamellite porphyry and granodiorite porphyry in Jiama deposits,Tibet,a magmatic evolution model has been set up.We concluded that Jiama quartz diorite-porphyrite-adamellite porphyry-granodiorite porphyry belong to a same consanguinity magma evolution series of arc magma geochemical affinity: being metaluminum,high potassium and calcium-alkaline.They are of the characters of both I-type(megnet-type-type) granite and O-type(translate to C-type) adakite.Their formation are related to the EM II enriched mantle formed by dehydration and metasomatism of the mantle wedge by the fluids from the Neo-Tethys plates.Due to the E-W collapse(about 15 Ma),the Lhasa terrane starts to stretch and thin,lead to melting of the EMⅡ-type enriched mantle under decompression and resulted the formation of O-type adakitic mafic magma rich in Cu,Au and so on.These adakitic mafic magma mixed with the granitic magma of high K and middle-high Fe/Mg features during ascend along normal faults in rift system,finally led to the formation of quartz diorite-porphyrite-adamellite porphyry-granodiorite porphyry assemblage in the deposits at the shallow crust.