西藏念青唐古拉山帕戈勒中新世二云母花岗岩成因机制及对稀有金属铍成矿能力的指示

Genetic mechanism and its implications for the capacity of rare beryllium mineralization of Miocene two-mica granite in the Pagele,Nianqingtanggula Mountain,Tibet

淡色花岗岩是研究浅部地壳物质成分和碰撞造山带地球动力学过程的理想对象,也是稀有金属成矿的重要载体。本文研究的帕戈勒二云母花岗岩发育在西藏冈底斯念青唐古拉复式花岗岩岩基的西南侧,以岩株和岩枝形式产出,局部偶见绿柱石。精细的锆石U-Pb定年显示,岩体结晶成岩年龄为18.4 Ma和17.6 Ma,形成于中新世。岩体具有高的SiO_(2)(72.78%~75.53%)和K_(2)O(4.83%~5.52%)含量,低的Al_(2)O_(3)(13.42%~14.60%)、MgO(0.10%~0.24%)、TFe_(2)O_(3)(0.86%~1.65%)含量,A/CNK为1.04~1.12,属于过铝质高钾钙碱性-钾玄质花岗岩。(^(87)Sr/^(86)Sr)i=0.712139~0.713545,εNd(t)=-9.0~-8.3,Nd二阶段模式年龄tDM2=1211~1468 Ma,^(206)Pb/^(204)Pb=18.797~18.841,^(207)Pb/^(204)Pb=15.755~15.785,^(208)Pb/^(204)Pb=39.425~39.519,ε_(Hf)(t)=-6.8~+2.0,Hf二阶段模式年龄tDM2值为970~1529 Ma,显示岩浆源区以中上地壳元古宇念青唐古拉群变质岩为主(70%~80%),混合有少量的幔源成分(20%~30%)。LREE/HREE=4.30~9.61,(La/Yb)_(N)=3.61~13.42,δEu=0.16~0.34,DI分异指数=95~97,K/Rb=76.51~116.39,且Rb、Th和U相对富集,P、Zr和Ti强烈亏损,Rb-Ba-Sr分异特征明显,暗示岩浆经历了强烈的钾长石、斜长石、锆石、磷灰石和黑云母分离结晶作用。结合区域地质资料,本文认为包括帕戈勒二云母花岗岩在内的念青唐古拉中新世花岗岩形成于印度大陆俯冲板片撕裂背景下,是软流圈地幔上涌诱发幔源岩浆沿着构造薄弱带上侵,并引发中上地壳泥质变质岩发生白云母脱水熔融形成壳源熔体,其与幔源岩浆混合并经历不同程度的结晶分异所形成。帕戈勒二云母花岗岩中局部偶见绿柱石,可能与富铍中上地壳部分熔融、强烈结晶分异和F-Be络合迁移等铍的富集机制有关。由这一富铍花岗质岩浆体系进一步演化形成的、分布在其外围的富铍花岗伟晶岩具有较大的铍成矿能力。

Leucogranite is an ideal object to study the composition of shallow crustal and geodynamic process of the collision orogenic belt,and also an important carrier of rare metal mineralization.The Pagele two-mica granite studied in this paper are developed in the southwest of the compound granitic batholith in the Nianqingtanggula Mountain,Gangdese.It is produced in the form of stocks and apophyses,and occasionally contains beryl.Precise zircon U-Pb dating shows that its crystallization age is 18.4 Ma and 17.6 Ma,corresponding to Miocene.The rock has high content of SiO_(2)(72.78%~75.53%),K_(2)O(4.83%~5.52%)and A/CNK(1.04~1.12),and low content of Al_(2)O_(3)(13.42%~14.60%),MgO(0.10%~0.24%),TFe_(2)O_(3)(0.86%~1.65%),and belongs to the peraluminous high-K calc-alkaline and shoshonitic granite.(^(87)Sr/~(86)Sr)_i=0.712139~0.713545,ε_(Nd)(t)=-9.0~-8.3,t_(DM2)=1211~1468 Ma,^(206)Pb/~(204)Pb=18.797~18.841,~(207)Pb/~(204)Pb=15.755~15.785,^(208)Pb/~(204)Pb=39.425~39.519,ε_(Hf)(t)=-6.8~2.0,t_(DM2)=970~1529 Ma,showing that the source was composed of main crust-derived melts(70%~80%)originating from the Proterozoic Nianqingtanggula Group metamorphic rocks in the middle and upper crust,and a small amount of mantle-derived melts(20%~30%).LREE/HREE=4.30~9.61,(La/Yb)_(N)=3.61~13.42,δEu=0.16~0.34,Differentiation Index(DI)=95~97,K/Rb=76.51~116.39.Rb,Th and U are relatively enriched.P,Zr,and Ti are strongly depleted and the Rb-Ba-Sr differentiation is obvious.They indicate that the magma may have undergone strong crystallization of K-feldspar,plagioclase,zircon,apatite and biotite.Combined with regional geological data,it is believed that the Miocene granite in the Nianqingtanggula area,including the Pagele mica granite,probably formed under the background of the tectonically weak belt in the upper-middle crust caused by tearing of the subducted Indian continent plate.The upwelling of asthenosphere induced the mantle magma to invade along the weak zone,and caused the dehydration and melting of muscovite in the pelitic metamorphic rocks in the upper-middle crust to form the crust-derived magma.Subsquently,the mantle and crust-derived magmas mixed and underwent different degrees of crystal differentiation.The rare occurrence of beryl in the Pagole two-mica granite may be related to the enrichment mechanism of partial melting of beryllium-rich upper-middle crust,strong crystallization differentiation and F-Be complexation.The beryllium-rich granitic pegmatite,evolved from the Pagole beryllium-rich two-mica granitic magmatic system,is likely to have great beryllium mineralization capacity.