【摘要】：在現(xiàn)今活動(dòng)的俯沖帶系統(tǒng)中,俯沖板片的后撤或回卷都可能導(dǎo)致弧前伸展作用,但弧前伸展作用是否局限于淺部構(gòu)造層次,還是涉及整個(gè)巖石圈,是理解島弧構(gòu)造演化和地球化學(xué)效應(yīng)的重要課題。在西藏岡底斯巖基南緣,保留著世界上典型的弧前盆地——日喀則弧前盆地。在該盆地的北緣,除了發(fā)育中新世閃長(zhǎng)玢巖脈外,還發(fā)育一系列近東西向展布的輝綠巖脈,形成時(shí)代為106.6±0.8Ma(鋯石U-Pb年齡)�；詭r脈在巖石學(xué)、礦物學(xué)和地球化學(xué)特征上較均一,具體表現(xiàn)為:(1)富集LREE,虧損HREE,但HREE平直;(2)強(qiáng)烈虧損Rb、Ba、Nb和Ta,微弱虧損Ti,但Zr和Hf不具虧損現(xiàn)象;(3)鋯石Hf同位素組成(εHf(t)=+11.7~+15.4)較高;和(4)包含捕獲的早侏羅世巖漿鋯石。上述數(shù)據(jù)表明:在日喀則弧前盆地形成過(guò)程中,伴隨著強(qiáng)烈的早白堊紀(jì)弧前伸展作用,誘發(fā)較虧損地幔發(fā)生部分熔融,形成不同于典型島弧巖漿巖的基性巖,可能是在新特提斯洋北向俯沖過(guò)程中,海溝后撤,在弧前形成強(qiáng)烈的地幔角流的結(jié)果。同時(shí),日喀則弧前盆地可能構(gòu)建于早侏羅紀(jì)結(jié)晶巖系之上。
[Abstract]:In the present active subduction zone system, the retreating or rewinding of the subduction plate may lead to the prearc extension, but whether the prearc extension is confined to the shallow tectonic level or involves the whole lithosphere, It is an important subject to understand the tectonic evolution and geochemical effects of island arc. In the southern margin of the Gangdis rock base in Tibet, the world's typical forearc basin-Xigaze forearc basin is preserved. In the northern margin of the basin, in addition to the developing diorite porphyrite veins, a series of near east-west diabase veins have been developed. The age of formation is 106.6 鹵0.8Ma (U-Pb age of zircon). The basic veins are more homogeneous in petrology, mineralogy and geochemistry, The main results are as follows: (1) rich in LREE, depleted in hree, but flat in HREE; (2) strongly depleted in RbbBaNb and Ta, with weak depletion of Ti, but not depleted in Zr and HF; (3) higher in zircon HF isotopic composition (蔚 Hf (t) = 11.7 ~ 15.4); and (4) containing trapped early Jurassic magmatic zircon. The above data indicate that during the formation of Xigaze forearc basin, along with the strong early Cretaceous prearc extension, partial melting of the depleted mantle was induced, resulting in the formation of basic rocks different from typical island arc magmatic rocks. It is possible that the trench retreated during the northward subduction of the Neo-Tethys ocean and a strong mantle corner current was formed in front of the arc. At the same time, Xigaze forearc basin may be built on the early Jurassic crystalline rock series.
【作者單位】： 中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所;中國(guó)地質(zhì)科學(xué)院礦產(chǎn)資源研究所;
【基金】：公益性行業(yè)科研專(zhuān)項(xiàng)(201511022) 國(guó)家重點(diǎn)研發(fā)計(jì)劃(2016YFC0600304) 國(guó)家自然科學(xué)基金項(xiàng)目(41425010)聯(lián)合資助
【分類(lèi)號(hào)】：P534.53;P542

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