雅魯藏布縫合帶澤當(dāng)段蛇綠巖的地球化學(xué)特征及構(gòu)造意義

1、雅魯藏布縫合帶澤當(dāng)段蛇綠巖的地球化學(xué)特征及構(gòu)造意義        雅魯藏布縫合帶代表曾經(jīng)存在于印度板塊與亞歐板塊之間的新特提斯古大洋殘余。整個縫合帶可分為西、中、東三段，澤當(dāng)段屬縫合帶東段并由澤當(dāng)蛇綠巖和一套島弧巖石組合混雜組成。作為東段最典型的剖面之一，對澤當(dāng)蛇綠巖的研究一直比較溥弱，其構(gòu)造環(huán)境也存在爭議。澤當(dāng)?shù)貐^(qū)不同于其他大多數(shù)僅以蛇綠巖為主體的縫合帶地段，除了蛇綠巖，該區(qū)還保存著一套島弧巖石，可能代表曾經(jīng)于新特提斯洋內(nèi)發(fā)育的島弧。新特提斯的構(gòu)造演化一直存在單一俯沖和雙重俯沖的爭論，關(guān)鍵問題就在于缺乏洋內(nèi)俯沖帶

2、和島弧存在的證據(jù)。從這個角度來說，對澤當(dāng)段縫合帶及蛇綠巖的詳細(xì)研究，不但有助于重建特提斯古洋盆格局，對進(jìn)一步認(rèn)識印度-亞歐板塊的匯聚和碰撞過程也有重要意義。澤當(dāng)蛇綠巖單元出露基本齊傘，包括地幔橄欖巖、輝長輝綠巖、塊狀和枕狀玄武巖。對澤當(dāng)蛇綠巖玄武巖進(jìn)行的Sm-Nd同位素分析，得一等時線年齡為175+20Ma，代表了蛇綠巖的形成時代。玄武巖的Nd(t)值為+7.0+7.3，表明原始巖漿來源于強(qiáng)烈虧損的地幔源區(qū)。澤當(dāng)蛇綠巖中的地幔橄欖巖以方輝橄欖巖為主，具有低Ti、Al，高M(jìn)g的巖石化學(xué)特征，REE表現(xiàn)為“U”型的分布型式，屬典型的殘余地幔橄欖巖。輝長輝綠巖樣品都具有LREE虧損的左傾分布型式和E

3、u正異常，其微量元素絕對含量很低，表現(xiàn)出明顯的Nb、Ta、Zr、Hf虧損和Cs、Rb、Sr、Ba、K富集。澤當(dāng)蛇綠巖殼層熔巖根據(jù)地球化學(xué)特征可分為兩組：玄武巖(接近N-MORB)和玄武巖(具IAT特征)。兩組巖石均表現(xiàn)為平滑的左傾LREE虧損型分布和基本相似的微量元素分布型式，但玄武巖的元素豐度明顯要高。玄武巖比玄武巖遭受到更強(qiáng)的俯沖作用影響，具有明顯的Nb、Ta虧損。澤當(dāng)蛇綠巖表現(xiàn)出島弧和MORB的雙重地球化學(xué)特征，屬于SSZ型蛇綠巖，結(jié)合區(qū)域地質(zhì)背景，推測其形成于俯沖帶之上的弧前盆地。首次從澤當(dāng)島弧巖石組合中厘定出一套埃達(dá)克質(zhì)英云閃長巖，是俯沖洋殼在角閃巖-榴輝巖相過渡帶部分熔融的產(chǎn)物。這

4、一認(rèn)識為新特提斯洋內(nèi)俯沖帶和島弧的存在提供了重要證據(jù)。在前人研究的基礎(chǔ)上，本次研究認(rèn)為澤當(dāng)?shù)貐^(qū)新特提斯洋存在北向的洋內(nèi)俯沖系統(tǒng)，澤當(dāng)蛇綠巖和澤當(dāng)島弧均為洋內(nèi)俯沖的產(chǎn)物。中侏羅世時，新特提斯洋內(nèi)發(fā)育一條幔內(nèi)型推覆韌性剪切帶，印度板塊和拉薩地塊對大洋巖石圈的雙向擠壓誘發(fā)了洋內(nèi)俯沖消減。俯沖板片向下消減作用的加強(qiáng)使得位于洋內(nèi)俯沖帶之上的大洋板塊發(fā)生局部拉張，形成擴(kuò)張脊。大量來自俯沖板片的水和大離子親石元素進(jìn)入虧損地幔楔，使之發(fā)生部分熔融，生成的巖漿沿著擴(kuò)張脊在海底噴發(fā)，形成了新的SSZ型洋殼和洋盆，時代為175Ma左右。同時，早期發(fā)育的老洋殼在北面開始沿拉薩地塊南緣俯沖消減，大約174Ma時在岡底斯

5、南緣生成了葉巴弧。由此形成了新特提斯洋的雙重俯沖格局。在這期間，年輕的熱的洋殼俯沖至75-85km深度左右時發(fā)生部分熔融并形成埃達(dá)克巖漿。俯沖板片的連續(xù)俯沖作用還導(dǎo)致了澤當(dāng)蛇綠巖北側(cè)洋內(nèi)島弧的形成，其活動時代約為156Ma。在晚白堊世，洋內(nèi)俯沖系統(tǒng)仰沖到南面的印度板塊之上，新特提斯洋洋盆停止擴(kuò)張，轉(zhuǎn)變?yōu)閱胃_格局。由于印度板塊向北強(qiáng)烈推擠，沿拉薩地體南緣的俯沖消減活動更加強(qiáng)烈，火山巖漿活動十分廣泛，進(jìn)一步形成了大量中酸性的鈣堿性島弧巖漿巖，如桑日島弧等。始新世時，印度板塊和亞歐板塊發(fā)生陸-陸碰撞，新特提斯洋關(guān)閉消亡，青藏高原開始隆升，縫合帶及相鄰地質(zhì)體被后期構(gòu)造運(yùn)動改造成如今的面貌The Ya

6、rlung-Zangbo surture zone marks the Neo-Tethyan remnants existed between the Indian plate and the Eurasian plate. The suture zone can be divided into three parts, namely, the western, middle and eastern segments. Zedong suture located within the eastern segment, comprise a suit of melange made of Ze

7、dong ophiolite and island arc volcanic assemblage. Although the ophiolite has undergone complicated tectonic movements, its sequence is basically completed, composed mainly of peridotite, diabase and gabbro dyke swarms and pillowed and massive basalts.The Sm-Nd isochron age of basalts in Zedong ophi

8、olite is 175±20Ma, indicating that the ophiolite formed in the mid-Jurassic. The eNd(t) values of basalts range in +7.0-+7.3, suggesting a strong depleted mantle source and no crustal contamination.The mantle peridotite mostly consists of harzburgite, showing low Ti, Al, high Mg, and the "

9、U" type REE patterns, and belonging to the residual mantle peridotite. The gabbro and diabase all have LREE-depleted patterns and positive Eu anomaly. The rece elements abundance is low, exhibiting the obvious depletion of Nb, Ta, Zr, Hf and enrichment of Cs, Rb, Sr, Ba, K. According to the geo

10、chemical properties, the crustal lava of Zedong ophiolite can be divided into two groups: basalt I (close to N-MORB) and basalt II (is of IAT). Two groups of rocks all indicate smooth LREE-depleted REE patterns and similar distributions of race elements, and the element contents of basalt I are high

11、er. Basalt II underwent the stronger subduction than ophiolite I, exhibiting obvious Nb and Ta depletion.Zedong ophiolite shows the compound geochemistry characterisitics of island arc and MORB, belonging to the SSZ-type ophiolite. Considering the geological setting, we conclude it formed in the for

12、e-arc basin above the subduction zone.The subducted-type adakites in the Zedong segment of the Yarlungzangbo suture zone were discovered and identified for the first time. Zedong adakites were produced by the partial melting of young and hot subducted oceanic crust in amphibolite-eclogite transition

13、 zone and by going through the mantle wedge. This work presents new evidence for the intra-Tethyan subduction and the previous suggestion about the existence of intra-oceanic island arc within Tethys.Based on the former studies, a north directed intra-ocean subduction system once existed in Zedong T

14、ethyan ocean, and the Zedong ophiolite and island arc are all the production of intra-oceanic subduction. In mid-Jurassic, there was a ductile nappe shear zone developing in Neo-Tethyan ocean, and at the same time, the bidirectional compressing of Indian plate and Lhasa block on the oceanic lithosph

15、ere induced the intra-oceanic subduction. The reinforce of slab subduction resulted in the local extension of oceanic plate above the subduction zone and the formation of spreading ridges. Plenty of H_2O and LILE entered the depleted mantle wedge, led to the partial melting. Along the spreading ridg

16、es, the magma extruded on the sea floor and formed the new SSZ-type oceanic crust and basin(175Ma). Meanwhile, the old oceanic crust started to subduct toward the Lhasa block, and produced the Yeba-arc at the south edge of Gangdese. The Tethyan double subduction situation came into being. During thi

17、s period, the partial melting of young and hot subducted oceanic crust at 75-85km produced he adaktitic magma. The subduction also led to the formation of the Zedong intra-oceanic island arc in the north, which is active at 156Ma.At late Cretaceous, the Neo-Tethyan ocean basin stopped to spread and

18、turned to be the single subduction situation, and then the intra-oceanic subduction system obducted onto the Indian plate in the south. Because of the northward compressing of Indian plate, the subduction along the south edge of Lhasa block was stronger. The volcanic magma activities were extensive, producing abundant intermediate-acid calcium alkaline island arc igneous rocks. In Eocene, the Indian