滇西高黎貢造山帶板塊架構與其構造動力之演化

The tectonic and geodynamic evolution of the Gaoligon orogeny belt, West Yunnan, China

邱宇平(CHIU Yu-Ping)

指導教授 ： 葉孟宛 ； 李通藝

國立臺灣師範大學/理學院/地球科學系/博士(2023年)

https://doi.org/10.6345/NTNU202300267

摘要

從東喜馬拉雅構造結(EHS)到滇西有著獨特的彎曲造山帶，它凸向被隱沒的歐亞板塊，這一點和西邊的印度-歐亞碰撞帶的其他邊界相異，也和東邊的新特提斯洋的海溝不同。位於滇西的騰沖地塊內有兩條彎曲造山帶，最顯眼的是位於騰沖地塊東緣的高黎貢造山帶；而在騰沖地塊西側有另一條規模較小的彎曲岩漿帶，其岩漿活動和特提斯洋隱沒作用相關。本研究藉由分析四期塑性變形事件的時間和動力機制，發現高黎貢造山帶是一條非典型的彎曲造山帶。最早的D1事件使早白堊紀(鋯石鈾鉛定年為118-78 Ma)的花岡岩變形，且在新特提斯洋閉合期間形成直立褶皺，在高黎貢造山帶北段為西北西-東南東走向，而在南段為南-北走向。淡色花岡岩質副片麻岩(06CG20)的鋯石鈾鉛定年結果結合主要元素分析結果顯示，沉積物源自124 Ma的花岡岩，之後經歷風化、侵蝕和沉積，在110 Ma到73 Ma之間沉積，然後在65 Ma深融形成S型花岡岩；隨後在45-40 Ma時於中度角閃岩相以上的環境發生混合作用(migmatization)。本研究對騰沖地塊南部東河岩體附近的角閃岩、花岡閃長岩、片麻岩和淡色花岡岩進行岩石學和地球化學分析。變質閃長岩和角閃岩等暗色體(melanosome)具有SiO2含量低 (50-60%)、融化溫度高(920-1100°C)和CaO/Na2O比值高 (變質閃長岩為4.35、角閃岩為1.40-2.03) 的特徵，代表這些暗色體是由角閃岩脫水之後的殘留物集合而成。中色體(mesosome)和新體(neosome)具有高Na2O+CaO含量但較低的K2O含量。本研究結合岩石學分析和板塊重建結果，提出在新特提斯洋板片回滾(rollback)和斷裂(break-off)造成地殼減薄時，地函熱源上湧，伴隨東河犁型斷層(D2)發生混合作用。
隨後，由於Great India板塊的碰撞，東河滑脫面於40-28 Ma期間在高度角閃岩相的環境形成西北-東南走向、向東北逆衝的褶皺與逆衝斷層帶(D3)，並產生近水平的S3A葉理。此為高黎貢造山帶成形的最初的階段。然而，位於東側的奧陶紀基盤阻擋此褶皺與逆衝斷層帶，因此形成了鏟型的逆衝剪切面。當這個鏟型的逆衝剪切面沿著奧陶紀基盤被侵蝕和出露後，使得高黎貢造山帶成為看似沿著奧陶紀基盤彎曲的造山帶，因此為一「非典型」的彎曲造山帶。在D3的後期(35-28 Ma)，褶皺與逆衝斷層帶的前翼(forelimb)被高角度向東傾的左移走滑剪切帶(D3B)重利用，成為因印度向北擠壓而向東南拖曳的塊體邊界。左移走滑剪切帶(D3B)的S/C組構由白雲母和蠕狀石(myrmekite)所定義，顯示為中到低度角閃岩相的變質環境。之後，板塊架構因藏南下部地殼拆層而發生劇烈變化。
從高黎貢片麻岩和同D4形成的長英質岩脈中挑出來的白雲母(34-31 Ma)、黑雲母(23 Ma)和鉀長石(33-21 Ma)的氬氬定結果可連結礦物的封存溫度建立降溫曲線，發現溫度從約34 Ma時的550°C降到32Ma時的350°C，在22 Ma時再降溫到275°C。再連結由同構造礦物定義的變形溫度和降溫曲線，結果發現了「從碰撞擠壓(collision-extrusion; D3B)過渡到地殼碎裂流(crustal cataclastic flow; D4)」的混合模式。陡峭的南-北走向、右移走滑斷層於28 Ma到15 Ma期間，在高黎貢造山帶北段形成高黎貢剪切帶(D4)，並成為該區域最顯著的構造。綠泥石化的黑雲母和形成串腸的矽線石沿著D4的S/C組構排列，顯示環境從角閃岩相退變質到綠片岩相。這反應了增厚的西藏重力垮塌，造成沿著EHS順時針旋轉的碎裂流為中到上部地殼的變形環境。
至於另一條規模較小的彎曲岩漿帶，XRF主要元素分析和鈾鉛鋯石定年結果顯示其為63-62 Ma形成的I型花崗岩(A/CNK值為0.94-1.02)。整合了整個區域的岩漿事件之後，發現岩漿活動有隨時間和空間向西從S型花崗岩過渡到I型花崗岩的現象，反應板片從低角度隱沒轉變為高角度隱沒的回滾過程；顯示新特提斯洋的隱沒作用的確發生在騰沖地塊下方，且可能導致彎曲的岩漿帶形成。因此，本研究提出不對稱的「不規則的板片側緣模型 (irregular lateral slab edges model)」來解釋騰沖地塊的彎曲岩漿帶。這個模式必須隨時間逐漸累積變化，最後在60 Ma時於隱沒帶兩側產生高角度的板片回滾和海溝後撤(retreat)，而位於隱沒帶中段的騰沖地塊則是位於相對突出和板片低角度隱沒的位置，也因此會發生板片從低角度回滾演變為高角度回滾的狀況。然而，西藏下方的新特提斯洋板片斷裂(~50 Ma)和藏南下部地殼拆層(~28 Ma)使得這個「不規則的板片側緣模型」的隱沒系統被破壞，導致東南亞的地體架構在新生代發生劇烈改變，啟動數條地殼尺度的大型走滑斷層。

關鍵字

高黎貢 ； 碎裂流 ； 彎曲造山帶 ； 混雜岩 ； 構造反轉 ； 板片彎曲

並列摘要

The curvature from Eastern Himalayan syntaxis via East Myanmar to West Yunnan has a unique curvature of convex towards the mantle wedge, which is different from other boundaries of India, even the other trenches of Neo-Tethys. Such convex curvature is also strongly present in the Tengchong block, located in West Yunnan. The most apparent convex is along the boundary between Tengchong and Baoshan blocks called the Gaoligong orocline; the Neo-Tethys-related pluton presents the minor orocline at the western Tengchong block. The timing and kinematic mechanism of four deformation events are deciphered that the Gaoligong orocline is an atypical orocline which is sculptured by a long geological time. The earliest D1 event deformed the Early Cretaceous granit (zircon U-Pb ages of 118–78 Ma), forming the upright folds with a WNW-ESE–striking in the north and an N-S–striking in the south during the closure of the Neo-Tethyan ocean.
Then, the Early Cretaceous granites underwent weathering and sedimentation, forming the granite after diagenesis and anatexis. Our U-Pb dating results from the leucogranitic gneiss (06CG20) indicated a sedimentary origin from the Cretaceous granites (124 Ma). They deposited during 110-73 Ma that later became an S-type granite during 65 Ma, followed by sillimanite-graded migmatization of Mogok metamorphic belt during 45-40 Ma. A suite of amphibolite, granodiorite, gneiss, and leucogranite in the vicinity of Donghe pluton in the southern Tengchong block are examined with petrological and geochemical analyses. The geochemical characteristics of low SiO2 (50-60%) content, relatively higher melting temperature (920-1100 °C), and CaO/Na2O ratio of metadiorite (4.35) and amphibolite (1.40-2.03) indicated that these rocks represent the concentrated restite with hornblende dehydration breakdown reaction of the melanosome portion. Most diatexite of neosome and mesosome showed high Na2O+CaO content but low K2O content compared to anatectic melt and melanosomes. This study proposes that the migmatization occurred along Donghe listric detachment (D2) development by crustal thinning and mantle upwelling as the Neo-Tethys slab rolled back and broke off.
The Donghe detachment was reactivated as a thrust system (D3) between 40 and 28 Ma as the greater India hard collided with the Tengchong block. The Gaoligong orocline was first developed during this event, which formed a NW-SE trending, top-to-the-NE sense of shear that folded the once subhorizontal foliation (S3A) and formed a fold-thrust belt under upper-amphibolite-facies conditions. The Ordovician basement was the eastern boundary to block the D3A detachment, thus forming a shovel-shaped thrust belt. The convex part of the Gaoligong orocline is the intersection lineation between the topography and shovel-shaped thrust belt, resulting in a map-view geometry of curvature, hence an “atypical” orocline. In the later stages of D3, the locally left-lateral strike-slip shear zone reactivated and overprinted the front limb of the fold-thrust belt with moderate NE-dipping, NW-SE–striking, left-lateral shear zones (D3B) that accommodated the southeastward extrusion of Indochina around 35–28 Ma. The sinistral sense of shear S/C fabrics defined by muscovite folia with foliation-bounded myrmekite indicates that deformation occurred under middle- to lower-amphibolite-facies metamorphic conditions. Then the tectonic setting has undergone drastic changes after the delamination beneath Tibet.
The new 40Ar/39Ar ages for muscovite (34–31 Ma), biotite (23 Ma), and K-feldspar (33–21 Ma) mineral separate from the Gaoligong group, and a matrix crosscutting quartzofeldspathic dyke produce a cooling path. By interlinking the synkinematic metamorphic conditions with the reconstructed cooling path, I find that the temperature dropped from 550 °C around ca. 34 Ma to 350 °C at ca. 32 Ma and dropped again to 275 °C around ca. 22 Ma, and a hybrid tectonic model indicating extrusion (D3B) followed by crustal flow (D4) is established. The 28 Ma to 15 Ma steep N-S–trending, right-lateral Gaoligong shear belt (D4 in the northern section) is the dominant structural feature in this region. Chloritization of biotite and boudinaged sillimanite along S/C1 fabrics indicate that the crystalline rocks retrograded from amphibolite-facies into greenschist-facies conditions. This reveals the mid- to up crustal environment of the clockwise cataclastic flow, which developed around the Eastern Himalaya syntaxis due to gravitational collapse after delamination of the thickened Tibetan Plateau.
As for the minor orocline with the curved pluton, the XRF and U-Pb analyses determines the most eastern and older I-type affinity (A/CNK=0.94-1.02) magmatism (zircon U-Pb age of 63-62 Ma). Thus, the westward younger magmatism reveals the process of slab rollback from the flat to steep subduction, implying that the subduction of the Neo-Tethys slab beneath the Tengchong block was related to curvature magmatism. As a result, the asymmetric “irregular lateral slab edges model” was proposed to answer the unique curvature of the Tengchong block. The time-dependent slab buckling at the edges controls the irregularity of the subduction zone resulting in the convex towards the Tengchong block side because the Tengchong block was located at the center of the subduction zone. The delamination beneath the Lhasa blocks could further result in the transpressional stress field in Southeast Asia and gradually destroy the “irregular lateral slab edges model” by tearing the slab, triggering several crustal-scale strike-slip shear zones eventually.

並列關鍵字

Gaoligong ； cataclastic flow ； orocline ； migmatite ； tectonic inversion ； slab buckling

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