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  • 學位論文

藏東南白堊紀至古新世花崗岩類與火山岩之地球化學及岩石成因

Geochemical Constraints on the Petrogenesis of Cretaceous to Paleocene Granitoids and Volcanic Rocks from SE Tibet

指導教授 : 鍾孫霖

摘要


新特提斯洋板塊向北隱沒至南亞大陸邊緣之下,所造成的一系列安地斯型岩漿活動,始於早侏儸紀(≥190 Ma),持續至早第三紀(~45 Ma)結束。此一島弧岩漿活動,在藏南拉薩陸塊南緣形成廣大的岡底斯岩體;然而,該岩體向東及向東南方向應如何延伸仍不明確。因此,本論文乃針對出露在青藏高原東南部的花崗岩,首次進行了較有系統的野外工作與地球化學研究。 研究地區位於藏東南波密和察隅兩地之間(東經95.5-97.5度,北緯28.5-30度),由西北向東南涵蓋了:阿扎貢岩體、德母拉岩體、察隅岩體、及沙馬岩體等四個主要岩體;此外,還針對研究區域中,出露於然烏地區的早白堊紀火山岩進行對比研究。 鋯石U-Pb定年結果顯示,本地區花崗岩類的形成年齡主要分布在白堊紀(~133-107Ma)和古新世(~60Ma)兩時段;就岩性而言,除了大範圍出露的鈣鹼性花崗岩類之外,在德母拉岩體和察隅岩體,分別發現了A型花崗岩和高分異的S 型花崗岩。所有的花崗岩類皆具有高場力鍵結元素虧損、大離子親石元素以及輕稀土元素族的富集等現象,蛛網圖分佈上,亦均顯示了隱沒帶或活動大陸邊緣島弧火成岩的地球化學特徵。A型花崗岩為一種在隱沒帶甚少出現的花崗岩類型,而本研究發現的德母拉岩體A型花崗岩,具有高鉀(Na2O+K2O= 7.1-8.3 wt.%;K2O= 4.5-5.5 wt.%;Rb= 491-713 ppm)、高的Ga/Al、Fe2O3T/MgO比值、重稀土元素富集[La= 20.1-54.5 ppm;(La/Yb)n= 1.5-4.9],明顯的Eu負異常[(Eu/Eu*)= 0.02-0.04],低Ba(10-46 ppm)、以及低Sr(6.5-11.5 ppm)等典型的A型花崗岩特徵。 波密-察隅地區花崗岩類總體而言具有富集的鍶-釹同位素組成 [εNd(T)介於-1.0至-13,ISr介於0.705-0.746之間];若與拉薩陸塊內兩條主要的侵入岩帶資料對比:即位於陸塊南部的岡底斯岩帶 [(1)白堊紀岡底斯:Nd(T)=+5.3至+2,~103-80Ma;及(2)第三紀岡底斯:εNd(T)=+5.3至-0.5,~65-45Ma;Wen,2007]和陸塊北部的北部岩漿岩帶 [εNd(T)= -1.0至-13,~150-50 Ma;Wen,2007],其同位素特徵類似拉薩陸塊北部岩漿岩帶。此外,然烏火山岩屬高鉀鈣鹼系列(玄武岩-英安岩:SiO2=46-69 wt.%; Na2O+K2O=4.07-6.35 wt.%),在同位素組成上,然烏火山岩具有較高的εNd(T)(+3至-6),而ISr分布範圍變化不大,介於0.705至0.707之間。 由鍶-釹同位素組成推斷,波密-察隅地區花崗岩類的岩漿源區主要有三,分別為:地幔楔、下部大陸地殼及上部大陸地殼。據此,我們提出兩階段岩漿分異的成因模式:(1)由地幔楔熔融上升之基性岩漿,侵入/底侵於下部大陸地殼底部或莫合面附近,經結晶分異及地殼混染後直接噴出地表,形成然烏火山岩;(2)上述分異岩漿,上升侵入於上部大陸地殼淺處造成的岩漿庫中,再經結晶分異及地殼混染,最終形成波密-察隅花崗岩體。此外,後者可能造成位於岩漿庫頂部或周邊之鈣鹼性圍岩(P≦4kbar;Patino-Douce,1997)的小程度部分融熔,因而形成德母拉岩體之A 型花崗岩。 就形成年代、地球化學以及同位素特徵等綜合分析,藏東南波密-察隅岩體,與拉薩陸塊的北部岩漿岩帶較為相似,較宜與之對比。因此,拉薩陸塊南部岡底斯岩體之向東以至東南的延伸能存在更南側的喜馬拉雅山脈或印度占領區內,猶待日後研究驗證。

並列摘要


Northward subduction of the Neo-Tethyan oceanic lithosphere (新特提斯洋板塊) beneath South Asia gave rise to an Andean-type convergent margin with arc magmatism starting since at least the early Jurassic (>190 Ma) and lasting until the Eocene (~45 Ma). Compared with the Gangdese (岡底斯) Batholith, i.e., the largest intrusive exposure thus formed in the southern Lhasa (拉薩) terrane, central part of southern Tibet, the temporal and spatial distribution of the arc magmatic product in its eastern/southeastern extension remains poorly understood. This thesis reports the first systematic investigation of granitoids from the Bomi (波密) to Chayu (察隅) area, between ~95.5°E and 97.5°E, and ~30°N and 28.5°N, the southeastern Tibetan plateau. From NW to SE, four batholiths, namely, Achakung (阿扎貢), Temula (德母拉), Chayu (察隅) and Shama (沙馬), were investigated. Moreover, a comparative study of early Cretaceous volcanic rocks from Ranwu (然烏) in the same region was also performed. Zircon U-Pb age data show that these granitoids were emplaced in the Cretaceous (~133-110) and Paleocene (~60Ma), respectively. In addition to the widespread calc-alkaline granitoids, A-type granites and highly fractionated S-type granites are observed in the Temula and Chayu batholiths, respectively. All the granitoids are characterized with depletions in the high field strength elements (HFSE), enrichment in the large ion lithophile elements (LILE) and light rare earth elements (LREE), and show spidergrams similar to those of typical arc magmas from subduction zones or active continental margins. The A-type granites, a rock type rarely reported in the convergent margin settings, are characterized by high alkalis (Na2O+K2O= 7.1-8.3 wt.%; K2O= 4.5-5.5 wt.%; Rb= 491-713 ppm), high Ga/Al, high Fe2O3 T/MgO, and high HREE [La= 20.1-54.5 ppm; (La/Yb)n= 1.5-4.9], coupled with large negative Eu anomalies [(Eu/Eu*)= 0.02-0.04], and low Ba (10-46 ppm) and Sr (6.5-11.5 ppm) contents. The granitoids from the Bomi to Chayu area, as a whole show heterogeneous Sr and Nd isotope compositions, with εNd(T) values varying from -1.0 to -13 and ISr ratios from 0.705 to 0.746, which correspond in general to those of S-type granitoids from the northern plutonic belt of the Lhasa terrane [εNd(T)= -4 to -13, aged ~150-85 Ma; Wen, 2007] and differ markedly from those of the I-type Gangdese Batholith [εNd(T)= +5.3 to +2, aged ~103-80 Ma; εNd(T)= +5.3 to -0.5, aged ~65-45 Ma; Wen, 2007]. The Ranwu volcanic rocks, have a wide range of compositions from basaltic to dacitic (SiO2= 46-69 wt. %), are calc-alkaline in nature. Relative to the neighboring granitoids, the volcanic rocks have higher εNd(T) (+3 to -6) and lower ISr ratios (0.705 to 0.707) that suggest involvement of the lower continental crust to have played an important role together with fractional crystallization in the petrogenesis. The overall Sr-Nd isotope compositions of the granitoids, furthermore, suggest three source components in the magma generation, namely, the mantle wedge, the lower continental crust and the upper continental crust. Consequently, we propose a two-stage magma differentiation model involving (1) deep differentiation and crustal contamination of mantle-derived mafic liquids that intrude/underplate in the lower part of the continental crust around the Moho and (2) additional differentiation and upper crustal contamination of such differentiated liquids as they rise and install in shallow-level magma chambers. While the former scenario explains how the Ranwu volcanic rocks were generated, the latter accounts for the emplacement and petrochemical characteristics of the Bomi-Chayu batholiths. The shallow intrusions, moreover, may have triggered low-pressure (P≦4 kbar; Patino-Douce, 1997) partial melting of calc-alkaline basement rocks, e.g., those atop the magma chambers, and hence produced the A-type granites in this part of the Neo-Tethyan subduction zone.Based on the age, compositional and isotopic constraints, it is concluded that the Bomi-Chayu batholiths probably correlate with the northern plutonic belt of the Lhasa terrane. In other words, the eastern/southeastern extension of the Gangdese Batholith, if existed, should be found further south within the Indian national territory that, however, still awaits future explorations.

參考文獻


劉振聲、王洁民 (1990). "川藏花崗岩的稀土元素地球化學特徵." 青藏高原地質
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楊曉明(2008)。緬甸晚新生代火山岩之地球化學及岩石成因〔碩士論文,國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342%2fNTU.2008.02870
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