土耳其東北部地區白堊紀至始新世火成岩之地球化學特性與岩石成因

Zircon U-Pb age and geochemical constraints on the petrogenesis of Cretaceous to Eocene magmatic rocks in Eastern Pontides, Turkey

陳嘉蕙(Jia-Huei Chen)

指導教授 ： 鍾孫霖

國立臺灣大學/理學院/地質科學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201700313

摘要

阿拉伯與歐亞大陸碰撞造山帶主要是由特提斯洋的閉合所造成，新特提斯洋的演化在土耳其東部地區主要的構造單元為北方的Pontides地塊與南方的Anatolide-Tauride地塊，此二地塊間的蛇綠岩套代表了新特提斯洋北支海洋地殼的殘留。為了解Eastern Pontides地區複雜的地質歷史，特別是白堊紀至始新世與新特提斯洋北支關閉相關的地質活動，本研究對此區域主要火成岩露頭進行採樣、鋯石鈾鉛定年與地球化學分析。鋯石鈾鉛定年的結果顯示研究區域白堊紀至始新世的岩漿活動可以分為三期，各期岩漿活動年代與地球化學特性如下：(1)早白堊紀(144-128 Ma)，由具鈣鹼性特徵的輝長岩至閃長岩組成。(2)晚白堊紀(87-77 Ma)：由具拉斑玄武岩質至高鉀鈣鹼性特徵的中性至酸性侵入岩，以及基性至酸性噴出岩所組成。(3)始新世早期(55-54 Ma) 與始新世中期 (43-40 Ma)：分別為具埃達克岩特徵，包含花崗閃長岩與流紋岩等酸性岩類；以及以高鉀鈣鹼性岩為特徵的基性至中性噴出岩與基性至酸性的侵入岩所組成。
整體而言，白堊紀至始新世之三期岩漿活動均呈現鈮 (Nb)、鉭 (Ta)、鈦 (Ti)的虧損，顯示曾經受到隱沒作用的影響。早白堊紀岩石呈現輕稀土元素富集的現象((La/Yb)n= 6.2-29.4)。晚白堊紀岩石則可分為稀土元素的斜率平坦((La/Yb)n =2.2-4.6)與輕稀土元素富集((La/Yb)n =8.3-17.3)兩種岩石序列；前者的基性岩幾乎沒有銪 (Eu) 元素負異常的現象，中酸性樣本則在兩種岩石序列皆普遍具有銪元素負異常之情形。始新世早期的樣本呈現輕稀土富集，重稀土元素明顯虧損之現象((La/Yb)n =62.2-92.6)。始新世中期的岩石則呈現輕稀土元素富集的現象((La/Yb)n =2.8-39.9)，銪元素負異常的現象明顯顯示於中酸性岩的樣本中。
早白堊紀岩漿活動可與同時期亞美尼亞之花崗岩類互相比對，本研究認為此期岩漿活動的發生與新特提斯洋北支的隱沒事件相關。晚白堊紀性質相異的兩種岩石序列被認為與新特提斯洋北支隱沒末期事件與其北側的黑海東側盆地 (East Black Sea Basin, EBSB) 的張裂相關。後續的陸塊縫合則造成始新世早中期的碰撞後岩漿活動，使得增厚的大陸下部地殼以及因前期受隱沒作用影響而被交代換質的大陸岩石圈地函發生部分熔融。

關鍵字

新特提斯洋演化 ； 阿拉伯與歐亞大陸碰撞造山帶 ； 鋯石鈾鉛定年 ； 鍶釹同位素

並列摘要

The Neo-Tethyan orogenic evolution of eastern Turkey in the Arabia-Eurasia collision zone involves two tectonic units, i.e., the Pontides in north and the Anatolide-Tauride in south, separated by at least two ophiolite belts representing relicts of the oceanic crust from the northern and southern branches of Neo-Tethys, respectively. To better understand the complex geologic history of Eastern Pontides, particular during the Cretaceous and Eocene when the northern branch of Neo-Tethys may have closed, a combined geochronological and geochemical analysis of main magmatic outcrops was conducted. The result, including specifically zircon U-Pb age and geochemical data of 26 samples, allows us to divide the magmatism into three stages: (1) Early Cretaceous (ca. 144-128 Ma) calc-alkaline rocks, composed of gabbro to granodiorite lithologies; (2) Late Cretaceous (87-77 Ma) tholeiitic to high-K calc-alkaline rocks, composed of diversified lithologies from diorite to granite and from basalt to rhyolite; (3) Early Eocene (55-54 Ma) adakitic rocks, exclusively of felsic composition such as granodiorite and rhyolite; and Late Eocene (43-40 Ma) high-K clac-alkaline rocks, also of diversified lithologies. The Early Cretaceous magmatism, which can be correlated to broadly coeval granitoids emplaced in Armenia, is attributed to the subduction following opening of the northern branch of Neo-Tethys. The second stage of magmatism that exhibits significant lithological and geochemical variations is interpreted as the products of ending subduction and subsequent closure of the northern branch of Neo-Tethys and the opening eastern Black Sea Basin. Under this framework, the two suites of Eocene magmatism are post-collisional activities that resulted from partial melting of the lower part of a collision-thickened juvenile crust and the subduction-metasomatized continental lithospheric mantle, respectively.

並列關鍵字

Neo-Tethyan evolution ； Eastern Pontides ； magmatic arc ； Zircon chronology ； Sr–Nd isotope

參考文獻

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