金門地區減薄大陸地殼中持續的高度變質作用

Sustained High-grade Metamorphism of Thinned Continental Crust in Kinmen Island

黃琮瀚(Tsung-Han Huang)

指導教授 ： 羅清華

國立臺灣大學/理學院/地質科學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202202659

摘要

中國東南部的大陸地殼分佈著大面積白堊紀時期的火成岩體以及張裂盆地，這些地質事件的形成被前人研究認為是在古太平洋板塊隱沒後撤的期間所發生的，而古太平洋板塊長時間的後撤讓中國東南部的大陸地殼從原先的主動大陸邊緣轉變為被動大陸邊緣，這樣地體構造轉換的過程就被位於此大陸邊緣上金門地區的基盤岩給記錄下來，因此金門地區的基盤岩就成為探討大陸地殼減薄過程中地殼內部變質溫壓演化的適合地點。前人研究根據金門地區花崗岩基盤構造變形的伸張應變特徵，提出此處的大陸地殼在花崗岩基盤侵入以後是在減薄的狀態，並在減薄的過程啟動這些複雜且高溫的變形事件，但由於缺乏決斷性的溫壓證據，因此本研究將從變質岩岩石學的觀點來探討大陸地殼減薄過程中地殼內部溫壓演化的歷史。
金門島基盤岩的核心由花崗岩體組成，並在花崗岩體的周圍零星分布不同的變質岩體，包含英雲閃長岩質的片麻岩、角閃岩、黑雲母片麻岩與矽線石雲母片岩。根據構造演化重建、岩象學以及變質岩岩石學的分析，雖然這些複雜的岩性源自於不同的母岩，但是它們都是在類似巴坎式溫壓變質作用 (Buchan-type metamorphism) 的溫壓特徵下一起經歷地殼的變形。在這些基盤岩從深部 (28.3-30.4公里) 出露到近地表 (<6.9公里) 的過程中共伴隨了六期的變形事件，並且溫壓演化的路徑是一個減壓、退變質的特徵。在基盤的花崗岩體侵入地殼以後，第一期的變形事件為公里規模的片麻岩隆穹，形成的溫度在644-725 °C且最大的壓力為7.9 kbar。第二期變形事件是地殼在重力垮塌的過程中所產生近水平的S型構造岩，其溫度環境與D1相同但是位於更低的壓力環境。隨著地殼持續的減薄以及岩體逐漸地上抬，片麻岩隆穹的東西兩側開始發育北北東-南南西走向的剪切褶皺帶 (D3)，其形成溫度為658-704 °C且最大壓力為5.5 kbar。隨後地殼的持續張裂以及基盤上抬至中部至上部地殼時發育了東北東-西南西走向具有左移特徵的伸張剪切帶 (D4)，其形成溫度為534-682 °C且最大壓力為4.6 kbar。由於基盤岩持續的上抬至地殼淺部壓力至少小於1.9 kar的地方，地殼開始轉變為脆性變形並且伴隨偉晶岩脈 (D5) 以及基性岩脈群 (D6) 的侵入。
從以上重建出的溫壓演化路徑顯示起初在D1以前至D3的期間，地殼經歷了減壓但是溫度範圍維持高溫的演化路徑，從最高的變質溫度725 °C與最大的變質壓力7.9 kbar，減壓成704 °C與5.5 kbar，隨後在D3end到D6的期間地殼還是在逐漸減壓的環境但是具有大規模的溫度下降，從在D3end時最高變質溫度682 °C與最大變質壓力4.6 kbar，到在D5以前的最低變質溫度534 °C與最低的變質壓力為1.9 kbar，最後在D5與D6期間溫度是低於534 °C、壓力是小於1.9 kbar。將重建出的地溫梯度曲線與一般大陸地殼的地溫梯度比較，若是在一般大陸地殼內部要達到這些最高的變質溫度，其地殼深度必須要在41.8至51.9公里處，這樣的深度明顯遠高於金門地區的基盤岩侵入深度以及後續的變質深度，因此金門地區在過去白堊紀時期具有較高的地溫梯度，在相對地殼的淺處就已經達到上部角閃岩相至粒變岩相的溫度區間。如此長時間的高度變質作用，加上伸張的應變特徵以及同時期的雙模式岩漿活動，並無法在擠壓且地殼增厚的造山帶中形成，相反的必須在大陸地殼持續伸張減薄的過程才有辦法形成這樣相對低壓但是持續高溫的地殼環境。

關鍵字

金門島 ； 伸張應變 ； 高度變質作用 ； 地殼減薄 ； 地質溫壓計

並列摘要

The continental crust of southeast China is featured by widespread Cretaceous magmatic complexes and extensional basins, which are attributed to the continual rollback of the subducted Paleo-Pacific plate. This long-lasting slab rollback transformed the original active continental margin into a passive one. Such a tectonic transition was recorded in the crystalline basement of Kinmen Island, along the SE Asia continental margin, making Kinmen Island a great candidate for exploring the metamorphic evolution of the continental crust during crustal thinning processes. Previous studies have reconstructed the structural evolution of the granitoid basements in Kinmen Island and proposed a continual crustal thinning setting, yet without conclusive evidence provided, which has been solved in the present study.
The crystalline basement of Kinmen Island comprises a granite core, surrounded by tonalitic gneiss, amphibolite, biotite gneiss and sillimanite mica schist. Based on a combination of structural, petrographic, and metamorphic petrology analyses, and despite these complex lithologies originating from different protoliths, it is revealed that the granite core along with the surrounding metamorphic rocks were all deformed at similar P-T conditions characteristic of Buchan-type metamorphism. A decompressional and regressive P-T path is revealed accompanying the identified six deformation events as the deep-seated crystalline basements exhumed from 28.3-30.4 km (pre-D1) to <6.9 km (D5-D6). A kilometer-scale gneiss dome (D1) formed after the intrusion of the granitoid basements at 644-725 °C/<7.9 kbar. A similar temperature range was maintained with pressure decreasing during the gravitational collapse of the continental crust with a generation of subhorizontal S-tectonite (D2). Further exhumation of the crystalline basements due to continual crustal thinning formed a NNE-SSW striking shear fold belt (D3) along the east and west limbs of the gneiss dome at 658-704 °C/<5.5 kbar. With further crustal extension and exhumation into the middle to upper crust, an ENE-WSW striking sinistral transtensional shear zone was developed throughout the basement at 534-682 °C/<4.6 kbar. Due to the continual exhumation into shallow levels (<1.9 kbar), the continental crust was further deformed by brittle fracturing along with intrusion of pegmatitic dykes (D5) and mafic dyke swarm (D6). The reconstructed P-T trajectory shows nearly sustained high temperatures along a decompressional path from 725 °C/<7.9 kbar (pre-D1 to D2) to 704 °C/<5.5 kbar (D3), followed by continual decompression but a rapid temperature drops from 682 °C/<4.6 kbar (D3end), 534 °C/1.9 kbar (pre-D5) to <300 °C/<1.9 kbar (D5 to D6). Compared with a normal continental geotherm, the corresponding depths to reach such peak metamorphic temperatures should be around 41.8-51.9 km, which is much deeper than the pressure conditions of Kinmen Island. It is therefore suggested that the continental crust of Kinmen Island had an elevated geotherm, which reached upper amphibolite to granulite facies conditions at shallower crustal levels. Such a sustained high-grade metamorphism since the emplacement of granitoids to D4 during crustal decompression along with the extensional strain pattern and contemporaneous bimodal magmatism cannot be produced at a contractional setting. Instead, a prolonged thinned crust under extension is preferred.

並列關鍵字

Kinmen Island ； extensional strain ； high-grade metamorphism ； crustal thinning ； geothermobarometer

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