亞美尼亞及高加索造山帶火成岩的地球化學特性與岩石成因

Abstract

阿拉伯與歐亞大陸板塊的碰撞造山，包含數個小地塊的拼貼或增積，因此被稱為土耳其式造山運動。此造山帶主要由高加索、伊朗以及安納托利亞高原所組成(簡稱為CIA地區)，碰撞前後均有廣泛分布的火成岩，前者為新特提斯洋向北隱沒所造成，後者約從11 Ma開始，形成機制多所爭議，本研究首先針對亞美尼亞境內的碰撞前與碰撞後火成岩進行地球化學與定年分析，討論岩石成因，再比對其他CIA地區碰撞後火成岩之地化性質與時空變化，綜合探討整個地區的碰撞後岩漿活動與地體構造演化。 本研究共分析採自亞美尼亞的12個碰撞前火成岩與35個碰撞後火成岩標本。結果顯示前者的形成年代介於57.5-26.5 Ma之間，屬中鉀鈣鹼性系列；後者的形成年代小於4.4 Ma，屬高鉀鈣鹼性系列。兩期火成岩皆具有輕稀土與其他高不相容元素的富集、高場力鍵結元素的虧損、並具有相似的鍶釹同位素組成(87Sr/86Sr ≈ 0.7040 to 0.7047; 143Nd/144Nd ≈ 0.5127 to 0.5129)。然而，碰撞後火成岩的鉀質與高不相容元素富集的程度明顯較高，根據稀土元素模擬計算，碰撞後火成岩之部份熔融程度(3-6%)較碰撞前火成岩(8-10 %)來得小，但兩者的地函源區均為尖晶石至石榴子石二輝橄欖岩過渡帶，約60-80公里深，且受過隱沒作用的富集，與其他CIA地區碰撞後火成岩之源區類似。 CIA地區的碰撞後岩漿活動不但普遍造成高鉀鈣鹼性系列岩石，還造成了埃達克岩與超鉀質岩。超鉀質岩目前只在伊朗西北部Saray火山被發現，噴發於11 Ma，具有相對富集的鍶釹同位素組成(87Sr/86Sr ≈ 0.7078; 143Nd/144Nd ≈ 0.5125)、由含金雲母之富集岩石圈地函發生小程度部份熔融而形成。埃達克岩分布較廣泛，約從6Ma肇始，由東安納托利亞經亞美尼亞到大高加索地區形成向東北逐漸年輕的噴發趨勢，並在伊朗西北部造成Sahand和Sabalan兩大火山，其鍶釹同位素組成均勻(87Sr/86Sr ≈ 0.7041 to 0.7050 and 143Nd/144Nd ≈ 0.5127 to 0.5128)，與其他碰撞後鈣鹼性火山岩無異、應屬碰撞增厚的基性底侵下地殼熔融所造成。 整個CIA地區之碰撞後鈣鹼性岩漿活動約始於11 Ma，有先向南再向東遷移的時空變化趨勢，推測是由於大陸碰撞起始地區東安納托利亞之下的新特提斯洋岩石圈先向南退卻(slab roll-back)、接著向東撕裂(tear migration, 約自6 Ma開始)、最後拆解(slab break-off, 約自2 Ma開始)的結果。此一地體構造改變，導致軟流圈上湧並造成岩漿活動的廣泛分布及時空變化。此外，CIA西南部的火山活動從2 Ma左右終止，推測是受到該地區岩石圈增厚的抑制，增厚的機制除了和碰撞相關的構造增厚之外，可能還包含新的岩石圈地函形成，後者為早期碰撞後岩漿活動在軟流圈頂部的熔融殘餘、具有耐熔的特性。因此，基於阿拉伯與歐亞兩大陸塊的異時斜向碰撞，碰撞後岩漿活動預期將會沿著札格洛斯縫合帶(Zagros suture)，向東南方向逐漸發展。

Armenia is located in the Arabia-Eurasia continental collision zone that has also been considered as the product of the “Turkic-type” orogeny involving accretion of a number of terranes. Cenozoic magmatism in this zone, named CIA (Caucasus-Iran- Anatolia) province in this study, took place in two main stages that, respectively, pre- and post-date the Arabia-Eurasia collision. Whereas the pre-collisional magmatism has been generally ascribed to the Neotethyan subduction, how was the cause or mechanisam of the voluminous post-collisional volcanism formed has long been an issue of debates. This study reports new ages and geochemical data of the pre- and post-collisional igneous rocks from Armenia. All the studied rocks are calc- alkaline and characterized by enrichment in LREE and other highly incompatible trace elements (e.g., Rb, Ba, Th, U), and depletions in the high field strength elements (e.g., Nb, Ta, Ti). These geochemical features, similar to those of coeval magmatic rocks from the CIA province, support the existence of a subduction- modified mantle that prevails throughout the Cenozoic. In Armenia, however, post-collisional rocks are more enriched in potassium and highly incompatible trace elements than pre-collisional ones. Post-collisional basalts [La=24-63 ppm; (La/Yb)N =5.8-20], for example, are more LREE-enriched than pre-collisional basalts [La=15-28 ppm; (La/Yb)N of 3.5-7.9]. All the Armenian rocks show rather uniform Sr-Nd isotopic ratios (87Sr/86Sr ≈ 0.7040 to 0.7047; 143Nd/144Nd ≈ 0.5127 to 0.5129), similar to the isotopic compositions reported in other CIA magmatic provinces. REE modeling suggests that Armenian pre- to post-collisional basaltic magmas were derived from a common mantle source that is located in spinel- to garnet-lherzolite transition region at ~60-80 km depth, with melting degrees being larger in the former (8-10 %) and smaller in the latter (3-6 %). The Armenian results, combined with our data from other parts of the CIA province and literature information from E. Anatolia, allow us to better constrain the temporal, spatial and geochemical variations in the CIA province. The post-collisional volcanism began at ca. 11 Ma, and it shows change in time and space, prevailing during 9-6 Ma in E. Anatolia or the southwestern part of the CIA volcanic province and then migrating eastward. No volcanism occurred in the southwestern CIA province since ~2 Ma. Along with the predominant calc-alkaline rocks, adakites and ultrapotassic rocks are observed in the CIA volcanic province. The adakites are small-volume but widespread, erupting with a northeastward-younging trend from E. Anatolia to the Greater Caucasus. They have uniform Sr-Nd isotope ratios (87Sr/86Sr ≈ 0.7041 to 0.7050 and 143Nd/144Nd ≈ 0.5127 to 0.5128), similar to those of the other CIA post-collisional volcanics, suggesting a common mantle source. The ultrapotassic rocks that were emplaced in Saray, NW Iran, as one of the earliest eruptions (~11 Ma), have more “radiogenic” Sr-Nd isotope ratios (87Sr/86Sr ≈ 0.7078; 143Nd/144Nd ≈ 0.5125). The adakites are interpreted as partial melts of eclogitized lower crust, formed by basaltic underplating during the Neotethyan subduction and thickened by the collision, and the ultrapotassic rocks as small-degree melts of the metasomatized lithospheric mantle. The driving force of the CIA post-collisional volcanism may be attributed to roll-back and then break-off of the subducted Neotethyan slab that, assuming an oblique/diachronous collision between Arabia and Eurasia, may have started from the northwest, i.e., beneath the southwestern CIA province, and propagated southeastward. Volcanism thus produced may later be ceased owing to the formation of new lithospheric mantle from below, as the melting residue, and subsequent crustal/lithospheric thickening caused by the continued collision. Under this framework, it is predictable that the post-collisional magmatism will eventually migrate southeastward along the Zagros suture zone.