喬治亞小高加索山區新生代火成岩之地球化學特性與岩石成因

Abstract

高加索山區、伊朗高原與安那托利亞高原 (合稱為CIA地區) 為阿拉伯板塊與歐亞板塊之間的碰撞帶。境內北與南各分佈一條縫合帶，分別為新特提斯洋北方分支在中生代晚期關閉所形成的Ankara-Erzincan/Sevan-Akera縫合帶與新特提斯洋南方分支於35 Ma關閉所形成的Bitlis-Zagros縫合帶。於兩次海洋關閉事件後，CIA地區均發生有岩漿活動。為了更進一步瞭解CIA地區新生代岩漿活動的形成機制，本研究針對地處Ankara-Erzincan/Sevan-Akera縫合帶北側、喬治亞小高加索山區的新生代火成岩進行定年與化學組成分析工作，並結合CIA地區大地構造事件進行岩漿形成機制的探討。 本研究由鋯石鈾－鉛定年與全岩氬－氬定年工作的結果結合同區域前人的研究，可將喬治亞小高加索山區的新生代岩漿活動分為始新世中期 (46.8-40.4 Ma)、漸新世晚期 (24.4 Ma)、中新世晚期 (9-5 Ma) 與新生代晚期 (< 3 Ma) 岩漿活動。由地球化學的分析結果顯示始新世中期、中新世晚期與新生代晚期 (< 3 Ma) 的火成岩相當分液，從基性至酸性均有分佈；漸新世晚期僅具有中酸性火成岩 (SiO2 = 58-66 wt.%)。由火成岩的K2O與重稀土元素的含量變化，則可將四期火成岩岩性作區分，分別為始新世中期的鈣鹼性系列火成岩與鉀玄岩質火成岩，漸新世晚期、中新世晚期與新生代晚期 (< 3 Ma) 埃達克質火成岩，而中新世晚期與新生代晚期 (< 3 Ma) 亦具有鈣鹼性系列火成岩形成。由微量元素的分析結果可見四期火成岩均有輕稀土元素與大離子半徑元素富集以及鈮、鉭、鈦元素相對虧損的情形，顯示岩漿源區曾受隱沒作用影響，此影響隨時間漸弱。全岩鍶－釹同位素組成與鋯石鉿同位素組成的分析結果則顯示四個時期的岩漿來源接近上部地函端元。由於始新世中期鉀玄岩質火成岩不具基性岩類，且此時期所有火成岩之鋯石鉿同位素組成變化顯示岩漿未混染古老大陸地殼，本研究認為鉀玄岩質的原岩應為角閃岩質的大陸下部地殼。由漸新世晚期、中新世晚期與新生代晚期 (< 3 Ma) 埃達克質火成岩MgO、鎳與鉻含量偏低的情形，本研究認為此三時期埃達克質火成岩為榴輝岩質的大陸下部地殼重熔的產物。而全岩鍶－釹同位素以及鋯石鉿同位素組成與上部地函端元相近的情況則顯示角閃岩與榴輝岩質的大陸下部地殼應為鄰近時期的火成活動之基性岩漿，經底侵作用滯留於大陸地殼底部所形成的產物。 結合CIA地區中生代晚期至新生代兩期海洋關閉事件，本研究認為於新特提斯洋北方分支關閉後，Ankara-Erzincan/Sevan-Akera縫合帶北側區域因海洋南北兩側陸塊的碰撞使岩石圈增厚，並於始新世中期發生拆層作用引發軟流圈上湧，上部地函因而發生部分熔融形成鈣鹼性系列岩漿。上升的鈣鹼性系列岩漿則使角閃岩質的大陸下部地殼熔融，造成同時期的鉀玄岩質岩漿活動；此期岩漿活動隨南新特提斯洋關閉帶動阿拉伯板塊與歐亞板塊於始新世末期發生碰撞而停止。隨兩板塊的擠壓，小高加索山區的岩石圈持續增厚，於漸新世晚期、中新世晚期與新生代晚期 (< 3 Ma) 發生拆層，使深達50公里而榴輝岩化的大陸下部地殼受熱重熔，形成三期的埃達克質岩漿活動，本研究認為中新世晚期與新生代晚期 (< 3 Ma) 的岩石圈拆層規模可能較大，因而造成同時期廣泛分佈的鈣鹼性系列岩漿活動。

The Caucasus-Iran-Anatolia (CIA) region in the Tethysides Orogenic belt was elevated by Arabia-Eurasia collision. The Ankara-Erzincan/Sevan-Akera suture zone and the Bitlis-Zagros suture zone in this region stand for the closures of northern and southern Neotethys system since late Mesozoic, and Cenozoic magmatism has been related to the subduction and termination of the Neotethyan system. In Georgia, located at the northern part of the CIA region and north of the Ankara-Erzincan/Sevan-Akera suture zone, the Cenozoic igneous rocks are widespread in the Lesser Caucasus region. This study combines a detailed geochemical analysis of Cenozoic igneous rocks in Lesser Caucasus of Georgia with tectonic events to comprehend the Cenozoic magma mechanism of northern CIA region. The previous research with the zircon U-Pb and whole rock Ar-Ar dating results in this study show that Cenozoic magmatism in Georgia can be divided into four main stages, i.e., middle Eocene (46.8-40.4 Ma), late Oligocene (24.4 Ma), late Miocene (9-5 Ma) and late Cenozoic (< 3 Ma). The first, third and fourth stages consist of basic to felsic rocks, but the SiO2 contents of the Oligocene volcanic rocks range from 58 to 66 wt.%. Rocks of four stages can be subdivide into three groups: middle Eocene calc-alkaline and shoshonitic rocks, late Oligocene adakitic rocks, late Miocene calc-alkaline and adakitic rocks, and the composing of late Cenozoic (< 3 Ma) is the same with the prior stage. Incompatible elements data show that all the Cenozoic igneous rocks display the “arc geochemical signatures” such as enrichment in LILE (e.g., Cs, Rb, Ba, K) and depletion in HFSE (e.g., Ti, Nb, Ta), and the signatures mitigate with stages. The Sr-Nd isotope data and the zircon Hf isotope data indicate that the magma source might be upper mantle. Because of lack of basic type and without assimilation of continental crust, we consider the source of middle Eocene shoshonitic rocks is amphibolitic lower crust. We also suggest the Cenozoic adakitic rocks with low MgO, Ni and Cr concentration are caused by remelting of eclogitized lower crust. Both types of mafic lower crust are form by mantle derived young basaltic underplates at the base of crust. The lithosphere of northern CIA region thickened after the closure of northern Neotethys in late Mesozoic. We think that the middle Eocene calc-alkaline magmatism of Georgia was induced by asthenospheric upwelling result from lithospheric delamination. The calc-alkaline magma provided as a heat source for the partial melting of the amphibolitic lower crust and produced the shoshonitic magma. The magmatism of middle Eocene ceased by Arabian-Eurasian collision after southern Neotethys closed in late Eocene. With the continental collision, the thickened basaltic underplating crust (> 50 km) of Lesser Caucasus transform into eclogite, and remelted into adakitic magma owing to delamination beneath the Lesser Caucasus in late Oligocene, late Miocene and late Cenozoic (< 3 Ma). The large scale of lithospheric removing in late Miocene and late Cenozoic (< 3 Ma) also triggered the widespread calc-alkaline volcanism in Lesser Caucasus of Georgia.