利用鋯石鈾鉛定年及鉿同位素研究伊朗新特提斯隱沒 帶岩漿演化與札格洛斯造山作用

The Neotethyan subduction and Zagros orogeny: constraints from zircon U-Pb age and Hf isotopic study of magmatic rocks from Iran

邱瀚毅(Han-Yi Chiu)

指導教授 ： 鍾孫霖

國立臺灣大學/理學院/地質科學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.00026

摘要

新特提斯洋岩石圈從侏羅紀中期開始向北隱沒，在伊朗與鄰近區域造成了大規模的島弧岩漿活動，然而伊朗境內最主要的Urumieh-Dokhtar島弧岩漿帶(Urumieh-Dokhtar magmatic arc: UDMA)一直以來缺乏精確的年代學研究與控制；除此之外，阿拉伯板塊與歐亞大陸開始碰撞的時間及地體構造模式還存有爭議。因此，本研究報導UDMA詳細且系統的鋯石鈾鉛定年與鉿同位素數據，並結合文獻資料，希望能更詳細地解析新特提斯隱沒帶演化和札格洛斯造山作用(Zagros orogeny)。同時分析伊朗其它的主要構造單元，例如：Sanandaj-Sirjan structural zone (SSZ)、Alborz Range、Central Iran及Sistan suture zone，與亞美尼亞地區的岩漿岩，以期望對此區域岩漿演化的時空關係能提出新的制約。
本研究發現：在大規模的新特提斯島弧岩漿形成之前，存在著新元古代晚期至寒武紀及晚三疊紀兩期主要的岩漿事件。其中，前者表現出虧損地函的同位素特徵，與Gondwana邊緣及Arabian-Nubian Shield的岩漿活動有其相關；後者的岩漿源區則明顯受到大陸地殼物質的混染，與古特提斯洋的隱沒和閉合有關。侏羅紀時期，廣泛的I型花崗岩類出現在SSZ帶，同時也出露在UDMA的中部和Lut block的北部，擁有不均質的鋯石鉿同位素組成[εHf(T) = +12 to -5]。在一段長時間的岩漿休止期後，晚白堊紀(81-72 Ma)花崗岩在UDMA東南部的侵入，表示岩漿活動有向內陸移動的趨勢。此期侵入岩漿在Jiroft及Bazman地區各別的鋯石鉿同位素組成有著相當的差異[Jiroft: εHf(T) = +15 to +11; Bazman: εHf(T) = +5 to -9]，後者暗示岩漿活動有明顯的大陸地殼物質混染。UDMA岩漿岩主要廣泛出現在始新世和漸新世期間(55-25 Ma)，比前人研究所認定的始新世岩漿大量噴發事件持續更久。這時期的UDMA岩漿大噴發與亞美尼亞地區的鈣鹼性岩石相似，代表此島弧岩漿往西北延伸，並且也同樣地發生在SSZ的西北部及Alborz的西部。其中，絕大多數的鋯石有虧損地函的鉿同位素特徵[εHf(T) = +17 to -1]，暗示新特提斯洋的隱沒作用形成了大規模的新生大陸地殼。除此之外，漸新世至第四紀時期(32-1 Ma)虧損地函來源的岩漿活動出現在Sistan suture zone的南部和Makran區域。相較之下，Central Iran的Saghand地區始新世花崗岩體有著相對較富集的鋯石鉿同位素組成[εHf(T) = +6 to -7]，指示此地區擁有古老的大陸地殼物質。整體而言，新生大陸地殼約從六億年前開始形成，經由後續的板塊拼貼及大陸碰撞作用後，到如今大範圍的存在於伊朗與亞美尼亞地區。
本研究顯示UDMA的岩漿活動有從西北向東南逐漸停止的趨勢，例如：Meghri地區的岩漿活動在早中新世(ca. 22 Ma)結束、Kashan地區的岩漿活動在中中新世(ca. 16 Ma)結束及Anar地區的岩漿活動結束在晚中新世(ca. 10-6 Ma)；這個觀察符合阿拉伯板塊與歐亞大陸的斜向與異時性碰撞的概念。約11百萬年以前，Saray地區開始碰撞後的火山活動，與鄰近Sahand (6.5-4.2 Ma)及Sabalan (≤0.4 Ma)的火山噴發，涵蓋了大部分的小高加索、伊朗西北部及安那托利亞高原東部等區域。

關鍵字

鋯石鈾鉛定年及鉿同位素分析 ； UDMA島弧岩漿帶 ； SSZ構造帶 ； 伊朗 ； 新特提斯隱沒作用 ； 札格洛斯造山作用

並列摘要

This study reports the first combined LA-ICPMS analyses of zircon U-Pb and Hf isotope compositions for magmatic rocks from several major domains of Iran, including the Urumieh-Dokhtar magmatic arc (UDMA), Sanandaj-Sirjan structural zone (SSZ), Alborz Range, Central Iran and Sistan suture zone. These results, together with the literature data and some unpublished ages of our team, better delineate the magmatic evolution related to the Neotethyan subduction and subsequent Zagros orogeny that resulted from the Arabia-Eurasia collision. Moreover, this study also identifies two major magmatic events that formed during the late Neoproterozoic to Cambrian and the Late Triassic before the occurrence of broad magmatic activities which are related to the Neotethyan subduction. The first magmatic event represents a depleted mantle-derived magmatic feature and it has associated with the magmatism that produced the peri-Gondwanan terranes and Arabian-Nubian Shield. The second magmatic event is attributed to the Paleotethys evolution and shows an obvious magmatic signature of crustal contamination.
The Neotethyan subduction-related magmatism was active during the Jurassic time, as evidenced by the presence of widespread I-type granitoids from the Middle to Late Jurassic (176-144 Ma) in the SSZ. These intrusive rocks, also exposed in the central part of the UDMA and the northern Lut block, show a heterogeneous isotopic affinity with variable zircon εHf(T) values between +12 and -5. After a protracted magmatic quiescence in the Early Cretaceous, the igneous activity renewed inland in the UDMA from which the Late Cretaceous granitoids (81-72 Ma) are found in the southeastern segment of the UDMA. These rocks from the Jiroft and Bazman areas have very different zircon εHf(T) values clustering from +15 to +11 and from +5 to -9, respectively, implying that the Bazman magmas were significantly mixed with the old crustal components. Then, the UDMA volcanism was most active and widespread during the Eocene and Oligocene (55-25 Ma), much longer lasting than the previously thought as just an Eocene pulse. Such a prolonged igneous “flare-up” event in the UDMA can be correlated to Armenia where the coeval calc-alkaline rocks are common. Similar ages are also obtained from the magmatic rocks in the northwestern SSZ and west Alborz. This magmatism, with mostly positive zircon εHf(T) values between +17 and -1, can further signify the formation of extensive juvenile crust in the regions owing to the Neotethyan subduction. The magmas with a primitive signature were also exposed in the southern Sistan suture zone and the Makran region during the Oligocene to Quaternary (32-1 Ma). By contrast, the ancient continental crust material was observed from the Central Iran, in Saghand area by the Eocene granitoids that yielded less radiogenic zircon Hf isotopes of εHf(T) values ranging from +6 to -7.
The UDMA magmatism ceased progressively from the northwest to the southeast, with the magmatic activities ending in the Early Miocene (ca. 22 Ma) in Meghri, in the Middle Miocene (ca. 16 Ma) in Kashan and in the Late Miocene (ca. 10-6 Ma) in Anar. The southeastward magmatic cessation is consistent with the notion of the oblique and diachronous continental collision between Arabia and Eurasia. The post-collisional volcanism started at ca. 11 Ma in Saray, east off the Urumieh Lake, which, along with the later eruptions in Sahand (6.5-4.2 Ma) and Sabalan (≤0.4 Ma) volcanoes, forms a compositionally unique component of the vast volcanic field covering much of the Lesser Caucasus, NW Iran and eastern Anatolia regions.

並列關鍵字

Zircon U-Pb age and Hf isotope ； UDMA ； SSZ ； Iran ； Neotethyan subduction ； Zagros orogeny

參考文獻

Agrad, P., Omrani, J., Jolivet, L., Mouthereau, F., 2005. Convergence history across Zagros (Iran): Constraints from collisional and earlier deformation. International Journal of Earth Sciences 94, 401-419.

Agard, P., Monie, P., Gerber, W., Omrani, J., Molinaro, M., Meyer, B., Labrousse, L., Vrielynck, B., Jolivet, L., Yamato, P., 2006. Transient, synobduction exhumation of Zagros blueschists inferred from P-T, deformation, time, and kinematic constraints: Implications for Neotethyan wedge dynamics. Journal of Geophysical Research 111, B11401, doi:10.1029/2005JB004103.

Aghazadeh, M., Castro, A., Omran, N.R., Emami, M.H., Moinvaziri, H., Badrzadeh, Z., 2010. The gabbro (shoshonitic)–monzonite–granodiorite association of Khankandi pluton, Alborz Mountains, NW Iran. Journal of Asian Earth Sciences 38, 199-219.

Aghazadeh, M., Castro, A., Badrzadeh, Z., Vogt, K., 2011. Post-collisional polycyclic plutonism from the Zagros hinterland: the Shaivar Dagh plutonic complex, Alborz belt, Iran. Geological Magazine 148, 980-1008.

Ahmadi Khalaji, A., Esmaeily, D., Valizadeh, M.V., Rahimpour-Bonab, H., 2007. Petrology and geochemistry of the granitoid complex of Boroujerd, Sanandaj-Sirjan Zone, Western Iran. Journal of Asian Earth Sciences 29, 859-877.