伊朗札格羅斯造山帶中部迪茲富勒海灣碎屑鋯石研究

Abstract

位處於伊朗西南方札格羅斯褶皺帶中的Dezful Embayment，是屬於中新世時期因為受到札格羅斯造山帶的影響所形成的前麓盆地。本研究利用雷射剝蝕感應耦合電漿質譜術(LA-ICPMS)將13個來自Dezful Embayment加上另外9個來自Urumieh-Dokhtar Magmatic arc (UDMA)及Sanandaj-Sirjan zone (SSZ)的沉積岩樣本進行碎屑鋯石的鈾鉛定年分析。在Dezful Embayment地區，樣本的採集主要在四個中新世以後的地層之中，由下至上依序為Gachsaran、Mishan、Agha Jari和Bakhtyari地層。分析的結果將與UDMA及SSZ地區所發表過的火成岩體定年資料相互比對，藉此對於Dezful Embayment的沉積物來源以及受阿拉伯與歐亞板塊碰撞所造成的地體構造解釋有著更明確的限制。在UDMA及SSZ的樣本數據中顯示出大量屬於始新世至漸新世(55-25 Ma)以及侏儸紀(176-144 Ma)的鋯石年齡組成，但是在Dezful Embayment，分析樣本的結果卻有著相當大的年齡組成差異，Embayment北部的樣本頻譜圖中顯示出數個主要峰值於早寒武紀(950-550 Ma)、晚石炭紀(323-299 Ma)、侏儸紀(176-144 Ma)和始新世至漸新世(55-25 Ma)時期，最後所提到的兩個年齡峰值說明了對於Dezful Embayment來說，SSZ和UDMA是十分重要的沉積物來源。相較之下，Embayment中部的樣本則主要以前寒武紀(2500 Ma、1850 Ma和950-550 Ma)的古老年齡為主，僅有少量的侏儸紀和新生代鋯石年齡，於此可推斷對於南方阿拉伯板塊各地區的沉積物來源存在著些許的差異。除此之外，本研究僅在Embayment北部的碎屑鋯石年齡頻譜圖中發現明顯的變化，出現在中新世晚期的Agha Jari地層之中。配合前人在Dezful Embayment西北方的Lurestan所進行的碎屑鋯石分析工作，發現相似的變化在Lurestan也有出現，但變化的時間點卻是在中新世中期的Gachsaran地層，早於本研究於Embayment北部的時間點判斷，藉由這樣的差異得以說明阿拉伯和歐亞板塊之間的碰撞是屬於不等時性的斜向碰撞。

The Dezful Embayment of the Zagros Fold-and-Thrust Belt (ZFTB) in southwest Iran is a foreland basin that formed as a flexural basin during the Miocene by the Zagros orogeny. This study reports a LA-ICPMS analysis of zircon U-Pb geochronology of thirteen sedimentary rock samples from the Dezful Embayment, and nine others from the Urumieh-Dokhtar Magmatic arc (UDMA) and the Sanandaj-Sirjan zone (SSZ). In the northern and central parts of the Dezful Embayment, samples were collected from four principal sedimentary sequences including, from bottom to top, the Gachsaran Formation, Mishan Formation, Agha Jari Formation and Bakhtyari Formation. Our new zircon ages, together with published geochronological data from the UDMA and SSZ, can be used to better constrain the source provenance and tectonic implication of the Dezful Embayment related to the Arabia-Eurasia collision following the closure of Neo-Tethys. Samples of the UDMA and SSZ are characterized by abundant zircon ages of Eocene to Oligocene (55-25 Ma) and Jurassic (176-144 Ma), respectively. In the Dezful Embayment, detrital zircon age populations show significant variations. The age spectra of the northern Embayment show Major peaks at the Precambrian (950-550 Ma), Late Carboniferous (323-299 Ma), Jurassic (176-144 Ma) and Eocene to Oligocene (55-25 Ma), with the last two age peaks suggesting an important source provenance from Eurasia, involving the SSZ and UDMA. By contrast, in the central Embayment, detrital zircons are dominated by Precambrian ages (2500 Ma, 1850 Ma, and 950-550 Ma), with rare Jurassic and Cenozoic ages. This suggests a different sedimentary source that we interpret to be the Arabian continent from the south. Moreover, gradual changes in the detrital zircon age spectra and thus in the source provenances are observed in different parts of the Dezful Embayment. The change in the northern Embayment occurred in the Agha Jari Formation, earlier than that in the central Embayment occurring between the Agha Jari and Bakhtyari Formations. This variation is consistent with the notion for oblique and diachronous collision between Arabia and Eurasia.