印度北部喀什米爾地區潘加爾火成岩區中皮爾潘加爾區域的地質年代學與地球化學之研究

在喜馬拉雅山區的早二疊紀火山序列，與新特提斯洋及細長狀的辛梅里亞大陸形成於同一時間，潘加爾火成岩區(Panjal Traps)是西部喜馬拉雅山區中與新特提斯洋有關的最大玄武岩區，而針對潘加爾火成岩區中皮爾潘加爾區域(Pir Panjal Range)的研究非常的稀少。本研究使用高精度化學侵蝕同位素稀釋熱電離質譜儀(CA-ID-TIMS)，針對玄武岩中分離出的鋯石進行鈾鉛定年，並對皮爾潘加爾南部的賈瓦哈爾隧道露頭(Jawahar Tunnel)的十個樣品進行全岩地化分析，以及採自潘加爾火成岩區不同露頭的樣品進行全岩錸鋨同位素分析。鋯石鈾鉛定年目標是賈瓦哈爾隧道露頭的玄武岩樣品-JT/9，定年結果為288.2±0.3百萬年，此一結果與前人利用雷射剝蝕感應耦合電漿質譜儀(LA-ICP-MS)，對斯里納加爾附近的流紋岩中的鋯石定年的結果相近。主要元素的結果顯示賈瓦哈爾隧道露頭的岩性可被區分為三個部分:(1)玄武岩 (二氧化矽 < 52 wt.%、Mg# < 56；(2) 玄武岩質安山岩(二氧化矽 = 52~60 wt.%、Mg# = 32~48)；(3) 酸性岩 (二氧化矽 = 64.0和70.1 wt.%、Mg# = 27和23)，並且可能與潘加爾火成岩區東部的格里爾萊文露頭(Guryul Ravine)有關。球粒隕石標準化的稀土元素序列、以及原始地函標準化的不相容元素序列均顯示:無論是基性岩或酸性岩都與格里爾萊文露頭所採集的樣本相似。銪、鍶與鋇的虧損顯示出此地區的岩漿應該經歷過長石的結晶分異，而鎂質低可能代表基性矽酸鹽礦物的結晶分異 (例如: 橄欖石、斜輝石)。來自潘加爾火成岩區不同露頭的87Os/88Os介於0.1230與0.2832之間，兩個來自皮爾潘加爾區域北部的樣品87Os/88Os則為0.1230及0.1256，這兩個數值與前人提出的原始上部地函的87Os/88Os相似，然而來自其他區域如:(皮爾潘加爾區域南部的樣品)皆擁有不同程度富集的訊號 (0.1343~0.2832) 。來自賈瓦哈爾隧道露頭的新數據顯示皮爾潘加爾區域南部可能與格里爾萊文露頭相關，而非與皮爾潘加爾區域北部有關。新的錸鋨同位素與前人的釹同位素結果，均顯示出此區域最初可能是球粒隕石質地函，並且逐漸從代表較富集岩漿源的賈瓦哈爾隧道與格里爾萊文區域，轉變成相對虧損的皮爾潘加爾區域北部。這個結果與此區域從大陸張裂逐漸演化成海盆擴張的地質背景一致。

關鍵字

印度；喀什米爾；大型洪流玄武岩；二疊紀；新特提斯洋

並列摘要

The Early Permian volcanic sequences in the Himalaya are contemporaneous with the opening of the Neo-Tethys Ocean and the formation of the Cimmerian ribbon-continent. The Panjal Traps are the largest exposure of the Neo-Tethyan rift-related basalt in the Western Himalaya. Few, if any, investigations of the Panjal Traps are focused on the Pir Panjal Range. This study presents a high-precision chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) zircon U-Pb age from a basalt, new whole-rock geochemical data of rocks collected near the Jawahar Tunnel located at the southern region of Pir Panjal Range and whole-rock Os isotopic data of rocks collected across Panjal Traps. The CA-ID-TIMS zircon U-Pb age was obtained from a basalt yielded a weighted-mean 206Pb/238U age of 288.2 ± 0.3 Ma (N = 4, MSWD = 1.5). This age is within the error of the zircon LA-ICP-MS age (289 ± 3 Ma) from the rhyolite collected in the Guryul Ravine section located near Srinagar. The major elemental compositions indicate that the samples from Jawahar Tunnel can be roughly divided into three groups (basaltic: SiO2 < 52 wt.%; Mg# < 56, basaltic andesite: SiO2 = 52 wt.% to 60 wt.%; Mg# = 32 to 48, silicic: SiO2 = 64.0 and 70.1 wt.%; Mg# = 27, and 23) and may correlate with the Guryul Ravine section in the eastern portion of Panjal Traps. The chondrite normalized rare earth element and primitive mantle normalized incompatible element plots show that both mafic and silicic samples have patterns similar to the rocks collected from Guryul Ravine. The Eu/Eu* values (0.61 to 0.81) and the depletion of Sr and Ba in the primitive mantle normalized incompatible element plot imply the magma underwent fractional crystallization of plagioclase. The low Mg# indicates the parental magma also underwent fractional crystallization of mafic silicate minerals (i.e., olivine and/or clinopyroxenes. The 87Os/88Os ratios in different parts of Panjal Traps sections are range from 0.1230 to 0.2832. Two samples from northern Pir Panjal Range have ratios of 0.1230 and 0.1256 which are similar to primitive upper mantle whereas samples from other sections including southern Pir Panjal Range show enriched values (0.1343-0.2832). The new data from Jawahar Tunnel imply the southern Pir Panjal Range might be correlative to the Guryul Ravine section instead of the northern Pir Panjal Range. The Re-Os and Nd isotopes suggest the source of the initial Panjal Traps was likely chondritic and that it transitioned from enriched at Jawahar Tunnel and Guryul Ravine to relatively depleted in the northern Pir Panjal Range. The results are consistent with a transition from continental rifting setting to ocean basin opening.

並列關鍵字

India ； Kashmir ； continental flood basalt ； Permian ； Neo-Tethys

參考文獻

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印度北部喀什米爾地區潘加爾火成岩區中皮爾潘加爾區域的地質年代學與地球化學之研究

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