Composite Mantle Xenoliths in Basaltic Pyroclastic Rocks from Tungchihsu, Penghu Islands, Taiwan Strait: Evidence for a Metasomatized Lithospheric Mantle beneath SE China

澎湖東吉嶼玄武質碎屑岩內之複合地函捕獲岩：華南岩石圈地函換質作用之證據

何恭算(Kung-suan Ho) ； 陳汝勤(Ju-chin Chen) ； 鐘三雄(San-hsiung Chung)

《Collection and Research》 19期 (2006/12) Pp. 49-76

https://doi.org/10.6693/CAR.2006.19.7

摘要

Three main occurrences of composite mantle xenoliths have been found in outcrops of basaltic pyroclastic rocks from Tungchihsu, the Penghu Islands, Taiwan Strait: (1) an amphibole-rich selvage in contact with carbonated peridotite xenoliths; (2) a thin amphibole-rich spinel websteritic vein crosscutting amphibole-free carbonated peridotite xenoliths; and (3) a clinopyroxene hornblendite band adjacent to the edge of spinel-garnet clinopyroxenite. In the first type, the amphibole-rich selvage commonly contains amphibole, phlogopite, and clinopyroxene, a lesser amount of apatite, and rare orthopyroxene and Ti-Fe oxide minerals. The available major and trace elements, and Sr-Nd isotopic data for the amphibole-rich selvages and their minerals suggest that their genesis may have been similar to that of the Tungchihsu group I pyroxenites. They may represent the products of megacryst precipitation from melts with a similar composition to the Penghu alkali basalt. It was also noted that the mineral composition varies systematically with respect to the distance from the edge of the peridotite xenolith. The iron and titanium contents are higher and magnesium and chromium contents are lower farther from the boundary. These relations indicate that the compositional variation of all phases may have resulted from melt-peridotite interactions. In the second type, the amphibole-rich spinel websterite consists of abundant kaersutite, orthopyroxene, and clinopyroxene with accessory fine-grained spinel. The pyroxenes of the vein have major-element compositions similar to enstatites and diopsides of the ambient peridotite, but are slightly richer in Ti and more depleted in Cr. In addition, the kaersutite in the vein has a higher TiO_2 content when compared with those of interstitial amphiboles of group II pyroxenite and mosaic amphiboles of group I pyroxenite. From petrographic evidence and mineral chemistry, we suggest that the vein may have formed from a relatively magnesium-rich, hydrous basic melt in the fracture of peridotite. In the third type, the spinel-garnet clinopyroxenite contains clinopyroxene, garnet, and spinel. The mineral constituents and compositions are similar to those of the Tungchihsu group II pyroxenites, and may have formed by crystallization of tholeiitic picritic magmas under high pressure. The clinopyroxene hornblendite is composed of interstitial amphibole (pargasite) and clinopyroxene. From mineral chemical data, we suggest that the interstitial amphiboles may have formed by a metasomatic interaction of clinopyroxene, spinel, and garnet of the spinel-garnet clinopyroxenite with a fluid enriched in Ti, Na, K, Ba, Sr, LREE, and H_2O. The lithospheric mantle of SE China was subjected to one or more metasomatic events which formed amphibole-rich selvages/veins and group I pyroxenites, and introduced amphibole and/or phlogopite into both peridotites of composite xenoliths and group II pyroxenites. The enrichment processes modified the chemical composition of the lithospheric mantle beneath SE China.

關鍵字

Composite mantle xenoliths ； Mantle metasomatism ； Melt-peridotite interaction ； Lithospheric mantle ； SE China

並列摘要

澎湖束吉嶼玄武質碎屑岩中發現三種不同產狀的複合地函捕獲岩:(1)富含角閃石岩體鑲嵌於碳酸鹽化橄欖岩包體外緣;(2)富含角閃石之尖晶石二輝岩岩脈貫入碳酸鹽化橄欖岩包體內;(3)斜輝角閃石岩與尖晶石－石榴石斜輝岩毗鄰共存。產狀一中，鑲邊岩體通常含有角閃石、雲母、斜輝石，和少量的磷灰石及鈦－鐵氧化物礦物。從全岩及其組成礦物之主要及微量元素含量和鍶－釹同位素組成研判，富含角閃石鑲邊岩體與東吉嶼第一群輝石岩有相似的成因，推斷可能是近似澎湖鹼性玄武岩成分的岩漿在地函內結晶的產物。另一方面，鑲邊岩體的礦物成分也隨著距離橄欖岩包體邊界的遠近而主系統性的變化。離橄欖岩包體愈遠者，鐵和鈦含量增高，鎂和鉻含量則降低。此一變化趨勢顯示，組成礦物的成分也受到岩漿與橄欖岩相互作用的影響。產狀二中，二輝岩岩脈含有豐富的鈦角閃石、直輝石、斜輝石和少量的細粒尖晶石。輝石的主要成分雖大致與周圍橄欖岩的頑火輝石和透輝石相似，但鈦含量稍高，鉻含量偏低。此外，岩脈內鈦角閃石的鈦含量不但比第二群輝石岩之填隙型角閃石，甚至較第一群輝石岩之嵌晶型角閃石還高。從岩相學和礦物化學推斷，富含角閃石岩脈可能是富鐵、富鈦之含水基性岩漿在橄欖岩裂隙中沈聚堆積的結果。產狀三中，尖晶石-石榴石斜輝岩的礦物種類和成分與東吉嶼第二群輝石岩相似，它可能是矽質苦橄質岩漿在高壓環境下結晶的產物。而斜輝角閃石岩是由填隙型角閃石和斜輝石所組成，因填隙型角閃石乃是斜輝石、尖晶石/石榴子石與富含鈦、鈉、鉀、鋇、鍶、輕稀土元素與水之換質溶液作用而來，因此，斜輝角閃石岩可能是部分尖晶石－石榴石斜輝岩發生換質作用形成填隙型角閃石與殘存斜輝石所構成的岩體。華南岩石園地函經歷了一次或一次以上的交代換質事件，不但形成富含角閃石鑲邊岩體/岩脈和第一群輝石岩，同時也在複合地函捕獲岩內之橄欖岩及第二群輝石岩中賦存角閃石和(或)金雲母。此等富化作用改變了華南地區岩石圈地函的化學組成。

並列關鍵字

複合地函捕獲岩 ； 地函換質作用 ； 岩漿－橄欖岩相互作用 ； 岩石圈地函 ； 華南地區