西伯利亞東南部Vitim火成岩區地函捕獲岩之地球化學特性

Abstract

Vitim火成岩區位於俄羅斯西伯利亞東南部貝加爾湖(Lake Baikal)東方約200公里處，為新生代玄武岩質熔岩流所構成的火山高原，部份熔岩流含有豐富的地函擄獲岩(xenolith)，提供良好的材料研究此區域大陸岩石圈地函(subcontinental lithospheric mantle, SCLM)的地球化學特性。此區域的玄武岩質熔岩流噴發時間分兩個時期，分別為中新世及更新世，本研究所採集兩個時期的樣本之鉀氬年代分別是第一期約為14 Ma及第二期約為0.65 Ma。 此區域之地函捕獲岩以二輝橄欖岩(lherzolite)為主，含有橄欖石、直輝石、斜輝石、石榴石或尖晶石、或兩者兼有。大部分二輝橄欖岩為原粒狀組織(protogranular)，部分呈現殘碎斑狀組織(porphyroclastic)；少數橄欖石有急折帶(kink-band)，部分石榴石邊緣有綠泥鎂鋁榴石(kelyphite)發育但無與水反應作用形成的kelyphite III出現，而液包體(fluid inclusion)、礦脈(vein)與含水礦物較少發現。斜輝石體積百分比大於10%，且大部分有蠕蟲狀反應圈(spongy rim)，反應此區域岩石圈地函相對較富化且曾受過後期熱事件、小程度部分熔融或是交代變質作用影響。此區域尖晶石與石榴石相變的過渡帶約為78公里(平衡壓力約為26 kbar)。第一期及第二期的地溫梯度有隨著時間越來越高溫的趨勢，與前人(Ionov, 2004)研究結果相同，暗示此區域下方軟流圈地函有逐漸上湧的現象。 Vitim二輝橄欖岩的橄欖石鎂值約為89.2至90.7，及全岩的氧化鎂、氧化鋁及氧化鈣含量與鄰近西伯利亞古陸的二輝橄欖岩相較，顯示此區域下方為相對較富化的岩石圈地函。全岩的氧化鋁、氧化鈣及氧化鈦的含量變化，以及橄欖石的鎂值與尖晶石的鉻值及石榴石氧化鋁含量的變化趨勢，顯示Vitim二輝橄欖岩受控於部分熔融作用。模擬不同部分熔融程度下斜輝石核心的釔與鐿元素含量及鋯與鈦元素含量變化，得知Vitim火成岩區下方為經歷1%到15%部分熔融的殘餘岩石圈地函。大部分斜輝石核心相較於邊緣的氧化鈣含量較低而氧化鋁含量較高，推測是受到後期交代變質作用或附近為氧化鋁含量較低的耐熔地函圍岩影響。 斜輝石的稀土元素分佈形態有三種：虧損輕稀土元素型(depleted LREE type)、富集輕稀土元素型(enriched LREE type)及過渡型(transitional type)。虧損輕稀土元素型指示其為地函發生部分熔融後殘餘的岩石，未受到後期交代質作用影響。富集輕稀土元素型指示其明顯受到後期交代變質作用影響。過渡型中，大多數斜輝石核心是虧損輕稀土元素型，邊緣是富集輕稀土元素型，表示斜輝石邊緣已受到交代變質作用但核心尚未受到後期交代變質作用影響。在有限的全岩分析資料中，所有二輝橄欖岩亦呈現富集型態，顯示後期交代變質作用顯著影響此區域的岩石圈地函。Vitim二輝橄欖岩受到交代變質作用的類型為cryptic metasomatism，但有少數二輝橄欖岩為modal metasomatism；而交代變質作用的介質主要為矽質岩漿，其中有含水礦物的二輝橄欖岩呈現富集的大離子半徑元素(large ion lithosphile element, LILE)及虧損的高場強元素(high field strength element, HFSE)，表示受到與隱沒作用有關的含水流體影響。綜合本研究之斜輝石及全岩的鍶釹同位素值與前人報導值(Ionov et al., 2005)，顯示第一期二輝橄欖岩為較富集和虧損的端元成分混合，第二期二輝橄欖岩的富集端元成分混合的比例減少，亦是反應此區域有軟流圈上湧現象的證據。

Located east of Lake Baikal in SE Siberia, Vitim volcanic plateau is made of the Cenozoic lava flows with abundant mantle xenoliths that can shed light on composition and evolution of the subcontinental lithospheric mantle (SCLM) beneath the region. There are two main episodes of basalt erupted in Miocene and Pleistocene, and the mantle xenoliths included in this study cover both episodes. The majority of mantle xenoliths are garnet-, spinel-, or garnet-spinel-bearing lherzolites, with a protogranular texture and to a lesser extent porphyroclastic texture. Kink bands and fluid inclusions are common in the olivines. Spongy rims in clinopyroxenes and kelyphites around garnets are common in these lherzolites, which are attributed to heating, partial melting, or mantle metasomatism. Most lherzolites have high modal clinopyroxene (> 10 vol.%), implying these lherzolites are relatively fertile. Employing the two-pyroxene thermometer and the Al-in-opx barometer of Brey and Kohler (1990) yields equilibration temperatures and pressures of 1100 – 1250oC and 20.9 – 28.5 kbar for garnet-bearing lherzolites, and 1100 – 1200oC and 20.6 – 25.7 kbar for garnet-spinel-bearing lherzolites. The equilibrium temperatures of spinel-bearing lherzolites are 850 – 1050oC by using Ca-in-opx thermometer of Brey and Kohler (1990). The depth of spinel-garnet transition zone is at least 78 km (about 26 kbar). The depth of lherzolites captured by the Miocene basalts are deeper that those in the Pleistocene basalts. Moreover, the Pleistocene Vitim geotherm is hotter than the Miocene Vitim geotherm that similar to Ionov et al. (2004), implying that the advancing asthenosphere upwelled from Miocene to Pleistocene. Their Fo contents of olivines and whole-rock major elemental compositions indicate relatively fertile SCLM beneath the Vitim volcanic field. The Cr# in spinel and Al2O3 in garnets correlate with Fo contents of olivines show strongly controlled by partial melting. The Vitim lherzolites experienced about 1% ~ 15% partial melting by using the cpx melting model. The CaO and FeO of spongy rims and cores of clinopyroxenes show obvious variations that could be caused by late metasomatism and interaction with refractory mantle wall-rock. Trace element distribution patterns of clinopyroxenes have three different types: depleted LREE type, enriched LREE type, and transitional type. Depleted LREE type indicates these mantle rocks are residue of partial melting without experiencing late metasomatism. On the other hand, enriched LREE type shows evidence of late metasomatism. In contrast, transitional type exhibits composite trace element patterns, i.e., depleted LREE pattern at the core of cpx and enriched LREE patterns at the rim. Available whole-rock geochemical data of the Vitim lherzolites also show enriched LREE pattern, indicating these mantle rocks had experienced metasomatism. Most lherzolites were affected by cryptic metasomatism, and fewer lherzolites were affected by modal metasomatism. Silicate melt is the most likely agent to metasomatize SCLM beneath the region. Lherzolites with hydrous minerals are obviously affected by hydrous fluid, and have enriched LILE and depleted HFSE. The Sr-Nd isotopic ratios of the Vitim lherzolites, combined with those previously reported (Ionov et al., 2005), display that lherzolites captured by the Miocene basalts have wider isotopic range from depleted mantle component to enriched components, whereas lherzolites in the Pleistocene basalts have less enriched component involved. It shows that lherzolites captured by the Miocene basalts from the deeper SCLM were prone to metasomatism by ascending melts, whereas those in the Pleistocene basalts at the shallower depth experienced less extent of metasomatism. The temporal variation reinforced advancing asthenosphere upwelled beneath the Vitim region from Miocene to Pleistocene.