富化岩石圈地函的地球化學特徵：以蒙古Tariat地函橄欖岩為例

Abstract

位於蒙古中部杭愛山區(Hangay Mountain)北緣的Tariat火成岩區，火山活動開始於上新世，持續活動至數千年前。許多火山口伴隨著火山岩流和碎屑岩在此區域廣泛分布，部份火山岩流富含來自地函的橄欖岩(peridotite)捕獲岩，因而提供良好的材料研究Tariat地區大陸岩石圈地函(subcontinental lithospheric mantle；SCLM)的物質組成、熔融過程和交代變質作用。 透過岩象觀察，此區域之地函捕獲岩以尖晶石二輝橄欖岩(spinel lherzolite)為主，組織型態為等粒狀(coarse-grained equigranular)，部分橄欖石具有急折帶微構造(kink-band)，顯示受到輕微的應力作用。另外，在少部分的橄欖岩中，發現具有: (1)含水礦物，如磷灰石(apatite)；(2)斜輝石中的液包體(fluid inclusion)；(3)斜輝石邊緣之蠕蟲狀反應圈(spongy vermicular aggregates)，皆明顯指示曾受到交代變質作用。絕大部分橄欖岩中的斜輝石體積百分比為10-15 %，且較少橄欖岩受到交代變質作用，因此反映此地區之岩石圈地函為富化(fertile)且無水(anhydrous)環境。主要元素方面，無論是全岩或礦物化學特徵，元素間對應關係和部分熔融後殘餘物質演化的趨勢相似。全岩鎂莫爾分率(Mg# = 100 Mg / (Mg+Fe)*100 )為0.89-0.91，顯示為地函物質經部分熔融後的殘餘岩石。由這些橄欖石在模式橄欖石比例對應橄欖石鎂莫爾分率的”Boyd diagram”分布顯示，除了兩個橄欖岩樣本具有較高的鎂莫爾分率外，其餘主要落在現今海洋橄欖岩趨勢(oceanic peridotite trend)上，暗示在此區域之岩石圈地函可能是富化的軟流圈地函 (fertile asthenospheric mantle)或是成分接近原始地函(primitive mantle)。以本研究中最富化且未受到明顯交代變質作用之橄欖岩樣本ST0802 (Mg# =0.89)，比較前人所計算出來的原始地函值(McDonough and Sun, 1995; Palme and O’Neill, 1985; Hart and Zindler, 1986; Ringwood, 1979)，其主要元素成份極為一致，代表此地區部份岩石圈地函成份可能極為接近原始地函的特徵。透過直輝石中Ca含量計算此區域尖晶石二輝橄欖岩之平衡溫度(Brey and Kohler, 1990) 為862°C 至 1041°C，亦落在前人所建立完成的Tariat地溫梯度(Ionov, 2007)之尖晶石二輝橄欖岩深度範圍內。 微量元素方面，藉由模擬全岩Ti-Yb與斜輝石Y-Yb和Zr-Ti在部份熔融後殘餘岩石的含量，表明此地區絕大部分尖晶石二輝橄欖岩為經歷小於10%部份熔融後的殘餘岩體，只有一個相對耐熔(refractory)之橄欖岩(Mg# = 0.91)為經歷較大程度部份熔融( > 10%)。從斜輝石之稀土元素分布圖(Rare Earth Element (REE) pattern)來看，可以分為虧損型式(depleted pattern)、富集型式(enriched pattern)和過渡型式(transitional pattern)。虧損型式指示典型地函部分熔融後殘餘的岩體，未受到或受輕微之後期交代變質作用影響。富集模式和過渡型式指示受到交代變質作用影響。其中，斜輝石之所以具有富集或過渡模式，推測乃和其距離岩脈遠近的不同位置所造成的差異。透過斜輝石中 (Pb/Nb)N - Ce/Pb、Ce/Pb - Ceb和(La/Yb)N - Ti/Eu作圖，顯示造成Tariat地區交代變質作用的介質主要有含水流體、矽質岩漿和碳酸鹽質岩漿，因此推斷此區域至少經歷三期以上交代變質作用。在整體不相容元素分佈圖中的高場強元素(High Field Strengh Element; HFSE)負異常亦應由含水流體交代變質所造成，可能和中亞造山帶伴隨的隱沒帶作用有關。 Sr-Nd同位素方面，具有斜輝石REE虧損型式的橄欖岩(未受到後期交代變質作用)，顯示較高的143Nd/144Nd (0.51282-0.51307)，和較低的87Sr/86Sr (0.70292-0.70434)，落在現今大西洋玄武岩(Pacific mid-ocean ridge basalt; P-MORB)和印度洋玄武岩(Indian mid-ocean ridge basalt; I-MORB)範圍內。最富化之橄欖岩ST0802接近地球初始組成值(bulk earth value)，與上述討論的主要元素特徵相符合，顯示Tariat地區之岩石圈地函不均質性。這些橄欖岩之Nd模式年齡(Nd model age)為0.9至2.3 Ga，推測此區域岩石圈地函自元古代(Proterozoic)就已經存在。相反的，具有斜輝石REE富集型式和過渡型式的橄欖岩(受到後期交代變質作用)，比起斜輝石REE虧損的橄欖岩含有較低143Nd/144Nd (0.51264-0.51291)和較高87Sr/86Sr (0.70335-0.70511)，指示這些橄欖岩為二元端成分混合的結果。一端元為虧損地函質(depleted MORB mantle; DMM)，另一端元為具有較高 87Sr/86Sr 和較低 143Nd/144Nd之特徵。最後，綜合所有地球化學特徵，在Tariat地區，存在古老且虧損的元古代岩石圈地函散布於年輕且富化的岩石圈地函，顯示此地區岩石圈地函的不均質性。

In the Tariat region, mantle-derived xenoliths of Tertiary basalts provide direct information on the nature of the lithospheric mantle. Most Tariat spinel lherzolites, with up to 15 vol.% of modal clinopyroxene (cpx), have neither hydrous minerals nor “melt pocket”, implying that some parts of the subcontinental lithospheric mantle (SCLM) are relatively fertile and anhydrous. Other cpx-rich (15-20 vol.%) lherzolites contain apatites, melt pockets and fluid inclusions in orthopyroxene (opx) and cpx porphyroblasts. Such mineralogical characteristis reflect modal metasomatism. Using the Ca-in-opx thermometry of Brey and Kohler (1990), the equilibration temperatures of Tariat spinel lherzolites range from 862°C to 1041°C, similar to that reported by Ionov (2007). Whole-rock Mg# (0.89-0.91; mainly between 0.89 and 0.90) and Fo contents of olivines in spinel lherzolites suggest that part of the SCLM beneath this region is fertile and similar to the estimated composition of primitive or fertile asthenospheric mantle. The mineral and whole-rock chemical compositions demonstrate that melt extraction was responsible for the geochemical variation of the Tariat lherzolites. Trace element (Y-Yb, Zr-Ti) modeling of cpx indicates that most Tariat spinel lherzolites had experienced less than 10% of melt extraction, except one lherzolite which requires a higher degree of melt extraction ( > 10%). Rare earth element (REE) patterns of cpx can be divided into three types: depleted, enriched and transitional types. The depleted patterns are typical of unmetasomatised, relatively fertile lithospheric mantle. The enriched and transitional patterns suggest that lherzolites had undergone enrichment. In the REE-depleted group of lherzolites, complementary Ti and Zr anomalies in opx (positive) and cpx (negative) have been observed as reported on mantle peridotites worldwide. In some lherzolites, cpx maybe REE-enriched, but other HFSE’s depleted, suggesting that the metasomatism could have occurred in a subduction zone. Plots of (Pb/Nb)N vs Ce/Pb, Ce/Pb vs Ce and (La/Yb)N vs Ti/Eu of cpx suggest that some lherzolites in the REE-enriched and transitional groups are most likely products of hydrous fluid, silicate and carbonatitic metasomatism. Therefore, at least three episodes of metasomatism are recognized in the SCLM beneath the Tariat region. Sr-Nd analysis in the Tariat lherzolites reveals ancient ages for the lithospheric mantle formation and suggests that some Proterozoic mantle domains have survived, and probably metasomatic enrichment.