| 摘要: | 太古代Olondo綠岩帶 (greenstone belt; OGB) 出露於俄國西伯利亞古陸最大的基盤阿爾丹地盾(Aldan Shield)之上。相較於其他產於世界各地的綠岩帶，其基性-超基性岩成份比例約佔整體超過30%，為一研究當時地函成份一重要的題材。本研究進行一系列Olondo綠岩帶 超基性-基性-酸性岩石完整的的全岩地球化學元素與同位素分析，以期制約Olondo綠岩帶的起源以及其在太古代形成時可能的地球動力學機制。超基性純橄岩(dunite)的錸-鋨同位素的 TMA 模式年代為 2960-3020 百萬年，與之前報導的Olondo綠岩帶的30億年形成年代相當。超基性岩以新鮮和蛇紋石化的純橄岩為主，主要地化成分包括P-鉑族元素 (P-PGEs) 相對於 I-PGEs 的明顯虧損，表明其為歷經大程度部分融熔後的虧損殘餘地函岩石。純橄岩亦呈現 U形的稀土元素 (REE) 分配型式，在微量元素分配型式的蛛網圖中呈現具有從正到負鈮(Nb)異常，指示其源區地函曾受後期交代變質作用。與大多數太古代超基性岩石的堆晶(accumulate)岩起源不同，Olondo綠岩帶純橄岩是大程度部分熔融（>30%）後的殘餘地函岩石，隨後被與隱沒帶相關的熔體或流體交代變質而造成所具有的地化特徵。另一方面，包括科馬提岩和矽質玄武岩的Olondo綠岩帶基性岩石亦呈現相似於殘餘純橄岩的地球化學特徵，強化此Olondo綠岩帶超基性－基性岩石形成過程中涉及的隱沒帶相關作用，即便其他基性岩石的形成仍與現今中洋脊和地函柱等的構造環境相關。矽質玄武岩具有從輕稀土虧損、近似隕石原始值到富集(LREE)的稀土元素分佈型式，另具有不等程度的鈮－鉭(Ta)負異常，表明其與現今虧損中洋脊玄武岩(N-MORB)和玻安岩(boninite)的地化性質相似，和現今典型隱沒帶所發現的蛇綠岩相當。這種具有較低 εNd(t)和鈮－鉭負異常的元素特徵很可能是與隱沒成分混合的結果，與觀察到的殘餘純橄欖岩的鈮虧損一致。鋁虧損的科馬提質玄武岩可能起源於深部地函，由其虧損的重稀土元素得以證實，應該是在石榴子石穩定的溫壓狀態的深度（>70 km in depth）下，需憑藉地函柱將更深的地函輸送至淺部與熔融。在Olondo綠岩帶中所發現的安山岩全岩鎂質從44到52，部分安山岩鎂質含量甚高，鉻和鎳含量亦高。由這些中酸性岩可辨識出埃達克岩。其呈現輕稀土富集的稀土元素分佈型式，和在蛛網圖的鈮和鈦的負異常，其岩石地化特徵類似於新生代埃達克岩，為隱沒板塊熔融所形成。Olondo綠岩帶的超基性-基性岩可能是見證地函柱、中洋脊海底擴張和隱沒帶等活動在中太古代時期發生的地函柱誘發隱沒起始過程的記錄。Olondo綠岩帶與中太古代的其他綠岩帶相似，是在地函柱－隱沒帶作用共生的環境中形成的。這表明中太古代時期可能標誌著早期地球從地函柱為主的地體動力演化轉變成板塊構造體系的過渡階段。;The Archean Olondo greenstone belt (OGB) is located on the Aldan shield, the largest basement of the Siberia craton. With well-preserved abundant mafic-ultramafic rocks, ≥ 30% in volume, the OGB is unique among other greenstone belts in the world. In this study, Ipresent a comprehensive geochemical and isotopic data for the OGB rocks, in order to better constrain their origin and the geodynamic process involved in their formation in the Archean time. Rhenium-osmium isotopic data of the ultramafic rocks yield TMA model age of 29603020 Ma, comparable to the formation age of the OGB at 3 Ga. The ultramafic rocks vary from fresh to serpentinized dunites, which are highly refractory residual mantle rocks evidently indicated by depletion in P-Platinum Group Elements (PGE) relative to I-PGEs. Fresh dunites show U-shaped rare earth element (REE) patterns, with positive to negative Nb anomalies, indicative of metasomatic overprint. Unlike having a cumulate origin for most Archean ultramafic rocks, the OGB dunites were mantle residues after high degree of partial melting (>30%), subsequently metasomatized by the subduction-related melt/fluid. On the other hand, the OGB volcanic rocks including komatiitic and tholeiitic basalts show geochemical characteristics relative to the residual dunites, reinforcing subduction-related processes involved in some of their formation, despite extra mid-ocean ridge and plume activities associated with other mafic rocks. Tholeiitic basalts yield variable REE patterns from depleted, flat, to enriched light rare earth elements (LREE) patterns, with variable Nb-Ta anomalies, indicating their similarities with modern N-MORB and boninites, comparable to those found in typical supra-subduction zone (SSZ) ophiolites. Such elemental characteristics with combined lower εNd(t) and negative Nb-Ta anomalies are most likely a result of mixing with subducted components, consistent with the observed Nb depletion in the residual dunites. The Al-depleted komatiitic basalts may have originated from deep mantle source, corresponding to garnet stability field, confirmed by their depletion in HREE and requires a mantle plume to transport and melt at deeper depth. Additionally, the OGB has documented the occurrence of magnesian andesites, andesites, rhyolites, and Nb-enriched basalts. Magnesian andesites show high Mg# (52) with elevated Cr and Ni content. Andesite-rhyolite display LREE enriched patterns and negative Nb and Ti anomalies, similar to Cenozoic adakites. They could be generated by melting of subducted slab. Nb-enriched basalts (NEBs) exhibit elevated concentrations of Na2O, P2O5, TiO2, and high Nb contents (>6 ppm). They are characterized by LREE enrichment with negative Nb anomalies. They are most likely the result of mantle wedge metasomatized by Olondo adakitic magmas during magma ascent.
The OGB ultramafic-mafic rocks could be a record to witness plume-induced subduction initiation processes such that mantle plume, sea-floor spreading and subduction were all in operation in the Mesoarchean time. The subduction initiation was triggered by a mantle plume, which also provided higher thermal conditions for slab melting. The NEB-Mg andesite-adakites assemblage is evidence of a young, hot subduction process.
The OGB, as some other greenstone belts in Mesoarchean, was formed in a combined plume-arc setting. This suggests that the Mesoarchean time might mark the transition stage from dominantly plume to plate tectonic regime on the Earth. |
