埃達克岩與花崗岩類中鋯石微量元素組成在岩石成因上的應用

Abstract

在地球演化相關研究中，鋯石長期以來被視為可以記述大陸地殼演化的時空膠囊。根據前人的地球化學分析結果知道大陸地殼平均組成在晚太古宙從TTG岩套轉變為花崗岩類，鑑於鋯石對親岩元素有較高的相容性，一般認為其有機會將岩漿的化學成分差異記錄在組成中。因為埃達克岩為TTG岩套的現代類比，所以本研究針對西藏南部後碰撞埃達克岩、岡底斯I型花崗岩類、蘇門答臘高/低總REE濃度花崗岩類以及多巴火山岩的岩漿鋯石進行電子微探和雷射剝蝕感應耦合電漿質譜原位分析，分別利用這兩個方法測量鋯石中三個主量元素和二十九個微量元素的含量，並比較埃達克岩與花崗岩類的鋯石化學組成。 結果顯示本研究的鋯石組成成分為31.4–34.8 wt.%的SiO2、60.2–68.6 wt.%的ZrO2、0.6–2.4 wt.%的HfO2，以及200–5000 ppm的稀土元素，並在稀土元素分配圖上呈現Ce與Eu的異常，而儘管埃達克岩相對花崗岩類的全岩組成具有明顯的重稀土元素虧損現象，但在鋯石稀土元素分配圖上樣本間幾乎不存在區別。並且任一項鋯石化學組成特徵皆未與全岩SiO2含量和ASI產生相關性，指示結晶分異作用不會對鋯石的組成造成顯著影響。本研究認為鋯石無法像全岩可以反映埃達克岩及花崗岩類的組成差異，除了晶格應變和電荷平衡要求的影響之外，亦可能起因於樣本中的鋯石是在晶漿環境中生長，周圍的岩漿已經演化到中後期，所以不同群體鋯石所記錄的岩漿組成並非平均熔體成分，且彼此差異很小；另一方面，先於或與鋯石同時結晶的礦物相(如磷灰石、獨居石等)也會降低熔體中輕稀土元素和中稀土元素的含量，所以會將岡底斯埃達克岩鋯石原本應該繼承的重稀土元素虧損特性消除掉。 雖然上述結果顯示埃達克岩與花崗岩類中的鋯石無明顯的化學成分差別，但本研究嘗試利用統計分析對龐大的鋯石微量元素組成資料庫進行處理，目的是為了客觀挑選出埃達克岩和花崗岩類鋯石中其他的成分判別因子，最後共選出Ti、V、Yb、Hf、Sc/Yb、U/Yb、Eu/Eu*、ΣHREE等八個判別因子。即使統計分析的結果無法利用雙變數圖分開埃達克岩和花崗岩類的鋯石群體，判別因子的線性組合仍可以提供一個有潛力的族群判別方式，將兩個鋯石群體分開。

Zircon has long been proposed as a time capsule of crustal formation. Concerning of its high capacity of lithophile elements, the dramatic change in crustal chemical composition in late Archean, i.e. from TTG suites to granitoids, may be recorded in zircon remnants. In this study, geochemical contents of zircons from adakites, a modern analogue of TTG suites, in southern Tibet were determined and compared with those in Gangdese I-type granitoids, Sumatra high/low ΣREE granitoids and Toba volcanics in order to examine the hypothesis. For zircons in each sample group, electron probe microanalysis and laser ablation-inductively coupled plasma mass spectrometry were conducted in order to provide in situ concentrations of 3 major- and 29 trace elements, respectively. Zircons in this study have a common composition of 31.4–34.8 wt.% SiO2, 60.2–68.6 wt.% ZrO2, and 0.6–2.4 wt.% HfO2 with ΣREE abundances of 200–5000 ppm, Ce positive anomaly and Eu negative anomaly in REE patterns. The REE patterns of zircons show little inter-sample discrepancy though there is significant difference in whole-rock HREE contents between adakites and granitoids. Since none of geochemical feature, including REE contents, of zircons in this study correlates with SiO2 content or ASI of corresponding host rocks, fractional crystallization has an insignificant impact on the compositional variation in zircons. In addition to the influence of lattice strain and charge balance requirements, zircons in these rock samples are proposed to crystallize from the magma mush, so they record the composition of evolved magma with least composition difference, not that of the bulk melt. More specifically, the pre-/co-existing mineral phases, i.e. apatite, monazite, etc., play a critical role in preferentially taking the LREE and MREE from melt, and eliminating the HREE depletion characteristics in residual melt and thus zircons of Gangdese adakites. With the aim of objectively identifying geochemical discriminants of zircons from adakites and granitoids, statistical analysis was used and then 8 parameters were selected, i.e. Ti, V, Yb, Hf, Sc/Yb, U/Yb, Eu/Eu*, ΣHREE. Despite the fact that the zircon populations of adakites- and granitoids-origins more or less overlap in any bivariate plot, the linear combination of discriminants provides a potential way to distinguish zircons from these two groups.