Host rock compositional controls on zircon trace element signatures in metabasites from the Austroalpine basement
Host rock compositional controls on zircon trace element signatures in metabasites from the Austroalpine basement
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DOI：
10.1016/j.gca.2005.10.001
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发表时间：
2006-02
影响因子：
5
通讯作者：
B. Schulz;R. Klemd;H. Brätz
中科院分区：
文献类型：
--
作者：
B. Schulz;R. Klemd;H. Brätz
新元古代和早古生代年龄的锆石群发生在高压角闪岩相单位的Austroalpine基底南部的陶恩窗的变基性岩。这些锆石的寄主岩石是具有N-MORB型特征的榴辉质斜长角闪岩、具有火山弧玄武岩特征的角闪片麻岩和碱性板内玄武岩斜长角闪岩。大块岩石岩浆微量元素模式保存在角闪岩相高压和随后的高温事件，以及绿片岩相叠印。LA-ICP-MS分析表明，这些锆石具有Ce正异常和Eu负异常，且在锆石稀土元素标准化模式中重稀土元素富集，具有典型的岩浆成因特征。锆石Y与HREE、Ce、Th、U、Nb和Ta具有良好的相关性，并可区分每种寄主岩石类型的成分场。低Th/U比值与低Th/U比值围岩中锆石的低Y和HREE丰度有关。这可能是一个主要的火成岩特征，不能归因于变质重结晶。锆石/寄主岩石元素比值的变化证实了离子半径和电荷控制了锆石中许多微量元素的丰度。微量元素比值-矿物/熔体分配系数-表明选择性抑制的Zr和Si的HREE和Y的锆石从N-MORB熔体结晶的替代。相关的寄主岩石和锆石微量元素浓度表明，变基性岩锆石不是捕虏晶，而是从镁铁质熔体结晶，由实际的寄主岩石。
Zircon populations of Neoproterozoic and early Paleozoic age occur in metabasites of a high-pressure amphibolite-facies unit of the Austroalpine basement south of the Tauern Window. The host rocks for these zircons are eclogitic amphibolites of N-MORB-type character, hornblende gneisses with volcanic-arc basalt signature, and alkaline within-plate-basalt amphibolites. Bulk rock magmatic trace element patterns were preserved during amphibolite-facies high-pressure and subsequent high-temperature events, as well as a greenschist-facies overprint. Positive Ce and negative Eu anomalies and enrichment of HREE in normalized zircon REE patterns, as analysed by LA-ICP-MS, are typical for an igneous origin of these zircon suites. Zircon Y is well correlated to HREE, Ce, Th, U, Nb, and Ta and allows discrimination of compositional fields for each host rock type. Low Th/U ratios are correlated to low Y and HREE abundances in zircon from low bulk Th/U host rocks. This is likely a primary igneous characteristic that cannot be attributed to metamorphic recrystallization. Variations of zircon/host rock element ratios confirm that ionic radii and charges control abundances of many trace elements in zircon. The trace element ratios—presented as mineral/melt distribution coefficients—indicate a selectively inhibited substitution of Zr and Si by HREE and Y in zircon which crystallized from a N-MORB melt. Correlated host rock and zircon trace element concentrations indicate that the metabasite zircons are not xenocrysts but crystallized from mafic melts, represented by the actual host rocks.