“Satellite monazites” in polymetamorphic basement rocks of the Alps: Their origin and petrological significance
“Satellite monazites” in polymetamorphic basement rocks of the Alps: Their origin and petrological significance
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DOI：
10.2138/am-2016-5477
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发表时间：
2016-05
影响因子：
3.1
通讯作者：
F. Finger;E. Krenn;B. Schulz;D. Harlov;D. Schiller
中科院分区：
文献类型：
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作者：
F. Finger;E. Krenn;B. Schulz;D. Harlov;D. Schiller
摘要变质岩中存在丰富的单独居石周围的Allanite-fluorapatite反应日冕。我们在这里报告了新一代“卫星”独居石颗粒在这些日冕中形成的特殊情况。本文以阿尔卑斯东部不同P-T区为例，利用电子探针、Th-U-Pb独居石测年和高分辨率BSE成像技术，研究了这种复杂矿物组合的成因及其岩石学意义。当含独居石的岩石在allanite稳定性场中变质（第一代独居石部分分解为氟磷灰石加allanite）时，卫星独居石颗粒形成，然后加热到允许氟磷灰石加allanite反反应的温度，形成围绕剩余的原始第一代独居石的次级卫星独居石颗粒。根据卫星全岩地球化学特征，单独居石可形成于上部绿片岩和角闪岩相条件下。在这三个例子中，尽管其中一个样品的变质温度达到了720°C，但继承的核心独居石都能抵抗再结晶和同位素重置。这表明在绿片岩相和角闪岩相多变质岩中，继承的单殖石和新形成的单殖石的单个颗粒可以并且经常会并排出现。原始的、遗传的独居石将优先保存在低钙、高铝的岩性中，在那里它分解成allanite + fluorapatite被抑制。相反，中钙或高钙含独居石的岩石在allanite稳定性场中经过一个强退退阶段后，将成为次生独居石再生的特别肥沃的土壤。在此基础上，可以制定多变质基底单独居石定年活动的具体采样策略。
Abstract Allanite-fluorapatite reaction coronas around monazite are abundant in metamorphic rocks. We report here special cases where a new generation of “satellite” monazite grains formed within these coronas. Using examples from different P-T regions in the eastern Alps, we examine the origin and the petrological significance of this complex mineralogical association by means of the electron microprobe utilizing Th-U-Pb monazite dating and high-resolution BSE imaging. Satellite monazite grains form when a monazite-bearing rock is metamorphosed in the allanite stability field (partial breakdown of first generation monazite to fluorapatite plus allanite), and is then heated to temperatures that permit a back reaction of fluorapatite plus allanite to secondary satellite monazite grains surrounding the remaining original first generation monazite. Depending on the whole-rock geochemistry satellite monazites can form under upper greenschist- as well as amphibolite-facies conditions. In each of the three examples focused on here, the inherited core monazite was resistant to recrystallization and isotopic resetting, even though in one of the samples the metamorphic temperatures reached 720 °C. This shows that in greenschist- and amphibolite-facies polymetamorphic rocks, individual grains of inherited and newly formed monazite can and often will occur side by side. The original, inherited monazite will preferentially be preserved in low-Ca, high-Al lithologies, where its breakdown to allanite plus fluorapatite is suppressed. Conversely, a medium- or high-Ca, monazite-bearing rock will become particularly fertile for secondary monazite regrowth after passing through a phase of strong retrogression in the allanite stability field. Based on this knowledge, specific sampling strategies for monazite dating campaigns in polymetamorphic basement can be developed.