Structurally-controlled hydrothermal alteration in the syntectonic Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi): Implications for REE and HFSE mobilities
Structurally-controlled hydrothermal alteration in the syntectonic Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi): Implications for REE and HFSE mobilities
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DOI：
10.1016/j.precamres.2015.08.016
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发表时间：
2015-10-01
影响因子：
3.8
通讯作者：
Demaiffe, Daniel
中科院分区：
文献类型：
--
作者：
Decree, Sophie;Boulvais, Philippe;Demaiffe, Daniel
布隆迪新元古代鲁武布上部碱性深成杂岩体（URAPC）位于东非裂谷西支沿线。它由过饱和和欠饱和正长岩以及浅层碳酸盐岩体（马通戈碳酸盐岩）组成，该碳酸盐岩体未露头，但已通过钻芯取样。 URAPC 的椭圆形地图轮廓指向同构造位点。岩浆侵位期间活跃的大剪切带适应了区域性的北东-西南缩短。晚期岩浆到热液成因的矿化特征与碳酸岩有关，碳酸岩本身包含致密的方解石脉网络。以锆石和钛铁矿巨晶形式出现的 HFSE 矿化可以在强烈而广泛的钾铁矿化区域中发现，这导致周围的正长岩转变为主要的钾长石 + 黑云母矿物组合（Inamvumvu 地区）。碳酸盐岩脉（被热液蚀变覆盖）存在于马通戈碳酸盐岩以北几公里处，位于正长岩的高度变形区域内。这些岩脉与钠长岩一起出现（由长石正长岩转变为以钠长石为主的共生岩），并且富含稀土矿物（独居石和箭岩-(Ce)）。许多岩浆（伟晶岩）岩脉和热液（石英+赤铁矿）矿脉也出现在 URAPC 的剪切带中。其中大多数都可以解释为张力裂缝。一些碳酸岩全岩样品的球粒陨石归一化稀土元素模式受到高度干扰，与岩浆后热液蚀变有关。热液脉/脉（碳酸岩内的方解石脉、变形区热液蚀变覆盖的碳酸岩脉、以及锆石和钛铁矿巨晶）矿物中的 HFSE 和 REE 分布证明了蚀变过程中 REE 的复杂行为。马通戈碳酸岩和碳酸岩脉的氧和碳同位素组成具有岩浆特征，为 7.2
The Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (URAPC), Burundi, is located along the western branch of the East African Rift. It comprises oversaturated and undersaturated syenites and a shallow-level carbonatite body (the Matongo carbonatite) that does not outcrop but has been sampled by drill-cores. The elliptic map contour of the URAPC points to a syntectonic emplacement. Large shear zones that were active during magmatic emplacement have accommodated a regional NE-SW shortening. Mineralization features of late-magmatic to hydrothermal origin are associated with the carbonatite, which, by itself, contains a dense network of calcitic veins. HFSE mineralization occurring as zircon and ilmenite megacrysts can be found in an area of intense and extensive K-fenitization, which lead to the transformation of the surrounding syenite into a dominant K-feldspar + biotite mineral assemblage (Inamvumvu area). Carbonatitic dykes (overprinted by a hydrothermal alteration) are present a few kilometers north of the Matongo carbonatite, within highly deformed zones in the syenite. These dykes occur along with Na-fenites (resulting from the transformation of the feldspathoidal syenite into an albite-dominant paragenesis) and are enriched in REE-minerals (monazite and ancylite-(Ce)). Many magmatic (pegmatoid) dykes and hydrothermal (quartz + hematite) veins also occur in shear zones in the URAPC. Most of them can be interpreted as tension gashes. The chondrite-normalized REE patterns of some carbonatite whole rock samples are highly disturbed, in relation to post-magmatic hydrothermal alteration. The HFSE and REE distribution in the minerals from the hydrothermal veins/dykes (calcitic veins within the carbonatite, carbonatite dykes overprinted by a hydrothermal alteration in deformed zones, and zircon and ilmenite megacrysts) attests for a complex behavior of REE during alteration. Oxygen and carbon isotope compositions of the Matongo carbonatite and the carbonatitic dykes have a magmatic signature, with 7.2