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首页> 外文期刊>Chemical geology >Depleted and enriched mantle sources for Paleo- and Neoproterozoic carbonatites of southern India: Sr, Nd, C-O isotopic and geochemical constraints
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Depleted and enriched mantle sources for Paleo- and Neoproterozoic carbonatites of southern India: Sr, Nd, C-O isotopic and geochemical constraints

机译：印度南部古生代和新元古代碳酸盐岩的贫化和富集地幔源：Sr，Nd，C-O同位素和地球化学约束

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摘要

Paleoproterozoic (Hogenakal) and Neoproterozoic (Samalpatti, Sevattur, Mulakkadu-Pakkanadu) carbonatites of Tamil Nadu, southern India, have been investigated for whole-rock geochemistry and Nd, Sr and C-O isotopes. These temporally distinct carbonatite complexes are located close to a tectonically active zone that marks the transition between cratonic non-charnockitic (low- to medium-grade) terrain to the north and the charnockitic mobile belt (granulite facies) to the south. The carbonatites are variably enriched in LREE; the Hogenakal carbonatites being extremely enriched, with the highest SigmaREE among the data while the younger carbonatites show variable enrichment levels and broadly comparable REE patterns. The Hogenakal carbonatites have coherent and typically mantle C- and O-isotopic ratios (delta(13)C(V-PDB)similar to-6parts per thousand and delta(18)O(V)-(SMOW)similar to8parts per thousand). The Neoproterozoic carbonatites are relatively enriched in C-13 (delta(13)C(V-PDB)=-5.3parts per thousand to -3.3parts per thousand) and also show a range of O-isotopic composition (delta(18)O(V-SMOW)=7.3parts per thousand to 15.4parts per thousand; two samples similar to25parts per thousand). Higher delta(18)O values indicate variable degree of post-magmatic low-temperature alteration. The older carbonatites have marginally positive epsilon(Nd) (+0.54 and +1) and significantly low initial Sr-87/Sr-86 ratios (0.70161 and 0.70174) while younger carbonatites have rather low epsilon(Nd) (-16.5 to -6.23) and higher Sr-87/Sr-86 ratios (0.70486-0.70658). The Sr-Nd isotopic data are compatible with a depleted mantle source for the Hogenakal carbonatites and an EM-I-type enriched mantle component for the younger group. This is the first report of the existence of depleted mantle beneath the southern Indian continental crust. The stable isotopic ratios are interpreted as suggesting a depletion event (crustal extraction) in the south Indian subcontinental mantle similar to2.6 Ga ago. The depleted mantle was subsequently enriched by metasomatic fluids under the influence of the subducting Dharwar plate (sediments and modified oceanic crust). (C) 2002 Published by Elsevier Science B.V. [References: 94]
机译：已对印度南部泰米尔纳德邦的古元古代（Hogenakal）和新元古代（Samalpatti，Sevattur，Mulakkadu-Pakkanadu）碳酸盐岩进行了全岩石地球化学和Nd，Sr和C-O同位素研究。这些在时间上截然不同的碳酸盐复合物位于构造活动带附近，该带标志着北部的克拉通非砂岩质（中低品位）地形与南部的砂岩质移动带（粒岩相）之间的过渡。碳酸盐岩富含LREE; Hogenakal碳酸盐岩极富集，在数据中具有最高的SigmaREE，而年轻的碳酸盐岩表现出可变的富集度和广泛可比的REE模式。 Hogenakal碳酸盐岩具有连贯且典型的地幔C和O同位素比率（δ（13）C（V-PDB）约等于千分之六和δ（18）O（V）-（SMOW）约等于千八分） 。新元古代碳酸盐相对富集于C-13（δ（13）C（V-PDB）=-5.3份/千至-3.3份/千），并且还显示出一系列O同位素组成（δ（18）O （V-SMOW）= 7.3份至15.4份;两个样本相似于25份）。较高的delta（18）O值表示岩浆后低温变化的程度可变。年龄较大的碳酸盐岩的ε（Nd）略微为正（+0.54和+1），且初始Sr-87 / Sr-86的比率显着较低（0.70161和0.70174），而年龄较小的碳酸盐岩的ε（Nd）较低（-16.5至-6.23） ）和更高的Sr-87 / Sr-86比（0.70486-0.70658）。 Sr-Nd同位素数据与Hogenakal碳酸盐岩的贫化地幔来源和年轻组的EM-I型富集地幔组分兼容。这是关于印度南部大陆地壳下地幔耗尽的首次报道。稳定的同位素比率被解释为表明印度洋次大陆地幔中的耗尽事件（地壳提取）类似于2.6 Ga以前。枯竭的地幔随后在俯冲的Dharwar板块（沉积物和改良的洋壳）的影响下被交代液富集。 （C）2002由Elsevier Science B.V.出版[参考文献：94]

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