Nd–Hf–Sr–Pb isotopes and trace element geochemistry of Proterozoic lamproites from southern India: Subducted komatiite in the source

نویسندگان

  • Ramananda Chakrabarti
  • Asish R. Basu
  • Dalim K. Paul
چکیده

The probable sources of some of the famous Indian diamonds are the 1.2 Ga old Krishna lamproites of Southern India, a rare Proterozoic occurrence of lamproites which are usually Cretaceous or younger in age. In this study we report Nd, Sr, Pb and Hf isotopes and multiple trace element concentrations of the Krishna lamproites. The goals are to evaluate mantle-processes and the petrogenesis of these ultrapotassic rocks of extreme chemical composition in light of these geochemical data, including their major element compositions. The Krishna lamproites show nearly uniform, parallel rare earth element (REE) distribution patterns with high concentrations and extreme light-REE enrichment (La/Yb(N)=41–88), high average concentrations of Ba (∼1200 ppm), Sr (∼1200 ppm), Zr (∼930 ppm), La (∼230 ppm), high U/Pb and Th/U ratios with notable absence of any Eu-anomaly. These rocks are typically porphyritic without any evidence of crystal accumulation, and have moderately high Mg-numbers (59–73) along with high Ni (average ∼301 ppm, highest 819 ppm) and Cr (average ∼183 ppm, highest 515 ppm) concentrations that show a positive correlation with MgO (wt.%), implying a role of olivine in the melt source. The low SiO2 content (lowest 37.8%, average 49%) and high Nb (average 147 ppm), Zr, Sr, as well as Ni and Cr in these rocks indicate lack of upper continental crustal contribution in the genesis of these rocks. The initial Pb-isotopic composition of these lamproites is unusual in that in a Pb/Pb vs. Pb/Pb plot, these rocks plot to the left of the 1.2 Ga geochron (age of emplacement), unlike most mantle-derived rocks. This Pb-isotopic signature and the superchondritic Nb/Ta ratios (average 23.6) of these rocks rule out their derivation from a metasomatized subcontinental lithospheric mantle. The high Pb/Pb at low Pb/Pb indicates an Archean component in the source of these rocks. We argue that this Archean crustal component, which produced the low-SiO2 lamproites along with the high Ni and Cr must have been ultrabasic, and we propose a model in which these lamproites formed by partial melting of metasomatized, subducted Archean komatiite in a peridotite mantle-source assemblage. In addition, these rocks display initial Hf isotopic compositions similar to Al-depleted komatiites, and high Nb/U, Nb/Th, and TiO2 as well as low Al2O3/TiO2 ratios (1.1–4.2) and average CaO/ Al2O3 of ∼1.6 that are also similar to Archean komatiites. This is also supported by the initial Pb isotopic composition of the Krishna lamproites, requiring evolution in a variably high U/Pb, Th/Pb reservoir early in earth history, possibly resulting from preferential segregation of Pb relative to U and Th in the sulfides of the komatiite. Chemical Geology 236 (2007) 291–302 www.elsevier.com/locate/chemgeo ⁎ Corresponding author. Fax: +1 585 244 5689. E-mail address: [email protected] (A.R. Basu). 0009-2541/$ see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2006.10.006 Au th or 's pe rs on al co py The Al-depleted subducted komatiitic component was enriched by carbonate metasomatism in the peridotitic mantle. This metasomatismwas responsible for the observed Nd–Hf isotope characteristics, specifically variable εNd(T) at relatively constant εHf(T) in the lamproites. This Nd–Hf-isotopic characteristic seems to be common in global lamproites of all ages. Our proposedmodel for the genesis of the Krishna lamproites involving a subducted komatiitic source may also be applicable for other global lamproites from cratonic settings, as older komatiite-bearing subducted crustal components were possibly ubiquitous in the architecture of ancient cratonic mantle. © 2006 Elsevier B.V. All rights reserved.

برای دانلود متن کامل این مقاله و بیش از 32 میلیون مقاله دیگر ابتدا ثبت نام کنید

ثبت نام

اگر عضو سایت هستید لطفا وارد حساب کاربری خود شوید

منابع مشابه

Origin and Source Evolution of the Leucite Hills Lamproites: Evidence from Sr–Nd–Pb–O Isotopic Compositions

Whole-rock major and trace element and O, Sr, Nd and Pb isotopic data are reported for 3.0–0.89 Ma lamproites from the Leucite Hills, Wyoming, USA. The two main groups of lamproites, madupitic lamproites and phlogopite lamproites, are geochemically distinct and cannot be related to one another by either fractional crystallization or crustal contamination. It seems likely that the geochemical di...

متن کامل

Trace element and Sr-Nd-Pb isotope geochemistry of Rungwe Volcanic Province, Tanzania: implications for a Superplume source for East Africa Rift magmatism

*Correspondence: Paterno R. Castillo, Geosciences Research Division, Scripps Institution of Oceanography, University of California, San Diego, 8615 Kennel Way, La Jolla, CA 92037, USA e-mail: [email protected] †Present address: Sæmundur A. Halldórsson, Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Reykjavik, Iceland The recently-discovered high, plume-like 3...

متن کامل

Hafnium isotopic variations in volcanic rocks from the Caribbean Large Igneous Province and Galápagos hot spot tracks

[1] We report Hf isotope compositions of 79 lavas that record the early ( 5–95 Ma) history of the Galápagos plume volcanism. These include lavas from the Caribbean Large Igneous Province (CLIP; 95–70 Ma), the accreted Galápagos paleo-hot spot track terranes (54–65 Ma) of Costa Rica (Quepos, Osa and Burica igneous complexes), and the Galápagos hot spot tracks (<20 Ma) located on the Pacific seaf...

متن کامل

Hafnium isotopic variations in volcanic rocks from the Caribbean Large Igneous Province and Galpagos hot spot tracks

[1] We report Hf isotope compositions of 79 lavas that record the early ( 5–95 Ma) history of the Galápagos plume volcanism. These include lavas from the Caribbean Large Igneous Province (CLIP; 95–70 Ma), the accreted Galápagos paleo-hot spot track terranes (54–65 Ma) of Costa Rica (Quepos, Osa and Burica igneous complexes), and the Galápagos hot spot tracks (<20 Ma) located on the Pacific seaf...

متن کامل

Elemental mixing systematics and Sr–Nd isotope geochemistry of mélange formation: Obstacles to identification of fluid sources to arc volcanics

We present major and trace element concentrations in conjunction with Sr–Nd isotope ratios to investigate the geochemical characteristics of mélange formation along the subduction zone slab–mantle interface. Mélange matrix of the Catalina Schist formed within an active subduction zone of the southern California borderland in Cretaceous time. Mélange formed through the synergistic effects of def...

متن کامل

ذخیره در منابع من


  با ذخیره ی این منبع در منابع من، دسترسی به آن را برای استفاده های بعدی آسان تر کنید

برای دانلود متن کامل این مقاله و بیش از 32 میلیون مقاله دیگر ابتدا ثبت نام کنید

ثبت نام

اگر عضو سایت هستید لطفا وارد حساب کاربری خود شوید

عنوان ژورنال:

دوره   شماره 

صفحات  -

تاریخ انتشار 2006