Madras granulites: Geochemistry and P-T conditions of crystallisation

  • Weaver B
  • Tarney J
  • Windley B
 et al. 
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Abstract

A study of the major and trace element geochemistry of Madras granulites has been made using 64 representative samples from the Pallavaram area. They can be divided into four geochemically distinct rock groups: meta-igneous pyroxene granulites, metagabbros and charnockites, and metasedimentary khondalites. The meta-igneous rocks, taken together, display a tholeiitic trend with marked Fe-enrichment, contrasting with the general calc-alkaline trends shown by Scottish Lewisian granulites. Geochemical differences between the pyroxene granulites and metagabbros suggest that whereas the former may represent original basaltic liquid compositions, the latter approximate more closely to cumulates. The charnockites mostly correspond to adamellite in composition, but the paucity of intermediate compositions emphasises the bimodality of the meta-igneous association, a common feature in other Archaean terrains. In terms of lithophile element concentrations of Ba, Rb, Sr and the light rare earths, the charnockitic rocks are broadly similar to modern Andean equivalents. However the concentration of yttrium is generally low, indicating significant though variable heavy rare–earth depletion and the influence of garnet or hornblende in their genesis. In common with other granulite terrains there is a trend towards increasing K/Rb rations with decreasing K2O, and K/Rb ratios of 1700 are recorded in some rock types. However there is a strong dependence of K/Rb ratio on mineralogy, those rocks with potassic phases having lower K/Rb ratios. Nevertheless, even the K-feldspar-rich charnockites have K/Rb ratios higher than the average crustal value. Microprobe analyses of coexisting pyroxenes and garnets in different mineralogical assemblages provide consistent P-T data (by several methods) defining the metamorphic crystallisation conditions as 720–840°C and 9–10 kb. Feldspar thermometry suggests crystallisation at about 680°C. These P-T conditions agree well with those recorded from other Precambrian granulite facies terrains. © 1978, Elsevier B.V. All rights reserved.

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Authors

  • B. L. Weaver

  • J. Tarney

  • B. F. Windley

  • E. B. Sugavanam

  • V. Venkata Rao

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