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Geochemistry and isotopic evolution of the Mesoproterozoic Cape Meredith Complex, West Falkland

ROBERT J. THOMAS^*,*, JOACHIM JACOBS and BRUCE M. EGLINGTON

^* Council for Geoscience, P.O. Box 572, Bellville, 7535, South Africa
FB Geowissenschaften, Universität Bremen, P.O. Box 330440, 28334, Bremen, Germany
Council for Geoscience, P. Bag X112, Pretoria, 0001, South Africa

^* Author for correspondence: bob{at}geobell.org.za

Whole-rock major and trace element geochemical and Rb–Sr/Sm–Nd isotopic data are presented for the Mesoproterozoic (~1.0 Ga) metamorphic and igneous rocks of the Cape Meredith Complex, West Falkland. The data indicate that the oldest rocks, the ~1.1 Ga supracrustal gneisses of the Big Cape Formation, which form three petrographic and geochemical groups (mafic amphibolite, quartz–plagioclase–biotite–hornblende intermediate gneiss and acid gneiss), probably represent a juvenile calc-alkaline, basalt–andesite–rhyolite volcanic sequence, with epsilon (NdT) values and ^NdT_DM ages of ~+3 to +6 and ~1100 to 1400 Ma respectively. It is argued on geochemical grounds that these metavolcanics were extruded in an island-arc at around 1120 Ma. The Big Cape Formation was intruded by granitoids during and after a collisional orogenic event at around 1090 Ma. The oldest, foliated, (G1) granodiorite was emplaced as thin sheets at approximately 1090 to 1070 Ma and is characterized by Nd values of ~+1.5 to 4 (T_DM = ~1200 to 1400 Ma), showing its juvenile nature. The ~1070 Ma (G2) syntectonic granitoid gneisses and ~1000 Ma G3 post-tectonic granites also exhibit juvenile characteristics ( Nd = ~0 to +5 and T_DM = 2200 to 1200 Ma, respectively). The granitoids show a time-composition evolution from Na-rich (G1) granodiorite to potassic, high-High Field Strength Element granites (G3). The geochemical and isotopic characteristics and geological evolution of the Cape Meredith Complex is comparable with that of the adjacent Gondwana crustal blocks in Natal (SE Africa) and Dronning Maud Land (East Antarctica), supporting models that demonstrate these areas evolved in a contiguous, juvenile arc environment prior to, and during, a major orogenic event at ~1.1 Ga. These events were associated with the birth of the Rodinian supercontinent. The three areas remained juxtaposed during Rodinia break-up and were subsequently incorporated into Gondwana in the same relative positions.

This article has been cited by other articles:

				G. S. KIMBELL and P. C. RICHARDS The three-dimensional lithospheric structure of the Falkland Plateau region based on gravity modelling Journal of the Geological Society, July 1, 2008; 165(4): 795 - 806. [Abstract] [Full Text] [PDF]