Geochemical Evolution of the Paleoproterozoic Vol-canic and Plutonic Rocks from the Omai Area, Guyana, South America: Implications for Tectonic History and Source Regions

摘要

Detailed geochemical studies have been carried out on the igneous and sedimentary rocks in the Omni area, part of the Paleoproterozoic Barama-Mazaruni greenstone belt, Guyana. Lithos-tratigraphical sequence has been divided in four complexes: volcano-sedimentary (VSC) , mafic-ultrnmafic (MUM), intrusive (IC) and several generations of mafic-ultramafic dykes (MUD) . Volcanic rocks range in composition from subalkaline basalt to high-silica rhyolites> with a gap between 60-65 wt% SiOz, suggesting a bimodnl suite. Major element criteria were used to sub-divide the rocks into thoieiites (TH, comprising MUC and part of the VSC) and calc-alkaline (CA, comprising IC and part of the VSC) . The TH rocks are characterized by REE with flat to moderately sloping Fractionation patterns [ (La/Yb)nc|,= 0. 9-5. 5, mean 2. 3], relatively high chondrite-normalized nbundances ( La = 3- 13 ppmt mean 9 ppm j LanCh = 12-55. mean 38), and flat to slightly sloping HREE profiles [ (Th/Lu)nch = 0. 7-1, 5, mean 1. 1] . They are geochemically evolved , but isotopically juvenile. Using the ratios of the immobile trace ele-ments, the tholeiitic rocks from Omai have the typical characteristics of immature intraoceanic island arcs or of the modern E-MORB basalts, but affected by a strong arc signature, common-ly reported for the back-arc basin basalts (BABB) . CA rocks are more evolved, with strong fractionated REEs, LREE- and LILE-enriched and HREE- and HFSE-depleted patterns, yielding the typical characteristics of the volcanic arc tectonic setting. The geochemical signa-ture of the Omai lithological complexes suggests either a mantle-derived magma source, which was active enough time to allow advanced degree of differentiation, or a progressive increase of contamination with a slightly older (Paleoproterozoic) crustal material, related to the closure of the Paleoproterozoic oceans in this part of the Guiana Shield.