Geochemistry and geochronology of the Ngualla volcanic and granitic rocks, SW Tanzania

The volcanic and associated granitic rocks of the Ngualla area in the Chunya District form part of the Lupa Terrane of the Palaeoproterozoic Ubendian Belt of southwestern Tanzania. On the basis of major and trace element composition, the volcanic rocks can be classified into 3 major groups: basaltic andesites–trachyandesites, (porphyry) dacites-trachydacites and rhyolites. The basaltic andesites-trachyandesites are characterized by high abundance of the LILE and low abundance of the HFSE. On chondrite normalized REE diagrams, they display enrichment of the LREE relative to the HREE and relative depletion of the MREE relative to the HREE (La/YbCN = 11.5 – 36.5, La/SmCN = 3.5 - 5.8) whereas on primitive mantle normalized patterns, the rocks are characterized by relative depletion of Ta, Nb and Ti relative to adjacent elements. On the basis of their trace element composition, the porphyry dacites-trachydacites can be further subdivided into two types: The Group I dacites–trachydacites are characterized by weakly negative Eu anomalies and high Sr contents and are generally similar geochemically to the basaltic andesites-trachyandesites. These similarities include high ratios of Sr/Y (21.3 – 57.3), La/Yb (27.4 - 68.4) and sub-chondritic Zr/Hf and Nb/Ta ratios of 37.8 – 43.9 and 7.14 – 20 respectively. They are, however, distinguished from the basaltic andesites-trachyandesites by their lower Ni and Co contents. The Group II porphyry dacites-trachydacites and associated rhyolites are distinguished from their Group I counterparts by their strongly negative Eu anomalies and the low Sr contents. The volcanic rocks have been intruded by low Sr peraluminous granites which are characterized by fractionated REE patterns (La/Yb = 8.2 – 41.6) and strong to moderately negative Eu anomalies. Like the volcanic rocks, the granites display depletion of Nb, Ta and Ti relative to adjacent elements in their primitive mantle normalized diagrams. Geochemical modeling of the REE abundances shows that the basaltic andesites-trachyandesites may have formed by 0.1% batch melting of garnet peridotite mantle, followed by 50% - 80% fractional crystallization involving the removal of clinopyroxene, hornblende, plagioclase and orthopyroxene in the proportion 61:20:4:15. On the other hand, the porphyry dacites-trachydacites are interpreted to have been derived by 8% - 13% partial melting of a lower crustal basaltic source leaving a residue with 55% plagioclase, 14% orthoclase, 18% clinopyroxene, 5% orthopyroxene and 8% hornblende. On the basis of their wide range of REE composition, low Sr contents and negative Eu anomalies, the granites are interpreted to have formed by the partial melting of metasedimentary sources leaving plagioclase as a residue phase. Zircon U – Pb geochronology shows that the volcanic and granitic magmatism in the Ngualla area occurred in two distinct pulses: eruption of the basaltic andesite –trachyandesites is dated at 1943 ± 20 Ma and was followed by the eruption of the dacites-trachydacites and associated ryholites at 1875 ± 12 Ma. The latter event was coeval with the emplacement of the granites at 1878 ± 15 Ma. Sm-Nd systematics reveal that the volcanic and magmatic rocks of the Ngualla area are characterized by negative εNd(t) values (-3.12 to -11.2) and Archaean TDM ages (2521 – 3274 Ma) which are 600 to 1400 Ma older than their respective emplacement ages indicating the incorporation of old Archaean material during their petrogenesis. Combined together, the array of geochemical and isotopic evidence shows that the volcanic and granitic rocks of the Ngualla area were formed in a Palaeoproterozoic continental margin between 1.87 and 1.94 Ga ago.