ALBERT

Description: The late-tectonic 511.4 ± 0.6 Ma-old Nomatsaus intrusion (Donkerhoek batholith, Damara orogen, Namibia) consists of moderately peraluminous, magnesian, calc-alkalic to calcic granites similar to I-type granites worldwide. Major and trace-element variations and LREE and HREE concentrations in evolved rocks imply that the fractionated mineral assemblage includes biotite, Fe–Ti oxides, zircon, plagioclase and monazite. Increasing K2O abundance with increasing SiO2 suggests accumulation of K-feldspar; compatible with a small positive Eu anomaly in the most evolved rocks. In comparison with experimental data, the Nomatsaus granite was likely generated from meta-igneous sources of possibly dacitic composition that melted under water-undersaturated conditions (X H2O: 0.25–0.50) and at temperatures between 800 and 850 °C, compatible with the zircon and monazite saturation temperatures of 812 and 852 °C, respectively. The Nomatsaus granite has moderately radiogenic initial 87Sr/86Sr ratios (0.7067–0.7082), relatively radiogenic initial εNd values (− 2.9 to − 4.8) and moderately evolved Pb isotope ratios. Although initial Sr and Nd isotopic compositions of the granite do not vary with SiO2 or MgO contents, fSm/Nd and initial εNd values are negatively correlated indicating limited assimilation of crustal components during monazite-dominated fractional crystallization. The preferred petrogenetic model for the generation of the Nomatsaus granite involves a continent–continent collisional setting with stacking of crustal slices that in combination with high radioactive heat production rates heated the thickened crust, leading to the medium-P/high-T environment characteristic of the southern Central Zone of the Damara orogen. Such a setting promoted partial melting of metasedimentary sources during the initial stages of crustal heating, followed by the partial melting of meta-igneous rocks at mid-crustal levels at higher P–T conditions and relatively late in the orogenic evolution.

Description: Deutsche Forschungsgemeinschaft

Description: Universität Hamburg (1037)

Description: Highlights • The syntectonic Otjimbingwe alkaline complex (Namibia) is ca. 545 Ma old. • Metasomatized mantle-derived melts are modified by AFC and accumulation processes. • The alkaline melts intruded during extensional tectonics during flat subduction. The ~545 Ma-old syn-collisional Otjimbingwe alkaline complex is composed of pyroxene-amphibole-biotite-bearing, mildly nepheline-normative to quartz-normative rocks ranging in composition from monzogabbro to monzonite, syenite and granite. The alkaline rocks have moderate to high SiO2 (50.5–73.0 wt%) and Na2O + K2O (5.1–11.5 wt%) and moderate to low MgO (6.6–0.2 wt%) concentrations. All samples have high large ion lithophile element (LILE: Ba up to 4600 ppm) and high-field-strength element contents (HFSE; Zr: 155–1328 ppm; Nb: 16–110 ppm; Ta: 1.4–7.1 ppm and Hf: 4–24 ppm) and have strongly fractionated LREE patterns ((La/Yb)N = 14–51). The most primitive members lack significant negative Eu anomalies. Mantle-normalized multi-element diagrams show depletion in Ba, Rb, Nb (Ta), P and Ti. The alkaline rocks have moderate radiogenic initial 87Sr/86Sr ratios (0.7061–0.7087) and unradiogenic initial ɛNd values (−3.9 to −6.1). This isotope signature, associated with high LREE/HFSE ratios indicates that the parental melts were generated in enriched portions of the shallow lithospheric mantle, which was probably affected by previous subduction zone processes. In addition, correlations between Sr and Nd isotopes indicate that some of these variations result from combined crustal assimilation and fractional crystallization (AFC) processes. A new model of flat subduction is presented that explains most of the unsolved problems in the orogenic evolution of the Damara orogen, namely (i) the absence of early intrusive rocks with a clear subduction zone setting, (ii) the absence of high-pressure rocks such as blueschists and eclogites, (iii) the unusual distribution of igneous rocks with a clear predominance of granite and granodiorite and (iv) the need for a asthenospheric window during a classical subduction to explain the high T/moderate P granulite facies conditions in the overriding plate.

Description: Highlights • New trace element and Sr-Nd-Pb isotope data for SVZ crustal and trench sediment samples. • New whole-rock Hf–O isotope data for crustal and trench sediment samples. • Nd model ages confirm a Mesoproterozoic crustal residence age for the Guarguaráz complex. • NSVZ trench sediments reflect the source composition of eroded mafic material. Abstract This paper provides new trace element and Sr-Nd-Hf-Pb-O isotope data on Neoproterozoic to Phanerozoic mid to upper crustal lithologies of the Andean basement in central Chile and western Argentina (33°-35°S; 69°-72°W). It also provides additional data on trench sediments being subducted offshore the northern segment of the Southern Volcanic Zone in Chile. Neoproterozoic metamorphic and igneous rocks from the Guarguaráz complex (Argentina; 33.6°S, 69.5°W), when back corrected to 350 Ma, display a narrow range in 87Sr/86Sri (0.713–0.718), 143Nd/144Ndi (0.5118–0.5121), εNdi (−8.1 to −1.1), εHfi (−11.4 to +1.2) and δ18O (9–13‰). Nd model ages (TDM = 1.08–1.65 Ga) for the Guarguaráz complex points to a Mesoproterozoic crustal residence age for these rocks. Metasedimentary rocks from the Carboniferous accretionary prism in central Chile (~34°S) overlap with these ranges, but differ by having lower initial 87Sr/86Sri (0.7052–0.7093) and higher δ18O (14–17‰). The Guarguaráz metamorphic and igneous rocks, when back corrected to 350 Ma, have similar Pb isotope ratios than the Chilean Carboniferous metasedimentary rocks (206Pb/204Pbi = 17.58–18.52 vs. 18.33–18.46; 207Pb/204Pbi = 15.50–15.64 vs. ~15.64; 208Pb/204Pbi = 37.70–38.36 vs. 37.98–38.18). Two Guarguaráz samples are shifted towards less radiogenic Pb isotope ratios, similar to samples representative of the Cuyania basement. This suggests that Chilenia hosts at least two geochemical components: (1) a component with unradiogenic Pb isotopes, similar to the Proterozoic Cuyania basement, and (2) a component with more radiogenic Pb isotopes, similar to Chilean Phanerozoic metasedimentary and igneous rocks. The ranges in Pb isotope ratios for the Chilean Mesozoic (206Pb/204Pbi = 18.44–19.86; 207Pb/204Pbi = 15.59–15.69; 208Pb/204Pbi = 38.30–40.30) and Miocene (206Pb/204Pbi = 18.43–18.57; 207Pb/204Pbi = 15.58–15.60; 208Pb/204Pbi = 38.33–38.46) igneous rocks are similar to those of the accretionary prism. The Mesozoic and Miocene intrusive rocks are characterized by low 87Sr/86Sri (0.704–0.708 and ~0.704, respectively) and high εNdi (−6.2 to +4.0 and + 3.9 to +5.9, respectively) and εHfi (+7.0 to +12.7 and + 8.5 to +10.8, respectively). They can be divided into two groups. Group (1), consisting exclusively of Mesozoic samples, has negative εNdi, 87Sr/86Sri 〉 0.706, elevated e.g., Ba/Th, Nb/Yb, Zr/Y and lower Nb/La, reflecting derivation from enriched (most likely overriding crust or mantle) material. Group (2), consisting of Mesozoic and Miocene rocks, has positive εNdi, εHfi, and lower initial 87Sr/86Sri than group (1) reflecting depleted mantle melts addition to the crust. Finally, Sr-Nd-O isotopic compositions of the trench sediments at latitude 33°-33.3°S are almost identical to those at latitude 35°-40°S, indicating a relative homogeneous material input along the SVZ, although there are subtle differences in REE and Pb isotopic compositions. Based on Nd–Hf isotopes, trench sediments offshore Chile (εNd〉 +1; εHf 〉 +2) and offshore Peru (εNd 〈 −2; εHf 〈 +1) have distinct compositions, reflecting the differences in input material. The positive εNd and εHf values suggest derivation from eroded depleted mantle-derived mafic material.

Description: Highlights • Synorogenic igneous rocks (Damara orogen) give evidence for crust-mantle interaction. • Assimilation of crustal material is widespread and includes lower crustal rocks. • A common subduction zone setting is unlikely. • Flat subduction tectonics may explain the chemical and isotope composition. Abstract The early-syntectonic 563.7±6.1 Ma old Oamikaub diorite (Damara orogen, Namibia) consists of metaluminous, magnesian, calc-alkalic to calcic diorites, granodiorites and granites. Associated gabbro-diorites and gabbros belong to the Neikhoes metagabbro. Linear major and trace element variations imply that the rock suite evolved through fractional crystallization processes involving amphibole, biotite, Fe-Ti oxides, zircon and apatite. Initial Sr (87Sr/86Sr: 0.7058-0.7123) and Nd (ε Nd: -2.1 to -18.8) isotopic compositions are highly variable and negatively correlated indicating that assimilation of crustal components occurred. Unradiogenic initial 206Pb/204Pb (16.23-17.23) and 207Pb/204Pb ratios (15.50-15.57) suggest derivation from or interaction with ancient crust with low U/Pb. Two gabbro-diorites have MgO, Ni and Cr abundances that are compatible with derivation of these rocks from upper mantle lithologies. Their initial ε Nd values (-2.1 and – 7.4) and 87Sr/86Sr ratios (0.7058 and 0.7076) imply derivation from an aged metasomatized lithospheric mantle. Other mafic samples have MgO abundances and compatible element concentrations that exceed the values commonly accepted for primary mafic melts implying some accumulation of clinopyroxene and amphibole. The granodiorites form a homogenous group in which the isotope data (initial ε Nd: -12.4 to -14.1; initial 87Sr/86Sr: 0.7083-0.7096) imply a lower crustal source. The granites are also magnesian and calc-alkaline but two of them are strongly peraluminous. Their isotope data (initial ε Nd: -13.2 to -18.8; initial 87Sr/86Sr: 0.7099 to 0.7123) imply derivation from more ancient sources, alternatively these samples gained their isotope systematics through extensive AFC processes from parental granodiorites. A common subduction zone environment as suggested from negative Nb-Ta anomalies in multi-element diagrams seems unlikely for all samples because of a lack of isotopically depleted signatures. The data from the Oamikaub diorite and other mafic complexes are better explained by a “flat” subduction model involving mainly continental mantle lithosphere and crust with limited, if any, melting of asthenospheric mantle.

Description: Age-progressive volcanism is generally accepted as the surface expression of deep-rooted mantle plumes, which are enigmatically linked with the African and Pacific large low-shear velocity provinces (LLSVPs). We present geochemical and geochronological data collected from the oldest portions of the age-progressive enriched mantle one (EMI)-type Tristan-Gough track. They are part of a 30- to 40-million year younger age-progressive hotspot track with St. Helena HIMU (high time-integrated U-238/Pb-204) composition, which is also observed at the EMI-type Shona hotspot track in the southernmost Atlantic. Whereas the primary EMI-type hotspots overlie the margin of the African LLSVP, the HIMU-type hotspots are located above a central portion of the African LLSVP, reflecting a large-scale geochemical zonation. We propose that extraction of large volumes of EMI-type mantle from the margin of the LLSVP by primary plume heads triggered upwelling of HIMU material from a more internal domain of the LLSVP, forming secondary plumes.

Description: Highlights • ~561 Ma old alkaline magmatism in the Damara Belt was caused by extensional tectonics. • Radiogenic isotopes constrain sources and AFC processes. • The alkaline rocks were generated by melting of enriched lithospheric mantle domains. • The lithospheric mantle was modified by ancient subduction zone processes. The 560.8 ± 2.4 Ma old Okamutambo Pluton is the easternmost intrusion of the syn-collisional Otjimbingwe Alkaline Complex (OAC; Damara Belt, Namibia) and consists of mafic to intermediate alkaline rocks that belong to the monzodiorite-syenite series. The studied samples are potassic (K2O/Na2O 〉 1) and have moderately high magnesium (MgO: 5.7–2.6 wt%), nickel (Ni: 66–26 ppm), and chromium (Cr: 223–62 ppm) concentrations. LILE (Ba: 2425–1243 ppm; Sr: 1359–941 ppm) and HFSE (Zr: 447–202 ppm, Nb: 32.8–16.4 ppm, Hf: 4.7–9.4 ppm, Ta: 1.2–2.0 ppm) contents are also high. Strontium and Nd isotope data reveal the existence of two magmatic suites indicating a multi-source origin. Group 1 monzonites-quartz monzonites have moderately evolved Sr and Nd isotopic compositions (initial 87Sr/86Sr: 0.7066 to 0.7073; initial εNd: −3.5 to −5.0) and radiogenic Pb isotope ratios (206Pb/204Pb: 17.65–18.02; 207Pb/204Pb: 15.62–15.67; 208Pb/204Pb: 38.19–38.32). In contrast, group 2 monzodiorites-syenites display more evolved Sr and Nd isotopic compositions (initial 87Sr/86Sr: 0.7088 to 0.7090; initial εNd: −6.7 to −7.1) but similar Pb isotope ratios (206Pb/204Pb: 17.63–17.82; 207Pb/204Pb: 15.64–15.66; 208Pb/204Pb: 38.21–38.37). Differentiation involved AFC processes in group 1 monzonites-quartz monzonites whereas group 2 monzodiorites-syenites were modified by fractional crystallization. Although second-order processes were operative, high total alkali contents, incompatible trace element concentrations in excess of bulk crustal values, and evolved isotopic compositions of the most primitive samples are source-controlled and provide insight into the origin of the parental magmas. With reference to experimental data from the literature, it is inferred that the Okamutambo alkaline rocks represent evolved melts that were generated through melting of enriched lithospheric mantle (phlogopite-lherzolite). The observed negative Nb-Ta and Ti anomalies and positive Pb anomalies in primitive mantle-normalized trace element patterns are in line with a mantle source that contains a recycled crustal component. Isotopic compositions indicate that mantle enrichment is an ancient feature that might be linked to Proterozoic subduction. At 561 Ma, the geodynamic regime in the Damara Orogen was mainly characterized by compression during convergence and continental collision between the Congo and Kalahari cratons which is difficult to reconcile with the generation of the OAC as alkaline magmatism is commonly associated with extensional tectonic regimes. The OAC is, however, associated with a major suture zone that may have been involved in localized transtensional tectonics during oblique flat subduction and thus enabled the generation and ascent of mantle-derived alkaline melts.

Description: Highlights • A HIMU-like volcanism belt along the southwest Africa. • The HIMU-like volcanic complexes form age-progressive volcanic tracks. • EMI and HIMU mantle plumes are from different domains in the lower mantle. Abstract The origin of carbonatitic and highly silica-undersaturated volcanism, common along the SW coast of Africa extending from Angola through Namibia to the tip of South Africa, is still poorly understood. Here we present new geochemical data (major and trace element and Sr-Nd-Pb-Hf-O-C isotopes) from the Agate Mountain calcio- to magnesio‑carbonatites (∼83 Ma), Dicker Willem calcio‑carbonatites (49 Ma) and Swakopmund basanitic plugs (76–72 Ma) along the coast of Namibia that were emplaced after the EMI (enriched mantle one) type Etendeka flood basalts. The trace element and isotopic composition of Agate Mountain carbonatites and Swakopmund basanites indicate that they were derived from a HIMU-type (high time-integrated 238U/204Pb with radiogenic Pb isotope ratios) magma source, similar to the St. Helena global HIMU endmember in the South Atlantic. The Agate Mountain carbonatites form part of the late-stage Walvis Ridge HIMU hotspot track overlying the EM1-type Walvis Ridge basement forming part of the Tristan-Gough hotspot track. The Dicker Willem carbonatites, however, extend to higher 206Pb/204Pb than St. Helena, but have similar 206Pb/204Pb to Mangaia HIMU lavas in the Pacific. Compared to Mangaia HIMU, the Dicker Willem carbonatites with mantle-type O and C isotopes have higher 207Pb/204Pb and 87Sr/86Sr but lower 143Nd/144Nd, suggesting it may represent a new HIMU endmember flavor. The HIMU carbonatitic and silica-undersaturated rocks form a belt of age-progressive volcanic tracks, including: 1) from the Walvis Ridge, through NW Namibia to central Angola, 2) from the Vema Seamount via Dicker Willem carbonatite to Gibeon kimberlites and carbonatites, 3) from the Namaqualand to Bushmanland and to Warmbad volcanic centers in northwestern South Africa, and 4) along the older end of the Shona EMI-type volcanic track extending into South Africa. Geochemical and seismic tomographic data suggest that the EMI and HIMU mantle plumes are generated from different geochemical domains at the base of the lower mantle. The Tristan-Gough, Discovery and Shona EM1 volcanic tracks are derived from a common low-velocity anomaly (superplume-like structure with three branching arms) ascending from the outer margin, possibly lower primoridal layer, of the African large low-shear-velocity province (LLSVP). Seismic low-velocity anomalies can be traced from beneath the belt of HIMU volcanism to an internal and shallower part of the LLSVP, located ∼900–1200 km east of the outer LLSVP margin and suggest that HIMU-type (possibly subducted oceanic lithospheric) material overlies EMI-type (possibly primordial) material in the internal part of the LLSVP.

Description: Highlights • Leucogranites from the Donkerhoek batholith (Namibia) are 530.6 ± 08 Ma old. • Leucogranites were generated from Kalahari craton basement rocks. • Extensive crystal fractionation, accumulation and assimilation modified the leucogranites. • The study highlights incremental growth of large-scale batholiths over ~30 Ma. Abstract Syn-tectonic 530.6 ± 0.8 Ma pegmatites and aplites from the Donkerhoek batholith in the Damara orogen (Namibia) are moderately to strongly peraluminous, ferroan, alkalic to calc-alkalic leucogranites. Major and trace element variations and strongly fractionated REE patterns with positive Eu anomalies indicate that the leucogranites represent highly fractionated melts that accumulated or retained feldspar that may account, in part, for their alkalic composition. Elemental variations imply that biotite, garnet, and feldspar were the main fractionating minerals. The pegmatites have lower 87Sr/86Sr (0.7053–0.7097) than—but similar unradiogenic initial εNd (−4.1 to −10) to—the least evolved Donkerhoek granites. Two aplites have similar εNd values but unreasonably unradiogenic 87Sr/86Sr ratios as a result of late-stage disturbance and associated overcorrection due to their extremely high 87Rb/86Sr ratios. Subtle variation in Nd isotope compositions coupled with LREE fractionation indicate limited AFC or contamination processes. Lead isotope compositions are more radiogenic than those from published Donkerhoek samples, indicating derivation from or involvement of a component with a considerable crustal residence time. Based on the alkalic to calc-alkalic and ferroan composition and the similarity in Nd–Sr isotopes, meta-igneous basement rocks from the nearby Kalahari craton are likely sources. This study confirms previous studies on the Donkerhoek batholith that have shown that giant batholiths consist of distinct magma batches that are derived from various sources. The new age constraints in conjunction with published ages show that the large-scale Donkerhoek batholith, with a spatial extent of 〉5000 km2, grew incrementally over a period of at least 30 Myr.

Description: The late-tectonic 511.4 ± 0.6 Ma-old Nomatsaus intrusion (Donkerhoek batholith, Damara orogen, Namibia) consists of moderately peraluminous, magnesian, calc-alkalic to calcic granites similar to I-type granites worldwide. Major and trace-element variations and LREE and HREE concentrations in evolved rocks imply that the fractionated mineral assemblage includes biotite, Fe–Ti oxides, zircon, plagioclase and monazite. Increasing K2O abundance with increasing SiO2 suggests accumulation of K-feldspar; compatible with a small positive Eu anomaly in the most evolved rocks. In comparison with experimental data, the Nomatsaus granite was likely generated from meta-igneous sources of possibly dacitic composition that melted under water-undersaturated conditions (X H2O: 0.25–0.50) and at temperatures between 800 and 850 °C, compatible with the zircon and monazite saturation temperatures of 812 and 852 °C, respectively. The Nomatsaus granite has moderately radiogenic initial 87Sr/86Sr ratios (0.7067–0.7082), relatively radiogenic initial εNd values (− 2.9 to − 4.8) and moderately evolved Pb isotope ratios. Although initial Sr and Nd isotopic compositions of the granite do not vary with SiO2 or MgO contents, fSm/Nd and initial εNd values are negatively correlated indicating limited assimilation of crustal components during monazite-dominated fractional crystallization. The preferred petrogenetic model for the generation of the Nomatsaus granite involves a continent–continent collisional setting with stacking of crustal slices that in combination with high radioactive heat production rates heated the thickened crust, leading to the medium-P/high-T environment characteristic of the southern Central Zone of the Damara orogen. Such a setting promoted partial melting of metasedimentary sources during the initial stages of crustal heating, followed by the partial melting of meta-igneous rocks at mid-crustal levels at higher P–T conditions and relatively late in the orogenic evolution.

Description: Highlights • The Okorusu complex in NE Etendeka have Gough-type isotopic composition. • Messum igneous complex in SW Etendeka show a Doros/Tafelkop-type composition. • Both Gough- and Doros-type components derived from the Tristan-Gough plume. • Doros-type volcanism is surrounded by Gough-type volcanism. • The head-stage of Tristan-Gough plume coincide with the concentric zonation model. Abstract The Etendeka large igneous province in central Namibia is believed to be caused by widespread melting of the Tristan/Gough mantle plume head between ∼137 and 123 Ma ago. To explain the observed compositional variations of the Etendeka flood basalts, a laterally-zoned plume head has been proposed. Here we present new (major and trace element and Sr-Nd-Pb-O-C isotope) geochemical data from the Okorusu and Messum carbonatitic and silica-undersaturated rocks. Okorusu carbonatites, located at the far eastern end of the Etendeka province, have a Gough-type enriched mantle one (EM1) composition, consistent with derivation from a common source with the northern Etendeka flood basalts, Walvis Ridge and Gough (southern) hotspot subtrack of the southern Atlantic Guyot Province including Gough Island. The Messum basanite, erupted directly after the Etendeka event near the central coast of western Namibia, has a different EM1 type flavor (with more radiogenic Nd, less radiogenic Sr and thorogenic Pb isotopes), similar to the Doros, Tafelkop and Horingbaai formations of the Etendeka flood basalts. Combining our new findings with published data from flood basalts, carbonatites and silica-undersaturated rocks from the region, we propose a concentric zonation model for the postulated plume head with the isotopically Gough-type EM1 plume mantle enclosing a blob of Doros-type EM1 plume mantle.