Two-step Gravity Inversion Reveals Variable Architecture of African Cratons

The lithospheric build-up of the African continent is still to a large extent unexplored. In this contribution, we present a new Moho depth model to discuss the architecture of the three main African cratonic units, which are: West African Craton, Congo Craton, and Kalahari Craton. Our model is based on a two-step gravity inversion approach that allows variable density contrasts across the Moho depth. In the first step, the density contrasts are varied for all non-cratonic units, in the second step for the three cratons individually. The lateral extension of the tectonic units is defined by a regionalization map, which is calculated from a recent continental seismic tomography model. Our Moho depth is independently constrained by pointwise active seismics and receiver functions. Treating the constraints separately reveals a variable range of density contrasts and different trends in the estimated Moho depth for the three cratons. Some of the estimated density contrasts vary substantially, caused by sparse data coverage of the seismic constraints. With a density contrast of Δ ρ = 200 kg/m3 the Congo Craton features a cool and undisturbed lithosphere with smooth density contrasts across the Moho. The estimated Moho depth shows a bimodal pattern with average Moho depth of 39–40 km for the Kalahari and Congo Cratons and 33–34 km for the West African Craton. We link our estimated Moho depth with the cratonic extensions, imaged by seismic tomography, and with topographic patterns. The results indicate that cratonic lithosphere is not necessarily accompanied by thick crust. For the West African Craton, the estimated thin crust, i.e. shallow Moho, contrasts to thick lithosphere. This discrepancy remains enigmatic and requires further studies.

Late Ediacaran occurrences of the organic-walled microfossils Granomarginata and flask-shaped Lagoenaforma collaris gen. et sp. nov.

New occurrences of flask-shaped and envelope-bearing microfossils, including the predominantly Cambrian taxon Granomarginata, are reported from new localities, as well as from earlier in time (Ediacaran) than previously known. The stratigraphic range of Granomarginata extends into the Cambrian System, where it had a cosmopolitan distribution. This newly reported Ediacaran record includes areas from Norway (Baltica), Newfoundland (Avalonia) and Namibia (adjacent to the Kalahari Craton), and puts the oldest global occurrence of Granomarginata in the Indreelva Member (< 563 Ma) of the Stáhpogieddi Formation on the Digermulen Peninsula, Arctic Norway. Although Granomarginata is rare within the assemblage, these new occurrences together with previously reported occurrences from India and Poland, suggest a potentially widespread palaeogeographic distribution of Granomarginata through the middle–late Ediacaran interval. A new flask-shaped microfossil Lagoenaforma collaris gen. et sp. nov. is also reported in horizons containing Granomarginata from the Stáhpogieddi Formation in Norway and the Dabis Formation in Namibia, and flask-shaped fossils are also found in the Gibbett Hill Formation in Newfoundland. The Granomarginata–Lagoenaforma association, in addition to a low-diversity organic-walled microfossil assemblage, occurs in the strata postdating the Shuram carbon isotope excursion, and may eventually be of use in terminal Ediacaran biostratigraphy. These older occurrences of Granomarginata add to a growing record of body fossil taxa spanning the Ediacaran–Cambrian boundary.

Late Paleoproterozoic mafic magmatism and the Kalahari craton during Columbia assembly

The 1.87–1.84 Ga Black Hills dike swarm of the Kalahari craton (South Africa) is coeval with several regional magmatic provinces used here to resolve the craton’s position during Columbia assembly. We report a new 1850 ± 4 Ma (U-Pb isotope dilution–thermal ionization mass spectrometry [ID-TIMS] on baddeleyite) crystallization age for one dike and new paleomagnetic data for 34 dikes of which 8 have precise U-Pb ages. Results are constrained by positive baked-contact and reversal tests, which combined with existing data produce a 1.87–1.84 Ga mean pole from 63 individual dikes. By integrating paleomagnetic and geochronological data sets, we calculate poles for three magmatic episodes and produce a magnetostratigraphic record. At 1.88 Ga, the Kalahari craton is reconstructed next to the Superior craton so that their ca. 2.0 Ga poles align. As such, magmatism forms part of a radiating pattern with the coeval ca. 1.88 Ga Circum-Superior large igneous province.

Supplemental Material: Late Paleoproterozoic mafic magmatism and the Kalahari craton during Columbia assembly

Geochronology, geochemistry, and paleomagnetic datasets. <br>

Supplemental Material: Late Paleoproterozoic mafic magmatism and the Kalahari craton during Columbia assembly

Geochronology, geochemistry, and paleomagnetic datasets. <br>

Two billion years of episodic and simultaneous websteritic and eclogitic diamond formation beneath the Orapa kimberlite cluster, Botswana

The Sm–Nd isotope systematics and geochemistry of eclogitic, websteritic and peridotitic garnet and clinopyroxene inclusions together with characteristics of their corresponding diamond hosts are presented for the Letlhakane mine, Botswana. These data are supplemented with new inclusion data from the nearby (20–30 km) Orapa and Damtshaa mines to evaluate the nature and scale of diamond-forming processes beneath the NW part of the Kalahari Craton and to provide insight into the evolution of the deep carbon cycle. The Sm–Nd isotope compositions of the diamond inclusions indicate five well-defined, discrete eclogitic and websteritic diamond-forming events in the Orapa kimberlite cluster at 220 ± 80 Ma, 746 ± 100 Ma, 1110 ± 64 Ma, 1698 ± 280 Ma and 2341 ± 21 Ma. In addition, two poorly constrained events suggest ancient eclogitic (> 2700 Ma) and recent eclogitic and websteritic diamond formation (< 140 Ma). Together with sub-calcic garnets from two harzburgitic diamonds that have Archaean Nd mantle model ages (TCHUR) between 2.86 and 3.38 Ga, the diamonds studied here span almost the entire temporal evolution of the SCLM of the Kalahari Craton. The new data demonstrate, for the first time, that diamond formation occurs simultaneously and episodically in different parageneses, reflecting metasomatism of the compositionally heterogeneous SCLM beneath the area (~ 200 km2). Diamond formation can be directly related to major tectono-magmatic events that impacted the Kalahari Craton such as crustal accretion, continental breakup and large igneous provinces. Compositions of dated inclusions, in combination with marked variations in the carbon and nitrogen isotope compositions of the host diamonds, record mixing arrays between a minimum of three components (A: peridotitic mantle; B: eclogites dominated by mafic material; C: eclogites that include recycled sedimentary material). Diamond formation appears dominated by local fluid–rock interactions involving different protoliths in the SCLM. Redistribution of carbon during fluid–rock interactions generally masks any potential temporal changes of the deep carbon cycle.

Congo-São Francisco at 1.11 Ga and the megacontinent Umkondia

&lt;p&gt;Currently three supercontinent cycles have been identified and existed supercontinents named from youngest to oldest: Pangea, Rodinia and Nuna/Columbia. Recently Wang et al. (2020) suggested that supercontinent amalgamation were each preceded by ~200 Myr by the assembly of long-lasting &lt;em&gt;megacontinent&lt;/em&gt; aking to Gondwana.&lt;/p&gt;&lt;p&gt;The Congo-S&amp;#227;o Francisco (C/SF) craton is a main building block in Gondwana due to its central location, but its participation to Rodinia is controversial. Salminen et al. (2018) presented 1.11 Ga paleomagnetic and geochronological data from a prominent Epembe-Huila swarm of gabbronoritic dykes in the southern part of the Congo craton in Namibia and in Angola. This paleomagnetic pole yields a relatively low paleolatitude for the C/SF craton at ca. 1.11 Ga and permits a direct connection between Congo and Kalahari cratons. This connection supports an earlier qualitative comparison (Ernst et al., 2013), that the mafic Epembe-Huila swarm was an integral component of the Umkondo Large Igneous Province (LIP). The 1.11 Ga Umkondo LIP is widespread across Kalahari craton, and coeval mafic magmatism has been identified in several of the world&amp;#8217;s other late Mesoproterozoic cratons: Laurentia, India, Amazonia, and Antarctica (Grunehogna). Were these coeval provinces spatially linked at the time of emplacement during the amalgamation of Rodinia? Robust paleomagnetic and geochronological data from Laurentia and Kalahari have demonstrated substantial separation between those two blocks at 1.11 Ga (Swanson-Hysell et al., 2015). However, based on similar tholeiitic magmatism Choudhary et al. (2019) proposed that Kalahari and C/SF together with Amazonia and northern India constituted &amp;#8220;Umkondia&amp;#8221; at 1.11 Ga. It has been proposed that Umkondia occupied an intermediary &amp;#8220;megacontinental&amp;#8221; role in the Nuna-Rodinia transition analogous to Gondwana in Rodinia-Pangea evolution (Wang et al., 2020). Contradicting Gondwana the proposed Umkondia was not long-lasting, since it has been proposed that Kalahari and Congo separated after 1.10 Ga to form a vast ocean (ca. 6000 km) during the formation of Rodinia and widespread juvenile intra-oceanic magmatism along the present-day central Brazil indicates a large ca. 0.94 Ga ocean between C/SF and Amazonia (Cordani et al., 2003).&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Choudhary et al. 2019. Precambrian Research 332, 105382.&lt;/p&gt;&lt;p&gt;Cordani et al. 2003. Gondwana Research 6, 275-283.&lt;/p&gt;&lt;p&gt;Ernst et al. 2003. Lithos 174 1-14.&lt;/p&gt;&lt;p&gt;Salminen et al. 2018. Geology 46, 1011-1014.&lt;/p&gt;&lt;p&gt;Swanson-Hysell et al. 2015. Geophysical Journal International 203, 2237-2247.&lt;/p&gt;&lt;p&gt;Wang et al. 2020. Geology 49, https://doi.org/10.1130/G47988.1&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Assessment of kimberlite diamond grade by the indicator minerals chemical composition in Lunda region of Angola

&lt;p&gt;To determine a diamond grade (ct/t) in the Lunda district kimberlites using the chemical composition of the KIM (indicator minerals) frequency of occurrence of their cluster groups (CG) we performed statistical analysis of the chemical composition of pyropes (3478 grains) of Cr-diopsides (714) and picroilmenites (1582) of the 6 kimberlite diamond deposits. Classification procedures of cluster and correlation &amp;#8211; factor analysis were used (Ivanov, 2017). Significant correlation coefficients were determined between the variations of KIM compositions and diamond content in kimberlites. Figure 2 shows the distribution of diamond contents in 6 kimberlite pipes, correlated with the distribution of pyropes G10 (Dawson et al., 1975), chromium diopsides CG S6, as well as CG of picroilmenites &amp;#8211; 12b and P12-16 in their frequency of occurrence, the interpretation of which is reduced to the following conclusions. The proportions of pyropes CG G10 in kimberlites of 5 pipes control the linear growth (R&lt;sup&gt;2&lt;/sup&gt;=0.97) of the diamond content in pipes to the center of the Saurimo structure, excluding the CAT-E42 pipe. With a relatively high diamond grade, the proportion of &amp;#160;G10 in this pipe is low, which may be related to the extremely low quality of its diamonds. In kimberlites. This indicator is typical for the Catoca and Luele pipes, with the maximum proportions of low-ferrous picroilmenites (11.0% and 13.9%). In the NE direction, the conditions for the preservation of diamonds in kimberlites decrease, which affects their low diamond grade (0.2-0.4 ct / t), which decreases exponentially (R&lt;sup&gt;2&lt;/sup&gt;=0.98) with an increase in the TiO&lt;sub&gt;2&lt;/sub&gt; content in picroilmenites. The proportion of CG S6 Cr-diopsides belonging to the high-pressure variety of the deep mantle lithosphere (coesite facies) (Sobolev, 1971) increases in the kimberlites of the central part of the Saurimo structure to 15-32% and controls the high diamond content of the Catoca, CAT-E42 and Luele pipes (Fig.&amp;#160;1). The established regularities of changes in the frequency of occurrence of CG KIMs in the NE-SW direction in the Lunda kimberlite region confirm the regional pyrope trend of N. V. Sobolev's diamond content and other KIMs correlations with the diamond content of kimberlites in this region. They also meet the &quot;rule of V. A. Milashev&quot; on reducing the diamond content of kimberlites to the periphery of regional structural units of kimberlite provinces (Zinchenko et al., 2016).&lt;/p&gt;&lt;p&gt;Sobolev N.V. Mineralogical criteria of diamond-bearing kimberlites. Geology and geophysics. No. 3. 1971, 70-80.&lt;/p&gt;&lt;p&gt;Dawson J.B., Stephens W.E. Statistical classification of garnets from kimberlites and xenoliths.J. Geol. 1975. 83, 589-60&lt;/p&gt;&lt;p&gt;Gurney D. D., Moore R. O. Geochemical correlation between kimberlite minerals and diamonds of the Kalahari Craton. 1994.,12&amp;#8211;24.&lt;/p&gt;&lt;p&gt;Ivanov A. S. Statistical analysis of indicator minerals of kimberlites. Proceedings of the XIII All-Russian Fersman Session. KSC RAS.&amp;#160; Apatity. 2017,&amp;#160; 172 -181.&lt;/p&gt;&lt;p&gt;Zinchenko, V., Felix J. T., Francisco J. Diamondiferous trend of the kimberlites in the Lunda region (Angola)//35th International Geological Congress Abstracts. Cape Town. South Africa. 2016.&lt;/p&gt;&lt;p&gt;&lt;img src=&quot;https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.0e3eea868d0066419941161/sdaolpUECMynit/12UGE&amp;app=m&amp;a=0&amp;c=7ecb3e7dd388e9edb98f52df0a9411a0&amp;ct=x&amp;pn=gnp.elif&amp;d=1&quot; alt=&quot;&quot; width=&quot;581&quot; height=&quot;346&quot;&gt;&lt;/p&gt;&lt;p&gt;&lt;img src=&quot;https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.960b89858d0069888941161/sdaolpUECMynit/12UGE&amp;app=m&amp;a=0&amp;c=cdf3e97fdd49a7a10a2127127c90c0ff&amp;ct=x&amp;pn=gnp.elif&amp;d=1&quot; alt=&quot;&quot; width=&quot;590&quot; height=&quot;127&quot;&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

The Magmatic–Hydrothermal Transition in Lithium Pegmatites: Petrographic and Geochemical Characteristics of Pegmatites from the Kamativi Area, Zimbabwe

ABSTRACT Lithium is a critical metal, vital for electrification of transport. Currently, around half the world's lithium is extracted from rare-metal pegmatites and understanding the genesis and evolution of these igneous rocks is therefore essential. This paper focuses on the pegmatites in the Kamativi region of Zimbabwe. A group of early pegmatites is distinguished from a late pegmatite suite which includes the ca. 1030 Ma Main Kamativi Pegmatite. Previously mined for tin, the mine tailings are now being investigated for lithium. Mineral-scale investigation of samples from the Main Kamativi Pegmatite has allowed recognition of a four-stage paragenesis: (1) an early magmatic assemblage dominated by quartz, alkali feldspar, spodumene (LiAlSi2O6) and montebrasite [LiAl(PO4)(OH, F)]; (2) partial alteration by widespread albitization, associated with growth of cassiterite and columbite group minerals; (3) irregular development of a quartz, muscovite, columbite group mineral assemblage; and (4) widespread low-temperature fluid-induced alteration of earlier phases to cookeite, sericite, analcime, and apatite. Whole-rock geochemistry indicates that the late pegmatites are enriched in Li, Cs, Ta, Sn, and Rb but depleted in Nb, Zr, Ba, Sr, and the rare earth elements relative to early pegmatites and country rock granitoids. A combination of field relationships and published dating indicates that the granitoids, and probably the early pegmatites, were emplaced toward the end of the ca. 2000 Ma Magondi Orogeny, whereas the late pegmatites are almost 1000 million years younger. The late pegmatites thus cannot be genetically related to the granitoids and are instead likely to have formed by partial melting of metasedimentary source rocks. The drivers for this melting may be related to crustal thickening along the northern margin of the Kalahari Craton during the assembly of Rodinia.