Two distinct igneous differentiation trends – the tholeiitic and calc-alkaline – give rise to Earth's oceanic and continental crust, respectively. Mantle melting at mid-ocean ridges produces dry ...magmas that differentiate at low-pressure conditions, resulting in early plagioclase saturation, late oxide precipitation, and Fe-enrichment in mid-ocean ridge basalts (MORBs). In contrast, magmas formed above subduction zones are Fe-depleted, have elevated water contents and are more oxidized relative to MORBs. It is widely thought that subduction of hydrothermally altered, oxidized oceanic crust at convergent margins oxidizes the mantle source of arc magmas, resulting in erupted lavas that inherit this oxidized signature. Yet, because our understanding of the calc-alkaline and tholeiitic trends largely comes from studies of erupted melts, the signals from shallow crustal contamination by potentially oxidized, Si-rich, Fe-poor materials, which may also generate calc-alkaline rocks, are obscured. Here, we use deep crustal cumulates to “see through” the effects of shallow crustal processes. We find that the tholeiitic and calc-alkaline trends are indeed reflected in Fe-poor mid-ocean ridge cumulates and Fe-rich arc cumulates, respectively. A key finding is that with increasing crustal thickness, arc cumulates become more Fe-enriched. We propose that the thickness of the overlying crustal column modulates the melting degree of the mantle wedge (lower F beneath thick arcs and vice versa) and thus water and Fe3+ contents in primary melts, which subsequently controls the onset and extent of oxide fractionation. Deep crustal cumulates beneath thick, mature continental arcs are the most Fe-enriched, and therefore may be the “missing” Fe-rich reservoir required to balance the Fe-depleted upper continental crust.
•Cumulates complement calc-alkaline and tholeiitic trends defined by magma studies.•Mid-ocean ridge cumulates are Fe-poor, balancing Fe-rich MORB.•Arc cumulates are Fe-rich, balancing Fe-poor arc magmas.•Fe increases in arc cumulates with increasing crustal thickness.•Arc crustal thickness may control primary melt H2O and Fe3+.
Most known porphyry Cu deposits formed in the Phanerozoic and are exclusively associated with moderately oxidized, sulfur-rich, hydrous arc-related magmas derived from partial melting of the ...asthenospheric mantle metasomatized by slab-derived fluids. Yet, whether similar metallogenic processes also operated in the Precambrian remains obscure. Here we address the issue by investigating the origin, fO
, and S contents of calc-alkaline plutonic rocks associated with the Haib porphyry Cu deposit in the Paleoproterozoic Richtersveld Magmatic Arc (southern Namibia), an interpreted mature island-arc setting. We show that the ca. 1886-1881 Ma ore-forming magmas, originated from a mantle-dominated source with minor crustal contributions, were relatively oxidized (1‒2 log units above the fayalite-magnetite-quartz redox buffer) and sulfur-rich. These results indicate that moderately oxidized, sulfur-rich arc magma associated with porphyry Cu mineralization already existed in the late Paleoproterozoic, probably as a result of recycling of sulfate-rich seawater or sediments from the subducted oceanic lithosphere at that time.
Types of rock, such as komatiite, which formed entirely or dominantly during restricted time periods in solar system history, are important indicators of how planetary bodies evolved in deep time. In ...this paper we characterize three different types of temporally-restricted anorthosites, and discuss their significance to the broad-scale evolution of planetary processes. Primordial anorthosites, which constitute the bulk of the lunar crust, were sampled during Apollo and Luna missions, and have subsequently been identified in our meteorite collections. They are characterized by very calcic plagioclase (An93–98), and all have magmatic crystallization ages >4.3Ga, suggesting that the earliest planetary crust-forming processes on the Moon, and possibly elsewhere, involved substantial, if not total melting. The “magma ocean” hypothesis, which has endured for nearly 50years, argues that the lunar anorthositic rocks represent global flotation cumulates of plagioclase, which crystallized after extensive precipitation and sinking of olivine and pyroxene. Another possibility, revitalized by critical filtering of age data, involves “serial magmatism”, whereby the lunar crust was constructed by younger, smaller, episodic magmatic events. On Earth, two distinct types of temporally-restricted anorthosites offer opportunities to understand the evolution of terrestrial geodynamics, tectonics and magmatic processes. Archean megacrystic anorthosites formed only between 3.73 and 2.49Ga as small bodies (<500km2) associated with the mafic magmatic assemblages of many greenstone belts. They are typified by accumulations of equidimensional, almost spherical megacrysts (1–30cm across) of calcic plagioclase (An61–94, avg. An80) in a mafic groundmass of broadly basaltic composition, and probably formed in shallow magma chambers. The unusually calcic plagioclase compositions may reflect crystallization from hydrous and/or Ca-rich basaltic parents that were fractionated derivatives of more primitive magmas, possibly even komatiites. Isotopic and geochemical data for many Archean anorthosite complexes indicate little to no evidence for contamination by silicic continental crust, suggesting magma formation in oceanic environments, most likely by subduction in oceanic arcs. Their temporal restriction might be explicable in terms of a hotter or wetter Archean mantle, especially if a genetic connection to komatiites can be demonstrated. Alternatively, higher Archean heat flow may have influenced oceanic crustal thickness, subduction rate, sites of magma generation and/or melt composition, producing favorable conditions for the generation of calcic megacrystic anorthosites. Proterozoic massif-type anorthosites are distinctly different from other anorthosite types, and occur as small plutons (1–10km2), to large (up to 18,000km2) composite batholiths, that are temporally restricted to a ~2000m.y. period between ~0.5 and ~2.6Ga. The rocks are dominated by lath-shaped, intermediate plagioclase (An30–70, avg. An53), with lesser pyroxenes and/or olivine. There is extensive evidence from petrology, geochemistry and isotope data for variable contamination with continental crust, and the most plausible physical model involves deep-crustal ponding of basaltic magmas, crystallization and sinking of mafic silicates, and eventual flotation of buoyant plagioclase. Massifs were constructed by the coalescence of plagioclase-rich mushes, which ascended to mid-crustal emplacement sites. The basaltic parent melts or magmas for massif-type anorthosites were mantle-derived; alternative models involving melting of crustal sources, mafic or otherwise, cannot be supported. Collective features most readily support an Andean-type continental arc setting, and careful geochronology is beginning to suggest that magmatism in individual massifs may have occurred over long time-scales of up to 100m.y. The onset of massif-type anorthosite formation at ~2.6Ga probably reflects global secular cooling of the Earth, resulting in increased lithospheric strength and crustal thickness, which promoted Moho-depth ponding and slow crystallization of basaltic magmas in continental arc environments. The apparent disappearance of anorthosite massifs at ~0.5Ga may also relate to global secular cooling that changed the thermal structure of subduction zones, and promoted the onset of high and ultra-high pressure metamorphism.
The Bellevue drillcore intersects ~3 km of Main and Upper Zone cumulates in the Northern Limb of the Bushveld Complex. Main Zone cumulates are predominately gabbronorites, with localized layers of ...pyroxenite and anorthosite. Some previous workers, using bulk rock major, trace and isotopic compositions, have suggested that the Main Zone crystallized predominantly from a single pulse of magma. However, density measurements throughout the Bellevue drillcore reveal intervals that show up-section increases in bulk rock density, which are difficult to explain by crystallization from a single batch of magma. Wavelet analysis of the density data suggests that these intervals occur on length-scales of ~40 to ~170 m, thus defining a scale of layering not previously described in the Bushveld Complex. Upward increases in density in the Main Zone correspond to upward increases in modal pyroxene, producing intervals that grade from a basal anorthosite (with 5% pyroxene) to gabbronorite (with 30–40% pyroxene). We examined the textures and mineral compositions of a ~40 m thick interval showing upwardly increasing density to establish how this type of layering formed. Plagioclase generally forms euhedral laths, while orthopyroxene is interstitial in texture and commonly envelops finer-grained and embayed plagioclase grains. Minor interstitial clinopyroxene was the final phase to crystallize from the magma. Plagioclase compositions show negligible change up-section (average An
62
), with local reverse zoning at the rims of cumulus laths (average increase of 2 mol%). In contrast, interstitial orthopyroxene compositions become more primitive up-section, from Mg# 57 to Mg# 63. Clinopyroxene similarly shows an up-section increase in Mg#. Pyroxene compositions record the primary magmatic signature of the melt at the time of crystallization and are not an artefact of the trapped liquid shift effect. Combined, the textures and decoupled mineral compositions indicate that the upward density increase is produced by the downward infiltration of noritic magma into a previously emplaced plagioclase-rich crystal mush. Fresh noritic magma soaked down into the crystallizing anorthositic mush, partially dissolving plagioclase laths and assimilating Fe-enriched pore melt. The presence of multiple cycles showing upward increases in density in the Bellevue drillcore suggests that downward magma infiltration occurred episodically during crystallization of the Main Zone.
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•The Kunene Complex’s lithological variations detected using Landsat 8 data and Alos DEM.•Semi-automatically extracted lineaments interpreted as magmatic layering.•Six remote sensing ...domains of distinct internal spectral response and architecture identified.•The Kunene Complex anorthosite suite made up of at least four distinct plutons spanning >60 Myr.•Potential to interpret magmatic architecture of large igneous bodies elsewhere.
The quasi-monomineralic composition and huge spatial extent of massif-type anorthosites make detecting lithological regions and boundaries challenging. We use processed Landsat 8 OLI multispectral images and ALOS digital elevation models integrated with field and petrographic observations to characterize the architecture of the ≥17,000 km2 Mesoproterozoic Kunene Complex (KC) anorthosite suite in Angola and Namibia. Images of false colour composite bands 6, 4 and 1 and band ratios 4/2 (ferric minerals), 6/5 (ferrous minerals) and 6/7 (OH-bearing minerals), as well as assessment of the PCA and MNF matrices of eigenvectors and eigenvalues from Landsat 8 data using available spectral libraries, have substantially improved the interpretation of the Kunene Complex rock types and structures. The dataset shows that the reflectance signature of KC anorthositic rocks is primarily a function of the degree of metasomatism, which is most significant in olivine-poor rock types. The weathering intensity of the olivine-bearing anorthosite substrate is another control on the remote sensing signal.
Our remote sensing and field-based approach has enabled us to divide the KC anorthosite suite into six distinct spectral and architecture domains and at least four distinct magmatic plutons when considering available high-precision geochronological data. The northernmost pluton (ca. 1380 Ma) of massive, olivine-bearing anorthosite shows distinct remote sensing signatures in the band ratio and MNF images marked by the dominance of OH-bearing and subordinate ferric minerals. The central pluton (1412–1400 Ma) is composed of NNE- to N-striking steeply dipping interlayered olivine-bearing and olivine-poor anorthosite, which correspond to ridges of dark-coloured, low albedo rocks with subordinate slightly oxidised ferrous mineral spectral signatures, and valleys of low albedo and OH-bearing mineral spectral signatures. A NNE-striking tectonic zone along a linear belt of KC granite gneiss separates the northern and central plutons. To the south is a NNW- to NNE-striking layered pluton (ca. 1390 Ma), marked by olivine-bearing anorthosite that forms ridges and metasomatized olivine-poor and pyroxene-bearing anorthosite that forms valleys. This domain is separated from the similarly layered and E–W-striking Zebra Lobe–Oryeheke anorthosite by a top-to-the north thrust zone. A small domain of ca. 1438 Ma olivine-bearing anorthosite sensu lato is situated to the SE of the main Kunene Complex.
This work highlights the variability in layered and massive textures, the extent of metasomatism, and the importance of internal tectonic boundaries in the architecture of the Kunene Complex anorthosite suite. Our approach can be applied in other anorthositic terrains and large igneous bodies elsewhere on Earth, especially those where outcrop is poor, or access is inhibited.
The limitations of many in situ microsampling techniques, especially when applied to isotopic analyses, may result in lower measurement precision and limit the ability to compare different isotope ...systems within the same phase or the same isotope system between phases. High‐precision, combined and complementary isotopic measurements are typically limited to whole‐rock samples or mineral separates at the expense of textural context. We demonstrate that laser ablation instruments could offer an effective, non‐mechanical alternative to micro‐milling for delineating and extracting single crystals, interstitial phases, and sub‐crystal scale features from soluble‐resin thick sections for column chromatography and high precision ID‐TIMS measurement. Systematic testing demonstrated that both modern and legacy laser instruments are capable of ablating through plagioclase, pyroxene and fused glass to depths of 200 μm. Modern instruments offer software benefits to identifying optically‐indistinct petrographic features, chemical zoning and avoiding alteration phases by using overlain high‐resolution datasets. Extracting delineated targets of varying geometry was achieved under binocular microscope using common mineral‐separation tools. Analysis of RM glass used as mineral proxies indicate (a) no significant thermal aureole on ablated margins that could affect measured parent‐daughter ratios, (b) no elevated risk of contamination compared with traditional mineral separation techniques, (c) excellent blanks for low‐yield analyses and (d) high‐precision ID‐TIMS results (87Sr/86Sr and 143Nd/144Nd) for sub‐100 ng Sr loads and sub‐10 ng Nd loads.
Key Points
LA instruments could offer a non-mechanical alternative for delineating and liberating mineral targets.
RM glass proxies demonstrated no significant deviation in parent/daughter analyte mass fractions or 87Sr/86Sr and 143Nd/144Nd ratios after ablation, extraction and chromatography.
This novel method combines the selectivity of LA with the high-precision and multi-isotope abilities of ID-IRMS.
Although Cu-sulfide mineralization within the Loolekop pipe of the Phalaborwa Igneous Complex has been extensively studied, certain aspects of the sulfide metallogenesis remain unclear. This research ...aims to constrain the relative timing and processes involved in Cu-sulfide mineralization within the Loolekop pipe through textural analysis and trace element composition of the three different types of sulfide assemblages in this intrusion. The first primary magmatic mineralization phase contains dispersed bornite with chalcopyrite exsolution and local traces of chalcocite and covellite related to fluid alteration, whereas the second primary phase is characterized by veins of chalcopyrite with cubanite exsolution. The third mineralization phase relates to leaching of primary sulfide assemblages by oxidizing fluids, resulting in desulfurization and the formation of valleriite (Fe
2+
,Cu)
4
(Mg,Al)
3
S
4
(OH,O)
6
and magnetite. Whole-rock ICP-MS analyses of platinum-group elements (PGE) indicate a mantle origin of the primary Cu-sulfides (bornite-chalcopyrite assemblages). In situ trace element analyses of sulfide assemblages also show evidence of external material input, most likely of crustal origin, during magma and sulfide liquid ascent, before decoupling of the sulfide liquid and the melt, although the exact source is unclear. The enrichment of Pd-group PGE (PPGE: Pd, Pt) over Ir-group PGE (IPGE: Os, Ir, Ru) in Cu-sulfide phases suggests the presence of an intermediate solid solution (iss)-monosulfide solid solution (mss) system. It is proposed that a Cu- and PPGE-enriched sulfide liquid fractionated from an IPGE-enriched mss, followed by the formation of an iss, and finally the extraction of a PPGE-rich Cu-liquid from the iss forming the bornite-chalcopyrite assemblages. Sulfide textures and the presence of chalcopyrite veins suggest that the iss was leached by high-temperature hydrothermal fluids at depth that remobilized and precipitated Cu along the fractures within the intrusion. Subsequently, all the primary magmatic phases were affected by late-stage fluids, which triggered their alteration into valleriite, also precipitating along fractures within the Loolekop pipe.
•We give new ID-TIMS and LA-ICPMS U–Pb ages, as well as detailed field observations from the Kunene Anorthosite Complex (KAC) of SW Angola.•High-precision U–Pb zircon and baddeleyite ages for ...anorthositic rocks from across the complex (n = 11) range between 1376 and 1438 Ma—a period of ~60 Myr.•Comagmatic pegmatoidal enclaves give ages of ~1500 Ma, indicating as much as 120 Myr between some of the earliest and latest crystallization products of the system.•We infer that the KAC represents a long-lived mushy magmatic system that was likely emplaced in distinct pulses.
Proterozoic massif-type anorthosites are voluminous, plagioclase-dominated plutonic suites with characteristic intermediate compositions (An50±10) that represent mantle-derived magmas ponded at Moho depths and crystallized polybarically until emplacement at mid-crustal levels. We present new ID-TIMS and LA-ICPMS U–Pb ages, as well as detailed field observations from the Kunene Anorthosite Complex (KAC) of SW Angola. Our main objective in this work is to contextualize a variety of important petrographic and geochronological features of the KAC that have implications for Proterozoic anorthosite petrogenesis and the geological evolution of the SW Congo Craton. High-precision U–Pb zircon and baddeleyite ages for anorthositic rocks from across the complex (n = 11) range between 1376 and 1438 Ma—a period of ~60 Myr—showing that such systems may be remarkably long-lived. Dating of magmatic zircons from a comagmatic pegmatoidal enclave in the KAC, which also contains high-Al orthopyroxene megacrysts (HAOM), gives an age of ~1500 Ma, indicating as much as 120 Myr between some of the earliest and latest crystallization products of the system. These enclaves provide a snapshot of the liquid line of descent of the anorthosites and are compelling evidence for the comagmatic nature of the HAOM. Field evidence for block structure, magma mingling, and extreme textural heterogeneity in the KAC supports emplacement as multiple influxes of magma, of various volumes, carrying variable proportions of crystals and autoliths from below. We infer that the KAC represents a long-lived mushy magmatic system that was likely emplaced in distinct pulses. These observations refine our understanding of Proterozoic anorthosite petrogenesis. They also provide evidence for an array of magmatic processes operating at various levels of crystallinity that may create textural and chemical heterogeneity in evolving magma products at lower- to mid-crustal levels.
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