Mafic-ultramafic Bodies Research Articles

Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications

The occurrence of high-pressure mafic–ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet–clinopyroxene–quartz coronas that formed after the breakdown of orthopyroxene–plagioclase–ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 system indicate a pressure–temperature (P–T) evolution from an isothermal (780°C) pressure up to c. 24kbar to decompression and cooling. Such a P–T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U–Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818Ma and Th/U ratios vary from 1.08 to 2.09. K–Ar dating of muscovite and biotite from biotite–kyanite–sillimanite gneiss and garnet–biotite gneiss yields age of 486±9Ma and 459±9Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal formation of the Betsimisaraka suture in north-eastern Madagascar.

Neoarchean arc magmatism followed by high-temperature, high-pressure metamorphism in the Nilgiri Block, southern India

The Nilgiri Block, southern India is an exhumed lower crust formed through arc magmatic processes in the Neoarchean. The main lithologies in this terrane include charnockites, gneisses, volcanic tuff, metasediments, banded iron formation and mafic–ultramafic bodies. Mafic–ultramafic rocks are present towards the northern and central part of the Nilgiri Block. We examine the evolution of these mafic granulites/metagabbros by phase diagram modeling and U–Pb sensitive high resolution ion microprobe (SHRIMP) dating. They consist of a garnet–clinopyroxene–plagioclase–hornblende–ilmenite±orthopyroxene±rutile assemblage. Garnet and clinopyroxene form major constituents with labradorite and orthopyroxene as the main mineral inclusions. Labradorite, identified using Raman analysis, shows typical peaks at 508cm−1, 479cm−1, 287cm−1 and 177cm−1. It is stable along with orthopyroxene towards the low-pressure high-temperature region of the granulite facies (M1 stage). Subsequently, orthopyroxene reacted with plagioclase to form the peak garnet+clinopyroxene+rutile assemblage (M2 stage). The final stage is represented by amphibolite facies–hornblende and plagioclase–rim around the garnet–clinopyroxene assemblage (M3 stage). Phase diagram modeling shows that these mafic granulites followed an anticlockwise P–T–t path during their evolution. The initial high-temperature metamorphism (M1 stage) was at 850–900°C and ~9kbar followed by high-pressure granulite facies metamorphism (M2 stage) at 850–900°C and 14–15kbar. U–Pb isotope studies of zircons using SHRIMP revealed late Neoarchean to early paleoproterozoic ages of crystallization and metamorphism respectively. The age data shows that these mafic granulites have undergone arc magmatism at ca. 2539.2±3Ma and high-temperature, high-pressure metamorphism at ca. 2458.9±8.6Ma. Thus our results suggests a late Neoarchean arc magmatism followed by early paleoproterozoic high-temperature, high-pressure granulite facies metamorphism due to the crustal thickening and suturing of the Nilgiri Block onto the Dharwar Craton.

Neoproterozoic crustal growth of the Southern Yangtze Block: Geochemical and zircon U–Pb geochronological and Lu-Hf isotopic evidence of Neoproterozoic diorite from the Ailaoshan zone

Neoproterozoic felsic igneous rocks associated with mafic-ultramafic bodies along the margins of the Yangtze Block, South China, can be used to constrain the continental crustal growth and secular evolution of the region. LA-ICPMS zircon U–Pb dating of the Adebo quartz diorite pluton in the Ailaoshan tectonic zone on the southern margin of the Yangtze Block gives the Neoproterozoic age of 800±7Ma and ɛHf(t) values in the range of −1.03 to +3.75 with two-stage model age of 1.3–1.6Ga. The pluton is characterized by relatively low SiO2 (60.97–64.41wt.%) and total alkalis (K2O+Na2O, 7.35–9.14wt.%) and high Al2O3 content (16.98–18.21wt.%) with mg-number of 36–39. REE-normalized patterns show enrichment in LREE with (La/Yb)cn of 11.36–19.77 and Europium negative anomalies with Eu/Eu*=0.61–0.74. The samples are characterized by negative Nb-Ta ((Nb/La) n=0.18–0.35) and P, Ti, Sr anomalies and high Y concentrations (33.79–41.04ppm) and low Sr/Y ratios (5.65–10.16). Their isotopic composition are similar to those of the Neoproterozoic mafic igneous rocks in the Ailaoshan zone and the southwestern Yangtze Block, indicating that the quartz diorite was produced by partial melting of mafic lower crust. The diorite also shows the similar geochemical characteristics with adakitic rocks from thickened lower crust or amphibolite and eclogite experimental melts. In combination with their arc-related geochemical signatures and synchronous developed adakitic rocks in the region, the Adebo quartz diorite pluton might be produced in a subduction-related tectonic setting during Neoproterozoic crustal growth along the margins of Yangtze Block.

The occurrence of platinum-group element and gold minerals in the Bon Accord Ni-oxide body, South Africa

Two samples from the enigmatic Ni-oxide body of Bon Accord (Barberton greenstone belt, South Africa) have been investigated with the hydroseparation technique to obtain heavy mineral concentrates. The concentrates contain abundant Pt, Pd, and gold minerals never reported before from the Bon Accord Ni-oxide body. The grains occur as: (1) minute inclusions (<3 μm) in trevorite (ideally NiFe3+O4) and (2) larger (5-70 μm) free aggregates liberated from the host phase. The first group comprises several PGM compounds of Pd-Sb, Pd-Sb-As, Pd-Cu-Sb, Pt-Sb, Pt-As-S, Ru-As-S, Ru-S, along with free grains of Ni-Fe-As. The second consists of sperrylite (PtAs2), members of the sobolevskite-kotulskite series, and electrum. These results are in good agreement with previous analyses of PGE-Au in bulk rock. Paragenetic relationships indicate that the PGM and electrum are of secondary origin, probably generated during low-temperature metamorphism of the Ni-rich mineralization. They have a terrestrial origin and are related with a low-sulfidation regime that usually accompanies hydrothermally driven serpentinization of mafic-ultramafic bodies. The ligands in the newly formed PGM (As, Sb, Bi, Te, and O) probably proceed from the same source of the hydrothermal solutions. In this model, the metals Ni-PGE-Au were original components of the primary mineral assemblage of the Bon Accord precursor, whereas As, Sb, Bi, Te, and O might have been contributed by the metasomatizing fluids, during nearsurface evolution of the ore body. The data on the high-grade heavy mineral concentrates, obtained by hydroseparation, have provided new knowledge about the mineral deportment of Pd, Pt, and Au.

Devonian to Permian evolution of the Paleo-Tethys Ocean: New evidence from U–Pb zircon dating and Sr–Nd–Pb isotopes of the Darrehanjir–Mashhad “ophiolites”, NE Iran

Middle to Late Paleozoic ophiolites, which are remnants of the Paleo-Tethys Ocean, are aligned in two main zones in northern Iran: Darrehanjir–Fariman–Mashhad, and Rasht in the north and Jandagh–Anarak ophiolites to the south. Our new U–Pb zircon dating results show that the ~200km long Darrehanjir–Mashhad mafic–ultramafic body is not a single ophiolite but a composite igneous complex composed of Permian pillow lavas and pelagic sediments in fault contact with a small outcrop of Devonian intrusive and ultramafic rocks. Darrehanjir intrusive rocks have U–Pb zircon ages of 380.6±3.7Ma and 382.9±3.7Ma respectively, ~100Ma older than published ages for gabbros and radiolarites intercalated with lavas near Mashhad and Fariman. Mantle peridotites from the Devonian complex contain low Cr# spinel, similar to that in MORB-type peridotites. Devonian Darrehanjir gabbros and Permian Mashhad sequences both have boninitic and calc-alkaline signatures, respectively. The δ18Ozircon values from the Devonian ferrodiorite (δ18Ozircon~4.6±0.3‰) are slightly lower than the 5.2 to 5.4‰ expected for MORB-type zircons whereas Devonian plagiogranitic zircons mostly have δ18Ozircon <5‰, perhaps reflecting involvement of hydrothermally altered crust. Similar, strongly positive values of zircon εHf(t) for plagiogranite (av. +14.9) and ferrodiorite (av. +13.8) indicate melt derivation from depleted asthenosphere. Darrehanjir–Mashhad ophiolitic rocks can be divided into groups with high εNd (>10.3) and low εNd (<5.4) for both Permian and Devonian suites. Most Darrehanjir–Mashhad rocks are characterized by radiogenic 207Pb/204Pb and 208Pb/204Pb, indicating the involvement of subducted terrigenous sediments in the source. The Mashhad–Darrehanjir mafic–ultramafic complex demonstrates that this part of Paleo-Tethys evolved from oceanic crust formation above a subduction zone in Devonian time to accretionary convergence in Permian time. Iranian Paleozoic ophiolites and oceanic igneous complexes along with those of the Caucasus and Turkey in the west and Afghanistan, Turkmenistan and Tibet to the east, define a series of diachronous subduction-related marginal basins that were active from at least Early Devonian to Late Permian time.

Magnetotelluric studies in the Central India Tectonic Zone: Implications for intraplate stress regimes and generation of shallow earthquakes

The Central India Tectonic Zone (CITZ) dissects the Indian Peninsula into the northern and southern crustal blocks. The CITZ has been a seismically active region since the Precambrian. Whereas the relatively deep crustal earthquakes near the Narmada faults in the eastern part of the CITZ have been well-investigated, the mechanisms for the shallow earthquakes in the western part remain unknown. Here we present results from a new magnetotelluric study to derive the crustal structure and to understand its implications. Our data show a thick and highly resistive (>500ohmm) crust in the south of Tapti River, as against a less resistive one in the north. These results in conjunction with heat flow values indicate that the crust below the southern part has stable continental cratonic signatures. On the northern side of the Tapti River, we infer the ascent of basaltic magmas from the mantle into the shallow crust and crystallization into layered intrusions. These mafic–ultramafic bodies could be a potential cause for the shallow earthquakes in the western part of the CITZ. The mafic–ultramafic bodies below the crust of CITZ would locally modify the intraplate stresses, which in turn would facilitate the occurrence of earthquakes due to reactivation of pre-existing faults. Thus, the large accumulation of strain energy in the deep crust beneath the region can be attributed to the presence of high stress bodies emplaced at depth during the Deccan Volcanic activity.

Multistage metamorphism of garnet orthopyroxenites from the Maowu mafic–ultramafic complex, Dabieshan UHP terrane, eastern China

Garnet orthopyroxenites of the Maowu mafic–ultramafic body occur in coesite-bearing paragneisses in the Dabieshan ultrahigh-pressure (UHP) metamorphic complex. We have distinguished six stages of metamorphism based on integrated mineralogical–petrological investigations: M1 – high-T/low-P metamorphism (˜1.4 GPa, ˜850°C); M2 – low-T/low-P metamorphism (˜1.4 GPa, ˜750°C); M3 – low-T/high-P metamorphism (2.1–2.5 GPa, 740–760°C); M4 – UHP metamorphism (5.3–6.3 GPa, ˜800°C); M5 – early retrogression (<3 GPa, >750°C); M6 – late retrogression (<2.3 GPa, <670°C). Detailed textures and mineral compositions indicate that the protolith of the Maowu garnet orthopyroxenites was a mantle refractory harzburgite or dunite that had been metasomatized by crust-derived silica-rich fluid or melt. M1–M2 defines an isobaric cooling P–T path, probably resulting from corner-flow in the mantle wedge above the subduction slab. M2–M4 defines an isothermal compressional P–T path, suggesting that the complex subsequently recycled into the deep upper mantle (up to ˜200 km depth) and underwent HP–UHP metamorphism. M4–M6 defines a retrograde P–T path, implying that the rocks ascended to crustal levels attending exhumation of the UHP terrane, and was continuously metasomatized by fluid derived from the continent country rocks.

Petrology, geochemistry and mineralization of the Las Águilas and Virorco mafic–ultramafic bodies, San Luis Province, Argentina

The layered mafic–ultramafic rocks in the Sierras de San Luis, Argentina (Las Aguilas, Las Higueras and Virorco), constitute a 3–5-km-wide belt that extends over 100 km from NE to SW. They carry a sulphide mineralization consisting of pyrrhotite, pentlandite and chalcopyrite, in veins and as disseminated to massive ore. Disseminated spinels are frequently associated with the sulphide minerals as well as platinum group minerals. A strong correlation between S, Ni, Co, Cu, Cr, Pt and Pd indicates the presence of one to three levels of mineralization within the ultramafic units. The maximum concentration of these elements coincides with the units containing platinum group minerals (PGM) and spinel group minerals. This clear relationship constitutes a good prospecting guide in the search for layers with high-grade ore, probably associated with deeper stratigraphic levels where ultramafic rocks are dominant. The bulk rock chemistry and concentrations of metals and platinum group elements as well as textural evidence suggest that the parental magma was mafic with tholeiitic affinities and MgO rich. The Las Aguilas layered mafic–ultramafic body and the remaining bodies in the area bear similar characteristics to well-known stratified complexes developed in extensional tectonic regimes, as it is the case of Jinchuan (China), Kabanga (Tanzania) and Fiambala (Argentina).

Geochemical Characteristics and Metallogenesis of the Qingkuangshan Ni‐Cu‐PGE Mineralized Mafic‐Ultramafic Intrusion in Huili County, Sichuan Province, SW China

Abstract:The Qingkuangshan Ni‐Cu‐PGE deposit, located in the Xiaoguanhe region of Huili County, Sichuan Province, is one of several Ni‐Cu‐PGE deposits in the Emeishan Large Igneous Province (ELIP). The ore‐bearing intrusion is a mafic‐ultramafic body. This paper reports major elements, trace elements and platinum‐group elements in different types of rocks and sulfide‐mineralized samples in the intrusion. These data are used to evaluate the source mantle characteristics, the degree of mantle partial melting, the composition of parental magma and the ore‐forming processes. The results show that Qingkuangshan intrusion is part of the ELIP. The rocks have trace element ratios similar to the coeval Emeishan basalts. The primitive mantle‐normalized patterns of Ni‐Cu‐PGE have positive slopes, and the ratios of Pd/Ir are lower than 22. The PGE compositions of sulfide ores and associated rocks are characterized by Ru depletion. The PGE contents in bulk sulfides are slightly depleted relative to Ni and Cu, which is similar to the Yangliuping Ni‐Cu‐PGE deposit. The composition of the parental magma for the intrusion is estimated to contain about 14.65 wt% MgO, 48.66 wt% SiO2 and 15.48 wt% FeOt, and the degree of mantle partial melting is estimated to be about 20%. In comparison with other typical Ni‐Cu‐PGE deposits in the ELIP, the Qingkuangshan Ni‐Cu‐PGE deposit has lower PGE contents than the Jinbaoshan PGE deposit, but has higher PGE contents than the Limahe and Baimazhai Ni‐Cu deposit, and has similar PGE contents to the Yangliuping Ni‐Cu‐PGE deposit. The moderate PGE depletions in the bulk sulfide of the Qingkuanghan deposit suggest that the parental magma of the host intrusion may have undergone minor sulfide segregation at depth. The mixing calculations suggests that an average of 10% crustal contamination in the magma, which may have been the main cause of sulfide saturation in the magma. We propose that sulfide segregation from a moderately PGE depleted magma took place prior to magma emplacement at Qingkuangshan, that small amounts of immiscible sulfide droplets and olivine and chromite crystals were suspended in the ascending magma, and that the suspended materials settled down when the magma passed trough the Qingkuangshan conduit. The Qingkuangshan sulfide‐bearing intrusion is interpreted to a feeder of Emeishan flood basalts in the region.

New interpretation of tectonic model in south Tibet

We present a new interpretation of tectonic evolution in southern Tibet that is drastically different from the existing models. A detailed tectono-sedimentary study crossing the Yarlung Zangpo ophiolite zone shows that many geological features are different from those commonly described in large subduction collision models. For example, no N–S oriented shear zones are found between the ophiolitic sequence and country flysch strata, whereas a conformable contact relationship is recognized between them. A tectonic window exists inside the ophiolite body in the Bailang region. Some intrusion-like mafic–ultramafic bodies occurred in the Renbu region, where the country strata are in sub-concordant contact with these bodies and show contact metamorphic aureole. Toward the west, the ophiolite zone was separated by flysch sequences into sub-parallel branches. In the Lhasa region, the sedimentary facies are similar on both sides of the Zangpo Valley, and have preserved an intact Mesozoic basin system. Instead of ophiolite rocks, volcaniclastic deposits occurred in the corresponding location of the ophiolite in the Zangpo Valley. Consequently, we conclude that the Zangpo ophiolite zone has a tectonic affinity of back-arc basin with its spasmodic growth of juvenile oceanic crust. The real tectonic suture, or the closure zone of the Neotethys, should be represented by the High Himalaya Central Gneiss Unit, which shows a large scale strong shearing in same orientation, high metamorphism and protracted re-mobilization. The oceanic crust subducted northward and split off the Himalaya continental front arc, created the Zangpo back-arc basin since Late Triassic. The collapse of the Zangpo back-arc basin by supra-subduction occurred since the Eocene. The final collision between India and the Himalayan arc took place since Late Eocene with a re-mobilized large shear system. The major mylonitic zones migrated progressively southward with bulk of shear slip absorbing the crust of north India and south Tibet.

U–Pb SHRIMP zircon geochronology, petrogenesis, and tectonic setting of the Neoproterozoic Baekdong ultramafic rocks in the Hongseong Collision Belt, South Korea

The Hongseong area, located in the western Gyeonggi Massif, South Korea, can be correlated with the northern margin of the South China block (Yangtze Craton). This area experienced Neoproterozoic igneous activity related to subduction before the amalgamation of Rodinia. Several isolated, lenticular, and serpentinized ultramafic–mafic bodies occur in the Hongseong area. The Baekdong body, one of the largest ultramafic bodies, has been highly deformed and metamorphosed to eclogite- and granulite-facies. The petrogenesis and tectonic environment of the Baekdong rocks are assessed using the composition of unaltered cores of spinel and olivine grains, and show that these rocks represent the mantle section of a suprasubduction ophiolite. The rocks originated from oceanic lithosphere that formed during the transition from nascent back-arc to mature island arc, related to subduction roll-back. During the back-arc stage, Al-rich spinel harzburgite formed through melt–rock interaction caused by the intrusion of magma. This magma was produced in small amounts, by less than 10% of partial melting of the wedge mantle. Subsequently, during the mature island arc stage, Cr-rich spinel dunite formed through melt–rock interaction caused by the intrusion of relatively evolved magma that formed by 30–35% partial melting due to a high input of volatiles from the subducted slab and sediments. The Baekdong ultramafic rocks, together with the Bibong ultramafic rocks, indicate that a suprasubduction tectonic setting prevailed before the amalgamation of Rodinia (at 860–890 Ma) in the Hongseong area, which may be an extension of the northern margin of the Yangtze Craton.

Reassessment of the volume of the Las Aguilas mafic-ultramafic intrusives, San Luis, Argentina, based on an alternative geophysical model

Abstract In the Sierra de San Luis, Central Argentina, a belt of small and discontinuous lenses of mafic-ultramafic rocks intrude a polydeformed basement and are thought to be the cause of a local increase of the metamorphic grade from amphibolite to granulite facies conditions. This assumption was especially based on forward modelling of a huge gravity anomaly centered over the Sierra de San Luis, which lead some workers to think that a vast volume of mafic-ultramafic rocks lay in shallow levels. Here, we propose an alternative model to explain this anomaly, in which the mafic-ultramafic intrusion is not the ultimate source. Therefore, there is no need to propose a bigger size than that observed in outcrops for the mafic-ultramafic bodies. The thermal effect of the emplacement of mafic-ultramafic sills and dikes on the host rocks was estimated applying a simple analytical solution (error function) for heating of a semi-infinite half space (the country rocks) in contact with a hotter sheet of finite thickness (the mafic-ultramafic intrusion). Results indicate that the effect of the intrusion of these hot mafic magmas is local, because beyond a few hundred meters from the contact zone temperatures never exceed 600 °C, and a few km from the intrusion they barely increase 50 °C relative to the initial temperature. These results, together with the preservation of primary igneous characteristics (such as rhythmic layering) being overprinted by metamorphic textural changes, indicate that the intrusion occurred before regional deformation. It is suggested that the thermal anomaly in the Pringles Metamorphic Complex could have been mainly caused by factors inherent to their geodynamic setting.

Platinum–palladium–gold mineralization in the Nkenja mafic–ultramafic body, Ubendian metamorphic belt, Tanzania

Significant and widespread enrichment of platinum, palladium, and gold has been found within the Nkenja mafic–ultramafic body located in southern Tanzania in the central part of the Ubendian metamorphic belt. This body is dominated by partly serpentinized chromitiferous dunite, wehrlite and olivine clinopyroxenite, which are tectonically intercalated with amphibolitized metagabbro. The dunites contain both disseminated and seam-type chrome spinel with an Al-rich composition. The seams are thin, impersistent and, together with enclosing dunite, often show deformation at granulite facies conditions. Forsterite contents of olivine in the dunite range from 87 to 92 mol%. Clinopyroxene in wehrlite and clinopyroxenite is diopsidic with significant contents of Al and Na. Clinopyroxene forms irregular bands and crosscutting veins in the dunite, as well as occurring as weakly dispersed isolated grains in the dunite. Elevated levels of Pt, Pd and Au occur in all ultramafic rocks, but not amphibolitized metagabbro, and there is a weak correlation between high abundances of platinum-group element (PGE) and chromitites. PGE values are erratically distributed and are associated with trace to minor amounts of disseminated sulphides (pyrrhotite, pentlandite, heazlewoodite, chalcopyrite and bornite). The abundances of all PGE are consistently anomalous, suggesting a primary igneous control by sulphides in ultramafic rock. However, there has evidently been a strong metamorphic and/or hydrothermal overprint on what was probably an original magmatic concentration of PGE-bearing sulphides. Geological mapping and petrological evidence, as well as the style of PGE sulphide mineralization, are consistent with the Nkenja ultramafic body being part of the crustal section of a dismembered Palaeoproterozoic ophiolite.

Siderophile and chalcophile metal variations in basalts: Implications for the sulfide saturation history and Ni–Cu–PGE mineralization potential of the Tarim continental flood basalt province, Xinjiang Province, China

It is well known that a genetic link exists between the formation of ultramafic-hosted Ni–Cu–PGE sulfide deposits and the eruption of associated chalcophile- and siderophile-depleted continental flood basalts. However, until recently little research has been undertaken on the Permian Tarim Basin continental flood basalt province in Xinjiang province, China. Here, we present new trace, siderophile and chalcophile element data from continental flood basalts that are well exposed and preserved in the Keping area of Xinjiang province and we discuss their sulfide saturation history and Ni–Cu mineralization potential. Ultramafic–mafic units dominated by basaltic rocks outcrop over an area of about 250,000km2 in the Tarim Basin; these Lower Permian basaltic rocks are hosted by the Kupukuziman and Kaipaizileike Formations, a mixture of basalts, trachybasalts, amygdaloidal basalts and basaltic pyroclastics.Major, trace and rare earth element (REE) data indicate that basalts within both formations are cogenetic, whilst low concentrations of Ni, Cu and platinum-group elements (PGE) indicate that they are chalcophile element depleted. The basalts have very low (Cu/Zr)PM and extremely low (Pd/Zr)PM and (Pd/Cu)PM ratios; both these ratios and the concentrations of Ni, Cu and the PGEs are invariant with changes in MgO concentrations. Furthermore, the magmas represented by the Keping basalts have very low calculated sulfur concentration at sulfide saturation (SCSS) capacities, and only needed to assimilate small amounts of sulfur-bearing crustal material to breach the SCSS and form immiscible sulfide liquids. This assimilation of crustal material appears to have occurred at depth; this is evidenced by the clear link between crustal contamination (as expressed by (Th/Nb)PM ratios) and the removal and depletion of chalcophile elements in the magmas that formed the basalts in the Keping area (as expressed by (Pd/Cu)PM and (Pd/Zr)PM ratios). The geochemistry of the basalts is consistent with an enriched mantle source; all of the basalts assimilated significant amounts of crustal material, became sulfur saturated and formed immiscible magmatic sulfides at depth; these magmatic Ni–Cu–PGE sulfides scavenged chalcophile elements from the magma and remained at depth whilst the chalcophile-depleted basalts were erupted.The identification of the segregation of magmatic sulfides from ultramafic–mafic magmas within the Tarim Basin indicates that the area should be considered highly prospective for mineral exploration for magmatic Ni–Cu–PGE deposits. Magmatic Ni–Cu–PGE sulfide enriched units may well exist in magma conduits or at the base of shallow magma chambers within the basin, and any ultramafic sills within the basin that are cogenetic and coeval with the Lower Permian Keping area basalts, for example the layered ultramafic–mafic bodies in the Bachu area, should be considered as prospective targets.

The Neoproterozoic layered mafic–ultramafic intrusion of Gabal Imleih, south Sinai, Egypt: Implications of post-collisional magmatism in the north Arabian–Nubian Shield

The Imleih layered mafic–ultramafic body is a Neoproterozoic intrusion covering about 45 km 2 in southern Sinai. The field relations as well as the fine grained chilled margins indicate that the layered intrusion is younger than the surrounding calc-alkaline syn-tectonic granodiorite and Iqna Shar ,a volcanics and older than the post-tectonic Iqna granite. There is a general consensus that there are no ophiolitic rocks younger than the syn-tectonic granitoids in the Arabian–Nubian Shield (ANS), which supports the non-ophiolitic origin of the Imleih layered intrusion. The studied layered intrusion is tilted to the north, with lower layers (peridotite and pyroxenite) exposed to the south and upper layers (cumulus gabbro and anorthosite) exposed to the north. Imleih rocks are cumulates, dominated by cumulus gabbro with subsequent amounts of peridotite, pyroxenite and anorthosite. Pegmatitic gabbro is locally observed. These rocks are unmetamorphosed. In the peridotite, the Fo content of olivine ranges from 0.81 to 0.83. Clinopyroxenes of the studied rocks range in composition from diopside to augite and have subalkaline affinities. Plagioclase in the cumulus gabbro is mostly bytownite (An 73-82). Two types of spinel are observed, chromite and pleonaste. Chromite has Cr# (Cr/(Cr + Al) of 0.60–0.81 whereas pleonaste have Cr# lower than 0.02. The present study revealed that the mafic–ultramafic complex of Gabal Imleih fractionated from a single batch of high Al-basaltic magma and evolved by fractional crystallization at the base of a Neoproterozoic magma chamber. Olivine and chromite were the first crystallized phases followed by pyroxene and plagioclase. Amphibole appears at nearly the same time as plagioclase. The Imleih mafic–ultramafic body was emplaced post-tectonically in a transitional phase between the calc-alkaline and alkaline magmatism.

Late Triassic subduction-related ultramafic–mafic magmatism in the Amasya region (eastern Pontides, N. Turkey): Implications for the ophiolite conundrum in Eastern Mediterranean

The eastern Pontides orogenic belt of northeastern Turkey offers critical clues on convergent margin tectonics associated with the late Mesozoic-early Cenozoic geodynamic evolution of the eastern Mediterranean region. Here we report the geology, petrology, geochemistry, Nd–Sr–Pb isotopes and U–Pb zircon ages from a ultramafic–mafic complex in Aksalur which forms part of a series of Alaskan-type ultramafic–mafic bodies in the Amasya region located at the south-western corner of the eastern Pontides orogenic belt. These ultramafic–mafic intrusions occur as minor (30–300 m in diameter), scattered, roughly circular bodies within the low-grade metamorphic rocks of the Tokat metamorphic massif. Petrographic studies characterise the rock types as gabbro and wehrlite, with the latter showing typical cumulate texture and mainly comprising serpentinized olivine, clinopyroxene, hornblende, mica and opaque minerals. The gabbroic rocks are characterized by relatively high initial ratios of 143Nd/ 144Nd (0.512618–0.512633) and low initial ratios of 87Sr/ 86Sr (0.70420–0.70435). Their initial Pb isotope ratios range from 17.306 to 17.771. The gabbros and cumulate wehrlites display geochemical distribution patterns analogous to the Alaskan-type intrusions that were derived from a subduction-related, high-alumina, hydrous basaltic parental magma in a convergent plate setting. LA-ICPMS analyses of zircons separated from the gabbros show high Th/U ratios (1.35–5.26) typical of magmatic origin. The U–Pb isotopic data define a weighted mean 206Pb/ 238U age of 203.3 ± 4.9 Ma. Detailed evaluation of the magma tectonics suggests that the Alaskan-type Aksalur ultramafic–mafic intrusions were accumulated in magma chamber(s) located at relatively shallow crustal levels. Two distinct magmatic pulses are identified: the first phase produced the gabbroic intrusions and the second phase was characterized by numerous small-scale werhlitic cumulate intrusions. In contrast to the previous models which assigned a mid-oceanic ridge-derived ophiolite affinity for ultramafic–mafic complexes in the eastern Pontides orogenic belt, our study reveals that these intrusions were derived from subduction-related high-alumina hydrous basaltic magmas transferred into the overlying relatively thin continental crust along deep-seated fractures.

Neoproterozoic crustal growth and reworking of the Northwestern Yangtze Block: Constraints from the Xixiang dioritic intrusion, South China

Neoproterozoic magmatism along the northwestern margin of the Yangtze Block, South China, produced voluminous adakitic and normal arc felsic intrusions associated with many mafic–ultramafic bodies. Dioritic plutons are rare but are important for understanding the crustal growth and secular evolution of the region. The Xixiang dioritic intrusion at Hannan, on the northwestern margin of the Yangtze Block, provides a good opportunity to investigate the petrogenesis of intermediate plutons and to document the Neoproterozoic continental crustal evolution of the region. The pluton has a zircon U–Pb age of 764 ± 9 Ma. Rocks from the pluton have relatively low SiO 2 (59.3–63.0 wt.%) and K 2O + Na 2O (5.8–6.2 wt.%) and high MgO (2.3–3.1 wt.%) and Al 2O 3 (15.8–17.1 wt.%). They have chondrite-normalized REE patterns enriched in LREE (La/Ybn = 7.2–14.7) with negative Eu anomalies. Their primitive mantle-normalized trace element patterns are characterized by enrichment of LILE and depletion of HFSE (Nb and Ta) with positive Pb and negative Ti anomalies. They have a narrow range of initial 87Sr/ 86Sr ratios (0.7033 to 0.7035) with positive εNd values (+ 1.94 to + 3.16). Their zircon εHf values range from + 5.5 to + 9.8 with an average of + 7.32. These isotopic data are similar to those of the Neoproterozoic mafic–ultramafic intrusions in the Hannan region, suggesting that the diorites were generated by partial melting of newly emplaced basaltic rocks. The diorites have Mg# values (50.3–54.4) higher than those of experimental melts of mafic igneous rocks. In addition, they contain Fe-rich clinopyroxene, suggesting that partial melts of the newly formed mafic lower crust interacted with the mafic residue. Based on its arc-related geochemical affinity and emplacement preceding voluminous adakitic magmatism, the Xixiang intrusion is proposed to have been generated in a Neoproterozoic arc setting during crustal growth and reworking that lasted from > 950 Ma to < 730 Ma, a period of more than 200 m.y.

U-PB SHRIMP ZIRCON DATING OF MESOPROTEROZOIC MAGMATIC ROCKS FROM THE SCOTTBURGH AREA, CENTRAL MZUMBE TERRANE, KWAZULU-NATAL, SOUTH AFRICA

Research Article| September 01, 2010 U-PB SHRIMP ZIRCON DATING OF MESOPROTEROZOIC MAGMATIC ROCKS FROM THE SCOTTBURGH AREA, CENTRAL MZUMBE TERRANE, KWAZULU-NATAL, SOUTH AFRICA B.M. EGLINGTON; B.M. EGLINGTON University of Saskatchewan, Saskatoon, Canada, e-mail: bruce.eglington@usask.ca Search for other works by this author on: GSW Google Scholar R.J. THOMAS; R.J. THOMAS British Geological Survey, Keyworth, Notts, NG12 5GG, United Kingdom, e-mail: bthomas@bgs.ac.uk Search for other works by this author on: GSW Google Scholar R.A. ARMSTRONG R.A. ARMSTRONG Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia, e-mail: Richard.Armstrong@anu.edu.au Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2010) 113 (2): 229–235. https://doi.org/10.2113/gssajg.113.2.229 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation B.M. EGLINGTON, R.J. THOMAS, R.A. ARMSTRONG; U-PB SHRIMP ZIRCON DATING OF MESOPROTEROZOIC MAGMATIC ROCKS FROM THE SCOTTBURGH AREA, CENTRAL MZUMBE TERRANE, KWAZULU-NATAL, SOUTH AFRICA. South African Journal of Geology 2010;; 113 (2): 229–235. doi: https://doi.org/10.2113/gssajg.113.2.229 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract New U-Pb zircon SHRIMP dates are presented for two critical meta-igneous rock units from the Scottburgh area, central Mzumbe Terrane, Natal Sector of the Namaqua-Natal Metamorphic Province (“Natal belt”). A sample of the oldest syntectonic granitic orthogneisses of the Mzumbe Terrane, the Mzimlilo Granite, gave a date of 1147 ± 8 Ma. This is interpreted to represent the crystallisation age and thus gives a reliable minimum estimate of the age of the first, collision-related tectono-metamorphic episode (D1) in the Mzumbe Terrane. The date is younger than the earliest foliation in the Tugela Terrane to the north, but is in accordance with previous Ar-Ar dating of shear zones in that terrane. This implies that the early fabrics in the Tugela Terrane were rejuvenated at this time. Magmatic zircons from melanorite of the Equeefa Suite provided a SHRIMP crystallisation date of 1083 ± 6 Ma. The Equeefa Suite comprises a large mafic-ultramafic body west of Scottburgh, associated with a swarm of deformed mafic (amphibolitic) dykes. The suite forms a valuable time marker, having been emplaced after D1 but during the early stages of the major, long-lived oblique sinistral shearing episode, which led to the final juxtaposition of the Mzumbe and Margate Terranes. This event has been dated between ca. 1.05 and 1.0 Ga from previous studies. The Equeefa date is thus consistent with the previous data and defines a period of possible crustal extension between the major tectono-thermal events. Both dates obtained provide important constraints on tectono-magmatic events in this part of the Natal belt and are consistent with previous data. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Kabanga Ni sulfide deposits, Tanzania: II. Chalcophile and siderophile element geochemistry

The Kabanga deposit constitutes one of the most significant Ni sulfide discoveries of the last two decades (indicated mineral resource 23 Mt of ore at 2.64% Ni, inferred resource 28.5 Mt at 2.7% Ni, November 2008). The sulfides are hosted by predominantly harzburgitic and orthopyroxenitic intrusions that crystallized from magnesian basaltic and picritic magmas. However, compared with other sulfide ores that segregated from such magmas (e.g., Jinchuan, Pechenga, Raglan), most Kabanga sulfides have low Ni (<1–3%), Cu (∼0.1–0.4%), and PGE contents (≪1 ppm), high Ni/Cu (5–15), and low Ni/Co (10–15) and Pd/Ir (2–20). Sulfides with higher metal contents (up to ∼5% Ni, 0.8% Cu, 10 ppm PGE) are found in only one unit from Kabanga North. The observed metal contents are consistent with segregation of magmatic sulfides from fertile to strongly metal-depleted magmas, at intermediate to very low mass ratios of silicate to sulfide liquid (R factors) of approximately 10–400. Sulfide saturation was triggered prior to final emplacement, by assimilation of up to 50% of the total sulfur in the intrusions from sulfide-bearing metasedimentary country rocks. Immiscible sulfide liquid was entrained by the magma and ultimately precipitated in dynamic magma conduits that formed tubular and sill-like mafic–ultramafic bodies characterized by abundant magmatic breccias, highly irregular layering, and frequent compositional reversals. The unusually large degree of crustal contamination and the low R factors render Kabanga an end-member in the spectrum of magmatic Ni sulfide ores.

Topolitossequências de solos do Alto Paranaíba: atributos físicos, químicos e mineralógicos

Pouco se conhece sobre a diferenciação pedogenética no Alto Paranaíba (MG), quando são comparados materiais de composição química tão variada, como tufitos, rochas ígneas alcalinas e ultramáficas e carbonatitos, todos de ocorrência na região. Dessa forma, este trabalho teve como objetivo caracterizar física, química e mineralogicamente os solos representativos de três topolitossequências do Alto Paranaíba. Para isso, foram descritos e coletados 11 perfis de solos, entre os municípios de Serra do Salitre, Patrocínio e Coromandel, representando as variações litológicas na faixa do contato geológico entre os grupos Bambuí, rochas vulcânicas ultramáficas e Araxá. Nas amostras de solos foram realizadas análises físicas e químicas de rotina, além de determinações de Fe, Al e Si após extração por ataque sulfúrico; Fe por DCB e oxalato; Fe, Ca, Mg, K, P, Ti e outros metais pesados após digestão total (ataque triácido); e determinação dos diferentes componentes da fração argila por difratometria de raios X. Os Latossolos do Alto Paranaíba são extremamente intemperizados e com índices Ki e Kr muito baixos, indicativos de solos ricos em óxidos de Fe e de Al, não possuindo uma filiação definida com os materiais de origem subjacente, indicando intensa pedoturbação e provável mistura com materiais alóctones. As assinaturas geoquímicas indicativas da natureza ultramáfica são os teores elevados de Cr, Ni, Mn, Fe e Mg. A mineralogia da fração argila dos Latossolos indica a coexistência de vermiculita com hidroxi-Al entrecamadas, caulinita, gibbsita e anatásio, evidenciando uma gênese policíclica dos minerais da fração mais fina e o elevado grau de intemperismo. Nos Cambissolos, a rápida dessilificação atual conduz à coexistência de gibbsita e óxidos de Fe com esmectitas e illitas, em virtude da rápida ação do intemperismo nos substratos de rochas máficas ou ultramáficas-alcalinas, pobres em sílica.