Boninite-like Rocks Research Articles

Mechanism of formation of podiform chromitite: Insights from the oxidation states of podiform chromitites and host peridotites from the Luobusa ophiolite, southern Tibet

Although the mechanism of formation of podiform chromitite is debated, oxygen fugacity (fO2) is likely one of the controlling factors, although the detailed mechanisms are poorly understood. To provide insight into this issue, we determine the Fe3+/∑Fe ratio of chromite via Mӧssbauer spectrometry, and from this calculate the fO2 of chromitites, dunite envelopes, dunite lenses, and their host peridotites in the Luobusa ophiolite in southern Tibet. The fO2 (−1.49 to −0.52 log units relative to the fayalite–magnetite–quartz buffer) and mineral chemistry of the host peridotites are similar to those of both abyssal and forearc peridotites. Higher fO2 values of the dunites and chromitites (−0.65 to −0.33, 0.69 to 1.13, and −0.46 to 0.64 log units for the dunite lenses, dunite envelopes, and chromitites, respectively) relative to those of host peridotites must have been inherited from their parental magmas. Both chromite compositions and the fO2 of the dunites and chromitites are similar to those in boninite-like rocks. Importantly, there is a previously unrecognized but marked decrease in fO2 from the dunite envelopes to the chromitites. This decrease is intimately related to the chromitite-forming processes and preferably explained as the result of the exsolution of oxidized fluids from high-fO2 magmas during subduction initiation.

Archean Boninite-like Rocks of the Northwestern Youanmi Terrane, Yilgarn Craton: Geochemistry and Genesis

Abstract Rocks with chemical compositions similar to Cenozoic boninites occur in many Archean cratons (boninite-like rocks), but they are rarely well-preserved, well-sampled, or presented within chrono- and chemo-stratigraphic context. This study provides a detailed description of the most extensive and well-preserved Archean boninite-like rocks reported to date. Within the 2820 to 2740 Ma magmatic suites of the northwest Youanmi Terrane, Yilgarn Craton, boninite-like rocks occur as two distinct units. The first boninite-like unit is thinner (several 10 s of m thick), occurs close to the base of the 2820–2800 Ma Norie Group and includes both volcanic flows and subvolcanic intrusions. The second boninite-like unit is thicker (locally several 100 s m), occurs near the base of the 2800–2740 Ma Polelle Group and consists of mainly fine-grained volcanic flows with local cumulate units. On average, major and trace element compositions for Youanmi Terrane boninite-like rocks are marginal between basalt, picrite and boninite and they have asymmetrically concave REE patterns, and Th–, Zr–Hf enrichments, similar to many Phanerozoic low-Si boninite suites, but at generally higher MREE–HREE contents. We report over 300 new whole-rock geochemical analyses, and 16 new Sm–Nd isotopic analyses, and associated petrographic evidence, including representative mineral compositions, which we support with published geochemical analyses and several decades of fieldwork in our study area. Comparison between Archean boninite-like rocks and Cenozoic boninites shows that most Archean examples had less depleted sources. We consider two possible petrogenetic models for the Youanmi Terrain examples: (1) they reflect variably contaminated komatiites, or (2) they reflect melts of metasomatised refractory mantle, analogous to Phanerozoic boninites. Trace element modelling indicates that crustal contamination could potentially produce rocks with boninite-like compositions, but requires an Al-enriched komatiitic parent liquid, for which there is no field evidence in our study area. Initial εNdT values in pre-2800 Ma rocks (εNdT -0·4 to +1·2) are on average slightly higher than those in 2800–2733 Ma examples (εNdT -3·2 to +1·2), compatible with increasing mantle metasomatism involving recycling of ≥ 2950 Ma crust. Integration of trace element and Nd isotopic data demonstrates that significant direct crustal assimilation was restricted to felsic magmas. The Th–Nb and Ba–Th systematics of mafic-intermediate rocks reflect fluid- and sediment-derived processes in the mantle, with boninite-like examples being linked primarily to fluid metasomatism. We compare the well-preserved igneous textures and mineralogy of Youanmi Terrane boninite-like rocks with those of their Phanerozoic counterparts, and based on studies of the latter, suggest that former had similarly hot, H2O-rich parent magmas. The association of boninite-like rocks in the Norie and Polelle Groups with coeval high-Mg andesites, sanukitoids and hydrous mafic intrusions of the Narndee Igneous Complex strongly suggests a metasomatised mantle source and subduction operating in the Yilgarn between 2820 and 2730 Ma.

Crustal architecture of the Shangdan suture zone in the early Paleozoic Qinling orogenic belt, China: Record of subduction initiation and backarc basin development

The early Paleozoic Shangdan suture zone in the Qinling orogenic belt (China) marks the main tectonic boundary between the North China and South China blocks. The mafic rocks along this suture zone make up three distinct geochemical groups: (1) MORB-like; (2) boninite-like, and (3) island arc type. The MORB-like basalts display the highest values of TiO2 (0.86wt.%–1.99wt.%), LREE depletion and no negative Nb–Ta anomalies. However, their enrichment in LILEs relative to N-MORB indicates subduction influence in their melt source. The boninite-like rocks display lower TiO2 (0.09wt.%–0.41wt.%) and FeO* (4.3wt.%–10.25wt.%) contents, lower Ti/V ratios and total REEs, and high Cr (303–1495ppm) and Ni (102–383ppm) values. These geochemical features collectively indicate that magmas of the boninite-like rocks were derived from partial melting of a refractory mantle source. However, distinctly different ages of the boninite-like rocks imply that not all boninite-like rocks in the Shangdan suture zone formed in association with subduction initiation. The 524Ma boninite-like doleritic rocks record the subduction initiation magmatism, whereas the younger boninite-like dolerites with more depleted geochemical signatures represent a magmatic stage associated with backarc spreading around 474Ma. The arc-type lavas display strong negative Nb–Ta and Zr–Hf anomalies, characteristic of intra-oceanic island arc rocks. The combined geological, geochemical and regional tectonic features of the ophiolitic rocks along the Shangdan suture zone in the North Qinling orogenic belt provide a record of subduction initiation, island arc development and backarc rifting in the early Paleozoic, reminiscent of the modern IBM arc-trench system.

Geochemistry and zircon U–Pb–Hf isotopic systematics of the Neoarchean Yixian–Fuxin greenstone belt, northern margin of the North China Craton: Implications for petrogenesis and tectonic setting

The Yixian–Fuxin greenstone belt in the Western Liaoning metamorphic terrane is situated in the middle segment of the northern margin of the North China Craton (NCC) and is composed mainly of plagioclase amphibolites, pyroxene amphibolites and hornblende plagioclase gneisses. LA-ICPMS zircon U–Pb isotopic dating of three representative samples of hornblende plagioclase gneisses reveals that the magmatic precursors of the Yixian–Fuxin greenstone belt were formed during 2589 Ma–2534 Ma and subjected to subsequent ~ 2490 Ma metamorphism. Lu–Hf isotopic analyses of the zircons dated from the three samples yield positive εHf(t) values of + 8.3 to + 2.7 and T DM from 2.76 Ga to 2.57 Ga. The youngest T DM value is close to their crystallization ages, suggesting that they were mainly derived from partial melting of the depleted mantle materials. These zircon chronological data and Lu–Hf isotopic features indicate an important ~ 2.5 Ga episode of crustal growth in the Western Liaoning metamorphic terrane. Based on whole rock geochemical characteristics, these meta-volcanic rocks in the Yixian–Fuxin greenstone belt may be subdivided into four groups. Samples in Group #1 show high MgO contents from 7.19% to 7.93% and Cr and Ni contents of 208–395 ppm and 113–210 ppm with Mg numbers (Mg#) of 53.4 to 55.1, and are characterized by slight LREE-depleted to flat REE patterns (La/Sm) N = 0.77–1.05, flat patterns on the primitive mantle normalized multiple trace elements diagram, and Nb/Ta ratios between 12.3 and 15.9, implying that they were derived from partial melting of depleted mantle source. Samples in Group #2 also display higher MgO (7.52%–14.51%), Cr (372–570 ppm) and Ni (240–320 ppm) contents with high Mg# (53.5–70.2), low TiO 2 contents (0.52%–0.64%), high Nb/Ta ratios (15.8–19.9), and have right-declined REE patterns with (La/Sm) N ratios from 1.73 to 3.6, and negative Nb, Ta and Ti anomalies, indicative of their derivation from partial melting of a metasomatized refractory mantle source. Samples in Group #3 show MgO content from 1.81% to 3.56% with Mg# of 35.9 to 43.5 and high Cr (1179–1280 ppm) and Ni (712–721 ppm) contents, lower Y and Yb contents (6.7–9.42 ppm and 0.64–0.88 ppm, respectively) and higher Nb/Ta ratios (18.4–21.8), and are characterized by the highest (La/Yb) N ratios (14.91–22.67) and negative Nb, Ta and Ti anomalies, revealing that they formed from partial melting of garnet-bearing oceanic slab, and the slab melts were contaminated by mantle peridotite and subjected to crustal contamination during their ascent. Samples in Group #4 exhibit high MgO content (5.98%–10.52%) with high Mg numbers (52.4–70.8), Cr (198–376 ppm) and Ni (112–177 ppm) contents and lower Nb/Ta ratios between 12.7 and 14.6, and have (La/Yb) N ratios ranging from 4.12 to 6.78 (lower than those of the Group #3) and strongly negative Nb, Ta and Ti anomalies, suggesting that they were derived from partial melting of mantle peridotite metasomatized by slab-derived fluids. Thus, the Yixian–Fuxin greenstones display geochemical features of the normal-mid-ocean ridge basalts (N-MORBs) (Group #1), boninite-like rocks (Group #2), adakite-like rocks (Group #3) and high magnesium andesites (HMA) (Group #4), suggesting that the protoliths of the greenstone belt might have formed in an active continental margin similar to the Andean-type orogen.

Isua supracrustal belt (Greenland)—A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology

The mafic–ultramafic units of the ∼ 3.8 Ga Isua supracrustal belt (ISB) in Greenland occur in a two-armed arcuate zone (eastern and western arms) and are grouped into two major tectonostratigraphic units based on their lithological and geochemical characteristics: (1) Undifferentiated amphibolites (UA), and (2) Garbenschiefer amphibolites (GA). The UA contains all major lithological units of a typical Penrose-type complete ophiolite sequence. The GA is composed dominantly of volcaniclastic and volcanic rocks, commonly found in immature island arcs. The available geochemical data from UA and GA show distinct differences between the two units. Compared with the geochemical evolution of some of the well known Phanerozoic ophiolites, the pillow lavas and associated dikes of the UA show a compositional range that is similar to typical MORB-type Ligurian ophiolites in the Western Alps–Apennines and those displayed by LIP-type Caribbean ophiolites. The GA is characterized by island arc tholeiite (IAT) to boninite-like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone (SSZ) ophiolites in the Mediterranean region. Our proposed geodynamic model for the ISB suggests that the UA was built by primary to differentiated, mantle-generated melts during seafloor spreading, little to moderately affected by subduction processes, and that the IAT to boninitic-like rocks of the GA formed at a later stage by melting from a strongly subduction-affected, depleted and hydrated mantle. Our interpretation of the ISB is that the UA and GA represent early and late stages, respectively, in the formation of a SSZ ophiolite. This implies that Phanerozoic-type plate tectonic processes, such as seafloor spreading and subduction, were operating by 3.8 Ga in the Palaeoarchean.

Geochemistry and geodynamic origin of the Mesoarchean Ujarassuit and Ivisaartoq greenstone belts, SW Greenland

The Ivisaartoq (ca. 3075 Ma) and Ujarassuit (ca. 3070 Ma) greenstone belts are the largest Mesoarchean supracrustal lithotectonic assemblages in the Nuuk region, SW Greenland. Both greenstone belts underwent polyphase deformation and amphibolite facies metamorphism, and were in due course variously dismembered. Pillow lavas, pillow breccia, magmatic layering, and relic sedimentary structures are well preserved in the Ivisaartoq belt. Volcanic rocks include basalts, and minor andesites and picrites. Boninite-like rocks occur in the Ujarassuit belt. Metasedimentary rocks in the Ujarassuit belt occur as thin layers (0.5–1.0 m) of biotite schists, and in the Ivisaartoq belt as ~ 500 m-thick biotite schists intercalated with minor quartzitic gneisses. There is no field evidence indicating that the Ivisaartoq and Ujarassuit supracrustal rocks were deposited on older continental basement, and their volcanic rocks do not exhibit any geochemical trends indicating contamination by Archean upper continental crust. Four groups of amphibolites (metavolcanic rocks) were recognized in the Ujarassuit greenstone belt on the basis of their REE and HFSE characteristics: (1) Group 1 is characterized by near-flat REE patterns (La/Sm cn = 0.77–1.14; La/Yb cn = 0.84–1.24) and moderate negative Nb anomalies (Nb/Nb⁎ = 0.60–0.79); (2) Group 2 displays LREE-depleted patterns (La/Sm cn = 0.53–1.02; La/Yb cn = 0.32–0.61) and pronounced negative Nb anomalies (Nb/Nb ⁎ = 0.32–0.67); (3) Group 3 consists of LREE depleted patterns (La/Sm cn = 0.69–0.84; La/Yb cn = 0.55–0.91) and absence of significant Nb anomalies (Nb/Nb⁎ = 0.92–1.15); and (4) Group 4 has concave-upward REE patterns (La/Sm cn = 1.64–2.42, Gd/Yb cn = 0.57–1.01) and large negative Nb anomalies (Nb/Nb ⁎ = 0.28–0.42). The depletion of LREE in Group 2 amphibolites appears to have resulted from mobility of these elements during metamorphism. Group 1, 3, and 4 amphibolites retain their near-primary geochemical signatures and their trace element patterns are comparable to those of Phanerozoic oceanic island arc tholeiites (IAT), normal-mid-ocean ridge basalts (N-MORB), and boninites, respectively. The trace element patterns of the least altered meta-ultramafic rocks (La/Yb cn = 2.48–1.35; Nb/Nb⁎ = 0.31–0.60) are comparable to those of modern subduction-related picrites. Amphibolites with an andesitic composition show large negative Nb anomalies (Nb/Nb⁎ = 0.21–0.55), enriched LREE patterns (La/Yb cn = 6.29–15.64), and fractionated HREE (Gd/Yb cn = 2.61–3.12) implying deep melting in equilibrium with residual garnet in the source. Biotite schists and quartzitic gneisses have low chemical indexes of alteration values (CIA = 46 to 62) and trace element characteristics indicating volcaniclastic sedimentary protoliths derived from poorly weathered felsic to mafic source rocks. Collectively, the geochemical features of metavolcanic and metasedimentary rocks suggest that the Ivisaartoq and Ujarassuit greenstone belts represent dismembered fragments of Mesoarchean supra-subduction zone oceanic crust formed in an arc-forearc-backarc tectonic setting.

Metamorphic rocks of the Murmansk domain (Kola Peninsula) as compared with the oldest rock associations of the northeastern Baltic shield, Canada, and Greenland

Rock associations characterized by heterogeneous sets of petrogeochemical parameters were compared by quantifying the degree of their similarity-dissimilarity and searching for discrimination trends between them. Using procedures specially developed for this purpose, it was demonstrated for the first time that the lithotectonic complexes of the Murmansk domain are fundamentally different from those of typical granulite-gneiss terrains and resemble the granite-greenstone terrains of the Baltic shield, Greenland, and Canada. Based on the whole data set, the Murmansk domain can be considered as a deeply eroded Archean granite-greenstone terrain retaining only the tonalite-trondhjemite-gneiss basement with abundant supracrustal enclaves. A trend of the compositional difference between the older and younger rock associations is similar to that between the tholeiitic and boninitic volcanic series. It was suggested that the petrogeochemical “age” trend reflects the initial stage of the compositional evolution of the metamagmatic rocks of the region from metamorphic rocks similar to tholeiites at the early stages to the Paleoproterozoic boninite-like rocks, which are believed to be linked to the unique PGE-bearing province of the northeastern Baltic shield. This implies that the specific metallogenic features of the region emerged already in the Archean, which supported the suggestion on the long duration of geological processes in the Early Precambrian.

Global Intracratonic Boninite-Norite Magmatism during the Neoarchean—Paleoproterozoic: Evidence from the Central Indian Bastar Craton

Although boninites and boninite-like rocks formed throughout Earth history and have been emplaced in different tectonic settings, they are most common in Phanerozoic settings where they are associated with subduction-related magmatism. However, Neoarchean—Paleoproterozoic noritic intrusions, which have bulk-rock geochemistry closely similar to boninites, occur in many Archean terrains, chiefly emplaced in intracratonic settings. Thus, noritic intrusions are probably derived from melts similar to those of boninites. The best examples of such boninite-norite associations occur in the Bushveld, southern West Greenland, East Antarctica, Wyoming, northwest Scotland, and the eastern Fennoscandian shield. A similar boninite-norite suite also is present in the central Indian Bastar craton. These high-Mg mafic igneous rocks were emplaced as dikes and volcanic rocks during Neoarchean—Paleoproterozoic time in an intracratonic rift. They contain high Si (>52 wt%), high Mg (>8 wt%), and low Ti (mostly <0.5 wt%) and are true boninites. Based on Al2O3/CaO ratios, these rocks may be classified as high-Ca and low-Ca boninites. The high-Ca rocks show typical boninitic geochemical characteristics, whereas the low-Ca variety shows geochemical characteristics similar to high-Mg norites. Bulk-rock compositions of these two varities of high-Mg igneous rocks suggest a comagmatic relationship through a high-magnesium mafic melt derived from a metasomatized, highly refractory mantle. Very close geochemical similarities exist between the Bastar and boninite-norite suites worldwide. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchean—Paleoproterozoic. Many Archean terrains were united as a supercontinent as expanded Ur and Arctica at this time, and its rifting gave rise to numerous mafic dike swarms, including boninite-norite.

Paleoproterozoic boninite-like rocks in an intracratonic setting from Central Indian Craton

Paleoproterozoic boninite-like rocks in an intracratonic setting from Central Indian Craton

Geochemistry and petrogenesis of Neoarchaean high-Mg low-Ti mafic igneous rocks in an intracratonic setting, Central India craton: Evidence for boninite magmatism

The southern Central India craton (also known as Bastar craton) has experienced several episodes of mafic magmatism during the early Precambrian time. These mafic igneous rocks are emplaced as dyke and volcanic rocks. A set of mafic dyke swarm and few exposures of mafic volcanics, both emplaced in the Neoarchaean time, contain high-SiO2 (>52 wt%), high-MgO (>8 wt%), and low-TiO2 (mostly <0.5 wt%) and classified as boninites on IUGS classification scheme. Al2O3/CaO ratio classifies these high-Mg mafic igneous rocks into high-Ca and low-Ca (Type 3) boninites. High-Ca boninites show typical boninitic geochemical characteristics, whereas low-Ca variety shows geochemical characteristics similar to the high-Mg norites. Siliceous high-magnesium basalts (SHMB) show similar geochemical properties as observed for the high-Mg norites. Early Precambrian high-Mg norites are reported from the rift setting environment but, on the other hand, boninitic magmatism represents an enigmatic style of magmatism and commonly formed at convergent margin settings. Most boninites are Phanerozoic in age and rarely reported from the Archaean terrains. The established geological and tectonic setting of the Archaean Bastar craton clearly suggests existence of a stable continental rift environment in the region since the Archaean time. Thus, report of Neoarchaean boninite-like rocks in an intracratonic setting is an important feature noticed in the Archaean terrain of Indian peninsula. A highly refractory mantle source region is required to form boninitic magma. Refractory lithosphere may be result of extensive extraction of mafic magma during Mesoarchaean. The preservation of refractory reservoir beneath the Bastar craton for long period of time corroborates a high degree of crustal stability during the Meso-Neoarchaean time. Trace element modelling suggests that high-Mg mafic igneous rocks under study are product of different pulses of boninitic magma produced by ∼20-25% melting of a refractory mantle source.

Petrogenesis of Neoarchaean volcanic rocks of the MacQuoid supracrustal belt: A back-arc setting for the northwestern Hearne subdomain, western Churchill Province, Canada

Rocks exposed in the MacQuoid-Gibson Lakes region, northwest Hearne subdomain, western Churchill Province, Canada comprise three major lithotectonic assemblages: the Principal volcanic belt; the metasedimentary MacQuoid homocline and; the Cross Bay plutonic complex. Neoarchaean supracrustal rocks of the belt range in age from <2745 to <2672 Ma and were intruded during the interval <2689 to 2655 Ma by diverse plutonic units ranging from gabbro through syenogranite, but greatly dominated by tonalite. Volcanic rocks occur only in the Principal volcanic belt and the MacQuoid homocline, are metamorphosed to amphibolite facies and vary from rare pillowed to common massive basalt and andesite, intercalated with less abundant, thin, dacitic to rhyolitic tuffs, lavas and volcaniclastic rocks. Basalt and andesite are dominated by subalkaline, FeO T-rich tholeiites with less common calc-alkaline rocks with higher SiO 2 contents and variable trace element contents. Felsic volcanic rocks exhibit calc-alkaline affinities and similarly diverse trace element abundances. The diverse trace element chemistry of the basalt and andesite supports their derivation from a heterogeneous mantle source(s) capable of generating MORB-, Arc-, BABB- and boninite-like rocks. Two geochemically distinct, arc-like suites were generated through contamination of the primary mantle-derived magmas either via assimilation of lower or middle tonalitic crust, or through contamination of their mantle source through subduction. Geochemical features of the felsic volcanic rocks indicate that these formed via both anatexis of crust in the amphibolite ± garnet stability field and via fractionation of more primitive progenitors in mid-upper crustal magma chambers. ɛNd t = 2680 Ma isotopic compositions cluster near depleted mantle, indicating that significant incorporation of older, >2700 Ma crust likely did not occur. ɛNd t = 2680 Ma values for three specimens, one from each of the Arc-like suites and one BABB-like basalt are slightly lower than the remainder, suggesting very minor incorporation of slightly older crust. These features imply that the processes that generated the MacQuoid supracrustal belt required simultaneous tapping of geochemically distinct mantle reservoirs with concomitant anatexis of sialic crust (garnet stability field) and fractionation of felsic magmas in upper crustal magma chambers. Shallow water deposition of abundant volcaniclastic rocks and semipelite along with minor conglomerate and quartzite was broadly contemporaneous with this magmatism. We envisage a geodynamic setting characterized by tectonomagmatic processes similar to those of modern supra-subduction zone back-arc marginal basins such as the Sea of Japan. Therein, an extensional, back-arc setting, likely proximal to continental crust, provides an explanation for a broad swath of diverse mantle-derived rocks intercalated with less common felsic rocks as well as an abundance of immature clastic metasedimentary rocks.

Plutonic boninite-like rocks in an anatectic setting: Tectonic implications for the Delamerian orogen in southeastern Australia

Research Article| April 01, 2003 Plutonic boninite-like rocks in an anatectic setting: Tectonic implications for the Delamerian orogen in southeastern Australia A.I.S. Kemp A.I.S. Kemp 1Department of Geology, Australian National University, Canberra 0200, Australia Search for other works by this author on: GSW Google Scholar Geology (2003) 31 (4): 371–374. https://doi.org/10.1130/0091-7613(2003)031<0371:PBLRIA>2.0.CO;2 Article history received: 28 Aug 2002 rev-recd: 19 Dec 2002 accepted: 20 Dec 2002 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.I.S. Kemp; Plutonic boninite-like rocks in an anatectic setting: Tectonic implications for the Delamerian orogen in southeastern Australia. Geology 2003;; 31 (4): 371–374. doi: https://doi.org/10.1130/0091-7613(2003)031<0371:PBLRIA>2.0.CO;2 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 SocietyGeology Search Advanced Search Abstract High-Mg, low-Ti diorites in the Cambrian–Ordovician Glenelg River Complex (southeastern Australia) have compositional affinities with the boninitic lavas erupted in intraoceanic arcs, but occur in an anatectic environment mingled with crustal melts. Boninitic rocks have not previously been recorded from such a setting. Field and petrologic evidence suggests that the parental boninitic liquids are approximated by hornblende diorites, crystallization from which yielded complementary cumulates and evolved tonalitic derivatives. The presence of these rocks, and associated arc-like intrusions, compromises collisional tectonic models for the Glenelg River Complex and instead implies early development of the terrane above a subduction zone. Boninitic magmas were subsequently trapped at deep crustal levels during convergent orogenesis. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrology of the Europe-Largest Burakovka early Paleoproterozoic layered pluton (Southern Karelia, Russia)

The Burakovka layered pluton of basic and mafic rocks is the largest intrusive massif in the Baltic Province composed of Si- and Mg-rich boninite-like rocks. The pluton consists of two individual bodies, each having its own internal structure, and contacting each other in their apical parts, known as the Aganozero and Shalozero-Burakovka bodies. Both bodies have a similar rock sequence including five dierentiated zones (upward): mafic rocks, pyroxenite, gabbro norite, pigeonite gabbro norite, and magnetite gabbro diorite (the latter found only in the Shalozero-Burakovka Body). Being generally similar to each other, these bodies dier notably in the styles of their cumulate stratigraphy and, to a lesser extent, in composition. The pluton is distinguished by the presence of markers - singular interlayers of high-temperature mafic cumulates emplaced in the sequence of lower-T formations. Their origin is believed to have been associated with the intrusion of fresh magma portions into the crystallizing magma chambers. The same mechanism is believed to have been responsible for a macrorhythmic pattern found in the southeastern portion of the Shalozero-Burakovka intrusive body. Using chemical and mineralogical data, it is shown that the bodies discussed were derived from similar high-Si and high-Mg magmas, except that the Aganozero Body was emplaced 50 million years later than the Shalozero-Burakovka intrusion: the former was dated (Sm-Nd isochron) 2372±22 Ma (Nd = 3.22 ± 0.13), and the latter, 2433±28 Ma (Nd = 3.14 ± 0.14). It is concluded that the Burakovka Pluton was a long-lived magma center which developed above a local mantle plume, the origin of which had been associated with the activity of a megaplume which had been responsible for the existence of the Baltic province throughout a period of 200 million years.

Archaean boninite-like rocks in an intracratonic setting

Boninites and high-Mg norites show considerable compositional overlap. While the petrogenesis of both may involve large ion lithophile element (LILE) replenishment of a mantle source, boninites appear restricted to convergent margin settings, while many high-Mg norite suites intrude intracratonic rifts. Rocks with boninitic compositions are extremely rare in Archaean terranes, where they are also interpreted to have formed at convergent margins. High-Mg, LILE-enriched basaltic rocks form a ca. 2950 Ma sub-volcanic sill within the 3010–2935 Ma Mallina Basin, of the Pilbara Craton, northwest Australia. They are spatially associated with low-Ti tholeiitic intrusions and a high-Mg diorite (sanukitoid) suite. They have a strong boninitic affinity, with high SiO 2 (52–54 wt%), Mg # (62–67) and Al 2O 3/TiO 2 (62–73), low TiO 2 (0.24–0.28 wt%) and strong LILE enrichments. Primitive-mantle normalised La/Gd and Gd/Yb ratios of ∼4.5 and ∼0.6, respectively, produce prominent ‘U-shaped’ normalised trace element patterns. Despite such close compositional affinity to boninite, these rocks probably formed more than 40 Myr after regional magmatism and 60 Myr after the last regional event linked to subduction. Geological evidence suggests that the Mallina Basin formed in an intracontinental setting, not at a convergent margin, and so high-Mg norite provides a more realistic analogy for the Mallina boninite-like rocks in terms of tectonic setting. In old terranes, the tectonic setting of rocks with boninite-like compositions must be interpreted with caution. Trace element modelling shows that contamination by felsic crust is a plausible explanation for LILE enrichments in the Mallina boninite-like rocks, but the scarcity of suitable contaminants makes this explanation unlikely. Petrogenesis of the boninite-like rocks was closely associated with that of the high-Mg diorite (sanukitoid) suite. LILE enrichments in high-Mg diorite require that the mantle beneath the entire Mallina Basin was variably metasomatically enriched by an earlier subduction event at, or before, ca. 3015 Ma. The refractory source for the boninite-like rocks may have also been enriched during this event.

Boninite-like rocks from the Palaeoproterozoic greenstone belt of Bogoin, Central African Republic: geochemistry and petrogenesis

Abstract The upper pillowed amphibolites of Bogoin (UPAB) are part of the supracrustal series of the Bogoin-Boali greenstone belt (western Central African Republic). They are underlain by Al-depleted komatiites, back-arc tholeiites and arc-related greywackes; they are interbedded with undepleted komatiites and overlain by banded iron formations (BIFs). PbPb and SmNd isotopic data yield an imprecise isochron age of about 2.3 Ga, which is believed to represent the time of greenstone deposition. No magmatic minerals are preserved in the UPAB, but pseudomorphs after olivine, Ca-poor and Ca-rich pyroxenes are present. The secondary paragenesis is related to upper greenschist facies metamorphism. Immobile element geochemistry suggests that UPAB are not siliceous high-magnesium basalts but are very similar to modern boninites. They are depleted in P and Ti, enriched in Sr and Zr and show high Zr/Sm, Sr/Sm, Al2O3/TiO2 and low Ti/V and Ti/Sc values. The REE patterns are U-shaped or LREE-depleted/HREE-enriched. The overprint of a magma mixing with the interbedded komatiites on a partial melting event can explain most of the differences noted between UPAB and modern boninites. UPAB genesis is explained by mixing depleted and enriched components. Sm/Nd isotopic data and major element contents suggest participation of a trondhjemite-like melt rather than sediments as the enriched component. The depleted component can be produced by remelting of a peridotitic residue following extraction of tholeiites similar to the tholeiites of the lower unit. The ascending diapir that yielded the upper komatiites may have furnished the necessary heat input to the remelting event. The presence of back-arc tholeiites, arc-related greywackes and boninite-like UPAB argues strongly for a compressive margin plate boundary setting for the Bogoin greenstone belt.

Copper and its alloys in 1975—some general observations : W. O. Alexander, Metals Mater., 3 (10) (1969) 396–402; 11 figs., 1 table

The mafic–ultramafic units of the ∼ 3.8 Ga Isua supracrustal belt (ISB) in Greenland occur in a two-armed arcuate zone (eastern and western arms) and are grouped into two major tectonostratigraphic units based on their lithological and geochemical characteristics: (1) Undifferentiated amphibolites (UA), and (2) Garbenschiefer amphibolites (GA). The UA contains all major lithological units of a typical Penrose-type complete ophiolite sequence. The GA is composed dominantly of volcaniclastic and volcanic rocks, commonly found in immature island arcs. The available geochemical data from UA and GA show distinct differences between the two units. Compared with the geochemical evolution of some of the well known Phanerozoic ophiolites, the pillow lavas and associated dikes of the UA show a compositional range that is similar to typical MORB-type Ligurian ophiolites in the Western Alps–Apennines and those displayed by LIP-type Caribbean ophiolites. The GA is characterized by island arc tholeiite (IAT) to boninite-like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone (SSZ) ophiolites in the Mediterranean region. Our proposed geodynamic model for the ISB suggests that the UA was built by primary to differentiated, mantle-generated melts during seafloor spreading, little to moderately affected by subduction processes, and that the IAT to boninitic-like rocks of the GA formed at a later stage by melting from a strongly subduction-affected, depleted and hydrated mantle. Our interpretation of the ISB is that the UA and GA represent early and late stages, respectively, in the formation of a SSZ ophiolite. This implies that Phanerozoic-type plate tectonic processes, such as seafloor spreading and subduction, were operating by 3.8 Ga in the Palaeoarchean.