Basin Basalts Research Articles

Age, Mineral Chemistry, and Geochemistry of Metamorphic Basement Rocks from the Southern Yap Arc, Western Pacific

Studying the basement characteristics and tectonic evolution of the proto-Yap Izu–Bonin–Mariana arc system can provide essential clues for understanding the subduction process in the Western Pacific Ocean. The Yap arc is a part of the Izu–Bonin–Mariana (IBM) arc, but the origin and formation time of the metamorphic basement rocks of the Yap arc still need to be determined. In this paper, we present the first systematic and detailed chronological, mineralogical, whole-rock major element and trace element research on metamorphic basement rocks from four stations in the Southern Yap arc and discuss the origin of the metamorphic basement rocks in the Southern Yap arc. The results show that the single mineral U-Pb ages of titanites and 40Ar/39Ar ages of amphiboles in the Southern Yap arc are 19.3 and 19.4 Ma, respectively, which are consistent within the error ranges, and these ages represent the metamorphic age of the basement rocks. The amphibolite is mainly produced via the metamorphism of back-arc basin basalt (BABB) and forearc basalt (FAB). The temperature and pressure conditions of this metamorphism are 446.5–641.6 °C and 0.24–0.73 GPa, the facies series of which is the medium P/T series. This study provides important data for understanding the Yap subduction system and the early evolution of the proto-IBM Arc.

Spatio-temporal evolution of the Paleo-Tethys in western Yunnan: Insights from mafic rocks in the Lancang tectonic belt

The Sanjiang orogenic belt in the southeastern Tibetan Plateau provides an excellent record of the Paleo-Tethys tectonic evolution. This study introduces new constraints on the spatio-temporal evolution of the Paleo-Tethys through zircon UPb ages, in-situ LuHf isotope data, and analyses of whole-rock major oxides, trace elements, and Sr-Nd-Pb isotopic compositions from Late Paleozoic mafic rocks in the Lancang tectonic belt. The mafic rocks from the Xiaoheijiang, Banpo, and Yakou areas were dated to approximately 281–267 Ma, 295–292 Ma, and 293–291 Ma, respectively. The Xiaoheijiang mafic rocks exhibit geochemical signatures resembling those of fore-arc basalt, characterized by low (La/Sm)N, relatively flat rare earth element (REE)-normalized patterns, positive εNd(t) (+5.6 to +10.1), and zircon in-situ εHf(t) values (+10.0 to +14.9). These features indicate an origin from a depleted mantle source with minor contributions from slab-derived components. The Banpo and Yakou mafic rocks display geochemical affinities to back-arc basin basalt, similar to the Okinawa Trough back-arc basin basalt. Their εNd(t) values range from +5.6 to +9.9 and εHf(t) values from +9.5 to +15.0, suggesting derivation from a mantle wedge source modified by slab-derived fluids or melts. In combination with available geochronological data concerning the Paleo-Tethys evolution along the Lancang tectonic belt, our findings support the hypothesis that the slow-speed, low-angle subduction of the Paleo-Tethys Ocean led to the formation of a forearc accretionary complex. The slab-derived fluids metasomatized the mantle wedge as subduction depth and angle increased, facilitating the development of the Lincang arc magmatism and the opening of the Banpo-Yakou back-arc basin. Continuous subduction promoted slab retreat under gravitational forces, inducing the upwelling of depleted mantle and the forming of forearc magmas.

Paraná basin basalt powder: A multinutrient soil amendment for enhancing soil chemistry and microbiology

This study evaluated the effects of Paraná Basin basalt powder, a mining byproduct from southern Brazil, on the macro- and micronutrient content of two soil types, Oxisol (OX) and Typic Quartzipsamment (TQ), as well as the metabolic activity of soil microorganisms. Basalt powder doses were determined based on CaO levels required to correct Ca2+ levels in each soil, ranging from 0 to 4 times the recommended amount. Soil samples were collected for analysis after 60 and 170 days of incubation to assess soil attributes. Additionally, a laboratory study examined soil basal respiration (SBR) after 168 days of greenhouse incubation. The results revealed that higher doses of basalt powder application led to increased pH, P, K+, Ca2+, Mg2+, Cu, B, and Si levels in OX, while decreasing Al3+ levels in the same soil. In TQ soils, the effects were more limited, showing specific increases in pH, P, S–SO42-, and Si levels. Moreover, there was a 21% increase in SBR in OX with higher basalt rates. These findings suggest that basalt powder has the potential to serve as an environmentally friendly solution for soil fertilization, positively impacting its chemical and biological properties.

A HIMU-like component in Mariana Convergent Margin magma sources during initial arc rifting revealed by melt inclusions

Compositions of island arc and back-arc basin basalts are often used to trace the recycling of subducted materials. However, the contribution of subducted components to the mantle source during initial arc rifting before back-arc basin spreading is not yet well constrained. The northernmost Mariana arc is ideal for studying this because the transition from rifting to back-arc spreading is happening here. Here we report major and trace element and Pb isotopic compositions of olivine-hosted melt inclusions from lavas erupted during initial rifting at 24°N (NSP-24) and compare them with those in active arc front at 21°N and mature back-arc basin at 18°N. NSP-24 high-K melt inclusions have highly radiogenic Pb compositions and are close to those of the HIMU end-member, suggesting the presence of this component in the magma source. The HIMU-like component may be stored in the over-riding plate and released into arc magma with rifting. HIMU-type seamounts may be subducted elsewhere beneath the Mariana arc, but obvious HIMU-type components appear only in the initial stages of arc rifting due to the low melting degree and being consumed during the process of back-arc spreading.

The relationship between iron redox states and H2O contents in back-arc basin basaltic glasses from the North Fiji Basin

Island-arc basalts (IAB) from convergent plate margins tend to have both higher iron oxidation states (higher Fe3+/Fe2+) and higher H2O contents than the mid-ocean ridge basalts (MORB) produced by seafloor spreading, which raises the question of whether these two characteristics are causally related. Back-arc basin basalts (BABB) may help with this question, in that they are products of seafloor spreading, like MORB, but sourced from supra-subduction mantle, like IAB. Here we examine the relationship between Fe3+/Fe2+ (determined by XANES spectroscopy) and H2O contents in BABB glasses from the North Fiji Basin (NFB). The glasses cover a range of compositions from 6.1 to 8.5 wt% MgO, and from 0.16 to 1.6 wt% H2O, and have also been analysed for trace elements and Sr-Nd-Pb-isotopes. Measured Fe3+/Fe2+ range from MORB-like to slightly higher values and are positively correlated with H2O and heavy-halogen contents (Cl, Br and I). The correlation is due to lower Fe2+ rather than higher Fe3+ compared to the MORB array. This suite of BABBs is not significantly enriched in trace elements other than H2O and the heavy halogens, with MORB-like Ba/Th and other incompatible-element ratios, suggesting that the H2O and halogens may have come from dehydration of subducted lithospheric serpentinites under subsolidus conditions. Na2O is unusually variable for a suite of samples from such a limited area, and is not correlated with H2O, precluding a relationship between H2O and degree of melting. The link between H2O and low Fe2+ may be explained by H2O-rich fluids entering the transcrustal magma plumbing system, where they increase the proportion of olivine and augite to plagioclase crystallizing during the crustal evolution of the magmas. In this situation, relatively small additions of H2O can have an enhanced effect on major-element chemistry if the crustal evolution proceeds by cycles of replenish-mix-tap-crystallize (RMTX) rather than by simple fractional crystallization without repeated replenishment and tapping. The absence of any increase in Fe3+ with H2O, together with the lack of any correlation between H2O and Fe3+/Fe2+ in MORB glasses generally, suggest that various mechanisms, including more extensive olivine fractionation, were responsible for elevating Fe3+/Fe2+ in H2O-rich basalts from convergent margin settings. Proxy methods based on concentrations of elements with redox-variable partition coefficients (V, Eu) are not sensitive enough to record the subtle variations in Fe3+/Fe2+ observable by XANES spectroscopy.

Subduction Within the Proto‐Tethys Ocean Revealed by Recognition of the Earliest Phanerozoic Intra‐Oceanic Arc, Northern Tibetan Plateau

AbstractThe possibility that the Proto‐Tethys Ocean may have undergone intra‐oceanic subduction during ocean closure remains poorly constrained due to a lack of geological evidence for a mature intra‐oceanic arc. Here we present new geochemical and geochronological data for potential arc‐related volcanic rocks adjacent to the accretionary complex and forearc basin in the North Qaidam collisional belt, northern Tibetan Plateau. The volcanic rocks are dominated by foliated basalt, andesite, tuff, and minor dacite with zircon U‐Pb ages ranging from 517 to 497 Ma. They show distinctive geochemical characteristics and can be subdivided into three groups: island‐arc intermediate‐basic volcanic rocks, back‐arc basin basalts (BABB), and dacites with intra‐oceanic arc affinity. The island‐arc volcanic rocks have variable εNd(t) values (+1.6 to +7.5) that decrease northward and were generated by partial melting of depleted mantle wedge modified by hydrous fluid and sediment melt. The BABBs have high εNd(t) values (+5.3 to +6.6) and formed through the melting of MORB‐like mantle, whereas the nearby dacites have positive εNd(t) values (+1.9 to +3.6) similar to the surrounding island‐arc volcanic rocks and were derived from partial melting of intra‐oceanic arc crust as a result of BABB underplating. Integrated analysis of the spatial‐temporal distribution of these volcanic rocks and the reconstructed intra‐oceanic arc‐trench system confirms the existence of the earliest Phanerozoic intra‐oceanic arc formed in response to north‐directed intra‐oceanic subduction. This unrecognized subduction of the Proto‐Tethys Ocean in the North Qaidam belt initiated at ca. 530 Ma, matured ca. 520 Ma, and terminated by ca. 480 Ma.

Paleozoic evolution of the Yukon-Tanana terrane of the North American Cordillera, NW British Columbia

Abstract The origins and primary relationships between tectono-stratigraphic units are fundamental to the terrane concept in accretionary orogens, but they are challenging to assess in metamorphic terranes. In NW British Columbia, three tectonically bounded metamorphic suites of the Yukon-Tanana terrane formed in distinct tectonic settings, based on high-spatial-resolution geochronology and immobile trace-element geochemistry. The Florence Range suite comprises late Neoproterozoic or younger to pre–latest Devonian metasedimentary rocks derived from continental crust, 360 ± 4 Ma calc-alkaline intermediate orthogneiss, and 357 ± 4 Ma amphibolite with oceanic-island basalt composition, consistent with rifting of a continental margin. The detrital signature is dominated by late Mesoproterozoic zircon, which indicates different sources than other parts of the Yukon-Tanana terrane. The Boundary Ranges suite comprises pre–Late Devonian metasedimentary rocks derived in part from a mafic source, amphibolite derived from subduction-zone metasomatized mantle, and 369 ± 4 Ma to 367 ± 7 Ma calc-alkaline felsic to intermediate orthogneiss. The Whitewater suite comprises meta-chert, graphite-rich metapelite, and amphibolite with back-arc basin basalt composition consistent with an anoxic basin near a volcanic source. Our data indicate that the Florence Range and Boundary Ranges suites were separate until at least the Early Mississippian and may have formed a composite terrane since the Permian, whereas the relationship with the Whitewater suite is uncertain. We compare the Paleozoic evolution of the Yukon-Tanana terrane in NW British Columbia with several modern analogues in the west and southwest Pacific Ocean.

GEOCHEMISTRY AND PETROGRAPHY OF METAMORPHIC SOLE AMPHIBOLITES FROM THE SLATINA QUARRY, MT. ZRINSKA GORA, CROATIA

The investigated Mt. Zrinska Gora amphibolites are a part of the metamorphic sole of the Banovina ophiolite complex. Mineral composition and other petrographic characteristics of amphibolites and adjacent peridotite were investigated by polarised microscopy and chemical analyses of rocks were obtained by a combination of inductively coupled plasma mass and emission spectrometry. The main minerals in these garnet-bearing amphibolites are amphibole and plagioclase with accessory garnet, sphene, spinel, opaque mineral ± clinopyroxene ± quartz ± actinolite ± zoisite-clinozoisite ± prehnite ± pumpelyite and ± clay minerals. Kelyphitic corona is developed around garnet. The amphibolites have nematoblastic, nematogranoblastic, porphyroblastic and porphyroclastic textures and homogenous to foliated structures. The presence of clinopyroxene in some of the investigated amphibolites points to their possible formation under P‒T conditions of upper amphibolite facies. The greenschist facies retrograde metamorphism and subsequent surface weathering of amphibolites is also evident. The chemical composition of rocks indicates that protoliths of most amphibolites were tholeiitic basalts, initially formed in island arcs and the adjoining back-arc basins. During the intraoceanic subduction in Jurassic Neotethys Ocean basalts/basaltic tuffs of island arc tholeiite affinity and back-arc basin basalts, positioned on the top of the down-going oceanic slab, were welded to the base of the hot over-riding mantle wedge, metamorphosed to amphibolite rocks and obducted together with ophiolites on the Adria continental margine. The new described amphibolite type having peridotite protolith characteristics originated as the consequence of the hydration of the bottom part of the overlying mantle wedge.

Nature and Origin of a Massive Sulfide Occurrence in the Karrat Group: Evidence for Paleoproterozoic VMS Mineralization in Central West Greenland

Abstract Mafic volcanic rocks of the Kangilleq Formation of the Paleoproterozoic Karrat Group host volcanogenic massive sulfide (VMS) mineralization in the area of central Kangiusap Kuua, central West Greenland. The mafic volcanic rocks display evidence of subaqueous, effusive eruption and redeposition by mass debris flows generated along fault scarps on the sea floor. A zone of semiconformable quartz alteration and disconformable chlorite alteration within hydrothermal breccias and mafic tuff breccias near the top of the volcanic sequence is interpreted to reflect a synvolcanic hydrothermal system. Conformable, massive to semimassive, and discordant, stringer-style sulfide mineralization is hosted within the quartz- and chlorite-altered volcanic rocks. The massive to semimassive sulfide mineralization is ~10 m thick and crops out along strike for ~2,000 m. The stringer zone is ≤10 m thick with individual sulfide stringers ranging in width from 5 to 90 cm. All sulfide zones are dominated by coarse pyrrhotite and pyrite, with trace amounts of sphalerite and chalcopyrite. The pillow lavas are subalkaline with geochemical characteristics typical of modern transitional to tholeiitic mid-ocean ridge or back-arc basin basalt. Trace element and Nd isotope data suggest that these lavas erupted in an epicratonic, back-arc basin. Characteristics of the host rocks indicate a period of localized rifting, volcanism, and VMS formation during genesis of the Karrat Group, which is dominated by siliciclastic rocks.

Paleoproterozoic rift-related volcanism of the Karrat Group, west Greenland: New insights from lithofacies analysis and geochemistry of the Kangilleq Formation

Mafic volcanic rocks of the Paleoproterozoic Kangilleq Formation of the Karrat Group of West Greenland represent a submarine mafic volcanic sequence, erupted and deposited on the Rae craton of Greenland. The formation ranges from <100 m-thick to ∼500 m-thick, and consists of mafic volcaniclastic rocks, pillow lavas and massive basalt lavas that have undergone greenschist facies metamorphism, yet preserve primary volcanic textures such as pillows, drainage cavities, lobes, flow banding, autoclastic breccias, hyaloclastite, scoria and layering.Rocks of the Kangilleq Formation overlie the Archean Qeqertarssuaq Complex and are interleaved with and overlain by the ca. 1950 Ma Nûkavsak Formation. Based on detailed (1:200) mapping and high-precision geochemistry, rocks of the Kangilleq Formation are subdivided into the Kangiusiap and Umiammakku members. The Kangiusiap member is dominated by mafic, scoria-rich breccias, has sulfide mineralization, and is bound at the bottom and top by the Nûkavsak Formation. This member has subalkaline, transitional to tholeiitic, mid-ocean ridge basalt (MORB) to back arc basin basalt (BABB) signatures with a Nd-isotope value similar to the depleted mantle at 1.88 Ga. The Umiammakku member is dominated by pillow lavas, with subordinate amounts of massive lavas and volcaniclastic rocks, overlies the Qeqertarssuaq Complex and is overlain by, and interleaved with, the Nûkavsak Formation. It is alkaline with characteristics similar to modern ocean island basalts and Nd-isotopic values slightly lower than the depleted mantle at 1.88 Ga.The rocks of the Kangilleq Formation represent non-arc suites or suites that have only minor subduction zone or crustal input, and display geochemical signatures similar to basalts found in modern intracontinental back arc settings. Lithofacies, bedforms, internal stratigraphy, and the identification of synvolcanic faults, suggest it is the product of volcanic eruptions during incipient rifting within a back arc setting, and seafloor mass wasting events at fault scarps related to rifting. Based on the geological characteristics and its petrogenetic and tectonic history, the Kangilleq Formation is interpreted to be correlative with the Bravo Lake Formation of the Piling Group of northeastern Canada and records incipient rifting of the southern margin of the Rae craton in the Paleoproterozoic.

Iron isotope heterogeneity in magmas of subduction zones and its origin

Iron isotope heterogeneity of subduction-related primitive magmas was traditionally thought to be predominantly attributed to the addition of slab-derived components. However, the specific subducted material responsible for inducing Fe isotope heterogeneity in the subarc-backarc mantle remains poorly constrained. Here we report Fe isotopic compositions of a series of backarc basin basalts (BABBs) from the Woodlark Basin (an intra-arc rift basin), Vate Trough (a nascent rear-rift basin), and Lau Basin (a relatively mature basin) in the southwestern Pacific. Even though these BABBs have various geochemical compositions ranging from mid-ocean ridge basalts (MORB)-like to arc-like, which should be affected by various amounts of the slab-derived fluids and/or sediment melts, their δ56Fe values (0.05–0.18‰) are within the range of global MORB (0.05–0.17‰). After correction for crystal fractionation, the primitive δ56Fe values (δ56Fe-prim) displayed no co-variation with indicators of subduction contributions (e.g., Ce/Pb, Nb/U, Ba/La, and Th/Yb) or source redox conditions caused by slab inputs (e.g., V/Yb, V/Ti), suggesting that the δ56Fe-prim values of these BABBs are rarely affected by subduction metasomatism and the change in oxygen fugacity caused by subducted input. Therefore, we proposed that the MORB-like δ56Fe values of the studied BABBs were primarily caused by mantle melting processes. Furthermore, we presented a compilation of available Fe isotopic data for BABBs and island arc basalts (IABs) and used these data to evaluate Fe isotope heterogeneity in subduction settings. In particular, compared with δ56Fe-prim of MORB, the δ56Fe-prim of BABBs from the Central Lau Spreading Centre in the Lau Basin and the IABs have lower values, whereas BABBs from the Rochambeau Ridges in the Lau Basin have higher δ56Fe-prim values. By comprehensively considering the factors affecting the variation in Fe isotopic compositions in magmatic rocks in subduction zones (e.g., crystal fractionation, partial melting of the mantle, redox conditions, and subduction-related metasomatism), we proposed that the varied Fe isotopic compositions in subduction settings could reflect their heterogeneous sources, which may be related to plate subduction. Specifically, during plate subduction, a melt-depleted refractory forearc mantle peridotite (with low δ56Fe values) could be dragged by subducted slab into subarc or backarc depths, resulting in light Fe isotopic enrichment in island arc magmas or BABBs; additionally, serpentinites (both in the slab and mantle wedge) can generate light Fe isotopic enrichment in island arc magmas. High δ56Fe values in the BABB (e.g., from the Rochambeau Ridges in the Lau Basin) may result from the toroidal influx of nearby upwelling plume components or the assimilation of hydrothermally altered oceanic crust.

Development of a back-arc basin from initial rifting to seafloor spreading: Constraints from Paleozoic basic rocks in the North Qinling accretionary orogen, central China

The progression from initial back-arc rifting to back-arc opening in ancient orogenic collages is important for reconstructing subduction histories, constraining tectonic switching, and understanding crustal growth and evolution. However, it is difficult to constrain this transition in ancient arcs. The Qinling-Dabie orogenic belt is one of the most important orogenic belts in eastern Asia, yet the history of Paleozoic accretionary processes in this belt remains equivocal, owing to a poor understanding of back-arc opening processes in the Erlangping unit. In this study, we present whole-rock geochemical analyses of mafic dikes in the Erlangping unit, along with U-Pb ages and Hf-O isotope compositions of zircon from these dikes. Zircon secondary ion mass spectrometry (SIMS) dating of these mafic dikes yielded U-Pb ages of 453 ± 3 Ma. The mafic dikes are calc-alkaline and are characterized by high K2O contents (1.16−3.16 wt%). They have enriched ([La/Yb]N = 4.3−14.9) light rare earth elements (REEs) and relatively flat heavy REE ([Dy/Yb]N = 0.9−1.5) patterns, and they exhibit enrichment in large ion lithophile elements (LILEs) but depletion in high field strength elements (HFSEs), resembling arc-like magmatism. These mafic dikes are characterized by relatively enriched initial 87Sr/86Sr (0.7049−0.7059), chondritic to slightly radiogenic εNd(t) (−0.36 to 1.33), and radiogenic whole-rock and zircon εHf(t) (+7.2−7.6 and +7.5−7.8, respectively). The zircons have δ18O values (5.0‰ ± 0.1‰) similar to those of normal mantle zircon. Accordingly, we interpret that the mafic dikes were derived from an enriched lithospheric mantle source metasomatized by subducted Proto-Tethys (Shangdan) ocean material. In contrast, previously reported ca. 440 Ma gabbros from the Erlangping unit are tholeiitic and have lower incompatible trace-element concentrations with less enrichment in LILEs and less depletion in HFSEs than the mafic dikes presented here. Whole-rock Nd and zircon Hf isotopic compositions suggest that these Silurian gabbros were derived from partial melting of a more depleted mantle with the involvement of asthenosphere, and they have a close geochemical affinity with back-arc basin basalts. In addition, trace-element ratios and geochemical modeling suggest higher melting pressures for the mafic dikes than the gabbros. The geochemical differences show systematic variations from island-arc basalt into back-arc basin basalt types, which is consistent with the magma source evolution of the Mariana Trough. Thus, we interpret that the ca. 454 Ma mafic dikes were emplaced when the infant arc split, and they record initial back-arc rifting in the Erlangping unit, whereas the Silurian gabbros subsequently formed during the opening of the back-arc basin. Our study provides a paradigm for deciphering the evolution of back-arc basins through the study of spatiotemporal geological and geochemical variations of mafic intrusions in ancient accretionary orogens.

Carboniferous back-arc bimodal rocks in West Kunlun during northward subduction of the Paleo-Tethys

ABSTRACT The widespread occurrence of Carboniferous Wuluate Formation volcanic rocks in the West Kunlun is closely related to the evolution of the Paleo-Tethys but has received little attention. In this study, we conducted detailed fieldwork on the Wuluate Formation in West Kunlun and performed zircon U‒Pb dating, whole-rock major and trace element analysis, whole-rock Sr-Nd isotope analysis, and in situ zircon Hf isotope analyses. The Wuluate Formation is characterized by a bimodal volcanic suite. Zircon U‒Pb dating indicates that the rhyolites erupted at ca. 348–353 Ma, which is coeval with the zircon U‒Pb dating result of 354 Ma for the basalt. The basalts show a tholeiitic magmatic evolution trend with flattened REE patterns and depleted whole-rock Sr-Nd and zircon Hf isotopes, indicating geochemical affinity to back-arc basin basalt (BABB). Trace element modelling results indicate that the primitive basaltic compositions fit well with the products of the DMM + sediments + H2O source under melting conditions of asthenospheric temperature and thin lithosphere thickness, and the REE modelling results show that the basalts underwent 40% to 95% crystallographic differentiation of olivine, monazite, hornblende, plagioclase, and magnetite. The silicic rocks are calcic to calcic-alkaline and meta/peraluminous. They exhibit flattened REE patterns and slightly enriched zircon Hf isotopes, which could originate from Precambrian metaigneous sources of West Kunlun. The Kungai bimodal suite is likely to have formed in a back-arc setting corresponding to the northward subduction of Paleo-Tethys. The subduction of Paleo-Tethys had already occurred, and a back-arc basin had developed before the Early Carboniferous.

Mantle-hosted ophiolitic chromitites from Colombia: implications for petrogenesis and geodynamic evolution

ABSTRACT Chromitite bodies can be found within ophiolitic mantle sections in three locations in Colombia: Medellín, Planeta Rica, and La Guajira. These chromitites are typically massive pods that are often surrounded by lateritic soil in Medellín and Planeta Rica, or small discoidal bodies in La Guajira. Inclusions in chromite are preserved to varying degrees in the three localities: olivine and pargasite are partially preserved in Medellín and Planeta Rica, whereas inclusions in La Guajira chromitites are completely altered to chlorite. Compositionally, chromite from the chromitites is high-Al in Medellín (Cr#[Cr/(Cr+Al) atomic ratio] = 0.41–0.51) and Planeta Rica (Cr# = 0.49–0.57), and high-Cr in La Guajira (Cr# = 0.75–0.76), whereas TiO2 contents are low in the three localities (0.12–0.43 wt%). Regarding trace elements composition, the chromitites are similar to others formed in supra-subduction zone settings; the chromite from the high-Al chromitites is similar to that in MORB/BABB, whereas chromite from the high-Cr chromitites has a strong boninitic affinity. Chromitites from Planeta Rica are richer in Ti, possibly due to the presence of nearby gabbroic bodies and impregnated peridotites. The alteration in chromite rims and silicate matrix is related to metamorphic processes. Bulk-rock platinum-group element (PGE) contents range from 30 to 487 ppb, and chondrite-normalized PGE patterns show the typical enrichment in IPGE relative to PPGE observed in most ophiolitic chromitites. The estimated melts in equilibrium with the different chromitites correspond to MORB-like melts for the Medellín and Planeta Rica chromitites and to boninites for La Guajira chromitites. However, the estimated TiO2 content in the melt is too low for a typical MORB, indicating back-arc basin basalts and a back-arc setting for the formation of the Medellín and Planeta Rica chromitites during the Late Triassic and the Late Jurassic, respectively. La Guajira chromitites are interpreted to have formed in a fore-arc basin related to subduction-initiation during Cretaceous times.

Geochemical constraints on the petrogenesis of mafic rocks (metadolerites) from the Proterozoic Shillong Basin, Northeast India: implications for growth of the Greater Indian Landmass

AbstractThe Paleo-Mesoproterozoic Shillong Basin of the Assam-Meghalaya Gneissic Complex is exposed in parts of Northeast India. The studied metadolerites are from the volcano-sedimentary sequence of Shillong Basin from the Borjuri area in the Mikir Massif. This episode of mafic magmatism can be correlated with the Columbia supercontinent formation and bears significance to its reconstruction. The present work discusses the field, petrography and geochemical characteristics of the metadolerites, which occur in close association with the quartzites of the Shillong Group of rocks (metasedimentary rocks of the Shillong Basin). Our data show distinctive characteristics of subduction-related magmatism exhibiting high LREE/HREE, large ion lithophile element/high field strength element ratios and pronounced negative Nb anomaly. Elemental ratios such as Zr/Ba (0.21–0.46), La/Nb (1.23–2.32) and Ba/Nb (30.08–56.90) point to a fluid-enriched lithospheric mantle source in a subduction regime. Metadolerites plot in the field of ‘back-arc basin basalts’ in tectonic discrimination diagrams reinforcing a subduction zone tectonic setting. The mafic rocks correspond to a 6–10 % partial melting of a mantle source incorporating spinel+garnet lherzolite. The metamorphic P-T of the metadolerites estimated from plagioclase-hornblende geothermobarometer (7–8 kbar, 664 °C) is indicative of amphibolite facies metamorphism in a medium P-T zone. Based on the comparative analysis of field observation, petrography, geochemistry and geological ages given by previous workers, we infer that the Shillong Basin represents a back-arc rift region and is the eastern continuation of the Bathani volcano-sedimentary sequence of the Chotanagpur Granite Gneiss Complex marking continuation of the Central Indian Tectonic Zone to the Mikir Massif.

A Unified Brightness Temperature Features Analysis Framework for Mapping Mare Basalt Units Using Chang’e-2 Lunar Microwave Sounder (CELMS) Data

The brightness temperature (TB) features extracted from Chang’e-2 Lunar Microwave Sounder (CELMS) data represent the passive microwave thermal emission (MTE) from the lunar regolith at different depths. However, there have been few studies assessing the importance and contribution of each TB feature for mapping mare basalt units. In this study, a unified framework of TB features analysis is proposed through a case study of Mare Fecunditatis, which is a large basalt basin on the eastern nearside of the Moon. Firstly, TB maps are generated from original CELMS data. Next, all TB features are evaluated systematically using a range of analytical approaches. The Pearson coefficient is used to compute the correlation of features and basalt classes. Two distance metrics, normalized distance and J-S divergence, are selected to measure the discrimination of basalt units by each TB feature. Their contributions to basalt classification are quantitatively evaluated by the ReliefF method and out-of-bag (OOB) importance index. Then, principal component analysis (PCA) is applied to reduce the dimension of TB features and analyze the feature space. Finally, a new geological map of Mare Fecunditatis is generated using CELMS data based on a random forest (RF) classifier. The results will be of great significance in utilizing CELMS data more widely as an additional tool to study the geological structure of the lunar basalt basin.

Geochemical and radiometric data for mafic rocks from the Guleman Ophiolite (SE, Turkey): New insights on the geodynamic evolution of the southern Neo-Tethyan ocean

The Guleman Ophiolite is located in the southeast of Turkey and mainly consists of harzburgites and dunites hosting economically viable chromitite bodies, and lesser amounts of cumulate and isotropic gabbro, diabase dykes, pyroxenite and locally plagiogranite. This study reports the combined whole-rock geochemical and radiogenic isotope (Sr and Nd) data for gabbro and diabase dykes, and in-situ zircon U-Pb geochronological data for plagiogranite and gabbro from the Guleman Ophiolite. Two different geochemical groups were determined based on immobile minor and trace elements, and radiogenic isotope characteristics of gabbro and diabase dykes investigated here. The first group is represented by boninites and displays highly depleted elemental patterns compared to NMORB, and spoon-shaped rare earth element profiles. Contrarily, the second group resembles back-arc basin basalts and is characterized by NMORB-like rare earth element patterns associated with negative Nb anomalies. The boninitic samples are characterized by less radiogenic 143Nd/144Nd(i) (0.51229 to 0.51267; ƐNd = −4.8 to +2.7) and more radiogenic 87Sr/86Sr(i) (0.70549 to 0.70758) isotope compositions compared to the back-arc basin basalt-like samples (143Nd/144Nd(i) = 0.51289 to 0.51296; ƐNd = +7.0 to +8.4; 87Sr/86Sr(i) = 0.70496 to 0.70629), indicating derivation from distinct and/or heterogeneous mantle source regions within a supra-subduction zone. The LA–ICP–MS zircon U-Pb analyses yielded concordant late Cretaceous ages for gabbro (82 ± 2.9 Ma) and plagiogranite (79.9 ± 0.8 Ma) samples. The overall geochemical, isotopic and geochronological data presented here suggest that the Guleman Ophiolite consists of forearc and initial back-arc spreading sections of oceanic crust, which may have been formed in an intra-oceanic subduction zone within the Berit Ocean (northern branch of the southern Neo-Tethyan ocean). Moreover, the generation of continental arc magmatism (i.e. Baskil and Esence Granitoids) was well-documented on the active margin of the Malatya-Keban Metamorphics to the north. Therefore the closure of this oceanic basin should have resulted in multiple coeval northward subduction events during the late Cretaceous.

Denudation of the Golan Heights basaltic terrain using in-situ 36Cl and OSL dating

While erosion rates of carbonates along the Dead Sea fault (DSF) and its margins have been determined in several studies, information about denudation of basalts along the DSF are absent. Here we report concentrations of in situ cosmogenic 36Cl in bedrock and sediment samples combined with OSL ages of terraces from the Meshushim basin, a typical basaltic basin in the Golan Heights, northern Dead Sea Rift (DSR).Denudation rates of exposed and soil-covered bedrock basalts, range from 12.3 ± 1.4 mm/ka to 176.1 ± 47.4 mm/ka, well within the range of DSR values, regardless of lithology and climate. In all locations where exposed and soil-covered bedrock were samples next to each other, the soil-covered basalt erodes faster. This is regardless of notable variations in average annual temperature and precipitation. This means that bedrock outcrops were not covered and exposed very frequently, and that chemical weathering at the bedrock-soil interface is a significant erosional factor.Sediments in the Meshushim basin are stored in soil pockets and blankets on the upper plateau, colluvial material on slopes, and alluvial terraces. The difference in 36Cl concentrations between soil-covered and exposed bedrock indicates coverage duration of ca. 3–6 ka. The residence time estimate of only several thousands of years for sediments stored in alluvial terraces is 170 ± 20 y to 4000 ± 290 y. Nowhere in the Meshushim drainage system did we find evidence for storage of sediment longer than a few thousands of years.36Cl concentrations in colluvial and alluvial sediments are similar to those measured in soil-covered basalts. This similarity suggests that sub-surface weathering of basalts is the major source for sediments in upper and central segments of the basin, concurrent with the higher denudation rates of buried bedrock. However, very low 36Cl concentrations measured in samples collected in the lower parts of the basin indicate involvement of landslides which instantaneously expose previously shielded sediments that were incorporated into the drainage system. The effect of landslides is also manifested by the high basin scale denudation rates calculated from alluvial samples in various locations, as well as the geomorphological profile of the Meshushim basin. Sediment yield calculated from 36Cl concentrations measured in this study suggest temporal changes in denudation rates over the Holocene and up to the last ∼60 ka, with higher past denudation rates and sediment yield even in most of the Holocene related to present rates and yields.Overall, the results of this study show that the fact that the Golan lithology is basaltic does not result in different denudation process and rate from other locations investigated along the Dead Sea Rift, which are mostly of carbonate lithology and that relief and climate are stronger parameters than lithology in controlling denudation.

Geochronology and geochemistry of basalts from the Yingchuan Formation, eastern Jiangnan Orogen: Implications for the Neoproterozoic tectonic evolution of the South China Block

The Neoproterozoic magmatism along the Jiangnan Orogen preserves significant information for understanding the tectonic evolution of the South China Block (SCB). We conducted a systematic zircon U–Pb geochronological and whole‐rock geochemical study on the basalts from the Yingchuan Formation in the eastern Jiangnan Orogen, SCB. The basalts yielded a weighted mean 206Pb/238U age of 817 ± 13 Ma, enriched in Rb, Th, U, LREE and HFSE (e.g., Zr and Ti). The basalts have positive εNd(t) (+1.9 to +2.1) and high (87Sr/86Sr)i (0.69853–0.70737), indicating a slightly depleted mantle source. The high La/Nb (2.14–2.35) and La/Ta (28.6–33.3) ratios indicate a lithospheric mantle source for the basalts. Their high Ba/Yb (7.85–44.3) and Th/Yb (0.59–0.74) ratios and high absolute Nb contents (7.64–11.6 ug/g) indicate that the lithospheric mantle source has been metasomatized by melts and fluids derived from a subducted slab. The basalts are characterized by (La/Yb) N ratios (3.0–3.5) and Sm/Nd ratios (0.24–0.26), comparable to intra‐continental back arc basin basalts (BABBs). Their Ta/Yb (0.14–0.16) and Th/Yb (0.59–0.74) ratios are similar to those of Okinawa Trough basalts and contemporary mafic rocks formed in the back‐arc basin in the Jiangnan Orogen. Combined with their source features, we propose that the basalts were formed in a back‐arc basin setting. The identification of BABBs from the Yingchuan Formation shows that there were Neoproterozoic back‐arc basins in the south‐eastern Yangtze Block, indicating that the final assembly between the Yangtze and Cathaysia blocks did not occur before ca. 817 Ma.

Oligo-Miocene subduction-related volcanism of the Loyalty and Three Kings ridges, SW Pacific: a precursor to the Tonga-Kermadec arc

The SW Pacific region contains several ridges and basins that are inferred to represent pre-Quaternary volcanic arcs and back-arc basins. The geology of these features is less well characterized than that of the active Tonga-Kermadec and Vanuatu arcs. We report new major and trace element, and Pb, Hf, Sr and Nd isotope data for 27 lavas dredged from the Loyalty and Three Kings ridges during the 2015 VESPA cruise of R/V l'Atalante. Low-K basalts were dredged from the seabed deeper than 3300 m, and high-K to shoshonitic suites from shallower ridge crests at 2000–3300 m. The samples are mainly basalts, with lesser trachybasalts, basaltic andesites, trachyandesites andesites, dacites, and one granite (anhydrous SiO2 and K2O + Na2O range from ∼47 to 64 and 1.5 to 11 wt% respectively). Trace element patterns allow discrimination of three geochemical signatures, identified as i) depleted, ii) transitional and iii) enriched, based on their light to heavy rare earth element (REE) ratios (with La/Sm ranging from 0.4 to 8). Depleted and transitional samples are basalts, featuring REE concentrations similar to MORB, but with high field strength element and large ion lithophile element contents, typical of back-arc basin basalts. The most enriched samples are basaltic andesites, andesites, trachyandesites and trachytes with island arc magma trace element signatures. Pb isotope ranges are limited (208Pb/204Pb ∼38 to 39.8, 207Pb/204Pb ∼15.51 to 15.64 and 206Pb/∼17.9 to 20.1), while Hf isotopes display more diverse compositions (εHf ranging from +7.7 to +14). Both Nd (εNd = 2.8–9.3) and Sr (87Sr/86Sr = 0.7026–0.7048) isotopes are correlated with Hf data. Trace element and isotopic compositions can be explained in terms of mixing between three distinct geochemical endmembers in the mantle resembling DMM, HIMU and EM-2 sources. Our study confirms voluminous subduction-related magmatism on the Loyalty and Three Kings ridges, mostly of Late Oligocene – Early Miocene age. The issue of polarity of subduction to generate these rocks remains open, but the composition-space-time distribution of the igneous rocks can be explained in the context of SW Pacific geodynamics using a west-dipping Pacific slab model.