Low Degrees Of Partial Melting Research Articles

Alkali basalt from the Seifu Seamount in the Sea of Japan: post-spreading magmatism in a back-arc setting

Abstract. We present geochemical and 40Ar∕39Ar age data for a peridotite xenolith-bearing basalt dredged from the Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An 40Ar∕39Ar plateau age of 8.33±0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it erupted shortly after the termination of back-arc spreading in the Sea of Japan. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element enrichment. The trace element features of the basalt are similar to those of ocean island basalt, although the Yb content is much higher, indicating formation by the low-degree partial melting of spinel peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM basalt differ from those of back-arc basin basalts in the Sea of Japan. The Sr–Nd isotopic composition of the SSM basalt suggests its source was depleted mid-ocean ridge mantle containing an enriched mantle (EM1) component. The SSM basalt was formed in a post-back-arc extension setting by the low-degree partial melting of an upwelling asthenosphere that had previously been associated with the main phase of back-arc magmatism.

GEOCHEMICAL CHARACTERISTICS OF THE UMAT TRACHYTIC PLUG: IMPLICATIONS FOR THEIR ORIGIN

The Umat trachytic plug is located in Bokkos area, Plateau State, part of Kurra sheet 189 (NE), North Central Nigeria. This study presents the geochemical results carried out on some samples of the Umat Trachytic Plug. The Umat trachytic hill extruded the Precambrian Basement granite-gneiss complex and in places, the Pan African Granites (Older Granites), Younger Granites Complexes and the Cenozoic volcanoes. A total of 10 rocks samples from the trachytic hill were analyzed for major, trace and rare earth elements using XRF and ICPMS respectively. Major, trace and rare element composition ratios and plots show that they have the same mantle source. They are alkali in nature which have been emplaced within plate volcanism comparable with Ocean Island Basalts as opposed to Mid-Oceanic-Ridge basalts which are tholeiitic. On the TAS classification diagram, the rocks expressed variation in their composition ranging from phonolite to tephriphonolite-trachyandesite. This is consistence with their trace element ratios such as Zr/Y; Ba/Nb, Ba/La, Zr/Nb, Th/Nb and La/Nb which are higher than those of primitive mantle and MORB but similar to those observed in the ocean island basalts. The average Zr/Y, Ba/Nb, Ba/La ratios are in agreement with a derivation related to OIB like mantle source. Similarly, their LREEs are highly incompatible in the mantle-melt system and LREE ratios should be close to those of the mantle source under batch melting condition. On chondrite-normalized REE patterns, the rocks exhibit strong LREE enrichment with (La/Yb) La/Ce ratios varying from 9.29 to 31. These ratios are higher than those of NMORB (0.5), EMORB (1.3) and OIB (12), indicating LREE enrichment which may be interpreted as due to low degrees of partial melting or reflecting an enriched fertile mantle source.

Evolution of lithospheric mantle in the north of Nain‐Baft oceanic crust (Neo‐Tethyan ophiolite of Ashin, Central Iran)

AbstractThis study is focused on a plagioclase‐bearing spinel lherzolite from Chah Loqeh area in the Neo‐Tethyan Ashin ophiolite. It is exposed along the west of left‐lateral strike‐slip Dorouneh Fault in the northwest of Central‐East Iranian Microcontinent. Mineral chemistry (Mg#olivine < ~ 90, Cr#clinopyroxene < ~ 0.2, Cr#spinel < ~ 0.5, Al2O3orthopyroxene > ~ 2.5 wt%, Al2O3clinopyroxene > ~ 4.5 wt%, Al2O3spinel > ~ 41.5 wt%, Na2Oclinopyroxene > ~ 0.11 wt%, and TiO2clinopyroxene > ~ 0.04 wt%) confirms Ashin lherzolite was originally a mid‐oceanic ridge peridotite with low degrees of partial melting at spinel‐peridotite facies in a lithospheric mantle level. However, some Ashin lherzolites record mantle upwelling and tectonic exhumation at plagioclase‐peridotite facies during oceanic extension and diapiric motion of mantle along Nain‐Baft suture zone. This mantle upwelling is evidenced by some modifications in the modal composition (i.e. subsolidus recrystallization of plagioclase and olivine between pyroxene and spinel) and mineral chemistry (e.g. increase in TiO2 and Na2O of clinopyroxene, and TiO2 and Cr# of spinel and decrease in Mg# of olivine), as a consequence of decompression during a progressive upwelling of mantle. Previous geochronological and geochemical data and increasing the depth of subsolidus plagioclase formation at plagioclase‐peridotite facies from Nain ophiolite (~ 16 km) to Ashin ophiolite (~ 35 km) suggest a south to north closure for the Nain‐Baft oceanic crust in the northwest of Central‐East Iranian Microcontinent.

The Neoproterozoic ophiolitic ultramafic rocks in Eastern Desert of Egypt: implications for petrogenesis and metasomatic processes

ABSTRACT This paper deals with the mineral chemistry and geochemistry of ophiolitic peridotites from three localities in the Eastern Desert of Egypt, namely; Wadi Muweilha, Wadi Arayis and Gabal Kurbiyay. The present peridotites show variable degrees of alteration, including; serpentinized peridotites, serpentinites and talc-carbonate rocks with small-scale outcrops of chromitites, serpentinized pyroxenites and chloritites. Cr-spinel together with olivine and pyroxene relics represent the most common primary phases in the studied rock types. The investigated rocks represent mantle residues that experienced high degrees of partial melting. The Cr numbers of Cr-spinels reveal that serpentinized pyroxenites exhibit lower degrees of partial melting (~35%) than serpentinites and chromitites (> 35%). The whole-rock trace and rare earth elements (REE) data together with the elevated Cr# of Cr-spinels (>0.6), high Mg# of olivines (Fo = 90.82–92.61) and depleted composition of clinopyroxene are consistent with a fore-arc supra-subduction zone environment. The enrichment of light rare earth elements (LREEs) relative to heavy ones (HREE) in samples of W. Muweilha and W. Arayis could be attributed to the induction of serpentinization-related hydrothermal fluids. In contrast, the studied samples of G. Kurbiyay are depleted in LREE compared with HREE in common with a previously formed abyssal peridotite that later modified in a fore-arc region. The serpentinized pyroxenites and chloritites are speculated a genetic relationship with the rest of the mantle suite. The former is produced by the assimilation of subducted sediments in arc-related magmas. The latter is considered to be products of retrograde metamorphism simultaneously with the obduction and thrusting of the ophiolitic ultramafics. The investigated Cr-rich chromitites were formed from boninitic melts resulting from the high-degree melting of the sub-arc depleted mantle in the presence of slab-derived fluids at a mature-arc stage.

The influence of igneous processes on the chromium isotopic compositions of Ocean Island basalts

We present the first stable chromium isotopic data for a suite of ocean island basalts (OIB) in order to investigate the Cr isotope fractionation during major igneous processes such as partial melting and fractional crystallisation. Twenty-one basaltic samples from Fangataufa Island (Tuamotu Archipelago, Pacific Ocean) have been analysed for major- and trace-element concentrations, and Sr, Nd and Cr isotopic compositions. They define two distinct series: medium to high-K calc-alcaline and low to medium-K calc-alcaline. The variations in incompatible elements such as La and Yb mostly result from varying degrees of partial melting of a mixed mantle source composed of two lithologies: garnet bearing peridotite and a recycled “fertile” component. The recycled component is also identified with the Sr and Nd isotopic composition of Fangataufa basalts. In contrast, the variations in compatible element contents such as Cr and Ni are governed by fractional crystallisation of a mixture of olivine, clinopyroxene and spinel. The samples analysed in this study are also characterised by small Cr isotope variations from −0.24 to −0.17‰. The Cr-poor samples have on average lighter Cr isotopic compositions compared to Cr-rich ones. The observed variations in the low-K series can be modelled by a Rayleigh fractionation model with a fractionation factor (Δ53Crcrystals-melt) of −0.010 ± 0.005‰. The fractionation is more limited than that observed in lunar basalts and two hypotheses may explain this observation: a change in crystallising phases (cpx + spinel on Earth and spinel on the Moon) and/or the difference in temperature and oxygen fugacity between the crystallisation of lunar and terrestrial basalts. The more primitive basalts from Fangataufa have an average Cr isotopic composition of −0.18 ± 0.01‰, lighter than the Cr isotopic composition of pristine mantle xenoliths. Chromium isotopes are therefore slightly fractionated during partial melting with the melts depleted in heavy Cr isotopes. The difference between silicate melts and mantle xenoliths indicates that partial melting could produce small but resolvable shift in mantle xenoliths Cr isotopic composition of up to 0.05‰. The covariation between δ53Cr and ε143Nd values in Fangataufa basalts can be explained by two processes: (i) lower degrees of partial melting produce lighter Cr isotopic composition in basaltic liquids relative to their sources and (ii) melts produced by low degree of partial melting have also experienced more fractional crystallisation.

Petrographic, Geochemical and Isotopic (Sr–Nd–Pb–Os) Study of Plio-Quaternary Volcanics and the Tertiary Basement in the Jorullo-Tacámbaro Area, Michoacán-Guanajuato Volcanic Field, Mexico

AbstractThe origin of the large diversity of rock types erupted along the subduction-related Trans-Mexican Volcanic Belt (TMVB) remains highly debated. In particular, several hypotheses have been proposed to explain the contemporary eruption of calc-alkaline and alkaline magmas along the belt. The Michoacán-Guanajuato Volcanic Field (MGVF) is an atypical, vast region of monogenetic activity located in the western-central part of the TMVB. Here we present new petrographic, geochemical, and isotopic (Sr–Nd–Pb–Os) data on recent volcanics in the Jorullo-Tacámbaro area that is the closest to the oceanic trench. TMVB-related volcanics in this area are Plio-Quaternary (<5 Ma) and mainly form a calc-alkaline series from basalts to dacites, with rare (<5 vol. %) alkaline rocks that range from trachybasalts to trachydacites, and transitional samples. Crystal textures are consistent with rapid crystallization at shallow depth and processes of mixing of similar magma batches (magma recharge). All of the samples exhibit an arc-type trace element pattern. Alkaline and transitional magmas have higher Na2O and K2O, lower Al2O3, and higher concentrations in incompatible elements (e.g. Sr, K, Ba, Th, Ce, P) compared to calc-alkaline rocks. Calc-alkaline rocks are similar isotopically to transitional and alkaline samples, except for a few low 87Sr/86Sr samples. Sr, Nd and Pb isotopes do not correlate with MgO or 187Os/188Os, indicating that they were not significantly influenced by crustal contamination. Isotopic and trace-element systematics suggest that the Tacámbaro magmas are produced by melting of a mantle wedge fluxed by fluids derived from a mixture of subducted sediments and altered oceanic crust. Alkaline and transitional magmas can be derived from a lower degree of partial melting of a similar source to that of the calc-alkaline rocks, whereas the few low 87Sr/86Sr calc-alkaline rocks require a lower proportion of fluid derived from oceanic sediments and crust. Volcanism at the trenchward edge of the MGVF was thus driven purely by subduction during the last 5 Ma, hence discarding slab rollback in this sector of the TMVB.

Partial melting, fractional crystallisation, liquid immiscibility and hydrothermal mobilisation – A ‘recipe’ for the formation of economic A-type granite-hosted HFSE deposits

A number of A-type granites host resources of the high field strength elements (HFSE), including the rare earth elements (REE), that have been considered for economic exploitation. The best known of them is the Strange Lake pluton (Canada) with a resource of 20 Mt grading 1.44 wt% REE2O3, of which ~50% are heavy rare-earth oxides. In this paper, we examine the various processes that are likely to be responsible for the hyper-enrichment of the HFSE, starting with the generation of a parental magma through a very low degree of partial melting of a mantle plume or a metasomatically enriched mantle, in both cases, below a continental crust (continental rift). We trace the evolution of this magma (olivine nephelinite) through fractional crystallisation of olivine and clinopyroxene followed by plagioclase, K-rich alkali feldspar and arfvedsonite to the final residues of hyper-enriched HFSE granitic pegmatites. In so doing, we show that at least two additional steps are essential to the process of HFSE enrichment. The first of these is the abrupt termination of plagioclase crystallisation prior to the final consumption of Ca or assimilation of a Ca source and the resulting exsolution of an immiscible Ca-fluoride melt that sequesters the LREE and Y. The second of these steps is prolonged crystallisation of arfvedsonite, which delays saturation of the magma with an aqueous phase and ensures the development of an agpaitic assemblage of Na-bearing HFSE minerals (e.g., Na-zirconosilicates and Na-titanosilicates). Finally, we show that saturation of the magma with an aqueous‑carbonic fluid, which evolves to a very acidic composition, is an essential ingredient in the ‘recipe’ for the formation of an economic A-type granite-hosted HFSE deposit.

Potassic/ultrapotassic intrusions at the southwestern margin of the Yangtze Craton, southwestern China: Petrogenesis and implications for the metal and fluid source of non-arc porphyry Cu–(Mo–Au) deposits

Magmatic rocks can reveal partial melting processes in the lower crust and mantle. Therefore, the petrogenesis of both ore-barren and ore-bearing magmatic rocks can provide key insights into the deep geological processes that control porphyry Cu–(MoAu) deposits (PCDs). Here, we investigate the petrogenesis of potassic/ultrapotassic igneous rocks in the southwestern Yangtze Craton (SWYC) to constrain their magmatic sources and assess their role in the formation of PCDs. These igneous rocks can be divided into three suites, ore-barren granite porphyry, lamprophyre, and ore-bearing quartz monzonite. UPb dating of zircons indicates that these potassic/ultrapotassic intrusions crystalized simultaneously between 35.4 and 33.6 Ma. The lamprophyres have low SiO2 contents and high MgO and K2O contents, exhibiting an ultrapotassic affinity. They are depleted in high field strength elements (HFSEs; Nb, Ta, Zr, Hf, and Ti) and enriched in large ion lithophile elements (LILEs; Rb, K, Sr, and Ba). The enriched Sr, Nd, and Hf isotopic compositions of the ore-barren lamprophyres suggest that they were derived from low-degree partial melting of lithospheric mantle that had been metasomatized by subduction-related fluids during the Proterozoic. The ore-barren granite porphyries have high SiO2 and K2O contents, with high-K calc-alkaline to shoshonitic characteristics. These rocks have low MgO, Cr, Ni, and Co contents. All samples exhibit high Sr/Y ratios, and highly fractionated rare earth elements (REEs) patterns with weak Eu anomalies. Combined with their low total REE contents (75–81 ppm) and εHf(t) values, which are similar to Neoproterozoic granulite xenoliths, we conclude that the ore-barren porphyries are adakites derived from thickened Neoproterozoic lower crust. The ore-bearing intrusions show variable SiO2 and high Na2O + K2O contents, plotting in the monzonite to quartz monzonite fields in a SiO2 vs Na2O + K2O diagram. These samples have much higher total REE contents (346–671 ppm) than the ore-barren intrusions. The homogeneous SrNd isotopic compositions and linear trends on Harker diagrams suggest that the magma that formed the ore-bearing rocks underwent fractional crystallization of mainly amphibole and accessory minerals. Water contents are estimated to have been 9–12 wt% at 0.8 GPa on the basis of the retention of plagioclase and low zircon saturation temperatures, which suggests that the primitive magmas of the ore-bearing intrusions were derived from water-fluxed melting rather than dehydration melting. The additional water, required to promote water-fluxed melting, was probably derived from underplated poassic/ultrapotassic mafic magmas. The high total REE contents, Sr/Y and (La/Yb)N ratios indicate that the primitive magmas of ore-bearing intrusions were probably derived from partial melting of potassic/ultrapotassic lower crust, formed by the underplating of mantle-derived potassic/ultrapotassic magma. These results, combined with previous research, show that mantle-derived potassic/ultrapotassic magmas can provide metals and fluids required for the formation of PCDs.

Genesis of the Guangshigou pegmatite-type uranium deposit in the North Qinling Orogenic Belt, China

The Guangshigou uranium deposit is an important pegmatite-type uranium deposit located in the North Qinling Orogenic Belt (NQB), China. The characteristics pertaining to its chronology, petrogenesis, and geodynamic setting remain unclear till date. In this study, we present systematic zircon U–Pb ages, Lu–Hf isotopic data, and mineral chemistry of uranium-rich biotite pegmatite dikes. In situ zircon U–Pb dating of the biotite pegmatite dikes yielded early Caledonian ages (422–417 Ma). The initial 176Hf/177Hf ratios of the zircons from the biotite pegmatites showed a narrow range (0.282378–0.282590), with εHf values ranging from −5.1 to 2.8 and Paleoproterozoic to Mesoproterozoic two-stage Hf model ages between 1723 and 1227 Ma. The bulk and mineral chemistries of the pegmatites also indicate that the pegmatite dikes were derived from low-degree partial melting of Qinling Group rocks with monazite as the residual mineral. Differences between the normal biotite pegmatite and the mineralized biotite pegmatite dikes with respect to mineralogy, texture, structure, and mineral chemistry indicate that the assimilation between the pegmatite dikes and the wall rock, may be the primary mechanism for the mineralization of the uranium, with the fractional crystallization process being another important mechanism. Amphibolite facies metamorphism, which was most likely caused by the exhumation of the subducted slab in the NQB, may have been a key factor in the genesis of the uranium-rich pegmatitic magma.

The passive margin of northern Gondwana during Early Paleozoic: Evidence from the central Tibet Plateau

The central–south domain of the Tibet Plateau represents an important part of the northern segment of Gondwana during the early Paleozoic. Here we present zircon U–Pb, Lu–Hf isotope, and whole–rock geochemical data from a suite of early Paleozoic magmatic rocks from the central Tibet Plateau, with a view to gain insights into the nature and geotectonic evolution of the northern margin of Gondwana. Zircon grains in four granitic rocks yielded ages of 532−496 Ma with negative εHf(t) values (−13.7 to −0.6). Zircon grains in meta–basalt and mafic gneiss yielded ages of 512 ± 5 Ma and 496 ± 6 Ma, respectively. Geochemically, the granitic rocks belong to high–K calc–alkaline and shoshonitic S–type granite suite, with the protolith derived from the partial melting of ancient crustal components. The mafic gneiss and meta–basalt geochemically resemble OIB (Oceanic Island Basalt) and E–MORB (Enriched Mid–Ocean Ridge Basalt), respectively. They were derived from low degree (∼5–10%) partial melting of an enriched mantle (garnet and spinel lherzolite) that was contaminated by upper crustal components. The parental magmas experienced orthopyroxene–dominated fractional crystallization. Sedimentological features of the Cambrian–Ordovician formations indicate that the depositional cycle transformed from marine regression to transgression leading to the formation of parallel/angular unconformities between the Cambrian and Ordovician strata. The hiatus associated with these unconformities are coupled with the peak of the early Paleozoic magmatism in Tibet Plateau, indicating a tectonic control. We conclude that the Cambrian–Ordovician magmatic suite and sedimentary rocks formed in an extensional setting, and we correlate this with the post–peak stage of the Pan–African orogeny. The post–collision setting associated with delamination, orogenic collapse or lithospheric extension along the northern margin of Gondwana, can account for the Cambrian–Ordovician magmatism and sedimentation, rather than oceanic subduction along the external margin. We thus infer a passive margin setting for the northern Gondwana during the Early Paleozoic.

Initial Pangean rifting north of the West African Craton: Insights from late Permian U-Pb and 40Ar/39Ar dating of alkaline magmatism from the Eastern Anti-Atlas (Morocco)

Numerous mafic dykes, sills and laccoliths crop out in the southern part of the Tafilalt basin (Eastern Anti-Atlas, Morocco). These rocks intrude the mildly folded Ordovician to Early Carboniferous formations, consisting mainly of lamprophyric dolerites and camptonites with minor gabbros and syenodiorites. Previous geochemical studies have shown that the Tafilalt magmatism of sodic-alkaline affinity has been produced by low degrees of partial melting from an enriched deep mantle source within the garnet stability field. However, the age and the geodynamic context of these rocks were presently unknown since no isotopic dating had so far been made of the Tafilalt dolerites. To resolve this issue, we present here the first 40Ar/39Ar biotite and U-Pb zircon dating from the Tafilalt alkaline magmatism. Three samples (biotite separates) yielded well-defined 40Ar/39Ar plateau ages of 264.2 ± 2.7 Ma, 259.0 ± 6.3 Ma and 262.6 ± 4.5 Ma whereas 206Pb/238U dating of zircon from one of these samples yielded an age of 255 ± 3 Ma. These ages coincide within the dating error, and indicate that this magmatism occurred in the late Permian. Considering geochronological and geochemical data, we propose that the Tafilalt magmatism reflects an early-rift magmatic activity that preceded the Triassic rifting heralded by the Central Atlantic Magmatic Province. This magmatic activity is recorded in both sides of the future Atlantic Ocean by small-volume alkaline magmatism that started in the late Permian and extends into the Triassic. The alkaline magmas are probably generated in response to an increase in the mantle potential temperature underneath the Pangea supercontinent.

Permian Tectonic Evolution in the Middle Part of Central Asian Orogenic Belt: Evidence from Newly Identified Volcanic Rocks in the Bilutu Area, Inner Mongolia

In this study, zircon U‐Pb ages, geochemical and Lu‐Hf isotopic data are presented for the newly identified volcanic rocks which were considered as Bainaimiao group in Bainaimiao Arc Belt (BAB), Inner Mongolia, which could provide important constraints on the evolution of the northern part of North China Block (NCB) and BAB. Basalt to basaltic andesite and andesite to dacite were collected from two sections, which showed eruption ages of 278.2±4.1 Ma and 258.3±3.0 Ma respectively. All samples are characterized by high abundances in Al2O3, LREEs, and LILEs, but depleted in HFSEs. Together with high Mg# ratios and low K/tholeiite to calc‐alkaline series, these features indicated that basalt to andesite was likely derived from relatively low degree partial melting of the subduction‐fluid related mantle in the spinel phase. And dacite was mainly from the partial melting of crust, then affected by mantle. All samples barely went through fractional crystallization process with the slight Eu anomaly. Compared with the contemporary basalt in NCB, rocks in BAB have a complex composition of zircon and a more positive εHf (t) value (–6.6–6.4), indicating that they had different magma sources of rocks. Though with different basements, NCB and BAB have become an integrated whole before 278 Ma. Therefore, it could be concluded that NCB and BAB belonged to the active continental margin and the PAO had not closed yet until late Permian and then it disappeared gradually and the CAOB developed into a condition of syn‐post collision.

Petrogenesis of the diamondiferous Pipe-8 ultramafic intrusion from the Wajrakarur kimberlite field of Southern India and its relation to the worldwide Mesoproterozoic (~1.1 ​Ga) magmatism of kimberlite and related rocks

Detailed mineralogy, bulk rock major, trace and Sr–Nd isotope compositions, and 40Ar/39Ar dating of the Pipe-8 diamondiferous ultramafic intrusion in the Wajrakarur cluster of southern India, is reported. Based on the presence of Ti-rich phlogopite, high Na/K content in amphibole, Al- and Ti-rich diopside, a titanomagnetite trend in spinel and the presence of Ti-rich schorlomite garnet and carbonates in the groundmass, the Pipe-8 intrusion is here more precisely classified as an ultramafic lamprophyre (i.e., aillikite). An aillikite affinity of the Pipe-8 intrusion is further supported by the bulk rock major and trace element and Sr–Nd isotope geochemistry. Sr–Nd isotope data are consistent with a common, moderately depleted upper mantle source region for both the Pipe-8 aillikite as well as the Wajrakarur kimberlites of southern India. A phlogopite-rich groundmass 40Ar/39Ar plateau age of 1115.8 ​± ​7.9 ​Ma (2σ) for the Pipe-8 intrusion falls within a restricted 100 ​Ma time bracket as defined by the 1053–1155 ​Ma emplacement ages of kimberlites and related rocks in India. The presence of ultramafic lamprophyres, carbonatites, kimberlites, and olivine lamproites in the Wajrakarur kimberlite field requires low degrees of partial melting of contrasting metasomatic assemblages in a heterogeneous sub-continental lithospheric mantle. The widespread association of kimberlite and other mantle-derived magmatism during the Mesoproterozoic (ca. 1.1 ​Ga) have been interpreted as being part of a single large igneous province comprising of the Kalahari, Australian, West Laurentian and Indian blocks of the Rodinia supercontinent that were in existence during its assembly. In India only kimberlite/lamproite/ultramafic lamprophyre magmatism occurred at this time without the associated large igneous provinces as seen in other parts of Rodinia. This may be because of the separated paleo-latitudinal position of India from Australia during the assembly of Rodinia. It is speculated that the presence of a large plume at or close to 1.1 ​Ga within the Rodinian supercontinent, with the Indian block located on its periphery, could be the reason for incipient melting of lithospheric mantle and the consequent emplacement of only kimberlites and other ultramafic, volatile rich rocks in India due to comparatively low thermal effects from the distant plume.

Petrogenesis and tectonic implications of the early Carboniferous volcanic rocks in West Junggar, NW China

Carboniferous magmatism in the Darbut region is critical for understanding the evolutionary history of the accretionary orogenesis of the West Junggar area in the Central Asian Orogenic Belt. In this paper, zircon U–Pb geochronological, whole‐rock geochemical and Sr–Nd–Pb–Hf isotopic analyses of the basalts, basaltic andesite, and dacite from the Carboniferous Tailegula Formation, Darbut region, have been performed. The magmatic zircons from basalt, basaltic andesite, and dacite yield concordia U–Pb isotope ages of 344 ± 4, 347 ± 3, and 340 ± 5 Ma, respectively, which are interpreted as the crystallization ages of these rocks. The basalts and basaltic andesite are characterized by an alkali composition; high Nb/Yb, Th/Yb, Ta/Yb, La/Sm, and Sm/Yb trace element ratios and Pb isotopic ratios; moderate (87Sr/86Sr)i ratios; low (143Nb/144Nb)i ratios; and positive εNd(t) (+0.66 − +4.73) and εHf(t) (+1.7 − +15.2) values. These characteristics indicate that the magmas were derived from mixed mantle sources, that is, enriched mantle sources and high U/Pb ratio mantle (HIMU) component sources with ≤10% partial melting of garnet and spinel lherzolite. The samples display pronounced light rare earth elements (LREEs) and incompatible element enrichment patterns with positive Nb–Ta anomalies and a high Pb content and Nb/La ratios but low (Th/Nb)N ratios and a lower degree of partial melting, implying that these rocks were formed in a continental extensional setting. Based on our results and that of previous studies, we conclude that the early Carboniferous volcanic rocks in the Darbut region formed by mantle plume or regional‐scale mantle upwelling in a continental extensional setting after closure of the Junggar Ocean.

Late Triassic porphyries in the Zhongdian arc, eastern Tibet: origin and implications for Cu mineralization

AbstractWhole-rock and Sr–Nd–Pb isotopic composition data, zircon Hf isotopic data and zircon U–Pb ages were obtained for the Late Triassic porphyries in the Zhongdian arc, eastern Tibet. These porphyries are intermediate and metaluminous and are enriched in large ion lithophile elements and depleted in high field strength elements. Moreover, they have weak negative Eu anomalies, high Sr and Ba contents, and high Sr/Y ratios. Different mineral geothermobarometers suggest that the porphyries in the Zhongdian arc crystallized atc. 640–829 °C and pressures of 2.1–2.8 kbar at depths shallower than 8 km. The porphyries have a calculated water content of 4.47–4.94 wt % and a relatively high magmatic oxygen fugacity. These porphyries were emplaced mainly at 230–203 Ma with a peak at 218–215 Ma. The Sr–Nd–Pb–Hf isotope data suggest that the porphyries in the Zhongdian arc were derived from a mixed melt of 50–65 % asthenospheric mantle and 35–50 % eclogite from the western Yangtze lower crust that experienced low-degree partial melting of 2–10 %. Subsequent fractional crystallization resulted in the decreasing trends of the major- and trace-element contents. The high Sr/Y and La/Yb values are the result of the low degree of partial melting of the western Yangtze lower crust rather than fractional crystallization, because no linear relationship was noted between Sr/Y or La/Yb and SiO2. The mixed melts from the lower crust and asthenospheric mantle provided a fertile magma source, and subsequent fractional crystallization under the favourable magmatic conditions of high water content and high oxidation state resulted in the formation of the porphyry Cu–Au deposits.

Pacific Lithosphere Evolution Inferred from Aitutaki Mantle Xenoliths

Abstract Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), major and trace element abundances, and 187Re–187Os systematics are reported for xenoliths and lavas from Aitutaki (Cook Islands), to investigate the composition of Pacific lithosphere. The xenolith suite comprises spinel-bearing lherzolites, dunite, and harzburgite, along with olivine websterite and pyroxenite. The xenoliths are hosted within nephelinite and alkali basalt volcanic rocks (187Os/188Os ∼0·1363 ± 13; 2SD; ΣHSE = 3–4 ppb). The volcanic host rocks are low-degree (2–5%) partial melts from the garnet stability field and an enriched mantle (EM) source. Pyroxenites have similar HSE abundances and Os isotope compositions (Al2O3 = 5·7–8·3 wt %; ΣHSE = 2–4 ppb; 187Os/187Os = 0·1263–0·1469) to the lavas. The pyroxenite and olivine websterite xenoliths directly formed from—or experienced extensive melt–rock interaction with—melts similar in composition to the volcanic rocks that host the xenoliths. Conversely, the Aitutaki lherzolites, harzburgites and dunites are similar in composition to abyssal peridotites with respect to their 187Os/188Os ratios (0·1264 ± 82), total HSE abundances (ΣHSE = 8–28 ppb) and major element abundances, forsterite contents (Fo89·9±1·2), and estimated extents of melt depletion (<10 to >15%). These peridotites are interpreted to sample relatively shallow Pacific mantle lithosphere that experienced limited melt–rock reaction and melting during ridge processes at ∼90 Ma. A survey of maximum time of rhenium depletion ages of Pacific mantle lithosphere from the Cook (Aitutaki ∼1·5 Ga), Austral (Tubuai’i ∼1·8 Ga), Samoan (Savai’i ∼1·5 Ga) and Hawaiian (Oa’hu ∼2 Ga) island groups shows that Mesoproterozoic to Neoproterozoic depletion ages are preserved in the xenolith suites. The variable timing and extent of mantle depletion preserved by the peridotites is, in some instances, superimposed by extensive and recent melt depletion as well as melt refertilization. Collectively, Pacific Ocean island mantle xenolith suites have similar distributions and variations of 187Os/188Os and HSE abundances to global abyssal peridotites. These observations indicate that Pacific mantle lithosphere is typical of oceanic lithosphere in general, and that this lithosphere is composed of peridotites that have experienced both recent melt depletion at ridges and prior and sometimes extensive melt depletion across several Wilson cycles spanning periods in excess of two billion years.

Differentiation in impact melt sheets as a mechanism to produce evolved magmas on Mars

Asteroid bombardment contributed to extensive melting and resurfacing of ancient (>3.5 Ga) Mars. Evidence from the largest impact structures on Earth and the Moon suggests that vertically thick impact melt sheets experience chemical differentiation. Recently observed materials in Gale crater are enriched in alkalis (up to 14 wt% Na2O + K2O) and silica (up to 67 wt% SiO2) with low MgO (<5 wt%). This differentiation trend has previously been attributed to fractional crystallization of mantle-derived magmas. The influence of impact melting on Mars' crustal petrology, however, has not been explored in depth. In this study, we investigate differentiation of impact-generated melts as a mechanism for petrogenesis of evolved rocks on Mars. We scale melt volumes for multiple impact magnitudes and employ the MELTS algorithm to model mineral phase equilibria in those melts as they crystallize. We consider a range of possible melt geometries, oxidation states (IW to QFM), crystallization regimes (equilibrium and fractional), and water contents (anhydrous to water-saturated). Moderate pressure and dissolved volatiles are required to produce alkaline differentiation trends in mafic melts. Large impacts that melt a water-bearing early basaltic crust provide a mechanism to generate wet magmas on Mars, consistent with some modeled Martian differentiation trends. Low-degree partial melting and/or an alkali-enriched source are needed to generate observed highly alkaline compositions, motivating future work to further assess whether these conditions can be achieved in impact-generated melts.

The Lazurnoe deposit in the Central Sikhote-Alin, Eastern Russia: Combined shoshonite-related porphyry Cu-Au-Mo and reduced intrusion-related Au mineralization in a post-subduction setting

The Lazurnoe porphyry Cu-Au-Mo deposit (>1Mt Cu-eq.) in the Mesozoic Central Sikhote-Alin orogenic belt (Eastern Russia) is located in an Early Cretaceous turbidite basin associated with a crustal-scale strike-slip fault zone, and was formed under the continental transform margin regime after the cessation of active subduction. The deposit is related to a magnetite-series, high-K calc-alkaline to shoshonitic, Early Cretaceous igneous suite that occurred in this tectonic setting, possibly due to asthenospheric mantle upwelling. The fertile magma was sourced from a subduction-modified mantle, under a low degree of partial melting, and was subjected to intense amphibole fractionation during its crystallization, the latter facilitating water and metal saturation. The parental igneous suite at Lazurnoe was coeval to ilmenite-series calc-alkaline igneous suites accompanied by reduced intrusion-related Au(±W) to W-Au deposits as well as to ilmenite-series, high-K calc-alkaline to shoshonitic suites accompanied by Sn-polymetallic mineralization.The deposit comprises magnetite-rich, quartz-K-feldspar to quartz-K-feldspar-biotite (potassic) alteration assemblages, containing bornite and chalcopyrite, which evolve to quartz-chlorite ± epidote ± amphibole (propylitic) assemblages, containing dominant pyrite with magnetite and chalcopyrite. Molybdenite-chalcopyrite-pyrite paragenesis dominates in quartz-sericite (phyllic) alteration assemblages that overprint and replace potassic and propylitic alteration. Potassic alteration assemblages formed from hot (~530–510 °C), high-salinity (45–48 wt% NaCl and 9–10 wt% CaCl2), sodic-calcic aqueous-chloride fluid. Gradual fluid cooling and dilution (from 29 to 31 wt% NaCl and 18–19 wt% CaCl2 through 24–25 wt% NaCl and 16–17 wt% CaCl2 to 12 ± 0.5 wt% NaCl and 12 ± 0.5 wt% CaCl2) is recorded for propylitic alteration assemblage. Another cycle of fluid exsolution from crystallizing magma can be suggested for phyllic alteration assemblages formed from a cooler (~390–310 °C), high-salinity (30–37 wt% NaCl and 5 ± 0.5 wt% CaCl2) aqueous-chloride fluid. The porphyry-style mineralization was overprinted by the late auriferous quartz-sulfide veins containing arsenopyrite, pyrrhotite and other sulfide minerals, together with native gold (locally > 100 g/t Au) and tellurides. These veins were formed from boiling aqueous-carbonic fluid that was characterized by elevated CO2 and CH4 contents, with subsequent separation of high-carbonic and aqueous-chloride phases, and the fluid cooling from ~390 to 250 °C. These veins can be related to another, more reduced, plutonic suite, which crystallized at a greater depth.

Crustal maturation through chemical weathering and crustal recycling revealed by Hf–O–B isotopes

Juvenile continental crust is dominantly formed at intra-oceanic arcs via subduction zone magmatism. However, it remains unclear how basaltic oceanic arcs convert to granodioritic continental compositions. Here we present zircon U–Pb and Hf–O isotope data, whole-rock B isotope compositions, combined with a synthesis of over 1100 geochemical analyses from magmatic rocks that span a wide range of emplacement ages (∼490–270 Ma), from the Junggar intra-oceanic arc of the southern Central Asian Orogenic Belt (CAOB). Geochemical data show that the Junggar evolved from a juvenile oceanic arc composition (low SiO2, K2O and Rb, and high MgO contents) to one closer to continental crust (high SiO2, K2O and Rb, and low MgO contents) at ca. 300 Ma. All samples show very high and uniform Hf isotope ratios with εHf(t) values from +10.6 to +14.4. Zircon δ18O values and whole-rock δ11B values, however, are highly variable. The Silurian – Carboniferous (pre-300 Ma) rocks have distinctly lower zircon δ18O values ranging from 5.16 to 6.72‰ with an average of 5.78‰, and high δ11B values (−7.5–+12.2‰), suggesting that they were derived from the asthenospheric mantle wedge, which supports the genesis of primitive intra-oceanic crust during that time interval. In contrast, the Early Permian (post-300 Ma) rocks display much higher zircon δ18O values (8.24 to 10.29‰) and lower δ11B values (−9.0 to −12.2‰), combined with the presence of Carboniferous inherited zircons, which requires a source component comprising young, weathered, volcanogenic sediments from the Carboniferous Junggar intra-oceanic arc. The Early Permian rocks were produced by low degrees of partial melting of all sources (ca. 10%) and element and Hf–O–B isotope mixing calculations indicate a contribution of more than ∼50% from weathered sediments in those sources. Thus, the evolution of the Junggar segment from a primitive basaltic intra-oceanic arc toward a young continent was related to the recycling of crustal residues of chemical weathering and the younger magmatism was generated by crustal melting of these weathering products after they were buried to lower crustal depths. Our study highlights that an intra-oceanic arc's own chemical weathering history promotes its transformation into continental crust during collisional events and clarifies the relationship between continental crust formation and intra-oceanic arcs through time.

Geochemical constraints on the petrogenesis of Triassic alkaline basalts of Sierra de Valle Fértil, Western Sierras Pampeanas, Argentina: implications for their origin, evolution and tectonic setting

The first geochemical and Sr-Nd isotope data of a series of Triassic basaltic rocks from Sierra de Valle Fértil, Western Sierras Pampeanas, Argentina, are herein presented. The mafic rocks are the result of continental intraplate magmatism related to an extensional tectonic setting after the climax of the Gondwanide orogeny (Upper Carboniferous-Permian). Major minerals phases of these rocks are olivine, clinopyroxene, plagioclase and Fe-Ti oxides. Olivine and clinopyroxene occur both as phenocrysts and as groundmass, while feldspar is mostly restricted to the groundmass. The rocks belong to a typical alkaline series displaying SiO2 contents of 43.9–51.6 wt%, Na2O between 3.1 and 5.4 wt%, K2O between 0.9 and 2.9 wt% and high TiO2 2.2–3.6 wt% and P2O5 0.5–1.2 wt% contents. Nepheline on its CIPW normative composition indicates that it crystallized from silica-undersaturated magmas. Samples have moderate to low Mg# (58.1–15.9) values and Cr and Co contents that indicate they are not primary magmas but have undergone fractional crystallization with earlier fractionation of olivine and clinopyroxene. Geochemical patterns are similar to those found in the ocean island basalts (OIBs), with enrichment of alkalis and incompatible elements: LILE, HFSE and LREE, strong fractionation of HREE relative to LREE [(La/Yb)N = 9.6–22.4], no depletion of Nb and Ta and absence of negative Eu anomaly. Observed LILE-enrichment is indicated by the alkaline nature of the basaltic rocks and suggests melting of an enriched mantle. The Eu and Sr positive anomalies, not related to plagioclase accumulation, argue against significant plagioclase fractionation and are original geochemical characteristics of the primary magma. The chemical features, together with high incompatible-element ratios, are consistent with low degrees of partial melting of a dominantly garnet-pyroxenite mantle source. The trace-element patterns and isotopic data with 143Nd/144Nd(i) (0.51257–0.51267), positive εNd(i) and low 87Sr/86Sr(i) (0.70292–0.70339) values, suggest a mantle source influenced by an enriched-component. Active subduction-related mechanisms promoted the conditions for decompression melting in the asthenospheric mantle. Primary basaltic magma experienced subsequent compositional modifications by fractional crystallization in shallow-level low-pressure magma storage reservoirs. There is no evidence of crustal contamination during magma ascent.