Crustal Cumulates Research Articles

Lower crustal copper enrichment and mobilization indicated by Late Permian amphibole gabbros and granodiorites in the eastern Kunlun Orogen, northern Tibet

Lower crustal fractional crystallization accompanied by sulfide saturation in thick continental arcs has been viewed as an essential step in forming porphyry Cu systems. These processes are supposed to lock Cu in the lower crust and, therefore, can be unfavorable to the generation of ore-forming magmas. In this study, we present in situ mineral major-trace elements, pyrrhotite S isotopes, and zircon UPb ages, as well as whole-rock major-trace elements and SrNd isotopes for amphibole gabbros and contemporary granodiorites in the eastern Kunlun Orogen to elucidate Cu variations during the transcrustal magmatic evolution of these rocks. The amphibole gabbros can be divided into two types based on field relationships with the granodiorites. These rocks were derived from an oxidized metasomatic mantle in a steep subduction setting at ∼250 Ma and experienced a transcrustal magmatic evolution. Orthopyroxene relicts, resorbed clinopyroxene, calcic plagioclase, high-Al2O3 amphibole, and magnetite in the amphibole gabbros of the first type are lower crustal cumulate minerals. Pyrrhotite with minor amounts of chalcopyrite, interstitial to cumulate amphibole or as inclusions in magnetite from the amphibole gabbros of the first type, is lower crustal Ni-rich monosulfide solid solution (mss) crystallized from immiscible sulfide melts with residual Cu-rich sulfide liquids. These lines of evidence together with field, mineralogical, and geochemical data indicate that the Cu-rich parental magmas experienced lower crustal MASH (melting, assimilation, storage, and homogenization) processes and became Cu-depleted. The evolved magmas after lower crustal evolution became moderately oxidized and Cu-normal according to amphibole, biotite, zircon, and whole-rock compositions. The moderately oxidized magmas scavenged and mobilized Cu from Cu-rich sulfide liquids to the upper crustal magma chambers where degassing dispersed the Cu and thus was unfavorable to porphyry Cu mineralization.

Sulfide resorption contributes to porphyry deposit formation in collisional settings

Production of Cu-rich melts via melting of sulfide-bearing lower crustal cumulates is thought to contribute to porphyry Cu formation in the post-collisional Gangdese belt (Tibet). We present new whole rock, including platinum group elements (PGE) data, for ore-causative granites (OCG), ore-related granites (ORG) and barren granites (BG) from the Gangdese belt. The OCG, ORG and BG show indistinguishable Cu, PGE and Cu/Ag behavior during differentiation down to ∼2 wt% MgO. These systematics, together with the occurrence of sulfides, demonstrate that the propensity for some Gangdese magmas to be ore-associated is unrelated to melting of sulfide-bearing cumulates or the relative timing of sulfide fractionation. The behavior of chalcophile and siderophile elements during differentiation of the OCG, ORG and BG diverges when melts evolve past 2–3 wt% MgO and exsolve H2O, S and Cl: the BG show a decrease in Cu, whereas the OCG and ORG show highly variable Cu, Au, and PGE trends. Because S degassing causes melts to become sulfur undersaturated and because Cu is highly soluble in Cl-rich fluids, any sulfides in contact with the melt or magmatic fluids following volatile exsolution would dissolve. Continual injection, cooling and ascent of magmas through the Gangdese crust is expected to have resulted in the development of regions of crystal-rich partially molten crust beneath the intrusive granites. We suggest that prior to porphyry mineralization, extension and erosion caused exhumation of these long-lived sulfide-bearing mush zones. This exhumation may have caused a change in magmatism style from volatile exsolution after the melts transited sulfide-bearing mushes (i.e., the BG), to volatile exsolution during magma transit through sulfide-bearing mushes (i.e., the OCG). We suggest that only the magmas and that exsolved fluids during their ascent through sulfide-bearing mushes were able to assimilate sufficient Cu-rich sulfides to fuel the formation of porphyry Cu deposits in the region.

Remobilization and eruption of an upper crustal cumulate mush at the Singkut caldera (North Sumatra, Indonesia)

Understanding the conditions and timescales of storage and remobilization of magma bodies in the upper crust is key to interpreting the signals of potential reawakening at active volcanoes. In this paper, we provide the first volcanological and petrochronological characterization of the Singkut caldera, a young volcanic system located in northern Sumatra (Indonesia), in close proximity to Medan, one of the country's most popoluous cities. Singkut formed at ∼44 ka during a VEI 6 explosive eruption that deposited at least ∼26 km3 of tephra (dense rock equivalent, DRE). The cataclysmic eruption was preceded by >200 ky of mostly effusive pre-caldera activity and followed by effusive to mildly explosive post-caldera activity. The lavas and pumices have high crystallinity (up to 62% crystals) with andesitic to dacitic bulk-rock composition and rhyolitic glass. Mineral textures and matrix glass compositions indicate resorption of quartz, plagioclase and zircon. Zircon crystallization ages show a complete overlap with the eruption ages in pre-caldera lavas, while a time gap in zircon crystallization (>50 ky) is identified in the caldera-forming tuff and post-caldera lavas. Ti-in-zircon thermometry shows that the Singkut magma body experienced a temperature increase starting approximately upon eruption of the pre-caldera lavas (∼254 ka). Such thermal perturbation determined progressive melting of mineral phases in the cumulate crystal mush, caused the resorption of the youngest zircon domains before the caldera-forming eruption, and hampered zircon crystallization between the caldera-forming eruption and the effusion of the post-caldera lavas (∼16 ka). Our data demonstrate how cumulate melting processes played a key role in leading the volcanic system towards a caldera-forming eruption and controlled the transitions in eruptive style between the effusive phases and the explosive climactic eruption.

Eoarchean ultramafic rocks represent crustal cumulates: A case study of the Narssaq ultramafic body, southern West Greenland

Several ultramafic enclaves found within the Eoarchean Itsaq Gneiss Complex (IGC) in southern West Greenland have previously been interpreted as representing mantle relicts. However, as Archean ultramafic rocks are frequently overprinted by metamorphism and late-stage metasomatism, it can be difficult to distinguish between peridotites representing mantle residues or primitive crustal cumulates. Therefore, detailed evaluation of individual Eoarchean peridotite occurrences is required to better constrain their origin. Here, we present new petrological observations and geochemical data for the >3.8 Ga Narssaq Ultramafic Body (NUB) in the IGC of SW Greenland. The NUB ultramafic rocks have high FeOt and Cr contents with flat chondrite-normalized trace element patterns and positive Cr-Mg correlations, which distinguish them from mantle rocks. In addition, the variations in olivine and spinel compositions are consistent with a fractional crystallization process. The above geochemical features suggest that the ultramafic rocks of NUB cannot be explained as mantle residues. Instead, their major element compositions are related to the spatially associated tholeiitic amphibolites, indicating that these ultramafic rocks likely represent crustal cumulates derived from high-Mg magmas. This interpretation is supported by thermodynamic modeling using MELTS, which shows that the compositions of the ultramafic rocks in the NUB can be reproduced via fractional crystallization of local high-Mg tholeiitic basalts, involving the accumulation of olivine, spinel, and possibly clinopyroxene, in conjunction with trapped melt. The NUB ultramafic rocks can thus form as a natural consequence of fractional crystallization of regular highly magnesian tholeiitic magmas, making a cumulate origin the simplest explanation for Eoarchean peridotites in general. This also implies that the formation of the NUB did not involve refertilization of subduction-like components as previously assumed, and thus did not require subduction to have taken place in the Eoarchean.

Petrology and Geochemistry of Adak Island Plutonic Xenoliths: Implications for Primitive Magma Generation and Crustal Differentiation in the Aleutian Island Arc

Abstract Deep crustal cumulates in arcs offer a window into the chemistry and crystallization conditions (P–T–H2O–fO2) of primitive basalts in the upper mantle and lower crust and can be studied in ancient exhumed terranes or in xenoliths erupted in young arc lavas. Here, we expand on previous studies and thoroughly characterize the extensive xenolith suites erupted from the Mt. Moffett and Mt. Adagdak volcanic centers (Adak Island, Central Aleutians), which range from primitive ultramafic cumulates to more evolved amphibole gabbros and hornblendites. We present detailed petrography as well as in situ trace and major element mineral chemistry. We use these data to calculate pressure, temperature, and fO2 estimates for the xenoliths, and compare these findings to experimental results to understand the crystallization sequence and P–T–H2O–fO2 under which the cumulates formed. The Moffett crystallization sequence is defined by early amphibole fractionation and an abrupt shift in oxide compositions from chromite to magnetite, while the Adagdak suite is characterized by simultaneous saturation of amphibole+plagioclase and oxide compositions that become increasingly aluminous before magnetite saturation. Olivine–spinel oxybarometry of the Adagdak xenoliths indicates that they are oxidized relative to mid-ocean ridge basalt (MORB:FMQ +0.1 to +2.1). Highly fractionated REE and elevated Sr/Y ratios are observed in clinopyroxene from the most primitive cumulates, consistent with a contribution from a basaltic eclogite melt. This basaltic eclogite melt is hypothesized to come from partial melting of the slab or through melting of basalt introduced into the subarc mantle through forearc subduction erosion. These signatures are greatly diminished in the more evolved lithologies, which can be explained through fractionation of plagioclase and amphibole. Our findings support the presence of a complex magmatic plumbing system beneath Adak, with Mt. Moffett and Mt. Adagdak volcanic centers tapping compositionally distinct sources. More broadly, our results are consistent with studies suggesting that low-degree basaltic eclogite melts through slab melting or forearc subduction erosion contribute to arc magmas in the Aleutians, although the associated geochemical signatures are easily obscured by differentiation in the crust.

Formation of lower arc crust by magmatic underplating revealed by high-precision geochronology

Abstract In modern plate tectonic regimes, continental crust is generated above subduction zones in magmatic island arcs. Models for continental growth—largely based on the modeling of geochemical processes that can transform mantle melts into the intermediate composition of bulk continental crust—have been hampered by a lack of definitive geochronology, which could clarify the temporal emplacement of igneous rocks at the base of island arc crust. The Kohistan Arc Complex (KAC) of Pakistan is a rare window into a nearly complete section of lower arc crust, revealing a sequence of igneous rocks representing discrete batches of magma underplated along the base of the arc crust. We present high-precision isotope dilution–thermal ionization mass spectrometry U-Pb geochronological data from zircon, and Lu-Hf and Sm-Nd isochron data from several mafic lower crustal cumulates within the KAC. These data establish a clear downward-younging age trend throughout this succession of cumulate complexes, demonstrating a total magmatic duration of ~20 m.y., corresponding to a rate of lower crust formation of up to 200–260 km3 km–1 Ma–1. These results provide the first direct evidence of a sequential process of underplating, revealing a fundamental mechanism responsible for building the root of island arc crust.

An Early Paleozoic Ultramafic Complex in the North Wulan Metamorphic Complex, North Qaidam: Contraints on the Nature of the Alaskan‐type Continental Arc Root

AbstractOrogenic peridotite is an important component of orogenic belts and retains crucial information on mantle magmatic activity, slab subduction, and melt or fluid metasomatism. To determine the source of the mantle‐derived parental magma of the peridotite and to investigate the metasomatism that it experienced, we undertook an integrated study of the petrography, whole‐rock major‐ and trace‐element compositions, in situ zircon U–Pb geochronology, and mineral major‐ and trace‐element compositions of an early Paleozoic ultramafic complex in the North Wulan area of North Qaidam. The Halihatu ultramafic–mafic complex is composed of dunite, pyroxene peridotite, and gabbro, which are characteristic of Alaskan‐type complexes. The dunite yields a weighted mean 206Pb/238U age of 479 ± 5 Ma (MSWD = 0.7), which reflects the age of the metasomatism rather than the crystallization age of the ultramafic magma. The peridotites have high Mg# (89.8–91.8) and Cr contents (2419–5190 ppm), low Al2O3 (0.20–1.68 wt.%) and Ni (289–1012 ppm) contents, and high olivine Fo contents (87–91), suggesting a large degree (~15%–22%) of partial melting of lithospheric ultramafic rocks followed by variable degrees of fractional crystallization of olivine and pyroxene. This is consistent with estimates of 15%–22.3% partial melting calculated using the Cr# of spinel crystals and with the low Yb (0.04–0.33 ppm) and Y (0.72–1.29 ppm) contents of clinopyroxene crystals. Whole‐rock trace‐element patterns show enrichment in large ion lithophile elements and depletion in high field strength elements, along with high Al2O3 (2.10–6.47 wt.%) and low TiO2 (0.01–0.21 wt.%) contents of clinopyroxene crystals, suggesting an arc magma cumulate trend. These features, along with the high olivine Fo contents (87–91 ppm), imply that the Halihatu peridotite is an Alaskan‐type crustal cumulates derived from Mg‐rich hydrous basaltic melts. The high estimated fO2 (FMQ +1.97 to FMQ +3.81) further supports the idea that they formed in an arc setting. The Ni/Co and Ni/Mn ratios and cumulate textures of the olivine, quenched boundaries between mafic and felsic melts, and the occurrence of tremolite and phlogopite reflect interactions between the Halihatu peridotite and injected silicate and carbonatitic melts in the lower crust. Therefore, we propose a new cumulate‐infiltration model for the petrogenesis of Alaskan‐type ultramafic complexes, which improves our understanding of the nature of Alaskan‐type continental arc root.

Earth's Earliest Phaneritic Ultramafic Rocks: Mantle Slices or Crustal Cumulates?

AbstractWhen plate tectonics initiated remains uncertain, partly because many signals interpreted as diagnostic of plate tectonics can be alternatively explained via hot stagnant‐lid tectonics. One such signal involves the petrogenesis of early Archean phaneritic ultramafic rocks. In the Eoarchean Isua supracrustal belt (Greenland), some phaneritic ultramafic rocks have been dominantly interpreted as subduction‐related, tectonically‐exhumed mantle slices or cumulates. Here, we compared Eoarchean phaneritic ultramafic rocks from the Isua supracrustal belt with mantle peridotites, cumulates, and phaneritic ultramafic samples from the Paleoarchean East Pilbara Terrane (Australia), which is widely interpreted to have formed in non‐plate tectonic settings. Our findings show that Pilbara samples have cumulate and polygonal textures, melt‐enriched trace element patterns, relative enrichment of Os, Ir, and Ru versus Pt and Pd, and chromite‐spinel with variable TiO2 and Mg#, and relatively consistent Cr#. Both, new and existing data show that cumulates and mantle rocks potentially have similar whole‐rock geochemical characteristics, deformation fabrics, and alteration features. Geochemical modeling results indicate that Isua and Pilbara ultramafic rocks have interacted with low‐Pt and Pd melts generated by sequestration of Pd and Pt into sulphide and/or alloy during magmatism. Such melts cannot have interacted with a mantle wedge. Correspondingly, geochemical compositions and rock textures suggest that Isua and Pilbara ultramafic rocks are not tectonically‐exhumed mantle peridotites, but are cumulates that experienced metasomatism by fluids and co‐genetic melts. Because such rocks could have formed in either plate or non‐plate tectonic settings, they cannot be used to differentiate early Earth tectonic settings.

The Evolving Chronology of Moon Formation

Defining the age of the Moon has proven to be an elusive task because it requires reliably dating lunar samples using radiometric isotopic systems that record fractionation of parent and daughter elements during events that are petrologically associated with planet formation. Crystallization of the magma ocean is the only event that unambiguously meets this criterion because it probably occurred within tens of millions of years of Moon formation. There are three dateable crystallization products of the magma ocean: mafic mantle cumulates, felsic crustal cumulates, and late-stage crystallization products known as urKREEP (uniform residuum K, rare earth elements, and P). Although ages for these materials in the literature span 200 million years, there is a preponderance of reliable ages around 4.35 billion years recorded in all three lunar rock types. This age is also observed in many secondary crustal rocks, indicating that they were produced contemporaneously (within uncertainty of the ages), possibly during crystallization and overturn of the magma ocean. ▪The duration of planet formation is key information in understanding the mechanisms by which the terrestrial planets formed.▪Ages of the oldest lunar rocks range widely, reflecting either the duration of Moon formation or disturbed ages caused by impact metamorphism.▪Ages determined for compositionally distinct crust and mantle materials produced by lunar magma ocean differentiation cluster near 4.35 Gyr.▪The repeated occurrence of 4.35 Gyr ages implies that Moon formation occurred late in Solar System history, likely by giant impact into Earth.

Alkaline rocks of the Bobaomby volcanic field point to a petrogenetic link between Comoros and northern Madagascar lithosphere

The Bobaomby volcanic field (10–11 Ma) is the northernmost volcanic area of Madagascar, and is a monogenetic volcanic field comprising outcrops of lava flows, dykes, scoria cones, tuff rings and plugs, widely scattered over an area of roughly 500 km2. The volcanic rocks range in composition from nephelinite, basanite and tephrite, through tephritic phonolite, to F- and Cl-rich peralkaline phonolite (MgO from 13 to 0.01 wt%), and the serial affinity varies from sodic to potassic. A few mica-amphibole-rich lamprophyric dykes have tephritic composition and ultrapotassic affinity. The mafic lavas host intrusive xenoliths with evident cumulate features (wehrlites, composite olivine gabbros s.l., amphibole clinopyroxenites and “kaersutitites”), as well as various types of mantle-derived xenoliths and xenocrysts in the most primitive rocks. The very wide compositional variations of the observed phases (olivine, clinopyroxene, amphibole, oxides, feldspars, feldspathoids, apatite, titanite, aenigmatite and other accessories) in lavas, dykes and cognate xenoliths are fully consistent with the variable degree of differentiation of the host lavas/dykes, and pointing out to limited open-system or polybaric crystallization. The mafic lavas have marked enrichment in incompatible elements and light rare-earth element (LREE) (e.g., Lan/Ybn = 19–27), whereas concave REE patterns are found in the peralkaline phonolites, as a result of removal of accessory titanite starting from tephritic phonolite magmas. The gabbroic/ultramafic xenoliths are interpreted as crustal cumulates of basanitic and tephritic magmas. Several liquid lines of descent in the basanites and tephrites are evident from the trace-element distribution, and from the differing geochemistry of the evolved rocks. The isotopic compositions reach extreme values (e.g., 206Pb/204Pb = 20.065 in the ultrapotassic lamprophyre) when compared to the rest of the Cenozoic/Recent Madagascan volcanic rocks, but similar to those of the Comoros archipelago, suggesting analogies of mantle sources and enrichment processes in the lithosphere of this volcanic archipelago. The origin of the Bobaomby mafic rocks is compatible from a derivation from low degree partial melting of an incompatible element-enriched peridotite source (possibly located in the lowermost lithospheric mantle) rich in volatile-rich phases (pargasite, locally also phlogopite and possibly carbonates), matching the sources of other Cenozoic volcanic areas throughout Madagascar, and perhaps Comoros.

Can Post-Subduction Porphyry Cu Magmas Form by Partial Melting of Typical Lower Crustal Amphibole-Rich Cumulates? Petrographic and Experimental Constraints from Samples of the Kohistan and Gangdese Arc Roots

Abstract Sulfide-rich hydrous arc cumulates in the lower crust are considered a likely source for porphyry Cu-forming magmas in post-subduction settings. However, it remains controversial whether or not post-subduction porphyry Cu magmas can be produced by partial melting of lower crustal arc cumulates. Here we performed detailed petrographic and geochemical studies on exhumed lower to middle crustal cumulates from the Gangdese and Kohistan arc sections, and then conducted partial melting experiments of the Gangdese sulfide-rich lithologies at 1.5 GPa and 800°C–1150°C using a piston cylinder press. Both the Gangdese and the Kohistan arc-root samples formed as cumulates in the lower to middle crust, but the Kohistan samples were subsequently brought into the garnet stability field during the India–Eurasia continental collision. Most of the Gangdese samples contain relatively high abundances of magmatic sulfides preserved in the form of Cu-poor monosulfide solid solution. In contrast, most of the Kohistan samples contain very low abundances of Cu-rich sulfides preserved as inclusions in garnet, probably due to intense metamorphic dehydration and up to 2 wt% partial melting in the garnet stability field. The original magmatic sulfides at Kohistan appear to have been removed mainly by metamorphic fluids rather than by partial melts, as the arc-root samples could have experienced only very low degree of partial melting and the sulfur solubility in experimentally produced partial melts is low (≤0.1 wt%) even at oxygen fugacities two log-units above the fayalite-magnetite-quartz buffer. Furthermore, the experimental partial melts contain much less K2O, MgO, Cl and ±S than typical post-subduction porphyry Cu-forming magmas. Hence, we conclude that the role of sulfide-rich arc cumulates in the formation of post-subduction porphyry Cu magmas has been severely overestimated.

Building of arc crust during Silurian subduction in the southern Paleo Asian Ocean: Constraint from mineralogy of the pyroxenitic-gabbroic suite in the Chinese East Tianshan

Arc lower crustal cumulates are significant in investigation of arc differentiation in the lower level of an island arc. The Silurian pyroxenitic-gabbroic suite in the Chinese East Tianshan represents a lithological assemblage of the basal to lower arc crust. This suite comprises, in the order of evolution, olivine websterite (ol-websterite), olivine and plagioclase clinopyroxenite (ol- and pl-clinopyroxenite), and amphibole gabbro (amph-gabbro). Pyroxenite are dominated by cumulus clinopyroxene with interstitial olivine. Amphibole is present in all these rocks, occurring either as coronas of pyroxene and olivine or as oikocrysts, implying a H 2 O-rich parental melt. Plagioclase occurs as veinlets cutting cumulus clinopyroxene in the pl-clinopyroxenite, revealing that it crystallizes totally later than clinopyroxene. Spinel ranges from chromite in the ol-websterite and ol-clinopyroxenite to pleonaste in other evolved rocks, revealing an aluminous differentiation trend. Olivine in the pyroxenite ranges in Fo from 64 to 86 mol%, averagely 7–15 Mg # units lower than coexisting clinopyroxene (79–93). Clinopyroxene shows a chemical trend from low Al 2 O 3 (<2 wt%) in the ol-websterite to high Al 2 O 3 (up to 6.5 wt%) in the pl-clinopyroxenite. All clinopyroxene has incompatible trace elements similar to those of clinopyroxene from typical arc lower crustal cumulates. All plagioclase is extremely calcic (An >83 mol%, up to 99 mol%), generally coupled with iron-rich olivine (Fo <76 mol%) in the studied rocks. Such distinct mineral compositions are typical of arc lower crustal cumulates and ascribed to the crystallization sequence of hydrous arc magmas at high pressure. The pressure for the formation of the pyroxenitic rocks is estimated to be 6–8 kbar using the two-pyroxene geobarometer, a preferred pressure for clinopyroxene crystallization coupled with high H 2 O content. In the stability field of clinopyroxene crystallization, olivine crystallizes from the interstitial melt but later than clinopyroxene, having unexpectedly lower Fo than Mg # of clinopyroxene. Meanwhile, plagioclase crystallization is totally suppressed with the residual magmas evolving towards highly aluminous, crystallizing spinel with a pure pleonaste composition. Upon saturation in plagioclase with decreasing pressures, plagioclase will acquire extremely calcic compositions due to the aluminous and hydrous nature of the residual arc magmas. The incompatible trace element compositions of the Silurian primitive magmas, estimated from the most primitive clinopyroxene (Mg # = 92–93), show selective enrichment in large ion lithophile elements relative to high field strength elements with an exception of depletion in Ba and low Nb/Ta ratios, implying a magmatic deviation from metasomatized mantle modified by aqueous fluids dehydrated from rutile-bearing slab. The Silurian arc lower crust was built by crystal fractionation and accumulation from hydrous arc basaltic magmas at 6–8 kbar of the lower crustal pressure. • The Silurian magmatic suite has mineral compositions similar to arc lower crust. • This magmatic suite formed from arc magmas under arc lower crustal pressures. • Mineral compositions suggest formation of arc lower crust by crystal fractionation.

A Showcase of Igneous Processes in the Urumieh-Dokhtar Magmatic Arc: the Miocene-Quaternary Collisional Magmatism of the Bijar-Qorveh Area, Northwest Iran

Abstract The Cenozoic Arabia–Iran continental collision was associated with emplacement of a large variety of magmatic rock types. This aspect is particularly evident in the Bijar-Qorveh area of NW Iran, where Miocene andesitic to rhyolitic rocks and Quaternary basic alkaline rocks crop out. The Miocene intermediate to acid products show radiogenic Sr and Pb isotopic compositions (87Sr/86Sri 0.70531–0.71109, 206Pb/204Pb 18.71–19.01, 207Pb/204Pb 15.66–15.73, 208Pb/204Pb 38.76–39.14), coupled with unradiogenic Nd isotopic ratios (143Nd/144Ndi 0.51223–0.51265). These characteristics, together with primitive mantle-normalised multielemental patterns resembling ‘subduction-related’ geochemical fingerprints, are considered ultimately derived from the Iranian plate mantle wedge, metasomatised during previous NE-directed Neothetyan Ocean subduction. The alkali-rich andesitic and dacitic rocks evidence both closed- and open-system differentiation, as typically observed for collisional settings in general. Both rock types display a high Sr/Y (37–100) and La/Yb (29–74) ‘adakitic’ signature that it is interpreted here with plagioclase (± amphibole) accumulation or melting of local mafic crustal rocks. Open-system processes involve recycling of crustal cumulates for pyroxene-rich andesite and biotite-rich dacite varieties and low-degree partial melting of the local crust for rhyolites. A radical change occurred during the Quaternary, when SiO2-understaturated to SiO2-saturated poorly evolved rocks (basanites, tephrites, alkaline and subalkaline basalts) were emplaced. The complete change of mantle sources suggests a phase of local extensional tectonics related with WNW-ESE right-transcurrent faults. The major oxide, as well as incompatible trace element and Sr-Nd-Pb isotopic fingerprint of these younger rocks is more akin to that of intraplate magmas, but still bearing some evidences for a variable contribution from a ‘subduction-modified’ mantle source. The NW-trend of increasing involvement of this subduction component is indicative of the strong tectonic control on magmatism. Additional lithotypes indicate the presence of open-system differentiation and remelting processes in the youngest phase of magmatic activity.

Geochemical variability among stratiform chromitites and ultramafic rocks from Western Makran, South Iran

The geochemical compositions of minerals from the Moho transition zone of ophiolites potentially can help to understand the magmatic evolution of the ophiolites, and subsequent mantle-melt interactions. The Jurassic-Late Cretaceous Makran ophiolite of south Iran comprises one of the most extensive tracts of oceanic crust which were scraped off and preserved in an accretionary complex. The Makran ophiolite records traces of MORB-OIB-type magmatism during the Jurassic, but mostly supra-subduction zone magmatic activity during the Late Cretaceous. Despite a few geochemical studies on the crustal rocks, the nature and geochemical signatures of the mantle rocks from this ophiolite remain controversial. The Sorkhband mantle-crust transition zone underlying crustal cumulates in the western Makran consists of stratiform chromitites, harzburgites, chromite-rich dunites and dunites, with crosscutting dikes of olivine websterite and olivine clinopyroxenite. Major- and trace-element compositions of clinopyroxene grains in olivine websterite and clinopyroxenite dikes indicate crystallization from melts similar to boninites and low-Ti fore-arc basalts. Spinel compositions in olivine websterite and clinopyroxenite dikes suggest crystal fractionation from boninitic or high- Mg# magmas have played a major role in the genesis these rocks.We propose a two-stage model for the formation of the Sorkhband dunites including (1) supra-subduction zone-related melt infiltrates through harzburgites in the mantle-crust transition zone to dissolve peridotite orthopyroxene and leave dunites with high forsterite-NiO olivines, and (2) boninitic melts accumulate and react with surrounding harzburgites to crystallize cumulate dunites with low-Mg# olivine and high-Ti spinels. We conclude that there have been temporal changes in the composition of mantle melts in the fore-arc mantle section of the Makran ophiolite during the initial subduction of the Neotethyan ocean beneath the Lut block during the Late Cretaceous.

Ocean crust accretion along a high-temperature detachment fault in the Oman ophiolite: A structural and petrological study of the Bahla massif

The Bahla massif exposes the lower crustal section of the Oman ophiolite located close to the thrust front of the Semail nappe. It is affected by intense faulting previously attributed to tectonic events that dismembered a classical ophiolitic sequence during or after the obduction. Here we show that most of this complexity is primary, inherited from syn-accretion tectonics. The crustal section is exposed in a 15 by 8 km tectonic enclave surrounded by mantle peridotite. Its northern boundary corresponds to a major, steeply dipping normal fault striking WNW-ESE, at low angle to the paleo-ridge axis. Movement along this fault was accommodated by intense plastic deformation of the crustal cumulates and adjacent mantle peridotites at temperature conditions ≥900 °C. The thickness of the deformed zone reaches several hundred meters. The flattening of the cumulate layering away from the fault is correlated to a decrease in the deformation intensity. Undeformed olivine-gabbro dykes cross-cut this “tectonic Moho” indicating that the tilting occurred before the end of the igneous activity. To the southwest, the crustal enclave is bounded by a NW-SE trending transtentional shear zone that was active in the amphibolite to greenschist facies and was intensely injected by syn- to post-kinematic gabbronorite and tonalite/trondhjemite dykes and plugs. The age of one felsic sample (95.214 ± 0.032 Ma, high-precision UPb zircon dating) is within error of the age of intrusive felsic intrusions into the mantle and lowermost axial crust from the length of the Oman ophiolite, which slightly post-dates the mean crystallization age of the Semail crust (V1 magmatism; 96.1–95.6 Ma). Other contacts are low temperature features including cataclastic faults, serpentine‑carbonate breccias and flat-lying décollements.Parent melts of the Bahla crustal cumulates were more siliceous and hydrous, i.e. more andesitic, than typical mid-ocean ridge basalt (MORB) as deduced from the frequent occurrence of early crystallizing orthopyroxene (opx) and late crystallizing amphibole. Some facies such as cumulate harzburgite and opx-troctolite have not been documented elsewhere in the Oman ophiolite and may be specific to the tectonic context in which the frontal massifs accreted. The chemical composition of the lower crustal cumulates can be accounted for by the hybridization in various proportions between MORB and a primitive andesite from a depleted source whose origin can be looked for in melts from a nascent subduction zone or from high temperature hydrothermal processes.The structure of the Bahla lower crustal section is reminiscent of the plutonic growth faults documented along present-day slow-spreading centres in both mid-ocean ridge and back arc settings. The distinctive characteristics of the Moho and lower crustal section in the Bahla massif are tentatively related to their position at the leading edge of the ophiolite, i.e. closer to the Arabian continental margin at the time of accretion than the massifs from the internal part of the ophiolite that have a more continuous and less deformed lower crust. It indicates that the style of crustal accretion may have changed during the opening of the oceanic basin from which the Oman ophiolite issued.

Origin of the J-M Reef and Lower Banded series, Stillwater Complex, Montana, USA

The origin and parental magma for layered cumulates in the Lower Banded series (LBS) and the J-M Reef Pd-Pt deposit of the Stillwater Complex remains poorly constrained. We present whole-rock lithogeochemistry and mineral chemistry from LBS rocks collected from drill holes and surface samples from the Mountain View area of the complex that in total span nearly the entirety of the LBS stratigraphy. Excess S, Pt, and Pd in the noritic and gabbronoritic cumulates of the LBS indicate that small amounts of high tenor sulfide liquid generated at very low degrees of sulfide oversaturation were ubiquitous parts of the cumulate assemblage. We show that a simple two-stage thermodynamic model of assimilation-batch crystallization of a komatiitic parental magma in the lower crust, produces a close match to a common suite of fine-grained gabbronorite dikes and sills that intrude both the complex and its footwall. After fractionating ultramafic cumulates in the lower crust, the model contaminated komatiitic liquid produces upper crustal cumulates by batch crystallization en route to or at the level of the intrusion. The modeled rocks have compositions and mineral assemblages closely resembling pyroxenite of the Bronzitite zone and both norite and gabbronorite cumulates in the lower LBS. The trends from the Bronzitite zone through Norite zone I and Gabbronorite zone I can be understood as the result of deposition of crystals from successive batches of the same contaminated parental magma, with an upward trend toward greater amounts of cooling before the separation of crystals from liquid. The olivine-bearing suite of Olivine-bearing zone I, which includes the J-M Reef, can be modeled by partial remelting of the same norite and gabbronorite cumulates due to a temporarily increased flux of hot, moderately less contaminated LBS parental magma that infiltrated partially molten cumulates because its density exceeded that of the interstitial liquid. This model suggests that infiltration of hot Mg-rich parental liquid into moderately PGE-enriched footwall cumulates may be fundamental to the formation of the extremely high tenor sulfide mineralization in the J-M Reef ore zone, and perhaps other reef-type deposits worldwide. The same metal tenors that would require silicate/sulfide mass ratios (i.e., R-factors) of 105 to 106 in a single stage of equilibration would be attained during this second stage of interaction by the incremental infiltration and passage of LBS parental magma through previously sulfide saturated cumulate mush.

Bulk Composition of Fast-Spreading Oceanic Crust: Insights from the Lower Cumulates of the East Pacific Rise and from Cocos–Nazca Rift Basalts, Hess Deep

Abstract Cores recovered by International Ocean Discovery Program Expedition 345 to the Hess Deep Rift (HDR) include lower crustal cumulates from the East Pacific Rise (EPR) and primitive basalts from the Cocos–Nazca Rift (CNR). This study presents major and trace element compositions of channel samples—the continuous strips of rock removed during core splitting—from this expedition. Consistently high Eu/Eu* anomalies (1·37–5·22) and strong correlations among major element oxides in samples of cumulates indicate that rock composition at the meter scale is controlled by the accumulation and segregation of plagioclase and olivine. However, constant Mg# (82·22 ± 0·66) among 13 samples through a ∼50 m interval suggests that this cumulus was host to percolating, replenishing melt(s). Modeling finds this rock composition to be in equilibrium with melts having Mg# = 58–61. This is identical to the mean value of EPR lavas (57·7 ± 6·2) and suggests that melt buffering by permeable crystal mush is a common and important process in controlling mid-ocean ridge basalt compositions at fast-spreading ocean centers. Analyses of the cumulates provide the most comprehensive composition of in situ, fast-spreading lower oceanic crust currently available. These are compiled with analyses of gabbros, dikes, and lavas from across the HDR to calculate the bulk composition of fast-spreading oceanic crust produced at the equatorial EPR. This bulk composition is strikingly similar to the composition of the primitive basalts from the CNR, and these compositions have nearly identical modeled fractional crystallization histories. Lower abundances of incompatible elements in the primitive basalt suggest that CNR magmatism is the result of the resumption of decompression melting in mantle that previously produced EPR crust. However, higher abundances of chalcophile elements in the CNR basalt point to a diversity of mantle melts that is not evident in calculations of the composition of bulk oceanic crust.

Petrogenesis of the Higashi-Akaishi Ultramafic Body: Implications for Lower Crustal Foundering and Mantle Wedge Processes

AbstractRecycling of ultramafic lower crustal cumulates via delamination or foundering is often invoked as a mechanism to return mafic material to the mantle during continental crust formation. These recycled pieces of the lower crust are rarely sampled but are preserved in several locations including the Kohistan and Talkeetna arc sections, Sierra Nevada and Colorado Plateau pyroxenite xenoliths and, as discussed here for the first time, the exhumed Higashi-Akaishi (HA) ultramafic body in Japan. The HA is located in the Besshi region of the Sanbagawa metamorphic belt in southwestern Japan and is dominantly composed of dunite with lesser garnet pyroxenite and harzburgite lenses. Although the petrogenetic history of the HA body is still debated, our new bulk major and trace element compositions, radiogenic isotope data, as well as petrologic and field observations, are consistent with a lower crustal cumulate origin for the HA dunite and pyroxenite, with a later slab-derived fluid overprint. Clinopyroxene and olivine in the foliated HA dunite have compositions consistent with ultramafic cumulates with high Mg#s (Mg# clinopyroxene = 0·94, Mg# olivine = 0·88), high NiO in olivine (∼0·26 wt %) and low-Al clinopyroxene. In addition, the bulk major element chemistry of the HA dunite and garnet pyroxenite follow systematic behavior in Mg# vs SiO2 wt %, similar to those observed in other lower crustal cumulate lithologies and corresponding intrusive lithologies, pointing to different liquid lines of descent for the corresponding melts. Our new thermobarometric estimates (peak pressure–temperature at 2·6 GPa, 713ºC) are consistent with a hot slab surface subduction path, rather than the lower crustal temperatures recorded in arc sections (Kohistan &amp; Talkeetna: 1 GPa, 800ºC). A pervasive slab-fluid influence is also indicated in the HA lithologies by LREE and Ce enrichments and strong Nb and Zr depletions. The trace elements and the pressure–temperature estimates, as well as the thermodynamic modeling results necessitate removal of the HA body from the lower crust and incorporation into cooler portions of a mantle wedge. At lower crustal conditions, the bulk density of the HA lithologies is greater than the background mantle, indicating the feasibility of lower crustal foundering into a mantle wedge where the HA was incorporated in the subduction channel to reach its peak conditions. Hydration of the HA body while in the subduction channel likely provided the change in density necessary to facilitate its rapid exhumation to the surface. Thus, the HA cumulate likely represents a piece of the subduction system that is rarely preserved, as well as a key component in the compositional evolution of the continental crust.

The importance of H2O in arc magmas for the formation of porphyry Cu deposits

Porphyry copper deposits (PCDs) hosted in subvolcanic intrusions at convergent margins are the primary world’s copper resources. However, the set of magmatic processes that lead to the generation of ore-bearing magmatic provinces remains unclear. In this paper we review the systematic of Cu evolution during arc magma differentiation using new and existing global compilations of whole rock geochemistry data. We trace the Cu evolution from primitive arc magmas through lower crustal plutonic to volcanic rocks. We focus on the well-known tholeiitic and calc-alkaline fractionation sequences, where arc tholeiitic series represents damp primitive melts (<2 wt% H2O) evolving with iron enrichment, and calc-alkaline series are wet primitive melts (>2 wt% H2O) that differentiate with iron depletion.Our study shows that the Cu concentration in primitive arc basalts (~80 ppm) is indistinguishable from that of primitive melts formed at mid-ocean ridges (MORBs) implying that Cu is mainly sourced from the mantle wedge in arcs with a limited contribution from the subducted oceanic lithosphere. A global compilation of plutonic rocks whole rock geochemistry (lower crustal cumulates and derivative melts) indicate no systematic difference in Cu concentrations between cumulates associated with tholeiitic or calc-alkaline series. Yet a complementary global compilation of arc volcanic whole rock geochemistry highlights the contrasting behavior of Cu in tholeiitic and calc-alkaline series during magmatic differentiation. In tholeiitic arc series, Cu shows an incompatible or compatible behavior during magma differentiation influenced by the crustal thickness. In calk-alkaline arc series, Cu is compatible during magma differentiation independently to the crustal thickness. This relates to the timing of sulfide saturation, which is controlled by the liquid lines of descent (LLD) and/or crustal thickness at redox conditions relevant for arc magmas.We demonstrate that the initial melt H2O content in primitive arc melts controls the LLD and the volume of remaining melt mass at fluid saturation. We show that the remaining H2O-saturated melt mass positively correlates with the total mass of Cu transferred into degassing fluids. The mass of extractable Cu ranges from ~3 to ~10 Mt (i.e., large PCD) for calc-alkaline series, and ranges from ~0.3 to ~2.5 Mt for tholeiitic series. The ore-forming potential of calc-alkaline arc magmas is at least ~4 to ~10 times higher relative to tholeiitic arc magmas. Despite the compatible behavior of Cu during magmatic differentiation, we propose that a single stage model for the formation of large economic PCDs (as opposed to multi-stage model for Cu-sulfides storage and remobilization) is most applicable for the calc-alkaline melts. The importance of the initial melt H2O content ultimately reflects the key role of flux melting associated with wet calc-alkaline series and high ore-forming potential, in opposition to decompression melting associated with damp tholeiitic series in arc.

Pyroxenite Xenoliths Record Complex Melt Impregnation in the Deep Lithosphere of the Northwestern North China Craton

AbstractTransformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere.The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ∼90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880–970 oC), and low-Mg# (70–80) plagioclase-bearing websterites with low equilibration temperatures (550–835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr–Nd (εNd = +11·41, 87Sr/86Sr = ∼0·7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ∼0·703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g., FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr–Nd isotopic ratios (εNd = -14·20– -16·74, 87Sr/86Sr = 0·7070–0·7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr = 0·706–0·711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle.Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian Ocean and the Late Mesozoic lithospheric extension of eastern Asia.