Open-system Magma Research Articles

Multi-scale isotopic heterogeneity reveals a complex magmatic evolution: An example from the wallundry suite granitoids of the lachlan fold belt, Australia

Open-system magmatic processes are expected to impart various sorts of isotopic heterogeneity upon the igneous rocks they produce. The range of processes under the “open-system” umbrella (e.g., simple two-component mixing, magma mingling, assimilation with fractional crystallization) cannot usually be uniquely identified using data from a single isotope system. The use of bulk-rock, mineral separate and in situ techniques and multiple isotope systems allows the characterization of isotopic variability at different sampling scales, illuminating details of the petrogenesis of a magmatic system. This approach has been applied to granitoids of the Wallundry Suite in the Lachlan Fold Belt, Australia. The Wallundry Suite exhibits variations in mineral assemblage, mineral composition and trends in bulk-rock major and trace element compositions consistent with the involvement of liquid-crystal sorting processes such as fractional crystallization. In situ paired O-Hf isotope data from zircon in six samples show an array indicating the isotopic evolution of the melt phase. Similarly, bulk-rock Sr-Nd-Hf isotope arrays support open-system magma evolution. These data combined with the petrographic observations and major and trace element geochemical variations suggest some form of assimilation-fractional crystallization process in the petrogenesis of the Wallundry Suite. Added complexity is revealed by two observations: 1) the isotopic variations are only weakly coupled to the lithology and major element compositions of the samples; and 2) there are distinguishable differences between the Hf isotope compositions of bulk-rock samples and those of the magmatic zircons they host. To varying degrees the rocks consistently show negative ΔεHfbulk-zrc values (i.e., the bulk-rock compositions have less radiogenic Hf isotope values than their coexisting zircons). The preservation of distinctly low Nd and Hf isotope ratios in an Fe-Ti oxide mineral separate suggests that the bulk-rock vs. zircon discrepancy is caused by the presence of unmelted components derived from a contaminant of continental origin (i.e., a rock with low Sm/Nd and Lu/Hf and thus unradiogenic Nd and Hf). Evidently, a complex interplay of assimilation, crystallization and melt segregation is required to account for the data. This investigation demonstrates that such complexity can, nevertheless, be disentangled through comparison of complementary isotope data at multiple sampling scales.

Temperature and Hf-O isotope correlations of young erupted zircons from Tengchong (SE Tibet): Assimilation fractional crystallization during monotonic cooling

Young zircons from crystal-poor volcanic rocks provide the best samples for the investigations of pre-eruption magmatic processes and for testing a possible relationship between zircon Eu anomalies and crustal thickness. We report trace element chemistry and Hf-O isotope compositions of young zircons from 3 Holocene volcanoes in the Tengchong volcanic field, SE Tibet, in order to provide insights into magma evolution processes and conditions for high-K calc-alkaline volcanic rocks in a post-collisional setting. As decreasing zircon Ti content and falling temperature, zircon Hf content and Yb/Sm increase whereas zircon Eu anomaly and Th/U decrease, indicating fractional crystallization of plagioclase and zircon during magma cooling. More importantly, zircon Hf isotope ratio (εHf values) increases with decreasing zircon Ti content and falling temperature (T), suggesting gradually increasing incorporation of relatively high εHf juvenile materials in the crystallizing zircons during magma evolution. Negative correlations between zircon εHf and zircon δ18O also support open-system magma evolution. Our data suggest fractional crystallization of a magma with simultaneous contamination by high εHf and low δ 18O juvenile (immature) crustal materials during monotonic cooling after zircon saturation. The low-T, high-εHf and low- δ 18O zircons may indicate the involvement of the early Cretaceous juvenile granitic country rocks during shallow magma evolution. Average Eu anomalies in zircons from young Tengchong lavas yield crustal thickness of 40.7 ± 6.8 km, consistent with present crustal thickness (42.5 km) determined by geophysical methods.

Geochemical and isotopic (Sr-Nd-Pb) signature of crustal contamination in Na-alkali basaltic magmas of South-East Turkey

Widespread sodic alkali basaltic activity developed from Early Miocene to Pleistocene, around the northern termination of the Dead Sea Fault Zone, and in the North-West foreland of the Arabian Plate, in South-East Turkey. Volcanic activity started within the Gaziantep Basin during the Early-Late Miocene (21.2-7.0 Ma), and further East, abundant magmatism brought to the formation of the Karacadağ Shield Volcano, starting from Middle Miocene throughout the Holocene (12.1-0.01 Ma). During the Pleistocene, volcanism developed also around the Anatolia-Africa-Arabia triple junction, along the Karataş-Osmaniye Fault, and along the northern segment of the Dead Sea Fault, in the Karasu Basin.Volcanic rocks emplaced in these areas mainly consist of abundant basalts and basanites, with subordinate hawaiites and basaltic andesites, mostly showing Na-alkaline affinity, whereas some of the older products show calc-alkaline signatures. As a whole, these volcanic rocks show LREE/HREE and HFSE/LILE ratios similar to those of OIB-like magmas. Some trace element and radiogenic isotope variations are observed amongst the less differentiated samples and they are explained by partial melting of a heterogeneous subcontinental mantle source at variable depths, mostly in the garnet stability field.The largest compositional variability observed, however, is attributed to differentiation processes, resulting in quite remarkable chemical and isotopic variations (e.g., 87Sr/86Sr = 0.70301-0.70529; 143Nd/144Nd = 0.51263- 0.51294; 206Pb/204Pb = 18.78-19.47; 207Pb/204Pb = 15.57-15.72; 208Pb/204Pb = 38.75-39.27). This implies the occurrence of open-system magma evolution, characterised by crystal fractionation at low pressure of mineral assemblage made up by olivine ± plagioclase ± clinopyroxene + apatite + Fe-Ti oxides, coupled with assimilation of crustal materials. The extent and the chemical modifications induced by this process are such that in some of the oldest and most evolved samples, the pristine Na-alkaline affinity is obliterated, and a secondary subalkaline signature is overprinted. This effect is less evident or negligible in more recent lavas, which preserve their primary features, being erupted in a well-developed fault system or in a long lasting, chemically homogeneous, magmatic feeding system.The magmatism studied here is quite unusual and not frequent in the frame of Na-alkaline intraplate volcanism, being characterised by abundant intermediate products and by evolutionary processes strongly affected by crustal contamination. In our view, this is due to the peculiar setting of the region, which is the locus of a significant mantle upwelling, accompanied by very limited or absent extensional rate.

Magmatic erosion of high-temperature-melting cumulates in the Bushveld Complex by chemical dissolution

• At least 10–20 m of floor cumulates are eroded away in the Bushveld complex. • These cumulates have very high melting temperatures (∼1400–1500 °C). • Low-temperature (∼1200 °C) replenishing melts cannot melt such cumulates. • The cumulates can, however, be chemically dissolved in these melts. • The replenishing melts were likely slightly superheated. The Bushveld Complex in South Africa shows spectacular examples of regional and local magmatic erosion of the floor cumulates by new melt batches that replenished the evolving magma chamber. Field observations indicate that, at some stratigraphic levels, at least, 15–20 m of pre-existing floor cumulates, often nearly monomineralic in composition (e.g., anorthosite or orthopyroxenite) were completely removed on a regional scale. What was the major agent of this erosion – (partial) melting or dissolution of the floor cumulates – remains poorly understood. Thermal melting appears to be a poor candidate because normal basaltic melts (∼1220–1260°C) cannot heat up the cumulates up to their melting temperature (∼1400–1500°C). We explored, therefore, the possibility of dissolution of these high-temperature-melting cumulates by slightly superheated (15°C above the liquidus) basaltic-andesitic melts that recharged the chamber and spread out laterally along its floor as basal flows. This was done using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems – the Magma Chamber Simulator. Our thermodynamic modeling shows that the superheated melts can digest up to 4.5–8.0 wt% of the bulk floor cumulates without inducing crystallization of the melts, despite them being much colder than the liquidus temperatures of these cumulates. This is equivalent to regional erosion of 15–24 m of the floor cumulates, given a basal melt layer of about 350 m thick. We conclude that the regional erosion of the high-temperature-melting floor cumulates in the Bushveld chamber has been mostly controlled by their chemical dissolution by replenishing superheated melts.

Chromite Composition and Platinum-Group Element Distribution in the Proterozoic Chimalpahad Anorthosite Complex, South India: Implications for Magmatic Processes and Discrimination of Tectonic Setting

Abstract The Proterozoic Chimalpahad Anorthosite Complex (CAC) is a deformed and dismembered complex in South India comprising of anorthosite-leucogabbro-gabbro-ultramafic rocks. It is located in the Nellore-Khammam Schist Belt (NKSB) within the contact zone between the Eastern Dharwar Craton (EDC) and the Eastern Ghats Belt (EGB). The chromitites occur as small dismembered lensoidal bodies within the ultramafic rocks. Presence of micrometric inclusions of iridium-like platinum-group minerals (IPGM), such as laurite (RuS2), erlichmanite (OsS2), irarsite (IrAsS), and Ir-Os-Ru alloys, is reported for the first time in the CAC chromites. The chromite cumulates preserve magmatic character (Cr#: 0.61 to 0.77; Mg#: 0.29 to 0.55), while disseminated chromites and rims of some grains (Cr#: 0.59 to 0.89; Mg#: 0.10 to 0.25) show postmagmatic modifications. The chromites are characterised by higher Cr and Al contents and lower Fe and Ti contents. The mineral chemistry of chromites such as Cr# and Mg# contents of Al2O3 (10.05–20.07 wt.%) and TiO2 (0.1 to 0.25 wt.%) typically suggests that the CAC chromitites and their ultramafic hosts were generated from the S-undersaturated boninite melt in a magmatic arc in suprasubduction tectonic setting. A general negative slope of bulk-rock PGE distribution with upward convex pattern is observed for the CAC chromitites in the chondrite-normalized plot. These geochemical and mineral chemical characters in addition to location of the chromitites and the CAC in the tectonic zone between Eastern Dharwar Craton and Eastern Ghats Mobile Belt point towards ophiolitic affinity of the CAC chromitites. Laurite (RuS2) grains are trapped close to the cores of chromites, whereas erlichmanites (OsS2) are trapped away from the cores. Textural relations indicate primary orthomagmatic nature of laurite and Or-Ir-Ru alloys and their precipitation prior to or concomitant with chromite from an S-undersaturated melt at ~1200–1300°C and log fS2 varying between −2 and −1.3. Ru-Os disulfide and erlichmanite crystallized subsequently with a gradual decrease in temperature in the differentiating ultramafic magma. Precipitation of irarsite and base-metal sulfides can be attributed to local and rapid fluctuations of temperature, fS2, and fAs during the formation of the chromitites due to influx of fresh batch of melts or crystallization in an open magmatic system. The compositional variations of laurite-erlichmanite record heterogeneous physicochemical environment during their crystallization. In addition to zoning, preservation of IPGMs in the CAC chromites points towards fast cooling of the magma possibly associated with rapid exhumation of the chromitites and their ultramafic hosts from the roots of the subcontinental lithospheric mantle (SCLM) towards their final emplacement into the crust. Occurrence of the CAC along a linear extension with the Kondapalli Ultramafic Complex and deformed remnants of ophiolites (Kandra and Kanigiri complexes) along the eastern margin of the Eastern Dharwar Craton (EDC) bordering the Eastern Ghats Belt (EGB) indicates this zone to be a Proterozoic convergent margin.

Complex Effects of Assimilation on Sulfide Saturation Revealed by Modeling with the Magma Chamber Simulator: A Case Study on the Duluth Complex, Minnesota, USA

Abstract Wall-rock assimilation can cause effective sulfide saturation in magmas and lead to the formation of base and precious metal sulfide deposits. Detailed assessments of how assimilation affects the sulfur content at sulfide saturation (SCSS) in magmas have been scarce because of the lack of suitable thermodynamic modeling tools. The Magma Chamber Simulator (MCS) is the first geochemical modeling software that accounts for thermodynamic wall-rock phase equilibrium in open magmatic systems experiencing recharge-assimilation-fractional crystallization. We used the MCS to model SCSS in a magmatic system corresponding to the parental melt of the Partridge River intrusion of the Duluth Complex, Minnesota. This intrusion hosts several Cu-Ni deposits in troctolitic and noritic rocks, which both show evidence of assimilation of the adjacent Virginia Formation black shale. Our simulations show that the dominantly troctolitic rocks can form via fractional crystallization if the parental melt is hydrous (≥ 1 wt % H2O), while gabbroic rocks dominate when the parental melt is H2O poor (≤ 0.14 wt % H2O). Formation of norite from the hydrous parental melt requires ~20–30% of selective assimilation of black shale partial melts or bulk assimilation of stoped blocks. In the fractional crystallization simulations, increasing the H2O content of the parental melt lowers SCSS. In the hydrous fractional crystallization scenarios, SCSS is lowered further by the depletion of FeO from the residual melt, owing to enhanced olivine stability. In the assimilation simulations, the residual melt in the magma subsystem becomes enriched in SiO2, Al2O3, K2O, and H2O with simultaneous depletion in FeO, MgO, CaO, and Na2O. These compositional changes promote sulfide saturation—an effect that is more pronounced in selective rather than in bulk assimilation scenarios. Trace element models, used as a proxy for the efficiency of sulfur assimilation, show that sulfur should behave as an incompatible element (DWR (S) ≤ 1) to wall rock in the selective assimilation simulations, i.e., enriched in early-assimilated wall-rock fluids and/or partial melts, in order to fulfill the natural sulfur isotope criteria of the Duluth Complex. Bulk assimilation may also be efficient enough to modify the sulfur isotope composition, but it requires a large amount of crystallization in the magma and is, hence, considered less likely to be the main process for sulfur assimilation. If wall-rock sulfur is effectively transported to the magma, in situ precipitation of sulfides without notable subsequent upgrading by dynamic processes could produce the sulfide grade of an average Cu-Ni deposit in the Partridge River intrusion.

Magma evolution of the South China Sea basin from continental-margin rifting to oceanic crustal spreading: Constraints from In-situ trace elements and Sr isotope of minerals

The South China Sea (SCS) has undergone a complete Wilson cycle. We studied the volcanic rocks from IODP Sites U1502, U1500, U1431, U1433, and U1434 that record the exact mantle dynamics of the SCS basin from continental rifting to oceanic crustal spreading. We present in-situ major and trace element data for minerals (olivine, clinopyroxene, and plagioclase) and melt inclusions; and in-situ Sr isotopic ratios for plagioclases from the volcanic samples. All SCS volcanic rocks belong to the tholeiite series. Most Site U1431 basalts are slightly depleted in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and similar to that of normal mid-oceanic ridge basalts (N-MORB). All clinopyroxenes and olivines, and the calculated equilibrium melts of clinopyroxene cores display depletion of LREEs and enrichment of heavy rare earth elements (HREEs) similar to N-MORB-type primary magma. Thus, these volcanic rocks are likely formed by the extension decompression partial melting of shallow depleted magma. The Sites U1502, U1500, U1433, and U1434 bulk-rocks and melt inclusions show flat LREE and HREE patterns similar to that of enriched MORB (E-MORB), likely attributed to the partial melting of a depleted mantle source with different degrees of lower continental crustal contamination. The increase in 87Sr/86Sr ratios from core to rim in Site U1500 plagioclases further prove the crustal contamination during magmatism. Site U1431 clinopyroxene and Site U1500 plagioclase show normal zoning textures and slight enrichment of incompatible elements in their rims but depletion in their cores, likely indicating significant crystallization of late-stage differentiated melts. Although some clinopyroxene and plagioclase were likely crystallized in a relatively closed environment, the existence of melt inclusions and the large Sr isotopic variations between the plagioclase rim and groundmass suggest a dynamic and open magmatic system that likely experienced melt–rock interactions and magma mixing processes. Melt supply rate changes from low to high during the SCS evolution process from initial rifting (Site U1500) to later spreading (Sites U1431 and U1433) indicating that: (1) the number of high-An plagioclase-hosted melt inclusions has gradually decreased; and (2) the zoning textures of clinopyroxene and plagioclase have gradually changed from simple to complex. The influence of mantle plumes and the upwelling of fertile subduction-related asthenospheric magma were negligible during the continental-margin breakup and initial SCS spreading, but played an important role during the late SCS spreading period, as revealed by the variation in Sr isotopic compositions of high-An plagioclase cores, the different mantle potential temperatures and source lithologies.

Thermodynamic limits for assimilation of silicate crust in primitive magmas

Abstract Some geochemical models for basaltic and more primitive rocks suggest that their parental magmas have assimilated tens of weight percent of crustal silicate wall rock. But what are the thermodynamic limits for assimilation in primitive magmas? We pursue this question quantitatively using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems—the Magma Chamber Simulator (https://mcs.geol.ucsb.edu)—and focus on modeling assimilation of wall-rock partial melts, which is thermodynamically more efficient compared to bulk assimilation of stoped wall-rock blocks in primitive igneous systems. In the simulations, diverse komatiitic, picritic, and basaltic parental magmas assimilate progressive partial melts of preheated average lower, middle, and upper crust in amounts allowed by thermodynamics. Our results indicate that it is difficult for any subalkaline primitive magma to assimilate more than 20−30 wt% of upper or middle crust before evolving to compositions with higher SiO2 than a basaltic magma (52 wt%). On the other hand, typical komatiitic magmas have thermodynamic potential to assimilate as much as their own mass (59−102 wt%) of lower crust and retain a basaltic composition. The compositions of the parental melt and the assimilant heavily influence both how much assimilation is energetically possible in primitive magmas and the final magma composition given typical temperatures. These findings have important implications for the role of assimilation in the generation and evolution of, e.g., ultramafic to mafic trans-Moho magmatic systems, siliceous high-Mg basalts, and massif-type anorthosites.

A Pilbara perspective on the generation of Archaean continental crust

Models for the generation and evolution of continental crust are informed by a detailed understanding of regional geology and geochemistry. This contribution therefore evaluates the increasing amounts of high quality geochemical data, on well-characterized samples, now available from the magmatic rocks of the Pilbara Craton, Western Australia. The East Pilbara Terrane is characterized by vertical dome and basin tectonics, and the younger (Mesoarchean) rocks to the west are associated with lateral tectonics. The craton stabilized at ~2.95 Ga (Hickman, 2012), and it was overlain by voluminous, dominantly mafic lavas of the Fortescue Group in an intra-cratonic setting at ~2.7 Ga. The magmatic rocks of the Pilbara Craton are bimodal in SiO2, with the volcanic rocks typically having <60% SiO2, and the plutonic rocks >60% SiO2, and such differences are in part linked to different depths of magma emplacement.The volcanic rocks are divided into two groups, with Dy/La > and < 0.6. The geochemistry of the former group is consistent with broadly closed system differentiation, whereas the low Dy/La rocks largely reflect open system magmatic processes, and contributions from high silica magmas. The high Dy/La rocks have restricted Th/Nb ratios similar to those of primitive mantle, whereas the low Dy/La rocks range to higher Th/Nb values. In most cases these higher Th/Nb values reflect open system magma differentiation, but in some of the younger volcanic suites (the Bookingarra Group and Cooneeina Basalt) these values appear to be associated with subduction. There is no evidence for garnet or amphibole in the generation of the volcanic rocks, however the composition of the plutonic rocks reflects the involvement of both garnet and plagioclase, during melt generation and differentiation.The high Dy/La rocks have Sm/Nd ratios similar to CHUR, but their Lu/Hf ratios are significantly sub-chondritic. Many of the Pilbara rocks from 3.5–3.0 Ga were generated from mantle sources with broadly chondritic Hf isotope ratios, and so in the preferred model the mean composition of the high Dy/La group was generated by ~20% melting of chondritic mantle. Apart from the high Th/Nb magmas at 3.05–2.97 Ga (Smithies et al., 2005), there are surprisingly few changes in the minor and trace element features of the magmas associated with the generation of the craton (3.5–3.0 Ga), and after it stabilized at ~2.95 Ga. The 3.05–2.97 Ga plutonic rocks in the granitic complexes also have elevated Th/Nb, suggesting that such ratios were inherited from their mafic source rocks. This observation implies that the mafic sources and the granitic rocks were of similar age, and that there was rapid conversion of young mantle-derived basaltic crust into felsic continental crust, with little contribution from older basement material. The volcanic rocks are characterized by relatively low water contents (~<2%) on the basis of their low maximum Al2O3 values, a mean Fe3+/ΣFe of 0.19 ± 0.01, and models in which the high Dy/La rocks were derived from primitive mantle. Such low water contents relative to modern subduction-related magmas are a feature of the magmatic rocks of the Pilbara Craton, and the development of ‘granitic crust’ appears to be linked to the development of higher water contents associated with subduction, locally commencing at ca. 3.1 Ga, and to globally dominant plate tectonics.

Structure and composition of picritic gabbro-dolerites of the Оctyabrskoye deposit central part

Issues relating to the genesis and deposit formation conditions in the Norilsk region are still debatable. At present, ore formation model in an open magmatic system is most popular. To study the actual process, the structure and composition of sulfide minerals in the Oktyabrskoye deposit central part were examined in two boreholes, RT-30 and RT-107, that reveal orebodies C-3 and C-4, respectively. Particular attention was paid to the structural patterns of disseminated ores in picritic gabbro-dolerites horizon. It has been established that sulfide inclusions are missing in the upper part of the horizon and their size and quantity gradually increase in depth, so that these inclusions grade into sulfide streaks in the lower part of the horizon. This indicates gravitational sulfide deposition in a closed magmatic system.

The basal dunite of the Precambrian mafic-ultramafic N\xe4r\xe4nk\xe4vaara intrusion: Petrogenetic considerations and implications to exploration

Several mafic-ultramafic layered intrusions were emplaced in the Fennoscandian Shield during wide-spread mantle-sourced magmatism at 2.5–2.4 Ga. The Näränkävaara intrusion (surface area 5 × 30 km2), northeastern Finland, contains a 1.5–2 km thick basal dunite (not dated), and a 1.5 km thick layered series (2436 ± 5 Ma). A newly discovered marginal series between the basal dunite and the layered series indicates that the basal dunite is older, and highlights the need for further study on their relationship. Along its southern basement contact, the basal dunite contains a 200–300 m thick zone of olivine ortho- and mesocumulates, but the bulk of it is composed of olivine adcumulates and lesser olivine-orthopyroxene heteradcumulates. Based on whole-rock geochemistry, the basal dunite is divided into a low-Fe zone (average FeOt 10.2 wt% and Ni 2250 ppm) and a high-Fe zone (average FeOt 12.5 wt% and Ni 1700 ppm). Both zones have high MgO (32–47 wt%) and varying Cr (830–5160 ppm) and Al2O3/TiO2 (16–26). Textural and geochemical layering is similar along the 30 km strike of the basal dunite. A LREE-enriched high-MgO basaltic parental magma composition (13–18 wt% MgO) is inferred for the basal dunite from olivine–melt mixing trends in orthocumulates. The dunite exhibits at least two geochemical reversals as well as abundant low-porosity adcumulates, poikilitic chromite, and bimodal olivine, suggesting formation in a high-volume open magmatic system. Significant similarity in major and trace element compositions with the Näränkävaara layered series and the Burakovsky intrusion and Vetreny belt extrusives in Russian Karelia suggests that the basal dunite belongs to the Fennoscandian 2.5–2.4 Ga mafic layered intrusions. An Archean komatiitic origin for the dunite body cannot be completely ruled out, however. Distinct Ni-depletion in olivine is found in the basal dunite from the low-Fe zone to the high-Fe zone (3200 versus 2200 ppm). This depletion does not correlate with Fo contents, which suggests that it is not related to olivine fractionation. The basal dunite may thus have potential for Ni-(Cu-Co-PGE) sulfide mineralization.

Magmatic Evolution following Damp Tholeiitic and Wet Calc-alkaline Liquid Lines of Descent: an Eastern Pontides (NE Turkey) Example

AbstractAssociations between tholeiitic and calc-alkaline arc magmatism with close spatial and temporal relationships can provide critical constraints on magma genesis and allow the reconstruction of subduction polarity at convergent margins. This study identifies two compositionally distinct intrusive series from the Yusufeli region in the Eastern Pontides arc, NE Turkey. The intrusive rocks from the Yusufeli intrusive complex were emplaced at 179–170 Ma and are dominated by the low- to medium-K tholeiitic series, with depleted Hf isotopic compositions. In contrast, the intrusive rocks from the Camlikaya intrusive complex were emplaced at 151–147 Ma and are characterized by the medium- to high-K calc-alkaline series, with relatively enriched Hf isotopic compositions. The Al-in-hornblende geobarometer reveals that the magmas of both intrusive complexes crystallized at upper crustal levels (∼150–250 MPa, ∼5–8 km). The presence of patchy-textured plagioclase and the widespread occurrence of coeval dykes and magmatic mafic enclaves indicate that the two intrusive complexes are derived from multiple magma pulses in open magmatic systems. The mineral crystallization order of amphibole and plagioclase, the trace elemental signatures (e.g. Sr/Y and Y), and rare earth element modeling collectively suggest that the Yusufeli intrusive complex was dominated by plagioclase and clinopyroxene fractionation with earlier plagioclase crystallization than amphibole, whereas the Camlikaya intrusive complex was dominated by the fractionation of amphibole accompanied by co-crystallization of plagioclase. Such significant differences in the fractionating mineral assemblages at comparable intrusion pressures can be attributed to different initial H2O contents of the Yusufeli and Camlikaya parental magmas, which ultimately control their distinct liquid lines of descent. In accord with thermodynamic modeling results derived using the Rhyolite-MELTS software, we propose that the Yusufeli intrusive rocks are derived from damp (∼1–2 wt% H2O) parental magmas formed dominantly by decompression melting of mantle wedge in a back-arc setting. In contrast, the wet parental magmas (&amp;gt;∼2 wt% H2O) of Camlikaya intrusive rocks are more hydrous and formed through flux melting of suprasubduction-zone mantle wedge. This conclusion, combined with the back-arc basin related Jurassic sedimentary and structural records previously determined in the Southern Zone of the Eastern Pontides, indicates that the geochemical compositions and spatial relationship of the Yusufeli and Camlikaya intrusive complexes are preferably explained by the southward subduction of the Paleotethys oceanic lithosphere in the Early to Late Jurassic.

Insights Into Magma Chamber Processes From the Relationship Between Fabric and Grain Shape in Troctolitic Cumulates

The strength of foliations defined by shape preferred orientation of plagioclase in troctolitic cumulates from the Layered Series of the Skaergaard intrusion, and the Rum Eastern Layered Intrusion, increases as the grains become more tabular, due either to the greater propensity of highly non-equant grains to be re-arranged by magmatic currents or tectonic disruption of poorly consolidated mush, or by the effects of a pre-existing shape preferred orientation on final grain shape in fully solidified rocks. The stratigraphic evolution of grain shape, microstructures and fabrics in the lowest 320 m of the Skaergaard Layered Series records the progressive inflation of the chamber to its final size. During the earliest stages of solidification, the extent of in situ nucleation and growth on the chamber floor decreased upward through the stratigraphy, due to the development of a thermally insulating blanket of mush on the floor. An upward increase in foliation strength as the chamber inflated to its final size was a result of the increasing strength of convection of the bulk magma and an increasing contribution to the floor mush of crystals derived from the walls of the enlarging magma chamber. Plagioclase in the troctolites in the open-system magma chamber of the Rum Eastern Layered Intrusion is generally more equant than that in the Skaergaard intrusion, perhaps related to the slower crystal growth on the margins of the continuously replenished Rum chamber. Significant sub-solidus modification of original igneous microstructures is observed in Rum troctolites from parts of the stratigraphy recording frequent replenishment events.

Geochemistry and Texture of Clinopyroxene Phenocrysts from Paleoproterozoic Picrobasalts, Karelian Craton, Fennoscandian Shield: Records of Magma Mixing Processes

This paper presents an integrated major and trace element data and crystal size distribution analysis for zoned clinopyroxene phenocrysts hosted in variolitic and massive picrobasalts of the Suisaari Formation, Karelian Craton, Eastern Fennoscandian Shield. Clinopyroxenes in variolitic and massive lavas occur as unzoned, reverse, and normally zoned crystal. Oscillatory-zoned clinopyroxenes are only observed in variolitic lavas. The obtained data were examined in order to evaluate the contribution of magmatic processes such as magma mixing, contamination and fractional crystallization to the formation of various zoning patterns of clinopyroxene phenocrysts. Clinopyroxene phenocrysts in both variolitic and massive lavas originate from similar primary melts from a single magmatic source. The obtained data on composition and texture of clinopyroxene phenocrysts together with the crystal size distribution (CSD) analysis suggest that crystallization of the massive lavas mainly involves fractionation in a closed magmatic system, whereas the crystallization of the variolitic lavas is determined by processes in an open magmatic system. The results provide novel information on the evolution of Paleoproterozoic magmatic systems in the Karelian Craton.

The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the Mismatch Between Erupted Mid-Ocean Ridge Basalts and Lower Crustal Gabbros

The trace element and isotopic compositions of mid-ocean ridge basalts (MORB) provide an important cornerstone for all studies seeking to understand mantle evolution. Globally there is a significant over-enrichment in the incompatible trace element concentrations of MORB relative to levels which should be generated by fractional crystallization. Thermal and geochemical constraints suggest that MORB require generation in open system magma chambers. However, the petrology of lower oceanic crustal rocks suggests instead that these enrichments maybe formed through reactive porous flow (RPF). Stable isotope compositions are process dependent and therefore provide an excellent mechanism to compare these contrasting models. This study presents the first neodymium (Nd) stable isotope compositions of Indian MORB and well characterized gabbroic rocks from the lower oceanic crust sampled at the Southwest Indian Ridge (Hole 735B). Indian MORB is extremely homogenous with a mean δ146Nd of −0.025 ±0.005‰ which is identical to the composition of Pacific MORB. Despite significant variability in the source composition of MORB globally (i.e. 143Nd/144Nd) their indistinguishable δ146Nd compositions suggests they were homogenized through the same process along the global ridge network. In stark contrast, oceanic gabbros have δ146Nd ranging from −0.026‰ to −0.127‰, doubling the natural variability in Nd stable isotopes observed in terrestrial rocks. Clinopyroxene separates possess variable δ146Nd but are isotopically heavier than the gabbroic whole rocks at the same major element compositions. These large variations in δ146Nd cannot be generated solely by the fractionation or accumulation of clinopyroxene and/or plagioclase. Hole 735B preserves widespread evidence of RPF which could induce kinetic isotopes fractionation during crystal growth. In clinopyroxene kinetic isotope fractionations will only induce ca. 0.02‰ variations therefore several cycles of dissolution and reprecipitation of isotopic signatures at grain boundaries are required to explain the range of δ146Nd observed in the gabbros. Given the large disconnect between the average composition of the lower crust (δ146Nd = −0.076‰) and MORB globally and the evidence of limited melt extraction into the upper crust at Hole 735B it is highly unlikely that the melts involved in RPF contributed in a substantial way to the Nd isotope composition of erupted MORB.

Geochemistry and geochronology of the shallow-level La Esperanza magmatic system (Permian-Triassic), Northern Patagonia

The La Esperanza plutonic-volcanic complex is the largest Late Paleozoic-Early Triassic composite magmatic system of northern Patagonia. This paper reports new SHRIMP U-Pb zircon ages and K-Ar muscovite dating as well as whole-rock geochemical data for selected units. In addition, we present some new and reprocessed whole-rock Sr isotopic compositions. On the basis of the new and published data, three compositionally and isotopically distinct high-K magnesian calc-alkaline series were distinguished. Two of these are characterized by high Ba-Sr: (i) biotite and muscovite bearing rhyolites and granites (265 ± 2 Ma; 260 ± 2 Ma) and (ii) metaluminous amphibole-biotite bearing granodiorites (273 ± 2 Ma), monzogranites (255 ± 2 Ma), dacites (253 ± 2 Ma), and slightly peraluminous granites (dated herein as 251 ± 2 Ma). There is also a low Ba-Sr series of high-silica metaluminous rocks (granites and acid dike swarms; 250 ± 2 Ma and ≈244 ± 2 Ma). Geochemistry coupled with geochronology revealed a pulsatory multi-sourced open magmatic system with mafic magma replenishment and reactivation of crystal mushes that occurred before upward migration to upper crustal levels. Mafic magmas alternated with crust-derived magmas incrementally assembled in subvolcanic levels over 30 Ma. Zircon crystallization and mica cooling ages in the granite units allowed detection of two magmatic lulls, between 270 and 265 Ma and between 260 and 255 Ma. Both episodes coincide with a period of exhumation in upper crustal levels. The new temporal and geochemical constraints allow correlation of the La Esperanza plutonic-volcanic complex with the Los Menucos Group (258-248 Ma), encompassing a volume of magmatism comparable to a moderately sized large igneous silicic province. These mid-to-late Permian to Middle Triassic rocks record the transition between subduction-related magmatism (>273 Ma) and post-orogenic extensional magmatism (<250 Ma) in the Gondwana margin. Even though this magmatism would be coeval with the proposed collision of the Patagonia terrane, no expected syn-collisional magmatism or associated deformation were found in upper crustal levels. However, the different nature and melting conditions of the inferred sources of the magmas that crystallized before 270 Ma, between 265 and 260 Ma, and from 255 to 245 Ma, suggest that the La Esperanza plutonic-volcanic complex was assembled during a 30 Ma period of major plate reorganization.

Interplay between volcanic and plutonic systems: A case study of the early Miocene Solarya Volcano-plutonic Complex in NW Anatolia (Turkey)

Linking volcanic and plutonic systems and examining them, together is critical to evaluate magma chamber processes and mantle-crust, interactions in lithospheric scale. Solarya Volcano-plutonic complex, (SVPC) is one of the major volcano-plutonic complexes located in NW, Turkey and it is remarkable for its coeval and cogenetic plutonic, hypabyssal and volcanic rocks. Solarya pluton (22 Ma) is an epizonal, pluton which consists of three main rock groups; K-feldspar, megacrystalline granodiorite, microgranodiorite and haplogranite. Plutonic rocks pass into hypabyssal association, represented by, granodiorite/quartz-diorite porphyry sheet intrusive rocks which were, injected into the ring and radial faults concurrently with the, emplacement of the pluton. Hypabyssal and volcanic rocks cut one another, and fragments of hypabyssal rocks are found in volcanic rocks. Volcanic, association, is formed from trachyandesite, andesite and dacite lavas, with associated pyroclastic rocks. New 40Ar/39Ar radiometric ages of, volcanic and hypabyssal associations display that, they have 40Ar/39Ar, biotite ages changing from 23.1 ± 0.2 to 21.0 ± 0.2 indicating that they, are coeval with Solarya Plutonic Association. Petrographical features of, SVPC display disequilibrium textures such as oscillatory zoning, sieve, and honeycomb textures in plagioclase, reverse mantled biotite and, hornblende crystals as indicatives of open system magma chamber processes, (mixing/mingling and/or AFC). Plutonic, hypabyssal and volcanic, associations of SVPC show similar major-trace and Sr-Nd-Pb-O isotopic, compositions indicating a common magma source and similar magma chamber, processes. They are medium to high-K calc-alkaline rocks and display, enrichment in LILE and depletion in Nb, Ta, P and Ti. The geochemical and, isotopic characteristics of SVPC are considered to reflect an enriched, mantle (EMII) or sub-continental lithospheric mantle source. Bulk mixing, model between ƐNd and 87Sr/86Sr isotopic data suggests that, the crustal, contribution to the enriched mantle source is about 25–45%. Evaluation of all these data suggests that, Solarya Volcano-plutonic, Complex with early Miocene age was developed under syn-convergent, extensional regime concurrent with the exhumation of other major volcanoplutonic, and core complexes (eg. Kozak, Menderes, Kazdağ, Çataldağ) in NW, Anatolia.

Chemical mass balance equations for open-system magma chamber processes that result in crystal zoning

Recent advances in micro-analysis have revealed that isotopic zoning profiles in igneous crystals reflect progressive changes in liquid composition during open-system magmatic processes. Consequently, the isotope ratio of the integrated (bulk) crystals and that of liquid are inevitably different. This study develops a mass balance model to describe the compositional relationships between the bulk crystals (total suspended crystals), liquid, magma (crystals plus liquid), and crystal rims during open-system magma chamber processes. The model incorporates the effects of concurrent magma recharge (or mixing), assimilation, magma extraction, and fractional crystallization. The mass balance differential equations for suspended crystals and liquid are solved simultaneously. The analytical solution of the equations gives a quantitative account of the evolution paths of trace elements and isotopes within total suspended crystals, liquid, magma, and crystal rims. The isotopic zoning profiles in phenocrysts vary markedly depending on the relative rates of recharge, assimilation, magma extraction, and fractional crystallization. The model can reproduce the 87Sr/86Sr zoning patterns in natural feldspars from different magma systems.

Geology of the western flanks of the Oktyabr’skoe deposit, Noril’sk district, Russia: evidence of a closed magmatic system

The Oktyabr’skoe deposit in the Northern Siberian Platform is one of the world’s largest deposits in terms of platinum-group elements and nickel reserves. The genesis of this deposit has been inferred from the inner geological structure of its central portion and remains a matter of long-lasting and heated discussions. The current dominant hypothesis is that the ores were produced in an open magmatic system. This study is devoted to only one aspect of the complicated problem of ore formation, i.e., the geological structure of the Kharaelakh intrusion to which the huge orebodies are related. Recently, new data have been acquired on the structure and composition of the western flanks of the Kharaelakh intrusion, which pinches out in this direction and does not coincide with basalts as has been suggested in many publications. A distinctive feature of this part of the deposit is that it is cut by numerous relatively thin intrusions of contrasting compositions (from picritic gabbro-dolerite to leucogabbro) that are spatially separated in contrast with the central portion of the deposit, where these rocks occur as discrete layers of a single intrusion. These bodies taper off westward and are “blind” apophyses of the intrusion to which various ore types are related. The inner structure of this part of the vertical section of the intrusion is analyzed based on detailed data on the core material recovered by a number of boreholes and on the chemical and mineralogical characteristics of the ores, which differ from those in the main orebodies in that they possess lower Cu and PGE concentrations (up to 3.5 wt% and 9 ppm, respectively) and hence result in the dominance of pyrrhotite and limited amounts (and smaller grain sizes) of platinum-group minerals. Peripheral sills of the Kharaelakh intrusion were formed by portions of initial magma with different melt/cumulus minerals proportions, forming separate bodies of picritic and olivine-bearing gabbro-dolerite. The last rock varieties comprise disseminated ore, in contrast with the central parts of the intrusion, where these ores are located in the picritic and taxitic horizons. No peripheral sills or dikes connected with lava flows have been found inside the studied area. These facts evidence ore formation in a closed magmatic system.

Molybdenum isotope variations in calc-alkaline lavas from the Banda arc, Indonesia: Assessing the effect of crystal fractionation in creating isotopically heavy continental crust

Recent studies report a large Mo isotope variability of up to 1‰ (expressed in δ98/95MoNIST3134) in convergent margin lavas. These isotopic variations have been associated with subduction zone processes and ultimately may account for heavy and variable isotope signatures in evolved continental crust. Arc lavas show both lighter and heavier Mo isotopic values when compared to the chondritic average (δ98/95MoNIST3134 −0.16 ± 0.02‰), with a concentration weighted mean isotopic composition (δ98/95MoNIST3134 +0.03‰) distinctly heavier than that of the mantle (δ98/95MoNIST3134~−0.21 to −0.16‰). The absence of isotopic fractionation during tholeiitic igneous differentiation indicates that heavy Mo isotope signatures in convergent margin rocks are intrinsic to subduction zone processes, caused either by recycling of subducted components, by Mo isotope fractionation during magmatic differentiation, or by a combination thereof. In order to gain a more detailed understanding of the Mo isotope variability in arc lavas, we have studied Mo isotopes and concentrations in calc-alkaline lavas sampled along the Banda Arc in Indonesia, an intra-oceanic subduction zone well known for variable contributions of subducting continental meterial.Banda Arc lavas exhibit Mo isotopic compositions covering a large range from −0.48 to +0.24‰ in δ98/95MoNIST3134. We find that, combined with published data from other subduction zones, the majority of arc data are isotopically heavier compared to its presumed mantle wedge source or mid-ocean-ridge basalts. Furthermore, arc lava Mo isotope signatures show an apparent covariation between indices of amphibole-clinopyroxene fractionation from the melt. Generally, heavier Mo isotopic signatures are associated with higher degrees of REE fractionation expressed as λ1, λ2, and Dy*/Dy, high Ca/Al, and higher Sc contents. These observations can be best explained with an open system magma reservoir where fractional crystallisation, recharge and eruption are tightly linked. Notable exceptions in the Banda region are four samples with isotopically light Mo. Three of these isotopically light lavas indicate the assimilation of an isotopically light Mo reservoir with heavy δ18O and radiogenic 87Sr/86Sr-206Pb/204Pb, tentatively identified as lower arc crust. Together, our data demonstrate, in line with previous findings, that magmatic processes modify the Mo isotopic composition in arc lavas, thus adding complexity to the Mo isotope systematics inherent to subduction zones.