Basaltic Partial Melting Research Articles

Serial interaction of primitive magmas with felsic and mafic crust recorded by gabbroic dikes from the Antarctic extension of the Karoo large igneous province

Two subvertical gabbroic dikes with widths of ~ 350 m (East-Muren) and ≥ 500 m (West-Muren) crosscut continental flood basalts in the Antarctic extension of the ~ 180 Ma Karoo large igneous province (LIP) in Vestfjella, western Dronning Maud Land. The dikes exhibit unusual geochemical profiles; most significantly, initial (at 180 Ma) εNd values increase from the dike interiors towards the hornfelsed wallrock basalts (from − 15.3 to − 7.8 in East-Muren and more gradually from − 9.0 to − 5.5 in West-Muren). In this study, we utilize models of partial melting and energy-constrained assimilation‒fractional crystallization in deciphering the magmatic evolution of the dikes and their contact aureoles. The modeling indicates that both gabbroic dikes acquired the distinctly negative εNd values recorded by their central parts by varying degrees of assimilation of Archean crust at depth. This first phase of deep contamination was followed by a second event at or close to the emplacement level and is related to the interaction of the magmas with the wallrock basalts. These basalts belong to a distinct Karoo LIP magma type having initial εNd from − 2.1 to + 2.5, which provides a stark contrast to the εNd composition of the dike parental magmas (− 15.3 for East-Muren, − 9.0 for West-Muren) previously contaminated by Archean crust. For East-Muren, the distal hornfelses represent partially melted wallrock basalts and the proximal contact zones represent hybrids of such residues with differentiated melts from the intrusion; the magmas that were contaminated by the partial melts of the wallrock basalts were likely transported away from the currently exposed parts of the conduit before the magma–wallrock contact was sealed and further assimilation prevented. In contrast, for West-Muren, the assimilation of the wallrock basalt partial melts is recorded by the gradually increasing εNd of the presently exposed gabbroic rocks towards the roof contact with the basalts. Our study shows that primitive LIP magmas release enough sensible and latent heat to partially melt and potentially assimilate wallrocks in multiple stages. This type of multi-stage assimilation is difficult to detect in general, especially if the associated wallrocks show broad compositional similarity with the intruding magmas. Notably, trace element and isotopic heterogeneity in LIP magmas can be homogenized by such processes (basaltic cannibalism). If similar processes work at larger scales, they may affect the geochemical evolution of the crust and influence the generation of, for example, massif-type anorthosites and “ghost plagioclase” geochemical signature.

Oblique continental collision and the formation of syn-collisional A-type granites: insights from the Early Jurassic Baoji granite suite in the Qinling orogenic belt, central China

ABSTRACT A-type granites are conventionally interpreted to be formed in late post-collisional or anorogenic settings. However, we firstly identified an Early Jurassic syn-collisional A-type granite suite in the Qinling orogenic belt: the Baoji granite suite. A comprehensive investigation into its mineralogical, petrological, geochronological, and geochemical characteristics can shed light on the genesis of syn-collisional A-type granites and geodynamic evolution of collisional orogeny. The Baoji granite suite contains two types of rocks: (1) A-type granites including biotite syenite, coarse-grained, porphyritic, and fine-grained syenogranites, and alkali-feldspar granite, and (2) magnesian monzodiorite; both types are coeval, with zircon U–Pb ages of ca. 190 Ma. The A-type granites yield A1-type trace element signatures and large variations in trace element contents, with Ba contents of 16.9–762 ppm, Sr of 11.0–211 ppm, Rb of 193–645 ppm, and Eu of 0.08–1.24 ppm. Their radiogenic isotope compositions (87Sr/86Sri = 0.70631–0.70637; εNd = −5.18 to −3.84; εHf = −5.01 to +2.76) are similar, and overlap those of Neoproterozoic OIB-like basic volcanics from the South Qinling belt. These characteristics show that the A-type granites originated from the anatexis of OIB-like lower crust and underwent fractional crystallization in a silicic magma reservoir. The monzodiorite has low SiO2 contents (49.82–53.29 wt.%) and enriched radiogenic isotopic compositions (87Sr/86Sri = 0.70554–0.70638; εNd = −11.0 to −5.28) contains abundant hornblende and biotite (~40%), and exhibits evidence of magma mingling. It also contains two groups of zircon: pristine and unzoned zircon that yields negative Hf isotope compositions (εHf = −9.42 to −3.14), and xenocrystic zircon that yields similar Hf isotope compositions (εHf = −3.54 to +1.85) to the A-type granites. These characteristics demonstrate that the monzodiorite was derived from the differentiation of basaltic partial melts of an enriched mantle source, and mingled with the A-type granitic magma during its ascent and emplacement into a silicic magma reservoir. Along with regional records of the Early Jurassic magmatism, deformation, metamorphism, and sedimentation, the genetic models for the Baoji A-type granites and monzodiorite suggest that the Middle Triassic to Early Jurassic oblique continental collision (ca. 230–190 Ma) produced coupled EW-trending strike-slip and NE-trending extensional faults at crustal levels and Rayleigh–Taylor (gravitational) convective instabilities at mantle depths in the Qinling orogenic belt, which controlled the production of the syn-collisional A-type granite suite. Our work also indicates that the A-type granites can be formed in syn-collisional setting in response to oblique continental collision and the resultant slow foundering of a high-density lithospheric root; therefore, we suggest that the syn-collisional A-type granites are evidence for oblique continental collision.

Early Paleozoic magmatic “flare-ups” in western Qinling orogeny, China: New insights into the convergence history of the North and South China Blocks at the northern margin of Gondwana

The geodynamic evolution of an accretionary orogeny commonly includes multiple magmatic episodes, which can be used to trace orogenic geodynamic processes using the geochronological and petrogenetic records preserved by igneous rocks. Paleomagnetic data indicate that the juxtaposed North China Craton (NCC) and South China Block (SCB) were located on the northern margin of the Gondwana supercontinent, but the geodynamic history of the two related blocks is still unclear. To address this issue, this paper integrates literature data with new geochronological, elemental, and isotopic data from early Paleozoic intermediate and silicic intrusions of the western North Qinling Terrane. We recognize three early Paleozoic flare-ups of intermediate–silicic intrusive magmatism in this area for the first time. Despite their similar lithology, the early Paleozoic igneous rocks formed during the three pulses have distinct geochemical compositions and petrogenesis, and record the evolution from subduction to continental collision. Geochronological data record: (1) An early Silurian (c. 441–434 Ma) pulse of diorite–quartz diorite–granite magmatism. The diorites and quartz diorites probably formed by fractionation of partial melts of mantle wedge modified by slab-derived hydrous fluids, while the granites formed by the partial melting of juvenile mafic lower crust. (2) A middle Silurian high-flux pulse of diorites, quartz diorites, and leucogranites (two-mica and muscovite granites) at c. 430–423 Ma. The diorites and quartz diorites were generated by crustal assimilation and fractional crystallization of basaltic partial melts of a pre-existing mantle source that had been altered by slab-derived hydrous fluids, and that was subsequently modified by slab-melts. The two-mica and muscovite granites formed by water-present melting of early–middle Silurian metamorphosed igneous rocks derived from enriched mantle and (meta-)graywackes, respectively. (3) A late Silurian (c. 415–409 Ma) magmatic pulse formed quartz diorites, A-type granites, and leucogranites (muscovite granites). The quartz diorites formed by differentiation of basaltic partial melts of enriched mantle, with a large contribution from depleted asthenospheric mantle. The A-type granites were generated by anatexis of juvenile mafic lower crust and subsequent fractional crystallization. The muscovite granites formed by biotite dehydration melting of (meta-)graywackes. The origins of these three early Paleozoic magmatic pulses were integrated with published regional paleomagnetic data and descriptions of sedimentary, metamorphic and igneous rocks, to provide a unique record of the geotectonic evolution of the Qinling Orogenic Belt that included: (1) slab rollback at c. 448–434 Ma; (2) initial continental collision at c. 433 Ma; (3) slab break-off at c. 430–423 Ma; (4) small-scale lithospheric foundering in a post-collisional setting at c. 415–409 Ma. Literature-derived geochronological records of collision-related magmatism and paleomagnetic data indicate that docking of the NCC and SCB with the northern Gondwana margin probably ended during the early Silurian (c. 433 Ma).

The role of water and compression in the genesis of alkaline basalts: Inferences from the Carpathian-Pannonian region

We present a new model for the formation of Plio-Pleistocene alkaline basalts in the central part of the Carpathian-Pannonian region (CPR). Based on the structural hydroxyl content of clinopyroxene megacrysts, the ‘water’ content of their host basalts is 2.0–2.5 wt.%, typical for island arc basalts. Likewise, the source region of the host basalts is ‘water’ rich (290–660 ppm), akin to the source of ocean island basalts. This high ‘water’ content could be the result of several subduction events from the Mesozoic onwards (e.g. Penninic, Vardar and Magura oceans), which have transported significant amounts of water back to the upper mantle, or hydrous plumes originating from the subduction graveyard beneath the Pannonian Basin.The asthenosphere with such a relatively high ‘water’ content beneath the CPR may have been above the ‘pargasite dehydration’ (<90 km) or the ‘nominally anhydrous’ (>90 km) solidi. This means that neither decompressional melting nor the presence of voluminous pyroxenite and eclogite lithologies are required to explain partial melting.While basaltic partial melts have been present in the asthenosphere for a long time, they were not extracted during the syn-rift phase, but were only emplaced at the onset of the subsequent tectonic inversion stage at ~8–5 Ma. We propose that the extraction has been facilitated by evolving vertical foliation in the asthenosphere as a response to the compression between the Adriatic indenter and the stable European platform. The vertical foliation and the prevailing compression effectively squeezed the partial basaltic melts from the asthenosphere. The overlying lithosphere may have been affected by buckling in response to compression, which was probably accompanied by formation of deep faults and deformation zones. These zones formed conduits towards the surface for melts squeezed out of the asthenosphere.This implies that basaltic partial melts could be present in the asthenosphere in cases where the bulk ‘water’ content is relatively high (>~200 ppm) at temperatures exceeding ~1000–1100 °C. These melts could be extracted even under a compressional tectonic regime, where the combination of vertical foliation in the asthenosphere and deep fractures and deformation zones in the folded lithosphere provides pathways towards the surface. This model is also valid for deep seated transpressional or transtensional fault zones in the lithosphere.

Network topology of olivine–basalt partial melts

Network topology of olivine–basalt partial melts

A geochemical study of the winonaites: Evidence for limited partial melting and constraints on the precursor composition

The winonaites are primitive achondrites which are associated with the IAB iron meteorites. Textural evidence implies heating to at least the Fe, Ni–FeS cotectic, but previous geochemical studies are ambiguous about the extent of silicate melting in these samples. Oxygen isotope evidence indicates that the precursor material may be related to the carbonaceous chondrites. Here we analysed a suite of winonaites for modal mineralogy and bulk major- and trace-element chemistry in order to assess the extent of thermal processing as well as constrain the precursor composition of the winonaite-IAB parent asteroid.Modal mineralogy and geochemical data are presented for eight winonaites. Textural analysis reveals that, for our sub-set of samples, all except the most primitive winonaite (Northwest Africa 1463) reached the Fe, Ni–FeS cotectic. However, only one (Tierra Blanca) shows geochemical evidence for silicate melting processes. Tierra Blanca is interpreted as a residue of small-degree silicate melting. Our sample of Winona shows geochemical evidence for extensive terrestrial weathering. All other winonaites studied here (Fortuna, Queen Alexander Range 94535, Hammadah al Hamra 193, Pontlyfni and NWA 1463) have chondritic major-element ratios and flat CI-normalised bulk rare-earth element patterns, suggesting that most of the winonaites did not reach the silicate melting temperature. The majority of winonaites were therefore heated to a narrow temperature range of between ∼1220 (the Fe, Ni–FeS cotectic temperature) and ∼1370K (the basaltic partial melting temperature). Silicate inclusions in the IAB irons demonstrate partial melting did occur in some parts of the parent body (Ruzicka and Hutson, 2010), thereby implying heterogeneous heat distribution within this asteroid. Together, this indicates that melting was the result of internal heating by short-lived radionuclides. The brecciated nature of the winonaites suggests that the parent body was later disrupted by a catastrophic impact, which allowed the preservation of the largely unmelted winonaites.Despite major-element similarities to both ordinary and enstatite chondrites, trace-element analysis suggests the winonaite parent body had a carbonaceous chondrite-like precursor composition. The parent body of the winonaites was volatile-depleted relative to CI, but enriched compared to the other carbonaceous classes. The closest match are the CM chondrites; however, the specific precursor is not sampled in current meteorite collections.

Three‐dimensional electrical resistivity model of the hydrothermal system in Long Valley Caldera, California, from magnetotellurics

AbstractThough shallow flow of hydrothermal fluids in Long Valley Caldera, California, has been well studied, neither the hydrothermal source reservoir nor heat source has been well characterized. Here a grid of magnetotelluric data were collected around the Long Valley volcanic system and modeled in 3‐D. The preferred electrical resistivity model suggests that the source reservoir is a narrow east‐west elongated body 4 km below the west moat. The heat source could be a zone of 2–5% partial melt 8 km below Deer Mountain. Additionally, a collection of hypersaline fluids, not connected to the shallow hydrothermal system, is found 3 km below the medial graben, which could originate from a zone of 5–10% partial melt 8 km below the south moat. Below Mammoth Mountain is a 3 km thick isolated body containing fluids and gases originating from an 8 km deep zone of 5–10% basaltic partial melt.

Trace-element modelling of mare basalt parental melts: Implications for a heterogeneous lunar mantle

The heterogeneous-source model of mare basalt formation indicates that Lunar Magma Ocean (LMO) overturn produced an uneven mixture of early-formed olivine and pyroxene, and late-formed, ilmenite-rich cumulates, which subsequently partially melted to give rise to mare magmas. These heterogeneous cumulate source regions would not only have been characterised by different mineral modal abundances, but also by different trace element compositions. The aim of this work was to investigate the petrology and geochemistry of a diverse suite of Apollo mare basalts, and utilise trace-element modelling in order to understand their petrogenetic history. Chemical modelling confirms that the mare basalts were produced by relatively small degrees of partial melting (<10%) of the LMO cumulates, and that the dominant melting type (batch vs. fractional) varies among different basalt groups. Similarly, single-source mineralogy cannot be applied to all mare basalt types, confirming that the lunar mantle was heterogeneous at the time of generation of mare magmas. Plagioclase is not required in the source of most mare basalts, with the notable exception of the Apollo 14 high-Al basalts. Addition of more than 1% plagioclase to the source of other basalts produces weaker negative Eu anomalies than those observed in the samples. AFC calculations demonstrate the compositional differences between materials assimilated into the Apollo 14 high-Al and Apollo 11 high-K mare basalt partial melts, highlighting the complexities of mare basalt petrogenesis.

Thermal history of the Earth and its petrological expression

article i nfo Non-arc basalts of Archean and Proterozoic age have model primary magmas that exhibit mantle potential temperatures TP that increase from 1350 °C at the present to a maximum of ∼1500-1600 °C at 2.5-3.0 Ga. The overall trend of these temperatures converges smoothly to that of the present-day MORB source, supporting the interpretation that the non-arc basalts formed by the melting of hot ambient mantle, not mantle plumes, and that they can constrain the thermal history of the Earth. These petrological results are very similar to those predicted by thermal models characterized by a low Urey ratio and more sluggish mantle convection in the past. We infer that the mantle was warming in deep Archean-Hadean time because internal heating exceeded surface heat loss, and it has been cooling from 2.5 to 3.0 Ga to the present. Non-arc Precambrian basalts are likely to be similar to those that formed oceanic crust and erupted on continents. It is estimated that ∼25-35 km of oceanic crust formed in the ancient Earth by about 30% melting of hot ambient mantle. In contrast, komatiite parental magmas reveal TP that are higher than those of non-arc basalts, consistent with the hot plume model. However, the associated excess magmatism was minor and oceanic plateaus, if they existed, would have had subtle bathymetric variations, unlike those of Phanerozoic oceanic plateaus. Primary magmas of Precambrian ambient mantle had 18-24% MgO, and they left behind residues of harzburgite that are now found as xenoliths of cratonic mantle. We infer that primary basaltic partial melts having 10-13% MgO are a feature of Phanerozoic magmatism, not of the early Earth, which may be why modern-day analogs of oceanic crust have not been reported in Archean greenstone belts.

S-wave velocity structure beneath the High Lava Plains, Oregon, from Rayleigh-wave dispersion inversion

The High Lava Plains (HLP) “hotspot” track is a prominent volcanic lineament that extends from the southeast corner of Oregon in the northern Great Basin to Newberry volcano in the eastern Cascades. With the age of silicic volcanism decreasing along track to the northwest, the HLP and Newberry volcano are a rough mirror image to the Eastern Snake River Plain and Yellowstone but, in the case of the HLP, at an orientation strongly oblique to North American plate motion. Since this orientation is incompatible with plate motion over a fixed hotspot, other proposed origins for the HLP, such as asthenospheric inflow around a steepening slab, residual effects of a Columbia River/Steens plume, backarc spreading, and Basin and Range extension, relate it to various tectonic features of the Pacific Northwest. To begin distinguishing between these hypotheses, we image upper-mantle structure beneath the HLP and adjacent tectonic provinces with fundamental-mode Rayleigh waves recorded by stations of the USArray Transportable Array, the recently-initiated HLP seismic experiment, the United States National Seismograph Network, and the Berkeley seismic network. We estimate phase velocities along nearly 300 two-station propagation paths that lie within and adjacent to the HLP and cross the region along two azimuths, parallel to and perpendicular to the HLP track. The dispersion curves, which typically give robust results over the period range 16–171 seconds, are grouped by tectonic region, and the composite curves are inverted for S-wave velocity as a function of depth. We also fit a single plane wave to phase delays across the region to identify laterally homogeneous subregions. The resulting variations in upper-mantle structure correlate with variations in surface volcanism and tectonics. The lowest velocities (∼ 4.1 km/s) occur at ∼ 50 km depth in the SE corner of Oregon, where there has been extensive basaltic volcanism in the past 2–5 kyr, and suggest uppermost mantle temperatures sufficient to produce basaltic partial melting. While the seismic velocities of the uppermost mantle beneath the volcanic High Lava Plains are low relative to the standard Tectonic North America (TNA) model, they are only slightly lower than those found for the adjacent northern Great Basin and they appear to be significantly higher than upper-mantle velocities beneath the Eastern Snake River Plain. Our results provide no evidence for a residual plume signature beneath the HLP region, leaving open questions as to the origin of the HLP volcanic track itself.

Three‐dimensional velocity structures of Mount Fuji and the South Fossa Magna, central Japan

We present the three‐dimensional structures of the P wave velocity (VP), S wave velocity (VS) as well as the P wave to S wave velocity ratio (VP/VS) beneath Mount Fuji and the South Fossa Magna, Japan, using arrival time data collected from 2002 to 2005 by a dense seismograph array. The high‐resolution data set and improved methodology reveal not only several velocity features that are consistent with previous studies but also important new details that clarify the velocity structures associated with volcanic processes beneath Mount Fuji and the collision tectonics of the South Fossa Magna. One such particular feature is a low‐VP, low‐VS and low‐VP/VS anomaly at depths of 7–17 km beneath Mount Fuji that corresponds with the locations of deep low‐frequency (DLF) earthquakes. The coincidence of the velocity anomaly and the DLF locations suggests that supercritical volatile fluid, such as H2O and CO2, may be abundant in the low‐VP/VS region and may play an important role in generating DLF earthquakes. This anomaly overlies a deeper low‐VP, low‐VS and high‐VP/VS anomaly at depths of 15–25 km that may represent a zone of basaltic partial melt. A low‐VP, low‐VS and low‐VP/VS anomaly is seen at depths of 6–14 km beneath Mount Hakone. Isovelocity surfaces (VP = 6.0 km/s and VS = 3.5 km/s) corresponding to the upper limit of hypocenter distribution below Mount Fuji may define the upper surface of the Philippine Sea plate whose existence in a seismic gap beneath Mount Fuji has been controversial.

Na depletion in modern adakites via melt / rock reaction within the sub-arc mantle

Interaction between slab-derived melt and mantle peridotite and the role of slab melt as a metasomatizing agent in the sub-arc mantle is being increasingly recognized. Adakite, the slab melt erupted on the surface, usually exhibits anomalously high MgO, CaO, Cr and Ni contents that indicate interaction with mantle peridotitite. Here we note that Cenozoic adakites have Na 2O contents below 5.8 wt.% with ∼95% samples lower than 5.0 wt.%, and are generally depleted in this component relative to experimental basalt partial melts (mostly beyond 5.0 wt.% and up to 9.0 wt.% Na 2O) produced under 1.5–3.0 GPa conditions that are most relevant to adakite production. We interpret the adakite Na depletion to be also a consequence of the melt / rock reaction that takes place within the hot mantle wedge. During ascent and reaction with mantle peridotite, primary adakite melts gain mantle components MgO, CaO, Cr and Ni but lose Na 2O, SiO 2 and perhaps K 2O to the mantle, leading to Na-rich mantle metasomatism. Selective assimilation of predominately mantle clinopyroxene, some spinel and minor olivine at high T/P has been considered to be an important process in producing high-Mg adakites from primary low-Mg slab melts [Killian, R., Stern, C. R., 2002. Constraints on the interaction between slab melts and the mantle wedge from adakitic glass in peridotite xenoliths. Eur. J. Mineral. 14, 25–36]. In such a process, Na depletion in the assimilated melt is the result of dilution due to the increase in melt mass. Phase relationships in the reaction system siliceous melt + peridotite and quantitative calculation suggest that assimilation of mantle clinopyroxene, olivine and spinel and fractional crystallization of sodic amphibole and orthopyroxene, under conditions of moderate T/P and increasing melt mass, is also an important process that modifies the composition of adakites and causes the Na depletion.

Northwest Africa 011: A “eucritic” basalt from a non‐eucrite parent body

Abstract— We have carried out a detailed petrographic, mineralogical, and trace element study of Northwest Africa (NWA) 011. This meteorite bears many similarities to the eucrites it was initially identified with, although oxygen isotopic compositions rule out a genetic relationship. Like many eucrites, NWA 011 crystallized from a source with approximately chondritic proportions of REE, although a slightly LREE‐enriched bulk composition with a small positive Eu anomaly, as well as highly fractionated Fe/Mg ratios and depleted Sc abundances (Korotchantseva et al. 2003), suggest that the NWA 011 source experienced some pyroxene and/or olivine fractionation. Thermal metamorphism resulted in homogenization of REE abundances within grains, but NWA 011 did not experience the intergrain REE redistribution seen in some highly metamorphosed eucrites. Despite a similarity in oxygen isotopic compositions, NWA 011 does not represent a basaltic partial melt from the acapulcoite/lodranite parent body. The material from which NWA 011 originated may have been like some CH or CB chondrites, members of the CR chondrite clan, which are all related through oxygen isotopic compositions. The NWA 011 parent body is probably of asteroidal origin, possibly the basaltic asteroid 1459 Magnya.

Origin and geodynamic evolution of Late Paleogene magmatic associations along the Periadriatic-Sava-Vardar magmatic belt

Along the Periadriatic Lineament in the Alps and the Sava-Vardar Zone of the Dinarides and Hellenides, Paleogene magmatic associations form a continuous belt, about 1700 km long. The following magmatic associations occur: (1) Eocene granitoids; (2) Oligocene granitoids including tonalites; (3) Oligocene shoshonite and calc-alkaline volcanics with lamprophyres; (4) Egerian-Eggenburgian (Chattian) calc-alkaline volcanics and granitoids. All of these magmatic associations are constrained by radiometric ages, which indicate that the magmatic activity was mainly restricted to the time span between 55 and 29 Ma. These igneous rocks form, both at surface and in the subsurface, the distinct linear Periadriatic-Sava-Vardar magmatic belt, with three strikes that are controlled by the indentation of Apulia and Moesia and accompanying strike-slip faulting. The geology, seismicity, seismic tomography and magnetic anomalies within this belt suggest that it has been generated in the African-Eurasian suture zone. Based on published analytical data, the petrology, major and trace element contents and Sr, Nd and O isotopie composition of each magmatic association are briefly defined. These data show that Eocene and Oligocene magmatic associations of the Late Paleogene Periadriatic-Sava-Vardar magmatic belt originated along a consuming plate margin. Based on isotopie systems, two main rock groups can be distinguished: (1) 87Sr/86Sr = 0.7036–0.7080 and δ18O = 5.9–7.2‰, indicating basaltic partial melts derived from a continental mantle-lithosphere, and (2) 87Sr/86Sr = 0.7090–72131 and δ18O = 7.3–11.5‰, indicating crustal assimilation and melting. The mantle sources for the primary basalt melts are metasomatized garnet peridotites and/or spinel lherzolites and phlogopite lherzolites of upper mantle wedge origin. The geodynamic evolution of the plutonic and volcanic associations of the Periadriatic-Sava-Vardar magmatic belt was related to the Africa-Eurasia suture zone that was dominated by break-off of the subducted lithospheric slab of Mesozoic oceanic crust, at depths of 90–100 km. This is indicated by their contemporaneity along the 1700 km long belt. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Origin and geodynamic evolution of Late Paleogene magmatic associations along the Periadriatic–Sava–Vardar magmatic belt

Along the Periadriatic Lineament in the Alps and the Sava–Vardar Zone of the Dinarides and Hellenides, Paleogene magmatic associations form a continuous belt, about 1700 km long. The following magmatic associations occur: (1) Eocene granitoids; (2) Oligocene granitoids including tonalites; (3) Oligocene shoshonite and calc-alkaline volcanics with lamprophyres; (4) Egerian–Eggenburgian (Chattian) calc-alkaline volcanics and granitoids. All of these magmatic associations are constrained by radiometric ages, which indicate that the magmatic activity was mainly restricted to the time span between 55 and 29 Ma. These igneous rocks form, both at surface and in the subsurface, the distinct linear Periadriatic–Sava–Vardar magmatic belt, with three strikes that are controlled by the indentation of Apulia and Moesia and accompanying strike-slip faulting. The geology, seismicity, seismic tomography and magnetic anomalies within this belt suggest that it has been generated in the African–Eurasian suture zone. Based on published analytical data, the petrology, major and trace element contents and Sr, Nd and O isotopic composition of each magmatic association are briefly defined. These data show that Eocene and Oligocene magmatic associations of the Late Paleogene Periadriatic–Sava–Vardar magmatic belt originated along a consuming plate margin. Based on isotopic systems, two main rock groups can be distinguished: (1) 87Sr/ 86Sr = 0.7036–0.7080 and δ 18O = 5.9–7.2‰, indicating basaltic partial melts derived from a continental mantle–lithosphere, and (2) 87Sr/ 86Sr = 0.7090–72131 and δ 18O = 7.3–11.5‰, indicating crustal assimilation and melting. The mantle sources for the primary basalt melts are metasomatized garnet peridotites and/or spinel lherzolites and phlogopite lherzolites of upper mantle wedge origin. The geodynamic evolution of the plutonic and volcanic associations of the Periadriatic–Sava–Vardar magmatic belt was related to the Africa–Eurasia suture zone that was dominated by break-off of the subducted lithospheric slab of Mesozoic oceanic crust, at depths of 90–100 km. This is indicated by their contemporaneity along the 1700 km long belt.

Calculation of Peridotite Partial Melting from Thermodynamic Models of Minerals and Melts. II. Isobaric Variations in Melts near the Solidus and owing to Variable Source Composition

We examine several issues related to peridotite partial melting by performing thermodynamic calculations using the MELTS algorithm. MELTS calculations suggest that the high alkali content of near-solidus melts leads to their having high SiO2 contents at 1 GPa, but that near-solidus enrichments in SiO2 become less pronounced with increasing pressure, such that 2% partial melts at 3 GPa have only ∼1 wt % more SiO2 than higher melt fractions (5–15%) at that pressure. Calculated near-solidus solid–liquid partitioning of TiO2 differs from that calculated and observed at high melt fraction, both at low and high pressure, reflecting variations in melt and mineral compositions as melting proceeds. Specifically, near the solidus, high liquid SiO2 and an abundant supply of charge coupling ions (Na+, Al3+) in clinopyroxene (cpx) makes Ti more compatible in the solid. We infer that similar effects are likely for other highly charged cations such as the high field strength elements (HFSE), U, and Th. To investigate the effects of heterogeneous source regions on major element chemistry of basaltic partial melts, we perform melting calculations for a range of peridotite compositions. For partial melts in equilibrium with spinel lherzolite residues, CaO/Al2O3 and CaO content increase with increasing temperature and decrease with increasing Na2O in the melt and Al/(Cr + Al) in coexisting spinel. Thus, at any given melt fraction, the CaO/Al2O3 ratios and CaO contents of partial melts are inversely correlated with the fertility or enrichment of the source and only weakly dependent on the source cao/A12O3 or CaO content. At fixed melt fraction, these effects of source depletion on melt composition (Na2O decreases, cao/A12O3 increases) are similar to the effects of increasing total extent of melting of a fixed source composition, and the two phenomena may therefore be difficult to discriminate on the basis of these melt variables alone. The effects of source variability on partial melting depend on whether compositionally distinct domains experience the same or different temperature–pressure paths. If they experience the same paths, as might be expected for small-scale heterogeneities, then enhanced melting of the enriched sources can yield partial melts with depleted characteristics such as low Na2O and high cao/A12O3 relative to partial melts of depleted sources at the same pressure and temperature. The differing compatibility of K and Ti in spinel peridotite minerals causes the K2O/TiO2 ratio of partial melts to vary inversely with the total extent of melting (and with FeO*), so K2O/TiO2 may not be a reliable indicator of source heterogeneity. Moreover, enriched high K2O/TiO2 sources probably produce melts richer in FeO than depleted low K2O/TiO2 sources at the same conditions. Consequently, the inverse correlation between regionally averaged K2O/TiO2 and melt FeO* observed by Shen & Forsyth (1995, Journal of Geophysical Research 100, 2211–2237) is probably more strongly influenced by variations in extent of melting than by source heterogeneity.

Calculation of Peridotite Partial Melting from Thermodynamic Models of Minerals and Melts. II. Isobaric Variations in Melts near the Solidus and owing to Variable Source Composition

We examine several issues related to peridotite partial melting by performing thermodynamic calculations using the MELTS algorithm. MELTS calculations suggest that the high alkali content of near-solidus melts leads to their having high SiO_2 contents at 1 GPa, but that near-solidus enrichments in SiO_2 become less pronounced with increasing pressure, such that 2% partial melts at 3 GPa have only ∼1 wt % more SiO_2 than higher melt fractions (5–15%) at that pressure. Calculated near-solidus solid–liquid partitioning of TiO_2 differs from that calculated and observed at high melt fraction, both at low and high pressure, reflecting variations in melt and mineral compositions as melting proceeds. Specifically, near the solidus, high liquid SiO_2 and an abundant supply of charge coupling ions (Na^+, Al^(3+)) in clinopyroxene (cpx) makes Ti more compatible in the solid. We infer that similar effects are likely for other highly charged cations such as the high field strength elements (HFSE), U, and Th. To investigate the effects of heterogeneous source regions on major element chemistry of basaltic partial melts, we perform melting calculations for a range of peridotite compositions. For partial melts in equilibrium with spinel lherzolite residues, CaO/Al_(2)O_3 and CaO content increase with increasing temperature and decrease with increasing Na_(2)O in the melt and Al/(Cr + Al) in coexisting spinel. Thus, at any given melt fraction, the CaO/Al_(2)O_3 ratios and CaO contents of partial melts are inversely correlated with the fertility or enrichment of the source and only weakly dependent on the source cao/Al_(2)O_3 or CaO content. At fixed melt fraction, these effects of source depletion on melt composition (Na_(2)O decreases, cao/Al_(2)O_3 increases) are similar to the effects of increasing total extent of melting of a fixed source composition, and the two phenomena may therefore be difficult to discriminate on the basis of these melt variables alone. The effects of source variability on partial melting depend on whether compositionally distinct domains experience the same or different temperature–pressure paths. If they experience the same paths, as might be expected for small-scale heterogeneities, then enhanced melting of the enriched sources can yield partial melts with depleted characteristics such as low Na_(2)O and high cao/Al_(2)O_3 relative to partial melts of depleted sources at the same pressure and temperature. The differing compatibility of K and Ti in spinel peridotite minerals causes the K_(2)O/TiO_2 ratio of partial melts to vary inversely with the total extent of melting (and with FeO*), so K_(2)O/TiO_2 may not be a reliable indicator of source heterogeneity. Moreover, enriched high K_(2)O/TiO_2 sources probably produce melts richer in FeO than depleted low K_(2)O/TiO_2 sources at the same conditions. Consequently, the inverse correlation between regionally averaged K_(2)O/TiO_2 and melt FeO* observed by Shen & Forsyth (1995, Journal of Geophysical Research 100, 2211–2237) is probably more strongly influenced by variations in extent of melting than by source heterogeneity.

Partial melting and melt migration in the acapulcoite-lodranite parent body

We review petrologic evidence that the acapulcoites and lodranites formed by < 1 vol% to probably 20 vol% whole rock partial melting of a chondritic precursor material. At low degrees of partial melting, only Fe,NiFeS cotectic melting occurred. Migration distances for partial melts were short, resulting in the formation of acapulcoites with essentially chondritic troilite and plagioclase contents, but achondritic textures. At high degrees of partial melting, both Fe,NiFeS and basaltic (plagioclase-pyroxene) partial melts formed, and the melts may have migrated out of the source rock. The partial melt residues, which are more or less depleted in Fe,NiFeS and plagioclase, are the lodranites. Melt migration was complex: most acapulcoites, which experienced relatively low degrees of partial melting, lost little if any of the partial melt. One acapulcoite, LEW 86220, represents a unique case in which Fe,NiFeS and basaltic partial melts appear to have migrated from a lodranite source region into a cooler acapulcoite region, where they were trapped. In cases of the relatively high degrees of partial melting experienced by lodranites, melts may have been partly, selectively, or totally removed from the rocks, and Fe,NiFeS and/or basaltic partial melts may have been removed to different degrees and may, in fact, have been trapped on occasion in greater than chondritic proportions. We model vein and dike formation and melt migration by calculating the excess pressures and vein and dike sizes for varying degrees of partial melting. Our calculations are broadly consistent with observations, indicating that melt migration is inefficient at low degrees of partial melting and extremely efficient at high degrees of partial melting. Although the size of the acapulcoite-lodranite parent body and the volatile contents of the chondritic precursor rocks are poorly constrained, the lack of basaltic rocks in the world's meteorite collections complementary to the lodranites suggests that basaltic partial melts may have been accelerated off the body by explosive volcanism of the type envisioned by Wilson and Keil (1991) and ejected into space. The diversity of rocks from the acapulcoite-lodranite parent body may provide a basis for better understanding the diverse range of spectral subtypes recognized among the S-type asteroids.

A petrologic and isotopic study of lodranites: Evidence for early formation as partial melt residues from heterogeneous precursors

We have conducted petrologic, chemical, and isotopic studies of lodranites in an attempt to constrain their genesis. Lodran, Gibson, Y-791491, Y-791493, Y-74357, Y-8002, Y-75274, MAC 88177, LEW 88280, EET 84302, FRO 90011, and QUE 93148 are classified as lodranites. Lodranites and acapulcoites are indistinguishable on the basis of oxygen isotopic compositions but are distinct in average grain sizes of their mafic silicates, with lodranites being significantly coarser-grained. Lodranites exhibit a diverse range of petrologic and mineralogic features: they range widely in mafic silicate compositions (Fa 3–13), plagioclase (0–11.4 vol%), Fe,Ni metal (0.5–20 vol%), and troilite (0.2–5.3 vol%) contents; and shock levels (S1–S4) . They appear to have experienced high peak temperatures and rapid cooling in the temperature range recorded by metallographic cooling rates (i.e., 700-350°C). The only dated lodranite, Gibson, cooled to Ar closure temperatures at 4.49 ± 0.01 Ga. Lodranites formed from chemically and isotopically heterogeneous precursors in which the mineral and oxygen isotopic compositions were correlated. Heating of their parent body to temperatures between ∼ 1050–1200°C resulted in formation of Fe,NiFeS and basaltic partial melts. Depletions of troilite and/ or plagioclase in most lodranites testify to the removal of some of these partial melts, although melt migration was complex. Lodranites appear to have experienced a complex cooling history of slow cooling at high temperatures, followed by rapid cooling at intermediate temperatures, possibly related to breakup of the parent body. Lodranites were liberated from their parent body during 1–3 impact events, with most having cosmic ray exposure ages of 5.5–7 Ma. The acapulcoites are samples from the same parent body but were heated to lower temperatures and, thus, experienced lower degrees of partial melting.

Dark clasts in the Khor Temiki aubrite: Not basalts

Abstract— The enstatite achondrite meteorites (aubrites) are ultramafic assemblages with highly variable bulk rare earth element (REE) compositions. An enrichment of REE in a dark clast from the Khor Temiki aubrite led Wolf et al. (1983) to suggest that such dark clasts could be the basaltic (i.e., enstatite‐plagioclase) complements to the ultramafic aubrites, with the relatively high REE contents resulting from the presence of plagioclase, which is a common carrier of the REEs. We have studied several dark clasts from the Khor Temiki aubrite and find no evidence for a basaltic character for such material. The microscopic character of the dark clasts is not significantly different from the main portions of Khor Temiki and consists either of highly brecciated material, containing a fine‐grained matrix, or of enstatite grains with abundant inclusions. We suggest that the dark clasts are shock‐darkened, heterogeneous Khor Temiki material that, by chance, contained variable trace contents of oldhamite (CaS), which has been shown to be a major carrier of REE in aubrites. We find that the REE contents of the clasts range from 0.1 to ∼20× CI. Most have negative Eu anomalies, but one has a small positive anomaly. Extensive searches have failed to identify basaltic material in Khor Temiki and other aubrites. The absence of basaltic material is consistent with, but does not prove, the model of Wilson and Keil (1991). They calculate that, on an asteroidal parent body &lt; ∼100 km in radius, a volatile‐rich basaltic partial melt erupted with a velocity greater than the escape velocity of the asteroid and, thus, was lost into space ∼ 4.55 Ga ago.