Separation Of Plagioclase Research Articles

Plagioclase Megacrysts in Mesoproterozoic Amphibolites from the New Jersey Highlands, USA: Indicators of Mixed-Source Magma and Fractionation Interruption in Anorthosite-Dominated Terrains

Rare amphibolite in the New Jersey Highlands containing plagioclase megacrysts ≤13 cm long forms bodies 0.5 to 2 m thick that preserve a penetrative metamorphic fabric and have sharp, conformable contacts against Mesoproterozoic country rocks. The megacrystic amphibolites were emplaced as thin dikes along extensional faults between 1160 and 1130 Ma. Amphibolites contain weakly zoned, subhedral andesine megacrysts (An29–44) in a matrix of plagioclase (An18–38), magnesio-hastingsite, biotite, diopside, Fe-Ti oxides, and apatite. The whole-rock major oxide composition of the megacrystic amphibolite matrix has high TiO2 (2.07 wt.% ± 2.0%), Al2O3 (17.03 wt.% ± 0.87%), and Fe2O3t (12.84 wt.% ± 3.2%) that represents an Al-Fe-rich mafic magma type that is characteristic of anorthosite associations globally. The whole-rock, rare earth element (REE) composition of the megacrystic amphibolite matrix is characterized by enrichments in light rare earth elements (LREEs) (La/YbN = 1.73–10.69) relative to middle (MREEs) and heavy (HREEs) rare earth elements (Gd/YbN = 1.30–1.85), and most samples have small positive Eu anomalies (Eu/Eu* = 0.95–1.25). The megacrystic amphibolite matrix is also enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs) (e.g., Ba/Nb = 24–22). Megacrystic amphibolites formed through partial melting of subduction-modified lithospheric mantle that fractionated olivine and plagioclase, producing a high-Al-Fe mafic magma. Plagioclase megacrysts formed through extraction of a plagioclase-rich crystal-liquid mush from anorthosite that mixed with mafic magma and collected in the upper part of the mid-crustal magma (depth of ~20 km based on Al-in-hornblende geobarometry) chamber through flotation. Periodic tapping of this mixed source by extensional fractures led to emplacement of the amphibolites as dikes and may have interrupted the extensive fractionation and plagioclase separation necessary to form voluminous anorthosite intrusions.

Discovery of the ∼35 Ma granite porphyry dikes in the giant Dachang Sn ore field: Tectonic and metallogenic implications

The world’s second-largest Dachang Sn ore field in the southwest of China exhibits genetic connections to the Late Cretaceous highly evolved S-type granitoids (93–91 Ma). In this study, we report the newly identified Eocene granite porphyry dikes with zircon U-Pb ages of 35–33 Ma at the Gaofeng Sn deposit from the Dachang ore field. The petrogenesis and mineralization potential are also constrained by employing whole-rock and zircon geochemical data. These Eocene dikes coincide with the emplacements of granitoids (35–30 Ma) along the Ailaoshan–Red River fault and its neighboring regions, indicating that they could be related to a similar lithospheric extension setting after the Indo-Asian collision. However, unlike those of the latter granitoids (35–30 Ma), which are grouped into I-type granites, the high-silica granite porphyry dikes show elevated ASI values (1.32–1.38) with high P2O5 (0.41–0.44 wt%) and normative-CIPW corundum (4.15–4.71 wt%) contents and could be classified as S-type granites. The strongly negative whole-rock Sr, Ba, and Eu anomalies signify separation of plagioclase and K-feldspar during magma crystallization, which is also confirmed by the zircon trace elements compositions. The Gaofeng Eocene granite porphyry dikes have a high initial zircon Tti-zr of 855 ℃, high Ba/Pb ratios (11.5–16.8), and low Pb contents (7.56–9.98 ppm) with low zircon εHf(t) values (−7.4 to −0.6), hinting at the dehydration melting of biotite from the metasedimentary basement. Additionally, the dikes are characterized by low oxygen fugacity (zircon Ce/Ce* <70; ΔFMQ from −6.0 to +0.6 with an average of −2.0) and show evidence of magmatic-hydrothermal interaction (whole-rock Nb/Ta < 5), in line with those of the Dachang Late Cretaceous fertile granites, suggestive of Sn mineralization potential. Therefore, it is recommended that attention should be paid to the Eocene granite porphyry dikes during future prospecting.

Petrogenesis of monzonites and leucogranites in the Ama Drime Massif: Implications for sources and differentiation of Himalayan leucogranites

Determining the sources and the evolution of the Himalayan leucogranites is crucial to deciphering the teotono-magmatic evolution history of the Himalayan Orogen. Various magmatic rocks, including the monzonites, two-mica leucogranites, and tourmaline leucogranites, are exposed as dykes in the Paleoproterozoic orthogneisses of the Ama Drime Massif (ADM), providing new insights into the Himalayan magmatism. This contribution presents systematic petrological, geochronological, and geochemical studies of these magmatic rocks from the ADM to constrain their emplacement timing, protolith, and petrogenesis. Geochronological results reveal two episodes of magmatism in the ADM, represented by the 13.5 Ma monzonites and the 10 Ma two-mica and tourmaline leucogranites. Despite having different ages, the two episodes of magmatism in the ADM both originated from the melting of the Paleoproterozoic orthogneisses. However, various magmatic rocks underwent different differentiation processes during emplacement. Based on mineral assemblages and geochemical features, the monzonites were formed by the accumulation of plagioclase, biotite, zircon, and monazite. For the leucogranites, the systematic variations in mineral assemblages, whole-rock geochemistry, and mineralogical chemistry demonstrate that the two-mica leucogranites gradually evolved into the tourmaline leucogranites by the separation of plagioclase, biotite, zircon, and monazite. Combining this contribution and previous studies, the Himalayan leucogranites might evolve from the peraluminous melts produced by the melting of Himalayan anatectic rocks according to fractional crystallization, with the formation of complementary cumulate rocks. The involvement of orthogneisses in the source suggests that the Himalayan leucogranites can not be simply classified into S-type granites.

How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H2O contents in nominally anhydrous minerals

Granulites from Holsnøy (Bergen Arcs, Norway) maintained a metastable state until fluid infiltration triggered the kinetically delayed eclogitization. Interconnected hydrous eclogite-facies shear zones are surrounded by unreacted granulites. Macroscopically, the granulite–eclogite interface is sharp and there are no significant compositional changes in the bulk chemistry, indicating the fluid composition was quickly rock buffered. To better understand the link between deformation, fluid influx, and fluid–rock interaction one cm-wide shear zone at incipient eclogitization is studied here. Granulite and eclogite consist of garnet, pyroxene, and plagioclase. These nominally anhydrous minerals (NAMs) can incorporate H2O in the form of OH groups. H2O contents increase from granulite to eclogite, as documented in garnet from ~ 10 to ~ 50 µg/g H2O, pyroxene from ~ 50 to ~ 310 µg/g H2O, and granulitic plagioclase from ~ 10 to ~ 140 µg/g H2O. Bowl-shape profiles are characteristic for garnet and pyroxene with lower H2O contents in grain cores and higher at the rims, which suggest a prograde water influx into the NAMs. Omphacite displays a H2O content range from ~ 150 to 425 µg/g depending on the amount of hydrous phases surrounding the grain. The granulitic plagioclase first separates into a hydrous, more albite-rich plagioclase and isolated clinozoisite before being replaced by new fine-grained phases like clinozoisite, kyanite and quartz during ongoing fluid infiltration. Results indicate a twofold fluid influx with different mechanisms to act simultaneously at different scales and rates. Fast and more pervasive proton diffusion is recorded by NAMs that retain the major element composition of the granulite-facies equilibration where hydrogen decorates pre-existing defects in the crystal lattice and leads to OH increase. Contemporaneously, slower grain boundary-assisted aqueous fluid influx enables element transfer and results in progressive formation of new minerals, e.g., hydrous phases. Both mechanisms lead to bulk H2O increase from ~ 450 to ~ 2500 µg/g H2O towards the shear zone and convert the system from rigid to weak. The incorporation of OH groups reduces the activation energy for creep, promotes formation of smaller grain sizes (phase separation of plagioclase), and synkinematic metamorphic mineral reactions. These processes are part of the transient weakening, which enhance the sensitivity of the rock to deform.

Geochronology, geochemistry and Sr–Nd–Hf isotopes of the Haisugou porphyry Mo deposit, northeast China, and their geological significance

The Haisugou Mo deposit is located in the northern part of the Xilamulun Mo–Cu metallogenic belt in northeastern China. The Mo mineralization mainly occurs as quartz-molybdenite veins within the Haisugou granite, which was emplaced into rocks of the Early Permian Qingfengshan Formation. Zircon U–Pb dating by LA–ICP-MS of the granite yields a crystallization age of 137.6±0.9Ma, suggesting emplacement during the peak time of Mo mineralization in eastern China, broadly constrained to ca. 150–130Ma, when tectonic stresses shifted from compression to extension. Whole-rock geochemical data suggest that the granite belongs to the high-K calc-alkaline series, and is characterized by relatively high LREE; low HREE; depletion of Ti, Ba, and Nb; and a moderate negative Eu anomaly. The zircon εHf(t) and whole-rock εNd(t) values for the intrusion range from +4.5 to +10.0 and +0.2 to +1.6, respectively, indicating that the magma originated from the juvenile lower crust source derived from depleted mantle, with some component of ancient continental crust. The granite is also characterized by initial (87Sr/86Sr)i ratios ranging from 0.7040 to 0.7074, which suggest some contamination by the upper crust during the ascent of the primitive magma. Moreover, it can be recognized from the whole-rock major and trace element data that significant fractional crystallization occurred during magmatic evolution, with the separation of plagioclase and K-feldspar. Because Mo is an incompatible element and tends to concentrate in the melt during crystallization, fractionation processes likely played an important role in the formation of the Haisugou Mo deposit.

Petrogenetic relationships between peralkaline rhyolite dykes and mafic rocks in the post-Variscan gabbroic complex from Bocca di Tenda (northern Corsica, France)

The Lower Permian complex from Bocca di Tenda (Corsica island, France) consists of a gabbroic sequence crosscut by chilled dykes ranging in composition from basalt to trachyandesite and peralkaline rhyolite. The gabbroic sequence is mostly composed of olivine gabbronorites, quartz gabbronorites/diorites locally displaying high ilmenite amounts, and hornblende-rich tonalites. The quartz gabbronorites/diorites and the hornblende-rich tonalites have similar initial eNd values (+0.9 to −1.1) and record a fractional crystallization process driven by separation of plagioclase, pyroxene, and ilmenite. The olivine gabbronorites have slightly higher initial eNd than the quartz gabbronorites/diorites and the hornblende-rich tonalites, thereby documenting that the early evolution of the melts that gave rise to the gabbroic sequence was controlled by concomitant fractional crystallization and crustal assimilation. The trachyandesite dykes are rare and rich in dark mica. The selected trachyandesite has initial eNd of +0.4, which is slightly lower than the eNd of the basalt dykes. The basalt and the trachyandesite dykes are most likely genetically related through a process of fractional crystallization controlled by segregation of plagioclase, clinopyroxene and minor ilmenite, and assimilation of crustal material. The peralkaline rhyolites have initial eNd values ranging from +0.3 to −0.3. Whole-rock chemical variations and trace element compositions of Na-amphibole (arfvedsonite) indicate that the peralkaline rhyolite dykes record a process of fractional crystallization mainly controlled by separation of alkali feldspar and minor ilmenite and arfvedsonite. A plausible petrogenetic hypothesis for the genesis of the peralkaline rhyolite melts implies a protracted process of fractional crystallization from the trachyandesitic melts. This fractionation process would be initially ruled by separation of plagioclase, dark mica, and minor ilmenite. An alternative hypothesis for the origin of the peralkaline rhyolite melts implies partial melting of nearly coeval amphibole-rich mafic intrusives, which formed by crustally contaminated mantle-derived melts. The genesis of the peralkaline rhyolites is in any case correlated with mantle-derived melts that experienced extensive crustal contamination.

Characterization of multiple lithologies within the lunar feldspathic regolith breccia meteorite Northeast Africa 001

Abstract– Lunar meteorite Northeast Africa (NEA) 001 is a feldspathic regolith breccia. This study presents the results of electron microprobe and LA‐ICP‐MS analyses of a section of NEA 001. We identify a range of lunar lithologies including feldspathic impact melt, ferroan noritic anorthosite and magnesian feldspathic clasts, and several very‐low titanium (VLT) basalt clasts. The largest of these basalt clasts has a rare earth element (REE) pattern with light‐REE (LREE) depletion and a positive Euanomaly. This clast also exhibits low incompatible trace element (ITE) concentrations (e.g., &lt;0.1 ppm Th, &lt;0.5 ppm Sm), indicating that it has originated from a parent melt that did not assimilate KREEP material. Positive Eu‐anomalies and such low‐ITE concentrations are uncharacteristic of most basalts returned by the Apollo and Luna missions, and basaltic lunar meteorite samples. We suggest that these features are consistent with the VLT clasts crystallizing from a parent melt which was derived from early mantle cumulates that formed prior to the separation of plagioclase in the lunar magma ocean, as has previously been proposed for some other lunar VLT basalts. Feldspathic impact melts within the sample are found to be more mafic than estimations for the composition of the upper feldspathic lunar crust, suggesting that they may have melted and incorporated material from the lower lunar crust (possibly in large basin‐forming events). The generally feldspathic nature of the impact melt clasts, lack of a KREEP component, and the compositions of the basaltic clasts, leads us to suggest that the meteorite has been sourced from the Outer‐Feldspathic Highlands Terrane (FHT‐O), probably on the lunar farside and within about 1000 km of sources of both Low‐Ti and VLT basalts, the latter possibly existing as cryptomaria deposits.

Geochemical, zircon U–Pb dating and Sr–Nd–Hf isotopic constraints on the age and petrogenesis of an Early Cretaceous volcanic-intrusive complex at Xiangshan, Southeast China

The Late Mesozoic geology of Southeast China is characterized by extensive Jurassic to Cretaceous magmatism consisting predominantly of granites and rhyolites and subordinate mafic rocks, forming a belt of volcanic-intrusive complexes. The Xiangshan volcanic-intrusive complex is located in the NW region of the belt and mainly contains the following lithologies: rhyodacite and rhyodacitic porphyry, porphyritic lava, granite porphyry with mafic microgranular enclaves, quartz monzonitic porphyry, and lamprophyre dyke. Major and trace-element compositions, zircon U–Pb dating, and Sr–Nd–Hf isotopic compositions have been investigated for these rocks. The precise SHRIMP and LA–ICP–MS zircon U–Pb dating shows that the emplacement of various magmatic units at Xiangshan took place within a short time period of less than 2 Myrs. The stratigraphically oldest rhyodacite yielded a zircon U–Pb age of 135 ± 1 Ma and the overlying rhyodacitic porphyry has an age of 135 ± 1 Ma. Three porphyritic lava samples yielded zircon U–Pb ages of 136 ± 1 Ma, 132 ± 1 Ma, and 135 ± 1 Ma, respectively. Two subvolcanic rocks (granite porphyry) yielded zircon U–Pb ages of 137 ± 1 Ma and 137 ± 1 Ma. A quartz monzonitic porphyry dyke, which represented the final stage of magmatism at Xiangshan, also yielded a zircon U–Pb age of 136 ± 1 Ma. All these newly obtained precise U–Pb ages demonstrate that the entire magmatic activity at Xiangshan was rapid and possibly took place at the peak of extensional tectonics in SE China. The geochemical data indicate that all these samples from the volcanic-intrusive complex have an A-type affinity. Sr–Nd–Hf isotopic data suggest that the Xiangshan volcanic-intrusive complex derived mainly from remelting of Paleo-Mesoproterozoic crust without significant additions of mantle-derived magma. However, the quartz monzonitic porphyry, which has zircon Hf model ages older than the whole-rock Nd model ages, and which has eNd(T) value higher than the other rocks, may indicate involvement of a subordinate younger mantle-derived magma in its origin. Geochemical data indicate that the various rocks show variable REE patterns and negative anomalies of Ba, Nb, Sr, P, Eu and Ti in the trace element spidergrams, suggesting that these rocks may have undergone advanced fractional crystallization with separation of plagioclase, K-feldspar and accessory minerals such as allanite. We suggest that this Cretaceous volcanic-intrusive complex formed in an extensional environment, and the formation of the Xiangshan mafic microgranular enclaves can be explained by the injection of mafic magma from a deeper seated mantle magma chamber into a hypabyssal felsic magma chamber at the crustal emplacement levels.

Petrogenesis and tectonic implications of Late Mesozoic granites in the NE Yangtze Block, China: further insights from the Jiuhuashan–Qingyang complex

AbstractThe Jiuhuashan–Qingyang complex is one of the Mesozoic granite complexes in the NE Yangzte Block, China. New petrographical and petrochemical data show that the complex comprises a dominant granodiorite–monzogranite, the Qingyang body, which was intruded by the Jiuhuashan granite body. The two are characterized by distinct mineral components and trace element patterns. Compared to the Qingyang granodiorite and monzogranite, the Jiuhuashan granite is enriched in Rb, Th, U, Nb, Ta, Hf, Yb and Lu, and depleted in Ba, Sr, Nd, Sm, Eu, Gd and Ti, which are ascribable to the separation of plagioclase and biotite, and crystallization of thorite and fergusonite during the magmatism. New LA-ICPMS zircon U–Pb dating suggests that the crystallization age of the Qingyang body is 139–133 Ma, and the Jiuhuashan granite followed at 127 Ma. Moreover, the new zircon U–Pb dates reveal that Archaean materials were involved in the formation of these magmas, and that a sodium-rich metasomatic event occurred at about 100 Ma. The CHIME monazite and zircon ages studied for the Jiuhuashan body agree well with the LA-ICPMS zircon ages. Integrating this information with previous studies for granites in the NE Yangtze Block and in the coastal area of SE China, we believe that all of these Late Mesozoic granites were produced under the tectonic regime of palaeo-Pacific plate subduction towards the SE China continent in a NW direction, but the granites in the NE Yangtze Block are basically derived by crustal melting with limited mixing of juvenile material during the magma generation.

Petrologic diversity of Plio-Quaternary post-subduction volcanism in northwestern Mexico: An example from Isla San Esteban, Gulf of California

AbstractDuring the Plio-Quaternary, numerous volcanic centres of contrasted compositions were emplaced in the Baja California Peninsula and adjacent Gulf of California. The 43 km2 Isla San Esteban (28o 42 N and 112o 34’ W), located in the central part of the gulf near the Pliocene extinct spreading centre of Lower Tiburón basin, is mostly made of andesitic, dacitic and rhyolitic subaerial lava flows, domes, and pyroclastic deposits. A new geological sketch map of the island is presented, as well as new K-Ar ages ranging from 4.5 to 2.5 Ma. Isla San Esteban lavas belong to two nearly contemporaneous magmatic series emplaced within an extensional tectonic framework related to the continental breakoff followed by the progressive oceanization along the Gulf of California. The first one is medium-K calc-alkaline, and its magmas evolved from basaltic andesites to rhyolites by fractionation of plagioclase, pyroxenes, titanomagnetite and apatite. The second one includes genuine adakites, and dacites derived from them through separation of plagioclase plus amphibole. We propose that these adakites originated from the partial melting of oceanic crust slivers, incorporated to the subcontinental lithospheric mantle during the Late Miocene opening of a slab window followed by the detachment of the deep part of the Farallon slab. The high thermal flux linked to the opening of the Gulf would have triggered their melting, as well as that of the subduction-modified mantle, which represents the likely source of the associated calc-alkaline lavas.

Distinguishing high‐alumina mare basalts using Clementine UVVIS and Lunar Prospector GRS data: Mare Moscoviense and Mare Nectaris

High‐alumina (HA) mare basalts are a unique group of the lunar sample collection. Sample geochemistry indicates that these basalts are derived from sources composed of late‐stage cumulates from the Lunar Magma Ocean (LMO). Their aluminous nature suggests their sources contained significant plagioclase, which has implications regarding the efficiency of plagioclase separation from earlier forming, mafic cumulates in the LMO to form the anorthositic lunar crust, and hence the heterogeneity of the lunar mantle. The Apollo and Luna missions sampled HA basalts from four different locations that are separated by 80 equatorial degrees (∼2400 km). Radiometric age dating of these samples demonstrates aluminous basaltic volcanism spanned over 1 billion years, suggesting HA basalts may be more prevalent on the Moon than implied by the sample population. Knowing their global occurrence would ultimately enhance our understanding of lunar evolution. Aluminous mare basalts occupy a unique location in Th‐FeO compositional space that suggests they can be identified using global remote‐sensing data of the Moon. We present our approach for distinguishing exposures of HA basalts using Clementine ultraviolet‐visible‐infrared (UVVIS) and Lunar Prospector Gamma Ray Spectrometer (LP‐GRS) data with constraints based on the FeO, TiO2, and Th abundances of Apollo and Luna HA samples. We identified 34 regions of interest (ROIs) where HA basalts could be a prominent component of the lunar surface. By analyzing the rims and proximal ejecta from small impacts (0.4–4 km in diameter) into the mare surface we characterized compositionally distinct basaltic units that make up the mare and thus determined which units represent HA basalt lavas. The results were used to generate maps that depict discrete mare units and classify their general basalt type. Here we focus on two ROIs: Mare Moscoviense and Mare Nectaris. Mare Moscoviense is composed of four basaltic units, two of which are HA candidates. Clementine UVVIS data of Mare Nectaris show evidence of up to three mare basalt units. One is the remnants of a mid‐Ti unit that capped earlier low‐Ti flows. The majority of the basin is filled by a compositionally indistinguishable low‐Fe, low‐Ti basalt. However, spectral profiles suggest there are two units. Regardless, the units both fit the criteria for a HA basalt.

Delayed accumulation of plagioclase in the Bushveld Complex

AbstractThe Upper Critical Zone of the Bushveld Complex consists of several cyclic units, each ranging from ultramafic to leucocratic. One view is that they were initiated by addition of, and crystallization from, relatively magnesian magma. Experimental studies on plausible parental magma compositions to this sequence show that plagioclase joins orthopyroxene in the crystallization sequence once the Mg# of the orthopyroxene has decreased below 83. Some norites from the Upper Critical Zone contain orthopyroxene with this Mg#, supporting the validity of the experimental studies. There are no orthopyroxene compositions with higher Mg# than 83 in the entire Upper Critical Zone in either the eastern or western limbs of the Bushveld Complex. This observation suggests that all these rocks formed from plagioclase-saturated magmas. If this interpretation is correct, pyroxenites in this succession are not the result of crystallization from a magnesian magma. Instead, they result from the mechanical separation of plagioclase and orthopyroxene, probably due to crystal sorting during settling, from a magma lying at the cotectic for these two minerals. Rocks exist in the Upper Critical Zone that contain non-cotectic proportions of cumulus plagioclase and orthopyroxene, again supporting models of crystal sorting during settling. In such a model, anorthosites result from the delayed accumulation of plagioclase relative to pyroxene, and not to formation from a magma saturated only in plagioclase.When traced from northwest to southeast in the western limb, there is a change in the relative proportions of plagioclase to orthopyroxene in the Upper Group 2 chromitite cyclic unit. In the northwest the unit is dominated by ultramafic rocks. In the southeast the plagioclase to pyroxene ratio exceeds that of the cotectic proportion, relations that may result from the lateral transport of suspended plagioclase grains to the southeast.

Subduction-related Jurassic andesites in the northern Great Caucasus

During the Jurassic the major tectonic units of the Great Caucasus (Bechasyn, Front Range, Main Range and Southern Slope zone) were affected by intensive magmatic activity. Magmatism within the Bechasyn zone, the northernmost unit, which represents the southern part of the Variscan-consolidated Skythian platform is considered here. With the beginning of the Early Jurassic this zone was reactivated by subsidence, accompanied by the deposition of epicontinental shallow water sediments. The Lower Jurassic portion of this sedimentary pile was intruded by numerous sills which display a clear temporal and spatial evolution. The older basic rocks are lower in the profile than the younger, more acidic rocks. A set of 75 samples, representing all exposed sills and their feeder-dikes, was analyzed for major and 21 trace elements. All samples appear more or less affected by alteration under lower greenschist facies conditions. However, these alterations essentially took place on local scales and did not affect the overall chemistry. According to their main element composition the rocks constitute a calc-alkaline series ranging from basaltic—andesitic to rhyolitic. Most of the samples are andesites. Chemically, these andesites closely resemble modern orogenic andesites occurring at convergent plate margins. Altogether, the field evidence and the chemical and mineralogical data obtained show the investigated rocks to be comagmatic and derived from basalt—andesitic initial melts by magmatic fractionation processes. Tholeiitic melts have to be considered as parental magmas, which according to the trace element characteristics of the basalt-andesitic rocks, were generated from an enriched peridotitic mantle source. 87Sr/86Sr isotope ratios and σ18O values confirm the mantle origin of this rock series. The observed compositional evolution can be explained as a result of olivine and clinopyroxene fractionation of the tholeitic melts followed by amphibole and plagioclase separation. 40Ar/39Ar measurements on biotite and plagioclase phenocrysts separated from these rocks vary between 190 and 180 Ma and thereby place the magmatic activity in the late Early Jurassic, in good agreement with the stratigraphic observations. Genetically, the calc-alkaline rocks are related to a subduction zone of the Andean type. Their chemical and isotopic compositions and their age setting corroborate the plate tectonic models for the evolution of the Caucasus orogenic belt during the Jurassic.

Is plagioclase removal responsible for the negative Eu anomaly in the source regions of mare basalts?

The nearly ubiquitous presence of a negative Eu anomaly in the mare basalts has been suggested to indicate prior separation and flotation of plagioclase from the basalt source region during its crystallization from a lunar magma ocean (LMO). Are there any mare basalts derived from a mantle source which did not experience prior plagioclase separation? Crystal chemical rationale for REE substitution in pyroxene suggests that the combination of REE size and charge, M2 site characteristics of pyroxene, fO 2, magma chemistry, and temperature may account for the negative Eu anomaly in the source region of some types of primitive, low TiO 2 mare basalts. This origin for the negative Eu anomaly does not preclude the possibility of the LMO as many mare basalts still require prior plagioclase crystallization and separation and/or hybridization involving a KREEP component.

Europium mass balance in polymict samples and implications for plutonic rocks of the lunar crust

From correlations of Sm concentration and Sm Eu ratio with Th concentration for a large number of polymict samples from various locations in the lunar highlands and the value of 0.91 μg/g for the mean Th concentration of the highlands surface crust obtained by the orbiting gamma-ray experiments, we estimate the mean concentrations of Sm and Eu in the lunar surface crust to be between 2 and 3 μg/g Sm and 0.7 and 1.2 μg/g Eu. The compositional trends indicate that there is no significant enrichment or depletion of Eu, on the average, compared to Sm relative to chondritic abundances; i.e., there is no significant “Eu anomaly” in average upper crust. Although rich in plagioclase (~70%), the upper crust does not offer evidence for a gross vertical separation of plagioclase from the final liquid from which it crystallized. This and the chondritic ratio of Eu Al in average highlands material imply that the net effect of the processes that led to formation of the lunar crust was to put most of the Al and incompatible elements in the crust. The concentration of Eu in most polymict samples from the highlands exceeds that which can be supplied by the fraction of KREEP necessary to supply the incompatible trace elements plus the fraction of plagioclase necessary to supply the Al, if the plagioclase has the composition of that in ferroan anorthosite. Among plutonic rocks, only plagioclase in rocks from the magnesian suite can supply the “excess” Eu in the polymict rocks. Owing to the intermediate value of the mean Mg Fe ratio of the crust, a significant fraction of the mafic rocks of the lunar highlands must have lower Mg Fe ratios than the norites and troctolites of the magnesian-suite of plutonic rocks. A large fraction of the plagioclase in the lunar crust is associated not with ferroan anorthosite, but with more mafic rocks ( e.g., norite, anorthositic norite). There is little evidence in the Eu data that the lunar crust ever consisted of a thick shell of nearly pure plagioclase, as envisioned in some formulations of the “magma ocean” model of its formation.

Petrogenesis of tin‐bearing granites of the Emuford district, Herberton tinfield, Australia

Late Palaeozoic biotite granites from the Emuford district of the Herberton tinfield form part of the Emu Suite, a group of fluorine‐rich, high‐silica, ilmenite series granites associated with extensive tin mineralization in the eastern Tate Batholith. The Emu Suite granites are enriched in F, Rb, U, Th, Y and Sn and depleted in TiO2, MgO, MnO, CaO, P2O5, Ba, Sr and Eu compared with many other silicic granites. Fractionation trends from early coarse grained to later medium and fine grained granites include increases in Al2O3, Na2O, F, Rb, Th, U, Hf, Y, Nb, W, Sn, Ga and HREE, and decreases in TiO2, MgO, Ba, Sr, Eu, La and Ce. These trends are compatible with an evolution via fractionation of a fluorine‐rich magma involving mainly separation of plagioclase and minor zircon and monazite. The Emu Suite granites form part of a much more extensive group of high‐Si granites associated with tin mineralization in the region. These tin‐bearing granites are amongst the earliest intrusions of a major Late Palaeozoic...

The origin of island arc high-alumina basalts

A detailed examination of the hypothesis that high-alumina basalts (HAB) in island arcs are primary magmas derived by 50–60% partial melting of subducted ocean crust eclogite shows that this model is unlikely to be viable. Evidence suggests that the overwhelming majority of arc HAB are porphyritic lavas, enriched in Al2O3 either by protracted prior crystallization of olivine and clinopyroxene, or by plagioclase phenocryst accumulation in magmas of basaltic andesite to dacite composition. Experimentally-determined phase relationships of such plagioclase-enriched (non-liquid) compositions have little bearing on the petrogenesis of arc magmas, and do not rule out the possibility that arc HAB can be derived by fractionation of more primitive arc lavas. Although models invoking eclogite-melting can match typical arc HAB REE patterns, calculations indicate that the Ni and Cr contents of proposed Aleutian primary HAB are many times lower than such models predict. In contrast, Ni vs Sc and Cr vs Sc trends for arc HAB are readily explained by olivine (+Cr-sp) and clinopyroxene-dominated fractionation from more primitive arc magmas. GENMIX major element modelling of several HAB compositions as partial melts of MORB eclogite, using appropriate experimentally (26–34 kb)-determined garnet and omphacite compositions yields exceptionally poor matches, especially for CaO, Na2O, MgO and Al2O3. These mismatches are easily explained if the HAB are plagioclase-accumulative. Groundmasses of arc HAB are shown to vary from basaltic andesite to dacite in composition. Crystal fractionation driving liquid compositions toward dacite involves important plagioclase separation, resulting in development of significant negative Eu anomalies in more evolved lavas. Plagioclase accumulation in such evolved liquids tends to diminish or eliminate negative Eu anomalies. Therefore, the absence of positive Eu anomaly in a plagioclase-phyric HAB does not indicate that plagioclase has not accumulated in that lava. In addition, we show that plagioclase phenocrysts in arc HAB are not in equilibrium with the liquids in which they were carried (groundmass). Exceptional volumes of picrite and olivine basalt occur in the Solomons and Vanuatu arcs; the presence in lavas from these and other arcs (Aleutian, Tonga) of olivine phenocrysts to Fo94, finds no ready explanation in the primary eclogite-derived HAB model. We suggest that most lavas in intra-oceanic arcs are derived from parental magmas with >10% MgO; fractionation of olivine (+Cr-sp) and clinopyroxene drives liquids to basalt compositions with <7% MgO, but plagioclase nucleation is delayed by their low but significant (<1%?) H2O contents. Thus evolved liquid compositions in the basaltic andesite—andesite range may achieve relatively high Al2O3 contents (<17.5%). The majority of arc basalts, however, have Al2O3 contents in excess of 18%, reflecting plagioclase accumulation. We give new experimental data to show that HAB liquids may be generated by anhydrous, low-degree (<10%) partial melting of peridotite at P<18 kb. Relative to arc HAB, these experimental melts have notably higher Mg#(69–72) and are in equilibrium with olivine Fo87–89. Only further detailed trace element modelling will show if they might be parental magmas for some arc HAB.

Early crystallization history of alkali olivine basalts, Diamond Craters, Oregon

Basaltic lavas form the isolated Quaternary volcanic centers of Diamond Craters and occur at Cow Lakes in S.E. Oregon. The exposed rocks are olivine and plagioclase phyric alkali olivine basalts. The holocrystalline groundmasses contain plagioclase, titanaugite, olivine, Fe-Ti oxides and minor apatite, and occur in the high-alumina (greater than 17%) petrographic province of the northwestern Great Basin. The whole rock and phenocryst compositions provide enough information to define the equilibrium state of the magmas. The solutions to the thermodynamic equations are valid for systems of known bulk composition and can be used to evaluate whether rock compositions are close to magma compositions. Two of the lavas from Diamond Craters have whole rock and phenocryst compositions consistent with their being natural melts. Rock compositions that reflect the effects of crystal separation (open systems) are recognized by significant differences in predicted versus observed phenocryst modes. The P-T conditions of phenocryst saturation and the crystallization history of the magmas can be inferred from petrologic data combined with thermodynamic modelling and mass balance constraints. The initial melts are inferred to have equilibrated at 0.8–1.0 GPa and 1210–1240°C. The majority of the lavas from Diamond Craters have compositions that reflect olivine and plagioclase crystal separation. These lavas can be related to two possible parent rock compositions by small amounts (2–8% by weight) of olivine and plagioclase separation.

Geochemistry of the North Mountain basalts (Nova Scotia, Canada)

The Early Mesozoic North Mountain basalts of Nova Scotia, Canada, are continental tholeiites similar in composition to numerous Triassic diabase dykes of the northern Appalachians. All these rocks probably originated during the initial opening of the Atlantic Ocean. The lavas have been affected by zeolite facies metamorphism which caused mobilization of some elements such as Li and Cu. The North Mountain basalts underwent extensive fractional crystallization dominated by pyroxene and plagioclase separation. Compared to oceanic tholeiites, these basalts have similar abundances of major, rare-earth and some high-field-strength elements, and corresponding element ratios, but they have higher contents of Th and K and associated elements. Their normalized trace-element patterns are closely similar to those of some other continental tholeiites and to the average continental crust, and display negative Nb anomalies. It is suggested that continental tholeiites were derived from an upper-mantle source similar to that for oceanic tholeiites but were affected by crustal contamination.

A Tale of Two Plutons: Petrographic and Mineralogic Constraints on the Petrogenesis of the Red Lake and Eagle Peak Plutons, Central Sierra Nevada, California

The Red Lake and Eagle Peak plutons are similar in age (K-Ar: 87-89 m.y.) and range in composition from granodiorite to granite. The Red Lake pluton is equigranular, has a locally greisened marginal zone, and shows only minor mineralogical and chemical zoning, whereas the Eagle Peak pluton is mineralogically, compositionally, and texturally zoned, with an equigranular margin and a porphyritic core. Modal and normative trends within each pluton are consistent with separation of plagioclase and mafic minerals leading to a residual liquid enriched in quartz and alkali feldspar. The formation of the prominent alkali feldspar megacrysts in the Eagle Peak core was promoted by plagioclase separation, $$H_{2}O$$ undersaturation, and a slower cooling rate relative to the Red Lake magma. Early loss of volatiles and a resulting increase in magma (melt + crystals) viscosity inhibited crystal segregation within the Red Lake pluton and prevented the development of zoning. Late stage aplites associated with both plutons formed from residual liquids created by crystal fractionation. Biotite and amphibole equilibria suggest internally buffered oxygen fugac-ity through most of the crystallization of the plutons. Comparison of mineralogic and whole rock compositional data with experimental studies on natural and synthetic sytems suggests a minimum intrusive temperature of 750°C and a load pressure of $$1.0 \pm 0.5 kb$$. Late stage hydrothermal alteration within each pluton reequilibrated feldspar compositions at subsolidus conditions. The presence of homogeneous cores in the plagioclase, early titanite crystallization in the Red Lake magma, and irregular hornblende compositions suggest refractory material was present when the magmas were intruded. Inferred source regions for the two plutons are amphibolite for the Red Lake magma and a more biotite-rich amphibolite for the Eagle Peak magma.