Underplating of Mesozoic Mantle-derived Magmas in Tongling, Anhui Province: Evidence from Megacrysts and Xenoliths

Lithological observations and mineralogical analyses on pyroxene and hornblende megacrysts and pyroxene and hornblende cumulates in xenoliths in the Mesozoic plutons of the Tongling region, Anhui Province, provide evidence for the magmatic underplating of mantle-derived alkali-olivine basalt at circa 140 Ma. The pyroxene and hornblende megacrysts and cumulates were formed through the AFC process at depths ranging from 27 to 35 km.

Melt Pockets in Garnet Megacrysts from Cenozoic Alkali Basalts of the Shavaryn Tsaram Vicinity,Mongolia

Garnet megacryst with a multiphase inclusion from intraplate alkali basalts of the Shavaryn Tsaram(Tariat,Mongolia)was the object of the study.This unusual aggregate consists of porous glass,Ti-rich biotite,orthopyroxene,spinel,clinopyroxene,olivine,and ilmenite.Win TWQ 2.32 thermodynamic simulation of this system revealed a few intervals of equilibrium.Pressure and temperature adjustment reflected in the paragenetic minerals of the melt pocket.The capture of already crystallised garnet megacryst was at P=0.8-1 GPa and T=1120-1160℃.Mineral crystallisation inside the melt pocket,accompanied by external inputs,occurred at P=0.75-0.95 GPa;T=790-1120℃.Symplectite assemblage formed in the garnet megacryst due to decomposition at(P=0.55-0.7 GPa;T=850-930℃).The study of the oxygen isotope content in primary garnet and biotite of the melt pocket showed that the δ^(18)O_(VSMOW) values are the same and correspond to that of typical mantle xenoliths.However,the chemical and microcomponent composition of the melt pocket minerals reveals a material that differs from basalts and peridotites.Thus,it has been revealed that the multiphase inclusion in the garnet megacryst formed not only on account of the garnet’s substance,but also due to the entrapped material of the Earth’s interior.

Helium isotope studies of the mantle xenoliths and megacrysts from the Cenozoic basalts in the eastern China

Helium isotope compositions of the mantle xenoliths and megacrysts in the Cenozoic basalts in the eastern China were measured. The samples were collected from Ludao of Heilongjiang, Huinan and Jiaohe of Jilin, Kuandian of Liaoning, Hannuoba of Hebei, Nüshan of Anhui, Dingan of Hainan. The 3He/4He ratios of the mantle xenoliths and megacrysts from the most areas were about 1×10-5 , and were similar to those of the MORB, thus reflecting the characteristics of the MORB-typed depleted mantle. The 3He/4He ratios of the mantle xenoliths from Jiaohe were 4.8×10-6 and the 3He/4He ratios of xenoliths from Hannuoba vary from 0.15×10-6 to 7.4×10-6 , obviously lower than those of the MORB, and even lower than the atmospheric helium isotope ratios, indicating that the continental mantle was strongly replaced in Jiaohe and Hannuoba areas. The helium isotope compositions of the mantle xenoliths and megacrysts in the same region vary in a very wide range. It is inferred that the mantle xenoliths and megacrysts were from different parts of the continental mantle. There were not necessary origin relations between the mantle xenoliths, megacrysts and their host basalts. An extremely high 3He/4He ratio of garnet megacryst from Hannuoba, Hebei was found.

Hydrogen isotope compositions of mantle-derived amphibole megacrysts from Qilin, Guangdong and its tectonic significance

Hydrogen isotope compositions of mantle-derived amphibole megacrysts from Qilin, Guang dong Province have been obtained by ion micro-probe. δD and H contents are constant both among different samples and within single sample, demonstrating that their formation condition is very stable.High δD values suggest the presence of a component recycled from crust which is possibly related to the subduction of Pacific Plate beneath Eurasian Plate in Mesozoic.

Megacrysts in the Cenozoic basalt of the Tuoyun Basin,Southwest Tianshan

Abundant megacrysts of clinopyroxene, amphibole, anorthoclase, and phlogopite are found together with deep-seated xenoliths in the Cenozoic basalt of the Tuoyun Basin, Southwest Tianshan. The megacrysts are mainly in the cone sheet formed at the early stage of the volcanic activity. Clinopyrox-ene megacrysts are located in the lower part of the profile, with amphibole and phlogopite megacrysts in the middle part and anorthoclase megacrysts in the upper part. The crystal integrity, absence of de-formation fabric and their relation to the host basalt suggest that they were crystallized from the host magma and quickly transported to the surface. The mineralogical studies imply that the clinopyroxene megacrysts are of Al-augite with higher Al2O3 (>9%). Amphibole megacrysts are kaersutite rich in TiO2 (>4.5%). Sulfide inclusions such as pyrrhotite occur in some clinopyroxene and amphibole megacrysts. Thermodynamic calculations reveal that pyroxene megacrysts formed under the temperature of 1185.85―1199.85℃ and the pressure between 1.53 and 1.64 GPa comparable to the crust-mantle boundary and amphibole megacrysts crystallized under the pressure of around 0.85 GPa, temperature about 1000℃ comparable to the depth of 30 km. Anorthoclase megacrysts crystallized under the pressure between 0.8―1 GPa,temperature about 900℃.The absence of Ti-rich inclusions such as rutile can be considered as an evidence of quick magma ascending. The P-T conditions estimated via py-roxene megacrysts and phenocrysts compose a P-T path with a steep slope. It can be considered as another evidence of quick magma ascending. However, the estimated temperatures for amphibole megacrysts are markedly lower than those for pyroxene megacrysts given the same pressure. It probably shows that the amphiboles have crystallized at the vanguard of magma and under the vola-tile-rich condition. Thus, we can conclude that the Cenozoic basalts are produced in an extensional tectonic setting and the processes governing crystallization and ascending of the megacrysts are very complex.

Almandine megacrysts from Yingfengling Cenozoic basalt in Leizhou Peninsula and their parental magma origin

The garnet megacrysts from Yingfengling basalts are characterlzed by high FeO (】20%), CaO (7.02% -8.16%) and low MgO (5.88%-10.87%). Significant composition variations are observed in these megacrysts, of which Ni, V, Sc, Co, and HREE are positively correlated with their Mg# and Zr, Hf, Ga, Y, Sr, Nb, Zn and LREE-MREE are negatively correlated with Mg#. Megacryst parent magma is a highly evolved residual melt with strongl depletion in Ti, Sr, Hf, Nb and HREE. This parental magma was generated by more than 60% of crystallization fractionation of clinopyroxene, garnet, plagioclase and ilmenite from quartz tholeiitic magma. It has not erupted to the surface, but stayed at the upper mantle and formed the megacrystic cumulate. Megacrysts and their host basalt are in disequilibrium.

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe,Arkhangelsk province,Russia:Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths (garnet peridotite,eclogite and clinopyroxene-phlogopite rocks) and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle (SCLM) and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation (Phl1) occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation (Phl2) occurs as rims and outer zones that surround the Phl1 grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phl1,within the garnet peridotite and clinopyroxeneephlogopitexenoliths,is characterised bylow Ti and Cr contents (TiO2<1 wt.%,Cr2O3<1 wt.% andMg#=100×Mg/(MgtFe)>92) typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr2O3 despite a wider range of TiO2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations (TiO2 > 2 wt.%,Cr2O3 > 1 wt.% and Mg#=100 × Mg/(Mg + Fe)< 92) and compositionally overlaps with phlogopite from polymict breccia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phl1 grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.

The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton:Links between mantle metasomatism,thermal state and diamond potential

Although the diamond potential of cratons is linked mainly to thick and depleted Archean lithospheric keels, there are examples of craton-edge locations and circum-cratonic Proterozoic terranes underlain by diamondiferous mantle. Here, we use the results of comprehensive major and trace-element studies of detrital garnets from diamond-rich Late Triassic(Carnian) sedimentary rocks in the northeastern Siberia to constrain the thermal and chemical state of the pre-Triassic mantle and its ability to sustain the diamond storage. The studied detrital mantle-derived garnets are dominated by low-to mediumCr lherzolitic(~45%) and low-Cr megacrystic(~39%) chemistries, with a significant proportion of eclogitic garnets(~11%), and only subordinate contribution from harzburgitic garnets(~5%) with variable CrOcontents(1.2–8.4 wt.%). Low-Cr megacrysts display uniform, “normal” rare-earth element(REE)patterns with no Eu/Eu* anomalies, systematic Zr and Ti enrichment(mainly within 2.5–5), which are evidence of their crystallization from deep metasomatic melts. Lherzolitic(G9) garnets exhibit normal or humped to MREE-depleted sinusoidal REE patterns and elevated Nd/Y(up to 0.33–0.41) and Zr/Y ratios(up to 7.62). Rare low-to high-Cr harzburgitic(G10) garnets have primarily “depleted”, sinusoidal REEpatterns, low Ti, Y and HREE, but vary significantly in Zr-Hf, Ti and MREE-HREE contents, Nd/Y(within 0.1–2.4) and Zr/Y(1.53–19.9) ratios. The observed trends of chemical enrichment from the most depleted,harzburgitic garnets towards lherzolitic(including high-Ti high-Cr G11-type) garnets and megacrysts result from either voluminous high-temperature metasomatism by plume-derived silicate melts or recurrent mobilization of less voluminous kimberlitic or related carbonated mantle melts, rather than the initially primitive, fertile nature of the Proterozoic SCLM. Calculated Ni-in-garnet temperatures(primarily within ~1150–1250 ℃) indicate their derivation from at least ~220 km thick Cr-undersaturated lithosphere at the relevant Devonian to Triassic thermal flow of ~45 m W/m^(2) or cooler. We suggest the existence of rare harzburgitic domains in the primarily lherzolitic diamond-facies SCLM beneath the northeastern Siberian craton at least by Triassic, whereas the abundance of eclogitic garnets, predominance of E-type inclusions in placer diamonds and specific morphologies argue for diamondiferous eclogites occurring within a ~50–65 kbar diamond window of the Olenek province by the same time.

Oxidation Experiment of Natural Megacrystal Clinopyroxene:Implications for Assignment of Mssbauer Spectra

The M?ssbauer spectra of natural megacrystal clinopyroxene are usually fitted by 4 sets of symmetric doublets, A-A', B-B', C-C' and D-D', respectively, in terms of increasing Qs value in literature. But the assignments of those doublets are quite different, except the D-D' doublet assigned to Fe3+at the lattice site M***1 in previous papers. Particularly, the assignment and interpretation of the C-C' doublet are diverse.

High-precision analysis of carbon isotopic composition for individual CO_(2)inclusions via Raman spectroscopy to reveal the multiple-stages evolution of CO_(2)-bearing fluids and melts

The carbon isotopic composition of CO_(2)inclusions trapped in minerals reflects the origin and evolution of CO_(2)-bearing fluids and melts,and records the multiple-stages carbon geodynamic cycle,as CO_(2)took part in various geological processes widely.However,the practical method for determination isotope composition of individual CO_(2)inclusion is still lacking.Developing a microanalytical technique with spatial resolution in micrometers to precisely determinate theδ^(13)C value of individual CO_(2)inclusion,will make it possible to analyze a tiny portion of a zoning mineral crystal,distinguish the differences in micro-scale,and possible to find many useful information that could not be obtained with the bulk extraction and analysis techniques.In this study,we systematically collected Raman spectra of CO_(2)standards with different d13C values(34.9‰to 3.58‰)at 32.0℃and from 7.0 MPa to 120.0 MPa,and developed a new procedure to precisely determinate theδ^(13)C value of individual CO_(2)inclusion.We investigated the relationship among the Raman peak intensity ratio,d13C value,and CO_(2)density,and established a calibration model with high accuracy(0.5‰1.5‰),sufficient for geological application to distinguish different source of CO_(2)with varyingδ^(13)CO_(2).As a demonstration,we measured theδ^(13)C values and the density of CO_(2)inclusions in the growth zones of alkali basalt-hosted corundum megacrysts from Changle,Shandong Province.We found the significant differences of density and d13C between the CO_(2)inclusions in the core of corundum and those inclusions in the outer growth zones,the d13C value decreases from core to rim with decreasing density:δ^(13)C values are from7.5‰to9.2‰for the inclusions in the core,indicating the corundum core was crystallized from mantle-derived magmas;from13.5‰to18.5‰for CO_(2)inclusions in zone 1 and from16.5‰to–22.0‰for inclusions in zone 2,indicating the outer zones of corundum grew in a lowδ^(13)C value environment,resulted from an infilling of low d13C value fluid and/or degassing of the ascending basaltic magma.

Mineral Chemistry and Crystallization Conditions of the Late Cretaceous Mamba Pluton from the Eastern Gangdese, Southern Tibetan Plateau

The Late Cretaceous Mamba granodiorite belongs to a part of the Mesozoic Gangdese continental magmatic belt. No quantitative mineralogical study has been made hitherto, and hence the depth at which it formed is poorly constrained. Here we present mineralogical data for the Mamba pluton, including host rocks and their mafic microgranular enclaves（MMEs）, to provide insights into their overall crystallization conditions and information about magma mixing. All amphiboles in the Mamba pluton are calcic, with ~B（Ca＋Na）〉1.5, and Si=6.81-7.42 apfu for the host rocks and Si=6.77-7.35 apfu for the MMEs. The paramount cation substitutions in amphibole include edenite type and tschermakite type. Biotites both in the host rocks and the MMEs collectively have high Mg O（13.19 wt.%-13.03 wt.%） contents, but define a narrow range of Al apfu（atoms per formula unit） variations（2.44-2.57）. The oxygen fugacity estimates are based on the biotite compositions cluster around the NNO buffer. The calculated pressure ranges from 1.2 to 2.1 kbar according to the aluminum-in-hornblende barometer. The computed pressure varies from 0.9 to 1.3 kbar based on the aluminum-in-biotite barometer which corresponds to an average depth of ca. 3.9 km. Besides, the estimates of crystallization pressures vary from 0.8 to 1.4 kbar based on the amphibole barometer proposed by Ridolfi et al.（2010）, which can be equivalent to the depths ranging from 3.1 to 5.2 km. The MMEs have plagioclase oscillatory zonings and quartz aggregates, probably indicating the presence of magma mixing. Besides, core-to-rim element variations（Rb, Sr, Ba, and P） for the K-feldspar megacrysts serve as robust evidence to support magma mixing and crystal fractionation. This indicates the significance of the magma mixing that contributes to the formation of K-feldspar megacryst zonings in the Mamba pluton.

Discovery of mantle and lower crust xenoliths from early Cretaceous volcanic rocks of southwestern Tianshan,Xinjiang

In Tuoyun area of southwestern Tianshan, mantle and lower crust xenoliths are present In the volcanic rocks with ages of 101-123 Ma. Mantle xenoliths include mineral megacrysts such as kaersutite and pargasite, feldspar, biotite, and rare pyroxene and rock fragments such as perodotite, pyroxenite, amphibolite, and rare glimmerite. Lower crust xenoliths are mainly banded and massive granulite. The volcanic rocks were produced by within-plate magmatism. Occurrence of hydrous and volatile mineral megacrysts, amphibolite, and some pyroxenite containing hydrous and volatile minerals indicates that mantle metasomatism was intense. Undoubtedly, this discovery is very important to understanding of the crust-mantle structure and geodynamic background in depth in southwestern Tianshan and geological correlation with adjacent regions.

Hydrogen isotope composition of mantle-derived mica megacryst from ion micro probe analysis

The hydrogen isotope composition of a mantle-derived mica megacryst from Cenozoic basanite from NUshan, Anhui Province has been determined by ion micro probe. The results demonstrate that δD and water content of the megacryst were heterogeneous on the micro scale, which resulted from reaction with meteoric water after being brought to the surface. The primary δD of mica megacrysts was about-23‰, suggesting the recycled crustal materials in its source. By combining these values with those of other researchers, it is believed that the hydrogen isotope composition of the mantle is heterogeneous at least on the large scale.

Oxygen isotope composition of mantle-derived materials from Anhui-Jiangsu basalt province, East China

OXYGEN isotope composition is an important key to an understanding of petrogenesis,mantleheterogeneity,crust-mantle interaction,etc.The great mass of data of the mantle-derived ma-terials published abroad have announced the δ18O heterogeneity of the mantle,the existence

REE AND Sr, Nd, Pb ISOTOPIC GEOCHEMISTRY OF HUANGYISHAN BASALT, KUANDIAN, LIAONING, NORTHEAST CHINA

In the light of major element geochemistry, mineral chemistry and REE and isotopic data, the small but apparent isotopic differences between the Cenozoic volcanic rocks east and west of the Tancheng-Lujiang fault are believed to be caused by the mixing and metasomatism of crustal and mantle material in tho mantle source region in response to Pacific plate subduction. The presence of phlogopite and pargasite in mantle xenoliths lends strong support to mantle metasomatism.

A Comparison between the ~1.08–1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa

Along the eastern margin of the Mesoproterozoic Namaqua metamorphic province(NMP) of southern Africa are a bimodal volcano-sedimentary succession, the ~1.13–1.10 Ga Koras Group, composed of rhyolitic porphyries and basaltic andesites, and the ~1.11–1.07 Ga late-to post-tectonic granitoids of the Keimoes Suite. This review examines existing whole-rock major-and trace-element data, along with isotope chemistry(with some new isotopic data), to investigate the role these two magmatic successions played in terms of post-collisional magmatism of the eastern NMP near the boundary with the Archean Kaapvaal Craton. The Keimoes Suite comprises variably porphyritic biotite monzogranites and granodiorites, with a charnockitic member. They are metaluminous to weakly peraluminous, ferroan, and calc-alkalic. They exhibit large ion lithophile(LIL) element enrichment relative to the high field strength elements(HFSE) with depletions in Ba, Sr, Nb, P, Eu and Ti, and enrichments in Th, U and Pb. Isotopic values(ε_(Nd)(t): 2.78 to-2.95,but down to-8.58 for one granite, depleted mantle Nd model ages(T_(DM)): 1.62–1.99 Ga, but up to 2.55 Ga;initial ^(87)Sr/^(86)Sr: 0.652 82–0.771 30) suggest derivation from weakly to mildly enriched(and radiogenic)sources of Meso-to Paleoproterozoic age, the former of more juvenile character. The Koras Group is characterized by a bimodal succession of calcic to calc-alkalic, magnesian and tholeiitic basaltic andesites and mostly metaluminous to peralkaline rhyolitic porphyries. Two successions are recognised, an older, lower succession that extruded at ~1.13 Ga, and a younger, upper succession at ~1.10 Ga. The rhyolitic porphyries of both successions show similar LILE/HFSE enrichment and the same element enrichments and depletions as the Keimoes Suite granitoids. The upper succession is consistently more fractionated in terms of both whole-rock major and trace element chemistry, and, isotopically, has a greater enriched source component(ε_(Nd(t):-0.69 to-4.26;T_(DM): 1.64–2.44 Ga), relative to the lower succession(ε_(Nd(t): 0.74–5.62;T_(DM): 1.28–2.12 Ga). Crystal fractionation of plagioclase and K-feldspar appears to have played a role in bringing about compositional variation in many of the granites. These were derived from partial melting of mainly igneous with subordinate sedimentary sources from mostly lower crustal depths, although some granitoids have indications of a possible mantle source component. The lower succession of the Koras Group was derived by partial melting of subduction-influenced enriched mantle giving rise to mafic magmas that fractionated to give rise to the rhyolitic porphyries. The upper succession rhyolites were derived by crustal melting due to the input of mafic magmatism. Crystal fractionation was the main compositional driver for both successions. The Keimoes Suite granitoids and the Koras Group are associated with extensional regimes subsequent to the main deformational episode in the eastern NMP.