Proportion Of Garnet Research Articles

Cretaceous evolution of the Lhasa-Qiangtang Orogen: insights from mafic igneous rocks

ABSTRACT The compositional changes of successively emplaced mafic magmatites provide insights into the evolution of orogenic belts, particularly the transformation processes and properties of the orogenic lithospheric mantle. Herein, we present new zircon U‒Pb ages and Lu‒Hf isotopes, as well as whole-rock major, trace element, and Sr – Nd isotope compositions from three mafic intrusions in the Asuo area, central Tibet, to clarify the evolution of the Lhasa – Qiangtang Orogenic Belt (LQOB). These mafic intrusions can be classified into calc-alkaline basalts and are enriched in large ion lithophile elements and depleted in high-field strength elements. Significant differences in the whole-rock trace element compositions, Sr – Nd isotope values, and U – Pb ages stratify them into two groups, namely Groups I and II, of which Group I rocks (ca. 105 Ma) show lower87Sr/88Sri = (0.70694–0.70701), [Sm/Yb]N = (1.51–1.54) and higher εNd (t) = (−0.74 to −0.61) than Group II (ca. 97–95 Ma) (87Sr/88Sri = 0.70759–0.70870; εNd (t) = –4.58 to −3.75; [Sm/Yb]N = 2.43–2.94). These features indicate that the mantle source exhibits a progressive increase in the proportion of garnet, along with significant addition of subduction-related components into the primary melts. One scenario for producing such trace element patterns and gradually enriched radioactive isotope signatures involves the extensive heating of an oceanic slab beneath the LQOB by upwelling asthenosphere, which leads to the extensive melting of sediments within the slab. Identification of ca. 105–95 Ma Asuo mafic rocks suggests their formation in a post-collisional setting with additional heat input from the deep mantle. These results provide new evidence supporting the occurrence of the Lhasa – Qiangtang collision within the framework of north-south bidirectional subduction.

Determination of skarn garnet compositions using Raman spectroscopy

AbstractGarnet is a key and defining mineral of skarns and associated metalliferous deposits. Variation in garnet composition, commonly expressed by the proportions of different end‐members, is widely used to determine the physical–chemical features of hydrothermal fluids. Skarn garnets from the Fujiashan W‐Cu‐Mo deposit, eastern China, were investigated by Raman spectroscopy and electron probe microanalysis to assess the quantitative correlation between Raman band positions and proportions of garnet end‐members. Compositions and Raman band positions of so‐called “grandite” garnet (Adr18–98Grs0–79), where Adr and Gr are the end‐members andradite and grossular, respectively, display a spatial zonation at Fujiashan that correlates with the distance from the contact between skarn and causative intrusion. Raman band positions determined in the ranges 234–246, 351–368, 369–375, 515–544, 814–826, and 873–882 cm−1 demonstrate moderate to very strong (R2 up to 0.99) linear correlations with mol% andradite and grossular components. This is attributed to homovalent substitution between Fe3+ and Al3+ in the octahedral site, which has an indirect effect on bond lengths and angles of Si‐O vibration, resulting in linear variation of Raman band positions. The band between 515 and 544 cm−1 is the most sensitive to compositional variation, and its position enables robust estimation of end‐member proportions within 10% of results calculated from electron probe microanalysis. This research highlights the potential of Raman spectroscopy as a rapid, powerful method to assess the composition of skarn garnet, enabling accelerated construction of spatial zonation models for skarns during skarn deposit exploration.

Response of trace elements to partial melting of felsic crust at high to ultrahigh temperatures: Implications for granite geochemistry

It is known that partial melting and melt extraction of crustal rocks result in chemical differentiation of the continental crust. But it is unknown how these two processes have affected the composition of granites due to a limited knowledge of trace element behaviors during crustal anatexis. In order to quantify this issue, a combined study of whole-rock and mineral trace elements and mineral modal proportions was conducted for high-temperature (HT) to ultrahigh-temperature (UHT) felsic granulites from the Tongbai orogen in central China. Reconstruction of LREE budgets suggests that LREE mainly reside in monazite in both HT and UHT granulites. As monazite is predicted to be not stable at UHT conditions in the modelling of previous studies, the presence of monazite in the Tongbai UHT granulites suggest that it was not sufficiently dissolved into anatectic melts. Considering monazite mainly occurs as interstitial grains, fast melt-residue separation may be the dominant factor for impeding its dissolution. Calculation of mineral/mineral trace element ratios indicates that K-feldspar has a higher capacity to accommodate Ba and Sr than biotite and plagioclase, with DBaK-feldspar/biotite of 2.5–3.0 and DSrK-feldspar/plagioclase values of 1.4–1.9, and ilmenite can preserve more Nb and Ta than biotite with DNbbiotite/ilmenite values of 0.07–0.1 and DTabiotite/ilmenite values of 0.04–0.05. In addition, partitioning of first row transition elements (FRTE) was constrained in the felsic granulites, with preference of Sc into garnet, and Co, V, Ni, and Cr into biotite. Modelling based on the presently constrained partition coefficients indicates that trace elements show consistent differentiation trends during anatexis of various felsic rocks, with enrichment of Sr, Ba, Nb, Ta and FRTE but depletion of Rb in anatectic restites. The high DNb/Ta values of biotite and ilmenite cause the residues produced by biotite dehydration melting to be commonly of higher Nb/Ta ratios than protoliths, indicating that the residues are potentially high Nb/Ta reservoirs. In contrast, the change of peritectic minerals due to various protolith compositions and melting conditions results in differential responses of Sr, Ba, and FRTE to partial melting, suggesting that these elements may be useful indicators to trace the petrogenetic process of granites. The correlation between Ba and Sc contents or between Sr/Ba and Sc/Co ratios is demonstrated to be a valid index in discriminating melts from metagreywackes and metapelites due to different modes of garnet, orthopyroxene, and K-feldspar in their anatectic residues. Moreover, as the proportions of garnet and orthopyroxene in the residues are significantly influenced by melting pressures, the Sc/Co ratio and Sc content of granites can be used to trace their anatectic depth.

Ca isotopic compositions of zoned granitoid intrusion: Implications for the emplacement and evolution of magma bodies

To investigate the mechanism of magma emplacement of the intruded zoned pluton, we report high-precision Ca isotopic data of 23 granitoids (including granodiorite and quartz diorite), 5 mafic enclaves, and 7 hornblende and plagioclase mineral pairs from the Early Cretaceous Fangshan pluton, Beijing, China. Δ44/40CaHbl-Pl of rocks span from 0.02 ± 0.15‰ to 0.23 ± 0.12‰, which likely reflects equilibrium inter-mineral isotope fractionation. The granodiorites exhibit ∼0.30‰ variation in Ca isotopic compositions, with δ44/40Ca ranging from 0.47 ± 0.08‰ to 0.77 ± 0.09‰. The diorites and the enclaves yield δ44/40Ca of 0.51 ± 0.03‰ to 0.64 ± 0.05‰, and 0.54 ± 0.06‰ to 0.66 ± 0.11‰, respectively, which are lower than that of the upper continental crust (0.72 ± 0.10‰). Ca isotopic variation cannot be ascribed to K decay due to their low εCa values (< ±1.0). Theoretical study showed that Hbl fractional crystallization could counteract Ca isotope fractionation effect of their co-existing Pl crystallization. Source heterogeneity cannot account for Ca isotopic variation because δ44/40Ca does not correlate to (87Sr/86Sr)i. Instead, the Ca isotopic variation is likely controlled by partial melting and residual mineralogy. Specifically, δ44/40Ca of the granodiorites are negatively correlated with (Dy/Yb)N, reflecting various magma pools derived from different crustal depths with clinopyroxene and garnet residua. Modeling calculations suggest that the low Ca isotopic signature can be caused by ∼10% partial melting of sources with various proportions of garnet and clinopyroxene (from 5% garnet + 95% clinopyroxene to 40% garnet + 60% clinopyroxene). The diorites intruded earlier than the granodiorites, which can be slightly contaminated by surrounding basement rock. Ca isotopic compositions of enclaves are similar to those of their host rocks. Considering the spatial structure of the Fangshan pluton, it should be assembled by at least four magma pulses derived from various crustal depths through ballooning. Our results show that Ca isotopes can be used as a potential tool to trace magma source depth and decipher the process of magma emplacement of intrusive pluton, which is important to understand the continental crustal growth.

Phase relations and melting of nominally ‘dry’ residual eclogites with variable CaO/Na2O from 3 to 5 GPa and 1250 to 1500 °C; implications for refertilisation of upwelling heterogeneous mantle

This study investigates the phase and melting relations of nominally ‘dry’ residual eclogites (Res2 and Res3), with varying bulk CaO/Na2O ratios (4 and 12, respectively), from ~160 (5 GPa) to ~90 km (3 GPa) depth. Garnet, clinopyroxene and minor quartz/coesite are subsolidus phases in both compositions. In contrast to Res2, in Res3, the proportions of garnet always exceeding those of clinopyroxene. This also leads to higher modal quartz/coesite in Res3 relative to Res2.In modelling melting along a near-adiabatic upwelling path with a mantle potential temperature of ~1360 °C, at 5 GPa, near-solidus andesitic Res3 partial melts are much less siliceous and sodic, and are more calcic and magnesian than the incipient dacitic melts of Res2. Continuously self-fluxed melting increases considerably from 4 to 3 GPa due to the increased breakdown of Ca-Eskolaite solid solution component in clinopyroxene along the adiabat. This causes a steepening of the solidus, but more-so for Res2 than for Res3. At 3 GPa, the near exhaustion of residual clinopyroxene causes higher melt productivity for Res3 (~60%) than for Res2 (~30%), despite both melts being of basaltic-andesite composition. Resulting Res3 melts are therefore significantly more calcic and magnesian, and less sodic than those of Res2 melts.As Res3 undergoes a higher degree of melting relative to Res2 during adiabatic ascent, Res3 eclogitic residues become significantly more refractory; with relatively higher Mg# and grossular in garnet, higher Mg# and Ca-tschermaks, and lower jadeite components of clinopyroxene, and higher garnet/clinopyroxene ratios than eclogitic Res2 residuals.In upwelling heterogenous mantle domains, the siliceous eclogitic melts formed within a body of eclogite will react with encapsulating mantle peridotite, effectively refertilising it and producing hybrid pyroxene- and garnet-rich rocks. Subsequent melting of these sources may lead to compositionaly diverse primitive mantle-derived magmas, with high Ca/Al and low Na/Ca signatures indicators of preferential melting of a heterogeneous mantle, previously refertilised by recycled Ca-rich oceanic crustal material, and primitive magmas with low Ca/Al and high Na/Ca derived from melting of mantle with a ‘normal recycled crustal material signature’. Thus, compositional magma diversity may directly reflect precursor compositions of the mantle source region.

Mesoproterozoic high Fe–Ti mafic magmatism in western Shandong, North China Craton: Petrogenesis and implications for the final breakup of the Columbia supercontinent

High Fe–Ti basic rocks (FeOt>12wt% and TiO2>2wt%) are rare in the geological record and display a different evolutionary trend (the ‘Fenner’ trend) from the normal ‘Bowen’ trend of SiO2 enrichment. In this paper, we present geochronological, geochemical and Sr–Nd isotopic data for the Fe–Ti-rich gabbros from western Shandong within the Eastern Block of the North China Craton. Zircon U–Pb dating indicates that they were emplaced at ∼1.21Ga. All high Fe–Ti gabbros are characterized by low MgO (<7wt%) and Mg numbers (<50), and high FeOt (>12wt%), TiO2 (>2wt%) and P2O5 (mostly >0.4wt%). Geochemically, they can be divided into two different series (the alkaline series for the Yishui gabbros and the sub-alkaline series for the Feixian gabbros). Both series of Fe–Ti-rich gabbros show similar depletion in Th, U, Nb and Ta, and have positive Eu anomalies. The main difference is that the alkaline series displays slightly more fractionated REE patterns and more depleted Sr–Nd isotopic compositions than the sub-alkaline series; it also lacks positive P anomalies. Elemental geochemistry indicates that both series were likely generated by the partial melting of lherzolitic mantle, with different proportions of garnet in their sources. The mantle sources were re-enriched through varying degrees of hybridization of lower crustal materials prior to partial melting. The results of modeling based on the Sr–Nd isotopic compositions show that their mantle sources could have formed from the hybridization of N-MORB-like depleted mantle through involvement of around 1% lower crustal components for the alkaline series and 2% for the sub-alkaline series: this accounts for their geochemical signatures, especially for the marked depletion in Th and U, and weak negative Nb and Ta anomalies. A combination of Fe-rich mantle source and subsequent fractional crystallization, coupled with low fO2 during magmatic evolution, likely resulted in the Fe–Ti enrichment trend in both series of gabbros. In combination with regional and global studies, their generation could be related to a prolonged superplume that led to the final breakup of the Columbia supercontinent during Mesoproterozoic time.

Phase relation and mineral chemistry in pyrolite to 2200°C under the lower mantle pressures and implications for dynamics of mantle plumes

Phase relations in pyrolite were studied at pressures of 24 and 30 GPa and temperatures up to 2200°C. At 24 GPa and 1600°C, an assemblage of Mg‐perovskite, magnesiowüstite, Ca‐perovskite, and garnet was observed, with the proportion of garnet increasing continuously with temperature. At 30 GPa, only the two perovskites and magnesiowüstite were observed, and no significant changes in mineral proportions with temperature were observed. Iron was almost equally partitioned between Mg‐perovskite and magnesiowüstite at temperatures below about 1800°C at 24 and 30 GPa. With increasing temperature, however, iron was concentrated more in magnesiowüstite at both of these pressures. The decrease of Al content in Mg‐perovskite with temperature, which is caused by an increase in the proportion of garnet, is likely to be responsible for the iron partitioning behavior at 24 GPa, while the minor elements (Na, Al, and Cr) in magnesiowüstite may play an important role in Fe partitioning at 30 GPa. The density difference between ascending hot plumes (ΔT = 200 and 400°C) and the surrounding mantle was calculated using the present experimental data. The density difference (ΔT = 400°C) at 30 GPa (806‐km depth) is 0.06 g/cm3 but increases to 0.22 g/cm3 at 24 GPa (670‐km depth). The large increase in the density difference that occurs in a hot plume at the uppermost part of the lower mantle originates from an increase of the proportion of garnet and may be a trigger for the plume's passage through the boundary between the lower mantle and the mantle transition zone.

Petrogenesis of the late proterozoic cura�� mafic dyke swarm, Brazil: Asthenospheric magrnatisrn associated with continental collision

The late Proterozoic Curacâ mafic dyke swarm is one of the more alkaline swaams of the Brazilian Precambrian. Intea-dyke chemical variations are characterised by higher abundances of incompatible elements in dyke margins than in the dyke centres, a feature that may have resulted from flowage differentiation, from progressive evacuation of liquids from zoned magma chambers, or possibly through dynamic melting of an uprising diapir. Inter-dyke chemical differences are best interpreted by different extents of melting of a mantle source with a small modal proportion of garnet. This source was heterogeneous and had the trace element compositions of ocean island basalts with Dupal and non-Dupa' characteristics. The emplacement of the Curaca dykes from 650–700 Ma may have been facilitated by the extensional tectonics perpendicular to the collision zone between the Sao Francisco craton and the Pernambuco-Alagoas massif during the evolution of the Sergipano orogen. The energy in the system was insufficient to promote much melting of, or interaction with, the continental lithosphere or overlying crust.

Upper-mantle travel times in Australia — A preliminary report

This paper gives a preliminary account of an experiment to determine upper-mantle travel times in Australia. A set of fifteen 14-track tape-recording units are being used along a profile from Darwin to Alice Springs in central Australia. In addition to the 14-track recorders, eleven 6-track units are being used both on-line and off-line as monitors. The sources used are natural earthquakes occurring to the north of Australia, especially Banda Sea events. Preliminary analysis of the records from the first sub-array has confirmed the high apparent velocities for P and S previously found for the distance range 800–1,800 km in central Australia. It is inferred that the high apparent velocities are due to a sharp increase in the proportion of garnet at a depth of about 90 km. Neither the P nor S arrivals from 800 to 1,800 km show any evidence of the shadow zones or offsets in the travel-time curve which would be associated with an upper-mantle low-velocity zone. It is suggested, however, that a low-velocity zone is not precluded by the observations, but that the associated offset may not occur in the first-arrival part of the travel-time curve.

The diopside-kyanite join at high pressures and temperatures

Initial mixes of composition (molecular proportions) 2 anorthite: 2 enstatite: diopside have been crystallized at 1200°C over a pressure range of 13.5–36 kb. The stable mineral assemblage changes from anorthite+clinopyroxene solid solution+orthopyroxene at 13.5 kb to clinopyroxene solid solution+garnet+minor quartz at 18 kb to clinopyroxene solid solution+garnet+minor quartz+minor kyanite at 22.5–36 kb. The proportion of garnet and its grossular content increases with increasing pressure. On addition of 1/2, 1 and 2 moles of jadeite to the initial mix, crystallization of garnet is markedly inhibited and the grossular content decreases. This study has application to the relative stability of clinopyroxene-kyanite or garnet-quartz assemblages in natural rocks such as eclogites and grospydites from kimberlite pipes.

The diopside-kyanite joint at high pressures and temperatures

Initial mixes of composition (molecular proportions) 2 anorthite: 2 enstatite: diopside have been crystallized at 1200°C over a pressure range of 13.5–36 kb. The stable mineral assemblage changes from anorthite+clinopyroxene solid solution +orthopyroxene at 13.5 kb to clinopyroxene solid solution +garnet+ minor quartz at 18 kb to clinopyroxene solid solution+ garnet+ minor quartz+minor kvanite at 22.5–36 kb. The proportion of garnet and its grossular content increases with increasing pressure. On addition of 1/2, 1 and 2 moles of jadeite to the initial mix, crystallization of garnet is markedly inhibited and the grossular content decreases. This study has application to the relative stability of clinopyroxene-kvanite or garnet-quartz assemblages in natural rocks such as eclogites and grospydites from kimberlite pipes.

An experimental investigation of the Gabbro-Eclogite transformation and some geophysical implications

A detailed experimental investigation of the gabbro-eclogite transformation in several basalts has been carried out. More than 200 runs at temperatures between 900 and 1,250°C and pressures up to 30 kbar were made, and the resultant phase assemblages studied by optical and X-ray techniques. In all the basalts studied, the transformation from the low-pressure gabbroic assemblage (pyroxene + plagioclase) into the high-pressure eclogite assemblage (garnet + pyroxene) was found to proceed via a transitional mineral assemblage characterised by co-existing garnet, pyroxene and plagioclase. This mineral assemblage closely resembles that displayed in nature by basic rocks in the garnet-clinopyroxene-granulite sub-facies. The width of the transitional assemblage (garnet granulite) varies from 3.4 to 12 kbar at 1,100°C in different basalts and the pressures required for incoming of garnet and elimination of plagioclase likewise vary widely according to the particular basaltic composition studied. These variations are caused by small differences in chemical composition, and are interpreted in terms of the principal mineral equilibria which are involved in the transformation. In passing from gabbro to eclogite via the garnet granulite assemblage, the proportion of garnet is observed to increase regularly with pressure, whilst the proportion of plagioclase decreases regularly. Thus the increase in density and seismic velocity from gabbro (ρ ~ 3.0 g/cm 3, V p ~ 6.9 km/sec) to eclogite (ρ ~ 3.5 g/cm 3, V p ~ 8.3 km/sec) is uniformly smeared out over the entire garnet granultte transition interval. The effect of temperature upon the pressures required for the gabbro-eclogite transformation is investigated. Our experimental results, together with the results of other workers on simple systems closely related to the gabbro-eclogite transformation, strongly indicate that eclogite would be stable relative to gabbro and garnet granulite throughout large regions of the normal continental crust. This conclusion has important tectonic implications. The bearing of the experimental results upon the hypothesis that the continental Mohorovičlć discontinuity is caused by an isochemical transformation from gabbro to eclogite is examined. It is concluded that this hypothesis must be rejected on the following grounds: 1. (1) Eclogite, not gabbro, appears to be stable throughout the continental crust. 2. (2) The experimental temperature gradient of the transformation is incompatible with inferred geophysical relationships between the temperature at the base of the crust and thickness of the crust. 3. (3) The transformation cannot explain the seismic velocity distribution in the crust. 4. (4) Minor changes in chemical composition strongly affect the pressure required for eclogite stability. This makes it difficult to understand the uniformity of crustal thickness in stable continental regions. 5. (5) The density of eclogite (3.5 mg/cm 3) is higher than the density of the upper mantle inferred from gravity observations. 6. (6) An upper mantle of eclogitic composition would be gravitationally unstable. The chemical composition of the continental crust is discussed in the light of the widely accepted geophysical model that it consists of a layer of granitic material overlying or passing gradually downwards into a gabbroic layer. Our experimental results are in conflict with the view that the lower crust is generally of gabbroic composition. It is more likely that the lower crust consists on the average of intermediate rocks in the eclogite facies. Although the gabbro-eclogite transformation is not believed to play a significant role in the structure of stable continental regions, it may be of major importance in tectonically active areas where the Mohorovičić discontinuity cannot be clearly recognized e.g., regions of recent orogenesis, continental margins, island arcs and mid-oceanic ridges. It is shown that the basalt-eclogite transformation may provide a tectonic engine of great orogenic significance. Large volumes of basalt, when extruded and intruded at and near the earth's surface may become transformed to eclogite under suitable circumstances on cooling. Because of the high density of eclogite, such large scale transformations would generate gravitational instability. Large blocks of eclogite would sink through the crust, dragging it down initially into a geosyncline, and later causing extensive deformation (folding). Because the density of eclogite (3.5 g/cm 3) is greater than that of the ultramafic rocks which make up most of the mantle (3.3 g/cm 3) blocks of eclogite will sink deep into the mantle, and may undergo partial fusion, leading to generation of andesitic and granodioritic magmas which rise upwards and intrude the folded geosyncline.